Before we start off by looking at what is heat treatment we need to shine a little light on the phases of steel. All phases have their distinct mechanical properties. In steels we have martensite a very hard and brittle phase; Pearlite is formed by a eutectic reaction and is composed of alternate layers ferrite and cementite. Bainite is another phase of steel and can be nearly as hard as martensite but is more ductile and like pearlite it is composed of ferrite and cementite but UNLIKE pearlite it has a completely DIFFERENT structure. In bainite, cementite precipitates exist embedded inside ferrite matrix. Like composites, cementite provides reinforcement while soft ductile ferrite surrounds the reinforcements. All these structures mentioned are derived formed from austenite, another phase of the iron-carbon system which found at temperatures above 727 degrees and can incorporate up to 2.14 wt% carbon. Individually speaking, ferrite is soft with an ability to be made magnetic under temperatures of 768 Celsius and it can only accommodate a very small percentage of carbon upt0 0.025 wt%. The excess amount of carbon in ferrite at temperatures below eutectic, i.e 727 degrees Celsius, forms cementite which is a compound, Fe3C. Now cementite is very hard and brittle and is mainly responsible for imparting strength to steels. It should be noted however that cementite is a metastable phase and although will remain as is at room temperatures indeterminately, but if soaked at temperatures between 650 and 700 degrees Celsius it will dissociate into ferrite and the excess carbon will form graphite. Moving on to why these different phases arise, it is all about stability. Whichever phase is more stable at the prevalent conditions (temperature, pressure, concentration of alloying additions) is more likely to exist. To learn more about this click here. Heat treatment is concerned with heating steels and subsequently controlling their cooling rates to acquire the desired phase in the steel. We can also have a combination of the above mentioned phases in our steels. In some cases the steel might contain a little bit of martensite, bainite and pearlite. Consider a steel with 0.6 wt% carbon which has been cooled from austenite phase and now contains both pro-eutectoid ferrite and pearlite and a bainite. By heating this steel to over 900 degrees Celsius and allowing it to soak until it forms a homogenous austenite phase. If required this austenite can be converted entirely in to martensite by rapidly quenching the steel. Or if need be, the steel can be converted to coarse pearlite upon slow cooling. You could transform this steel to bainite (usually some alloy additions other than carbon are required) by quenching it to say 400 degrees Celsius and allow a sufficient soaking time for transformation to take place. With the right cooling conditions you could, for instance, get both martensite and pearlite. Consider steel containing 0.78 wt% carbon. Quenching it to 200 degrees would get you martensite but if quenching is stopped, so will the transformation to martensite and remaining austenite would turn to pearlite. Continuous cooling transform diagram or just CC diagrams are devoted to the science of heat treatment. Each alloy has its own CC diagram with temperature on the vertical axis and time on horizontal axis. Using these diagrams, you can obtain variables like cooling rates which will allow you to manufacture whatever phases you want in your steel. If you want more on this subject please write to us in the comments below or contact us on one of our email addresses we will try to get back to you as soon as we can.
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