PHYSICAL METALLURGY OF STEEL Chapter II

Like I said before, that Piping & Fabrication will continue this post of Physical Metallurgy of Steel Chapter II, and here we getting more and more of information of piping materials.

It is the effective use by the metallurgist of the knowledge contained on this and similar phase diagrams that allows for the manipulation of properties of engineering materials by varying their chemistry and heat treatment. For steel, the principal phases and their properties are briefly summarized in the following list:

Austenite: A single-phase solid solution of carbon in gamma iron (FCC). It exists in ordinary steels only at elevated temperatures, but it is also found at room temperatures, but it is also found at room temperature in certain stainless steels (e.g., 18 Cr–8 Ni type) classified as austenitic stainless steels. This structure has high ductility and toughness.

FIGURE A3.18 Iron-carbon equilibrium diagram

Ferrite: Alpha iron (BCC), containing a small amount of carbon (0.04–0.05 percent) in solid solution. This phase is soft, ductile, and relatively weak. Cementite: Iron carbide, Fe3C, a compound containing

PHYSICAL METALLURGY OF STEEL (chapter I)

After trying to make this mood back, I mean good mood, now I'll try to make one step ahead, even still I don't where to start but with Piping & Fabrication, I will try to be patient until that chance will comes to me. And now we discuss about Physical Metallurgy of Steel chapter I.

Like all other metals, iron and steel are crystalline in structure, composed of atoms in a fixed lattice. As noted earlier, iron may exist in one of two cubic forms, bodycentered (BCC) or face-centered (FCC).

At room temperature, pure iron is composed of a body-centered cubic lattice. In this form it is known as alpha iron, also called ferrite, which is soft, ductile, and magnetic.When heated above about 1415_F (768_C), alpha iron loses its magnetism but retains its body-centered crystalline structure. This temperature is called the Fermi temperature. The crystal structure changes to face-centered cubic at about 1670_F (910_C), at which temperature alpha iron is transformed to gamma iron, the FCC form, and remains nonmagnetic. As temperature rises further, another phase change occurs at 2570_F (1410_C), when delta iron is formed. This phase is again body-centered like that of the low-temperature alpha iron. It is stable to the melting temperature. In cooling very slowly from the liquid state, the phases reappear in reverse order.

The solid-state transformations of atomic structure,

General Definition of Steel

The General Definition of Steel as Piping Material will explain by Piping & Fabrication more detail and this is the explanation of Steel.
Steel is defined as an alloy of iron with not more than 2.0 weight percent carbon. The most common method of producing steel is to refine pig iron by oxidation of impurities and excess carbon, which have a greater affinity for oxygen than iron.

The principal reduction methods used are the basic oxygen process (BOP) and the electric furnace process, each representing a type of furnace in which the refining takes place. The BOP primarily uses molten pig iron as the initial furnace charge; the electric furnace can use a charge of selected steel scrap. Another process, called the basic open-hearth process, is no longer in use in the United States. Although it constituted the major steel producing process for decades, it has succumbed to the more advanced and economical BOP and electric furnaces.

The pig iron is reduced to the desired steel composition through use of acid and/or basic reactions with fluxing agents, heat, oxygen, and time.

METALLIC MATERIALS ON PIPING MATERIALS

Metallic Materials is one of piping materials that Piping & Fabrication will expose today, and with this post I hope we all know what is Metallic Materials on Piping Materials.

Metals are divided into two types: ferrous, which includes iron and iron-base alloys; and nonferrous, covering other metals and alloys. Metallurgy deals with the extraction of metals from ores and also with the combining, treating, and processing of metals into useful engineering materials. This section presents the fundamental metallurgical concepts and practices associated with the most common engineering metals, and outlines metallurgical considerations appropriate in the selection process of metals for piping system construction.

Ferrous Metals
Metallic iron, one of the most common of metals, is very rarely found in nature in its pure form. It occurs in the form of mineral oxides (Fe2O3 or Fe3O4), and as such it comprises about 6 percent of the earth’s crust. The first step in the production of iron and steel is the reduction of the ore with coke and limestone in the blast furnace. In this process, the oxygen is removed from the ore, leaving a mixture of
iron and carbon and small amounts of other elements as impurities. Coke is the reducing element and source of heat. The limestone (CaCO3) acts as a fluxing agent which combines with impurities of the ore in the molten state and floats them to the top of the molten metal pool, where they can be removed as slag. The product removed from the blast furnace is called pig iron and is an impure form of iron containing about 4 percent carbon by weight percent. Liquid pig iron cast from the blast furnace is sometimes used directly formetal castings. More often, however, the iron is remelted in a cupola, or furnace, to further refine it and adjust its composition.

Cast Iron
Pig iron that has been remelted is known as cast iron, a term applicable to iron possessing carbon in excess of 2 weight percent. Compared with steel, cast iron is inferior in malleability, strength, toughness, and ductility. On the other hand, cast iron has better fluidity in the molten state and can be cast satisfactorily into

Hardenability and Property Stability in Piping Materials

Hardenability and Property Stability is one of the important in Piping Materials and also Piping & Fabrication will expose this topics more deep and deep for our knowledge.

Hardenability
This is a property of certain steels that allows them to be strengthened, or hardened, by heat treating. In carbon and alloy steels, for example, this hardening is accomplished by heating the material to a temperature above about 1550_F (843_C), where the material completely changes its crystal structure from BCC to FCC. When this is followed by rapid cooling or quenching, usually in water or oil, the result is a crystal structure akin to the original BCC, but distorted along one of the unit cell directions. In the case of steels the result is amartensitic structure possessing a lattice, termed a body-centered tetragonal (BCT), with a larger volume per unit cell than the starting BCC.

The maximum hardness achieved in a quenched structure is primarily a function of the steel’s carbon content: the higher the carbon content, the greater the hardness. The depth into the material to which a high hardness is achieved for a given quenching operation is a function of the total alloy content within the steel. The substitutional alloying element nickel has perhaps the strongest effect on increasing the depth to which hardness extends.3 Other elements creating similar if less potent effects are manganese and boron, substitutional and interstitial alloying elements, respectively. Standard specimens and procedures have been adopted for testing the harden ability of steels. The test rates a combination of the highest hardness achievable and the depth to which significant elevation of hardness occurs. It is called the JominyEnd-Quench test and is performed using a 1-in-diameter cylindrical specimen machined from the metal in question and heated to a temperature in its austenitic phase (FCC) region.

Other Metallurgical Properties of Metals

In addition to the properties already described on Piping & Fabrication, other characteristics of metals can have an important effect on the design process. These may profoundly affect the uniformity, achievable level, or stability of mechanical strength and ductility over long periods of usage. and the rest of Other Metallurgical Properties of Metals is here.

Grain Size
Upon solidification from the molten state, metals take crystalline form. Rather than a single, large crystal, the material consists of many small crystals that initiate independently and nearly simultaneously from separate nuclei sites. These individual crystals are called grains, and their outer surfaces are called grain boundaries. Grains form initially during the solidification process, but they may also reform, grow, or rearrange while in the solid state.

FIGURE A3.15 Sketch illustrating individual grain growth from nuclei and dendrites

Some properties of many engineering metals are very dependent on grain size (Fig. A3.15). For example, austenitic stainless steels, such as Type 304 (18% Cr-8% Ni-Fe), possess excellent creep strength when the material has a coarse grain structure, but very poor strength with fine (small) grains. If this same austenitic material is plastically cold-worked, these grains will become distorted and possess high levels of lattice strain and residual stress. Subsequent heat treatment can cause the crystal lattice to reform unstrained grains initiating at lattice defects which act as nuclei. The process, called recrystallization, results in an initially very small grain size as the nucleated stress-free grains begin to grow. If heavily strained material is placed into elevated temperature service at temperatures sufficient to cause recrystallization, it will initially exhibit good creep strength until the grains begin to reform, upon which the result is very poor creep rupture strength. The material will only return to its prestrained creep-strength level if additional heat treatment is performed, resulting in further grain growth.

Grain size is a material characteristic that is sometimes directly inspected in the base material testing and certification process. The test entails retrieving a piece of the material and then metallographically polishing and etching the specimen with a weak acid solution, which reveals the grain boundaries under magnification.

The test is described in ASTM Specification E 112. Grain size can be measured and reported a number of ways. The most commonly used method involves reporting grain size as an ASTM grain size number (n), corresponding to the exponent of the following equation:

ASTM has correlated this grain size number, which increases as grain diameter decreases, to a series of photographs representing the grain structure at 100 magnifications. The grain size number can then be estimated by visual comparison. Examples of this comparative standard are shown in Fig. A3.16. Fine grained carbon and low alloy steels tend to possess better notch toughness and ductility than coarse grained steel. As noted earlier, as operating temperature increases into the creep regime, engineering material strength properties are usually enhanced with coarser grains. Although this is an oversimplified (and perhaps overstated) rule of thumb, it is important for the engineer to take grain size into account for critical structures.

Density, Thermal Conductivity, Thermal Expansion, Specific Heat on Physical Properties of Metals

On this post today we will talk about Physical Properties of Metals such as Density, Thermal Conductivity, Thermal Expansion, Specific Heat, and all those thing will explain here only at Piping & Fabrication.
Physical properties are those, other than mechanical properties, that pertain to the physics of a material. Physical properties of importance to the materials and design engineer are material density, thermal conductivity, thermal expansion, and specific heat.

Density. 

Density is the ratio of the mass of a material to its volume. In vessel and piping design, the density of a construction material versus its strength per unit area of cross section is often an important consideration.

Thermal Conductivity. 

This is the characteristic ability of a material to transmit energy in the form of heat from a high-temperature source to a point of lower temperature. The ability to transmit heat is usually expressed as a coefficient of thermal conductivity (k) whose units are a quantity of heat transmitted through a unit thickness per unit time per unit area per unit difference in temperature. For example:

The lower the value of k, the more resistant the material is to the flow of thermal energy. Good insulators possess low coefficients of thermal conductivity. Thermal conductivity is a function of the temperature of the material. For example, the coefficient of thermal conductivity of carbon steel decreases as its temperature increases, thereby decreasing its ability to transfer heat energy. Austenitic stainless steels, on the other hand, increase in k value with temperature. However, they remain lower than carbon steels in normal piping system temperature ranges.

Thermal Expansion. 

Expressed as the coefficient of linear expansion, thermal expansion is a ratio of the change in length per degree of temperature, to a length at a given standard temperature (such as room temperature, or the freezing point of water). The units of the coefficient are length of growth per unit length per degree of temperature. The value of the coefficient varies with temperature.

Specific Heat. 

This is a measure of the quantity of heat required to raise a unit weight of a material one degree in temperature.

Elevated Temperature Tensile and Creep Strength

When Piping Materials now already inside the Elevated Temperature Tensile and Creep Strength, Piping & Fabrication still waiting something more and more, only for your satisfaction.

FIGURE A3.12 Effect of alternating stresses with and without corrosion

Tensile tests are performed at elevated temperatures to characterize the material’s yield and ultimate tensile properties at potential use temperatures above room temperature. A heating chamber is combined with a conventional tensile testing machine, and special strain measuring extensometers are used that are capable of withstanding the test temperatures. Generally, as temperature increases, yield and ultimate strengths decrease, and ductility increases.

FIGUREA3.13 Creep time versus elongation curves at a given temperature

Creep is defined as the time-dependent deformation of a material that occurs under load at elevated temperatures. The test is performed by holding a specimen, similar in configuration to a tensile specimen, at a uniform temperature and a constant load (usually using a dead weight) and allowing the specimen to gradually elongate to ultimate failure. The practice is defined in ASTM Specification E 139. The simplest test method records only the applied stress (based on original test specimen cross section), time to failure, and total elongation at failure. This is called a stress rupture test. If periodic measurements of strain accumulation versus test duration are also taken, the test is referred to as a creep-rupture test.

A representation of typical creep strain-versus-time data is shown in Fig. A3.13. Three stages of creep behavior are exhibited. Upon initial loading, instantaneous straining occurs. Almost immediately, the rate of creep strain accumulation (creep rate) is high but continuously decreasing. The test then progresses into a phase where the strain rate slows and becomes fairly constant for a long period of time. Finally, with decreasing load-bearing cross section of the specimen due to specimen stretching and necking, applied stress begins to increase steadily, as does the creep rate, until failure occurs. These three regions are termed the primary, secondary, and tertiary stages of creep. The intent of safe design practice is to avoid the third stage, where strain accumulations are rapid and material behavior less predictable.

FIGURE A3.14 Typical stress-to-rupture curves

After accumulating a number of rupture data points (i.e., time to failure of a metal at various applied stresses), the data is generally represented as a stress rupture curve (Fig. A3.14). Each curve represents the time to failure at various applied stresses, at a given test temperature. Another useful property that can be measured in these tests is the creep rate during the second stage of creep, for a given applied stress and temperature. This, along with time to onset of the tertiary creep stage, are useful properties
to the design engineer and are used in establishing allowable design tension stresses in design codes.

Metals that experience creep will accumulate a progressively larger amount of microscopic damage to the structure of the material. Damage is first observed microscopically as small cavities, or voids, that begin appearing in the grain boundaries of the metal, particularly at triple points (i.e., where three grains come together). Further progression of damage entails formation of more voids along many of the adjacent grain boundaries, until ultimately they link together to form grain boundary microcracks. With more time, these form larger macrocracks that lead to ultimate failure of the metal component. The determination of a metal’s degree of creep damage, and its consequence on the continued safe operation of the component, has developed into a sophisticated science referred to as component condition assessment, or estimation of remaining life. This will be addressed in more detail later in the chapter.

A practice essentially identical to cyclic fraction life summation used in fatigue design can be employed in material creep analysis to estimate the percentage of creep life expended. Here the individual life fraction corresponds to the amount of time a component spends at a given stress and temperature, compared to the total time to failure given on the stress rupture curve for the same applied stress and temperature. All of these fractions for all the operating conditions are then added together, and compared to an appropriate design limit (1.0 or less).

Fatigue Resistance of Piping Materials

The ability of a metal to resist crack initiation and further propagation under repeated cyclic loading is a measure of its fatigue resistance. Everything with Piping & Fabrication will continuing the explanation of this post and this is full explanation of fatique resistance. 

Several standardized test methods have been developed to test metals, machined to particular geometries, where applying a repeating load range. Loads are generally applied through bending, cantilevered, or push-pull load application in suitably outfitted testing machines. Either constant applied stress or strain ranges can be employed to determine material response.

The most common representation of fatigue test data is an S-N curve, relating stress (S) required to cause specimen failure in a given number of cycles (N) (Fig. A3.10a). These tests are generally performed on smooth specimens, but they can also be run with stress-concentrating mechanisms such as notches machined into the specimen surface. The effect of stress concentrations on fatigue life cycles can also be estimated from the smooth specimen S-N curve by calculating the intensified stress due to the particular geometry, and intersecting the curve at that point on the stress axis.

FIGURE A3.8 Transition temperature range and transition temperature in Charpy impact test

FIGURE A3.9 Drop-weight test specimen with brittle weld deposit
on specimen face; machined notch to act as crack starter.
Impact load applied from side opposite weld deposit

As the applied load range decreases, ferritic steels exhibit a point at which an infinite number of cycles can be absorbed without causing failure. This level of stress is called the endurance limit. Many of the other metals do not exhibit this behavior, but rather exhibit an increasing, but finite, number of cycles to failure with decreasing cyclic load (Fig. A3.10b).

FIGURE A3.10 S-N curves that typify fatigue test results (a) for testing medium-strength
steels and (b) showing typical curve shape for ferrous and nonferrous materials. SL is the
endurance limit

The fatigue resistance of a material at a given applied stress or strain range is a function of a number of variables, including material strength and ductility. Results may vary significantly for different surface finishes, product forms of the same material (Fig. A3.11), material internal cleanliness, test specimen orientation, and levels of residual stress, among other factors. Variations in the test environment can also have a profound effect on test results (Fig. A3.12). Therefore, fatigue test results characteristically exhibit significant scatter.

Fatigue design curves are generated from test data by applying large safety margins to the average property curve. In U.S. design codes, the fatigue design curve is commonly generated by taking the lesser of 1/20 times the cycles to failure, or 1/2 of the stress to cause failure. A new curve is constructed taking the lower bound of these two factored curves.

FIGURE A3.11 Fatigue characteristics (S-N curve) for cast and wrought 1040 steel in the normalized
and tempered condition, both notched and unnotched. R. R. Moore rotating beam tests, Kt 2.2

When considering metal fatigue in design, a further safety margin is often also applied against the cycles-to-failure at a given stress amplitude. For example, if a component is continuously cycled over the same stress range, a design limit on allowable cycles may correspond to the cycle life multiplied by a factor such as 0.8. This is a common safety margin employed in vessel and piping design. As is normally the case, components may experience a wide variety of cyclic stress ranges, at various temperatures, over their life. The effect of this array of cyclic parameters on fatigue life can be estimated by an approach referred to as life fraction summation. In this design practice, the percentage of life used up in cycling at a certain stress range is calculated, corresponding to the ratio of the number of actual service duty cycles to the total number of cycles to failure at that stress range. This calculation is performed for all of the various stress ranges/duty cycles anticipated. The fractions thereby calculated are summed and compared to the design limit (1.0 with no safety margin, or 0.8 or some other value depending on the design safety factor that applies).

Piping Material with Hardness and Toughness of the Material's Ability

Continuing the Piping Material with Hardness and Toughness of the material's ability and Piping & Fabrication will give the detail information of it.

Hardness

This is a measure of the material’s ability to resist deformation, usually determined by a standardized test where the surface resistance to indentation is measured. The most common hardness tests are defined by the indentor type and size, and the amount of load applied. The hardness numbers constitute a nondimensioned, arbitrary scale, with increasing numbers representing harder surfaces. The two most common hardness test methods are Brinell Hardness and Rockwell Hardness, with each representing a standardized test machine with its own unique hardness scales. Hardness loosely correlates with ultimate tensile strength in metals (Fig. A3.5). Approximate hardness conversion numbers for a variety of material types, including steels, can be found in ASTM Specification E140.

FIGURE A3.4 Offset method of determining yield strength

Toughness
Sudden fracture, exhibiting little ductility in the vicinity of the break, occurs in certain metals when load is rapidly applied. The capability of a material to resist such a brittle fracture is a measure of its toughness. Highly ductile materials (those possessing an FCC lattice, for example) exhibit considerable toughness across a full range of temperatures. Other materials, such as BCC-based carbon steels, possess a level of toughness that is dependent on the metal temperature when the load is applied. In these metals, a transition from brittle to ductile behavior occurs over a narrow range of temperatures.

 

FIGURE A3.5 Conversion chart for Brinell and Rockwell
hardness numbers, giving corresponding tensile strength for
steel. Based on hardness conversion table

The two most common methods used to measure metal toughness are the Charpy Impact test, defined in ASTM specification E 23, and the Drop-Weight test, defined in ASTM E 208. The Charpy test employs a small machined specimen with a machined notch that is struck by a pendulum weight (Fig. A3.6). The energy loss to the pendulum as it passes through and breaks the specimen (Fig. A3.7), measured in kilojoules or ft _ lb of force, is a measure of the toughness of the specimen. Typical impact behavior versus test temperature is shown in Fig. A3.8.

The Drop-Weight test is similar in principle but employs a larger specimen with a brittle, notched weld bead used as the crack starter (Fig. A3.9). A weight is dropped from a height onto the specimen, which had been cooled or heated to the desired test temperature. The test determines the nil-ductility transition temperature (NDTT), defined as the specimen temperature when, upon striking, the crack propagates across the entire specimen width. Propagates across the entire specimen width.

 

FIGURE A3.6 Charpy (simple beam) subsize (Type A) impact test specimens

The Charpy brittle transition temperature (sometimes called the Charpy fix temperature) and the Drop Weight NDTT are both important design considerations for those materials that can exhibit poor toughness and that may operate in lower temperature regimes. In pressure vessel and piping design codes, limits are placed on material minimum use temperature based on adding an increment of margin over and above the Charpy fix or NDTT. Operating at or above this elevated temperature is then usually sufficient to avoid brittle, catastrophic failure, as for example is the case when at a temperature on the ‘‘upper shelf’’ of the Charpy Vnotch toughness-versus-temperature curve.

 

FIGURE A3.7 Charpy V-notch specimen placement during strike by testing anvil

Mechanical Properties of Metals, Modulus of Elasticity, Yield Strength, etc on Pipe Systems

Mechanical Properties of Metals is one of the important part on pipe systems in this case is piping materials which is contain Modulus of Elasticity (Young's Modulus), Yield Strength, Ultimate Tensile Strength, Elongation and Reduction Area, and that's all of thing will explain in this chapter only at Piping & Fabrication.

Mechanical properties are critically important to the design process. They are defined as the characteristic response of amaterial to applied force. The standardized test methods for measuring these properties are described in ASTM specifications. Properties fall into two general categories, strength and ductility. Some properties, such as material toughness, are dependent on both strength and ductility. The most widely known and used material properties, as defined by ASTM, are described in the following paragraphs.

FIGURE A3.2 Tension-test specimens

Modulus of Elasticity (Young’s Modulus). 

The modulus of elasticity is the ratio of normal stress to corresponding strain for tensile or compressive stresses. This ratio is linear through a range of stress, known as Hooke’s law. The material behavior in this range is elastic (i.e., if the applied load is released the material will return to its original, unstressed shape). The value of the slope in the elastic range is defined as Young’s Modulus. The modulus of elasticity is measured using the tension test, the most widely used test applied to engineering materials. The test consists of applying a gradually increasing load in either tension or compression, in a testing machine, to a standardized test specimen (Fig. A3.2). The applied load is continuously monitored, as is test specimen elongation or contraction under load. These measured quantities are generally represented on a coordinate axis, called a stress-strain curve (Fig. A3.3). The modulus of elasticity and other strength properties are established from this curve. Values of the modulus of elasticity for a number of construction materials are given in Table A3.1

FIGURE A3.3 Stress-strain diagram

Yield Strength. 

When a specimen is loaded beyond the point where elastic behavior can be maintained the specimen will begin to deform in a plastic manner. Most materials do not abruptly transform from purely elastic to purely plastic behavior. Rather,

Chemical Properties of Metals on Pipe System

Chemical properties are herein defined as those material characteristics that are dictated by the elemental constituency of the solid. The explanation will continue with Piping & Fabrication and let's continue. This is usually measured by the relative atomic weight percent of the various elements (metals or nonmetals) or compounds within the material. Metals are not usually used in their pure form. Rather, secondary elements are purposely added to improve or modify their behavior. This addition of secondary elements is called alloying, and the elements added fall into two categories, based on the relative size of the atoms. Atoms significantly smaller than those of the parent metal matrix fit into spaces between the atoms in the lattices’ interstices and are called interstitial alloying elements. Carbon added to iron, creating steel, is the most common example. Larger-sized atoms will substitute for parent metal atoms in their matrix locations, thus the name substitutional alloying elements. Examples of this include zinc substituting for copper atoms in copper, creating brass; and tin substituting for copper atoms, creating bronze alloys.

Pure metals possess relatively low strength. Adding an alloying element will increase the strength of a metal’s atomic matrix because the atomic lattice is strained locally by the foreign atom, creating a larger impediment for the sliding of planes of atoms across one another during plastic flow. This is true whether the alloying element is interstitial or substitutional; however, the former generally serve as better lattice strengtheners. Strength properties are often improved to the detriment of ductility. Proper alloying, combined with appropriate metal processing and heat treatment, results in optimization of material properties. Elements are also added to metals to improve or modify their corrosion or oxidation characteristics, or to improve manufacturability (e.g., machine ability) and/or electrical properties, among other effects. However, it is important to note that alloying done to optimize one material property may act to the detriment of others.

Carbon steels, the most common of the construction materials, always contain the elements carbon, manganese, phosphorous, sulfur, and silicon in varying amounts. Small amounts of other elements may be found either entering as gases during the steel-making process (hydrogen, oxygen, nitrogen), or introduced through the ores or metal scrap used to make the steel (nickel, copper, molybdenum, chromium, tin, antimony, etc.). The specific effect of each of these elements on steel properties will be addressed later in the chapter. Addition of significant quantities of the interstitial element carbon will result in high strength and hardness—but to the detriment of formability and weld ability. A great amount of research has gone into the development of the principal metals used in piping design and construction; thus the specification limits must be vigorously adhered to in order to assure reliability, predictability, and repeatability of material behavior.

The number of elements alloyed with a parent metal, and the acceptable range of content of each, are identified in the material specification (e.g., ASTM, API, ASME). Tests appropriate for determining the elemental constituency of an alloy have been standardized and are also described in ASTM specifications. The material specifications also stipulate whether the chemical analysis of an alloy may be reported by analyzing a sample of the molten metal, or taken from a specimen removed from the final product. The former is commonly referred to as a ladle analysis, and the latter as a product or check analysis. This ‘‘chemistry’’ of a construction material is reported on a material test report which may be supplied by the material manufacturer upon request.

MATERIAL PROPERTIES OF PIPING MATERIALS

FIGURE A3.1 The three most common crystal structures in metals
and alloys. (a) Face-centered cubic (FCC); (b) body-centered
cubic (BCC); (c) hexagonal close-packed (HCP).

The behavior of piping material can be understood and predicted by studying a number of properties of the material. Appreciation of how a material will perform must extend all the way down to the atomic components of the material. Metals are crystalline in structure, composed of atoms in precise locations within a space lattice, and Piping & Fabrication will explain more and more about Material Properties of Piping Materials.

The smallest component of the crystalline structure is called a unit cell, the smallest repeating building block of the material. For example, iron and iron-based alloys exist in two unit cell forms, the body-centered cubic (BCC) and the face centered cubic (FCC) structure, shown in Fig. A3.1. They are differentiated in the way the atoms are arranged in repeating patterns. The body-centered cubic structure is represented by a cube with atoms at all eight corners, and one atom in the center of the cube. The face-centered lattice is represented by atoms at the eight corners of the cube, plus one atom located at the center of each of the cube’s six faces. The crystal structure naturally assumed by a material dictates some of the fundamental properties of the material. For example, FCC materials are generally more ductile than BCC materials. This is basically because FCC crystals are the most tightly packed of metallic structures and, as such, allow for more planes of atoms to slide across one another with the least amount of resistance (the fundamental atomic motion involved in what is called plasticity).

Metallic materials consist of these and other ordered crystal structures. Some metals, most notably iron, change their crystal structure as temperature varies. Structure may also change as certain other elements are added in the form of alloying additions. These changes are used to advantage by metallurgists and are the basis for developing and manipulating important material behavior, such as the heat treatability of carbon and low alloy steels.

Plastics may be defined as synthetic materials whose chief component is a resin or resin equivalent. The term plastic covers a very broad range of materials that contain, as an essential ingredient, one or more organic polymetic substances. They possess large molecular weight, formed by the chemical combination of carbonhydrogen atom chains (monomers to polymers). The atomic structure is thus ordered and predictable, but dissimilar from that of metals. Many plastics have greater strength per unitweight than metal, but suffer due to lower impact strength, chemical stability, and thermal and aging stability. However, plastics fill an important niche in the piping engineer’s repertoire.

Ceramic materials are composed of the oxides of metal arranged in ordered atomic structures similar to that of metals. The atomic constituents are electronically different, resulting in rigid, predictable behavior, but with an inherent lack of plasticity compared to metals.

Glasses form the other extreme of the atomic structure spectrum. Their atomic makeup is essentially that of a liquid; the structure is actually a solid with no ordered arrangement of atoms. These atomic characteristics (i.e., the natural arrangement of the atoms, as well as the specific elements involved and their electronic characteristics) establish the fundamental properties of engineering materials. The properties that a engineer requires to design and construct a piping system are a manifestation of the longerrange effects of atomic structure. These properties fall into three categories: chemical, mechanical, and physical.

PIPING MATERIAL ON PIPE SYSTEMS

The selection of materials for piping applications is a process that requires consideration of material characteristics appropriate for the required service. Now Piping & Fabrication will go with Piping Material on Pipe Systems, and here there rest of the articles. Material selected must be suitable for the flow medium and the given operating conditions of temperature and pressure safely during the intended design life of the product. Mechanical strength must be appropriate for long-term service, and resist operational variables such as thermal or mechanical cycling. Extremes in application temperature can raise issues with material capabilities ranging from brittle fracture toughness at low temperatures to adequacy of creep strength and oxidation resistance at the other end of the temperature spectrum.

In addition, the operating environment surrounding the pipe or piping component must be considered. Degradation of material properties or loss of effective load-carrying cross section can occur through corrosion, erosion, or a combination of the two. The nature of the substances that are contained by the piping is also an important factor.

The fabric ability characteristics of the materials being considered must also be taken into account. The ability to be bent or formed, suitability for welding or other methods of joining, ease of heat treatment, and uniformity and stability of the resultant microstructure and properties all of a given piping material contribute toward or detract from its attractiveness and economy. The selection process should lead to the most economical material that meets the requirements of the service conditions and codes and standards that apply.
Applicable design and construction codes such as the ASME Boiler and Pressure Vessel Code and the ASME B31 Pressure Piping Code identify acceptable materials for piping systems within their jurisdiction. These codes specify the design rules, allowable design stresses, and other properties required to accomplish the design task. However, the information supplied is generally only adequate and intended to assure safe operation under the thermal and mechanical conditions expected under steady-state and sometimes (as in nuclear construction) cyclic operation.

These codes do not directly and explicitly address the many other environmental and material degradation issues that should be considered by design and materials engineers in arriving at a piping system that is not only safe to operate but will offer long-term, reliable service and function. Thus, simply designing to ‘‘the Code’’ when selecting materials can sometimes lead to premature end-of-life of piping system components.

This chapter will attempt to identify the important metallurgical characteristics of piping materials and how they can affect or be affected by operation of all of the other materials available to the engineer. Carbon and low-alloy steels come closest to being the ideal construction material. Due to the fact that the majority of piping applications employ iron-based metals, these will be emphasized in this chapter

CONCRETE, CEMENT, AND CEMENT LINED PIPE ON PIPING SYSTEM

Finally, all the job that I have to do is finish, even only for today, at least I can take some rest. and continue with Piping & Fabrication with Concrete, Cement and Cement Lined Pipe which will explain all the details, only here at this blog.

Nonreinforced Concrete Pipe
Nonreinforced concrete pipe for the conveyance of sewage, industrial waste, and storm water is made in sizes from NPS 4 to NPS 36 (DN 100 to DN 900). It is produced in accordance with ASTM Specification C14, Standard Specifications for Concrete Sewer Storm Drain and Culvert Pipe. Nonreinforced-concrete drain tile is used for land drainage and for subsurface drainage of highways, railroads, airports, and buildings. It is made in sizes from NPS 4 through 36 (DN 100 through 900) in accordance with ASTM Specification C412, Standard Specification for Concrete Drain Tile, and AASHO M178, Standard Specification for Concrete Drain Tile. Drain tile is available in the standard quality, extra-quality, and special-quality classifications. Perforated concrete pipe used for under-drainage is made in accordance with ASTM Specification C444, Specifications for Perforated Concrete Pipe. This pipe is also made in sizes NPS 4 through 36 (DN 100 through 900) and is available in the standard-strength and extra-strength classification. Concrete irrigation pipe, used for the conveyance of irrigation water under low hydrostatic heads and for land drainage, is made in sizes NPS 4 through 24 (DN 100 through 600) in accordance with ASTM Specification C118, Standard Specifications for Concrete Pipe for Irrigation or Drainage. Nonreinforced-concrete irrigation pipe for use with rubber-type gasket joints is made for conveyance of irrigation water at water pressures of 1 bar (35 ft of head) or higher depending on the diameter. Such pipe is made in sizes NPS 6 through 24 (DN 100 through 600) in accordance with ASTM Specification C505, Specifications for Nonreinforced Concrete Irrigation Pipe with Rubber Type Gasket Joints. Physical and dimensional requirements of standard-strength bell-and-spigot nonreinforced-concrete sewer pipe are tabulated in Table A2.26.

Jointing.
Rubber-gasketed joints for C14 and C76 pipe are covered by ASTM Specification C443, Joints for Circular Concrete Sewer and Culvert Pipe, Using Flexible, Watertight, Rubber-Type Joints.

Reinforced-Concrete Pipe
Reinforced-concrete pipe for the conveyance of sewage, industrial wastes, and storm water and for the construction of culverts is made in sizes from NPS 12 to 144 (DN300 through 3600). Reinforced-concrete pipe may or may not be manufactured for use with rubber gaskets to seal the joints. It is usually manufactured in accordance with the following specifications:
•    ASTM C76, Specifications for Reinforced Concrete Culvert, Storm Drain and Sewer Pipe
•    AASHO M170, Specifications for Reinforced Concrete Culvert, Storm Drain and Sewer Pipe
•    Federal SS-P-375-Pipe, Concrete (Reinforced, Sewer)
Reinforced-concrete pipe may be made with either tongue-and-groove or belland-spigot joints. When made for use with rubber gaskets, the joints must conform to ASTM Specification C443 or AASHO Specification M198, Specifications for Joints for Circular Concrete Sewer and Culvert Pipe, Using Flexible Watertight, Rubber-Type Gaskets. Concrete pipe is available also in both an arch and an elliptical cross section. These pipes are made in accordance with the following specifications:
•    ASTM C506, Specifications for Reinforced Concrete Arch Culvert, Storm Drain and Sewer Pipe
•    ASTM C507, Specifications for Reinforced Concrete Elliptical Culvert, Storm Drain and Sewer Pipe
In each of the standards covering reinforced-concrete pipe, five strength classes are defined in terms of minimum three-edge bearing load at a crack width of 0.01 in (0.25 mm) and at the ultimate strength of the pipe. The strength class required for a given installation is determined by computing the earth load and live load which will be transferred to the pipe under the conditions anticipated. This load is then converted to an equivalent three-edge bearing load by dividing it by a bedding factor. The bedding factor depends upon installation conditions and is always greater than 1.0.

TABLE A2.26 Physical and Dimensional Requirements of Class 1, Bell-and-Spigot Nonreinforced Concrete Sewer Pipe (ASTM C1488; for Class 2 and Class 3 refer to ASTM C14)

Cement-Lined Steel, Ductiles, and Cast-Iron Pipe
Cement-lined pipe is well established for use in cold-water lines. Substantial quantities of cement-lined steel pipe are used for other applications where corrosion is more of a problem. The largest user, by far, is the petroleum industry in oil field flow lines, pipelines, tubing, and casing. Cement-lined pipe is particularly suitable for these applications because of the presence in the oil fields of saltwater, hydrogen sulfide, carbon dioxide, and other corrosive material. Other applications include lines in salt works for handling brine, discharge lines in coal mines for carrying highly corrosive sulfur water, lines in paper and pulp mills for handling diluted acids and corrosive waste liquids, and lines in process plants where water or other liquids must be kept free from iron contamination or rust. Cement-lined pipe is generally joined with screwed seal rings which prevent the corrosive liquid from coming in contact with steel. Flanged joints are also extensively used. Some prefabrication is done of piping assemblies involving welding of the steel joints. Field joining of the preassembled welded assemblies is then done with flanged ends. Cement of course must not be at the pipe ends being welded. After welding, these ends are filled with mortar.

Grooved Segmented Ring Coupling and Flange Joints on Pipe Systems

Huff.... I felt so boring today, may be because I'm to tired, but for Piping & Fabrication I will keep posting, and we reach to the explanation of Grooved Segmented Ring Coupling and Flange Joints. I hope this will make us more understand about piping systems because now I want to take a rest for a while.

Grooved Segmented-Ring Coupling
The type of split coupling shown in Fig. A2.34 is used with either ductile cast-iron or steel pipe that has grooves near the ends which enable the coupling to grip the pipe, in order to prevent disengagement of the joint. The couplings are manufactured in a minimum of two segments for small pipe sizes and several segments for large pipe sizes. Grooved-end fittings are available for use with the couplings. With proper choice of gasket material, the joint is suitable for use above- or underground with nearly any fluid or gas. The joint’s advantages are its

FIGURE A2.34 Victualic coupling for grooved end cast iron steel pipe

FIGURE A2.35 Screwed-on cast-iron flange

FIGURE A2.36 High-hub cast-iron flanges with bitumastic
to protect the exposed threads

•    Ability to absorb minor angular and axial deflections
•    Ability to increase gasket sealing force with increased system pressure
Refer to AWWA C.606, Standard for Grooved and Shouldered Joints.
•    Simplicity for rapid erection or dismantling for systems requiring frequent disassembly.
The coupling is also available in a style where grooving of the pipe ends is not required. Joint separation is prevented by the use of hardened steel inserts (teeth) which grab the mating pipe ends.

TABLE A2.25 Standard Dimensions of Class 125 Flanged Joints for Silver Brazing with
Centrifugally Cast Pipe

Flanged Joints
Flanged ductile or cast-iron pipe is used aboveground for low and intermediate pressures in water-pumping stations, gas works, power and industrial plants, oil refineries, booster stations for water, and gas and oil transmission lines. Cast iron flanges usually are faced and drilled according to ASME B16.1. For flanged joints in a ductile iron pipe, refer to ASME B16.42, ANSI/AWWA C110/A21.10, C111/A21.11, C115/A21.15, and C153/A21.53. Cast-iron pipe is made both with integrally cast flanges and with threaded companion flanges for screwing onto the pipe (as shown in Figs. A2.35 and A2.36). In the latter case, the outside diameter of the pipe conforms to iron pipe size (IPS) dimensions to allow for the threads provided. It is available in sizes NPS 3 (DN 50) through NPS 24 (DN 600) and in length to 18 ft (5.5 m). For lengths less than 3 ft (1 m), in sizes NPS 3 (DN 50) through NPS 12 (DN 300), the flanges may be cast integrally with the pipe, rather than screwed on the pipe, at the manufacturer’s option. Standard dimensions of flanged joints for silver brazing are shown in Table A2.25.

Ball and Socket Joints, Universal Pipe Joints, Compression Sleeve Joints on Pipe Systems

Let us continue again with All about Piping, Fabrication, Welding, etc. only at Piping & Fabrication, and now let us know more about Ball and Socket Joints, Universal Pipe Joints and Compression Sleeve Joints. and here the rest of them.

Ball-and-Socket Joints

FIGURE A2.31 Ball-and-socket mechanical joints

For river crossings, submarine lines, or other places where great flexibility is necessary, ductile cast-iron pipe can be obtained with ball-and-socket joints of the mechanical-gland types, as shown in Fig. A2.31. Provision is made for longitudinal expansion and contraction, and a positive stop against disengagement of the joint is a feature of the design. As much as 15_ angular deflection can be accommodated without leakage. This pipe is heavy enough to remain underwater where laid without requiring river clamps or anchorage devices. The pipe may be pulled across streams with a cable, since the joints are positively locked against separating, or it may be laid directly from a barge, bridge, or pontoons, without the services of a diver. The mechanical ball-and-socket joint is suitable for use with water, sewage, air, gas, oil, and other fluids at considerable pressure. Either bell-and-spigot or mechanical (gland-type) fittings can be used with this line of pipe, although the integral ball present on the spigot end of some designs has to be cut off before the pipe can be inserted in a regular bell.

Universal Pipe Joints

FIGURE A2.32 Universal cast-iron pipe joint

This type of cast-iron pipe joint (shown in Fig. A2.32) has a machined taper seat which obviates the need for caulking or for a compression gasket.

JOINING DUCTILE OR CAST-IRON PIPE

After I've got new task from the Boss which is I have to do safety patrol, that means I have to control the safety of the employee when they work such as erection, brazing, welding, chamfering, etc, and that's make my times reduced to take a post for this Piping & Fabrication. But that's doesn't matter because I will keep posting and today we talk about Joining Ductile or Cast Iron Pipe and here we go.

Bell-and-Spigot Joint
This joint for underground cast-iron pipe was developed as long ago as 1785. Standard dimensions are shown in Table A2.22. The joint may be made up with lead and oakum, sulfur compounds, or cement. Lead and oakum constitute the prevailing joint sealer for sanitary systems. Belland spigot joints are usually reserved for sanitary sewer systems. These joints are not used in ductile iron pipe.

Mechanical (Gland-Type) Joint
This modification of the bell-and-spigot joints, as designated in Federal Specification WW-P-421 and ANSI/AWWA C111/A21.11, is illustrated in Table A2.23. This joint is commonly used for low and intermediate-pressure gas distribution systems, particularly those conveying natural gas or dry manufactured gas. Mechanical joints are also used for water lines, sewage, and process piping. In the mechanical (gland-type) joint shown in Fig. A2.30, the lead and oakum of the conventional bell-and-spigot joint are supplanted by a stuffing box in which a rubber or composition packing ring, with or without a metal or canvas tip or canvas backing, is compressed by a ductile cast-iron follower ring drawn up with bolts. In addition to making an inherently tight joint even under considerable pressure, this arrangement has the advantage of permitting relatively large lateral deflections (3 1/2* to 7*), as well as longitudinal expansion or contraction. For more details, refer to AWWA C600, Standard for Installation of Ductile-Iron Water Mains and Their Appurtenances.

FIGURE A2.30 Mechanical (gland-type) joint

Tyton Joint
The Tyton joint is designed to contain an elongated grooved gasket. The inside contour of the socket bell provides a seat for the circular rubber in a modified bulbshaped gasket. An internal ridge in the socket fits into the groove of the gasket.

Know Brazed Joints More on Piping & Fabrication

Know Brazed Joints More on Piping & Fabrication is one of the post that I will gave to all of you now and hope that would be useful.

Brazed Joints
Lap or shear-type joints generally are necessary to provide capillary attraction for brazing of connecting pipe. Square groove butt joints may be brazed, but the results are unreliable unless the ends of the pipe or tube are accurately prepared, plane and square, and the joint is aligned carefully, as in a jig. High strengths may be obtained with butt joints if they are properly prepared and brazed.

However, owing to the brittleness of the brazing alloy, they are not normally applicable. The alloys generally used in brazing exhibit their greatest strength when the thickness of the alloy in the lap area is
minimal. Thin alloy sections also develop the highest ductility. For brazing ferrous and nonferrous piping with silver and copper-base brazing alloys, the thickness of the brazing alloy in the joint generally should not be more than 0.006 in (0.15 mm) and preferably not more than 0.004 in (0.1 mm). Thicknesses less than 0.003 in (0.07 mm) may make assembly difficult, while those greater than 0.006 in (0.15 mm) tend to produce joints having lowered strength. The brazing of certain aluminum alloys is similar in most respects to the brazing of other materials. However, joint clearances should be greater because of a somewhat more sluggish flow of the brazing alloys. 

FIGURE A2.28 Eccentric insertion of consumable insert ring in pipe welded in the fixed horizontal pipe position.

FIGURE A2.29 Examples of typical fillet welded joints

For aluminum, a clearance of 0.005 to 0.010 in (0.12 to 0.25 mm) will be found satisfactory. Care must be exercised in fitting dissimilar metals, since the joint clearance at brazing temperature is the controlling factor. In these cases, consideration must be given to the relative expansion rates of the materials being joined.
The length of lap in a joint, the shear strength of the brazing alloy, and the average percentage of the brazing surface area that normally bonds are the principal factors determining the strength of brazed joints. The shear strength may be calculated by multiplying the width by the length of lap by the percentages of bond area and by taking into consideration the shear strength of the alloy used. 

TABLE A2.22 Standard Dimensions of Bell-and-Spigot Joints for Pipe Centrifugally Cast in
Metal Molds

TABLE A2.23 Standard Dimensions of Mechanical (Gland-Type) Joints

An empirical method of determining the lap distance is to take it as twice the thickness of the thinner or weaker member joined. Normally this will provide adequate strength, but in cases of doubt, the fundamental calculations should be employed. Such detailed determinations are generally unnecessary for brazed piping, since commercial brazing fittings are available in which the length of lap is predetermined at a safe value. For brass and copper pipe, cast or wrought bronze and wrought copper fittings are available. A bore of correct depth to accept the pipe is provided and midway down this bore may be a groove into which, at the time of manufacture, a ring of brazing alloy is inserted. Since the alloy is preplaced in fittings with such a groove, separate feeding of brazing alloy by hand is generally unnecessary.

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Consumable Insert Ring and Fillet Weld on Pipe Systems

I though I will felt fresh after take a holiday even only for a day, but the fact is the whole body is feel so numb, so sick, that's because I push my body too much when I looking for the rent house for me and my family, so the post of Piping & Fabrication delayed. But now Piping & Fabrication continue with Consumable Insert Ring and Fillet Weld on Pipe Systems.

Consumable Insert Rings. 

The chemical composition of a piping base metal is established primarily to provide it with certain mechanical, physical, or corrosion resisting properties. Weld ability characteristics, if considered at all, are of secondary concern. On the other hand, the chemical composition of most welding filler metals is determined with primary emphasis on producing a sound, high-quality weld. The steelmaking process employed in the manufacture of welding filler metals permits closer control of the composition range, which is usually considerably narrower than would be practical for the piping base metal where much larger tonnages of steel are involved. On some base metals, the welding together by fusion of only the base-metal compositions may lead to such welding difficulties as cracking or porosity. The addition of filler metal tends to improve weld quality. However, in inert gas tungsten-arc welding, the addition of welding filler metal from a separate wire, which the welder feeds with one hand while manipulating the tungsten arc torch with the other, is a cumbersome process and interferes with welding ease. The welder may leave areas with lack of penetration, which generally are considered unacceptable as can be seen, e.g., in the rules of the ASME Boiler and Pressure Vessel Code. Since some types of serious weld defects are detected only with difficulty during inspection (if they are detected at all), it is extremely important to provide the easiest welding conditions for the welder to produce quality welds.

FIGURE A2.26 Commercial consumable insert rings used in pipe welding (MIL-I-23413).Style D: for NPS 2 and larger. On Schedule 5 for NPS 5 and larger; style E: for NPS less than 2. On Schedule 5 for NPS less than 5.

One technique to produce high-quality welds is to employ consumable insert rings of proper composition and dimensions. Consumable insert rings which are available commercially are shown in Fig. A2.26. The three primary functions of consumable insert rings are to (1) provide the easiest welding conditions and thereby minimize the effects of undesirable welding variables caused by the ‘‘human’’ element, (2) give the most favorable weld contour to resist cracking resulting from weld-metal shrinkage and hot shortness, or brittleness, in hot metal, and (3) produce metallurgically the soundest possible weld-metal composition of desirable strength, ductility, and toughness properties.

The best welding conditions are obtained where the flat-land and extended U-bevel preparations are used. These joint preparations are particularly helpful where welding is done in the horizontal fixed pipe position (5G), since they ensure a flat or slightly convex root contour and provide by far the greatest resistance to weld cracking in those alloys particularly susceptible to microfissuring.

The weld-root contour conditions to be expected from different bevel preparations and consumable insert rings are illustrated in Fig. A2.27. Where sink is not acceptable, it is considered obligatory to use consumable insert rings with the special flat-land or extended U-bevel preparation. In horizontal-rolled (IG) and vertical-position (2G) welding, the insert ring should be placed concentrically into the beveled pipe. In horizontal fixed-position (5G) welding, the insert ring should be placed eccentric to the centerline of the pipe (as shown in Fig. A2.28). In this position, the insert ring compensates for the downward sag of the molten weld metal and aids in obtaining smooth, uniform root contour along the inner diameter and the joint.

Fillet Welds
Circumferential fillet-welded joints are generally used for joining pipe to socket joints in sizes NPS 2 (DN 50) and smaller. Figure A2.29 illustrates three typical fillet-welded joints. These types of welds are subjected to shearing and bending stresses, and adequate penetration of the pieces being joined is essential. This is particularly important with the socket joint, since the danger of washing down the end of the hub may obscure, by reason of fair appearance, the lack of a full and sound fillet weld. This condition is one which cannot be detected in the finished weld by the usual visual inspection. Additionally, a 1/16-in (1.5-mm) gap (before welding) must be maintained between the pipe end and the base of the fitting to allow for differential expansion of the mating elements.

There are service applications in which socket welds are not acceptable. Piping systems involving nuclear or radioactive service or corrosive service with solutions which promote stress corrosion cracking or concentration cell action generally require butt welds in all pipe sizes with complete weld penetration to the inside of the piping.

Know More about Backing Rings on Piping Systems

The Blogger has unavailable status yesterday, so Piping & Fabrication can not continue the post for this blog, but now I will continue and now we will Know More about Backing Rings on Piping Systems.

backing rings

Backing rings are employed in some piping systems, particularly where pipe joints are welded primarily by the shielded metal-arc welding process with covered electrodes. For example, a significant number of pipe welds for steam power plants and several other applications are made with the use of backing rings. On the other hand, in many applications backing rings are not used, since they may restrict flow, provide crevices for the entrapment of corrosive substances, enhance susceptibility to stress corrosion cracking, or introduce still other objectionable features. Thus, there is little, if any, use made of backing rings in most refinery piping, radioactive service piping, or chemical process piping. The use of backing rings is primarily confined to carbon and low-alloy steel and aluminum piping. Carbon-steel backing rings are generally made of a mild carbon steel with a maximum carbon content of 0.20 percent and a maximum sulfur content of 0.05 percent. The latter requirement is especially important since high sulfur in deposited weld metal (which could be created by an excessive sulfur content in such rings) may cause weld cracks. Split backing rings are satisfactory for service piping systems. For the more critical service applications involving carbon- and low-alloy steel piping, solid flat or taper-machined backing rings are preferred in accordance with the recommendations shown in Pipe Fabrication Institute Standard ES1 and illustrated in Fig. A2.24 and Table A2.21.

When a machined backing ring is desired, it is a general recommendation that welding ends be machined on the inside diameter in accordance with the Pipe Fabrication Institute standard for the most critical services and then only when pierced seamless pipe that complies with the applicable specifications of the American Society for Testing and Materials is used. Such critical services include high pressure steam lines between boiler and turbines and high-pressure boiler feed discharge lines, as encountered in modern steam power plants. It is also recommended that the material of the backing ring be compatible with the chemical composition of the pipe, valve, fitting, or flange with which it is to be used. Where materials of dissimilar composition are being joined, the composition of the backing ring may be that of the lower alloy.
On turned-and-bored and fusion-welded pipe, the design of the backing ring and internal machining, if any, should be a matter of agreement between the customer and the fabricator. Regardless of the type of backing rings used, it is recommended that the general contour of the welding bevel shown in Fig. A2.24 be maintained.
When machining piping for backing rings, the resulting wall thickness should be not less than that required for the service pressure. Wherever internal machining for machined backing rings is required on pipe and welding fittings in smaller sizes and lower schedule numbers than those listed in Table A2.21, weld metal may have to be deposited on the inside of the pipe in the area to be machined. This is to provide satisfactory contact between the machined surface on the pipe inside and the machined backing ring. For such cases, the machining dimension should be a matter of agreement between the fabricator and the purchaser.

Whenever pipe and welding fittings in the sizes and schedule numbers listed in Table A2.21 have plus tolerance on the outside diameter, it also may be necessary to deposit weld metal on the inside of the pipe or welding fitting in the area to be machined. In such cases, sufficient weld metal should be deposited to result in an ID not greater than the nominal ID given in Table A2.21 for the particular pipe size and wall thickness involved.

Experience indicates that machining to dimension C for the pipe size and schedule number listed in Table A2.21 generally will result in a satisfactory seat contact of 7/32 in (5.5 mm) minimum (approximately 75 percent minimum length of contact) between pipe and the 10_ backing ring. Occasionally, however, it will be necessary to deposit weld metal on the inside diameter of the pipe or welding fitting in order to provide sufficient material for machining a satisfactory seat. In welding butt joints with backing rings, care should be exercised to ensure good fusion of the first weld pass into the backing ring in order to avoid lack of weld penetration or other types of stress-raising notches.

Just to remembering you, all this post is connected with the others and you better read those one to, thanks!