Heat Treat Society Online Member Community

To all:

Im new to the field and am interested into some insight of DI vlaues as they pertain to heat treatment. I am learning as I go but many have aided me with valuable knowledge so far. We do not have a metallurgist anymore so I have tried to step up as a career path change from industrial electrician with induction heating experieence. My question is we are always in caution of a higher DI as it makes our product harder to mahcine prior to heat trearment via induction. I have searched to see what concerns may be if our Di value is lower. Not that it puts us out of spec or tolerance but Im just curious as to its possibility of concern.

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Rodney Recore
Plant Process Metallurgist
Gasport NY
(716) 998-9823
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Hello Rodney,

    The DI is the ideal critical diameter, it is a number that represents the hardenability, properly the diameter of a bar of a given steel that would get 50% martensite in its core when quenched with an ideally drastic quench. Put in other words, the DI relates how deep you can harden a material by quenching a completely austenitised section of steel. Moreover, the DI by itself does not reflect how hard (e.g. in HRC) the steel could get, but how deep you can find martensite. 

What dictates the surface hardness is the carbon content on the steel, and how hardness decreases going deep into the material is a function of the chemistry of the steel (hardenability, DI) and the quench procedure (medium, temperature of quench medium, agitation, etc). So answering your question, if you have a lower DI steel, you could expect shallower cases, or lower hardness at the core.

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Jose Mariano Flores Herrera
Heat Treat Engineer
Caterpillar
San Nicolas de los Garza
5218717957482
mariano@flores-h.com
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