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Tomv
User Rank
Iron
In process control mandatory
Tomv   1/29/2014 9:52:41 AM
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Derating is not a good option for dealing with hydrigen embrittlement due to the unpredictability of potnetial failure.  The problem must be controlled/eliminted in process.  I know of circumstance in which a critical flight control component fialed due to hydrogen embrittlement under strictly vibratory loading - no compression or tensile load at all.  Reuslt was the death of a number of individuals.

bdcst
User Rank
Platinum
Re: Process cost
bdcst   8/10/2012 12:17:43 PM
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Well, hydrogen embrittlement is  a long known issue that has played havoc with metals at nuclear power plants and has lead to fuel rod cadding failures along with reduction of the rating of some containment vessels.

Dave Palmer
User Rank
Platinum
Re: Process cost
Dave Palmer   8/6/2012 11:50:08 AM
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@TJ McDermott: I would strongly recommend against derating hydrogen embrittled parts, if by derating you mean using the affected parts in a lower-stress application.  The problem is that it's extremely difficult to predict what level of stress a hydrogen-embrittled part will fail at.  Even residual stresses from the forming process may be enough to produce microcracks, which may propagate later in fatigue.  If I knew that a part was likely to be hydrogen embrittled, I wouldn't recommend using it in any application.

The same line of argument applies to baking parts as a re-work method after a problem has been found.  As I mentioned, it's important to bake parts as soon as possible after plating or welding.  Some aerospace specifications require that this be done within one hour.  The more time passes, the less effective baking will be.  This is not so much due to the hydrogen being any more difficult to remove (although you will hear this claim), so much as the fact that microcracks may have already formed as a result of residual stress.  Obviously, once cracks have formed, no amount of baking will heal them.

As naperlou pointed out, prevention of process-induced hydrogen embrittlement depends on having a good quality system in place.  If you're plating or welding high-strength parts, you need to ensure that they are always baked at the proper temperature within the specified length of time.

tekochip
User Rank
Platinum
Great Story
tekochip   8/6/2012 10:44:58 AM
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Great story, that's an obscure and dangerous failure mode.


TJ McDermott
User Rank
Blogger
Re: Process cost
TJ McDermott   8/6/2012 10:11:28 AM
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Alternatively, it might mean derating the usable strength of the parts in order to eliminate the process cost.

naperlou
User Rank
Blogger
Process cost
naperlou   8/6/2012 9:09:18 AM
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This is an interesting example of a failure mode that is not going to be easy to predict.  It seems that, short of testing a sample of the parts after each process, one can only track the parts after production and remedy situations as they occur.  This requires a detailed tracking of the products after delivery and detailed reports of problems.  In general, quality control systems do this.  By linking those databases with design data in a PLM system, another big theme of Design News lately, one can avoid the problem in the future. 



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