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.

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

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