Stainless steels are more than just steel that doesn’t stain. They’re well-known for their corrosion resistance and are 100 percent recyclable. When stainless steels reach the end of their useful lives, 80 percent of them are recycled. Thanks to continuous improvement in the chemistry and heat treatment of stainless steels, the family of metals have become commonplace in a wide range of applications.
Duplex stainless steels are a subset of stainless steels. The name itself signifies it is a mixture of two different phases – ferrite and austenite. Compared to traditional stainless steels, the duplex family boasts improved strength and corrosion resistance over conventional ferritic and austenitic stainless steels.
With these advantages, duplex stainless steels find their way into offshore oil and gas activities, which depend on equipment that can operate under severe conditions. With rising oil and gas exploration activities and an increase in deepwater exploration, the demand for duplex stainless steels is rising.
In addition to the oil and gas industry, other prominent end users of duplex and super duplex stainless steels include the construction and chemical industries and waste-water treatment plants. And, with continuous effort and research, today’s duplex stainless steel products are stepping into new arenas, such as nuclear power and the production of hydro turbine pumps.
Duplex stainless steels are classified from lean to hyper based on chemistry (Table 1). Lean duplex stainless steel has corrosion resistance equal to conventional austenitic stainless steels with enhanced strength. It is primarily used in construction industries, including areas requiring high strength like bridge construction and for tie bars.
Similarly, standard (22 percent Cr, PREN = 32-36) and high-alloyed (25 percent Cr, PREN < 40) duplex stainless steels have better corrosion resistance and strength. Super duplex (PREN = 40-45) and hyper duplex (PREN > 45) stainless steels have very high corrosion resistance coupled with very high strength.
For various sensitive applications like marine, chemical and oil and gas engineering, super duplex grades, such as SAF 2507, UR52N, DP3W and Zeron 100, are used. In high-strength steels containing martensite and ferrite, the steels are susceptible to sulphide stress cracking and so they are replaced with 22 percent Cr and 25 percent Cr duplex stainless steel.
Welding is an inevitable process in all manufacturing sectors. It’s critical for a range of repair work and, of course, is necessary for the construction of a myriad of products and structures, but when proper care isn’t taken while welding, defects can result that lead to the rejection of parts and components.
Initially, the handling of duplex stainless steels was found to be difficult due to their complex microstructure and the formation of various secondary precipitates in the microstructure. Continuous research, however, led to ways to make duplex stainless steels easier to weld. This was achieved by the development of various electrodes with high nickel and nitrogen contents.
Duplex stainless steels can be welded using various methods, including SMAW, GMAW and GTAW. During welding, high heat is generated and transferred to the adjacent areas of the weld region. When duplex stainless steel is exposed to temperatures above 300 degrees C, it becomes embrittled due to formation of various secondary phases.
Furthermore, duplex stainless steels are highly prone to 475 embrittlement and sigma phase embrittlement. Because 475 embrittlement occurs between 400 and 500 degrees C and sigma phase embrittlement occurs when steel is exposed between 700 and 1,000 degrees C, proper care must be taken during welding. This is in part due to why welding of duplex stainless steels consumes more time and attention than other steels. Before getting started, however, there are a few requirements to be aware of when it comes welding duplex stainless steels:
- Fixturing of component in correct position
- Understanding the procedure qualification record (PQR) and welding procedure specification (WPS)
- Ensuring work environment cleanliness and safety measures are in place
- Establishing the welding process and parameters
- Determining base metal and filler metal details as well as gas requirements
- Understanding the weld joint design and suggested position for welding
- Ensuring that duplex stainless steel is in solution annealed condition before welding or contains minimum ferrite
- Calibrating equipment and welder qualification
- Carrying out post-weld heat treatment and NDT requirements
Duplex stainless steels are highly brittle when sigma phase forms. So, exposing the steel with sigma again to a high temperature gradient will initiate cracking during and after welding. Therefore, it is always preferred to weld duplex stainless steels in a solution annealed condition. Unless care is taken to avoid sigma phase formation before welding, duplex stainless steels are highly susceptible to crack initiation.
Even once the steel has been welded properly with all requirements in place – and especially if the weld metal thickness is very high – duplex stainless steels must still be handled with utmost care until post-weld heat treatment can be completed. Probability of sigma phase formation is high when the welding region is large enough. Carrying out post-weld heat treatment immediately after welding is preferred to avoid delayed cracking. Sigma phase formed during the welding process will dissolve during post-weld heat treatment and, thus, the material regains its steel properties.
Even with proper care taken during welding of duplex stainless steels, close control during post-weld heat treatment is to be considered. Any issues in the heat treatment cycle, such as temperature deviations or heat leakage, may also lead to differential heating of component resulting in sigma phase formation. Eventually, sigma phase formation causes cracking during quenching after post-weld heat treatment.
Case in point
If duplex stainless steels are to be treated as a potential material of the future, care must be taken during each and every processing step. To highlight the measures that must be taken, the following case study demonstrates the importance of handling after welding.
In this example, super duplex stainless steel was welded using the SMAW process following proper PQR and WPS. After welding, no cracking was observed in the base metal or the weld metal. After welding, post-weld heat treatment was carried out, and a crack formed after quenching (see Figure 1). This was the first time the cracking of duplex stainless steel after post-weld heat treatment had been witnessed, so further analysis was made. Conversely, Figure 2 shows an acceptable microstructure.
To better understand the scenario, the microstructure of the steel was studied further using an optical microscope. The microstructure showed sigma phase formation in both the weld metal and the base metal, as shown in Figure 3. A fine sigma phase formed when the steel was exposed to a temperature range of 700 to 850 degrees C for just a few minutes. Blocky and large sigma phases formed when the steel was exposed to a temperature range between 900 and 1,040 degrees C for a few to several minutes. Here, the microstructure showed formation of blocky ferrites, indicating that the steel may be exposed in the temperature range of 950 to 1,040 degrees C during heat treatment.
Analysis indicated that the crack might have been due to heat fluctuations during the heat treatment process. Based on this, post-weld heat treatment was carried out again and the resulted microstructure showed austenite and ferrite, as seen in Figure 4.
Thus, the following few points were drawn out for the proper handling of duplex and super duplex stainless steels during processing.
- Proper PQR and WPS are to be followed during welding of duplex stainless steels and it should only be welded by a qualified welder.
- Sufficient dwell time must be given during multiple welds to avoid high heat generation and high penetration.
- The selection of shielding gas during welding using GMAW and GTAW processes must be carefully considered.
- The formation of deleterious phases, like sigma phase, contribute to cracking if present in large quantity and as big particles.
- To achieve a good duplex stainless steel weld, an annealing treatment is advised. Its purpose is to dissolve all the undesirable phases that precipitated during the manufacturing process and to obtain the desired duplex microstructure. In this regard, ensure uniform temperature throughout the furnace so that components are exposed at the desired temperature.
- In addition to the deleterious sigma phase, the formation of carbides and nitrides in equilibrium has to be considered. The duration of the isothermal treatment has to be long enough so that the whole cross-section of the casting is effectively annealed.
While the overall welding process may require extra time and care, the characteristics of duplex stainless steels offer significant benefits. As research continues and as welders become more familiar with the material, it will be inevitable to see duplex stainless steels applied to a growing list of industries.