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Martensite is not an equilibrium structure. When martensite in a steel is heated below the eutectoid temperature, the stable α and Fe3C precipitate. This process is called tempering. The decomposition of martensite in steels causes the strength and hardness of the martensite to decrease while the ductility and impact properties are improved.
At low tempering temperatures, the martensite may form two transition phases — a lower carbon martensite and a very fine nonequilibrium ε–carbide, or Fe2.4C. The steel is still strong, brittle, and perhaps even harder than before tempering. At higher temperatures, the stable α and Fe3C form and the steel becomes softer and more ductile. If the steel is tempered just below the eutectoid temperature, the Fe3C becomes very coarse and the dispersion-strengthening effect is greatly reduced. By selecting the appropriate tempering temperature, a wide range of properties can be obtained. The product of the tempering process is a microconstituent called tempered martensite.
Martensite is hard and brittle. Tempering is a heat treatment applied to hardened steels to reduce brittleness, increase ductility and toughness, and relieve stresses in the martensite structure. It involves heating and soaking at a temperature below the eutectoid for about one hour, followed by slow cooling. This results in precipitation of very fine carbide particles from the martensitic iron-carbon solution, and gradually transforms the crystal structure from BCT to BCC. This new structure is called tempered martensite. A slight reduction in strength and hardness accompanies the improvement in ductility and toughness. The temperature and time of the tempering treatment control the degree of softening in the hardened steel, since the change from untempered to tempered martensite involves diffusion.
Tempering is a process of hardening glass and metals, especially steel. First, the steel is heated to a high temperature. Next, it is quenched (cooled rapidly) by plunging it into water, oil, or other liquid. Then, it is heated again to a temperature lower than that used before quenching it, and is allowed to cool slowly.
Tempering changes the internal structure of the steel. Different uses of steel require different properties, such as varying degrees of hardness, strength, and toughness. To obtain those properties, the structure of steel is changed by tempering it in different gases at various temperatures and for various lengths of time.
Thin films of iron oxide form on steel that is being heated in the tempering process. Those films have different colors, known as temper colors, which vary with the tempering temperature.
Glass is tempered in a somewhat similar way. It is heated until it becomes almost soft, then chilled by blasts of air or by plunging it into oil or other liquids. Glass which has been tempered may be up to five times as hard as ordinary glass. It may be used to hammer nails into wood.
Following quenching, the work-piece is in its hardest but brittle condition and therefore requires a further thermal treatment (tempering, or drawing), to produce the optimum balance of properties. This consists of re-heating the work-piece to a lower temperature and holding for a specific time. The choice of time and temperature depends upon the amount of tempering or ‘softening back’ the work-piece requires. Hardening of engineering steels (in the carbon range 0.3 to 0.55%, ranges between 800 and 900 deg.C.. Tempering is generally carried out between 400 and 700 deg. C. Tools made from higher alloy steels are also hardened and tempered but require significantly higher temperatures, up to 1300 deg. C. for hardening and multiple tempering treatments are often required.
For any steel analysis and quenching medium there is a section size, above which the work-piece will not satisfactorily through harden. This is known as the limiting ruling section and is the main design parameter that needs to be considered, in combination with the geometry and property requirements of the work-piece, when specifying a hardening and tempering treatment. As quench severity increases, as it does if air is replaced by oil, and oil is replaced by water, the limiting ruling section increases for a particular steel composition. However, the use of more severe quenching is limited in turn by the increased risk of distortion or cracking during quenching, due to the higher thermal stresses induced in the work-piece.
Although it can be applied to bars, forgings and castings, hardening and tempering is often left to a late stage of manufacture, in order to minimise manufacturing costs and maximise the properties produced. Often, the only post treatment operation is grinding. As a result, the environment within the furnace surrounding the work-piece has to be controlled, in order to prevent unwanted chemical changes, such as oxidation, decarburisation and scaling, adversely affecting the work-piece surfaces. For this reason Bodycote operate vacuum furnaces and controlled atmosphere pit and continuous furnaces, sealed quench furnaces and fluidised beds, all of which are suitable for hardening and tempering with complete control of surface chemistry.
The need for precise control of temperature and time, as well as environmental chemistry, has led to the application of computerised control systems for most types of furnace. This is a development that was pioneered in the UK by Bodycote and continues to be an important tool for both process improvement and cost control.