Mechanism and measurement of work hardening of austenitic stainless steels during plastic deformation
Austenitic stainless steels work-harden significantly during cold working. This can be both a useful property, enabling extensive forming during stretch forming without risk of premature fractures and a disadvantage, especially during machining, requiring special attention to cutting feeds and speeds.
What is work hardening and why is it a particular problem in the austenitic family?
Work hardening is the progressive build up in the resistance to further work or deformation. One result of this is that the tensile properties (proof and tensile strength) increase with cold work.
This only happens during cold working. During hot working the steel is continually being 'self-annealed'.
There are two mechanisms operating in the austenitics.
'Normal' work hardening occurs as 'dislocations' (naturally occurring line defects that enable metals to be ductile) in the atomic lattice move during plastic deformation. The stress fields around the dislocations interact as the dislocations 'tangle' so that more force is then required to move them. The face centred cube (fcc) atomic structure in metals such as aluminium, copper, austenitic stainless steel etc. also results in more energy being needed to keep the deformation process going, as 'partial dislocations' try to move through the lattice together. This not so marked in the ferritic (bcc) structure.
The austenite structure or phase is also unstable during cold deformation and breaks down to the much stronger, less ductile martensite phase.
In this condition an austenitic stainless steel becomes slightly ferro-magnetic, as the martensite formed is 'ferro-magnetic' ie it will attract a permanent magnet.
These combined effects are reversible by solution heat treatment generally by heating to 1050/1120C and cooling quickly.
Measure of work hardening - 'n' value
Work hardening begins after the steel has 'yielded' and begins to plastically deform. During tensile testing, a plot of stress against strain produces a curve as plastic deformation progresses. The slope of a logarithmic plot of stress against strain gives the 'n' value.
For ferritic stainless steels types, n values are approximately 0.2, which do not vary with strain level.
The austenitic stainless steels have two n value ranges, depending on the amount of strain.
A 'stable' austenitic (higher nickel types) would have values of n around 0.4 at low strains and 0.6 at higher strains. These grades are suitable for deep drawing.
In contrast less stable grades would have comparative values of 0.4 at lower strains and 0.8 at higher strains. These grades are more suitable for stretch forming. This is because as stretching procedes, the sheet thins uniformly, resisting localized thinning and premature fracture in the walls of pressings.
The work hardening properties are also reflected in the difference between proof strength (yield strength) and tensile strength. The values for austenitics are wider apart than the lower work hardening ferritics.
Anisotropy (directional differences forming properies) - 'r' value
The drawability is also affected by the anisotropy of the sheet ie the differences in strain in the plane of the sheet compared to the reduction in thickness. This 'r' value (strain ratio) is around 1 for austentics and between 1 and 2 for ferritics. The higher the value the better the sheet resists thinning and so on this basis the ferritics would be expected to draw better than the austenitics.
This strain ratio can vary in relation to the rolling direction of the sheet. When it does the material is said to have 'planar anisotropy' and this can be used to predict how the sheet will form ears during drawing. The austenitics are less prone to this anisotropy than ferritics (ie their properties are less directional in the plane of the sheet). So with their lower proof strengths and higher work hardening rates the austenitics are usually considered better for sheet drawing and forming operations than the ferritics.