Grade 11 hardness after welding/PWHT

[CWEng]

We have a new ASME Section VIII job on 1.25CR-.5Mo material where the hardness requirement (weld/HAZ/base metal) is 225 HV10, which is not out of the ordinary for Grade 11 material (at least for B31 pipe with a higher PWHT temperature). However, it is a lower hardness requirement than previous customers where we met their hardness requirements (250 HV10 max) with a 1 hour PWHT at 1275F and 150F preheat on 3/8" thick material. We did meet 225 HV10 with 3 hours at 1275F PWHT on the same material. But our new job requires base/weld metal be qualified for the PWHT cycle needed to build the equipment, the possibility of a repair cycle, plus 3 PWHT cycles left over for the owner. So if we have to qualify 5 PWHT cycles at 3 hours, 15 hours seems to be a bit excessive (for this job with 1.38" wall pipe/fittings).

So I'm looking at options to get hardness lower.
On this job, the customer also requires 300F preheat. Will that be as effective as the additional 2 hours PWHT required on the first job to get hardness down to 225 HV10?

If we do have to do 15 hours PWHT, will that be long enough to start reducing the impact tests values (40 ft-lb average at 0F required per API 934C) or tensile strength of the weld below 75 ksi required for SA-387 Grade 2?

Note we are using GTAW and GMAW with either ER70S-B2L or ER80S-B2

 

[metengr]

First, why are you using 3 hours per PWHT cycle, at 1.38" wall thickness assuming full penetration weld deposits you are looking at 1.5 hours maximum per PWHT cycle, which translates to 7.5 hours x 80% of aggregate time per Section IX = 6 hours total time.

To answer your questions:
1. Elevated preheat will slow cooling and help to reduce hardness of the weld region.
2.I would be concerned with impact toughness degradation versus reduced tensile strength. I would expect your incoming material tensile properties are above the minimum specified and there would be sufficient margin to accommodate reduction based on additional PWHT cycle time.

 

[CWEng]

Hi Metengr,
We originally qualified for Job 1 (3/8" wall) at 250V max with 1 hour PWHT, but customer 1 required qualification of 1 additional cycle PWHT in case of repairs, and one cycle left for the owner. So we also qualified with 3 hour PWHT. With the 3 hour PWHT we also met the 225 HV10 max.

So now we have Job 2 (1.38 wall), and have to requalify on thicker material. So I'm trying to cut down on the potential iterations of experimenting with preheat vs. number of hours for PWHT, since we don't have a lot of time to qualify job 2. You know how customers are. So 3 hours is kind of a baseline PWHT time where we have met the hardness before, but I'd like to cut it down if I can without running the risk of failing by being too hard (which higher preheat should help with, but I'm not sure how much). This would also cut down on the risk of PWHT too long and impairing the toughness. Just wondering if there is any information or experience out there for preheat vs hardness, or PWHT time vs impacts/hardness to see which way might be most expedient. You make a good point about 80% PWHT time in ASME IX, but I think our customer requirements don't follow that rule.

 

[metengr]

Preheat will certainly aid to slow cooling and reduce harmful transformation products that can lead to higher hardness and cracking. For toughness I would suggest you use a temper bead method for welding with stringer beads and a 50% bead overlap and increase the heat input after the first pass. You don't need to specifically follow the temper bead method only parts of it to improve impact toughness by grain refinement in the BM HAZ.

This is not a precise science and that is why you qualify a welding procedure, just for this type of scenario, and evaluate what happens to the weld region.

I failed to mention EPRI may be of help with the free document below regarding handbook of Grade 11 steels

https://www.epri.com/#/pages/product/000000000001013358/

 

Too late for this work but you can mitigate hardness by specifying a maximum carbon content in your base metals. Carbon is the dominant element in the carbon equivalent formula, a measure of base metal hardenability when welding (i.e., heat-affected zone).

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[CWEng]

I did have a copy of the EPRI handbook, and they always have useful information, but not quite what I was looking for. We might play around with the temper bead on the cap since the highest hardness is always right there in the HAZ. And we did specify to have our vendor supply the lowest carbon heat available, but I don't know what that is yet. so thanks for the suggestions.

I found some time last week, and did some cooling rate calculations in accordance with PPD-6335720, and figured out that with the thicker plate, that the preheat and even increased heat input won't slow the cooling rate at 800C below what we welded the 3/8" plate with. So it looks like preheat alone won't get me where I want to go. I did notice that if I increase the PWHT temp from 1275 by just 10 degrees, the LMP for a 2 hour PWHT is greater than the LMP for a 3 hour PWHT at the lower temperature. So looks like I'll either have to PWHT a hair hotter, use temper bead caps, or just go with the longer PWHT time.

 

p.s., don't forget sulphur - lower S helps toughness whatever the alloy.
I presume from the description of your job that the so-called tramp elements are already restricted by spec.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[CWEng]

Yes, X-factor elements, S, Cu, Ni and V were all restricted.

 

Not exactly relevant, but I once investigated massive cracking in welded P11 piping, about 12"Ø x 5/8" wall IIRC. Weld hardness was north of 350 HV, a higher number than any I could locate on the available CCT diagram. It turned out they had welded the long seam in a single pass (maybe there was a small root pass) from one side using GTAW. No idea if this efficient manufacturer is still in business today...

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."