EFFECT OF TANTALUM ON THE IMPACT RESISTANCE OF 12% CR STEELS SUBJECTED TO THERMOMECHANICAL TREATMENT

Abstract

The effect of tantalum on the impact resistance 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 brittle-ductile 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 entire fracture toughness versus temperature curve is 30-50 J/cm2 higher over the entire temperature range, including the upper and lower shelf energies, and the brittle-ductile 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 initial 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.