Introducing Stainless Steel & Comparing International Grade Numbers
It is generally quite easy to say, from a careful visual examination of a steel sample, that it is a stainless steel brand, but to go beyond that simple statement is rarely possible. And yet the difference between them may be considerable, because the name "Stainless Steel" is a common designation for a very large group of alloyed steels. By examining the strips under a microscope, however, it is possible to sort them out and to give them more exact names.
Just as with pastry, the properties of stainless steels depend on two things, ingredients and preparation. With stainless steel, the connects of chromium and nickel are the decisive factors, and so enable us to distinguish two main classes of stainless steels, chrome steels and nickel-chrome steels, respectively.
Types of Stainless Steel & Microstructure of Stainless Steel
Chrome steels have a chromium content varying between 12% - 30%. In nickel-chrome steels the percentages are, 4% - 35% nickel, and 12% - 30% chromium, respectively. These two main groups can each be subdivided into two groups according to the structure of the alloy.
In chrome steels, the structure is determined by the relative proportions of carbon and chromium. Chrome steels with 12% - 13% chromium are "Ferritic", if the carbon content is below 0.09%, and "Martensitic", if it is higher. In steels with 15% - 18% chromium, carbon contents up to 0.12% produce a ferritic structure, pure martensite not being obtained until the carbon content reaches 0.4% - 0.7%. If the chromium content exceeds 30%, the steel is ferritic irrespective of carbon content.
In nickel-chrome steels, the structure depends mainly on the nickel content, being "Austenitic-Ferritic" at low contents of nickel, 4% - 8%, and pure "Austenite" at higher nickel content.
Ferrite is iron which contains practically no carbon. The light parts of the micrograph are ferrite crystals. These areas are surrounded by dark lines, which are slightly curved.
The dark points within the ferrite crystals are carbide particles which give the steel its hardness. The carbide precipitation should be situated inside the crystals and not along the boundaries, as this causes a greater susceptibility to corrosion by chemicals, and jeopardizes the strength of the material by lowering the inter-crystalline cohesion.
Ferritic steels are magnetic. Their structure undergoes no changes when they are rapidly quenched at high temperature, that is to say, they cannot be hardened. They are easily machined with cutting tools. Their ductility is good, but they are not very suitable for welding.
Chemical resistance is good against water, steam, food stuffs, and weak chemicals. Steels having a higher chromium content show rather good creep strength, ie. tensile strength at high temperature, even in the presence of sulphurous gases.
Steels with approx. 17.5% chromium and 0.08% carbon are used in the manufacture of pressed and welded equipment for low corrosion stresses, such as barrels and drums, kitchen sinks, etc.
Steels with approx. 14.5% chromium and 0.08% carbon, and with a small amount of molybdenum, are used for screens in the pulp and paper industries, etc. - without the molybdenum, for spoons, meat hooks, parts for steam and water turbines, injectors for diesel engines, etc.
Steels with approx. 24% chromium and 0.20% carbon are used for heat-resistant equipment in the sulphur, sulphite, and sulphurous-acid industries.
Martensitic steels have a higher carbon content than ferritic steels. The carbide particles are closely packed in a matrix of ferrite. The boundaries between the ferrite crystals are seldom clearly defined. As shown by the micrograph, the carbide particles are rather small, rounded and evenly distributed in the steel.
Martensitic steels are magnetic and can be hardened. After tempering, they are easily worked with cutting tools. They are also easily welded.
The chemical resistance is highest in the hardened state because of the homogeneous structure obtained by hardening, that is, there are no points on the surface of the material more susceptible to chemical attack than the rest of the surface.
Steels with approx. 14% chromium and 0.09% carbon are suitable for knives, armatures, etc. - with a carbon content of 0.20%, for components in steam machinery, cast furnace parts such as hearths, and annealing boxes - with carbon content of 0.30%, for edge tools such as knives and scissors, for various other instruments, and for springs.
Steel with approx 17% chromium, 0.20% carbon, and about 0.7% nickel,are used for constructional elements subjected to more severe corrosive attack, such as shafts and rotors in compressors for nitreous gases, in nitric-acid pumps, etc.
In austenitic steels the grain boundaries are narrow straight lines. In this case also, we find precipitation of carbide in the crystals, though to a lesser degree than ferrite, and generally well within the crystals.
Austenitic steels are non-magnetic at room temperature and in an annealed condition. They cannot be hardened in the normal manner - a considerable increase of hardness may, however, be achieved through cold working. They are readily machined in the cold state and are also very suitable for welding.
During welding, carbide may be precipitated in the material on both sides of the weld (in parts which have attained a temperature of about 900°C), and this precipitation may give rise to intergranular corrosion mentioned above, the so-called "Weld Decay". The remedy for this defect is to anneal the complete weld, whereby the precipitated carbide reverts into solid solution in the austenitic matrix.
The corrosion resistance against most acids, bases alkalis, and salt solutions is remarkably good, as are heat resistance and creep strength. The corrosion resistance is highest in the annealed steel because of its more homogeneous structure.
Steel with approx. 18% chromium and 8% nickel are suitable for household utensils, for interior architectural purposes, for equipment in breweries and dairies, and in the nitric-acid and cellulose industries, etc. - with approx. 1.5% molybdenum, for chemical equipment, and for exterior architectural designs, etc. - with approx. 2.7% molybdenum for equipment subjected to severe corrosive attacks, as in the sulphite and dyeing industries, and in pharmacy.
Steels with 20%- 25% chromium, and 20% - 23% nickel, are used for heat resistant equipment, both in the form of sheet and castings: charge baskets for furnaces, muffles, case hardening boxes, conveyors in furnaces, parts for furnaces in the nitrogen industry, salt-bath tanks, etc.
(Duplex & Super Duplex Stainless Steels eg., UNS S31803, UNS S32750, etc.)
As the name implies, this structure is mixed ferritic and austenitic, in a proportion that may vary within rather wide limits. The chromium content, however, must be about 23% - 24%, a lower value giving a purely martensitic structure.
The dark mass in the micrograph is ferrite, the light parts austenite. The dark points in the austenite are carbide which generally appears near the ferrite. The boundaries between the crystals may be straight or curved.
Austenitic-ferritic steels are magnetic. Their structure remains unchanged when quenched, that is, they cannot be hardened. They are extremely well adapted to casting, less so to rolling.
Chemical resistance is excellent, even at the highest temperatures. Steels with approx 26% chromium and 4% nickel are suitable for shafts, armatures, pumps etc., and as heat-resistant material in furnace linings and heat baffles, for lead-bath tanks, case hardening boxes, etc.
Steel with approx. 26% chromium, 5% nickel, and 1.5% molybdenum, are employed in the cellulose industry for digesters and bleaching equipment, and in the food production industry for pumps, valves, and fittings.
According to the result desired, there are different types of heat treatment - soft annealing, hardening, tempering, and quench annealing. The type of heat treatment must be chosen with regard to the actual type of steel. A short account of the methods employed is given below;-
Soft-annealing: means a heating of the steel to such a temperature that re-crystallization takes place. The temperature must be raised rather slowly, and the final temperature maintained for several hours, before the work is allowed to cool. By this treatment, the extremely small carbide particles are assembled into larger, rounded particles, and the steel becomes softer and more easily machined with cutting tools.
Hardening: The process consists of heating to a high temperature, followed by rapid quenching. The heating must be carried so far that structural changes take place, and the quenching should be sufficiently rapid to prevent the steel from returning to its original structure. The hardened structures, therefore, a forced state.
Tempering: denotes heating of the hardened steel to a temperature well below the hardening temperature, followed by cooling in air. The structure changes successively as the temperature rises, and the stresses in the material are relieved. The hardness decreases, and the toughness increases simultaneously. Hardening plus tempering to a high temperature produces a steel which is, at the same time, hard and ductile.
304 being used in industrial and dairy meaning we would all have food poisoning if it corroded or contaminated.316 rods can be used with pure argon to weld 304 but it would be the weld that then started to contaminate,back purging with argon would do nothing to stop this,the only way around is to use 304 as the filler welded with pure argon,i very much doubt that anybody goes to the trouble of back purging exhausts as its expensive but thats also an option.