Machining of duplex and super duplex stainless steels

24th November 2020

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Duplex and super duplex stainless steels are considered by some to be harder to machine than other stainless steel grades. Whilst this may be true, it does not mean that they cannot be successfully machined into intricate finished components if the correct parameters are observed.

Compared with most other grades of stainless steel, duplex and super duplex stainless steels have considerably higher strengths. This property alone will impose larger forces on the cutting tool, increasing the potential for damage and abrasive wear.

Duplex and super duplex stainless steels are also more likely to work harden than austenitic, ferritic or martensitic stainless steels. Therefore, the machined surface becomes harder whilst the tool is wearing. Perhaps counterintuitively for a more challenging grade, this phenomenon can be mitigated by taking the deepest cut possible in order to remove the work-hardened layer from the previous pass, without compromising the quality of the machined finish.

The combination of both high strength and high elongation in duplex and super duplex stainless steels makes chip breaking more difficult. Long chips can become wound on to the part, negatively impacting surface finish.

Compared to other steels, stainless steels have approximately a third of the thermal conductivity properties. Considerably less heat leaves the cutting process together with the chip and more heat is transferred into the insert, necessitating a lower cutting speed for stainless steel materials, compared to carbon steels. Low thermal conductivity also leads to built-up edges, where material smears onto the insert. This then rips away the coating of the insert, deteriorating the insert edge.

Using stronger and more rigid machines, higher-performance inserts, a plentiful flow of coolant, the correct speeds and feeds will help in the machining of duplex and super duplex stainless steels. Optimum settings are now more widely available from a number of tooling suppliers.

In addition to machining settings, the specific grade of duplex and super duplex stainless steel can also influence the process. Langley Alloys is a distribution partner of Sandvik for their range of duplex and super duplex stainless steels. A number of these are supplied as their enhanced Sanmac® specification. Machinability has been improved without jeopardizing properties such as corrosion resistance and mechanical strength. The non-metallic inclusions in Sanmac steels are of great significance to the improved machinability. In addition to sulphides, Sanmac steels contain oxide inclusions that improve chip breaking and reduce tool wear. Machining is more consistent, allowing improved rates of productivity, less rework and improved part quality.

Langley Alloys stocks 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).