Effect of austenitic stainless steel composition and heat treatment on cold formability
Introduction
Alloying elements that help stabilize 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The nickel level was increased in the superseded BS1449 grade 304S16 compared to the 304S15 level (around 8.0%).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 optimize 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.
Effect of cold work and heat treatment on the magnetic permeability of austenitic stainless steels
Introduction
Austenitic stainless steels are generally non-magnetic with magnetic permeabilities of around 1.0. Permeabilities above 1.0 are associated with the amount of either ferrite or martensite phases present in the 'austenitic' steel and so depend on:
- cold working and heat treatment conditions
-
composition effects
Cold working and heat treatment
Cold working of austenitic stainless steels can partially transform austenite to martensite. As martensite is ferromagnetic, cold worked austenitic stainless steels can show a degree of 'pull' towards a magnet. This usually occurs at sharp corners, sheared edges or machined surfaces but can be detected on wrought products such as rods or bars which may have been cold straightened, following the final hot rolling or annealing in the mill.
The degree to which this occurs depends on the compositional effects of austenite stabilising elements. High nickel or nitrogen bearing grades tolerate more cold working before localised increases in permeability are noticed.
These increases in permeability can be reversed by full solution annealing (at temperatures around 1050 / 1120 °C with rapid cooling). This transforms any cold-formed martensite back to austenite, the non-magnetic phase, which is then retained on cooling.
The best austenitic stainless steel types for low permeability applications are those with high austenite stability as these have low permeability in both annealed or cold worked conditions. These include the nitrogen bearing types, 304LN (1.4311) and 316LN (1.4406) or the high nickel types such as 310 (1.4845).
Austenitic steels are referred as non-heattreable in conventional sense i.e. they can't be hardened by transformation hardening mechanism (quench & temper)
The only suitable method to increase strength/ hardness of austenitic grades is cold working.
In limited uses a surface treatment such as nitriding can also be tried but at the loss of corrosion resistance, however.
Following heat treatments apply to Austenitic grades:
Solution heat treatment
Stress relieving
Best Regards,
Iqbal
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of sandy kumar
Sent: July 30, 2009 3:49 AM
To: materials-welding@googlegroups.com
Subject: [MW:2849] effect of heat treatment on Austentic Steel
Can anyone please tell about the effect of effect of heat treatment on Austentic Steel?
Regards
Sandeep kumar
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