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/ 1120 °C 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 relievingaustenitic stainless steel products is not normally needed.
Effect of residual stresses
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.
The approach to heat treatment selection
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||Stainless Steel Grade Types|
|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|
|Severe SCC risk in service||Note 1||A,B||B,A|
|Some risk of SCC in service||C||A,B,C||B,A,C|
Standard carbon steel 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.
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
|A||1050 / 1120 °C, slow cool|
|B||900 °C, slow cool|
|C||1050 / 1120 °C, fast cool|
|D||210 / 475 °C slow cool (approx. 4 hours per 25mm of section)|
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)