Types of steel
Stainless steels are iron, chromium and carbon based alloys that may contain other elements such as Ni, Mo, Si, etc, whose main characteristic is their resistance to corrosion.
This characteristic is due to the passivation property of these steels in the presence of an oxidizing enviroment (therefore also in air). Passivation consists in the formation of an invisible layer of oxide, which varies according to the chemical composition of the steel, the type of heat treatment and the type of oxidizing environment which forms a protective barrier to continuation of oxidation and therefore corrosion, and which is rectified immediately if removed.
The grades of stainless steels are:
• Austenitic Steels
• Martensitic steels
• Ferritic steels
• Duplex / Super Duplex Steels
Contain Cr (16-20%), Ni (7-18%) and in some types Mo (2-6%) with a carbon content that is usually less than 0.08%; the presence of stabilizing elements, such as Ti, further improves their resistance to corrosion, particulary intergranular corrosion, combined with greater resistance to high temperatures.
These are basically plain chromium steels (11-18%) that may contain small quantities of other elements. The main characteristic of these steels is their ability to improve their mechanical properties through a hardening and tempering heat treatment.
These are steels containing chromium (12-27%) and with a carbon content generally of 0.20% possibly with minor additions of other elements. Their main characteristic is that they have a ferritic structure at any temperature and, therefore, their mechanical properties cannot be improved through heat treatments.
DUPLEX / SUPER DUPLEX STEELS
The term Duplex delivers from the concept that they have a two-phase microstructure consisting of grains of both ferritic and austenitic stainless steel within formulated in the same material. The term Super Duplex denotes highperformance Duplex steel based on elevated contents of chromium, nickel and molybdenum to improve pitting corrosion resistance.
CLASSIFICATION OF CARBON STEELS
The steels which we use as listed by EN 10253-2 standards, are classified in 2 ways:
• CARBON STEELS: P235TR1, P235TR2, P235GH, P265GH, P265GH, P265NL, P355NH, P355NL1, L290NB, L360NB, L360QB, L415NB, L415QB, L450QB.
• LOW ALLOY STEELS, when their alloy content (chrome, nickel, molybdenum, etc) is less than 5% whether as the sun of elements or as single value if only one element is present:
• HIGH ALLOY STEELS, when their alloy content (chrome, nickel, molybdenum, etc) is greater than 5% whether as the sun of the elements or as single value if only one elements is present:
• STEELS FOR USE AT HIGH TEMPERATURES: P235GH, P265GH, 16Mo3, 13CrMo4-5, P355NH.
• STEELS FOR USE AT LOW TEMPERATURES: P265NL, P355NL.
• HIGH-YIELD POINT STEELS: L290NB, L360NB, L415QB, L450QB.
Heat treatment is the operation (or series of operations in the case of complex treatments) during which one or more thermal cycles are carried out on the steel, i.e modifying the temperature within specific time limits. During a thermal cycle, the steel is usually heated to a certain temperature, held at this temperature for a certain time and then left to cool to room temperature in various ways according to the effects required.
Normalizing consists in heating to a temperature above AC3 (= temperature at which, during heating, trasformation of the ferrite into austenite begin) for a to time sufficient allow complete austenization of the material followed by cooling in still or forced air. This process is usually carried out on hot worked rough parts in order to obtain a finer, more even grain so that the steel is in optimal conditions for subsequent heat treatments.
The purpose of annealing is to soften the steel so that it is suitable for mechanical and/or plastic machining, to eliminate residual stresses and to eliminate the effects of plastic deformation, of welding or of a previous heat treatment. There are a number of used cycles (Sub critical, isothermal annealing), whose annealing cycles must be selected according to the hardness and structures required for a specific type of machining.
The hardening treatment includes austenitizing heating followed by cooling to a temperature below MS (= temperature at which, during cooling, transformationof the austenite into martensite starts) a speed which allows into martensite, a very hard brittle structure.
When hardened, steel is characterized by a high level of hardness and a low level of toughness.
A subsequent treatment is therefore required to produce a greather or lesser modification of the martensitic hardening structure, eliminating its stresses and brittleness. This treatment, known as tempering, envolves heating to a temperature below AC1, (= temperature at which, during heating, austenite starts) to form holding this temperature for a certain time and finaly, cooling to room temperature
in an appropriate container.
This process consists in heating to a temperature usually between 1000 and 1100°C. The part is held at this temperature for a time sufficient to eliminate the structural alterations caused by previous machining and also to obtain maximun complete if as possible “solubilization“ of the carbides of the austenite; subsequent cooling, in air or water, must be fast enough to prevent reprecipitation of the carbides which, in the case of slow cooling, usually occurs in the range between approx. 450 and 850°C. Maximum softening of austenitic stainless steels is obtained with this treatments.
This treatment consists in heating to normal hardening temperature followed by fast quenching in a salt bath at a temperature of around 10°C - 30°C above Ms until complete transformation of the austenite.
Stress relieving, which consists in heating to temperatures below 250°C, is used for casehardening or air-hardening steels in order to restrict and possibly eliminate residual hardening stresses while maintaining high level hardness. In this way, there are no appreciable modifications to the structure.