Stress Relieving Heat Treatment for Austenitic Stainless Steel

Unlike martensitic stainless steel, the austenitic stainless steel are not hardenable by heat treatment as no phase changes occur on heating or cooling. Softening is done by heating in the 1050/ 1120C range, ideally followed by rapid cooling. This is of course the complete opposite to martensitic steel, where this sort of treatment would harden the steel.

Apart from inter-stage annealing during complex or severe forming operations, for many applications, final stress relieving austenitic stainless steel products is not normally needed.

Cold worked austenitic stainless steels will contain some 'strain induced' martensite, which, as well as making the steel partially 'ferro-magnetic', can also reduce the corrosion resistance. A highly stressed cold worked structure may also have lower general corrosion resistance than a fully softened austenitic structure.

The main hazard is stress corrosion cracking (SCC), which relies on tensile strength as part of the failure mechanism. Stress relieving removes such residual tensile stresses and so improves the SCC resistance.

The other main reason for stress relieving is to provide dimensional or shape stability. The risk of distortion can be reduced during forming or machining operations by stress relieving.

A full solution anneal stress-relieving heat treatment will re-transform any martensite formed back to austenite. (This will also give the lowest magnetic permeability possible for any particular grade.) Slow cooling is advisable to avoid introducing distortion problems or residual thermal tensile stresses and so the risk of sensitisation during a slow cool may have to be accepted.

The temperature ranges used in stress relieving must avoid sensitising the steel to corrosion or the formation of embrittling precipitates. As a general guideline, it is advisable that the range 480-900C is avoided. The low carbon (304L or 316L) or the stabilised (321 or 347) types should not be at risk from corrosion sensitisation during stress relieving treatments.

Stress relieving treatments for austenitic stainless steel

The table shows alternative treatments in order of preference.

Process or Corrosion Hazard	Steel Grade Types
Process or Corrosion Hazard	Standard Carbon 304, 316	Low Carbon 304L, 316L	Stabilised 321, 347
Annealing following severe forming	C	A,C	A,C
Forming interstage annealing	C(A,B)	A,B,C	B,A,C
Post welding heavy sections and/or high service loading applications	C	A,C,B	A,C,B
Dimensional stability	D	D	D
Severe SCC risk in service	Note 1	A,B	B,A
Some risk of SCC in service	C	A,B,C	B,A,C

Note 1
Standard carbon grades are susceptible to intergranular corrosion (ICC) on slow cooling treatments. Fast cooling treatments are not advisable as residual tensile stresses could result in SCC.

Note 2
Treatment B is also intended to reduce the risk of "knife-line" attack in the stabilised grades. This form of attack is due to the solution of titanium or niobium carbides at higher annealing temperatures.

Heat Treatment Codes

Code

Treatment Cycle

1050 / 1120C, slow cool

900C, slow cool

1050 / 1120C, fast cool

210 / 475C slow cool (approx. 4 hours per 25mm of section)