超超临界火电机组用P92钢管硬度异常原因分析

Cause analysis for abnormal hardness of P92 steel pipe used in ultra-supercritical thermal power unit

对用于某超超临界火电机组改造项目的硬度出现异常的P92钢管进行了实验室取样分析。结果表明:钢管生产厂家对P92钢管端部进行高温加热矫正处理且未采取回火处理是导致该P92钢管组织和硬度出现异常的主要原因,其所采用的高温加热矫正温度应为950~1050℃。高温加热矫正处理使材料中的碳化物出现了回溶,合金元素重新固溶到基体中,使得固溶强化作用加强,导致硬度显著增加;在缓冷至室温过程中,由于相变过程中产生较大的内应力,导致组织发生动态再结晶,但未形成明显的板条状回火马氏体亚结构,使得晶内形核位置较少,碳化物等第二相只能在再结晶晶界处沿晶析出,最终得到正火马氏体组织。

A P92 steel tube with abnormal hardness,which was used in the transformation project of an ultra-supercritical thermal power unit,was sampled and analyzed in laboratory.The results show that the end of the P92 steel pipe was high temperature heating correction treated at 950-1050℃not followed by a necessary tempering treatment by the steel pipe manufacturer,which is the main cause of the abnormal microstructure and hardness of the P92 steel tube.During the heating treatment at high temperature,the carbides in the steel and the alloying elements are redissolved into the matrix,which enhances the effect of solid solution strengthening and leads to a significant increase in hardness.Then during slow cooling to room temperature,significant internal stress is formed in the phase transformation process,which results in dynamic recrystallization of the microstructure.But there is no tempered martensite with obvious lath substructure formation,thus the nucleation sites inside grains are less,the secondary phase such as carbides can only precipitate along the grain boundary,and normalization martensite microstructure is finally obtained.