Effect of tempering temperature on microstructure and mechanical properties of laser solid formed 300M steel

• The microstructure of LSFed 300M steel with tempering treatment changes little.
• There are two kinds of tempered martensite.
• The carbides in general tempered martensite transforms from ε-carbide to cementite.
• The hardness of 300M steel presents incomplete linear correlation with the strength.
• The strength of the tempered LSFed 300M steel reaches the forgings standard.

The microstructure and mechanical properties of laser solid formed (LSFed) 300M steel with tempering treatment was investigated. The microstructure of laser solid formed 300M steel with tempering treatment mainly consisted of the tempered martensite, bainite and a small amount of retained austenite. When the tempering temperature increased from 250 °C to 350 °C, the size of martensite lath and martensite blocks changed little. There are two kinds of tempered martensite. With increase of the tempering temperature, ε-carbide in the general tempered martensite transformed to cementite. In another kind of tempered martensite, ε-carbide were found to be precipitated along specific growth directions, with at least two variants for carbide precipitation. The hardness presented an incomplete linear correlation with the tensile strength and the yield strength for LSFed 300M steel. The hardness in LSFed 300M steel had not shown a significant change with increase of the tempering temperature. The tensile strength and yield strength increased first till the tempering temperature reaches 290 °C–310 °C then decreased with further increasing the tempering temperature. Especially, the strength dramatically decreased when the tempering temperature was above 310 °C. However, the elongation and percentage reduction of area changed little with increase of the tempering temperature. The tensile fracture of LSFed 300M steel with tempering treatment presented a ductile fracture pattern.