Investigation Into the Effect of Post Weld Heat Treatment on Thermo-Mechanically Controlled Rolled Pipeline Steels

Practical options for routing new or diverted pipelines are often limited, which in some cases can lead to the construction of pipelines in potentially seismic or land-slip areas. Designers have advocated the use of heavy wall pipe in these areas; with the potential requirement for girth weld post weld heat treatment (PWHT) to reduce the residual stresses and temper any hard microstructures created during construction welding. A large proportion of high strength, heavy wall pipe is manufactured using 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 TMCP plate, but provide limited guidance 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 response to PWHT of two TMCP pipeline steels used within the UK gas transmission network has been investigated to confirm if the expected degradation in properties occurs and attempt to quantify the scale of any effect. The pipe material tested was L450MB and L555MB (equivalent grade to API5L X65 and X80), with dimensions 1219 mm × 25.4 mm and 1219 mm × 22.9 mm respectively, manufactured using the UOE process and submerged arc welding. The girth welds were manufactured using a manual metal arc process at the National Grid Pipeline Maintenance Centre in the UK. The PWHT parameters were selected to represent the top end of the temperature and time ranges available in existing standards and literature, theoretically testing the worst case scenario in terms of any possible deleterious effect. The project incorporated a detailed test program of the pipe, longitudinal weld and girth weld in both the as-welded and PWHT condition. The paper describes the tests performed and the results obtained. The mechanical properties of both pipe grades met the minimum requirements of the associated specifications after PWHT. However, a general deleterious effect of PWHT was observed, most significantly for the L555MB material and particularly the longitudinal seam weld. 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. 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.