[TowerTalk] Hydrogen Embrittlement
kb0fhp at comcast.net
Tue Nov 15 10:08:12 EST 2005
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.....
-------------- Original message --------------
> Thank you, Scott, for that excellent and learned summary of hydrogen
> 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?
> Scott, kb0fhp, writes:
> <<..For a Steel to be susceptible to Hydrogen Embrittlement, there are
> 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
> 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...>>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
> Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
> and ask for Sherman, W2FLA.
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