Post Weld Heat Treatment (PWHT)

Post Weld Heat Treatment Services
During the welding process, the two metal pieces being joined are subject to extreme temperatures and can cause the crystalline structure of the metal to pass through various metallurgical phases. As a result, hardening (and embrittlement) of the metal can occur to varying degrees (usually dependent on carbon content). Heat treatment is designed to reduce the hardness in the heat affected zone of the metals and also increase ductility in these sections. Various pressure vessel codes contain the specifics regarding the procedures for post-weld heat treatment. Heat is usually held for one hour per inch of thickness of the metal. The temperature used is based on the "P-number" of the metals. P-numbers are assigned based on the chemical composition of the metals. Holding temperatures can range from 1100-1350 °F (593-732 °C).

What is PWHT?
Post Weld Heat Treatment Services (PWHT) is defined as one of heat treatments done after welding/machining to improve the Chemical-Mechanical properties of weldments / machined surfaces. In concept, PWHT covers many different potential treatments. However, in steel fabrication, most common procedure used is Stress Relieving.

When is PWHT required?
Machining and/or Welding induce stresses in parts. The bigger and more complex the part, the more the stresses. These stresses can cause distortions in the part in long term. If the parts are clamped in service, then cracking could occur. Also whole locations can change causing them to go out of tolerance. For these reasons, stress relieving is often necessary.

i. Stress induced by welding:
As a result of welding process used to join metals together. The base material near the weld metal and the heat-affected zones transform through various metallurgical phases. Depending upon the chemistry of the metals in their areas. Hardening occurs in various degrees, depending mainly upon the carbon content. This is particularly very true in the heat - affected zone adjacent to the weld metal deposit. The resultant stresses are highest due to melting and solidification. Stress, due to welding is of magnitude roughly equal to the yield strength of the base material.

ii. Stress induced by machining:
Machining induces stresses in parts. The bigger and more complex the part, the more stresses. These stresses can cause distortions in the part in long term. Cracking could occur and location changes causing parts to go out of tolerances. Stress Relieving heat treatment is used to reduce the stress that remain locked in a structure as a consequence of the various manufacturing processes. Stress Relieving, as the name implies, is designed to relieve these imposed stresses by reducing the hardness and increasing the ductility, and by these, reducing the danger of cracking in the weldments.

How is Stress Relieving Done?
Stress Relieving is done by uniformly heating- fabricated equipment or the vessel or vessel part to a sufficiently high temperature, but below the lower transformation temperature range, then subjecting it to a thermal retardation for a sufficient time depending upon the material thickness and then finally uniformly cooling it which is also of utmost importance.

Advantages of Post Weld Heat Treatment - Stress Relieving

  • A much greater dimensional stability is obtained and maintained
  • The potential of stress induce cracking is reduced.
  • Metallurgical structure is improved.
  • Strength of the Material and Life of the equipment is enhanced.
  • Relaxes residual stresses
  • Relaxes thermal stresses
  • Tempers (softens)
  • Removes diffusible hydrogen

Advantage of Weld Pre-heats

  • Reduces the levels of thermal stress
  • Compensates for high heat losses
  • Minimizes the rate of weld hardening
  • Reduces the porosity of the weld
  • Reduces hydrogen cracking
  • Improves microstructure of heat affected zone