Modified Tool Steel? 6

[metalman8357]

I was under the impression that we were using S2 tool steel (0.4-0.55% Carbon) but I've learned today that we are actually using a modified tool steel with 0.67-70% Carbon. Our parts are experiencing brittle failure and we've decided to temper back to 42-46HRC from our previous specification at 52-56HRC. Now I'm wondering if this increased carbon content is the reason that we are seeing brittle failure. My understanding was that S2 was known for retaining reasonable ductility in the hardened condition which gave it it's toughness and shock resistant properties. By increasing the carbon content, we may be losing these benefits?

 

[redpicker]

In general, yes, higher carbon lowers toughness, so you could expect an increase of brittle fracture. However, the rest of the alloy mix is important, too.

One of the problem with going from 0.40-0.55 % carbon to 0.67-0.70 % carbon is that you will increase the likelihood of quench cracking. If these are the parts you posted about here, Identifying Quench Cracks, then you may consider oil quenching. 0.67% Carbon is a bit high to be water quenching.

I would also want to investigate the reason the modified chemistry was chosen as well as the specified hardness range. If the thought behind rasing the carbon was to increase the hardness of the parts, then lowering the hardness becasue of the carbon content is probably not the way to go.

rp

 

[metalman8357]

RP,

The mandrels were failing and fractographic analysis revealed brittle failure. The hardness was specified at 52-56HRC. I was asked to check out the issue, and I was told the material was AISI S2 tool stool. I decided to have test samples made that were tempered back to 42-46HRC to increase the toughness. My understanding was that S2 had purposely low carbon content to retain increased toughness and shock resistance even at high hardness values. I learned today, however, that we are actually using a modified S2 with 0.67-0.70 carbon content. Like you stated, lowering the hardness isn't necessarily the right approach to increasing toughness given the high carbon content. I am suggesting that we return to S2 tool steel with a hardness profile of 48-52HRC. Thoughts?

 

[metalman8357]

Modified Composition
C = 0.72
Mn = 0.58
P = 0.010
S = 0.009
Si = 1.25
Cr = 0.18
Ni = 0.06
Mo = 0.49
V = 0.16

 

[metman]

metalman8357,

I agree with redpicker and your comments.

You can expect minimal plastic deformation (brittleness) at HRC 55 or greater.

It appears that you need a way to verify that the chemistry received is what you ordered unless you know that the modified chemistry was what was actually ordered.

Design for RELIABILITY, manufacturability, and maintainability

 

[TVP]

Does the mandrel only require surface hardness in order to resist wear? If so, then it would be better to harden S2 tool steel with the regular C content to 42-46 HRC and use some type of surface treatment (ferritic nitrocarburize, Cr plating, TiN or similar type PVD coating, etc.) together with an appropriate lubricant to minimize wear. If the mandrel is experiencing unforeseen and undesirable bending loads, then this will be a difficult problem to resolve using only heat treatment of the mandrel.

 

[metalman8357]

I havent looked at any hardenability data for S2 tool steel but I was under the impression that it had a shallow hardenability when compared to, say S7. That being said, would it seem reasonable to achieve a through hardness of 42-46HRC for a 0.34" diameter mandrel for regular C content S2 tool steel? I havent spoken with our mechanical engineer on why a modified S2 with increased carbon content was selected, but my best guess is that it was chosen since we are normally through hardening these to 52-56 HRC. I like TVP's suggestion of using regular S2 with a wear resistant surface treatment. Regular S2 hardened to 42-46HRC should have much, much more toughness than modified S2 with 0.7% carbon hardened to 52-56HRC.

 

[TVP]

S2 is normally water quenched, but even oil quenching should be sufficient for 100% transformation with a diameter of only 0.34" (8.6 mm). But not with very much margin for error.

 

[LeanGreenMfg]

Sounds like shock has become a factor, in which regular carbon tool steels often fail. Have you tried a higher alloy (tough) tool steel like... X38CrMoV5-1 (1.2343) and X40CrMoV5-1 (1.2344) they are hot-work tool
steels for universal use. Should be about the same price, and hardened using the same advice above. I'm a manufacturing guy from aerospace, not a materials "expert", so maybe TVP and metalman can verify?