Could We Apply PWHT After Getting in Service

I would suggest you cut a sample of the failed tube and take it to a metalugist to see if the grain structure in the cracked areas is significantly different from the parent material. The problem might not be PWHT only the material itself might not be up to the service. If there is a big difference and PWHT will bring it to an acceptable condition then replace all the failed sections with new ones. Otherwise if money is the problem gauge out and re-weld and do PWHT it should be fine but this will give you more down time.

Is it necessary to apply PWHT for 3" Sch40 Carbon Steel welds? From which code requirements that needs PWHT? PWHT Not required per ASME codes for this size and material.

Do you have PQR and WPS for these welds? You may use mistake electrode or pipe material could be alloy. Normally cracks not occur on HAZ for this material.

It looks miss material-electrode application. Make sure application per PQR&WPS and then make material analysis form weld, pipe, HAZ. After that you have to re-weld all weld joints.

Regards,

Eyup Tan

First PWHT can not be used for 3" sch 40 pipe. It can only be done for 20" pipe and above. For 3" sch 40 pipe, NDT is used. Make sure that the test meet ISO standard before use.

"Godspower"

Where did you get this idea? "First PWHT can not be used for 3" sch 40 pipe. It can only be done for 20" pipe and above. For 3" sch 40 pipe, NDT is used. Make sure that the test meet ISO standard before use."

1. "First PWHT can not be used for 3" sch 40 pipe." Wrong!! PWHT (Post Weld Heat Treating) can and is used for all sizes of pipe. The pipe size and Schedule is not the prime reason for PWHT.

2. "It can only be done for 20" pipe and above." Wrong!! Same answer.

3. "For 3" sch 40 pipe, NDT is used." It is "NDE" (Non-Destructive Examination) not NDT and the examination of any size pipe does not accomplish the required "Stress relieving" or "Normalizing" of steel pipe that proper PWHT accomplishes. It sounds like you may not know what NDE and PWHT really is, True?

4. "Make sure that the test meet ISO standard before use." Their project may or may not be an "ISO Standard" Project.

You mentioned about "suffering leaks due to repeated crack in the HAZ area" What is joint type on this piping system? Groove or socket welds? The possibility of cracking can be due to 1. stress corrosion cracking due to acid / alkaline attack 2. fatigue of joint due to residual stresses Before you can decide what to do, the following can be considered: 1. What is the welding process and heat input of welding and hardness of the welds (typically)? Is the cracking ready caused by stress corrosion cracking? Metallurgical examination on weld section and pipe material can be considered? 2. What is the WPS used? Multi beads (welds) is being used, consumable? What hardness, Was PWHT part of the parameters? 3. In addition to assessment as per item 1 above, there is possible - to study the corrosion rate that happened in the service duration? - Was the pipe material undergoing normalization e.g. meeting NACE requirement? - Would the whole piping system still be feasible to be used as it is after putting in service? - If material is suitable for the service fluid, what was the hardness of weld made? this leads to consider PWHT the joints to stress relief regardless of sizes and thickness 4. Check at which location the crack joint is happening? - Subjective to cyclic service due to fatigue, e.g. pipe connecting to rotating equipment? - If so, what are the assembly / fit-up quality or workmanship standard? i.e. How the pipe being join to the equipment, have the assembly / fit-up alignment being monitored? forced to join with lots of misalignment including horizontal and vertical alignment? 2. Potential of additional stresses due to lacking of pipe supports, vibration, kicking and process upsetting? Sufficient rooms for expansion and contraction?

Gents, Already PWHT had been applied during the line construction as a service requirements. Design temperature of this line is 100 deg. C and this requires applying PWHT for carbon steel materials according to the "caustic Soda Service Graph". But these cracks may give signs for inappropriate application of PWHT. Another point is that the line has a general corrosion (thinning) in some areas. I think that I have to make an assessment for the pipe thickness and the feasibility of the line. But the concern is in the repeated cracks in the HAZ, are there some inspection activities required to be conducted for other welds and could I apply local heat treatment again for welds again. Is it applicable to apply PWHT after getting in service for months?

What is the PWHT soaking temperature and cooling rate? There is some recommendation of NACE requiring the soaking temperature to be around 630+-around 10 Deg C. Material is also undergone complying NACE requirements. Consumable used is recommended if for GTAW / GMAW should be something like ER-70S-2~4, SMAW is E60XX or E70XX, etc for FCAW & SAW, Hardness should be monitor if other types is used. For assessment of the concern of repeatative defective / cracks in the welds, you can use any NDT (Non-Destructive) testing method which will have no harm to weld e.g UT / PAUT, RT (preferable X-ray), MT (for external surface only if needed). This will give you the idea of how bad the condition is. As mentioned in my earlier comment, you may first need to assess to find find out its root causes i.e. Material? WPS / Consumable? PWHT & Harness? What extend is the attack of SCC? If it is widely and seriously attack by SCC and badly attack (crack) intergranular, then I believe it is better to re-fabricate the whole system. Otherwise, I believe re-PWHT to the correct temperature and controlled hardness is acceptable.

This looks like stress coorrosion cracks and when I googled it (not much familiar with caustic service so can not vouch for it ).

I got this

Corrosion by caustic (sodium or potassium hydroxide) at all concentrations is easily handled at room temperature with a variety of metals and alloys, including carbon steels. It becomes increasingly more corrosive with increasing temperature and concentration. The useful safe limit of carbon steel is approximately 150^oF/65^oC, both with regard to caustic stress corrosion cracking (CSCC) and corrosion. Stainless steels are more resistant to general corrosion compared with carbon steel; however, they can suffer CSCC at approximately 250^oF/121^oC.

As a general rule, the resistance to caustic solutions increases with increasing nickel content. Susceptibility to caustic SCC is dependent on several variables, including alloy content, caustic

If you continue you will get some useful info in this article.

Also there are some good articles (one from Dow and a few others if you search -

"carbon steel stress corrosion crack caustic service"

UD15