In maraging steels Fe and 18 to 22% Ni is using, this steel is quenched fully martensite is forming depending upon Ni content. Ni is a austenite stabilizer, why retained austenite not forming in these steels.
The iron–nickel equilibrium diagram presents that when the alloy with 18% nickel is held at high temperature (above 650 °C), its structure consist entirely of austenite with FCC austenite structure. While at temperatures below 350 °C, a unique BCC structure is obtained regardless of the cooling rate. Moreover, the aging at about 520°C for 3 hours develops reverted austenite in 18% Ni maraging steels.
Maraging stainless steel usually does contain retained austenite. The content of retained austenite is dependent on the amount of stabilizing alloying elements (Ni, N, Mn..) of that phase. It is actually very hard to create a BCT structured steel without it containing any retained austenite. In your case, probably the amount of retained austenite is small (1%) that you might wont be able to detect it on XRD, for example, due to its limit of detection.
In maraging steels, due to their higher Ni content, elevates the martensite start (Ms) temperature which promotes the easy transformation of austenite to martensite. The martensite formed in maraging steel is different from the one we observe in plain carbon steels. The martensite formed in plain carbon steel is formed by quenching. This martensite so formed is hard and brittle with a BCT structure. However, in the case of maraging steel, martensite formation occurs even with air cooling. This is due to the lower carbon content and higher Ni concentration. The martensite so formed is soft with a BCC structure.
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