I don't believe forgings have the same level of susceptibility to hydrogen induced cracking as do the rolled skelp products. Maybe there is the presence of lap seams in the forging that might behave similarly to HIC-sensitive sites.
I've seen a sphere [hammer-formed] and an 11-ft elipsoidal head [spun and forged] both ruined by HIC. 'True' hot-forging like valve bodies are not as HIC succesible because the small internal porosity cavities, inclusions, and other open defects that will trap hydrogen have been forge-welded shut. With no place for hydrogen to accumulate, there doesn't seem to be any problem with HIC. A good x-ray, prefferibly CR, will tell you if there are any open defects in your valves.
It's not so much about the defects, it's about the inclusion shape morphology and the microstructure. In fact, trapping hydrogen at voids is beneficial. It's the hydrogen at inclusions and a surrounding microstructure with low fracture toughness that is the key. That's why plate materials tend to have the higher susceptibility: the inclusions are elongated and sharp, and they tend to be located in a banded microstructure of low temperature transformation products with low fracture toughness.
Having said all that, the only way to finally prove the forging is to test it. I have seen valve and flange forgings fail, but there has been no post mortem to see whether the steelmaking was at fault.
I am not sure that my understanding of HIC is in line with that. I can certainly agree with the idea that a metal's microstructure that exhibits a predisposition towards accumulating needle-like inclusions at grain boundaries (such as manganese sulfide) will be prone to cracking, but in my mind that is more an SSCC issue, not as much a HIC issue. My understanding is that HIC has more to do with dissociation of hydrogen-bearing compounds (such as H2S in water) that liberates atomic hydrogen which is small enough to permeate into the steel, whereupon if conditions and chemistry are right, molecular hydrogen can form at voids to promote a step-wise blistering across the metal thickness.
I guess what you are saying is that while the mechanisms are different, the conditions (chemically and micro structurally) giving rise to them are similar. OK, upon typing this, that makes sense to me now.
Can you elaborate a bit on how the entrapment of molecular hydrogen at voids is beneficial? Is it sort of like the hydrogen sponges out the other trace elements that otherwise might precipitate at grain boundaries?
Metallurgy is not my thing...kind of a black art to me.
OK, it's just a case of thinking about fracture. Another name for HIC used to be hydrogen induced pressure cracking. The pressure build up, as you rightly point out, providing the crack driving force. However, for there to be a crack progression, the fracture toughness of the material has to be exceeded. The 'sharpness' of elongated inclusions helps to create a stress intensification and the low temperature transformation microstructure has a low resistance to fracture. If the hydrogen becomes trapped at other sites away from these susceptible areas, it is not available to build up pressure. For SSC, the stress is externally applied and cracking tends to proceed through thickness rather than in planes parallel to the surface. We could start to consider SOHIC - stress oriented HIC - but then we start to get off topic.
