Super 13% Cr steels are used for offshore applications and have to meet strict tensile strength, yield strength, toughness, elongation, and hardness specifications. The mechanical properties of these steels are strongly dependent on the proportions of retained austenite and martensite, and consequently small changes in heat treatment parameters result in major changes in properties. In this work, a detailed investigation of the effect of heat treatment parameters on microstructural features, hardness and tensile properties of the commercial supermartensitic 13Cr-5Ni-2Mo steels in the asreceived and re-heat treated state were undertaken. The re-heat treatment was performed by re-austenitising the samples at 950 °C for 2 h, air-cooling, and then tempering (single and double temper) in the range of 550-700 °C. The effects of tempering temperature, time, and their combination P (P = (273+T (°C) x (40 + login t (h)) x 1000'1), the Holloman-Jaffe parameter, on retained austenite volume fraction, hardness and tensile properties were investigated. Retained austenite content increased with P to a peak value at P-36.9 and then decreased due to the formation of fresh martensite. However, the second temper increased retained austenite due to the re-transformation of fresh martensite to austenite. This resulted in refined grain size and a high dislocation density. An increase in P gave a decrease in the C and Ni content in austenite on tempering while the austenite grain size increased. The combination of these two effects led to a decrease in the stability of the retained austenite with P, as shown by the increased M. For the first temper at P > 36.9, the austenite present after the second temper was more stable than after the first temper as a result of re-distribution of C and Ni from the martensite to the austenite. 0.2% proof strength and hardness were inversely related to retained austenite content with P. Both hardness and 0.2% proof strength decreased linearly with increase in retained austenite content, but elongation showed the reverse trend. However, the slope of the relationships depended on whether fresh martensite was present or not. The results have shown that retained austenite with volume fraction between 16-30 vol% and size < 208 nm gives the optimum combination of strength, ductility and hardness.
|Publisher||University of Sheffield|
|Source Sets||Ethos UK|
|Type||Electronic Thesis or Dissertation|
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