Tempering may also be used on welded steel, to relieve some of the stresses and excess hardness created in the heat affected zone around the weld. This is explained by the reduction in the amount of retained austenite and because cementite is harder than martensite . 6 illustrates the volumetric changes in the piece and the distribution of stresses from the surface to the centre at different stages in cooling. The austenite to martensite leads to largest expansion. Hardening • In hardening, the steel is heated 30 to 50oC above Ae3 temperature in case of hypoeutectoid steels and 30 to 50oC above A1 temperature in case of hyper-eutectoid steel, kept at that temperature for some time, followed by quenching at a rate faster than the critical cooling rate of the steel. The cold worked austenitic stainless steels by this treatment recrystallise to result in low hardness but with good corrosion resistance. In spheroidized steel, the cementite network breaks apart and recedes into rods or spherical shaped globules, and the steel becomes softer than annealed steel; nearly as soft as pure iron, making it very easy to form or machine. Austempering is a technique used to form pure bainite, a transitional microstructure found between pearlite and martensite. Hardening followed by tempering is intended for improving the mechanical properties of steel . Using liquid ntirogen with the same hardening temperature gets about 63.5 Rc as-quenched. Less danger of quenching cracks appearing in the article. As it is impossible to dissolve all the carbides in austenite, some finely dispersed carbide (such as vanadium carbide) are allowed to remain undissolved intentionally to inhibit austenitic grain growth at such high temperatures of austenitising. On quenching, coarse grained martensite with little amount of undissolved cementite, and a large amount of retained austenite are obtained. Tempered structures have high toughness and ductility, the value of which in the hardened state is nearly zero. The depth of the hardening is usually taken as the distance from the semi martensite zone (50% martensite + 50% troostite). At 260 °C, ε-martensite completely reversed to austenite on heating. In the initial stage , steel is oxidised by the chemical reaction on its surface ,oxidation proceeds by the diffusion of oxygen atoms through the scale to the metal or by diffusion of atom(ions)of the metal through the scale to the surface. Such cooling is called quenching. However, steel is sometimes annealed through a process called normalizing, leaving the steel only partially softened. This family of stainless steels displays high toughness and impressive resistance to elevated temperatures. Therefore, a hardenability range or band is not a simple curve plotted against each grade of steel. As the presence of carbides in austenitic class of steels is always undesirable and detrimental to properties, the carbides are eliminated by heating the steel to higher temperatures to dissolve these carbides, and obtain homogeneous austenite at that temperature. If the white cast-iron has a hypoeutectic composition, it is usually tempered to produce malleable or ductile cast-iron. To make steel harder, it must be heated to very high temperatures. Cooling at a higher rate than the critical value enables the austenite to be supercooled to the martensite point . They provide a lower cooling rate in the martensitic transformation temperature range , especially when concentrated solutions are applied ,due to their boiling point. At the moment of immersion at the molten caustic alkali, the film breaks off or dissolves and bares the metal surface. Ferrite zone lowers the mechanical properties of steel not only after hardening but after tempering as well . Precise control of time and temperature during the tempering process is crucial to achieve the desired balance of physical properties. If a steel is cooled slowly, the temperature distribution across the section of the part can be regarded to be uniform; thermal and structural volume change then, occur uniformly and simultaneously throughout the section. The most widely employed quenching media are water , various aqueous solution ,oil , air and molten salts. Fig 6 : Cooling rate and time for different quenching media. Tempering at higher temperatures, from 148 to 205 °C (298 to 401 °F), will produce a slight reduction in hardness, but will primarily relieve much of the internal stresses. The pack method involves packing into a steel box the parts to be hardened along with a compound of charcoal or coke to which carbonates have been added. This forms a crust of borax which becomes a continuous protective layer at high temperature . Quenching in two media is widely employed in the heat treatment of carbon steel tools (taps, dies, milling carters etc) of a shape unfavourable as regards cracking and warping. Tempering can further decrease the hardness, increasing the ductility to a point more like annealed steel. The last defect lowers the quenching capacity. Dependent on the type of material, appropriate cooling rates vary from very fast (water quench) to very slow (air cool). The resultant as-quenched hardness of the steel is less, because of: 1. Two-step embrittlement, however, is reversible. At low temperature ( 200֯ C) , the cooling rate in oil is approx. O1 Tool Steel is the original oil-hardening, “non-shrinking” tool steel that can be hardened to the Rockwell C 65 range from a low austenitizing temperature. 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