借助光学显微镜、扫描电镜分析对比了FB2马氏体耐热钢在焊接热模拟前后的组织状态,认为FB2钢在快速加热条件(≥100 ℃/s)下的奥氏体相变是切变型的,表现出奥氏体记忆效应;而在慢速加热条件(≤5 ℃/s)下其奥氏体相变是扩散型的,该过程是受原子短程扩散控制的,并且无奥氏体记忆效应发生。FB2钢在焊接过程中特殊的相变过程是其焊接热影响区呈现出“无粗晶区”的主要原因。结合已有的文献报道,初步提出了B元素改变FB2钢在加热过程中奥氏体相变类型的机理模型,进一步发展了现有的研究结果。

The improvement of steam parameters in fossil power plants requires the development of new kinds of 9% Cr martensitic heat-resistant steels, among which FB2 steel is a 100×10^-6 (mass fraction) boron-containing steel and mainly used for manufacturing components with thick walls operating at high temperatures above 600 ℃. In the alloy system of martensitic heat-resistant steels, boron plays an important role in suppressing type IV crack of weld joints by the formation of heat affected zone (HAZ) with no fine grains in the normalized and intercritical zones, where there exhibit fine grains in conventional 9%Cr heat-resistant steels with no boron such as P91 steel. In this work, the formation process of HAZ in FB2 steel was investigated. The microstructures before and after thermal simulation were compared using OM and SEM. It was concluded that the austenization of FB2 steel at rapid heating rates (≥100 ℃/s) took place by shear mechanism, demonstrating austenite memory effect; while at slow heating rates (≤5 ℃/s), the austenization was by atom short range diffusion mechanism, without austenite memory effect. The special phase transformation of austenization is the main cause for the formation of HAZ with no coarsened grain in the overheated zone. Based on the previous results reported by other researchers, a preliminary model was proposed to describe how boron atoms change the austenite transformation type of FB2 steel during heating process, which developed the previous ideas about the phenomenon.