Cause Analysis of High Hardness of 12Cr1MoVG Steel Pipes with Large Diameter and Thick Wall for High Pressure Boiler and Optimization of Heat Treatment Process

The hardness of 12 Cr1 MoVG steel pipes with large diameter and thick wall for high pressure boiler was too high.The problems of high hardness were analyzed by means of hardness test,mechanical property test and metallographic examination.The results show that the structure of steel pipes were not uniform after heat treatment,which resulted in the mechanical performance of steel pipes was not uniform,and the hardness of some areas did not meet the design requirements.According to the characteristics of the power plant unit,three heat treatment process schemes were designed including rotating internal spray cooling after normalizing,direct water cooling after normalizing and “precooling+water cooling+air cooling”after normalizing.Finally,the tempering temperature and precooling time were controlled within the reasonable range by selecting the process of“precooling+water cooling+air cooling”,so that the steel pipe hardness met the design requirements.