Effect of austenitizing temperature on microstructure and mechanical properties of medium carbon spring steel

• Austenitizing at 860 °C refines PAGs and martensite blocks. • Refined PAGs increase dislocation density and high-angle grain boundaries. • Variant analysis shows fewer low-misorientation pairs (V1&V4) at 860 °C. • Increased HAGBs and refined blocks enhance strength and ductility. • Yield strength rises from 1563 MPa to 1648 MPa without ductility loss. The influence of austenitizing temperature on the microstructure and mechanical properties of medium carbon spring steel was systematically studied. Microstructural characterization showed that decreasing the austenitizing temperature from 900 °C to 860 °C effectively refined prior austenite grains from 8.8 μm to 5.3 μm, reduced martensite block thickness from 0.9 μm to 0.7 μm, and increased the fraction of high-angle grain boundaries (HAGBs) from 70 % to 76 %. Crystallographic variant analysis revealed increased boundary densities for V1&V2, V1&V3(V5), V1&V4, and V1&V6 pairs in the 860 °C sample with refined PAGS, along with elevated frequencies of V1&V3(V5) and V1&V6 pairs classified as HAGBs, consistent with the K-S orientation relationship. These results demonstrate that reducing the austenitizing temperature from 900 °C to 860 °C significantly affects martensite block size, HAGB fraction, and crystallographic variant pairs, leading to enchanced yield strength from 1563 MPa to 1648 MPa and total elongation from 9.3 % to 9.7 %.