Hardenability

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The hardenability of a metal alloy is its capability to be hardened by heat treatment. It should not be confused with hardness, which is a measure of the material's resistance to indentation or scratching. It is an important property for welding, since it is inversely proportional to weldability, that is, the ease of welding a material.

The hardenability of ferrous alloys, namely steels, is a function of the carbon content and other alloying elements, the grain size of the austenite, and the cooling rate. The relative importance of the various alloying elements is calculated by finding the equivalent carbon content of the material. The fluid used for quenching the material influences the cooling rate due to varying thermal conductivities and specific heats. Substances like brine and water cool much more quickly than oil or air. Additionally, if the fluid is agitated, cooling occurs even more quickly. The geometry of the part also impacts the cooling rate; if the surface area to volume ratio is high, the material will cool faster.

To test the hardenability of a ferrous alloy, a Jominy test is used. A round metal bar of standard size is transformed to 100% austenite through heat treatment. It is then quenched on one end with room temperature water. As a result, the cooling rates throughout the material will vary significantly, being highest at the end being quenched. The hardenability is then found by measuring the hardness throughout the bar. The farther away from the quenched end that the hardness exists, the higher the hardenability.

References

• Kalpakjian, Serope, et al. (2001). Manufacturing Engineering and Technology. Pearson Education.