Generally ASS are not stress relieved after welding due to the chances of sensitization i.e carbide precipitation in the temperature ranges of 425 - 900 degrees C.
However in certain critical applications for dimensional stability in long service, ASS have to be stress relieved after welding.
We have infact successfully carried out SR of ASS (304LN material)at 400 degreesC and 980 degrees C in certain critical jobs as per client requirement.
Stress relieving at temperatures below 400°C is an acceptable practice but results in only modest stress relief. Stress relieving at 425 to 925°C significantly reduces residual stresses that otherwise might lead to stress corrosion cracking or dimensional instability in service. One hour at 870°C typically relieves about 85% of the residual stresses. However, stress relieving in this temperature range can also precipitate grain boundary carbides, resulting in sensitisation that severely impairs corrosion resistance in many media.
To avoid these effects, it is strongly recommended that a stabilised stainless steel (grade 321 or 347) or an extra-low-carbon type (304L or 316L)or with Nitrogen addition like 304LN be used, particularly when lengthy stress relieving is required.
Full solution treatment (annealing), generally by heating to about 1080°C followed by rapid cooling, removes all residual stresses, but is not a practical treatment for most large or complex fabrications.
Low Temperature Stress Relieving:
When austenitic stainless steels have been cold worked to develop high strength, low temperature stress relieving will increase the proportional limit and yield strength (particularly compressive yield strength). This is a common practice for austenitic stainless steel spring wire. A two hour treatment at 345 to 400°C is normally used; temperatures up to 425°C may be used if resistance to intergranular corrosion is not required for the application. Higher temperatures will reduce strength and sensitise the metal, and generally are not used for stress relieving cold worked products.
Regards
Prem Nautiyal
Godrej, Mumbai.
On Thu, 30 Jul 2009 21:07:01 +0530 wrote
>Can anyone please tell about the effect of effect of heat treatment on
>Austentic Steel?
>
>Regards
>Sandeep kumar
>
>>
>
PREM S NAUTIYAL
CELL : 9820313278
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