Study On Solid-state Phase Transition Of Three Typical Iron-based Alloys At Different Cooling Rates

Rapid or sub-rapid solidification widely exists in the welding process,additive manufacturing,atomizing spray deposition,laser cladding and so on.In many cases,materials still undergo one or more solid phase transitions after liquid-solid transformation.It is very important to understand the solid-state transformation under the rapid or sub-rapid solidification conditions,which contributes to improve the microstructure and performance of the component made by these techniques.However,due to low cooling rate and heating temperature(or phase zone),the past research about solid-state phase transition is hard to achieve it.In this work,we studied the effect of cooling rate and heating temperature(or phase zone)on solid-state transformation of three typical iron-based alloys including industrial pure iron(single component material),Fe-10Cr(binary alloy),and Fe-20Mn-9Al-1.2C(multielement hypoeutectoid alloy).In the experiment,a high-precision thermal expansion instrument(DIL805 L)was used to extend the cooling rate to 500?/s and analyze the influence of the phase transition in the high temperature on the subsequent phase transition.This research is useful to understand the solid phase transition process and microstructure evolution in the sub-rapid solidification.The experimental results about industrial pure iron show that with the heating rate increasing to 300?/s during the heating process,the onset temperature for the ferrite(?)?austenite(?)transformation increases by about 26?.During the cooling process,with the cooling rate increasing to 150?/s,the onset temperature for the???transformation decreases by about 74?.When the cooling rate is560?/s,the onset temperature for the???transformation decreases by about112?.For the industrial pure iron,the increase of cooling rate has a greater effect on the onset temperature of transformation than the heating rate.According to kinetic analysis,when the cooling rate increases from 0.1?/s to 100?/s,ferrite growth tends to change from the three-dimensional to the two-dimensional growth mode.The experimental results about the Fe-10Cr binary alloy show that when the alloy is heated to the austenite phase zone of 1000? and then cools,the critical cooling rate for martensite transformation is between 10?/s and 20?/s,and when the cooling rate increases to 100?/s,only the??M(martensite)transformation occurs.However,when the Fe-10Cr binary alloy is heated to the high temperature ferrite(?)phase zone of 1300?,the critical cooling rate for martensite transformation is between 0.1?/s and 2?/s,and only the????M transformation is observed,when the cooling rate increases to 20?/s or more.It can be seen that the heating temperature or phase zone has a great influence on the critical cooling rate for martensite transformation in the Fe-10Cr binary alloy.Analysis shows that the significantly coarsening of grains at high temperature is the main reason for the clearly decreasing of critical cooling rate of martensite transformation.Kinetic analysis shows that when the sample slowly cools at a cooling rate of 0.1?/s,ferrite grows in a three-dimensional mode,when the sample is heated to the high temperature ferrite zone and then cools,martensite grows in two-dimensional mode during the??M phase transformation,but when the sample is heated to the austenite zone and then cools,the martensite grows in three-dimensional mode due to the fine grains.The Fe-20Mn-9Al-1.2C multielement hypoeutectoid alloy is heated at a rate of50?/s to 800? for 5 minutes and then cools.When the cooling rate is 0.1?/s,a large amount of ferrite and carbide are formed in the microstructure.When the cooling rate increases to 100?/s,the carbide content decreases significantly,which indicates that the eutectoid transformation,austenite?ferrite+carbide,is significantly suppressed.However,the alloy is heated to 1000? has less ferrite content with the cooling rate in the range 0.1?/s to 100?/s.When the cooling rate is 0.1?/s,some carbides in micron scale are formed at the austenitic grain boundary.When the cooling rate increases to 10?/s,the precipitation of micron grade carbides is suppressed.In summary,the change of cooling rate within the range of 0.1?/s to 560?/s not only affects the transformation temperature and new phase growth mode of industrial pure iron,but also changes the phase transition products of Fe-10Cr binary alloy and Fe-20Mn-9Al-1.2C multielement hypoeutectoid alloy when the cooling rate exceeds a certain critical value,and the critical cooling rate is closely related to the heating temperature or phase zone.It can be seen that when binary or multivariate alloys are cooled at a sub-rapid cooling rate within the range of 10~0?/s to 10~4?/s,its microstructure is decided not only by the cooling rate,but also by the heating temperature or phase zone.