In this article we will discuss about the procedure adopted for heat treatment of general engineering steels.
Heat Treatment of Carbon Steels:
Carbon steels in the range of 0.25% to 0.60% carbon find large number of applications in the form of bars and forgings. These are commonly given normalising treatment. The temperatures for normalising are given in table 12.1 (b).
To obtain optimum properties, i.e., high ratio of yield strength to ultimate tensile, high elongation, reduction in area and impact strength, carbon steels are hardened and tempered (in range 500-700°C). As carbon steels have low hardenability, only thin sections are given this treatment.
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Steels having carbon 0.35 and 0.50% are used for large number of applications after hardening and tempering, particularly of 0.50% carbon for axles and bolts. Oil quenching may be used, but for thicker sections, water quenching is used, such as for crankshafts of motor-car engines.
Heat Treatment of Plain Carbon Steels:
Plain carbon steels may be heat treated to improve the microstructure and the properties for specific applications. For example, the heat treatment of cast carbon steels breaks the dendritic structure to yield fine-grained steels, improving thereby the microstructure and the properties. Normally, the steel castings shall be normalised after a high temperature homogenising, which is followed by annealing at a lower temperature to refine the grain size.
The heat treatments used in processing the wrought steel products are:
1. Annealing:
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Full annealing is given to steels to improve the machinability, or formability as the heat treatments results in coarse pearlite. Sub-critical annealing or spheroidising annealing is done when severe cold-forming is to be done. Process annealing is most commonly used for cold-rolled sheets strips and tubes.
2. Normalising:
As, it is difficult to control the grain size of hot-rolled steels, a normalising treatment from the usual low austenitising temperature results in a fine uniform grain size to improve ductility and toughness.
3. Quenching and Tempering:
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Plain carbon steels have low hardenability. Sections thicker than 12.5 mm are difficult to be through-hardened. But, small tools and fasteners can be given such a treatment.
4. Austempering:
As this treatment avoids additional tempering treatment, and results in obtaining lower bainite, and which is able to induce high toughness at a hardness of 50 HRC in certain steels, thin sections below 5 mm of carbon steels can be given this treatment.
Heat Treatment of Alloy Steels:
Alloy steels are used to obtain higher and uniform strengths with good toughness in sections thicker than obtainable in plain carbon steels. Table 12.4 (a) gives annealing temperatures of some alloy steels. Table 12.4 (b) gives comparison of mechanical properties of some alloy steels in process annealed and in spheroidised annealed state.
Heat Treatment of Ball Bearing Steels:
Bearing balls, rings and rollers require high hardness, wear resistance and resistance to contact fatigue, and high crushing strength. Ball bearing steels require prolonged soaking at the forging temperature before they are forged. Similarly a prolonged annealing is essential in order to spheroidise the free carbide particles to increase its machinability. The presence of nonmetallic-inclusions specially on the surface is highly undesirable as these act as stress-raisers to cause premature fatigue failure.
The carbides should be uniformly distributed in the structure. The annealing of ball bearing steels should result in a homogeneous microstructure of fine-grained pearlite with fine dispersion of alloy carbides. The hardness at this stage is 187-207 BHN, which ensures high machinability.
The cheapest bearing balls, rollers are made out of high carbon steels having carbon 1.2 to 1.5%. As these balls get worn off soon with corrosion attack, commonly, the ball bearing steels contain 1-1.4% Cr too.
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A steel having 1% C, 1% Cr of 51 mm diameter can be interrupted quenched in water, i.e., quenched in water for a minute and then withdrawn from it till surface attains temperature of 200°C from the heat in the core and then put in water again, leads to a soft and tough core of hardness 450 VPN with the surface having a high hardness of 750 VPN (hardened zone is about 2.54 mm).
Large-size rings and rollers are made from chromium-nickel carburising grade steel (C = 0.20%; Cr= 2%, Ni 4%). It is carburised at 940-970°C to get carburised case of 5 to 10 mm depth, and is finally oil-quenched from 800°C followed by tempering at 160°C.
Ball bearings for highly corrosive atmospheres are made from AISI 440C having 0.95-1.20% carbon; 16-18% Chromium, 0.75% molybdenum.