Effect of Tantalum on the Impact Toughness of 12% Cr Steels Subjected to Thermomechanical Treatment

The effect of tantalum on the impact toughness of 12% Cr steels with different tantalum content (12CrTaNb and 12CrNb) was investigated in impact tests in the temperature range from –40 to +120°C with determining the ductile-brittle transition temperature as the temperature in the middle between the upper and lower shelf energies. Both 12% Cr steels were thermomechanically treated by alternating 1050°C annealing and forging in addition to standard normalizing treatment with high-temperature tempering. It was found that for Ta-alloyed 12% Cr steel, the total fracture toughness versus temperature curve is 30–50 J/cm² higher across the entire temperature range, including the upper and lower shelf energies, and the ductile-brittle transition temperature is 10°С lower. The main structural parameters of Ta-alloyed 12% Cr steel which can have a beneficial effect on toughness are a smaller prior austenite grain size, a larger average size but lower density of M₂₃C₆ carbide particles along low-angle martensite lath boundaries, and a larger volume fraction of VX carbonitrides.