Induction hardened steel

The principles for selecting steel for induction hardening are similar, but not identical, to those for steel hardened by other methods. The hardenability of ordinary carbon steel increases with increasing carbon content. For example, steel with a 0.2% carbon content, or 1020 steel, can only be hardened to HRc 48, while steel with a 0.45% carbon content, or 1045 steel, can be hardened to HRc 60. Adding alloying elements such as chromium, nickel, molybdenum, and tungsten can improve the hardenability of steel by shifting the ridge of the S-curve to the right.

Induction hardened steel

For example, during through hardening, ordinary carbon steel requires rapid cooling or quenching rates to avoid the ridge of the S-curve and prevent softening. Due to slow heat conduction and slow cooling, the inner layer beneath the surface hardened layer may soften due to incomplete martensitic transformation. Even during surface hardening, heat conducted from the center of the workpiece to the surface tends to slow the surface cooling rate, similar to the effect of forming a soft pearlite surface. Quenching with a quenchant more intense than oil often results in excessive deformation. Therefore, by adding a small amount of alloying elements, oil quenching can minimize deformation while still achieving the desired hardness.

Another reason for adding alloying elements is to improve the mechanical strength of the steel in terms of tensile, yield, impact, and fatigue properties. The desired hardness is achieved by improving strength.

Through quenching

Surface induction heating significantly influences the choice of steel. Because only the surface layer is heated, there are no problems with incomplete phase transformations below the surface. Furthermore, rapid surface heating has been shown to allow for rapid quenching without the risk of deformation due to internal stresses. Because the center of the workpiece is neither heated nor quenched, its original strength is retained. These two effects mean that plain carbon steel can often replace more expensive alloy steels quenched using other heating methods.

Another reason why plain carbon steels are preferred for induction heating is that they dissolve quickly into solid solution and dissolve at lower temperatures than alloy steels. This allows for shorter heating times while reducing heat losses to the center of the workpiece and improving productivity. A further reason is the ability to use water quenching, which is superior to oil quenching. Preliminary heat treatment of steel is crucial because most induction heating processes use short heating times, which are insufficient to achieve uniform diffusion in steels with coarse or uneven grains. Free ferrite often remains after heat treatment, causing soft spots. Annealing, spheroidizing annealing, and similar preliminary heat treatments should be avoided whenever possible.

The ideal preliminary heat treatment is normalizing, which produces a well-distributed, uniform pearlite structure. Annealing before induction hardening produces a deep hardened layer but a low surface hardness (Rockwell HRC 52). Normalizing before induction hardening produces a medium hardness (Rockwell HRC 53). Tempering before induction hardening produces a thin hardened layer (0.75 mm) with a very high hardness (Rockwell HRC 62). Normalizing before induction hardening actually produces a good metallurgical structure with a deep hardened layer. Some steels require a short pause between heating and quenching to complete the carbon dissolution process. To eliminate this pause, other hardenable steels (SAE 1000 and 1300 series steels) have been developed.