Improved impact toughness by multi-step heat treatment in a 1400 MPa low carbon precipitation-strengthened steel

Abstract

Multi-step heat treatment including intercritical treatment plus traditional tempering is proposed to tailor strength and toughness in a low carbon precipitation-strengthened steel. Microstructure and mechanical properties has been systemically studied. Result shows that multi-step heat treatment is effective to improve the toughness (from ~8 J to ~38 J) with limited sacrifice of yield strength (from ~1429 MPa to ~1311 MPa). Nano-sized retained austenite was obtained located at boundaries, and its volume fraction is ~6.1 pct. after multi-step heat treatment, which is thought to be responsible to the improvement of toughness. Meanwhile, high angle boundary (>15 deg.) length increases from 0.7959 μm to 0.9157 μm per square micron. Increased boundary length results in equivalent grain size decreases from ~1.11 μm to ~0.61 μm. The decreased equivalent grain size is beneficial to the increment of strength and toughness. Moreover, reduced Cu segregation is also responsible to the increment of toughness, and slightly decrease in strength could come from decreased dislocation density, coarsening precipitates and reduced solute content in tempered martensite.