Nonferrous Material & Heat Treatment Methods

Still I try to focus with this Piping & Fabrication blog, even I post this blog rarely, but I think that would be enough to gave some information about Piping System.

Nonferrous Materials. Bending and forming of nonferrous materials may result in undesirable work-hardening. Some nickel alloys may be subject to carbide precipitation when hot bent or formed. Materials that can be hardened by precipitation require other considerations. Depending on the final use, it may be desirable to perform some type of postbending or forming heat treatment. Because of the great many new materials being developed and used, it is suggested that the user contact the material manufacturers or material associations for their recommendations on the specific material and service.

Heat Treatment Methods. Shop heat treatments are most often carried out in specifically designed heat treatment furnaces, but local stress relieving of welds may also involve induction, resistance, or torch heating. Above critical heat treatments, such as annealing, normalizing, and normalizing and tempering for ferritic steel and carbide solution heat treatment for austenitic stainless steels, are performed in large heat-treatment furnaces. These same furnaces are also used for stress-relieving heat treatments of ferritic steels. Such furnaces are generally fired with natural gas, propane, or low-sulfur oil. Depending on their design, they may attain temperatures up to 2300°F (1260°C) which covers the entire spectrum of temperatures commonly encountered in piping applications. Heating and cooling rates and holding temperatures are automatically controlled. Larger furnaces may have two or more zones, each independently controlled. Records of furnace zone temperatures and material temperatures are obtained using recording potentiometers.

When assemblies are too large or furnaces are not available, local stress relieving of individual welds may be accomplished in the shop using electrical induction, electrical resistance, or gas torch heating. Induction equipment involves alternating current frequencies of the order of 60 to 400 Hz. Induction generates heat within the wall of the pipe. This has the advantage of amore uniform temperature through the thickness with greater uniformity at the lower frequencies. The heat treatment cycle is controlled automatically with thermocouples attached directly on or adjacent to the weld. The weld and

thermocouple are covered with insulating material. The induction field is generated in copper cables or solid or water-cooled copper coils external to the insulation.

Resistance heating involves the use of direct current in suitable lengths of nichrome heating wire. Various configurations and sizes of prefabricated heating elements consisting of heating wires separated by ceramic beads are available commercially. Depending on the size, wall thickness, and desired heating temperature, multiple heating units and combinations of elements may be needed. The weld and heating elements are covered with insulating blankets to retain the heat. Since heating is from one side, a somewhat wider heating band on the outside may be needed to assure that the inside of the pipe attains the required temperature. Thermocouples attached directly to the weld or adjacent to it are used to control heating, holding, and cooling temperatures. Torch heating can often be used for stress relieving, but where controlled heating and cooling rates are mandated, it may be less than satisfactory. Single torches may be used for pipe up to about NPS 3 (DN 80), but ring burners are needed for larger sizes. Exothermic heating has been used in the field and is discussed in the section ‘‘Installation.’’

Austenitic Stainless Steels

Austenitic Stainless Steels. Today Piping & Fabrication will talk about and this is the rest of post. Austenitic stainless steels do not undergo phase changes like the ferritic steels. They remain austenitic at all temperatures and so heat treatments usually do not apply. When austenitic stainless steels are to be used in corrosive services, cold working and heating for bending may significantly lower their corrosion resistance. Cold working may result in residual stresses, and heating operations can result in sensitization. Both factors contribute to intergranular stress corrosion cracking (IGSCC). When austenitic stainless steels are heated in the range of about 800 to 1600°F (430 to 870°C), carbon in excess of about 0.02 percent will come out of solution and diffuse to the grain boundaries where it will combine with adjacent chromium to form chromium carbide (Cr23C6). This phenomenon is called sensitization. These grain boundaries are then preferentially attacked by corrosive media. The heat treatment often applied to cold-worked and sensitized stainless steels to restore corrosion resistance is a carbide solution heat treatment. In this procedure, the material is heated to a temperature above the sensitization range, usually about (1950 to 2100°F (1065 to 1150°C), and held there sufficiently long to permit the carbides to dissolve and the carbon to go back into solid solution. The material is then removed from the furnace and rapidly cooled through the sensitization range, preferably by quenching in water. The rapid cooling does not give the carbon sufficient time to come out of solution, and corrosion resistance is restored to the sensitized area.

Heat treatment cycles

Obviously carbide solution heat treatment is limited by the furnace size and quenching facilities. It is most freqently applied to bends but is also useful in reducing sensitization and residual stresses in welds.

HEAT TREATMENT

Now after all the busy time I had pass, so I'll try to post this Piping & Fabrication with the topic Heat Treatment.

Purpose. Heat treatment during piping fabrication is performed for a variety of reasons (i.e., to soften material for working, to relieve fabrication stresses, to restore metallurgical and physical properties, etc.). During fabrication, ferritic steels undergo phase changes during heating and cooling, while the austenitic stainless steels and nonferrous piping materials do not; consequently differing criteria must be applied.

Ferritic Steels. Ferritic steels undergo a phase change on heating and cooling during fabrication operations because their principal component (iron) is allotropic; that is, it undergoes a change in crystalline structure with temperature. At room temperature iron favors a body-centered cubic (BCC) structure called alpha iron, but on heating to 1670°F (910°C) it changes to a face-centered cubic (FCC) structure called gamma iron and subsequently at 2534°F (1390°C) it reverts to a BCC called delta iron. The addition of carbon to the iron to form steel and additions of other elements such as chromium, manganese, molybdenum, and nickel to form alloys modify the temperatures at which transformation occurs and the manner in which the crystalline structure forms into grains.

As an example, a melt of 0.30 percent carbon steel will first begin to solidify as delta iron and a liquid, then at about 2680°F (1479°C) to an interstitial solid solution of carbon in gamma iron called austenite.At about 1500°F (815°C) this will transform into a mixture of austenite and ferrite, which at 1333°F (721°C) becomes ferrite and pearlite. Ferrite is alpha iron which contains small amounts of carbon (up to a maximum of about 0.02 percent) in solid solution. The excess carbon not in solid solution with the ferrite forms as iron carbide (Fe3C) or cementite. The cementite forms as thin plates alternating with ferrite. This structure is known as pearlite. The temperatures at which the transformations occur are called critical temperatures or transformation temperatures. The lower critical temperature, usually designated A1, is that point on heating where the BCC ferrite and pearlite phase begins to transform to FCC austenitic structure, and the upper critical temperature, A3, is the temperature at which the transformation is complete. Between these two points the structure is a mix of ferrite-pearlite and austenite. These temperatures are of importance in postbending and postwelding heat treatments as well as qualification of welding procedures.

The critical temperatures are a function of chemical composition and as such will vary with alloy. As an example, for 9Cr-1Mo-V, the lower critical is located between 1525 and 1560°F (830 and 850°C), and the upper critical is between 1650 and 1725°F (900 and 940°C). Some approximate methods of calculating critical temperature are found in Welding Metallurgy34 and The Making, Shaping and Treating of Steel.

Critical temperatures are affected by heating and cooling rates. An increase in heating rate will serve to increase the transformation temperatures, while an increase in cooling rate will tend to depress them. The more rapid the rate of heating or cooling, the greater the variation from the critical temperature at equilibrium conditions. Most sources will indicate the lower and upper critical temperatures on heating as Ac1 and Ac3, respectively, and the upper and lower on cooling as the Ar3 and Ar1, respectively. In the case of our 0.30 percent carbon steel, cooling from the austenite phase through the critical range at a rate of 50°F/h (28°C/h) or less will result in the soft, ductile ferrite-pearlite structure. On the other hand, extremely rapid cooling from the austenite phase down to temperatures 600°F (316°C) or lower can result in an extremely hard structure called martensite. This is because the austenite FCC crystals did not have time to transform to BCC ferrite and cementite.

Heat treatments which are applied to ferritic steels are related to the critical temperatures and depending on which is applied will have differing results. These are annealing, normalizing, normalizing and tempering, and stress relieving.  Annealing is used to reduce hardness, improve machinability, or produce a more uniform microstructure. It involves heating to a temperature above the upper critical or to a point within the critical range, holding for a period of time to assure temperature uniformity, then following with a slow furnace-controlled cooling through the critical range. Normalizing is used to refine and homogenize the grain structure and to provide more uniform mechanical properties and higher resistance to impact loadings. It involves heating to a temperature above the upper critical temperature, holding for a time to permit complete transformation to austenite, and cooling in still air from the austenitizing temperature. A normalized structure may be pearlitic, bainitic, or even martensitic depending on the cooling rate. If there is a concern for excessive hardness and attendant low ductility, a tempering treatment may follow the normalizing treatment. Tempering involves heating to a temperature below the lower critical and slowly cooling to room temperature, much like a stress relief. The degree of tempering depends on the tempering temperature selected. The higher the tempering temperature, the greater the degree of softening.

A stress-relieving heat treatment is primarily intended to reduce residual stresses resulting from bending and welding. It involves heating to a temperature below the lower critical; holding for a predetermined time, depending on thickness and material, to permit the residual stresses to creep out; and then slowly cooling to room temperature.

Brazing and Soldering

Everything that Piping & Fabrication try to explain is only for the information to all of us, and hope that will help us to have more knowledge about piping system. and now we will talk about Brazing and Soldering.

Brazing. For services involving the ASME Boiler and Pressure Vessel Code or the B31 Code for Pressure Piping, brazing procedures and brazers must be qualified in accordance with ASME Section IX similar to welding procedures and welders. See the section ‘‘Procedure and Personnel Qualification.’’ There are a great many types of brazing processes. In establishing a brazing procedure, consideration must be given to the ability of the filler metal to produce suitable physical properties, its melting point and wettability, possible base metal and filler metal interactions, loss of base metal properties, increased sensitization to corrosion, increased hardness in the base metal due to brazing temperature, and the need for postbrazing heat treatments.

Since most piping materials can be welded, the use of brazing for joining is rather limited. It is most often used for joining coppers and for combinations of metals which cannot be welded. Brazing is a process wherein the base metals do not melt, the filler metal has a liquidus above 840°F (450°C), and the filler metal wets the base metal and is drawn into the joint by capillary action. Although butt or scarf joints can be used, a lapped joint with an overlap of 3 times the thickness of the thinner member gives the best joint efficiency and ease of fabrication. It should be noted that typical copper or brass fittings have a depth of socket based on the strength of tin-lead solders. When brazing is used, only a small percentage of that depth is needed. Required clearance between the faying surfaces usually vary from 0.001 to 0.010 in (0.025 to 0.25 mm) depending on the filler and flux combination used during the operation. The flux melts upon application of heat and is displaced by the molten filler metal. Flux residue should be removed after the operation is complete. Silver, copper-phosphorus, and copper-zinc filler metals are most often used for copper brazing.

Torch brazing is commonly used for fabrication and installation of copper piping systems. For torch brazing, the type of fuel gas selected is a function of the melting temperature required to melt the filler metal. For piping joints NPS 2 (DN 50) and larger, use of a second torch to preheat may be desirable. In brazing metals with differing coefficients of expansion, it is preferable that the metal with the higher expansion coefficient form the socket and the metal of the lower expansion coefficient form the pipe or tube. Clearance between the parts at room temperature must be adjusted so there will be a suitable clearance at brazing temperature. On cooling, the greater contraction of the socket will put the joint in a compressive stress state.

Soldering. Unlike welding and brazing, ASME Section IX has no requirements for qualification of soldering procedures or personnel. Soldering is much like brazing in that the base metals are not melted, the faying surfaces are wetted by the filler, and the filler is drawn into the joint by capillary action. However, the melting point of the filler metal is lower than 840°F (450°C) usually between 450 and 500°F (230 and 260°C). Since the strength of soldering filler metals is considerably less than that of brazing fillers, a longer overlap is required to develop a joint equal to base metal strength. A clearance of about 0.003 in is preferred for optimum strength. A good soldered joint depends again on the cleanliness of the faying surfaces. Fluxes are used to assist in the wetting action by removing tarnish films and to prevent oxidation. Rosin fluxes and organic fluxes are used for most materials. Inorganic fluxes may be required for certain other materials that can be soldered, while in some cases precoating of the material with a surface that can be soldered may be required. Most piping applications use tin-lead solders. These range in composition from 5 percent tin, 95 percent lead to 70 percent tin, 30 percent lead, with 50 percent tin, 50 percent lead the most common. Tin-antimony and tinsilver solders are also frequently used. For soldering aluminum, tin-zinc and zincaluminum are used.

Clad, Metal-Coated, and Lined Pipe

Clad, Metal-Coated, and Lined Pipe.
With the continuous of post from Piping & Fabrication, Clad, Metal-Coated and Lined Pipe will explain here. There are instances when it is economically desirable to construct a piping system from relatively inexpensive material but with an interior surface having corrosion- or erosion-resistant properties. Clad pipe may be made by seam welding of clad plate, by weld metal overlay of the inside surface, or by centrifugal casting of a pipe with two metal layers. Lined pipe is made by welding a linear, sometimes as strips, to the inside surface of the pipe. Metal-coated pipe is made by dipping, metal spraying, or plating the entire pipe.

Before choosing construction which requires welding of clad, lined, or metalcoated pipe, such factors as filler metal compatibility, filler metal strength relative to the base metal strength, dilution of base metal into the finished weld, and need for postfabrication heat treatment must be considered. Because it is not possible to cover the great many combinations of base metals and cladding, lining, or metal coatings, some examples of the more common applications will be given. For corrosion services, a carbon steel base material, clad or lined with austenitic stainless steel, is often used. The cladding is usually about 3⁄32 to 5⁄32 in thick. Where the inside of the weld is accessible, the preferred method is to weld the base metal from the outside with carbon steel filler metal, back-gouge the root from the inside, and weld the root from the inside with two or more passes of austenitic filler metal to minimize dilution from the base metal.  Where the inside surface is not accessible, a backing strip of the same composition as the cladding, fillet welded to the cladding on the upstream side may be used.
The root weld between the two clad surfaces and the austenitic backing strip is then made with austenitic filler metal. The root weld can also be made with the GTAW process using austenitic filler or preplaced inserts. The carbon steel should be removed for a sufficient distance back to preclude dilution into the root weld. In most instances, the balance of the weld is usually made with austenitic filler metal since it is not good practice to deposit carbon steel or low-alloy steel directly against the stainless steel deposit. See Fig. A6.22b. In some cases, nickel-base alloys are used for cladding where high-temperature corrosion is involved. The joints may be treated much like the austenitic cladding, except that appropriate nickel-base filler metals are used.

Some services require the use of carbon steel pipe nickel plated on the inside surface. Since the plating is relatively thin, different approaches are needed. First, as much fabrication as possible should be done prior to plating. For joints to be welded after plating, the ends to be prepared for welding should be buttered with nickel filler metal and machined to the required contour prior to plating. The root weld is made using the GTAW process with nickel filler metal.

Examples of welding clad

Some occasions require the use of aluminized pipe. Steel pipe is prefabricated and coated with aluminum by immersion in a bath of molten aluminum or by metal spray. Where the inside of the weld will not be accessible for metal spray, one method of joining is to counterbore the ends and use a solid machined backing ring which is fit and welded into one side of the joint prior to coating. After coating, the weld is made using an appropriate base metal process and filler, taking care not to blister the aluminum coating on the underside of the backing ring.

Galvanized steel pipe is often used for external corrosion applications. Since welding of galvanized pipe releases toxic vapors and since the welded area most often cannot be regalvanized, welding of galvanized pipe is not recommended. It is preferable that the assemblies be fabricated with provisions for mechanical joining in the field and then galvanized.

For services involving erosion, carbon steel pipe is often lined with cement or some type of abrasion-resistant material which cannot be welded. In this case the joints are butted together to minimize the gap between the adjacent linings. The weld is then made between the two carbon steel weld bevels, recognizing that full penetration through the carbon steel joint may not be achieved and that additional thickness may be necessary for strength. The gap between the adjacent linings is usually not a problem if only erosion is present.