Effects of Mo, Cr and Nb on microstructure and mechanical properties of heat affected zone for Nb-bearing X80 pipeline steels

The microstructure and mechanical properties of welding heat affected zone (HAZ) of three typical X80 pipeline steels, i.e. Mn–Cr–Nb, Mn–Mo–Nb and/or Mn–Cr–Mo–Nb steels, have been studied using the welding thermal simulation method on a Gleeble-3500 thermal simulator. The results show that the chemical compositions and welding process parameters have significant effects on the microstructure and properties of HAZ. With increase of the cooling rate, the amount of microstructure transformed at lower temperature increases, and the microstructure becomes finer, furthermore, the strength and toughness of HAZ show a rising trend. Within the range of the peak temperature from 600 to 1350 °C, there are two brittle zones and one major strength weakening zone in HAZ, especially for high-Nb added Cr steel, weaken of HAZ is very serious. Effects of the key chemical elements such as Mo, Cr and Nb on microstructure transformation of HAZ have been discussed. Although these three alloy system steels in this work display good weldability, the Mn–Mo–Nb and/or Mn–Cr–Mo–Nb steels show better combination of strength and toughness of HAZ compared with the Mn–Cr–Nb steels. Therefore, for high-Nb steels, it is necessary to add suitably Mo to improve the toughness and strength of HAZ, especially, increase the strength.