Hello Al,
thank you - once more - for your excellent reply!
Yes! This what you have described in the upper part of your response was - by now - my personal knowledge as well. Even that martensite can emerge, caused by the relatively well-known mechanisms. And that was the reason for that I was so surprised as I have read what the fellows have stated in their paper, even that - at least in my interpretation - hydrogen appears to be a cause for martensite formation as well!
And as you have so excellently stated by saying:
"The interface between the two can have a mushy zone that is martensitic and subject to hydrogen cracking should atomic hydrogen migrate to the martensitic area."
From the results of the paper I can confirm hereby, that even this is - amongst others - a supposed cause for the "disbonding" of the clad layer as a separation from the parent metal. At least as far the investigations treated within the paper have yield.
To be honest with you. I was glad to reading that neither the greatly appreciated fellows Jon (jon20013) nor Prof. Giovanni Crisi (thanks for replying Giovanni!) nor you my friend have ever heard of this. That naturally becalms me, but however it is nonetheless interesting what the gentlemen who have written the paper might have meant by writing so.
Thank you also Al for writing:
"It would be interesting to know the particulars of the case you read about."
If you don't mind, I would like to state hereinafter some of even the particulars you have mentioned. Since these are - at least from my humble point of view - extraordinary interesting.
Well, the paper I have mentioned has the title:
"Disbonding of austenitic stainless steel cladding following high temperature hydrogen service"
was written by M.F. GITTOS et al, as a year 2007 contribution to the IIW Commission IX (Behaviour of metals subjected to welding). The paper (IIW IX-H-653-07) deals with the investigations of a phenomenon which is particularly called "disbonding" what means a kind of removal of the deposited clad layer (different austenitic fillers were used) away from the substrate, as an effect from the component's shut down from service conditions. The components themselves which have been investigated are - just as you have correctly assumed already - high pressure vessels (hydrodesulphurisers, hydrocrackers, heat exchangers and vessels in coal conversion plant) made of 200(!) mm (~ 7,9 inch) thick 2.25Cr-1Mo steel grades. The inwards vessel surface was clad with different grades of austenitic filler materials
· 309L/347
· 309Nb
· 309LMo/308L
as well as a Ni-Base filler of a grade
· NiCr (65.6% Ni/17.9%Cr/3.58%Mn)
Different welding processes were used for the cladding operations as there were:
· Submerged Arc Welding (High- and Low Current Conditions)
· Manual Metal Arc Welding
· Electroslag Welding
· High Speed Submerged Arc-Strip Welding
· Submerged Arc Series-Arc Welding (not further explained but I assume it means SAW Multiple Wire Welding) and
· High Speed Submerged Arc Strip Welding
The service temperature was described as being up to ~ 450°C (~ 840°F) and the service hydrogen partial pressure is mentioned with up to 175(!) bar (~ 2,5 KSI). The vessels are being PWH-treated after manufacturing and it was found out that even this procedure has a considerable influence upon the later disbonding susceptibility (due to martensite formation, tempering etc.).
As you have already assumed Al, the major problem of disbonding is physically a kind of hydrogen embrittlement mechanism. This mechanism is agreed - in this particular application of components - as being based upon the diffusion of hydrogen through the clad-layer (and partially vessel wall) while the component is in service and a subsequent cracking "...on cool down to normal ambient temperature."
What finally has been found out by the extensively performed investigations, as recommendations to decrease the susceptibility for disbonding or increase the disbonding resistance respectively, was (concluded) as follows:
· NiCr Cladding
· Manual Metal Arc deposited cladding
· Finishing temperature for PWHT below 650°C (~ 1200°F)
· High ferrite in the first-layer cladding
· Increased cladding thickness
· Martensite in the first-layer cladding
· Use of vanadium-modified parent steel [according to (*)]
The authors state as well the following procedures to minimize disbonding susceptibility on already existing - in service - vessels as follows:
· Employing hydrogen release heat treatments before cooling
· Applying an additional low temperature PWHT
For the case of fabricating new vessels GITTOS and co-authors recommend the following measures as being most efficient:
· Avoiding low ferrite in the first-layer deposits
· Applying duplex (double) PWHT
· Use of V-modified steels
So far in a short overview the content of the very interesting paper. As you can see, the basis of the "disbonding" mechanism is even comparable with what you have already assumed (you are great Al!!) by saying:
"The interface between the two can have a mushy zone that is martensitic and subject to hydrogen cracking should atomic hydrogen migrate to the martensitic area."
Even this - beside my own humble interpretations of the results of the treated investigations - was the reason for that I was so surprised when I could read what the authors have stated what I have already posted in my original topic:
"A further possible mechanism for martensite formation could be hydrogen-induced transformation caused by the high hydrogen concentrations which develop at the interface (between parent metal and clad layer) during and subsequent to hydrogen charging. In this case, martensite formation would occur on cooling from hydrogen charging."
Since even this is something what I have never heard before. Hydrogen as possible "initiator" for martensite formation? Hmmm, strange...
Unfortunately even this particular statement was not further explained within the paper (at least as my English language knowledge has let me recognize this correctly), what was the reason for me to humble request help from my great fellows in the forums.
However, perhaps our outstandingly appreciated fellow Jeff (js55) may have mercy with me and can point me or us in the right direction by estimating what the authors of the paper might have meant by their statement. And I am absolutely sure, if Jeff has never heard about this, nobody may have ever heard about hydrogen as a martensite transformation initiator. :-)
Thank you once again and all the best to you and all the others,
Stephan
(*)
· SHIMOMURA, J. et al: "Improvement in resistance to disbonding of stainless steel-overlaid 2.25Cr-1Mo steels". ISIJ International,4, 1991 (pg. 379-386)
· BROUWER, R.C.: "Hydrogen distribution through the wall of pressure vessels made of conventional and V-modified steels" International Journal of Pressure Vessels and Piping, 56, 1993, (pg. 133-148)
· CAYWARD, M.S. et al: "Evaluation of hydrogen disbonding of stainless cladding for high temperature hydrogen service" Corrosion 94, Paper 518, NACE, 1994
· FUSARI, F. et al: Properties of strip surfaced overlays on the new generation of parent metals for the petrochemical industry" Welding in the World, 36, 1995, (pg. 173-180)