Mn:S ratio in High Strength Steel Bolts 5

[sunshine419]

Please I would appreciate if experts in the house could help.

I have some high strength steel bolts (M48 x 210mm) of grade 8.8 which had failed in service (along their bolt-heads). My initial suspect was manufacturing defect. Does anyone have an idea if ‘hot shortness’ could be a possibility? Also, would want to know the minimum Mn/S ratio required for this sort of bolt.

Finally, I had tried to check BS ISO 898-1 but found that it is only limited to max bolt size of M39. Does anyone know which standards these M48 bolts could be covered?

 

[Wrenchbender]

Hot shortness? I think that description is for high temperature failure close to the forging temp, although most modern steels have enough Mn to preferentially make globular MnS instead of flat FeS. I've seen rules of thumb on ratios and i'm sure someone will chime in. Some mills also use Ca additions for the same reason. Good reason to check the spec chemistry (as you are attempting, but I can't help you there either). At least you have a consistent failure at a stress concentration. Could stress corrosion be a factor?

 

[dbooker630]

SAE J1199 does not show anything for Mn/S ratio. For class 8.8 studs resulfurized steel is permitted (0.13 max), otherwise for class 8.8 max sulfur is 0.058 for plain carbon steel. For 8.8 alloy steels (sizes over M24) max sulfur is listed as 0.045. You should next get the sample analyzed for chemistry.

 

[sunshine419]

Wrenchblender,
thanks for the info but stress corrosion cracking is less likely as there was no corrosive environment. The bolts were used to clamp two identical shells weighing.

dbooker630,
do you know if there is an equivalent BS spec of SAE J1199?

In addition, the analysis gave C,Mn,S,and P values as 0.47, 0.7, 0.033 and 0.033% with Cr,Mo and Ni additions as 0.58, 0.04 and 0.16 respectively.

 

[swall]

Sunshine49--could you elaborate a bit on the failure mode? Method of manufacture?

 

[TVP]

sunshine,

SAE J1199 is essentially equivalent to BS EN ISO 898-1. There are some small differences, but in general it uses the same or similar requirements for mechanical properties, chemical composition, etc. Most ISO standards that include sizes > M39 say something like this:

Property class
d ≤ 39 mm: 5.6, 8.8, 10.9
d > 39 mm: as agreed

International standards
d ≤ 39 mm: ISO 898-1
d > 39 mm: as agreed

So if it says 8.8 somewhere, then it is essentially referencing ISO 898-1, because otherwise 8.8 is meaningless.

With regards to hot shortness, no, this is not directly applicable as a failure mechanism for a threaded fastener. It pertains to hot working of steel during manufacture of plates, bars, etc. Mn:S ratio should not be a problem in general, and certainly isn't based on your reported composition (0.7% Mn, 0.033% S). The usual limit is ~ 8:1 for hot shortness to be an issue.

With regards to fastener failure modes, there are several things to investigate when there is head-to-shank separation:

1. Delayed fracture due to hydrogen embrittlement
2. Stress corrosion cracking
3. Manufacturing defect such as quench crack, forging lap/burst, grainflow runout, etc.
4. Dimensional problem such as head-to-shank transition radius too small (radius should be 1.6 mm minimum [ISO 8676 or similar], excessive runout between thread axis and underhead bearing surface/flange, etc).

You should have a proper metallurgical analysis performed so that you can understand the failure mode.

 

[metengr]

Get a proper failure anlaysis performed otherwise all speculation.

 

[sunshine419]

Swall,the bolts were forged.

TVP, ISO 898-1 only specify the mechanical properties for sizes d>39. On the chem properties, no mention was made wrt to d>39.

I was also thinking the limits for hot shortness was 20:1. Do you please have a reference on ratio being ~8:1 as stated?

I think the bolts had failed based on forging laps as a dull gray oxide scale was seen below the failed areas.

The shank and threaded areas were however unaffected.

 

[Maui]

If you are able to post images of the failures, and any microstructural photographs that have been taken, I would encourage you to do so. They would help a great deal in narrowing down the possible root cause(s).

Maui

http://www.EngineeringMetallurgy.com

 

[sunshine419]

Guys,
I have been able to get photos.
Please see for yourselves and any suggestions would be appreciated.

 

[metengr]

sunshine419;
To be honest, the macrophotographs do not provide any 'smoking gun' as to the actual cause of failure. The appearance of the defect along the surface seems to follow the flow pattern of the grains during heading. Beyond this observation, I would be guessing, and you should not be paid to guess or speculate.

I could eliminate the following - forging lap, stress corrosion crack, seam defect in the bar used to form the head, and possibly a quench crack because of orientation and appearance. Beyond this you still have possible chemistry issues related to the bar, segregation, loss of temperature control during heading, etc.

 

[CoryPad]

Was this part welded? There appears to be a weld seam between the shank and head.

 

[mrfailure]

I concur with Metengr's original comment - you really cannot tell what happened to this bolt without a metallurgical failure analysis. More background would be required as well - such as what type of service the bolts saw, how long it took before failure was noticed, etc. You also need to know how deep the crack runs and what head structure through the crack looks like.

Regarding bolt size: My interpretation of ISO 898-1 is the M39 size limitation only applies to tensile load testing of full-size bolts. That is a practicality limitation. You can still use bolts of larger size - you just have to test using machined tensile specimens.

 

[Guest102023]

First get a failure analysis, or at least a determination of the mechanism (they're not the same thing though).