Effect of Preheating Temperature on Critical Fracture Stress of Welded Joints of X100 Pipeline Steel

X100 pipeline steels were welded by manual arc welding using E9010-G cellulose electrode at different preheating temperatures (80, 120, 160℃). The microstructure and hardness of heat affected zone (HAZ) of the joint were studied and the critical fracture stresses were measured by implant test. The cold cracking sensitivity of the joint was analyzed. The results show that the microstructure of HAZ of the welded joint mainly consisted of lath-like bainite-ferrite (B_F). With the increase of preheating temperature, the HAZ microstructure was refined, the amount of brittle M-A constituent decreased and the B_F lath became narrow and distributed tightly. The critical fracture stresses of welded joints increased with the rise of preheating temperature while the hardness of HAZ decreased. At the three preheating temperatures, the critical fracture stress of the welded joint was higher than 0.75σ_s (where σ_s was yield strength of base material) and the HAZ hardness was lower than the critical hardness of cold cracks, indicating a relatively high resistance to cold cracking of the welded joint.