18-8 Stainless Fasteners Too Hard

[kmenke]

We're finding a portion of the 18-8 stainless steel fasteners exceeding the hardness requirements of ASTM F593; which is HRC 32 for the cold-worked condition. I'm curious on the potential problems and issues that could be encountered with a stainless steel bolt with a core hardness of HRC 33-35.

It hasn't been tied to one manufacturer, but we've seen some correlation once the carbon content exceeds 0.4% (which is still below the maximum requirement).
Thanks

 

[MintJulep]

The following is an excerpt from my disapproval of a contractor's proposal to substitute 18-8 fasteners for the specified AISI 304 fasteners:

The term “18-8 stainless steel” is colloquial, loosely grouping a number of AISI stainless steel grades, but also describing a wide range of material that may not be controlled by any governing technical standard. There is no recognized technical standard controlling the content of “18-8 stainless steel”, the only “requirement” for inclusion in the 18-8 group is that the material contain “about” 18 percent chromium and 8 percent nickel. There is no limit on the amount of other elements that may be present, some of which may introduce undesirable properties to the alloy if allowed in greater than trace amounts.

 

[kenvlach]

MintJulep,
Agree with you in general, but in this specific case, ASTM F593 has an Alloy Group 1 (304, 304L, 305, 384, 18-9LW, 302HQ)* corresponding to 18-8 compositions.
*There are some additional, free-machining alloys permitted, but only with purchaser's agreement.

kmenke,
ASTM A593 Alloy Group 1 has 8 subdivisions, with 3 having a hardness limit HRC32 (there are also 4 softer grades & 1 harder: max HRC36). So, do you mean F593C, F593D or F593B?
I suspect that the hardness limits are a safety factor, to preserve an amount of elongation. Elongation decreases rapidly as hardness & YS increase, from 40% for F593A (hardness B85 max) to 12% for F593A (HRC36 max).
ASTM A593, Table 3, indicates that elongation testing isn't mandatory for all grades, so a maximum hardness limit is a de facto minimum elongation limit. And, hardness is a quick & easy test.

I presume & hope "carbon content exceeds 0.4%" is a typo and that you really meant 0.04% C.

 

[EdStainless]

If the fasteners still have adaquate elongations I wouldn't worry.
Hitting strength levels with various C and N levels can be a real trick.
There are tubing specs where you can accept based on hardness, but not reject. If the hardness is out of spec you must conduct tensile tests.

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[kmenke]

Sorry about the miscue on the carbon content; it was meant to be 0.04%.

We are dealing with the F593C and F593D, which would be the Group 1, cold-worked condition. As of now, we have tested approximately 80 different lots (incluidng five manufacturers), and have seen that about 25% of the parts exceed the allowable hardness limit. We haven't had any parts fail the elongation, but have had a few exceed the maximum allowable tensile strength.

Thanks for the further insight.

 

[kenvlach]

"a core hardness of HRC 33-35" seems a bit odd, as the core should be nearer the lower hardness limits (B80, B95), with the rolled threads near the max. hardness (depends a lot on the diameter). F593C & F593D bolts are "Headed and rolled from annealed stock thus acquiring a degree of cold work; sizes 0.75 in. and larger may be hot-worked and solution-annealed." -- ASTM F593.

"a few exceed the maximum allowable tensile strength" --
Perhaps manufactured from strain-hardened stock, so possibly meet the F593A specifications (HRC36 max, higher strengths, lower elongation)? But, even if so, then are mis-labelled.

Due to liability issues, the out-of-spec lots should be failed.