Induction hardening

Heating a component by induction, followed by oil or water quenching.

With steels having carbon contents of 0.4/ 0.5%, it is possible to obtain a hard case for wear resistance or to increase fatigue strength by means of induction hardening. A copper induction coil is made to surround the workpiece and the surface temperature is raised to above the upper critical temperature in a few seconds, by the heating effect of the induced electromagnetic current in the surface of the workpiece. A quenching spray follows the inductor and provides rapid cooling to produce full transformation of the heated surface.

The depth of heat penetration and therefore the hardening effect is proportional to the frequency of current in the inductor, the power generated, the steel composition of the workpiece and the heating or dwell time. Thus, for one generator operating at a given frequency it is possible to obtain various case depths. Considerable skill is required in ‘setting up’ the workpiece to be hardened, in order to obtain the ideal combination of dwell time and quench delay, such that an optimum case to core hardness profile is produced. Once programmed, modern handling units are capable of operation by less skilled staff.

There are two main methods of induction hardening, ‘single shot’ hardening, where the whole area to be hardened is heated at once e.g. small gears or shafts, spun inside the induction coil and the complete periphery heated and quenched. Alternatively, the workpiece can be traversed, as with long shafts, where the area to be hardened is progressively heated and quenched by a moving coil followed by a quench ring, or in the case of gears the tooth by tooth method of hardening. Surface hardnesses between 50 and 60 Rc are achievable by this process, depending upon the steel composition of the work piece.

Since induction hardening only uses electrical energy to heat the surface zone of a component, it is the most energy and therefore cost effective method for surface hardening many components. It has, as a piece part operation, the disadvantage that for small volumes of components it can be labour intensive. High frequency (HF) induction sets are used to heat treat small components up to 2 inches in diameter or localised flank hardening of areas of larger components and medium frequency (MF) induction sets are used to heat treat larger components. The HF method is particularly suitable when large volumes of components of relatively simple shape, such as pins, bushes, studs, and camshafts are required to be hardened. Automated handling equipment can be readily applied and the resulting hardening facility may be easily incorporated into a manufacturing line next to machining and finishing stations. The effectiveness of induction hardening depends upon the manufacture of a close fitting copper induction coil, which involves considerable product knowledge and skill. Electronic control of the power input provides control of the temperature regime but the induction method has the disadvantage that the point effect of sharp edges causes local overheating and may even lead to localised melting. Therefore care has to taken when components have sharp edges or contain details such as threads or circlip grooves. Quenching is achieved by a linked quenchant spray system, which closely follows the heating coil as they both traverse the component surface, normally using proprietary oil mixtures or polymer quenchant. Hardened depths of up to 1mm are usually achieved with the HF method, whereas MF sets can economically provide hardened depths of up to 5mm. The latter process is applied to large components such as shafts and gears, which may be flank hardened tooth by tooth.