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.