Effect of austenitic stainless steel composition and heat treatment on cold formability
Introduction
Alloying elements that help stabilise the austenitic phase reduce the tendency of the austenitic stainless steels to work harden. Nickel additions have been used traditionally to do this, but nitrogen also has a profound affect on stability of the austenite phase.
Deep drawing compositions
The nickel level was increased in the superseded BS1449 grade 304S16 compared with the 304S15 level, (around 8.0%). This enabled the 304S16 grade with around 8.5% nickel to be used for deep drawing applications. Both these grades are covered in BS EN 10088-2 as 1.4301, but the higher nickel variant can be specified as a ‘deep drawing’ grade. In contrast, the higher nickel variants of 316, (1.4435), were developed for improved ‘selective’ corrosion resistance, originally in pharmaceutical applications, from lower ferrite levels. The ‘standard’ 316 type, (1.4401), with around 11% nickel should be suitable for deep drawing.
The BS1449 grade 305S19 with its 11.0 – 13.0 % nickel range is even more stable when cold worked. One application for this grade is for temper rolled strip for springs where low magnetic permeability is required. BS EN 10088-2 covers this grade as 1.4303. An alternative is the Sandvik strip grade ’13RM19′ with 6% manganese and 0.25% nitrogen in addition to 7% nickel.
Stretch forming compositions
Stretch forming application grades would normally have ‘standard’ nickel levels, (around 8.0 / 8.2%), but if the sheet is intended for stretch forming the manufacturer / supplier should be informed, as the final heat treatment / process line speed conditions are adjusted to optimise the mechanical properties. The grain size of cold rolled stainless steel sheet is usually fine enough, (around ASTM 7-8), to avoid ‘orange peel’ surface roughening during pressing.
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