Metallurgical effects on the dynamic and failure properties of shock loaded ChromX 9120 high strength steel

The strength-ductility synergy of a high chromium martensitic steel with a unique microstructure was investigated under dynamic extremes. The effects of phase transformation and dynamic tensile loading on failure in pure iron and steels under extreme dynamic environments are not well understood. Therefore, a series of in-situ time-resolved and soft recovery plate impact shock experiments were conducted to develop a better understanding of the metallurgical effects on the dynamic behavior and failure properties of ChromX® 9120 high strength steel. ChromX® 9120 is a low carbon-high chromium steel, that exhibits high strength, high ductility, and superior corrosion resistance. Samples were shock loaded below and above the solid-solid phase transformation (bcc-to-hcp) stress, which was determined to be 10.985 GPa. The Hugoniot Elastic Limit (HEL) was measured to be 1.751±0.022 GPa and a 21% precursor decay was observed between the 2 mm and 12 mm thick samples. The spall response reveals an increase in corrected spall strength from 3.685 GPa to 3.893 GPa between 6 GPa and 9 GPa peak shock stress, then a significant decrease to 3.100 GPa across the solid-solid phase transformation stress at a peak shock stress of 12 GPa. The corrected spall strength then increases thereafter to 3.301 GPa at a peak shock stress of 25 GPa. Results derived from shock recovered samples show evidence of grain boundary strengthening due to dislocation pile-up at the untransformed nano-sheets of austenite boundaries separating the martensite laths. The dominant failure mode was determined to be void nucleation, growth, and coalescence but at higher magnifications, some evidence of quasi-cleavage fracture is revealed. High chromium rich regions were determined to deflect and arrest propagating cracks, hindering crack growth. In totality, the results derived from these experiments will advance the state-of-the-art pertaining to low carbon-high chromium alloy steels.