Special Considerations for Welding of Thick Duplex Stainless Steel

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1 Special Considerations for Welding of Thick Duplex The 5 th China International Duplex Congress Gary Coates Beijing, China, 26 Nov. 2015

2 Why should we be concerned about welding of thick duplex SS? Because when it goes wrong, there can be serious consequences! Duplex 2205 REAC Header Box Cracking Failure was due to sulfide stress cracking, which initiated at a high ferrite, high hardness weld root and followed the fusion line of a weld joining the header box top plate to the plug sheet. REAC Reactor Effluent Air Cooler Fire at ConocoPhillips Refinery Leaks at duplex welds caused vapour leaks and explosion / fire 2

3 REAC vessels scope of the problem There have been 8 publically reported serious incidents worldwide This resulted in a joint MTI (Materials Technology Institute) and API (American Petroleum Institute) activity to identify the root cause and prevent other failures. Many 2205 REACs were operating without any problems References: API Technical Report 938-C Use of Duplex s in the Oil Refining Industry, 3 rd EDITION, February 2015 MTI Project Welding Duplex Alloys Only a few of the REACs have failed, but where they have failed in service, they have failed with serious consequences 3

4 REAC vessels A REAC is used in hydroprocessingwhich involves hydrogen at high temperature and pressure. Sulphur and nitrogen compounds are converted by a catalyst in the first stage reactor to hydrogen sulphide and ammonia. As the effluent stream from the reactor cools down, the ammonia and hydrogen sulfide combine to form solid ammonia bisulfide. In the absence of liquid water, the ammonia bisulphidecondenses directly from the vapor phase to form a crystalline solid which can result in rapid plugging. To prevent this plugging, water is injected ahead of the REAC. This prevents fouling, however, a highly corrosive ammonium bisulphide solution is created. High rates of carbon steel failures led the industry to select duplex 2205 (S31803 or S32205) for REAC and associated piping as the most economical alloy choice meeting the corrosion requirements. There have been changes to the operating conditions of REACs which had resulted in the selection of 2205 duplex 4

5 REAC vessels history and investigation Higher ferrite and high hardness in the 2205 welds were noted in many of the failures reported. Sometimes only one weld pass had the high ferrite, but it was the weld exposed to the corrosive conditions. Sulphide Stress Cracking is the principle failure mechanism. Some REACs are made of Ni-Cr-Mo Alloy 825 (N08825), and have not reported any issues. 825 is not susceptible to the same issues as the 2205 duplex alloy. However 825 is more expensive and not as strong as Some welds were not according to specification 5

6 REAC vessels history and investigation Some of the failures were associated with thinner material welded to thicker material, including tube-to-tube sheet welds. Some of the failures were associated with difficult to produce welds because of limited access, which also meant difficulties to adequately inspect. Failures were mostly in the welds with a few in the HAZ 6

7 Welding theory 2205 welding when duplex weld metal solidifies in the molten weld pool, it solidifies as 100% ferritic, and transforms in the solid state to a mixture of austenite and ferrite, if the cooling rate is adequately slow (20-50 o C/s). Ideally the result would be a 50/50 mixture of austenite and ferrite. The filler metal is enriched in nickel to help ensure a faster transformation, and it is important not to lose nitrogen from the weld pool, as it helps to transform the ferrite to austenite at a faster rate. Many factors can cause welding to go wrong 7

8 REAC vessels specifications Specifications for 2205 welding often require max. 65% ferrite in the weldment (weld and HAZ), but this can be hard to guarantee in production welds. There is also variability in the ferrite content from one area to another. Often it is important to simulate in the weld qualifications the exact conditions of the weld to be performed, especially in unusual circumstances such as thick material to thin. The weld qualification tests should look at variations that might occur in welding parameters (e.g. min. & max. heat input) and their effect on properties. In many cases, there was a specification, but it was not adequate to prevent out-of-specification welds 8

9 REAC vessels specifications It should be noted that in addition to welding issues, failures were often associated with REACs that had: - more severe process conditions (pressure, T, level of sulphides) - higher stresses caused by misalignment or poor design Sometimes there were other contributing factors to failures 9

10 Special considerations when welding thicker ( 25mm) 2205 duplex SS Evaluate the consequences of failure of each weld, and design the testing and inspection accordingly. Ensure high quality base metal by using S32205 base metal instead of S31803, and qualify each piece of metal via ASTM A923 or similar. Select proper joint design and ensure joint preparation results in a uniform joint. Cleanliness prior to welding is very important. The welding engineer must understand the metallurgy of duplex SS and analyse thoroughly, designing the testing and inspection protocol 10

11 Special considerations when welding thicker ( 25mm) 2205 duplex SS Choice of Welding processes generally high heat input processes such as SAW, FCAW, GMAW and larger diameter SMAW electrodes are preferred to lower heat input processes such as GTAW or laser. SMAW electrodes tend to have higher lower average ferrite than processes using wire. Choice of purge and torch gas which will affect the resulting ferrite content. Pre-heat and Interpass temperature. Choice of filler metal nitrogen in the higher end of the range. There are some cases where you may wish to select another filler metal than The welding engineer must understand the metallurgy of duplex SS and analyse thoroughly, designing the testing and inspection protocol 11

12 Special considerations when welding thicker ( 25mm) 2205 duplex SS Weld procedure qualification -is it directly applicable to the weld being made? - have all the necessary tests been specified? -are you suitably testing each weld pass to ensure correct properties? - are all possible types of repair welds qualified? Welder qualification testing -is the welder properly qualified to weld duplex SS? -is the welder trained to understand the welding differences between austenitic and duplex SS? The welding engineer must understand the metallurgy of duplex SS and analyse thoroughly, designing the testing and inspection protocol 12

13 Special considerations when welding thicker ( 25mm) 2205 duplex SS Production weld testing and inspection -can you ensure that the welds were made according to the approved procedure? -if not, can you test them properly to ensure the properties are acceptable? The welding engineer must understand the metallurgy of duplex SS and analyse thoroughly, designing the testing and inspection protocol 13

14 Special considerations when welding thicker ( 25mm) 2205 duplex SS The main rules for welding thicker 2205 are not different than when welding standard thicknesses, but the consequences of bad welding may be more significant. If a fabricator / engineering / end user is not willing to follow or specify procedures / inspection / testing adequately, as there is an extra cost associated with this, perhaps duplex SS should not be used. Summary 14