Effect of bainite/martensite mixed microstructure on the strength and toughness of low carbon alloy steels

Bainite/martensite mixed mic restructure and full martensite (M) microstructure were obtained respectively by air-cooling and oil quenching the experimental steels. The effect of tempering temperature and microstructure on the strength and toughness of the steels was investigated. TEM micrograph shows that after air-cooling the microstructure of the low carbon Mn-Cr alloy steel (steel A) is typical bainite/martensite (B/M) mixed microstructure, and the microstructure of the low carbon Mn-Si-Cr alloy steel (steel B) is carbide free bainite/martensite (CFB/M) mixed microstructure. The bainite amount in mixed microstructure is about 20% by Formaster-F simulating the tensile and impact specimens cooled in air. The impact toughness of steel A with B/M is higher than that with M under as-quenched condition. The tempered martensite embrittlement onset temperature of the steel B with CFB/M is above 360 degreesC, while that with M is about 220 degreesC. The instrumented Charpy impact testing shows that the higher crack propagation work leads to the higher impact energy in both steels. So without tempering, the steel A with B/M can be used. The steel B with CFB/M must be tempered before using, especially after tempering under 340-360 degreesC.