Properties and Sulfide Stress Cracking Resistance of Coarse-Grained Heat-Affected Zones in V-Microalloyed X60 Steel Pipe

摘要

The effect of weld cooling rates and postweld heat treatment on microhardness, microstructure, tensile properties, andsulfide stress cracking resistance was studied. Evaluation of heat-affected zone (HAZ) properties after postweld heat treatment (PWHT) is important not only for selection of suitable chemical composition of microalloyed steel pipes, but also for development of welding procedures, including PWHT, for sour service. In this study, coarse-grained heat-affected zones (CGHAZ) were induced in V-microalloyed X60 steel pipe specimens using a Gleeble weld thermal simulation. Cooling rates from 5 to 80℃/s were used to represent a broad range of welding conditions. The CGHAZ specimens were subjected to PWHT at temperatures from 635 to 670℃ (1175 to 1238°F) for 3 to 15 hours. Microhardness and microstructure of the CGHAZ were evaluated in the simulated "as-welded" and PWHT condition as functions of weld cooling rate and PWHT schedule. The tensile properties and sulfide stress cracking resistance (SSC) of CGHAZ with selected hardness level were determined as well. The maximum hardness of the CGHAZ increases from 233 to 392 HV-10, and its microstructure changes from ferrite with aligned second phases to martensite by increasing the cooling rate from 5 to 80℃/s. None of the PWHT schedules used in this study induced hardening of the CGHAZ of the microalloyed steel. However, the rate of softening may have been retarded because of VC secondary hardening. For a holding time between 3 and 15 hours, the softening effect induced by the PWHT can be described by the Larson-Miller tempering parameter. The apparent activation energy for tempering in the different softening regimes observed during PWHT was determined. The behavior of the CGHAZ during tensile testing depends on the degree of overmatching between the CGHAZ and the base material. Coarse-grained heat-affected zones with a maximum hardness of 264 HV did not crack during exposure to sour testing according to procedure specified in standard NACE TM0177 (Method A).