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11. Free machining stainless steels grades


The traditional ‘free machining’ stainless steel grades 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 steels 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.

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British standard free machining grades

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as alternatives to some of the popular grades, but grade rationalisation has resulted in some of the options being withdrawn from standards.

Un-Treated Type Machinability Addition BS Free Machining Grades Nearest BS EN 10088 Grade
420 S 416S21 1.4005
416S37 1.4029
Se 416S41
431 S 441S29
Se 441S49
304 S 303S21 1.4305
Se 303S41
321 S 325S21
316 Se 326S36
316 S 1.4598

BS EN 10088 grades with permitted sulphur additions

Although purpose designed free machining grades are limited in the European standard, BS EN 10088 does allow machinable grade variants with 0.015 / 0.030% S additions to some grades.
Selenium additions are not allowed in this European standard.

1.4006 410 type – 0.08 / 0.15 %C
1.4021 420 type – 0.16 / 0.25 %C
1.4028 420 type – 0.26 / 0.35 %C
1.4031 420 type – 0.36 / 0.42 %C
1.4057 431 type – 0.12 / 0.22 %C
1.4112 440B type – 0.85 / 0.95 %C
1.4125 440C type – 0.95 / 1.20 %C
1.4542 17/4 PH type
1.4307 304L type
1.4541 321 type
1.4401 316 type
1.4404 316L type

Proprietary enhanced machinability 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, 2205 (1.4462) duplex and 17-4 PH (1.4542) precipitation hardening 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. Many manufacturers of stainless steel bars market these enhanced machinability grades with brand names, for example:

Company Brand
Acerinox Roldamax
Cogne IMCO
Outokumpu PRODEC
Schmolz and Bickenbach Ugima
Valbruna Maxival

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.

Disadvantages of free machining grades

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 steels 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 BSEN 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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