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Excessive hardness on 9 Chrome Pipe Welds

Excessive hardness on 9 Chrome Pipe Welds

Excessive hardness on 9 Chrome Pipe Welds

Have welded piping 9 chrome using electric preheat to contol and monitor interpass temperatures with approved and demonstated weld procedure. Many hardness readings after PWHT are in excess of Code allowable of 241 BHN. All the obvious variables have been checked, ie materials & welds PMI'd as well as calibration of all equipment.

Looking for someone that has seen this problem in the past, determined the cause, and and how it was resolved.

RE: Excessive hardness on 9 Chrome Pipe Welds

Suspect the problem is in the PWHT.  What is material thickness, PW heat temp, and hold time?  Have you had a replication polishing done across the weld?  Also, have you done a hardness traverse across the weld and if so, how far into the HAZ does the hardness prevail?

RE: Excessive hardness on 9 Chrome Pipe Welds

The material is 5" sched 40 (.258"), 1375 deg F +- 25, 2 hour hold.

RE: Excessive hardness on 9 Chrome Pipe Welds

If you want to anneal and moderate the hardness traverse, you will need to increase the temperature (1500 - 1700F depending on alloy) and increase the hold time (could be as much as 8-10 hrs).  Not sure of your application so be careful that what I'm telling you does not detrimentally affect your desired properties.

RE: Excessive hardness on 9 Chrome Pipe Welds

pwht holding time & temperature are to be increased; filler metal is to be choose carefully (e.g. excessive carbon content) content

RE: Excessive hardness on 9 Chrome Pipe Welds

The 1375 temp is correct.  We recently welded 9Cr pipe (8" sch 40) using a E505-16 electrode.  Post weld heat treatment was the same as you had described and we did not have any trouble with the hardness.  Was your cool down rate properly controled.  When was the electric heating pads removed.  We generally leave the weld wrapped until the weld reaches a temp of 100F, verified by the PWHT chart on your stress machine.

RE: Excessive hardness on 9 Chrome Pipe Welds

Check the actual chemistry of your filler metal (not just MTR's).  There are 2 possibilities here.  

1) There were some problems in the last few years with mismarked filler material coming from out of the country (Mexico, if I remember right).  

2) There are 2 types of 9Cr materials, P9 and P91 (or if dealing with tubing, T9 or T91), with the appropriate filler material for each.  The difference between them is P9 (or T9) contains NO vanadium while the P91 (or T91) does.  

We ran across the second case about 5 or 6 years ago and found that some filler material manufacturers made no distinction in the AWS designation for these very different materials.  Some manufacturers labeled the vanadium containing rod E505-Mod (or ER505-Mod for GTAW) while some just called it E505, the same as for P9 (non-vanadium) base material.  Even a little vanadium will greatly increase the hardness of the weld, even after proper PWHT.

RE: Excessive hardness on 9 Chrome Pipe Welds

Normally what happened is that the cooling rate are too fast. I have 10 years of experience in the heat treatment industry. You have the right holding time and soak temperature, what you have to do slow the rate speed in the cooling process. For example:

From 1375ºF to 1175ºF at 100ºF/hour
From 1175ºF to 800ºF at 200ºF/hour
From  800ºF to 400ºF at 300ºF/hour

If you need a procedure, i can send a copy.


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