Hydrogen Embrittlement (associated topic to chrome plating)
Posted: Fri Dec 14, 2007 4:53 pm
Just as an interesting aside.... We sometimes are not aware of the subtle things that go on behind the scenes in mfr'g aircraft parts. In a discussion by participants at another site I sometimes visit, hydrogen embrittlement of aviation fasteners came up.
The questioner posed this:
Soon after receiving a new batch of custom aerospace screws, I received a complaint from some of our technicians that the screws were failing after torquing and needed some help from engineering to investigate the problem. I thought, no problem. This should be one of the easiest investigations I’ve ever come across – they were probably just over-torquing them and needed someone to re-calc. for a proper pre-load. I expected to be a hero when I left.
After examining the properties of the screw, the clamping surface properties and the torque values that they were using, I was a bit confused as to why the screws were breaking. The torque values seemed to produce about 60-70% of a max. acceptable preload – which was OK since that was reflective of how effective the screws had to be. Nevertheless, this was not the most confusing aspect of the problem – The screws only failed around 20 – 30 minutes after torquing.
I verified the screw’s capabilities by tensile testing several for Fyt and Fut and then torqued many more to failure. The data was what one would expect it to be – given the material and screw size. These screws are 4340 steel heat treated to 160ksi Fut min. Does anyone have an idea of what is going on? Could it be that the screws were not tempered properly after heat treating? Thanks in advance.===========
One of the respondents gave a little insight to what can go wrong when specifications are scant/non-existant or assumed:
The key words You used "new batch of custom aerospace screws" raised the hair on my neck. "Spec parts", [NAS, MS, AS, M, etc] have very rigid fabrication processes and tracking requirements [paper-work out-the-gazoo for each production run. "Custom parts", made to customized specs, for a specific client are another story.
A wise (no-so-old) fastener engineer summarized the business of custom aero-hardware in the following statement.
"You deserve to get what You asked for [IE: on the drawings or spec sheet]. However, if You don't ask for very much, then You have no right to 'expect' very much [high quality and performance for a bargin]."
I suspect that several things MAY have happened, if this is truly custom hardware.
Your drawings/specs lacked clarity. There are very good specs for steel [male] fasteners available that may be liberally referenced for fundamental processing methods, tracking, batch testing, documentation, etc. NAS4002 stands-out immediately for fasteners heat treated to 160-KSI. Also, several other documents deal explicitly with fastener metallurgy, grain flow, etc; while several other documents deal with plating processes for high strength steel parts [note: embrittlement is usually a problem with steel parts heat treated above 160-KSI...You never stated what the heat treat was supposed to be].
So where am I going with this? Your problem description is very "hazy", since the nature [shape, composition, coating, etc] of the "screws" was not defined for us. PS: composition does NOT just mean 4340 steel... it means:
4340 or 4340M [X] steel per AMSxxxx, heat-treated to XXX-to-XXX-KSI per xxxxxxxx, maximum grain size X, inclusions and allowed microstructure anomalies of xxxxxx, forged head per XXXX, rolled threads per XXXXX, rolled root radius of xxxx, ground to a machined finish of XX-RA, Magnetic particle inspected before and after plating per XXXX, grade [or class XX], etc....
So what could have gone wrong???
Machined featured VS forged/rolled features
Improper heat treatment [too high, too low, wrong quenchant, etc]. NOTE: AMS2759/1 suggests that tempering temperatures for 4340 vary, depending on initial quench hardness, etc.
No grain controls [size, impurities, flow, etc].
No batch testing for basic metallurgy or embrittlement. NOTE: if actually accomplished on a random sampling of parts, this embrittlement should have been "immediately obvious" to your processor. NOTE: most specs require a "sampling quantity" be included with each production run... IF You ordered a 1000 parts, then X% should have been added to the lot by the processor for destructive testing... if demanded by QA control requirements.
Embrittlement due to improper plating processes [too thick, too much current, improper plating solutions, too long, etc]. OH yeah was it coated with cadmium, nickel, Ni-Cd, Zn-Ni, Zinc, Silver, etc???
Embrittlement relief bake gone bad? Perhaps... but which way...
(a) Pre-plating stress-relief bake not accomplished [or perhaps not specified]?
(b) No plating stress-relief bake required by spec... but improper heat treatment over strengthened parts to point where it was mandatory??
(c) Too low a bake temperature and/or too-short a bake time to eliminate hydrogen? Note: very thick/dense platings demand longer bake times to force H-atom thru the coating.
(d) Too high bake temperature causing liquid-metal embrittlement [cadmium embrittlement]?
NOTE: IF DONE PER AN ESTABLISHED PROCESS, AND PROPERLY DOCUMENTED your [legitimate] processor/vendor should be forthcoming in helping You analyze the failures and come to a "satisfactory" compensation [warranty] for the entire batch of parts. IF you discover holes in the paper trail, or lack of cooperation on the part of Your vendor, then do as others have suggested... send ~100 off to a reputable metallurgy lab for analysis... and scrap the rest... and notify the FAA.
NOTE: You may discover, brutally, that the vendor/processor did every thing EXACTLY as you specified... and the parts failed. This MAY turn-out to actually be Your problem because of the way You spec'ed-out the fabrication process [refer to the "rule" I stated in my intro, way-above].
Lots of things to consider in this situation…=================
"...and scrap the rest...."
Next time you see some "aircraft quality bolts" for sale on Ebay.... think twice about how much money you might actually be wasting your life over.
The questioner posed this:
Soon after receiving a new batch of custom aerospace screws, I received a complaint from some of our technicians that the screws were failing after torquing and needed some help from engineering to investigate the problem. I thought, no problem. This should be one of the easiest investigations I’ve ever come across – they were probably just over-torquing them and needed someone to re-calc. for a proper pre-load. I expected to be a hero when I left.
After examining the properties of the screw, the clamping surface properties and the torque values that they were using, I was a bit confused as to why the screws were breaking. The torque values seemed to produce about 60-70% of a max. acceptable preload – which was OK since that was reflective of how effective the screws had to be. Nevertheless, this was not the most confusing aspect of the problem – The screws only failed around 20 – 30 minutes after torquing.
I verified the screw’s capabilities by tensile testing several for Fyt and Fut and then torqued many more to failure. The data was what one would expect it to be – given the material and screw size. These screws are 4340 steel heat treated to 160ksi Fut min. Does anyone have an idea of what is going on? Could it be that the screws were not tempered properly after heat treating? Thanks in advance.===========
One of the respondents gave a little insight to what can go wrong when specifications are scant/non-existant or assumed:
The key words You used "new batch of custom aerospace screws" raised the hair on my neck. "Spec parts", [NAS, MS, AS, M, etc] have very rigid fabrication processes and tracking requirements [paper-work out-the-gazoo for each production run. "Custom parts", made to customized specs, for a specific client are another story.
A wise (no-so-old) fastener engineer summarized the business of custom aero-hardware in the following statement.
"You deserve to get what You asked for [IE: on the drawings or spec sheet]. However, if You don't ask for very much, then You have no right to 'expect' very much [high quality and performance for a bargin]."
I suspect that several things MAY have happened, if this is truly custom hardware.
Your drawings/specs lacked clarity. There are very good specs for steel [male] fasteners available that may be liberally referenced for fundamental processing methods, tracking, batch testing, documentation, etc. NAS4002 stands-out immediately for fasteners heat treated to 160-KSI. Also, several other documents deal explicitly with fastener metallurgy, grain flow, etc; while several other documents deal with plating processes for high strength steel parts [note: embrittlement is usually a problem with steel parts heat treated above 160-KSI...You never stated what the heat treat was supposed to be].
So where am I going with this? Your problem description is very "hazy", since the nature [shape, composition, coating, etc] of the "screws" was not defined for us. PS: composition does NOT just mean 4340 steel... it means:
4340 or 4340M [X] steel per AMSxxxx, heat-treated to XXX-to-XXX-KSI per xxxxxxxx, maximum grain size X, inclusions and allowed microstructure anomalies of xxxxxx, forged head per XXXX, rolled threads per XXXXX, rolled root radius of xxxx, ground to a machined finish of XX-RA, Magnetic particle inspected before and after plating per XXXX, grade [or class XX], etc....
So what could have gone wrong???
Machined featured VS forged/rolled features
Improper heat treatment [too high, too low, wrong quenchant, etc]. NOTE: AMS2759/1 suggests that tempering temperatures for 4340 vary, depending on initial quench hardness, etc.
No grain controls [size, impurities, flow, etc].
No batch testing for basic metallurgy or embrittlement. NOTE: if actually accomplished on a random sampling of parts, this embrittlement should have been "immediately obvious" to your processor. NOTE: most specs require a "sampling quantity" be included with each production run... IF You ordered a 1000 parts, then X% should have been added to the lot by the processor for destructive testing... if demanded by QA control requirements.
Embrittlement due to improper plating processes [too thick, too much current, improper plating solutions, too long, etc]. OH yeah was it coated with cadmium, nickel, Ni-Cd, Zn-Ni, Zinc, Silver, etc???
Embrittlement relief bake gone bad? Perhaps... but which way...
(a) Pre-plating stress-relief bake not accomplished [or perhaps not specified]?
(b) No plating stress-relief bake required by spec... but improper heat treatment over strengthened parts to point where it was mandatory??
(c) Too low a bake temperature and/or too-short a bake time to eliminate hydrogen? Note: very thick/dense platings demand longer bake times to force H-atom thru the coating.
(d) Too high bake temperature causing liquid-metal embrittlement [cadmium embrittlement]?
NOTE: IF DONE PER AN ESTABLISHED PROCESS, AND PROPERLY DOCUMENTED your [legitimate] processor/vendor should be forthcoming in helping You analyze the failures and come to a "satisfactory" compensation [warranty] for the entire batch of parts. IF you discover holes in the paper trail, or lack of cooperation on the part of Your vendor, then do as others have suggested... send ~100 off to a reputable metallurgy lab for analysis... and scrap the rest... and notify the FAA.
NOTE: You may discover, brutally, that the vendor/processor did every thing EXACTLY as you specified... and the parts failed. This MAY turn-out to actually be Your problem because of the way You spec'ed-out the fabrication process [refer to the "rule" I stated in my intro, way-above].
Lots of things to consider in this situation…=================
"...and scrap the rest...."
Next time you see some "aircraft quality bolts" for sale on Ebay.... think twice about how much money you might actually be wasting your life over.