Welding of Duplex stainless steel, Duplex stainless steel to Carbon steel/ Low alloy steel
Duplex stainless steel is type of stainless steel (Click here to know about types of Stainless Steel) having a microstructure consisting of 50% ferrite (α) and 50% austenite (γ) in theory, although ferrite and austenite is having an acceptance range. During solidification, the duplex solidifies as ferrite, which then partially transforms to austenite when the temperature decreases further.
The 50%: 50% microstructure is achieved at room temperature after water quenching from the solution annealing temperature by the steel manufacturers. This two-phase microstructure provides the duplex stainless steels with important mechanical and corrosion properties and is strongly dependent on the heat treatment and chemical composition (suitable balance between austenite-formers nickel & nitrogen and ferrite-formers chromium & molybdenum).
Understanding Duplex Stainless Steel
Temperature is a key-parameter that drives the microstructure of the duplex materials in both HAZ and weld metal. Inappropriate temperature control can lead to the following issues:
- The formation of detrimental phases and
- The formation of an unbalanced microstructure.
Chemical Properties of Duplex Stainless Steel
Table 1 shows the chemical composition of various types of duplex stainless steel.
Mechanical Properties of Duplex Stainless Steel
The mechanical properties of Duplex are higher than the equivalent austenitic grades, their yield strength is about the double of those of classical grades. Table 2 shows the mechanical properties for various Duplex Stainless Steel grades.
Ferrite / austenite phase balance control
Temperature plays an important role in the phase balance of duplex stainless steels:
- In HAZ: in case of over-heating, additional ferrite will be formed. If the cooling rate is too high, the ferrite will not retransform into austenite, leading to an unbalanced microstructure. If the cooling rate is appropriate (depending on the heat input and thickness of the material), austenite will be formed. In addition, it is important to note the austenite stabilizing action of nitrogen that will reduce the potential excess of ferrite in HAZ.
- In weld metal: in order to avoid too high ferrite content, the filler material must be over-alloyed in nickel which will promote austenite formation. For instance, 2209 (9%Ni) is recommended to weld 2205 grade (5%Ni). This document provides lists of filler materials recommended to weld each duplex grade.
Selection of Welding Filler wire for Duplex stainless-Steel Welding:
Filler wire/ Electrode for Duplex stainless steel welding for various grades, Duplex to carbon steel, low alloy steel and other combinations can be easily find using the below table:
Minimum and Maximum Heat Input for Duplex Stainless-Steel Welding
Below Table is the general guidelines on welding heat input for the satisfactory mechanical results in the weldment. Heat Input can be calculated using our ONLINE HEAT INPUT CALCULATION TOOL.
Pre-heating & post-heating for Duplex Stainless-Steel Welding
Preheating of duplex steels is not recommended and can have a detrimental impact on the properties of the welded assembly. To remove moisture, the duplex steel can be heated up to 100°C (210°F).
As a general rule, the interpass temperature should be limited to 150°C (300°F) for lean duplex and duplex steels, and 100°C (210°F) for super-duplex steels. Too high interpass temperatures will decrease the cooling rate. In this case, intermetallic phase, nitrides or carbides precipitation is likely to occur in the weld metal.
Post-heating is not recommended and must be avoided if the temperature exceeds 290°C (550°F).