Recommended PWHT temperature (clad pipes)

Date 12-15-2016 13:45

The stress relief temperature (subcritical) typically used for the X70 would be high enough to sensitize the austenitic stainless. I assume the austenitic stainless is being used due to corrosion issues. Stress corrosion cracking due to the sensitization could become a concern if halide ions are present in the fluid. Other electrolytes can be an issue as well.

Cladding type 316 directly to the X70 could also be a problem. Usually it is prudent to deposit a couple of layers of type 309 first and then the type 316 for metallurgical reason. Still, thermal stress relief can cause the 316 to be sensitized.

Al

By DaLi64 Date 12-16-2016 09:13

Thanks for your answer. The purpose of the pipeline is to transport hydrocarbons, so the clad is there to prevent corrosion of the base (X70) steel. I understand that sensitization may occur in the the temperature range 450-850oC. So a PWHT temperature should be lower than that?

By 803056

Date 12-16-2016 14:26

Typical stress relief temperatures are 600 degrees C or slightly above. That puts you right in the danger zone.

By DaLi64 Date 12-16-2016 14:57

Thanks again, Al.

By waqasmalik (**)

Date 12-19-2016 17:34

Al
The question asks about low carbon grade of 316. Does the problem of sensitization induced by stress relieving will be there? I read somewhere in TWI website that at 0.03% C, time required for carbide precipitation is 8 hrs compared to regular grades having 0.08% C.

By 803056

Date 12-20-2016 01:12 Edited 12-20-2016 01:24

True, but the use of L grades of austenitic stainless helps mitigate the formation of M23C6, it does not eliminate it entirely.

Another potential issue is any ferrite present (iron from the carbon steel can contribute ferrite) can form sigma phase in short order when heated to an elevated temperatures such as that experienced during the thermal stress relief operation.

One may have to live without the thermal stress relief and utilize a transition layer of 309 or 310 between the carbon steel and the 316L cladding.

Al

By Eutectic (**)

Date 12-20-2016 10:33

DaLi64,

What is the purpose of the PWHT. To what code are you welding?
PWHT on x70 Low C steel pipe would be purly stress relief.
how do you plan to restore the cladding on the ID?

Ussually 625 clad and joints with 625 consumable with no PWHT.
or joint with 309L and no PWHT on the 316 clad but still you need to plan for restoration more than PWHT.

By DaLi64 Date 12-21-2016 08:58

The main purpose of the PWHT will be to remove as much hydrogen (subsea pipeline) as possible from the weld and the heat affected zone, but also for stress relief. In order to reduce the level of hydrogen in and adjacent to the weld, a large temperature should be used. However, a large temperature will adress critical issues like sensitization and a degraded clad-base interface. So it is not easy to chose such a temperature. A very complicated part of the weld is the so-called triple point where the weld material, base material and clad has a common interface. I must also add that I work on numerical simulations of welding, and to get input from knowledgeable people who have good experience from welding is valuable to me.