Grade 91: Mehanicals vs Welding Process- general feedback

Date 03-30-2016 10:56

Good afternoon to whom it may concern,
Thought I would just give some feedback. Might not be a surprise to some but may assist others.
I am currently involved with Grade 91 fabrication and we have had some interesting occurrences.
We have been blessed with a project with a client requirement of 41J. Because we all know that toughness you don’t need will improve your creep properties. And that is where the shyt hits the fan.
I have seen it many times now but it never seizes to amaze me how people can focus on the front end qualification and paperwork but neglects to follow through during actual fabrication.
This fact resulted in unfortunate (but academically fortunate) additional weld tests with incremental PWHT.
So the feedback:
For the consumable brands we used and the specific Heat input ranges
-GTAW: had excellent toughness even at a PWHT of 740°C for 60min, >150J WM and HAZ.
-SMAW: had consistent toughness above 30J for a 740°C for 60min. but soak temperature definitely played an important role here yielding 38J for 750°C for 60min and only after 90min at 750°C did we achieve the elusive 41J.
-SAW: The only thing I can say was that the impact values were not predictable at all. (thank you @JS55 I confirmed what you already knew and told me back in my post of 2014 of CVN vs HJP) We completed 2 coupons testing increasing temperatures and times and there were seemingly no rhyme or rhythm. Some coupons passed at 750°C for 90min and then the very next set of 760°C for 120min failed dismally. I am talking <20J.
I am still investigating this but I have some theories and would like some feedback from some of the more experienced contributors.
-First: is that you could probably write a degree on the setup of parameters for SAW welding of Grade 91 material.
-Second: PWHT is not the biggest contributor to improving impact properties for Grade 91.
-Third: I suspect that even the slight wire feed speed (WFS) variance as a result of constant Voltage changes your pool size and solidification structure to such an extent that properties will vary from one inch to the next
-Forth: I don’t think it is a good idea to weld with 4.0mm wire if your goal is impact properties.
-Fifth: When all is said and done fit for purpose and sound engineering should prevail. Fabricators and clients should understand why they request certain properties and the implication this has on fabrication and future life of the equipment.
Cheers and thanks for all the great posts and topics, I often kick myself for having so little time to spend on this forum.
CHeers

By js55

Date 03-30-2016 19:19

You have a really good grasp of what it takes to make Grade 91 work. You also have a really good grasp of what you need to do to improve properties. This, unfortunately is a rare thing.
SAW, there is no other way to put it, is a *****. Yes, it can vary a great deal depending upon welding parameters but is overall not going to be very good no matter what. Small wires, flat beads, rapid travel speeds, REHEATING OF PRIOR BEADS, are all critical for this stuff. I also stay away from agglomerated fluxes.
What temp did you test at?
Back in the olden days they didn't worry about CVN's much with this stuff since it is always used in a high temp applications. Who gives a rats ass about CVN's at 1000deg? The thinking has changed dramatically.
PWHT can be tricky with this stuff because while you are relieving stress you may also be getting precipitations that could cause it to go the other way. I have not confirmed this macroscopically but I have had long cooks that actually INCREASED strength over shorter ones with the same EXACT materials. I have no other explanation.
It is absolutely critical to get a 100% martensitic microstructure. You can do macros, but this is impractical in production for obvious reasons.

By Eutectic (**)

Date 03-31-2016 04:52

Thanks for the reply, I believe I have learned a little bit thanks to your previous nudge,
Test temperature was room temperature, about 23degC. Failed to mention that other mechanicals was great. I did notice one phenomena that also tickled me a bit. When performing only Base material heat treatment UTS and Hardness in increments of 3hours at 770Deg C up to 18 hours the was secondary hardening/strengthening but al values was >630MPa.

However the base material hardness 25mm away from the weld joint of the welded specimens exhibited a definite reduction in hardness for similar heat treatments.

This leads me to believe that the influence of the cumulative welding passes on base material properties, beyond the conventional HAZ region, was quite significant.

The aim is to do some TEM, SEM work on the mechanical specimens in due time to see if anything can be gained to enable reverse engineering to parameter setup. Once done I will send you th results, May hap try and publish a light hearted article...
Cheers