Hardening and Tempering with Induction

What is Hardening?

Hardening is a process that increases the hardness of a material while decreasing its ductility. This technique is used to strengthen surfaces that are subject to high wear, thereby enhancing the overall longevity of parts. There are various methods of hardening, each suitable for different materials. One commonly used technique is induction hardening, which is often applied in a specific process known as quenching and tempering.

What is Quenching & Tempering?

Quenching and tempering is a hardening process specifically applicable to medium to high-carbon steel. Initially, the steel is heated to a high temperature, causing its crystal structure to transform from ferrite to austenite. In this state, the steel can absorb more carbon than it can under normal conditions. The steel is then rapidly quenched using water, oil, or a water-polymer solution in induction equipment. This rapid cooling prevents the formation of carbon precipitates, ensuring the steel achieves the desired hardness.

As the steel cools to a lower temperature, it tries to revert to its original low-temperature state. However, because it has absorbed more carbon than its original state can accommodate, the steel transforms into a different crystal structure known as martensite. Martensite is exceptionally hard due to its distorted crystal structure and high carbon content. While the steel achieves high hardness at this point, it also becomes quite brittle.

Tempering is the subsequent step, involving reheating the steel to a lower temperature to slightly reduce hardness and, consequently, brittleness. The specific temperature for tempering depends on the desired hardness level. Once the target hardness is achieved, the steel undergoes another quenching process to prevent any residual heat from further affecting the tempering process.

Quenching & Tempering with Induction

Induction technology is predominantly employed in Quenching and Tempering procedures due to its notable advantages. Achieving the desired hardness profile in this process requires exceptional precision in both heating and quenching the steel. Even slight variations, such as heating for a moment too long or quenching at an incorrect temperature, can lead to significant disparities between parts. Therefore, maintaining precise control over the process is crucial, and induction technology excels in providing superior control compared to other heating methods.

The automatic handling and fixturing of components for heating and quenching contribute to high production rates and consistent results across various parts. Induction is the fastest method for hardening and heat treating, resulting in minimal distortion, no surface decarburization, a fine-grain microstructure, and meticulously controlled hardness patterns. For any facility aiming to elevate production to the next level, induction emerges as the ideal solution.

Induction Case Hardening

Selective induction case hardening can enhance the performance of parts by providing hybrid mechanical properties: hardness on wear surfaces and ductility in the core for impact resistance.

Selective induction hardening can rapidly apply heat to a specific area of a part, resulting in a layer, or case, of hardened material. This is ideal for parts that experience high stress during operation and require a combination of mechanical properties, such as high yield strength, fatigue resistance, and wear resistance.

The precise hardness pattern can be controlled by adjusting the frequency used, induction coil geometry, power level, and the part's position in the coil. Radyne systems ensure highly consistent hardness patterns from part to part due to industry-leading precision. Rotation during heating ensures a uniform case.

Progressive Induction Hardening | Induction Scanning

Frequently, parts require surface hardening in selected areas to provide optimal performance and long service life. Induction hardening, using progressive heating and quenching, offers an economical way to process shafts and other parts.

By progressively passing a steel shaft through a heating coil and into a water quench, the outer surface can be heated, quenched, and hardened without affecting the core. When a completely uniform case is required, it is often necessary to rotate the shaft. The coil and the associated water quench typically form a single unit, as the position of the water quench relative to the inductor is crucial. The water supply is often fed through the coil itself, as illustrated.

Controlled scanning of shafts through the induction coil and quench ring while rotating produces consistent case depths over adjustable lengths of the shafts, all within an automated cycle.

Control of the case depth depends on the power input from the generator and the downward speed of the shaft through the induction coil. The length of the hardened section is controlled by a pyrometer, which is set to meet specific requirements. Once the desired case depth and hardness patterns are achieved by adjusting the equipment parameters, these settings can be consistently applied to ensure a repeatable process across all parts.

While the described application involves surface hardening, this method is adaptable for other heat-treating operations that require progressive heating, such as annealing and tempering.