|Moly Ferritics - Fabrication|
Thermal processing and fabrication
Annealing is achieved by heating to the following temperatures for 90 minutes per 25mm thickness (3.5min/mm) followed by air quenching. Controlled atmospheres are recommended in order to avoid excessive oxidation of the surface.
Stress relieving after welding is not normally required. Should this be necessary, temperatures between 200°C and 300°C are recommended for 60 minutes per 25mm thickness (2.5min/mm).
Uniform heating of the steel in the range of 950°C to 1 050°C is required. The finishing temperature should be below 750°C and the steel should be cooled rapidly between 550°C and 400°C to prevent 475 embrittlement. Extended holding times above 1 000°C should be avoided as excessive grain growth may occur and ductility may be detrimentally affected.
All hot working operations should be followed by annealing and then pickling and passivating to restore the mechanical properties and corrosion resistance.
Moly ferritic stainless steels have good formability characteristics with useful mechanical properties. Their good ductility allows them to be readily formed by bending and deep drawing. They do not undergo significant work hardening when cold formed. U-434 is slightly more prone to roping than the other moly ferritics.
The moly ferritic stainless steels are prone to grain growth in the heat affected zone of weldments. As such, the tensile, fatigue and toughness properties in the welded condition are relatively poor. They should thus not be used for applications where tensile or dynamic loading will be experienced.
The moly ferritics are generally limited to a combined thickness in the welded condition of 2mm for U-434, 2.5mm for U-436 and 3mm for U-444. Edge welds are not recommended.
The use of austenitic filler metals such as types 308L, 309L or 316L will improve the ductility of welds to some extent but all welding procedures should nevertheless endeavour to maintain minimum heat inputs.
The weld discolouration should be removed by pickling and passivating to restore maximum corrosion resistance.