Hydrogen Embrittlement Information

[Kwan]

Greetings,

I have been involved a couple times with hydrogen embrittlement bake oven failures. The last failure was due to power failure. 180+ ksi steel, low HE plating, about 7 hrs at 375F, oven failure, temp dropped to 195F over about 7.5 hrs, then oven restart and bake for 25 hrs at 375.

The time requirement before bake is 8hrs min. Does the initial bake "hold" the parts in suspension as far as HE is concerned? Or is there a chance that HE occurred during the 7.5 hr cool down?

Does anyone have a source for more specific information concerning the HE process in metals?

Thanks in advance.

 

[TVP]

Kwan,

I am not sure exactly what you mean by "'hold' the parts in suspension". The information that I can provide is the following:

1. Hydrogen embrittlement would not occur during the 7.5 hour cool down. The hydrogen would have been introduced prior to baking. All baking does is remove or redistribute the hydrogen.

2. It is possible that there is still sufficient hydrogen remaining in the steel to cause delayed fracture. This is because the part was only baked for 7 hours.

3. SAE AMS 2759/9B is the aerospace standard for hydrogen embrittlement relief. 8 hours is only required for some types of plating-- Cr, Sn, Au, & Ag only require 3 hours of baking. If the embrittling potential of your plating operation is low, perhaps 7 hours was sufficient from a product standpoint, even if it does not meet the requirements of the SAE standard.

4. Perhaps you should perform some type of test for embrittlement/delayed fracture using a statistically relevant sampling of the parts. I am most familiar with automotive standards like SAE/USCAR-7 and GM3661P, which require testing the parts with a sustained (static) load for a period of time to evaluate the susceptibility to delayed fracture. An example would be to place a bending load on a flat part or shaft that is equivalent to say 70-80% of the ultimate tensile strength and see if the parts fracture within 100 hours.

5. There is a great deal of information available on delayed fracture and hydrogen embrittlement. Perform a keyword search using Google Scholar or Scirus. The best info is from ASTM meetings and steel producers like Kobe and Nippon Steel that have developed improved alloys for delayed fracture resistance.

 

[Kwan]

Thanks for you help TVP.

The required bake time is 23 hours. When I said "hold" I mean, does the initial bake prevent the hyd from causing embrittlement. Thus, since the plate to bake time requirement is 8 hours and the initial bake delayed the onset of embrittlement, perhaps I still meet the requirement?

Anyway, I shall look up these references you mentioned. The parts in question are landing gear parts. I will not disposition these because I am not sure of the effects of initial bake, cool down, then bake again. I need to do some serious homework for future issues such as this.

 

[swall]

To take TVP's idea of testing a little further, you could take some notched tensile bars (typically used to evaluate plating baths for hydrogen embrittlement) and run them through the plating bath used for the parts. Then run a couple through the standard bake and run some through a bake cycle that would simulate the interrupted bake the parts saw. Then perform a 200 hour sustained load test on the bars and see if there is any difference.There is an ASTM standard (F 519??) that describes the notched tensile bars.

 

[TVP]

I completely forgot about all of the ASTM standards on this subject. Thanks for the reminder swall. THe following links provide some information on the various ASTM standards related to hydrogen embrittlement:

ASTM F 519

http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/F519.htm?L+mystore+psvm1794

ASTM F 1459

http://www.astm.org/cgi-bin/SoftCar...PAGES/F1459.htm?L+mystore+psvm1794+1129744331

ASTM F 1624

http://www.astm.org/cgi-bin/SoftCar...PAGES/F1624.htm?L+mystore+psvm1794+1129744331

ASTM F 1940

http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/F1940.htm?L+mystore+psvm1794