Alternate PWHT to Produce Tempered Martensite? 1

[tc7]

We are repairing a 4-inch pipe manifold by replacing one weldneck flange. It is unusual in that the flange is 4130 (normalized) and the pipe is A106 (mild steel). We would normally PWHT at ~1250 deg F for an hour (I don't have my WPS on this handy but I think that is close). This manifold won't fit in our ovens, so we need an alternate method. I came across some old correspondence that allowed a PWHT method which called for an increase preheat & interpass of 900 deg F then the PWHT consisted of holding the interpass temp of 900 deg for two hours followed by wrapping and slow cooling. This was claimed to produce a tempered martensite structure in the 4130.

Does this method have any merit?
Thankyou

 

[metengr]

tc7;
First off, the flange would most likely not be tempered martensite. A normalized 4130 depending on thickness would most likely contain a combined structure of ferrite, martensite and bainite and carbides. In other words, it is air cooled and not liquid quneched.

So, I really don't see a problem with local PWHT (using resistance heat) applied to the weld region provided you remain below the original tempering temperature that was applied to the 4130 steel.

 

[metengr]

tc7;
One additional comment, if you don't have specific heat treatment information regarding the 4130 steel, other than it is specified in a normalized condition with no tempering after normalizing, I would use a PWHT range of 1100-1200 deg F, not the alternative method you described.

 

[tc7]

Thanks for the advice metengr; I was not at all comfortable with the 900 deg PWHT method. I am looking into special contoured pads for a resistance heat system and do the local PWHT that way, as you suggest.
Regards.

 

[stanweld]

tc7,
Was 4130 specified for the flanges? Prior to about 1980, it was not uncommon to find small bore flanges machine fabricated from 4130 and 1045 bar stock, espcially those which would be mated to X52 pipe in the natural gas industry. The specification for those flanges did not have chemistry restrictions. We normally found these flanges during welding due to their propensity for cold cracking. We replaced them with appropriate material whenever they were discovered.

If carbon steel flanges would be acceptable, why not replace the 4130 flanges with SA-105 flanges?

 

[tc7]

Stan-
we were all set up to do this work on 2nd shift thinking the flanges were typical forged A105 flanges and only thought to check the chemistry with a spark test when our fitter just happened to make mention to me that while prepping the end, it seemed harder than normal. Then sure enough we found the specs did call for these to be 4130 and the project engineer won't change it.

What do you think caused your cold cracking? We'll be using SMAW with 8018-B2 rod and PWHT as mentioned above - any concerns?

 

[stanweld]

tc7,
In our cases (gas utility operations) we thought that we were getting low carbon steel equivalent to X-52 and welded it as such using E6010 or E7010 to make the welds without preheat. Cold cracking was pretty much inevitable.

You should have no real concerns with proper preheat and PWHT. I would normally use E7018 to make this weld though, since it is much less susceptible to cold cracking than the "B2" filler and provides some "forgiveness" should the welder not correctly apply the preheat and maintain the minimum interpass temperature.