Heat Treatment
Heat treating can be defined as the heating and cooling of metals or metal alloys in some manner that will alter their metallurgical structure and change their mechanical properties.
Hardening is usually thought of when we say heat treat. But any form of thermal process is a form of heat-treating. The goal of all thermal treating is to induce a phase change, complete a phase change or reduce stresses caused by a phase change or cold working.
Cryogenic quenching or treatments are done to steels to complete the austenite to martensite phase change, but any material that undergoes a phase change upon cooling can benefit from cryogenic treatments.
Tempering is a method to reduce the stresses induced by the austenite to martensite phase transformation, and stress relieving is usually performed after cold working.
Annealing has several categories, and includes spheroidizing, and normalizing. Each of these thermal treatments will be discussed in the next chapters. The rule of thumb is to heat the part and hold or "soak" it for one hour per inch of thickness.
I would like to mention here that any heating of a steel over 1200º F. will cause it to decarburize in an uncontrolled atmosphere. That is to say the exposed surface will lose all or part of its carbon. What happens at heat-treat temperatures is this: Carbon doesn't really like to be in steel. It would rather be with oxygen. If an oxygen atom hits the steel it will form CO. If CO2 hits the steel it will form CO + CO. If water vapor hits it you will get H2 + CO. I have seen decarb as deep as 0.020 inches deep not having ANY carbon and another 0.050 inches deep of partially decarburized steel. FYI, this is written as 0.020"FF - 0.070"TAD. FF means Free Ferrite and TAD means Total Affected Depth.
Decarburized steel is not good for obvious reasons. It's recommended to heat treat a part before its final dimension. That is to say, allow some tolerance to machine off or remove in some way, the decarburized material.
Heat treating can be defined as the heating and cooling of metals or metal alloys in some manner that will alter their metallurgical structure and change their mechanical properties.
Hardening is usually thought of when we say heat treat. But any form of thermal process is a form of heat-treating. The goal of all thermal treating is to induce a phase change, complete a phase change or reduce stresses caused by a phase change or cold working.
Cryogenic quenching or treatments are done to steels to complete the austenite to martensite phase change, but any material that undergoes a phase change upon cooling can benefit from cryogenic treatments.
Tempering is a method to reduce the stresses induced by the austenite to martensite phase transformation, and stress relieving is usually performed after cold working.
Annealing has several categories, and includes spheroidizing, and normalizing. Each of these thermal treatments will be discussed in the next chapters. The rule of thumb is to heat the part and hold or "soak" it for one hour per inch of thickness.
I would like to mention here that any heating of a steel over 1200º F. will cause it to decarburize in an uncontrolled atmosphere. That is to say the exposed surface will lose all or part of its carbon. What happens at heat-treat temperatures is this: Carbon doesn't really like to be in steel. It would rather be with oxygen. If an oxygen atom hits the steel it will form CO. If CO2 hits the steel it will form CO + CO. If water vapor hits it you will get H2 + CO. I have seen decarb as deep as 0.020 inches deep not having ANY carbon and another 0.050 inches deep of partially decarburized steel. FYI, this is written as 0.020"FF - 0.070"TAD. FF means Free Ferrite and TAD means Total Affected Depth.
Decarburized steel is not good for obvious reasons. It's recommended to heat treat a part before its final dimension. That is to say, allow some tolerance to machine off or remove in some way, the decarburized material.