3/8" THK 4130 BM Help

[MrGezus]

Good morning! I have done several 4130 procedures in the past, however I am having trouble with the mechanicals and testing on a thinner specimen.

Codes:
API 6A
DNV OS E101
ASME IX
NACE MR0175

I have tried using SMAW/FCAW/GMAW-spray.
Preheat: I have tried 400 & 600
SMAW: 10018 D2 (1/8")
GTAW Root: ER100S-G
I've had welders try welding wall->wall->middle fill all the way out.
*EDIT* PWHT 1190F @ 8 Hours and 1175F at 4 hours.
The biggest problem I am having is with the NACE hardness requirement of 250 Vickers in the HAZ. I've used propane torches for preheat and resistance ceramic blankets both which were wrapped in insulating material during and after welding to control heat loss. As far as I know the cooling rate has a big effect on the HAZ hardness.

Both tests I have tried were using a roll out on a piece of pipe. I could use some advice so I don't just keep trying to make something happen that may or may not work. It could just be something that I am missing.

 

[DVWE]

No PWHT?

 

[MrGezus]

Sorry, yes. PWHT I have tried 1190F and 1175F for 8 and 4 hours respectively.

 

[DVWE]

What is the C content of the base metal you are having issues with? Compared with your previous successful tests?
What hardness values are you getting with the PWHTs you have tried?
How about tensile values at those PWHTs?
What is the welding heat input? On thinner/smaller diameter pipes, welders tend to want to go faster and turn down the amperage, which would directly affect your hardness in the HAZ.
What are your cooling rates after welding?

 

[MrGezus]

See below in RED.

What is the C content of the base metal you are having issues with? 0.69

Compared with your previous successful tests? 0.673 / 0.643

What hardness values are you getting with the PWHTs you have tried?

How about tensile values at those PWHTs? Tensile, I have not had a problem with really. However, lately I have tried it a few times and just tested the hardness before proceeding to spend money on the rest of the testing.

What is the welding heat input? On thinner/smaller diameter pipes, welders tend to want to go faster and turn down the amperage, which would directly affect
your hardness in the HAZ. I just checked it out and most passes (all of 6) were around the 24-28k range.

What are your cooling rates after welding? This I have never actually measured. We wrap it in insulating wool after retaining a preheat of around 500 for an hour after weld completion. It is always still warm in the morning.

I have come across customers who will allow us to do a Rockwell survey if the Vickers does not meet the 250 mark. However NACE MR0175-2 states "The HRC method may be used for welding procedure qualification if the design stress does not exceed two thirds
of SMYS and the welding procedure specification includes post-weld heat treatment. The use of the
HRC method for welding procedure qualification in all other cases shall require the agreement of the
equipment user." Is the two thirds SMYS a hard requirement? I have had other companies say it's fine.

 

[DVWE]

Is the two thirds SMYS a hard requirement?

I'm assuming yes, but that's up to your client and/or the end user.

Everything looks pretty good to me other than the welding heat input, that does look a little low. I have run similar qualification tests and my KJ/in was in the range of 50-60, while stringently keeping the preheat at or above 400F. Do you have to perform impact testing also? The reason I ask is that you will begin to walk a fine line if you have to meet impact requirements and begin increasing your heat input.

The reason I asked about the tensile test is because the only other way to decrease your hardness will be to increase your PWHT temperature.

 

[DVWE]

I just caught that your C is 0.69? Typical range is 0.28 - 0.33

 

[CoryPad]

I believe those are Carbon Equivalent (CE) numbers, not carbon concentration [C].

 

[MrGezus]

Yes, sorry. I read it as "what is your CE". I do have to do charpy impacts but only at -50 which they are plenty high enough to warrant a heat input increase. Would you recommend slowing down on the travel speed or increasing the settings on the machine?

 

[DVWE]

At ~25 KJ/in, and six passes on 3/8" thick pipe, your welders are probably putting in small stringers. Simply have them put in larger stringers with a slight weave; this will slow them down a little. Your KJ/in target should be around 50.

What impact values are you required to achieve at -50F?

You can also try a PWHT temperature increase to around 1250F and cut your time at least in half. If you have any left over coupons you should try that.

 

[MrGezus]

20/15 on the CvN. I cannot go above 1200F because of customer requirements and NAVE MR0175 unfortunately. Most of our customers limit this. Thank you for the info. I'll have them slow down.

 

[racookpe1978]

Why did you choose 8 hour PWHT? Is that the whole cycle (heatup, maintain, cool down?

It appears (scaling) that the total wall thickness is <18 mm. true?
The 3/8 thickness is wall thickness, right? Heat pads are the outside, and the center is insulated.

Are you actually annealing the part, not performing the PWHT only as you expect?

Which ASME Sect IX page are you working from?

 

[MrGezus]

I chose the 8hr pwht cycle (8 hours at temp) because I was hoping just to add this 3/8" wal;l with CVN to an existing procedure. Other than that there was no particular reasoning. This particular one was written long ago before I was here and this is what QC would like to do.

 

[racookpe1978]

PWHT should be as short as possible, and never too long, and at the right temperature to reduce the STRESSES induced right across the weld.

Too hot for a short time, or "almost too hot" for too long a time, and you re-anneal the metal; or, worse, you completely screw up the crystal structure across the whole heated zone.

Typically, a hard finish is characterized by a quick cooling to "freeze" the carbon and other alloys in the crystal borders between each grain. (It's more complex than that, but I hope you get my point.) Go too long at a hot temperature, and the crystals grow and the inclusions disperse as the move through the grains.

Don't get hot enough, and the stresses induced by the welding remain. But 3/8 thick is very thin, and at that thickness, I've never needed more than an hour at temperature for the alloys my company welds.