In this article we will discuss about:- 1. Meaning of Stainless Steels 2. Properties of Stainless Steels 3. Heat Treatment.
Meaning of Stainless Steels:
Stainless steels are stainless as these have a minimum of 11.5% chromium in them, which having more affinity for oxygen than iron has, forms a very thin, protective and stable oxide (probably Cr2O3) film on the surface. This film is continuous, impervious and passive to stop further reaction between the steel and the surrounding atmosphere.
Thus, chromium imparts in the steels corrosion resistance, oxidation resistance and pleasing appearance. (The layer could be of oxygen, or other element, bound by chemisorption-electronic forces to metal). Apart from the essential element chromium, the stainless steels also have additions of nickel, molybdenum and manganese to enhance other properties and improve the corrosion resistance.
Properties of Stainless Steels:
Stainless steels have become versatile because of combination of following properties:
1. Good Corrosion and Oxidation Resistance:
A fresh surface of stainless steel immediately reacts with the oxidising medium to form the protective (Cr2O3) film which stops further reaction. Thus, the steels are resistant to corrosion and oxidation in an oxidising atmosphere. In less oxidising mediums like hydrochloric acid or sulphuric acid, the damaged oxide film may not get repaired easily, and corrosion may occur so is true of reducing atmospheres.
2. Good Creep Strength:
Austenitic stainless steels have good high temperature creep resistance due to their low stacking fault energy, which does not allow cross-slip of screw dislocations.
3. High resistance to scaling and oxidation at elevated temperatures.
4. Wide range of strength and hardness.
5. High ductility and formability.
6. Excellent pleasing appearance.
7. Good weldability and machinability.
8. Good low temperature properties as austenitic stainless steels do not undergo ductile/brittle transition.
Thus, from utensils, cutlery items and many more domestic items (wash-basins, etc.), to vessels, pipings, heat exchangers and storage tanks in chemical and related industries, to structural materials in nuclear reactor, constructional material for chemical reprocessing plants dealing with irradiated nuclear fuel, etc. are wide and diverge range of applications of stainless steels.
Heat Treatment of Stainless Steels:
Most of the stainless steels are subjected to one of the following types of heat treatment cycles.
Some heat treatment cycles have already been given with relevant steels:
1. Stress Relief:
Stress relief heat treatment reduces the residual stresses present in the stainless steels induced during mechanical deformation (such as bending, machining) or during thermal processing (such as quenching, welding etc.).
The stress-relief heat treatment is carried out as depending on the aim such as:
(a) The steels are heated below 370°C. i.e., below the lower limit of sensitisation to improves the elastic properties of the cold drawn materials.
(b) The steels are heated to temperature above 700°C to reduce the susceptibility to stress-corrosion cracking.
(c) To obtain the best corrosion resistance, the steels are heated to below 370°C.
Ferritic stainless steels are annealed by heating them to a temperature higher than 770°C, soaking and then cooling either in furnace, or air, or water.
The aims of annealing could be one or more:
(a) To relieve welding stresses.
(b) To relieve cold-working stresses.
(c) To obtain a more homogeneous structure by removing the patches of the transformation products.
3. Solution Treatment or Quench Annealing Treatment:
The austenitic stainless steels are quite often given the solution-annealing or quench-annealing treatment to obtain a single phase homogeneous austenite. In this treatment, the steels are heated to 1050-1120°C. Any chromium carbide, if present, gets dissolved in austenite, which has high solubility of carbon at this temperature.
After solutionising, i.e., homogenising, the steel is cooled rapidly through the critical range of 900°C to 400°C by quenching in water, oil, or air to avoid the re-precipitation of carbides. The choice of the cooling medium depends on the section-thickness, as the small thin sections may be even air cooled. Furnace cooling is never done.
4. Hardening and Tempering:
This treatment is similar to plain carbon and alloy steels. Of the common stainless steels, martensitic types of stainless steels are given hardening and tempering treatment (the other two common types-ferritic and austenitic-do not undergo phase change and are thus not given this heat treatment).
Depending on the carbon content, the martensitic stainless steels are heated to a temperature to 950-1050°C, and then quenched in oil or air. Due to the presence of large amount of alloying elements, (here chromium), these steels have high hardenability to obtain martensite even by air cooing at least in thinner sections. After hardening, the steels are tempered by heating to a temperature between 100° to 700°C depending upon the combination of hardness and toughness required.
5. Stabilising Treatment:
Stabilising treatment is given to the stabilised grades of austenitic steels (steels having strong carbide forming elements like titanium, niobium, columbium, etc. than chromium). The heat treatment consists of heating the austenitic steels to 870°C to 900°C, and holding there for 2 to 4 hours, followed by rapid cooling in air, oil, or water.
The aim of heating to and soaking is to precipitate out all the carbon from austenite as carbides of titanium, niobium, columbium and leave no carbon remaining dissolved in austenite. The basic aim is to prevent subsequent precipitation of chromium carbide during their service period.
6. Post-Weld Heat Treatment:
The welded stainless steels are many times given heat treatment to improve the mechanical properties, or to improve the corrosion resistance. Depending on the requirement, or any other heat treatment may be given.