Duplex and super duplex stainless steels can be machined into intricate finished components, provided the correct parameters are observed. They are considered harder to machine than other stainless grades, but that does not mean they cannot be finished successfully.
Higher strength, higher cutting forces
Compared with most other grades of stainless steel, duplex and super duplex grades have considerably higher strength. That alone imposes larger forces on the cutting tool, increasing the potential for damage and abrasive wear.
Work hardening
Duplex and super duplex stainless steels are also more likely to work harden than austenitic, ferritic or martensitic grades, so the machined surface becomes harder as the tool wears. Counterintuitively for a more challenging material, this can be mitigated by taking the deepest cut possible to remove the work-hardened layer from the previous pass, without compromising the quality of the machined finish.
Chip breaking
The combination of high strength and high elongation makes chip breaking more difficult. Long chips can wind onto the part and spoil the surface finish.
Low thermal conductivity
Stainless steels have roughly a third of the thermal conductivity of carbon steels. Less heat leaves the cutting process with the chip and more is transferred into the insert, so cutting speeds must be lower than for carbon steels. Low thermal conductivity also leads to built-up edges, where material smears onto the insert, ripping away its coating and deteriorating the cutting edge.
Getting good results
Stronger, more rigid machines, higher-performance inserts, a plentiful flow of coolant and the correct speeds and feeds all help in the machining of duplex and super duplex stainless steels. Optimum settings are now widely available from a number of tooling suppliers.
The specific grade also influences the process. We are a distribution partner of Alleima for their range of duplex and super duplex stainless steels, a number of which are supplied to the enhanced Sanmac® specification. Machinability is improved without compromising properties such as corrosion resistance and mechanical strength. The non-metallic inclusions in Sanmac steels are key: alongside sulphides, they contain oxide inclusions that improve chip breaking and reduce tool wear. Machining is more consistent, allowing higher productivity, less rework and improved part quality.
We stock duplex and super duplex stainless steels between 1/2" (12.7mm) and 16" (406.4mm) diameter in the most popular grades, including duplex S32205 (F51, 1.4462), SAF2507 (F53, S32750, 1.4410), S32760 (F55, 1.4501) and Ferralium® 255-SD50 (F61, 1.4507).