Mn:S ratio in High Strength Steel Bolts
Mn:S ratio in High Strength Steel Bolts
(OP)
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?
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?





RE: Mn:S ratio in High Strength Steel Bolts
RE: Mn:S ratio in High Strength Steel Bolts
RE: Mn:S ratio in High Strength Steel Bolts
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.
RE: Mn:S ratio in High Strength Steel Bolts
RE: Mn:S ratio in High Strength Steel Bolts
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.
RE: Mn:S ratio in High Strength Steel Bolts
RE: Mn:S ratio in High Strength Steel Bolts
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.
RE: Mn:S ratio in High Strength Steel Bolts
Maui
www.EngineeringMetallurgy.com
RE: Mn:S ratio in High Strength Steel Bolts
I have been able to get photos.
Please see for yourselves and any suggestions would be appreciated.
RE: Mn:S ratio in High Strength Steel Bolts
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.
RE: Mn:S ratio in High Strength Steel Bolts
RE: Mn:S ratio in High Strength Steel Bolts
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.
RE: Mn:S ratio in High Strength Steel Bolts