Martensite Formation from Reverted Austenite at Sub-zero Celsius Temperature
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t martensitic stainless steels are essentially Fe-Cr-Ni alloys which contain a very low fraction of interstitially dissolved C and N.[1–3] These steels are used for heavy-section water turbine components, pump and valve bodies, and wellhead equipment in the oil and gas industry.[4] Soft martensitic stainless steels are known for their resistance against CO2 corrosion, good weldability, good ductility at high strength, and excellent impact toughness, even at sub-zero Celsius temperatures.[5,6] As for the case of supermartensitic stainless steels[1] and ferritic Fe-Ni alloys,[7] the excellent impact toughness at low temperature is partially owed to the low content of interstitials, which leads to low hardness,[1] and partially to the presence of, so-called, reverted austenite, which forms a fine ‘‘composite’’ microstructure with tempered martensite.[7–9] Soft martensitic stainless steels show impact values of approx. 70 to 110 J at 190 K,[5,6,10] supermartensitic stainless steels even in excess of 100 J.[1,2,11] Consequently, these
F. NIESSEN is with the Technical University of Denmark, Danish Hydrocarbon Research and Technology Centre, 2800, Kgs. Lyngby, Denmark. Contact e-mail: [email protected] M. VILLA and M.A.J. SOMERS are with the Technical University of Denmark, Department of Mechanical Engineering, 2800, Kgs. Lyngby, Denmark. Manuscript submitted April 4, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
materials appear particularly suitable for sub-zero Celsius applications.[1,5,6,12] Reverted austenite forms during inter-critical annealing treatment between A1 and A3, where both ferrite and austenite can co-exist under thermodynamic equilibrium conditions. After austenitization and cooling to room temperature, the microstructure consists of lath martensite; heating to and annealing within the inter-critical temperature regime promotes partial reversion of martensite to austenite in the form of austenite films along martensite lath boundaries.[13] Austenite formation is accompanied by the partitioning of alloying elements in an attempt to establish thermodynamic equilibrium. In particular, the diffusion-controlled Ni partitioning governs the overall kinetics of the transformation.[14,15] Because of the partitioning of alloying elements, reverted austenite is more stable than the homogeneous austenite formed during austenitization and can be retained at room temperature.[8,15,16] The material used in the present work is a 15Cr-5.8Ni-1Mo-0.9Mn steel (in wt pct), grade EN 1.4418. The content of interstitials was 0.034 wt pct C and 0.032 wt pct N, respectively. The formation of reverted austenite is presented in Figure 1. The initial ‘‘as normalized’’ condition (i.e., austenitized and quenched) was obtained by austenitization at 1190 K for 0.6 ks and quenching at an average rate of approx. 1 K.s1. Subsequent inter-critical annealing consisted in heating to a temperature in the range 898 to 973 K, followed by cooling to room temperature. The heating rate was constant and equal to 0.25 K.s1; the cooling rate wa
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