Surface Engineering by Expanded Austenite
Austenitic Stainless Steels
The stainless steels that were the focus of the investigations were that of the 300 series, known as austenitic stainless steels (ASS). These are the most commonly used types of stainless steel worldwide. ASS always contain the elements iron, chromium and nickel, with the occasional presence of small amounts of molybdenum and nitrogen, and are the most corrosive resistant of all of the steel series.
Austenite is a phase also known as the gamma (γ) phase which occurs in an iron-carbon alloy above the eutectoid temperature of 723°C and below approximately 1500°C.
Note: the phase diagram and transition temperatures will be different for stainless steel
The transition from the ferrite phase is known as a polymorphic transformation, which is defined as changes to crystal structure in the solid state; i.e. changes to the microstructure. These transformations allow for the mechanical properties of the material to be altered. For example phase transformations alongside controlled heat-treatments can results in great increases in the strength or toughness of the material.
The main change that occurs to the crystalline structure during a polymorphic transformation from ferrite to austenite is a change in the arrangement of the atoms within the crystal, this being a change from a body centred cube (BCC) structure to a face centred cube (FCC) structure.
The difference being that within a cubic unit cell, a BCC structure has an atom positioned at each corner of the cube as well as one in the centre. The means there is a total of 2 atoms per unit cell. This can be pictured below.
Whereas an FCC structure has atoms located at each of the eight corners of the cube as well at the centre of each face of the cube, as pictured below. This gives a total of 4 atoms per unit cell.
The atomic packing factor (APF), that being the ratio of the total volume of the atoms within the cube to the volume of the cube itself, is greater in a FCC structure than compared to a BCC structure; 0.74 compared to 0.68 respectively.
Therefore an FCC austenite structure can hold approximately 100 times the amount of carbon (up to 2wt %) than a BCC structure is able to. This greater solubility is due to the interstitial positions being much larger in an FCC structure.
The change in atomic structure to FCC results in an increase in toughness, ductility and weldability, as the shear stresses that have to be overcome for dislocations to move are lesser for a closer (more densely) packed plane. But these properties come at the expensive of poor corrosion cracking resistance and a lesser hardness. However the hardness can be recovered via other hardening techniques post transformation.
Unlike other ferritic and martensitic stainless steels, austenitic steels cannot be hardened by heat treatment, but only by cold work. So in order to retain the favourable properties of the austenised stainless steel, another method must be used if some of the initial hardness is to be regained. This method is carburising.
