This article discusses the post weld heat treatment (PWHT) process and its effectiveness in reducing the residual stresses and improving the mechanical properties of weldments.
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What is PWHT?
Post weld heat treatment (PWHT) refers to any heat treatment performed after machining/welding to improve the mechanical and chemical properties of the machined surfaces/weldment. Welding induces residual stresses in a structure, which cause distortions in the structure in the long term. Moreover, the magnitude of stresses increases in more complex or bigger parts/structures.
During the welding process when the metals are joined together, hardening occurs in different degrees based on the carbon content in metals, specifically in the heat-affected zone near the weld metal deposit. The resultant residual stresses in the structure increase substantially due to solidification and melting and their magnitudes become almost equal to the yield strength of the base material.
Typically, PWHT is used to reduce the residual stresses and enhance the resistance to brittle fracture. Additionally, PWHT can also lead to material strength enhancements and hardness reduction. Stress relieving and post-heating are the most commonly used PWHT procedures. Application and code requirements as well as service environment primarily drive the need for PWHT.
Post Heating
Post heating is utilized to decrease the potential for hydrogen-induced cracking (HIC). HIC typically occurs when the weldment has a sensitive microstructure, a high level of stress, and/or a sufficient level of hydrogen.
In ferritic steels, hydrogen embrittlement occurs at temperatures close to ambient temperature. Cracking in such susceptible microstructures can be prevented by diffusing the hydrogen from the welded region immediately after welding before the steel is cooled down to room temperature.
The steel must be heated from the interpass temperature to the post heat temperature and held at the post heat temperature for a specific duration. Typically, the post-heating temperature is maintained at 230oC for one hour per inch of thickness.
Post heating can be a code requirement as both The American Society of Mechanical Engineers (ASME) Section III and the National Board Inspection Code (NBIC) have such provisions.
Moreover, post-heating is often needed for critical repairs, such as for the repairs defined under the fracture control plan (FCP) for nonredundant members of the American Association of State Highway and Transportation Officials/American Welding Society (AASHTO/AWS) D1.5 bridge welding code. However, post-heating is not required where the causes of hydrogen cracking are absent.
Stress Relieving
Stress relief heat treatment is applied to relax the stresses locked in a welded structure due to the manufacturing processes. The residual stresses are relaxed by heating a structure uniformly to a sufficiently high temperature, then subjecting it to thermal retardation for a specific duration based on the material thickness, and finally uniformly cooling the structure.
However, the heating temperature must remain under the lower transformation temperature range. For instance, carbon steels are often heated at 600 to 675 °C for one hour per inch. Thus, stress relief heat treatment reduces the risk of cracking in the weldments by reducing their hardness and increasing their ductility. The other major benefits of stress relieving PWHT include improvement in the metallurgical structure and relaxation of thermal stresses.
Importance of PWHT
In a study published in the journal Procedia Engineering, researchers investigated the effect of PWHT parameters, including holding time and holding temperature, on the relaxation of welding residual stresses in electron beam welded P91 steel plates to optimize the process.
A numerical thermo-elastic-plastic model was developed to simulate the plate welding, while the PWHT was implemented using Norton creep law and the residual stresses were determined after relaxation. The holding temperature was kept constant at 770°C and the holding time was varied to assess the effect of holding time on the residual stress relaxation.
Similarly, the holding time was kept constant at 1.5 h to determine the residual stress relaxation at 550oC, 650oC, and 770oC holding temperatures. The findings demonstrated that the holding time was significant for only up to two hours and no considerable relaxation in the residual stress was observed after that duration.
On the contrary, a higher relaxation in residual stresses was observed with the increasing holding temperature, indicating that the influence of holding temperature was significantly higher than the holding time in residual stress relaxation. However, the maximum holding temperature that can be used for PWHT is 770oC, as the holding temperature must remain below the Austenite start temperature.
Disadvantages of PWHT
Loss of strength
Excessive holding times or temperatures for stress relieving PWHT can reduce the strength of the material. Moreover, tempering treatments can also reduce the strengths of the quenched and tempered materials. Thus, temperature and time must be well controlled to prevent loss of strength.
Collapse or Distortion
The stress relief or tempering temperatures significantly lower the strength of the material. Thus, a structural component can buckle or distort if it experiences some load during the PWHT cycle, leading to extensive and costly damages. Hence, all structures that are being exposed to high temperatures during PWHT must be supported properly to prevent distortion.
Recent Studies on PWHT
In a recent study published in the Journal of Manufacturing Process, researchers investigated the effect of heat treatment modification during the PWHT on the mechanical properties and microstructure of dissimilar welds of Super304H and T91.
Tungsten inert gas welding was utilized with ERNiCr-3 as filler to fabricate the dissimilar metal welds. Subsequently, two PWHTs, including post-weld direct tempering (PWDT) and post-weld normalizing and tempering (PWNT), were performed. PWDT was performed for one hour at 760°C, while PWNT was performed for 30 min at 1050 °C and then tempered for one hour at 760oC.
The findings displayed a more uniform microstructure after PWNT compared to PWDT. Moreover, PWDT heat treatment led to coarse chromium-carbides at grain boundaries, indicating sensitization on the Super304H side, while the PWNT heat treatment resulted in small chromium-carbides, indicating a delay in the sensitization phenomenon.
The susceptibility of the fine-grained heat-affected zone for type IV cracking was identified and eliminated through the PWNT treatment on the T91 side. Additionally, the improved microstructure after PWNT treatment compared to PWDT treatment was well correlated with the improved mechanical properties after PWNT treatment.
For instance, the toughness, % elongation, tensile strength, and 0.2% proof stress for PWNT were 132 MJ/m3, 22%, 691 MPa, and 384 MPa, respectively, while the values of the same parameters for PWDT were 109 MJ/m3, 20%, 649 MPa, and 324 MPa, respectively. However, failure was observed in the weld metal region for both treatments due to coarse niobium/titanium intermetallic carbides.
To summarize, PWHT is one of the crucial processes that can be used after welding to effectively reduce the residual stress and improve the mechanical properties of the weldment.
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References and Further Reading
Venkata, K. A., Kumar, S., Dey, H.C., Smith, D.J., Bouchard, P. J., Truman, C. E. (2014). Study on the Effect of Post Weld Heat Treatment Parameters on the Relaxation of Welding Residual Stresses in Electron Beam Welded P91 Steel Plates. Procedia Engineering, 86, 223-233. ISSN 1877-7058. https://doi.org/10.1016/j.proeng.2014.11.032
Kumar, R., Varma, A., Kumar, Y. R., Neelakantan, S., Jain, J. (2022). Enhancement of mechanical properties through modified post-weld heat treatment processes of T91 and Super304H dissimilar welded joint. Journal of Manufacturing Processes, 78, 59-70. ISSN 1526-6125. https://doi.org/10.1016/j.jmapro.2022.04.008
Funderburk, R.S. (1998). Key Concepts in Welding Engineering. Welding Innovation Volume XV, No. 2. http://www.jflf.org/v/vspfiles/assets/pdf/keyconcepts4.pdf
Post Weld Heat Treatment (PWHT). [Online] https://www.welderdestiny.com/post-weld-heat-treatment.html (Accessed on 07 August 2022)
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