Strength of stainless steel bolt 4

[TJK1]

Shigley's text book states that the proof load of a bolt is roughly the same as the yield strength. Does anyone know what the proof load or yield strength would be for a typical 3/8" diameter, 300 series stainless steel hex head bolt, the kind you would buy at a hardware store. McMaster-Carr shows 316 stainless bolts as having a minimum tensile strength of around 70ksi, but that I assume is the ultimate tensile strength. I am looking for the yield or proof strength. Shigley also says to tighten a grade 8 steel bolt to 90% of its allowable proof load. Would you also tighten a stainless steel bolt to 90% of its proof load?

 

[Cockroach]

There are many 300 series stainless steel types. It depends on exactly what you are supplied with, condition wise.

I would phone a supplier and ask for an MTR for your size and material bolt intended for use. Yes, typically Atlas T316 SS comes in at around 75 ksi tensile, 25 ksi yield (ASTM A276 for bars, hot finished or annealled).

Again, this is a typical mechanical property for wrought austenitic stainless steel, pg 230, The Metals Black Book, Volume 1-Ferrous Metals (CASTI Publishing Inc).

Shigley is talking about a SAE J429 Grade 8 bolt which is a medium carbon steel alloy, quenched and tempered. This means the bolt has 150 ksi tensile, 130 ksi yield. Therefore the strengths are TWICE that of stainless steel. I wouldn't be taking any SS fastener to ninety proof preload!

I recommend you talk to a bolt supply house for guidance.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[TJK1]

Thanks Cockroach. If I assume the allowable proof load is only 25ksi for the stainless steel bolt, what percentage of that would you normally think it should be tightened to? Shigley also says that you should tighten a grade 8 bolt to 90% of its proof load. If you tighten it to less than that, does that mean that the bolt will not be able to take as much of an external tension load?

 

[Cockroach]

You can approximate the required preload noting that for a bolt of material 70 ksi UTS and 21 ksi YS a 3/8 diameter thread can be torqued to:

a) Course thread form: 14.68 ft lbf,
b) Fine thread form: 16.74 ft lbf.

This presupposes the fasteners are new, uncoated without lubrication. In this case the standard coefficient of friction is assumed to be 0.14 and that the final tightening sequence is achieved smoothly and slowly. At full torque value, the preload is approximately 90% fastener yield strength.

I would use a safety of 80% to 85% for stainless steel on these figures noting that stainless is more susceptable to thread pickup and galling. I would recommend a lubricant, teflon free base, something like an anti-seizing bronze grease.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[TJK1]

Thanks Cockroach, but what would happen if I tightened the stainless steel bolt to only 50 or 60% of the torque values you gave me. Would that cause the bolt not to be able to take as much of an external tension load? We may not be able to use a torque wrench and may end up tightening it with just a rachet wrench.

 

[CoryPad]

TJK1,

The proof stress for fasteners is defined as 90% of the yield stress.

There are many stainless steel alloys used for fasteners, and many conditions for each alloy. ASTM F 593 Standard Specification for Stainless Steel Bolts, Hex Cap Screws, and Studs has several classes for alloys that meet 316 chemistry. For example, condition A has a tensile strength of 70 ksi (as measured on machined test specimens), and a yield strength of 30 ksi.

The proof strength would be 27 ksi. I would use 100% of proof as a starting point since this fastener has huge plastic strain capacity and won't be sensitive to torquing slightly beyond the yield stress. You should tighten the fasteners as closely as possible to the proof strength to maximize the resistance to external forces, including loosening.

I think you should test any torque values before using them on actual joints. Untreated stainless steel has a high friction coefficient (~ 0.3 if there is no lubricant), so using a lubricant is a must. With a molybdenum disulfide lubricant (commonly used for stainless fasteners), I think a torque ~ 11 ft lbf would be suitable (assuming standard hex head, standard hole, and coarse threads).



Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[arto]

If you need high yield strength stainless bolts, get strain hardened i.e., ASTM A-193 B8 Class 2 {125ksi Su, 100ksi Sy]. Strain Hardened have the head marking grade ID underlined. Use neverseez to prevent galling.

 

[sreid]

You might look at NAS1351N and NAS1352N (A286) bolts.