Toughness improvement in coarse grain heat affected zone of X70 pipeline steel by accelerated cooling

Abstract

For in-service welding, a large amount of heat loss through the pipe wall is created by the flowing gas, resulting in accelerated cooling of the weld. In the present paper, the thermal simulated specimens were used to investigate the microstructure and mechanical properties in coarse grain heat affected zone (CGHAZ) of in-service welding X70 pipeline steel. The simulated results indicate that the embrittlement phenomenon in the CGHAZ is not serious for X70 pipeline steel at cryogenic temperature by accelerated cooling. Microstructure analysis suggests that the improvements in CGHAZ toughness can be explained not only by the amount of martensite–austenite constituents, but also by the presence of austenite films. From all the results obtained, a mechanism for toughness improvement among CGHAZs is proposed and discussed.

Keywords:

• X70 pipeline steel
• Gleeble simulation
• Accelerated cooling
• Microstructure
• Hardness
• Fracture toughness

The authors would like to acknowledge the supports of the National Natural Science Foundation of China (grant no. 51074174) and the Natural Science Foundation of Shandong Province (grant no. ZR2010EM041).