Thermal simulation of single thermal cycle for high strength steel pipe

Single thermal cycle simulation tests were carried out for X80 high strength steel pipes from three steel mills by a Gleeble 3500HS thermal simulation test machine, and coincidence degree of the thermal simulation curve with the set curve under heat inputs of 6–30 kJ/cm was observed; The relationship between different heat inputs and microstructure, impact toughness and hardness of steel pipe CGHAZ (coarse grain heat affected zone) was studied by metallographic examination, impact test and hardness test. The results show that with the increase of heat input, original austenite grain size increases gradually, the lath bainite ratio decreases and the granular bainite ratio increases. The impact toughness of C steel pipe is lower than those of A and B steel pipe, and the impact toughness of CGHAZ from the three steel pipes show different trends: for A steel pipe CGHAZ, impact toughness increases first and then decreases, with the highest value of 270–320 J under 20–25 kJ/cm; for B steel pipe CGHAZ, impact toughness decreases slightly; for C steel pipe CGHAZ, impact toughness increases, with the highest value of 260–300 J under 25 kJ/cm. As the heat input increases, the hardness of three X80 steel pipes CGHAZ shows a decreasing trhighend, and C steel pipe has the largest decreasing range.