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Metal and Metallurgy engineering FAQ

Heat Treatment

How do I heat treat high speed and tool steel alloys? by Maui
Posted: 11 Nov 03 (Edited 19 Sep 06)

The tool that you produce is only as good as the heat treatment that it receives, and there is no such thing as an acceptable shortcut in the heat treating of high speed or tool steels. Heat treating is an inherently dangerous process, and should be performed by a trained professional whenever possible. There are four steps that should be followed in any heat treating process. They include in order: preheating, austenitizing, quenching, and tempering.

1.) Preheating provides two important benefits. Since most tool and high speed steels are sensitive to thermal shock, a sudden increase from room temperature to the austenitizing temperature of 1500F/2250F may cause these tools to crack. Secondly, there is a phase transformation that the steel undergoes as it is heated to the austenitizing temperature that produces a change in density or volume. If this volume change occurs in a non-uniform manner, it can cause distortion of the tools. This problem is especially evident where differences in geometry or section size can cause some parts of the tool to transform before other parts have reached the aim temperature. The material should be preheated to just below this critical transformation temperature, and then held long enough for the entire cross-section of the part to equalize. Once the part is equalized, then further heating to the austenitizing temperature will allow the material to transform while undergoing a minimum amount of distortion.

2.) The austenitizing temperature that is selected depends strongly upon the alloy content of the steel. The aim properties including hardness, tensile strength, grain size, etc. also factor into the temperature that is chosen. In the annealed microstructue, the alloy content of the steel is primarily contained in the carbide particles that are uniformly distributed as tiny spheres. This condition is typically referred to as a sheroidized annealed microstructure. The idea behind austenitizing is to re-distribute this alloy content throughout the matrix by heating the steel to a suitably high temperature so that diffusion can take place. Higher temperatures allow more alloy to diffuse, which usually permits a higher hardness. (This is true as long as the temperature does not exceed the incipient melting temperature of the steel.) If lower austenitizing temperatures are used, then less diffusion of alloy into the matrix occurs. The matrix is therefore tougher, but may not develop as high a hardness. The hold times that are used depend upon the size of the part and the temperature that is used.

3.) Once the alloy content has been redistributed throughout the matrix, the steel must be cooled fast enough to fully harden it. This process is called quenching. By quenching the steel properly, a new phase transformation occurs, and the microstructure changes from austenite to martensite. How rapidly this process must take place depends upon the chemical composition of the alloy. Generally, lower alloy steels such as 01 must be quenched in oil in order to cool fast enough. Higher alloy content steels can develop fully hardened properties by undergoing a slower quenching process. For some alloys, cooling in still air is sufficient. Other mediums that are frequently used for quenching include water, brine, and salt bath. Whatever quenching process is used, the resulting microstructure is extremely brittle and under great stress. If the tool is put into service in this condition, it would likely shatter like glass. Some tools will even spontaneously crack if they are left in this condition. For this reason, tools that are quenched and cooled to hand warm (about 100F/150F) should be tempered immediately.

4.) Tempering is performed to soften the martensite that was produced during quenching. Most steels have a wide range of temperatures that can be used for tempering, and the one that is chosen depends upon the aim hardness. Most tool and high speed steels require several tempers before the part can be put into service. This is because these alloys will retain a certain percentage of austenite when they are quenched, and during the first temper some of this retained austenite will transform to untempered martensite. By performing a second temper, this new martensite is softened, thus reducing the chance of cracking. But by tempering a second time, some of the remaining austenite is transformed to untempered martensite, and so the process may need to be repeated several times.

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