Free Machining Stainless Steel

The traditional 'free machining' stainless steel have been based on sulphur (S) additions in the UK or alternatively, selenium (Se) additions, which have been favoured in the United States. The addition of these elements is in the region of 0.15 - 0.35%. In practice, it is impossible to obtain the selenium treated steel from European producers.

These additions form manganese rich sulphides (selenides), which provided they are evenly distributed through the steel, act as chip breakers for machining operations and so can offer higher machining speeds and improved cutting tool life.

These 'non-metallic' inclusions can also provide a source of solid lubricant to the tool / workpiece interface which may in turn also result in improvements in surface finish.

Free-machining types have been specified as alternatives to some of the popular grades, but grade rationalisation has resulted in some of the options being withdrawn from standards.

Although purpose designed free machining grades are limited in the European standard, EN 10088 does allow machinable grade variants with 0.015 / 0.030% S additions to some grades.

Proprietary sulphur and selenium grades have been marketed, including ferritic 430 and martensitic 440 types.

Proprietary alternatives based on controlled non-metallic inclusion levels are also available. These are usually based on calcium de-oxidisation steelmaking techniques and include grades based on 304, 316 austenitic and 2205 (1.4462) duplex types. The levels of calcium or oxide type inclusions are not part of the grade specification, the grades being identified only by their proprietary names.

Other additions deliberately made to enhance machinability include copper. This helps improve machinability of austenitic types by reducing the cold work hardening tendency. (Copper works like nickel as a powerful 'austenite' phase stabiliser, reducing the formation of strain induced martensite during cold working). The cold forging grades are copper bearing types.

The 'machining' grades may not perform as well as the 'standard' grades, from which they are derived. When specifying and working with these types of stainless steel it is important to bear in mind that they can be inferior to the un-treated types.

This is summarised below:

Lower pitting and crevice corrosion resistance in chloride environments. (303 is only grade tested to have failed nickel release synthetic sweat tests to EN 1811)

Inferior SCC (stress corrosion cracking) and corrosion fatigue resistance

Lower ductility and toughness

Poorer hot workability (forging)

Poorer cold workability (cold forming)

More limited weldability

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