Heat Treatments by Basic Definition
A heat treatment is an engineered solution that enhances or modifies the physical and/or mechanical properties of metal. Specific — and very much controlled — heating and cooling methods at specific temperatures and periods of time can dramatically alter the physical and mechanical properties without changing the product shape.
Heat treatments are often applied to improved tensile strength of materials, it can be used to modify its ductility or change its elongation properties. It is often applied to relieve residual stresses caused by external working, such as bending, welding, or machining.
Heat treating is often used to modify the material hardness which comes about through a change in the internal atomic structure.
The most common forms of onsite heat treatments that we apply are:
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Hydrogen Bake-Out
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Refractory Drying
Post Weld Heat Treatment
By far our most common heat treatment application is “Post Weld Heat Treatment” (PWHT).
PWHT in Accordance with AWS D10.10M
Post Weld Heat Treatment is performed after welding, generally at a higher temperature and with different objectives than preheat/inter-pass heating.
PWHT of carbon and low alloy steels is typically performed below the lower critical transformation temperature and is therefore referred to as subcritical. The lower and upper critical transformation temperatures indicate where the crystal structure of steel begins and finally completes a change from body centered cubic (BCC) to face centered cubic (FCC) upon heating and the reverse upon cooling. In simple terms the molecular structure of the metal will reform and change configuration if the temperature is allowed to reach the upper critical transformation. The temperature and duration at which the material is treated is where/how the mechanical properties of the metal are change - such as hardness and ductility.
The primary benefits of PWHT are tempering, relaxation of residual stresses and hydrogen removal. When applied appropriately PWHT helps to prevent hydrogen induced cracking, improves dimensional stability, improves ductility and notch toughness and most importantly improves corrosion resistance.
Keep in mind excessive or inappropriate PWHT temperatures and/or long holding times can adversely affect your mechanical properties such as decreased tensile strength, reduced creep strength, and poor notch toughness caused primarily by embrittlement due to precipitate formation.
The influence of PWHT on the materials properties is a function of the weld and base metal composition and the prior thermal and mechanical processing of the base metal. If PWHT is run at higher than specified temperatures and / or longer specified soak times the work piece can develop an undesirable grain structure, suffer carbon precipitation which can undermine the mechanical tensile/yield and elongation properties of the material.
Post weld heat treatment is generally specified and governed by codes and standards such as ASME, API or ASTM but are also defined through client specific manufacturing processes such as welding procedures, or machinability criteria, or repair requirements defined by governing codes and standards. PWHT is also generally triggered by material type, thickness and critical or severe operational environments. These fabrication codes provide detailed requirements regarding PWHT.
The need for PWHT based upon service environment is not always treated by fabrication codes and standards. Instead guidance may be found in recommended practices regarding service environment and are often defined by client/end user experience and hence custom specifications which meet or exceed generally defined codes and standards.
The application of a PWHT, whether directed by code or desired by a customer, is a critical evolution and requires appropriate scrutiny of quality control checks and balances to ensure the PWHT process can meet the require engineering specifications. It ensures the work piece is correctly reconditioned, giving the customer the specified levels of hardness, strength, and ductility. Attention to each phase of the process is essential to ensuring the correct cycle/ start/end temperatures, ramp rates, soak temperatures and soak duration are specified, executed, documented, and published so the desired material performance is achieved.
Understanding the Areas of the Work Piece
When dealing with the work piece there are some important areas that you need to be aware of and key terms, and they are: Weld area (w), heat affected zone (HAZ), nominal thickness (t), the diameter (D), and radius (R).
Weld Area & Heat Affected Zone
The terms Weld Area and Heat Affected Zone (HAZ) are defined in accordance with D10.10M: (i) the Weld Area is the "Widest width of butt or attachment weld." The Heat Effected Zone (HAZ) as seen in the below figure is described as, the area of the base material on the metal which has had its microstructure and properties altered by welding, heat effected cut zone, induction bending or work hardened area. The PWHT process is intended to restore the HAZ to a similar grain structure and mechanical properties exhibited by the parent material.
Stress Relief Heat Treatment
Residual stress is an internal stress that is not a result of externally applied loads. If stress buildup in the weldment is excessive, the fatigue life of the metal is reduced.
Importance of Stress Relief Heat Treatment
Cold working, hot rolling, grinding, quenching treatments, welding, and thermal cutting can all induce residual stress into metal. The nature of residual stress, its distribution, and prediction of the level within a metal is a complex and not completely understood phenomenon.
The welding process creates a prime source of residual stress caused in part by the rapid thermal expansion and contraction created in a very localized area where molten steel is created in excess of 3000°F and rapidly cools to a solid from the resultant cooler surrounding area. The metal expands as it is brought to a molten state. As the molten weld pool solidifies along the joint, there is resistance to its shrinkage by the already solidified weld metal and the un-melted base metal adjacent to the weld. This cooling process creates a very fine grain structure and results in a tensile strain in the longitudinal and transverse directions of the weld. Distortion is often the result, and if the stress is excessive, buckling, stress corrosion cracking, and shortened fatigue life are possible.
All welds will have some residual stress, and it will never be totally reduced to zero strain. Heat input, base metal thickness, cooling rate, restraint of the weldment, and welding process play roles in the level of residual stress induced into a weldment.
Tempering
Tempering is a heat treating process which is used to increase the toughness of iron-based carbon alloys. Tempering is often performed to control material hardness after normalizing and quenching. Tempering is done at temperatures well below the normalizing state and used to control yield and tensile strength, reflected in a measurable hardness range. In general, higher tempering temperatures result in reduced tensile performance but improved ductility and elongation performance. The performance benefit and response depends on both the specific composition of the alloy and on the desired properties in the finished product.
Annealing Heat Treatment
Annealing, in metallurgy and materials science, is a heat treatment that alters a material to increase its ductility and to make it more workable. It involves heating a material to above its critical temperature, maintaining a suitable temperature, and then cooling. Annealing can induce ductility, soften material, relieve internal stresses, refine the structure by making it homogeneous, and improve cold working properties.
Various types of Annealing processes are common, and often involve applying heat at extreme temperatures, followed by rapid cooling using a variety of specified techniques.
Normalizing Heat Treatment
Normalizing of steel* is a heat-treating process that is often considered from both thermal and microstructure standpoints. In the thermal sense, normalizing is an austenitizing heating cycle followed by cooling in still or slightly agitated air. Typically, the work is heated to a temperature about 55 °C (100 °F) above the upper critical line of the iron-iron carbide phase diagram, above Ac3 for hypo eutectoid steels and above Acm for hypereutectoid steels. To be properly classed as a normalizing treatment, the heating portion of the process must produce a homogeneous austenitic phase (face-centered cubic, or FFC, crystal structure) prior to cooling. Normalizing is also frequently thought of in terms of microstructure. The areas of the microstructure that contain about 0.8% C are pearlitic (lamellae of ferrite and iron carbide). The areas that are low in carbon are ferritic (body-centered cubic, or bcc, crystal structure). In hypereutectoid steels, proeutectoid iron carbide first forms along austenite grain boundaries. This transformation continues until the carbon level in the austenite reaches approximately 0.8%, at which time a eutectoid reaction begins as indicated by the formation of pearlite. Air-hardening steels are excluded from the class of normalized steels because they do not exhibit the normal pearlitic microstructure that characterizes normalized steels.
*ASM Handbook Volume 4, Heat Treating (ASM International), Published: 1990, Pages: 35-41.