follow-up, Molydisulfide lubricants and high strength steel bolts
02-20-2003, 08:38 AM
#1
Racer
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follow-up, Molydisulfide lubricants and high strength steel bolts
Some time ago in a thread about lubricating bolts I posted a caution about "Moly"something products. This is a follow up post about Molybdenum-disulfide and Sulfide Stress Corrosion Cracking of high strength steel bolts (referred to as SSCC, SSC or SCC in various forums & publications). The text below was taken from the corrosion section of our company discussion forum. In summary:
1) Molydisulfide is a source of one element of the Sulfide Stress Corrosion environment.
2) Lubricants appied where "dry" torque is specified may result in overstressing bolts.
3) Add water to the above and the overstressed bolt is prone to cracking failure.
Please note crack growth rates in Stress Corrosion Cracking mechanisms are very high and that failure is usually catastrophic because there is precious little time to detect the problem by routine inspections. I prevent by not applying lubricants unless specified, preferential use of water repellant grease form, and challenging the need for the very low friction coefficient of these products.
Disclaimer: I am not an expert on this subject, I am stating opinions and passing on information that I think is useful to the Rennlist paticipants. IMHO, YMMV etc. etc.
Enjoy <img border="0" alt="[burnout]" title="" src="graemlins/burnout.gif" />
Nol
quote"
Sulfide Stress Corrosion Cracking on Hi Strength steel.
From memory: Incident investigation following a failure of flange bolts on offshore facility in mid-1990's concluded that the cause of bolt failure could be attributed to the molydisulphide lubricant. In this instance molydisulphide combined with the bolt material in a moist environment allowed corrosion to be accelerated through the presence of sulphides. Apparently when molydisulphide is applied in paste form (grease base) then it is not so problematic because grease repels water. In this instance water had been able to penetrate because the molydisulphide was a spray application. I think there was also reference to the spray application resulting in over-tensioning of the bolts through overspray onto the bolt bearing surface and hence incorrect torque readings which contributed to overtensioning and the eventual failure. Also possibly the corrosion environment exascerbated by the lack of airflow in the boltholes - water wouldn't dry or oxygen deficiency or something but we're getting to the limits of my memory now.
Recommendation resulting from this was immediate discontinuation of the use of molydisulphide as a lubricant (not sure how far-reaching this was in terms of spray- or grease-application or marine environments)
"unquote
1) Molydisulfide is a source of one element of the Sulfide Stress Corrosion environment.
2) Lubricants appied where "dry" torque is specified may result in overstressing bolts.
3) Add water to the above and the overstressed bolt is prone to cracking failure.
Please note crack growth rates in Stress Corrosion Cracking mechanisms are very high and that failure is usually catastrophic because there is precious little time to detect the problem by routine inspections. I prevent by not applying lubricants unless specified, preferential use of water repellant grease form, and challenging the need for the very low friction coefficient of these products.
Disclaimer: I am not an expert on this subject, I am stating opinions and passing on information that I think is useful to the Rennlist paticipants. IMHO, YMMV etc. etc.
Enjoy <img border="0" alt="[burnout]" title="" src="graemlins/burnout.gif" />
Nol
quote"
Sulfide Stress Corrosion Cracking on Hi Strength steel.
From memory: Incident investigation following a failure of flange bolts on offshore facility in mid-1990's concluded that the cause of bolt failure could be attributed to the molydisulphide lubricant. In this instance molydisulphide combined with the bolt material in a moist environment allowed corrosion to be accelerated through the presence of sulphides. Apparently when molydisulphide is applied in paste form (grease base) then it is not so problematic because grease repels water. In this instance water had been able to penetrate because the molydisulphide was a spray application. I think there was also reference to the spray application resulting in over-tensioning of the bolts through overspray onto the bolt bearing surface and hence incorrect torque readings which contributed to overtensioning and the eventual failure. Also possibly the corrosion environment exascerbated by the lack of airflow in the boltholes - water wouldn't dry or oxygen deficiency or something but we're getting to the limits of my memory now.
Recommendation resulting from this was immediate discontinuation of the use of molydisulphide as a lubricant (not sure how far-reaching this was in terms of spray- or grease-application or marine environments)
"unquote
02-20-2003, 11:38 AM
#2
Yup,
I understand why people always seem to put some "grease" on a bolts before tightening.
But, as you say is true. It allows them to be overtightneded and causes other problems as well.
There are different tricks for this or that proven through time, but as a rule always follow what the manufacturer says. They desinged it and know. If it doens't specifically mention adding something, then don't. And if it does..do it.
I understand why people always seem to put some "grease" on a bolts before tightening.
But, as you say is true. It allows them to be overtightneded and causes other problems as well.
There are different tricks for this or that proven through time, but as a rule always follow what the manufacturer says. They desinged it and know. If it doens't specifically mention adding something, then don't. And if it does..do it.
06-26-2011, 09:04 AM
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1st Gear
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Friction Coefficient
I do not profess to be an expert in these matters but I have had to consider them in my working life. A friction coefficient has to be taken into consideration when making up a fastener to a certain torque. If the manufacturer states using a lubricant then use the specified one (or one with similar lubricating properties). This will either come with the lubricant, or can be found on the manufacturer's (of lubricant) website or by direct contact.
But to come back on thread, I prefer to use a thread lubricant as I may need to disassemle this equipment in the future (or somebody else for sure). There are tables available to tell you the reduction of torque required to give you the same tension on a bolt between dry, oiled or various lubricants that give specified friction coefficients.
e.g. A one inch NC bolt requires a torque of 1,332 ft/lbs dry, using a lubricant with a friction coefficient of 0.06 requires a torque of 245 ft/lbs to give the same tension.
I hope that this makes sense and that it helps.
But to come back on thread, I prefer to use a thread lubricant as I may need to disassemle this equipment in the future (or somebody else for sure). There are tables available to tell you the reduction of torque required to give you the same tension on a bolt between dry, oiled or various lubricants that give specified friction coefficients.
e.g. A one inch NC bolt requires a torque of 1,332 ft/lbs dry, using a lubricant with a friction coefficient of 0.06 requires a torque of 245 ft/lbs to give the same tension.
I hope that this makes sense and that it helps.
