There are two very important temperatures when hardening steel - the temperature of the onset of martensitic transformation Mn and the temperature of the end of the martensitic transformation Mto.
Role of austenite grain boundaries
When austenite transforms on cooling to some other phase, then this new phase is always first formed or nucleated at the old grain boundaries of austenite. for instance, if pearlitic steel U8 was first heated to full austenite at a temperature 850 ° C, and then cooled in air to 650 ° C, then small pearlite grains will form at the old austenite grain boundaries immediately after the temperature drops below 727 ° C. When the temperature reaches 650 ° C, these grains will start to grow, but there will still be some austenite between them. If the sample is kept at 650 ° C, then pearlite grains will grow until all the austenite ends.
Influence of the degree of supercooling of austenite
Consider also alternative heat treatment, when the sample is cooled from temperature 850 ° C in hot liquid at temperature 650 ° C. this treatment provides cooling of the sample to a temperature 650 ° C much faster, than when cooled in air. The sequence of decomposition of austenite will be the same - small pearlite grains originate at old austenite grain boundaries and grow into the remaining austenite, until it's all over.
What to change compared to air cooling? Faster cooling will give three differences:
- pearlite grains will grow faster;
- the distance between cementite plates in pearlite will be much less;
- pearlite grains will be smaller, since more of them will be generated at the boundaries of austenite grains.
Pearlite growth and martensite growth
Now suppose, that U8 steel is cooled in a water tank at an even lower temperature, eg, at room. At such a low cooling temperature, martensite formation and significant steel hardening can be expected.. What is common and what are the differences between the formation of martensite at room temperature and pearlite at temperature 650 ° C? Like pearlite, martensite will begin to form along the austenite grain boundaries, but unlike pearlite, martensite grows into austenite at a tremendous rate. Perlite grows into austenite at a rate of about 50 μm / s at 650 ° C and even slower at higher temperatures. Martensite grows into austenite at almost the speed of sound 4510 m / s (in steel, not in the air) at any temperature, at which it is formed. Besides, unlike perlite, which completely replaces austenite simply by holding the sample for a sufficiently long time at a low temperature.
Martensitic transformation temperature range
Martensite will not replace all austenite., until the quenching temperature is below the temperature, which is called the temperature of the end of the martensitic transformation Мto.
Moreover, martensite will not start to form at all, until the quenching temperature drops below the temperature of the beginning of the martensitic transformation Mn.
If the quenching temperature is between Mn themto, then only part of the austenite turns into martensite, and the remaining austenite - retained austenite - will continue to be in the steel.
The figure below represents the graph, which illustrates the dependence of the amount of formed martensite on the quenching temperature.
Figure - Dependence of the amount of martensite on the quenching temperature
Badge M50 temperature indicated, at which the 50 % martensite. If the steel is cooled to a temperature of M50, then 50 % austenite in it will turn into martensite. Moreover, this will happen within milliseconds after reaching the temperature M50. However, the remaining 50 % austenite, which surround martensite, will remain, as retained austenite until then, until the temperature changes.
Phase diagram of steel and martensite
It should be noted, what the phase diagram of steel predicts, that all austenite must disappear, when the steel temperature drops below temperature A1. Moreover, she demands, so that below point A1 the steel consisted of ferrite and cementite. However, with hardened steels, both of these rules are violated - the phase diagram correctly reflects the dependence of the phases on temperature only with sufficiently slow cooling.
Martensite - metastable phase
At high cooling rates, a new phase appears - martensite, which the phase diagram does not predict. Such phases are called metastable., that is, unstable. If martensite is heated, like on vacation, then it goes to more stable phases. These stable phases should already be on the phase diagram - heating of martensitic structures to temperatures below A1 transforms also martensite, and retained austenite into a mixture of ferrite and cementite.
Source: John D. Verhoeven, Steel Metallurgy for Non-Metallurgists, 2007