^ Metallographic structures of duplex showing both austenite and ferrite phases present. Duplex derives its name from this unique grain structure.
Article By Team Ward (Bob Besh, Chris Grice, Bill Huffman, Adam Renstrom and Jon Ward) Ward Vessel & Exchanger Corporation, Charlotte, North Carolina, USA
The article explains best practices to ensure the welding of duplexes avoid such factors as premature equipment failure, downtime, maintenance, and unnecessary expenditure in the long run, before concluding with a few final words on the fabrication of duplex stainless steels.
The low nickel content and the high strength of duplex stainless steels allows for design with thinner sections of material, providing the potential for cost savings in raw materials and welding. In addition, customers in the petrochemical and chemical processing Industries often realize further savings due to lower transportation costs of lighter equipment as well as fewer repairs than encountered with less corrosion resistant options.
But as attractive as duplex stainless steels are for process industries applications, they require special attention to detail when it comes to fabrication, particularly welding. A variety of factors affect the ability to create a satisfactory weldment with duplex alloys when they are selected as one or both of the joining members. These include:
- Alloy compatibility
- Engineering the joint
- Welding processes
- Welding procedures
- Welder qualification.
Understanding alloy compatibility is a good first step
While duplex stainless steels are regularly joined to themselves or compatible alloys without encountering problems, there are materials that present significant challenges, which should be addressed prior to welding. For example, the materials’ coefficients of thermal expansion (CTE), heat dissipation, and the fusion grain structure all need to be considered when joining duplex alloys to carbon steel and low-alloy steel. Differences in CTE between alloys can impart residual stresses from fabrication into the welded joint above and beyond what would be there when joining materials with similar properties.
Thermal conductivity, or how quickly heat dissipates from a joint during fabrication (measured in BTU-in/hr-ft2-F), can also impact the quality of the weldment. Since carbon steel is a much more efficient conductor of heat than duplex stainless steels, the difference in heat dissipation exacerbates thermal expansion as rates rise at higher temperatures. The consequences of excessive heat buildup coupled with differing thermal coefficient of expansion can include:
- Distortion of material
- Potential for buckling
- Combined stresses exceeding the tensile strength of the material (cracking).
To avoid potential problems that could lead to premature equipment failure, it’s important to select a fabricator with extensive knowledge of duplex alloys to other materials. A thorough understanding of all the details, such as grain structure and the impact of martensite on weld quality, can have a significant impact on the success of a project. Before settling on the low price bidder, ask potential fabricators to discuss previous duplex stainless steel projects that they have worked on and the materials that they have joined. Paying a little extra for a fabricator with extensive experience could save you downtime, maintenance, and money in the long run.
The importance of qualification/testing in the welding process
The roles of temperature and shielding gas in duplex stainless steel welding
Heat input, shielding gas, and interpass temperature must all be factored into any duplex stainless-steel welding project. Although there are other things to take into account when fabricating equipment with duplex stainless steel alloys, these are the important elements to consider when developing a set of welding procedures:
- Heat input is a major contributor to the thermal cycle of the weldment and, therefore, assists in proper grain formation during cooling. The specific heat input for a weldment has to be controlled within the value reflected by the welding procedure qualification. Too much heat will negatively affect the grain structure and corrosion resistance, while too little heat can cause problems with weld penetration and fusion.
- Shielding gas plays an important role in welding of duplex stainless steels. Duplex is unique in that nitrogen plays a major role in both the formation of austenite as well as the pitting resistance of the weldment (as little as 0.05% in the lean duplexes and up to 0.40% in some of the super duplexes.). For maximum corrosion resistance, 2% nitrogen in the shield and no more than 5% in the purge is recommended. This prevents the escape of atomic nitrogen during welding. Excessive nitrogen can cause embrittlement of the material, and all production gases should be closely monitored. Duplex is more sensitive to atmospheric gases and, therefore, extra care should be taken with welding equipment and selection of gas lines, flow rates, gas lenses, water-cooled MIG guns, etc. to control any possible contamination of the shielding gas.
- Interpass temperature is the temperature at which subsequent weld runs (each passage of an electrode or torch) are deposited. With duplex stainless steel alloys, proper control of interpass temperature limits the thermal cycle of the weldment and controls the weld metal’s microstructural development. Cooling below 250°F, and in some cases less, prevents excessive heat buildup, which can result in formation of intermetallic phases. The higher the grade of duplex (for example, super duplex), the more sensitive it is to heat input during welding. On critical path welds, maintaining a tighter range (higher start temperature and lower interpass temperature) can result in more uniform microstructure and minimize grain structure issues caused by multipass welds.
Getting a duplex project right requires more than basic welding skills
- Test for detrimental intermetallic phases using ASTM A923 Method C test.
- Consider running the full range of testing performed in the procedure qualification.
- Monitor heat input, along with interpass temperature, during welding of the coupon as well as production welding.
- Final testing of the weldment for ferrite between 40% and 65% or tighter ranges depending on the application.
A final word on fabricating with duplex stainless steels
For more information about Ward Vessel & Exchanger’s capabilities and experience building long-lasting petrochemical and CPI equipment with duplex stainless steels and other alloys, visit wardvesselandexchanger.com.