Repair welding of vessel overlay

[James01]

I have a carbon steel 75.0mm thick pressure vessel with a 3.0mm 904L overlay. Internal inspection has revealed a pit defect in the overlay with a diameter of 5.0mm and a maximum depth (taking account of measurement uncertainties) of 2.5mm.

The vessel was heat treated during construction.

I have a weld overlay procedure with a max interpass temp of 135degC and a preheat temperature of 50degC using a propane torch.

The question is: How likely is it that the carbon steel shell at the weld location could experience temperatures close to the lower critical temperature?

 

[metengr]

First off, the preheat temperature should be increased to at least 150 deg C, not 50 deg C for this type of repair. The interpass should be increased to 250 deg C maximum.

Per your stated conditions, most likely you will have a zone of heat affected material in the carbon steel base material at the location of weld repair. By my calculations, you only have about .5mm (0.020") of remaining weld deposit, which is not enough material to avoid contact with the carbon steel substrate.

What are your service conditions that would allow you to use a temper bead type weld repair with elevated preheat? Do you have any specific hardness requirements for the carbon steel base material because of corrosion concerns related to process conditions?

 

[stanweld]

ASME Section VIII, Div.1, Note 5 of Table UCS-56 for P-1 steels does not mandate PWHT after application of the corrosion resistant weld overlay provided a 200F minimum preheat is applied for the first layer, when the base metal thickness exceeds 1 1/4 inches.

If you are concerned with meeting NACE MR0178 hardess requirements in the P-1 HAZ, I would advise the use of a welded mockup and measure hardness in the HAZ of the carbon steel base plate. In selecting base plate for the mockup, try to obtain it with a carbon equivalent as near as possible to, or greater than that of the vessel plate.

 

[James01]

The vessel is NACE MR0175 compliant, there are no other specific hardness requirements. The vessel design code is British standard BS 5500 Cat 1.

One of our Metallurgists is happy with the weld procedure described above due to the small area to affected and that the area will be filled with two parallel runs.

What we have noticed is that the heat sink is such that after the 50degC preheat is applied the surface cools in seconds.

We have taken a wall thickness measurement from the outside and the reading is 81.0mm, how much of that is carbon and how much is overlay is not determinable, however we are sure that there is more than 0.5mm of the linning remaining.

 

[SJones]

Firstly, why worry about ISO 15156 (MR0175) compliance on a clad vessel? There will be no corrosion process to affect the carbon steel. Secondly, the welding requirements for 904L need to be followed and I can't see a preheat higher than 50 deg C and a high interpass being beneficial for corrosion resistance. I'd support your metallurgist.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[metengr]

I totally disagree with your low preheat. First, you have the potential for hydrogen or delayed cracking in the carbon steel HAZ. Second, by increasing preheat you slow down the rate of cooling, thus reducing your chances of forming an adverse microstructure in the carbon steel base metal HAZ. Sorry, but I would not endorse this lower preheat welding procedure especially if you have only a 0.5mm weld layer. If you were welding on 904L base metal, different condition.

 

[SJones]

Ok then Met, ask james what the CE of the carbon steel is? If his company has gone the ISO 15156 route (unnecessarily so) they may have gone the whole hog and asked for HIC resistance. I wouldn't expect a CE much above 0.39 in that case. Now, if this were a high restraint butt weld, I may have a similar wariness, however, this is overlay and (I hope) it is a minimum 2 layer weld build up with a reasonable run length that should give a beneficial thermal cycle regime. Of course, we may all be discussing this without the input of the authority that will recertify the vessel. Nevertheless, why jeopardise the very material put onto the carbon steel to protect it?

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[SJones]

We should also inquire as to whether the vessel is actually made from clad steel or if it is overlaid and whether the CRA layer has gone through the heat treatment as that could well have degraded its corrosion resistance anyway.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[James01]

Steve

The base metal is Superelso 500 and the CE is between 0.4 & 0.45 and the 3mm clad 904l. The cladding was subjected to the PWHT. The degradation of the cladding by the PWHT process is minor in terms of the corrosion resistance requirements.

I am the certifying authority and as such am not too concerned about the possibility of SCC or HIC as once the cladding has been restored there is no possibility of chlorides or Hydrogen reaching the base metal. What does concern me slightly is the long term effect of a local hard spot. I have asked for a mock up weld to and get macros & hardness.

The defect that is being repaired is an original fabrication defect that was picked up during the vessels 1st routine inspection.

Jim
Principle Mechanical Eng

 

[stanweld]

I would not worry about hydrogen embrittlement of the C.S. HAZ. The 904L deposited weld metal will absorb all the hydrogen since it has such a high affinity for it. Also, since the repair is so small, I would assume that it would be made with the GTAW process.

 

[metengr]

Agree with stanweld if GTAW is used. However, with a CE of 0.4-0.45, I would still recommend a local preheat of at least 100 deg C, 50 deg C for this shell thickness and local repair is too low. Of course, my recommendation assumes that you do not have enough remnant cladding to avoid partial transformation of the carbon steel base material in the repair area.

 

[SJones]

Curiouser and curiouser! A high strength steel more akin to offshore structures being used for a pressure vessel (notionally in sour service) clad (or weld overlaid) with 904L that is then subjected to a heat treatment cycle that has the possibility to degrade the corrosion performance. By the way, it's the effect of the chlorides and H2S on the 904L that will be your first problem. Should that lead to a breach then, one assumes, you will have big trouble (why else the need for cladding if carbon steel corrosion rates aren't excessive)from general corrosion, or HIC, or both. The mock up route is the way to go to qualify the welding procedure but beware the status of the rest of the 904L! Good luck.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/