The temperature of the onset of martensitic transformation in steels is highly dependent on the carbon content of austenite.
Martensitic transformation and carbon content
On the picture 1 the temperatures of the beginning and end of the martensitic transformation Mn themto for ordinary carbon steels depending on the carbon content. Temperature Mto has a fairly significant spread. Steel is hardened most often in water at room temperature.. This temperature is marked with a horizontal line Troom. This line makes it possible to estimate at what carbon content complete quenching at room temperature is possible, or how much retained austenite will be in the steel after quenching.. According to the picture 1 hardening at room temperature even with carbon content 0,3-0,4 % become incomplete, since at such a carbon content the temperature Tto drops below room temperature.
Retained austenite and carbon content
The percentage of retained austenite in the steel is determined by the X-ray method. On the picture 2 shows the results of such measurements of the volume fraction of retained austenite in hardened ordinary carbon steels, depending on the carbon content in them. Just like the temperature Tto there is a wide range of data. for instance, for steel with carbon content 1,4 % the percentage of retained austenite ranges from 28 to 45 %.
The graph in the figure 2 makes it possible to draw the following conclusions:
1) Steel with full lath martensite (less carbon content 0,6 %) will not have a significant amount of retained austenite.
2) Fully lamellar martensite steels (carbon content over 1 %) will have a significant amount of retained austenite. The higher the carbon content, the greater the amount of retained austenite.
Quenched U8 steel with carbon content 0,77 % will have a mixed lath-lamellar structure of martensite and contain 6-10 % residual austenite. Generally speaking, it is very difficult to see through an optical microscope the residual austenite between the martensite plates until then, until its content is about 10 %.
Source: John D. Verhoeven, Steel Metallurgy for Non-Metallurgists, 2007