Features of microstructure, phase composition and strengthening capability of stainless steels with 13 – 17 % Cr

The paper considers the study of the features of structure and phase transformations in high-strength, resistant to carbon dioxide corrosion, complex alloyed steels of martensitic, austenitic-martensitic and martensitic-ferritic classes with 13 – 17 % Cr. Influence of the alloying on crystallization and solid state phase transformations was revealed in the temperature range of hot deformation and heat treatment using thermodynamic modeling and experimental study. The effect of quenching temperature on the phase composition and microstructure was analyzed as a result of X-ray diffraction phase analysis, optical and transmission electron microscopy. It was found that increase of nickel content leads to growth of retained austenite fraction resulting in significant decrease of yield strength along with high tensile strength and elongation. To obtain predominantly martensitic microstructure in martensitic-austenitic steel, the multistage heat treatment is proposed including quenching, intermediate annealing for precipitation of dispersed carbides and tempering forming final mechanical properties. The composition of precipitated carbides was evaluated by X-ray microanalysis. The results of the tensile test for steels with martensitic and martensitic-ferritic microstructure showed that required strength grade (σ0.65 ≥ 862 MPa; σв ≥ 931 MPa) was reached after heat treatment including quenching and tempering. Multistage heat treatment including quenching, intermediate annealing and final tempering was resulted in required strength properties of high-nickel martensitic-austenitic steel with 15 % Cr.