Hardness to Yeild strength table

[ATVextreme]

I am trying to calculate the strength of a heat treated part made from 8620. I have the hardness profile of the case and core measured from a lab. I have a hardness conversion table for steel to convert Rc to UTS. Is this correct or should I use a table specific to my material. I would rather use yield strength than UTS, but it is all I can find. Where can I find tables for specific alloy steels that relates hardness to yield strength and UTS. Any help is appreciated.

 

[desertfox]

Hi

I found this on google, it suggests that hardness to strength charts are empirical and depending how accurate you want to be suggests that charts for a specific material may need to be referred to.

Regards

desertfox

http://books.google.co.uk/books?id=...rength conversion for specific steels&f=false

 

[metengr]

Correlations between hardness and strength are generally to ultimate tensile strength and not yield strength for steels.

 

[OGMetEngr]

Take a look at the paper titled "Correlation of Yield Strength and Tensile Strength with Hardness for Steels" by Pavlina and Van Tyne (taken from 20 yrs thesis work at the Colorado School of Mines). They make the correlation for different microstructures and yield/tensile ratios. If you graph their equations alongside the ASTM A370 table, they match quite well. Standard error for tensile and yield are similar when grouped according to microstructure or YS/TS ratio.

 

[TVP]

Use the general conversion of hardness to UTS. The yield strength will be ~ 90% of the UTS for a quenched and tempered microstructure. If you need more accuracy than this, then you need to perform more detailed testing.

 

[ATVextreme]

Thank you for the feedback. Does this mean that regardless of the specific alloy of steel that the strength calculations are based on the heat treat hardness specifications. I guess I assumed that different alloys at the same hardness would have a different YS and perform differently. I was wanting to compare different materials for an application. Specifically we have a part being made of 8620 and it is carburized. We want to make it stronger and tougher with out changing it dimensionally. I would like to know if we change the material to 4340 or 9620 without changing the heat treat callout would there be a change in strength that could be calculated. If the Case hardness ,case depth, and core hardness have the same values then would the parts be very similar in strength? The current hardness specification for this thin walled part is Rc60-64 to a depth of 0.8-1.1mm. The core is Rc35-45. The minimum wall thickness for this bearing cage is 4mm.

Does the core hardness simply need to increase to gain more strength regardless of material?

 

[swall]

For strength calculations in a carburized part, such as your's, you have to consider the strength of both case and core and how much of the cross section is comprised of each. If you are just looking for a breaking strength or bending strength to compare carburized versions of your part with through hardened grades, you may be well suited by devising some sort of proof load test on the part.

 

[ATVextreme]

Yes swall, I have the microhardness profile measured through the case and core. I have modeled the cross section in Microsoft Excel using the varing hardness profile at each depth interval. To compare a part through hardened made of 4340 with a thinner Nitride case I would model acordingly. I can handle the math, but I was struggling with values to use for UTS or YS based on different materials heat treated to specific hardness values. Knowing that the YS is approximately 90% of UTS helps. I am looking for % increase in strength from current production as opposed to achieving a specific load. I think the bottom line is that I will use the values from the general table I found that relates Rc to UTS for Steel. As far as understanding the effect of different alloys regarding toughness I will look at Izod versus hardness. I found this link that looks very helpful.

http://www.emjmetals.com/PDF/BBook/Mechanical Properties and Hardenability.pdf

Ultimatey any production changes will be tested real world before implemented in any quantity. I just want to get a best estimate before ordering samples. Samples and tests take weeks to months and math models only take hours or days and cost much less. Thanks again for the information.

 

[swall]

ATVextreme--yes, you are aware of the issues involved and your analysis may provide a good insight into the ultimate strength of the part. I would caution you about the nitrided 4340, however as nitride cases are for wear resistance rather than load carrying. I would just use the core hardness of the 4340 to derive the yield strength of the part and ignore the nitride layer.

 

[ATVextreme]

The part is a bearing surface and requires the surface to be hard. Not sure if 4340 through hardened to Rc50 would be hard enough surface for wear with out the Nitride? Also not sure what the upper limit of 4340 is with out becoming too brittle.

 

[swall]

One of my previous employers used a 4340 variant, 300M, for a thrust bearing (roller type). Hardness, as I recall was around Rc 53-55. Price and availability concerns with 300M if you are looking at small quantities. One upgrade to 8620 you might consider is 9310, carburized.

 

[ATVextreme]

Yes, I am familiar with 300M and it is on my short list of material options and thank you for the 9310 suggestion. Back to the origonal question. To justify the expense of the 300M on paper by showing an increase in strength over 4340. If the hardness call outs are the same according to this thread then one would expect the part to have the same strength. Unless I can find a table that says 300M has a higher impact / izod at the same hardness than the 4340. How do you show the performance improvemnt on paper?