Microstructure evolution of 022Cr25Ni7Mo4N duplex stainless steel during isothermal treatment

Abstract

The microstructure evolution of 022Cr25Ni7Mo4N duplex stainless steel after isothermal treatment for 2-20 h at 1100℃ was studied. The morphology evolution of austenite grain was observed and the grain size was quantitatively characterized. The change of element content in austenite and ferrite phase was measured. The effect of microstructure evolution on the anisotropy of volume fraction of ferrite phase and low temperature impact toughness of test steels was analysed subsequently. The results show that with the increase of holding time, there is obvious accumulation, growth and coarsening of austenite grain, accompanied by significant changes in grain morphology. The volume fraction of rod-like crystal grains with a/b value above 4.0 dropped from nearly 20% to below 5%, and that of equiaxed grains with a/b value between 1.0 and 1.9 increases significantly, while the number of grains with size above 20 μm increases rapidly. The prolongation of the holding time makes molybdenum and chromium elements diffuse and enrich to the ferrite phase further, which leads to the increase of pitting resistance equivalent number (PREN) value in ferrite phase. The significant decrease in the volume fraction of slender rod-like austenite crystallites is the primary reason for the improvement in anisotropy of austenite volume fraction and the increase of low temperature impact toughness of test steel.

Keywords

• duplex stainless steel
• isothermal treatment
• grain morphology
• anisotropy
• impact toug-hness