Steel parts are hardened to obtain certain properties, such as tensile strength, fatigue strength and wear resistance, required in specific applications to achieve optimum performance. Hardening may be done either in the selected areas of the workpiece or throughout the entire part section.
The induction hardening procedure involves heating a part to a temperature higher than the austenitizing temperature to achieve a completely austenitic microstructure, and then cooling the steel at a rate fast enough to transform the austenite to a fully martensitic structure. A wide variety of steel parts lend themselves to hardening by means of induction heating followed by quenching.
Induction surface-hardened parts having a hard outer case and ductile core are strong and typically have high compressive stresses at the surface. Compressive stresses are important for improving the fatigue properties of the component, allowing the delay of crack initiation and its propagation during service [1].
Steel selection for parts to be induction hardened plays a critical role in determining if the required properties can be achieved in the hardening process.
Steel selection
The selection of the proper steel grade for use in a particular application depends on the specifics of the component's working conditions, required hardness and cost. Plain-carbon steels and low-alloy steels are the least expensive steels being successfully used for a variety of hardening applications. It is important to remember that the carbon content of steel plays a critical role in the determination of the maximum achievable hardness, as well as how it affects the amount of retained austenite and steel hardenability.
Medium-carbon steels (e.g., AISI 1038 to 1055) are the most common steels used in industry. These steels are used to make, for example, a variety of transmission components (shafts, gears, etc.), engine components (camshafts, crankshafts, connecting rods, rocker arms, etc.) and fasteners (bolts, screws, studs, etc.).
Low-carbon steels, on the other hand, are used where toughness rather than high hardness is required, such as in clutch plates and pins for farm equipment.
A wide application range of high-carbon steels in industry is limited due to the low ductility, poor machinability and higher cost of these steels compared with medium-carbon steels. At the same time, there is a variety of applications including valve-spring wire, drill bits, and other cutting tools, where high-carbon steels (such as AISI 1060 to 1080) provide a noticeable advantage over medium- and low-carbon steels. It is wise to remember that some high-carbon steels have a tendency toward cracking during heating as well as quenching. Therefore, process parameters should be modified when induction hardening certain high-carbon steels.
Proper prior microstructure important
It is imperative to mention the importance of having "favorable" metal conditions prior to induction hardening. An example of a favorable initial metal structure is a quenched and tempered martensitic structure having a hardness of 30-34 HRC, which leads to fast, consistent metal response to hardening with the smallest shape/size distortion and minimum amount of grain growth. Normalized, fine grain structures are also often considered as preferable microstructures.
In contrast to quenched and tempered and normalized structures, steels having large carbides (spheroidized structures, for example) generally have a poor response to induction hardening, resulting in the necessity of having prolonged heating and requiring higher hardening temperatures. The existence of large carbides may also lead to hardness data scatter [1].
Although plain-carbon steels, being the least expensive steels, are widely used in industry, there are many engineering applications where the properties of plain-carbon steels and low-alloy steels are not adequately suitable for meeting a particular engineering requirement or combination of requirements. Such requirements could include the necessity to increase the depth of hardness, to reduce the grain growth and to improve corrosion resistance, abrasion resistance, etc. It is often not only required to achieve just a certain property of the steel component, but also to achieve a combination of properties that are often contradictory. For example, it might be necessary to obtain a combination of strength, toughness and ductility, as well as to improve the mechanical properties of the steel at elevated or low temperatures. Alloy steels are also often induction hardened.
Steels having high carbon contents are more prone to cracking. Although the carbon content of steel has the great influence on its properties, there also are other elements that affect properties and crack sensitivity. The extent depends upon the amounts and combinations of elements present.
For example, the amounts of sulfur and phosphorus in carbon steel should be minimized to avoid brittleness and crack sensitivity. Sulfur reacts with iron, producing hard, brittle iron sulfides (FeS) that concentrate at grain boundaries. FeS also has a relatively low melting temperature. This combination can lead to grain boundary liquation, increasing brittleness and sensitivity to intergranular cracking [1,2]. FeS in carbon steels is minimized by the addition of manganese to form manganese sulfides (MnS) that are distributed within grains rather than at grain boundaries, creating a less brittle microstructure.
Certain alloy steels even with medium carbon content (AISI 4140 and 4340, for example) are prone to cracking under rapid cooling during spray quenching, and, therefore, require special considerations. This situation can be amplified when hardening complex shaped parts.
Part-geometry factor
Complex-shaped parts having geometrical stress raisers (for example, holes, sharp corners, grooves, undercuts, diameters changes, etc.) can present some challenges to avoid cracking and to obtain the required hardness pattern. Sharp corners and poor chamfering or rounding of holes and edges can lead to the occurrence of local overheating, resulting in excessive thermal stresses and excessive grain growth, which could cause the formation of cracks. Often product engineers prefer to have lower hardness in the undercut area. That is, they sometimes prefer having martensitic-bainitic structure instead of a fully martensitic microstructure. This provides some ductility in the undercut area and allows reducing the probability of cracking and premature failure.
Parts consisting of a combination of thick and thin sections present a different challenge due to nonuniform heating characteristics. For example, during heating, the thick sections may not come up to the required temperature as quickly as thin sections. This condition is addressed by using induction coil profiling and special process settings to help overcome the problem [1].
Thickness variations in parts having a complex geometry can create some difficulties in obtaining the required heat treat pattern during quenching as well. For example, the cooling rate of thinner sections is markedly different than that for massive sections, which can require the use of special adjustments in inductor design and/or quench blocks.
The proper choice of design parameters (applied frequency, power density, coil geometry, steel selection, etc.) allows the heat treater to obtain the required heat treating pattern even in cases when undesirable combination of electromagnetic end effect and thermal edge effects seem unsuitable for hardening by induction.
Figure 2 shows an induction machine for hardening the working surface of wrench jaws, as well as etched surfaces, which reveal the different hardness patterns of parts having appreciable differences in section thicknesses near the hardened surface area.
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