In marine, O&G, MOU construction: A large amount of high strength, heavy wall pipe is manufactured using a thermo-mechanically controlled processed (TMCP) steel plate.
It is generally accepted that these steels are not designed for subsequent PWHT and may suffer some degradation in their mechanical properties when re-heated above critical temperatures.
A number of standards, specifications and technical publications acknowledge the influence of PWHT on mechanical properties of the TMCP plate, but provide limited guidance (depend on supplier quality) on how it will affect the properties.
Others refer directly to a detrimental effect, such as BS 4514-1, which states “when PWHT is carried out it may be necessary to verify the properties of the pipe and the welded joints affected by the heat treatment” and EN10208-2 which states “subsequent heating of TMCP material above 580°C may lower the strength”.
The magnitude of any effect is considered a function of the material chemistry, TMCP parameters, welding consumable and PWHT parameters. PWHT on TMCP pipe materials should be restricted.
Nevertheless, where PWHT cannot be avoided, testing should be performed on the specific material to clearly define the magnitude of any deterioration in properties at the proposed PWHT temperature and thermal cycle by Testing and Examination and recorded in a qualified system or lab test.
- Stress Relieve (PWHT): Welding introduces high residual stresses into metals in the region around the weld. This can result in reduced fracture toughness properties and susceptibility to corrosion mechanisms such as Stress Corrosion Cracking. (SCC) The stress relieve is probably the most often performed post-weld heat treatment, particularly on carbon steels and carbon-manganese steels. Stress relieve is normally a code requirement for pressure equipment above a certain thickness. (Each code has different rules for determining when a stress relieve is required.) For carbon steels, the stress relieve is typically around 580°C – 650°C. Please note that stress release treatment at the higher end of the range could reduce the strength of TMCP steel. When post-weld heat treatment is required for TMCP steels, it should preferably be done at the lower end of the range.