Microstructural characterization and mechanical properties of super 13% Cr steel.

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.