Study on Grain Growth Tendency and Its Correlation with Recrystallization and Precipitation in High‐Nb 444‐Type Ferritic Stainless Steel during Brazing

Abstract

This study elucidates the determining role of initial recrystallization degree on grain growth tendency during the brazing process (heat treated at 1100 °C and held for 15 min) of high‐Nb 444‐type ferritic stainless steel used in automotive exhaust systems. The incomplete recrystallized grains induced by insufficient annealing of cold rolled stainless steel plate are more likely to promote abnormal grain growth in the actual brazing process. The simulation experiment results of grain growth tendency show that a matrix with 3.7% deformed grains and nonuniform grain size causes abnormal grain growth during reheating, and the starting temperature of grain coarsening decreases from 1100 to 1000 °C. During reheating, the deformed grains induces a rapid precipitation of Nb‐rich particles at low temperature, and the volume fraction is higher than that of the sample with more completed recrystallization. In addition, the study also finds that an increase in Nb content can increase the precipitation temperature of Laves phase, induce the formation of Laves phase distributed along ferrite grain boundaries in high‐Nb sample during brazing, and inhibit the coarsening of ferrite grains.