T95 welding and post weld heat treatment 2

[acrmnsm]

We are considering having some joints welded on to 7 5/8ths T95 linepipe steel, does anyone have any experience of welding T95, and if we can PWHT to get it back to anything like the yield of the pipe? We do not need to be NACE compliant on this project.

Normally T95 is brought to its yield by heat treatment anyway right? (Ie Austenetiseng and quench, is it then tempered?)
(I have tried to find info to learn a bit about the metallurgy of T95 so if anyone could point to a resource I would appreciate it. I was trained as a metallurgist but don't have specific knowledge of linepipe steels. So any book/refs would be a great help.)

At some point we will get advice but I guess I want to know more about it so we can consider if it is at all possible.

 

[redpicker]

As far as I know, T95 is a casing and tubiung grade, not a line pipe grade. Check out API 5CT for applicable informaton. Different manufacturers use different chemistries, but it usually is a modified 4130 with increased Mn, Cr, and Mo, and tightly controlled P & S levels, perhaps with Ti and Cb added. It really wasn't designed for welding.

Typical uses do not include welding. Welding, if performed, would have to be conducted under very controlled conditions of preheat and post heat and even with such controls, it would be wise to MPI/UT inspect as well as a hardness test on the HAZ for each weld. With Cr around 1% and Mo around .7%, even with 0.33 carbon cracking is likely.

rp

 

[denlow60]

I think there is no T95 but T91

 

[redpicker]

T95 is a Casing and Tubing grade, an OCTG product (Oil Country Tubular Goods), developed as a high strength (95 KSI Yield Strength), H2S resistant grade. Casing and tubing are typically not welded, but assembled with threaded couplings. At 7-5/8" diameter, I'd assume casing, as that is pretty big for tubing.

Line Pipe is also an OCTG product, but has decidedly different specificaiton reqruiements than Casing and Tubing. Line Pipe steels are typically designed for weldability and are welded during installation (some types are welded during fabrication, too). 7-5/8 could also be a line pipe size, so the OP could have a situation where they are trying to use some casing as a substitute for line pipe. That is what I assumed in my original response. Welding T95 casing with procedures used for line pipe is a bad idea.

T91 is a piping grade (typically used in steam plants for steam piping). I believe it is also typically welded during installation. It has a higher alloy content, but lower carbon content, than T95, but because of the high alloy content, I also believe special welding procedures are also necessary, but I don't have much experience with T91, so I could be wrong about that.

rp

 

[metengr]

rp is correct. T (tube) or P (pipe) or plate and even cast 91 (12A) is a creep strength enhanced ferritic steel with 9% Cr- 1% Mo - .25%V (with controls on Al, N and Ti) that is weldable but requires care for preheat maintenance and post weld heat treatment.

 

[acrmnsm]

Apologies, I knew it was OCTG Casing. Was suffering brain fade on the linepipe thing.
Thanks for info.
We make specialist casing connections where some parts are welded, our client wants us to use T95 in this project so I am trying to learn a bit more about it, and if we can weld it. There is no room for a threaded connection.

 

[redpicker]

Well, in that case, if you are capable of welding P-110 and Q-125, the procedure for T-95 will be similar. As you have noted, you will loose the H2S resistance in the weld zones, and the preheat/postheat temperatures may need adjusting, but the procedures will be similar. If you aren't capable of welding P-110 or Q-125, then you may have some difficulty.

rp

 

[acrmnsm]

Now having read API 5CT (thanks Rp for that ref) I wionder if I should rephrase my question:

The steel is given its properties by austenitising, quench, and it is then tempered at 649C.
So post weld, if we apply the same heat treatment that was applied during manufacture, will we be able to bring it back to similar levels?
I have a feeling that welding effects will change the carbon content locally and other composition and so effect Ms and the volume fraction converted to martensite.
As I said we do not need NACE compliance, but the recovery of a decent proportion of yield and toughness would be advantageous.

I understand we will need to get more detailed "official" advice on this and will have to do some weld qualification etc, but as I said I am just looking to expand my own understanding and see if it is feasible.

 

[metgist]

I believe welding does much more than changing the carbon concentration locally. If T95 has a tempered martensite structure (like P91) which ensures better (high temperature?) properties, welding will certainly destroy that very microstructure (because it involves melting/solidification and results in a casting micrstructure). The primary aim for PWHT of P91 (and I guess for T95 too) is to get the tempered martensite back. And for P91, I think they carry out the PWHT at a temperature somewhat lower than the original tempering temperature (to avoid formation of fresh martensite). For T95 it could be 10-20 C lower than 649 C, but you need to check the appropriate standard to be sure.

 

[acrmnsm]

I believe welding does much more than changing the carbon concentration locally. If T95 has a tempered martensite structure (like P91) which ensures better (high temperature?) properties, welding will certainly destroy that very microstructure

Indeed, sorry if I did not make that clear, this is of course implicit in the problem, hence the need to heat treat to try and regain the martensite microstructure. Martensite formation during heat treatment depends upon (amongst other things) the composition. My question is will the welding have such an effect on the local composition around the weld that might result in a change in fraction of martensite upon austenitising/quench. (It is assured that it will, but hopefully there is answer out there as to how this has been dealt with in T95 before i.e. what welding rods to use, or perhaps a modified heat treatment, not to mention pre-heat etc). What I am trying to establish is if there is an existing weld procedure for T95. If there is not, then if anyone has the experience/confidence with similar materials to say that it is likely/possible, then we can employ the appropriate welding metallurgist to help develop a weld procedure..

 

[acrmnsm]

for example RP has pointed towards P110 and Q125, which we have welded P110 successfully before.

 

[redpicker]

You really need to consult with qualified personnel who can see exactly what you are doing and what you are trying to achieve. It appears you do not fully understand the situation.

The T95 has a high alloy content, so high that when it cools from welding temperatures, it is likely to crack. The cracking is due, in part, to the cooling rate of the material adjacent to the weld. Heating the pieces to a specified "preheat" temperature prior to welding will slow this cooling rate since the cooling rate of the weld zone is based on based on the temperature difference between the welding temperature and base material temperature. With a higher base material temperature, the cooling rate is slower.

This solves one problem, cracking in the heat affected zone (HAZ) during cooling from welding temperatures. The base material in the HAZ that was exposed to temperatures above the Ac3 will still transform, and likely with a large percentage of martensite. Regardless of the micorstructure in the HAZ, it is highly likely that it is harder and has lower toughness than the base material, and is under considerable residual stress. To address these issues, the weld zone needs to be heated to a temperature below the original tempering temperature. Note the 649C (1200F) temperature you mention is a minimum and the actual tempering temperature will usually be much higher. This secondary heating (post-weld heating, or postheat) will lower the hardness of the HAZ and relive some of the residual stresses created by welding, greatly lowereing the likelihood of cracks developing in the HAZ.

You mention the type of welding rods, which is also important. This depends on strength needed in the weld. Even though the base material may have a minimum yield strength of 95,000 PSI, it is unlikely you will have a weld deposit with this strength level. There are filler metals that will produce that strength, but at those strength levels, their toughness is generally too low. Again, you need some professional help to determine what materials and processing will be needed; there is just too much informaton needed to be able to do this over a message board.

rp

 

[acrmnsm]

redpicker thanks for the comments.
As I have mentioned I understand that I do not understand it, hence asking here and mentioning a few times that we would be seeking the help of a welding metallurgist.
I was already aware of the cracking issue and pre and post heat.

I'm not sure I have quite phrased my question correctly. Could we, post weld, re do the austenise ,quench, and temper that was done originally to get the T95 to its finished state?

 

[redpicker]

Could we, post weld, re do the austenise ,quench, and temper that was done originally to get the T95 to its finished state?

Could you? I am not sure why you couldn't. I am a bit unclear on why you would want to, however. The strength of the finished weldment isn't going to be much different than what you would get with following proper welding procedures. If you re-austenitize and quench, you have to deal with distortion and cracking and, unless you perform mechanical testing of the material after your re-heat treatment, you will be unsure of the actual mechanical properties, away from the weld. In generaly, the weld should be located in a low-stress area of the part, so as long as the weld zone is not excessively hard or brittle, you should be fine. Again, it depends on the design.

There are some applicatons where the weld zone is given a local quench and temper operation after welding, but this is generally done under highly controlled conditions to obtain very specific results.

rp

 

[acrmnsm]

Ha ha, I'm not sure either...
It is a V high value product and we wanted to be sure there were no issues.
Anyway thanks for all your pointers, lots of good advice, I'm off to see the welder.

 

[blacksmith37]

T-95 is not intended for welded integral joints. It normally goes through 2 Q & T cycles; the final temper being above 1350 F, (and grain size of 12 to 16 ). You would have to perform NACE TM 01-77 on the weldments. My guess your casing will no longer be T-95.

 

[acrmnsm]

We do not need to be NACE compliant on this project.