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.