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Thermal processing and fabrication
Annealing of the duplexes is achieved by heating to between 1 020°C and 1 100°C for 90 minutes per 25mm thickness (3.5min/mm) followed by quenching in an agitated water bath down to room temperature. Controlled atmospheres are recommended in order to avoid excessive oxidation of the surface.
The duplexes can be stress relieved at 525°C to 550°C for 60 minutes per 25mm thickness (2.5min/mm) in special cases but it is preferable to fully anneal. Stress relieving contributes significantly to improving the resistance to stress corrosion cracking by lowering the residual tensile stresses.
The duplexes can be readily forged, upset and hot headed. Uniform heating of the steel in the range of 1 150°C to 1 250°C is required. Initial hot working should be affected without large reductions or change of shape (especially if upsetting or staving up). Once the material starts to flow, progressively more deformation can be accomplished. The finishing temperature should not be below 950°C. If the temperature after forging is still above 1 000°C, rapid cooling (water quenching) can be carried out directly from the working temperature. Otherwise, all hot working operations should be followed by annealing and pickling and passivating to restore the mechanical properties and corrosion resistance.
The duplexes have good formability, but due to the higher proof strength, more power is required for most cold forming operations than austenitic stainless steels. Roll forming can be readily applied to the duplexes, but loadings will be about 60% higher than for mild steel and slower speeds should be used. Severe deep draws may require an intermediate anneal. Cold bending reduces the maximum gauge capacity of the machine by about half, compared with austenitics. The minimum inner bend radius for the duplexes is three times the plate thickness and four times is recommended. Severe bends should be carried out transverse to the rolling direction. The duplexes exhibits greater spring back than mild steel and this should be compensated for by slight over bending.
The high strength that makes the duplexes useful in many applications also reduces their machinability. U-2001 has the best machinability of the duplexes and is similar to the U-316L types. In general, for the duplexes, cutting speeds are approximately 20% slower than those for U-304. Machine tools should be ground to close tolerances to avoid the risk of excessive work hardening in the outer layer of the stock. Larger tools should be used to give stability and efficient heat dissipation. Tools with large rake angles, sharp edges and smooth surfaces reduce the work hardening and the risk of built up edges. Relatively large feed rates and cutting depths minimise the work hardening of the surface layer. A suitable cutting fluid should be used to minimise the risk of built up edges. The work should be flooded to ensure maximum heat removal.
The duplexes have good weldability in most applications, provided that the recommended procedures are adopted. They can be welded with most standard welding methods (MMA/SMAW, MIG/GMAW, TIG/GTAW, FCAW, SAW and PAW). If the duplexes are autogenously welded, the fabrication should be solution annealed to restore the desirable duplex microstructure and hence the toughness. Only welding consumables specifically specified for the duplexes should be used to ensure that the deposited metal has the correctly balanced duplex microstructure. 2209 filler welding electrodes are recommended for optimum properties. Nitrogen, added to the shielding gas, will also assist in ensuring adequate austenite in the microstructure.
The heat input should be controlled to between 1 and 2kJ/mm in order to keep the Heat Affected Zone (HAZ) narrow and to ensure there is at least 20% austenite in the HAZ. The interpass temperatures should not exceed 150°C. The lower coefficient of thermal expansion of the duplexes, compared to austenitic stainless steels, reduces distortion and the associated stresses.
Preheating, although not essential, is beneficial on thicker gauge sections. Typical preheat temperatures are between 100°C and 250°C. Post-weld heat treatment is not normally required, but solution annealing will restore the toughness and confer the optimum stress corrosion cracking resistance to the fabrication.