Design for Thru Hardening or Not?

[mewhg]

I have been thinking about the post on 1045 vs. 4340

http://www.eng-tips.com/viewthread.cfm?qid=190661&page=2

and have some questions about design methodology.

Lets just say one is designing a machine component and is considering a range of steels. Why is it totally necessary for the component to be hardened all the way through? I understand that a tool such as a die/punch where the tool will be ground throughout its lifetime for sharpening purposes and consistent properties are desired through the tool cross section.

Take for instance a machine shaft or a component in a high speed machine under moderate impact and light wear. If one uses a steel that will not through harden the part, will there be favorable compressive stresses in the surface of the non-thru hardened component similar to a carburized part? If the surface gets hard enough with a partially hardened part isn't that enough? Wouldn't a fully through hardened part be more inclined to fracture?

I remember reading an ad I think in a trade magazine where someone redesigned a large track pin for Caterpiller. CAT had been using an alloy steel for the pin that was carburized then Q/T. The new pin was made from 1065 if my memory serves me well. The point the article made was that after heat treat the 1065 pin had a softer core that assisted in fatigue resistance and the harder surface was sufficient to withstand service conditions. Also the 1065 pin was cheaper to make and heat treat.

 

[swall]

You start by understanding the stresses acting on the parts. Then you know how strong the part needs to be. If you don't have size constraints, you can use unheat treated steel. If you must make a compact part that will see high stress, you need to strengthen it by heat treating. Then you address special requirements, such as wear resistance and fatigue resistance. Many factors lead up to choosing a steel for a particular application.

 

[EdStainless]

One issue can be that in some situations the mixed structure that you will have in the core may harm your overall performance. Since the properties of the core will be somewhat uncontrolled you have to able to tolerate that.
My hunch is that the Cat part mentioned was seeing some wear and the load was in pure shear. It might be different if the part say impact loading.

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[swall]

Yes, getting back to that Cat part. The previous part, carburized, quenched and tempered was probably made from 8620 steel, a carburizing grade. This gives you a hard case for wear resistance and a tough core to support the case layer. The 1065 replacement was undoubtedly induction hardened to achieve the hard case layer. The core would have not been hardened. As long as you properly choose the case depth and base material, induction hardening can make a perfectly satisfactory replacement for a carburized part at lower cost.