[TowerTalk] Hydrogen Embrittlement

[kb0fhp@comcast.net]

These alloys are susceptable to hydrogen embrittlement.  But the black oxide process (hot) involves the immersion in a series of caustic salts at 295F.  Essentially, the caustic (NaOH) reacts with the iron surface creating an adherant iron oxide (typically Fe3O4-magnetite).  This reaction releases hydrogen at the surface of the steel. However, there may be inhibitors or other stuff in the hot bath that would not cause the formation of hydrogen - similar to the low hydrogen form of cadmium plating.  If you suspect hydrogen embrittlement, the best thing to do is to do a hydrogen embrittlement stress-relief for 24 hours at 325-375F.  

I tried looking up some information on stress-relief after black-oxide coating, but was not able to readily find it.  I would ask your black-oxide supplier technical people.  They love questions like that.....

Scott

-------------- Original message -------------- 

> Thank you, Scott, for that excellent and learned summary of hydrogen 
> embrittlement! 
> 
> In my work, I make many custom tools, typically from AISI D2 and A2 tool 
> steels, at hardnesses typically from Rc45 to Rc55. However, I usually don't 
> have them plated (The Gov't tends to shy away from the toxic entanglements 
> of CAD plating) and few have weldments. At these hardnesses, steels tend to 
> be more brittle even without secondary effects. 
> I do frequently use black oxide coating (hot process). Will black oxiding 
> encourage the hydrogen embrittlement effect? 
> 
> Thanks, 
> 
> --...MARK_N1LO...-- 
> 
> 
> Scott, kb0fhp, writes: 
> 
> <<..For a Steel to be susceptible to Hydrogen Embrittlement, there are 
> several 
> things that must occur at the same time: First you have to have a 
> susceptible material; second, you have to have hydrogen present dissolved in 
> the metal; and lastly, you need a low strain rate. Static tensile loads are 
> appropriate. 
> 
> In general, the all steels are susceptible. Cr-Mo steels, i.e., 4140 and 
> Cr-Ni-Mo (4340) are some of the most susceptible - if they are heat treated 
> to a tensile strength level above 200KSI. This means, in general, heat 
> treated to above Rockwell C 43. Practically all steels are susceptible 
> above HRC 43. The tower materials used are typically cold rolled, annealed 
> and pickled, or hot rolled, then galvanized AISI 1018 steel. The hardness 
> of this material is on the order of HRC 25 - not susceptible. The Cr-Mo 
> masts most people use today are also cold rolled and annealed AISI 4140 or 
> 4340 at approximately 135 ksi (HRC 28) - again not susceptible....SNIP...>> 
> 
> 
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> 
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