Welding of Corrosion Resistance Alloys
Introduction
Corrosion Resistant Alloys (CRA) are available with many different properties that have been designed to be suitable for various corrosive environments. There are several corrosion mechanisms that need to be considered for the correct material to be selected, these include:-
- Atmospheric corrosion
- Uniform corrosion
- Pitting & crevice corrosion
- Intergranular corrosion
- Chemical corrosion
- Microbial corrosion
- Stress corrosion cracking
There is also the combined effect of corrosion with other mechanisms such as corrosion fatigue and erosion corrosion that are equally important and should be considered.
As there are different alloy systems available that have specific properties for each application it is essential that all the correct environmental conditions are known so that the optimum material selection can be made. Many environments are very demanding and alloy selection and performance in service are of the utmost importance.
It is also usual that an alloy will be fabricated for service. Therefore it is equally important that all welds are completed in a competent manner in order that the full alloy properties are maintained as much as possible to give the optimum life in service. In the majority of cases a weldment will give a reduction in corrosion resistance compared to the parent material and if it is also incorrectly welded this could lead to premature failure and even catastrophic failure both in the weldment and HAZ (heat affected zone).
The metallurgy of most alloys is complex. A good level of understanding is required to ensure suitable weld parameters are established for an optimum weld quality for the intended service. In most cases either a design code or a customer specification will demand that a Weld Procedure Qualification (PQR, WPS) and a Welder Qualification (PQR) are established to show adequate mechanical performance. However, design codes do not necessarily establish the metallurgical integrity of a weld. As it is not possible to carry out any non-destructive examination (NDE) that will evaluate the metallurgical condition of the weld, it is prudent to require that a corrosion test be used as a tool to establish the integrity of the weld produced. X-Ray of weldments is routinely completed to ensure there are no inclusions or mechanical defects in the weldment.
Most safety instructions for welding of Metals and Alloys are similar and require consideration of handling and lifting, working with electrical equipment, necessity for good ventilation and extraction, working in enclosed spaces, risk of fire and explosion and require the correct Personal Protection Equipment (PPE) be worn.
Preparation for welding
It is important to ensure that all materials to be welded are correctly identified and this can be facilitated using either visual and identification checks or by the use of PMI techniques. It is also essential that all materials are clean and are in the pickled condition. The fabrication shop area should be clean and preferably be remote from any Carbon steel fabrication to prevent any contamination by airborne dust. Ideally the fabrication of all CRA material should be in a separate facility to any carbon steel fabrication. All tools should only be used on CRA materials and never transferred to stainless after being used on carbon steel.
Weld preparation should be wide enough to allow adequate access but not overly wide to increase distortion as both austenitic Stainless steels and Nickel alloys have a high coefficient of expansion and will distort excessively. There are a number of publications that are available that will give guidance on weld preparation and care should be taken that all codes and standards are complied with.
Components for welding should generally be in the solution annealed condition. Edge preparation should be carried out by machining, grinding, plasma or waterjet cutting but all oxide or any cutting fluid should be fully removed and thoroughly cleaned prior to the start of welding. Oxide can be cleaned by local grinding and all materials shall be degreased to remove any cutting fluids etc. All marker pen, thermal crayon, dye penetrants, shall all be thoroughly cleaned from the weld zone prior to welding. Cold solvent, vapour degreasing and alkaline degreasers may all be used but if using alkaline cleaners ensure that the piece is fully washed and dried before welding. Thorough cleaning of an area of at least 50mm either side of the weld fit up is usually sufficient.
Good fit up is essential as excessive mismatch between items can create crevices which will further reduce corrosion resistance of the completed weld. Good design of a component which provides adequate access to all joints that require welding, allowing optimal weld preparations to minimise the quantity of weld metal and hence minimise distortion.
Most of the alloys have a matching consumable available which will be compatible for the alloy to be welded. In certain instances, some alloys may be welded with an overmatching weld consumable that will give improved weld performance in certain environments. More information is given in the applicable section later. Certain alloys are also used for heat resisting applications and fillers that are meant for use with heat resisting alloys are not suitable for corrosion resisting alloys. Heat resisting alloys generally have a higher carbon content for additional strength at temperature which will reduce corrosion resistance as they will sensitise components by forming carbides and tying up important alloying elements that should be in solution.
All common welding processes can be used to weld CRA materials but lower heat input processes such as GMAW, (MIG) and GTAW (TIG) are preferred as they have less segregation and being gas shielded rather than flux shielded will have lower levels of inclusions, that are known as sites to initiate corrosion. In pipe welding or single sided closure welds it is usual for the root runs to be GTAW welded as this will give the optimum root profile, where access to weld from both sides is not possible.
Purging is extremely important as heat tint is known to reduce corrosion resistance. Control of oxygen content in the purge to extremely low levels is very beneficial and proprietary systems that will enable low levels of oxygen are available commercially.
The use of backing rings is not generally recommended for single sided welds. These can trap slag, inclusions and can leave a lack of penetration which will act as a crevice and reduce corrosion resistance.
Arc strikes in the weld zone and weld spatter can also act as crevices if not removed. Post weld grinding to remove these or the use of proprietary anti-spatter compounds can help reduce the effects of these. Stop start defects can also create crevices so correct welding techniques need to be reinforced to prevent these.
It is also important that any attachment welds are either protected on the reverse side to prevent any oxidation, or alternatively if this is not possible then any oxide is removed prior to service as this will improve corrosion resistance markedly.
Welding of austenitic and super austenitic stainless steels
Standard austenitic grades 309, 321, 347 & 253MA supplied by NeoNickel are readily weldable as the filler composition being less than 15% Nickel allows a small amount of ferrite in the deposited weld metal that will mitigate against solidification cracking. Further information is available in the applicable Weld Data Sheet or the relevant welding stainless steel section.
Austenitic stainless steels have a high coefficient of thermal expansion so practices to minimise distortion by good joint design, pre-setting, frequent and strong tacks will help to make this controllable.
Alloy 20 is an austenitic stainless alloy that has more than 15% Nickel but is not a true Nickel base alloy. It is impossible to amend the composition to introduce any ferrite into the weld deposit on solidification and so alternative methods to minimise solidification cracking are required. This is achieved by adequate cleanliness of the weld area, filler materials with low levels of residual elements and modest heat inputs with good joint design.
Super Austenitic stainless steel, of the type AL-6XN® (6%Molybdenum) are more difficult to weld than standard austenitic grades and an overmatching 625 weld consumable is recommended. This is required due to micro-segregation of alloying elements during cooling of the weld, particularly Molybdenum that will reduce corrosion resistance. The higher alloyed 625 fillers with 9% Molybdenum will ensure that even with segregation, the weld will maintain the 6% Molybdenum level and match the base material corrosion properties. Modest heat inputs are recommended to minimise precipitation of carbides and intermetallic precipitates such as sigma and Chi, to minimise the propensity to hot cracking and distortion and to help prevent excessive segregation of molybdenum. Care with thick section base material where excessive grain growth from previous processing can be an issue that will magnify the effects of Molybdenum segregation in the HAZ.
Cleanliness of the weld area is essential to minimise hot cracking and prevent contamination of the weld zone.
Autogenous welding of high alloy austenitic stainless steels is not recommended unless the weld is to be solution treated and quenched after welding due again to reduced corrosion resistance from segregation issues.
Precipitation hardening alloys should be welded in the solution annealed or over aged condition with a matching filler material followed by post weld heat treatment.
Detailed guidance on each alloy is available in the relevant manufacturers Weld data sheet and further information is available in the section on welding of stainless steels.
Welding of duplex stainless steels
Duplex and Super Duplex stainless steel are readily weldable and having a duplex microstructure then solidification cracking is not an issue. Duplex stainless steels have a coefficient of thermal expansion similar to the Ferritic steels and is much less than an austenitic stainless steel and is more controllable. Controlled heat inputs are still required to prevent the formation of intermetallic precipitates.
As a weld cools relatively fast then there is less time for the required levels of austenite to be formed in the weld zone. Duplex stainless are therefore welded with a weld filler over-alloyed with Nickel to promote austenite reformation in the weld metal. In the HAZ modern base metals with minimum levels of nitrogen are required to ensure satisfactory austenite levels. The concern is less with Super duplex alloys as most have a minimum PREN of 40 (Pitting Resistance Equivalent Number PREN =Cr% + 3.3 *(Mo% + 0.5W%) =16*N%) which is sufficient. The issue is more relevant to 22% Standard duplex grades where a minimum PREN of 35 is required to ensure satisfactory performance of the HAZ.
A further issue with duplex stainless steels is the propensity to Nitrogen loss in the weld root during solidification of single sided welds, which will reduce corrosion resistance. The most common method is to use a shielding gas with nitrogen additions to ensure minimum levels of nitrogen are achieved to maintain corrosion resistance.
Detailed guidance on each alloy is available in the relevant manufacturers weld data sheet and further information is available in the section on welding of stainless steels.
Again autogenous welding is not permitted unless special techniques are employed or solution heat treatment is carried out to ensure correct austenite levels are achieved in the weld zone.
It is possible to weld 22% Chromium duplex grades with Super Duplex which can improve performance.
Welding of nickel alloys
Nickel alloys are reasonably easy to weld, with modest heat inputs needed to prevent solidification cracking and scrupulous cleanliness required to prevent pick up of contaminants that can exacerbate any cracking. Filler materials that have low residual elements will also help to minimise cracking.
Precipitation hardening Nickel alloys are usually welded in the solution annealed condition or over aged condition with a post weld heat treatment to achieve the final mechanical properties in the weld zone.
Further detailed information is available in the in the relevant weld data sheet and the Welding of Nickel alloys section.
Weld overlay
Rather than an expensive solid CRA another option is to weld overlay a more cost effective material with a high alloy weld on the working surfaces. A common weld overlay is a 625 weld overlaid onto a cheaper carbon steel substrate but other combinations can be used dependent on the application.
It is common to weld overlay with a minimum of 3 layers in order that no dilution of the weld metal is on the working surface otherwise corrosion resistance will be compromised. It is recommended that a procedure qualification is produced which requires a chemical analysis of the final layer machined back to ensure that the method used is satisfactory.
It is important that compatible materials are chosen for the layer and substrate as any post weld heat treatment of a finished component is difficult, it is also important to ensure that the HAZ has adequate toughness and that the two alloy systems are compatible.
Distortion of the component is a major concern as extensive stresses can be built up from the welding which will release and cause distortion on machining or any further cutting operation.
Cleaning & pickling
Welding will always produce a slight oxide layer or discolouration which will reduce corrosion resistance of the weld. Effort should be made to ensure that purging is efficient as possible in order to minimise discolouration but is not always possible to achieve an oxide free weld. Pickling or cleaning of the final weld will improve corrosion resistance. It is recommended that this should be carried out where possible by the use of immersion pickling, pickling paste or light cleaning/grinding.
NDE
As weld quality is very important in achieving optimum weld corrosion resistance, and to ensure the absence of crevices caused by poor welding it is recommended that NDE is carried out. The method used, either visual surface examination or volumetric examination should be dictated by the severity of the application.
References
Guidelines for the welded fabrication of Nickel alloys for corrosion resistant service. NIDI publication 11012.
AL-6XN® alloy Fabrication Manual, Bulletin 203, Rolled Alloys.
Alloy 20 Fabrication Manual, Bulletin 205, Rolled Alloys.
Guidelines for welding of ZERON® 100 SDSS, Rolled Alloys.
Disclaimer
The information presented here has been prepared for general circulation. Although we believe it to be correct, it is up to each individual to check all facts and ensure any information is relevant to their application. NeoNickel cannot be held responsible or be liable in any way as a result of use of this information. NeoNickel reserve the right to amend, revise or add any information to this document without prior notice.
