Bolt Hardness?

[ggt1_02]

I am doing an informal failure analysis of a fractured bolt for an acquaintance. It appears that the bolt failed from poor surface finish at the cam hole. The bolt was not shot-peened post machining. My searching has shown that this supplier has an issue with bolts fracturing at this location.

I took a case measurement of the bolt and the case depth was only 0.17mm with a surface hardnees of 60HRC and a core of ~31HRC. I do not know what the material is but am guessing that it is 9310 as it is a budget brand and is case carburized with zinc phosphate coating.

What is the spec for case depth of the 9310 bolts?

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[Todd.K]

I'm not sure you should expect a standard heat treat for a non standard material. Other than believing it's a cheaper and more available material to substitute rather than an engineered design change...

C158 is .010" to .014"

[ggt1_02]

I'm not sure you should expect a standard heat treat for a non standard material. Other than believing it's a cheaper and more available material to substitute rather than an engineered design change...

C158 is .010" to .014"

I never thought about there not being a spec due to it not being the standard material, thanks. I am used to everything we make or use having hard specs and knowing what they are made of before hand.

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[lysander]

The print calls for a Carpenter 158 bolt with a case depth of .010" to .014", with a hardness of Rockwell 15N89/90.5, or around 58 to 60 on the Rockwell C scale.

If you have the core hardness of the failed item you should be able to back out the ultimate tensile strength from that, it should be in the 160 KSI range.

Other bolts made from 9310 (namely M60 bolts) call for a core hardness of Rockwell C 32 to 44, and a surface hardness of Rockwell A 80 (Rockwell C 57-58) and a case depth of 0.022 to 0.032", with a fine grain structure. This gives a ultimate yield of around 160 KSI as well.

If they failed to shot peen the cam pin area, the question is: "What else did they not do, or do wrong?" AISI 9310 steel should be cryo-treated (-100 F or lower) for fatigue resistance after carburizing, but before tempering, and should not be carburized in a cyanide bath or be nitrocarburized (from the M60 bolt print). (NOTE: Cyanide bath carburizing is acceptable for C158, per the print.)

[ggt1_02]

The print calls for a Carpenter 158 bolt with a case depth of .010" to .014", with a hardness of Rockwell 15N89/90.5, or around 58 to 60 on the Rockwell C scale.

If you have the core hardness of the failed item you should be able to back out the ultimate tensile strength from that, it should be in the 160 KSI range.

Other bolts made from 9310 (namely M60 bolts) call for a core hardness of Rockwell C 32 to 44, and a surface hardness of Rockwell A 80 (Rockwell C 57-58) and a case depth of 0.022 to 0.032", with a fine grain structure. This gives a ultimate yield of around 160 KSI as well.

If they failed to shot peen the cam pin area, the question is: "What else did they not do, or do wrong?" AISI 9310 steel should be cryo-treated (-100 F or lower) for fatigue resistance after carburizing, but before tempering, and should not be carburized in a cyanide bath or be nitrocarburized (from the M60 bolt print). (NOTE: Cyanide bath carburizing is acceptable for C158, per the print.)

Thanks for the info, we had discussed that the core microstructure we were observing was possibly due to a slack quench. If they did not cryo treat it that would explain it. I didn't have any of the images when I originally posted, attached are the fractured surface, surface condition and case 200x and core 1000x microstructures.

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[Clint]

Thanks for the info, we had discussed that the core microstructure we were observing was possibly due to a slack quench. If they did not cryo treat it that would explain it. I didn't have any of the images when I originally posted, attached are the fractured surface, surface condition and case 200x and core 1000x microstructures.

How would you characterize that core micro structure?

[lysander]

One of the good things about 9310 is that you can get 100% martensite at a fairly low hardness, something you can't easily do with high carbon content alloys. This gives good toughness. However, the high nickel content gives high percentage of retained austenite in the case. Retained austenite lowers the hardness; therefore the fatigue and wear properties are adversely affected. If you are seeing RHc 60 in the surface, it would appear that the freezing may have been done. Freezing the part to -100 F for a few hours will normally transform all the austenite to martensite.

The carbon and nickel content of Carpenter 158 is similar to 9310, so it would seem that the above should be true of that alloy as well, however, I haven't seen freezing recommended for that alloy....

[lysander]

How would you characterize that core micro structure?

It would appear to be fine grained.

[bruin]

AISI 9310 steel should be cryo-treated (-100 F or lower) for fatigue resistance after carburizing, but before tempering, and should not be carburized in a cyanide bath or be nitrocarburized (from the M60 bolt print).

This is interesting, as nitrided 9310 AR bolts are more common nowadays. Any insights as to why the process was prohibited for M60 bolts? Would any drawbacks also apply to AR bolts?



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[lysander]

This is interesting, as nitrided 9310 AR bolts are more common nowadays. Any insights as to why the process was prohibited for M60 bolts? Would any drawbacks also apply to AR bolts?



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No idea.

Maybe they didn't want the surface that hard and brittle.