The contraction of the surface layers is resisted by the central part. The centre has expanded in stage II. It is during this stage, the greatest danger of cracking exists (that is why, a thumb rule is used in industry: put the piece in tempering furnace to minimise danger of cracking as tempering induces ductility in surface before centre transforms to martensite). 6.3 (a) illustrates structure of Hadfield manganese steel casting having intense carbide precipitation formed on cooling in mould from casting temperature: (i) Decrease in ductility and impact strength. The adhering film of gas/vapour appreciably reduces the cooling process and results in general decrease in hardness, or may result in soft spots as compared to ground parts. Mild steels (< 0.3% carbon) tend to be difficult to harden (with not much increase of hardness), because critical cooling rate is attained with difficulty, and that too in very thin sections by using drastic cooling, which may cause distortion and cracks. Such holes may be packed with wet asbestos, clay, or steel inserts to avoid hardening inside them. Since the amount of carbide is different in different types of steels, the soaking time thus depends on the grade of the steel. The fast cooling prevents precipitation again of carbides from austenite. The surface has little chance of plastic deformation as it has brittle martensite (unyielding). the hardness shall be low. Based on calculated values, heating time to hardening temperature of 850°C in an oil-fired muffle furnace can be obtained from Fig. Heating Time 5. There are many different types of surface hardening processes but typically, you take a base metal with a low carbon content and then combine it with a thin layer of another metal alloy that has a higher carbon content and is also more durable. Such treatment in ‘Hadfield’ Mn steel is many times called ‘water- toughening treatment’. There are many different types of surface hardening processes but typically, you take a base metal with a low carbon content and then combine it with a thin layer of another metal alloy that has a higher carbon content and is also more durable. Austenitising Temperature for Highly Alloyed Steels: In these alloy steels, austenite is a stable phase from room temperature to high temperatures, i.e., austenite does not undergo phase transformation; neither on heating, nor on cooling, i.e., no grain refinement is possible by phase change. 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 (Fig. In stage III, as the centre is thermally contracting, the surface (martensite formed) is almost at room temperature, prevents the contraction as much as it should. Structural stresses are the stresses, which develop due to non-uniform volumetric expansion, due to phase change (mainly austenite to martensite) and at different times, when the steel is rapidly cooled. These steels also undergo phase transformation, and thus, are heat treated to get martensite. 6.4 b), decreasing the corrosion resistance of the regions causing intergranular corrosion (Fig. More so because much higher thermal stresses are induced due to quenching from a much higher temperature. The presence of high carbon, not only aggravates by lowering Ms temperature, but also increases the brittleness of martensite, increasing the tendency to quench cracking. There are different types of metal-hardening processes, such as heat treatment. On quenching, coarse grained martensite with little amount of undissolved cementite, and a large amount of retained austenite are obtained. If the temperature of austenitising of hyper-eutectoid steels is increased, but still below Acm temperature, correspondingly increased amount of cementite is dissolved in austenite (whose carbon content then becomes higher than 0.77%), grain growth of austenite may occur, as the cementite barriers to the motion of grain boundaries essential for grain growth have largely dissolved. Under right conditions, both type of stresses get superimposed to become larger than the yield strength to cause warping, but when tensile stresses become larger than tensile strength, quench cracks can occur. Hence, no internal stress is set up. The greatest danger now is to produce a tensile crack in the internal central part, but cannot come to surface because of prevalent compressive stresses in the surface. Plagiarism Prevention 5. 6.2 a). The stresses that develop in a quenched part, as a result of unequal cooling, which causes temperature gradient and resultant non-uniform volume changes, are called thermal stresses. Content Guidelines 2. In stage V, the centre is contracting thermally and the surface is almost at the room temperature, which leads to decrease in stress levels, and many tines it may even reverse (b-VI). These steels on slow cooling as in castings or even on heating (to 500° – 800°C) precipitate carbides, generally on the grain boundaries of austenite. Hardening is done of steels containing more than 0.3% carbon as the gains in hardness are most substantial in these steels. This is because, the usually compressive nature of internal stresses obtained in these methods, make the surface layers to bear greater amount of tensile stresses, i.e., the strength of the surface is increased. Introduction to Surface Hardening of Steels* Revised by Michael J. Schneider, The Timken Company, and Madhu S. Chatterjee, Bodycote SURFACE HARDENING, a process that includes a wide variety of techniques (Table 1), is used to improve the wear resistance of parts without affecting the more soft, tough interior of the part. Hardened steel. Structural stresses are developed due to two main reasons: (i) Austenite and its transformation products have unequal specific volumes, leading to a change in volume when transformation occurs. (iii) Presence of double phase, instead of single phase austenite, further accelerates corrosion by forming micro-galvanic cells. The surface and the centre, undergo these changes to varying extent and at different times. In it, transformation takes place simultaneously to martensite in surface and to pearlite in the centre in stage II. On quenching, austenite transforms to fine martensite but the undissolved nodules of cementite remain unchanged. Each metal hardening process includes three main steps: heating, soaking and cooling the metal. 6.4 a) to dissolve these carbides, and obtain homogeneous austenite at that temperature. The above range of austenitising temperature for hypo-eutectoid steels, results in single phase, fine grained and homogeneous austenite, which on quenching transforms to fine-grained (very fine needles/plates), hard martensite, which is desired to be obtained in hardening. Copyright 10. Coarse grained martensite is more brittle and 1-2 units less hard. Surface hardening is a heat treating service for reinforcing a metal surface. by cleaning with wire brushes, or in sand blasting machine, as their presence interferes with the quenching process and decreases hardness. Such steels cannot be austenitised at temperatures above Acm/eutectic temperature (as in austenitic class of steels), because these steels shall then, burn and melt as these are ledeburitic steels. The main aim of this process is to make the only external surface of steel hard and inner core soft. Hyper-eutectoid steels, when heated in the above range, i.e., just above Ac1 have fine grains of austenite and small nodules of proeutectoid cementite (the network of cementite has been assumed to be broken). Shallow hardening steel in which transformation occurs simultaneously at the surface and the centre. It is the process of carburisation i.e., saturating the surface layer of steel with carbon, or some other process by which case is hardened and the core remains soft. Dissolution of cementite leads to very rapid grain growth of austenite. Internal stresses are always produced due to non-uniform plastic deformation.
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