The Repeatable Times of Quenching and Tempering

Repeated quenching is a common occurrence. If the initial quenching fails or the tempering is excessive, re-quenching or re-tempering becomes necessary. However, repeated quenching can lead to quenching cracks due to the accumulation of quenching stress and coarsening of crystal grains, which increases hardenability. Consequently, the chances of deformation and quenching cracks also increase.

JIS standards (specifically JIS G0303 Inspection General Rules, Re-testing of Mechanical Properties) limit the repetition of heat treatment (practically referring to quenching) to twice, including the initial attempt, totaling three times. Tempering, on the other hand, has no limit on the number of repetitions, as long as the desired hardness or strength is achieved.

It’s important to note that annealing between quenching cycles changes the situation. If repeated quenching is performed without intermediate annealing (or process annealing), the amount of deformation will increase. If repeated multiple times, the workpiece may become spherical and eventually crack. Therefore, repeated quenching without intermediate annealing is limited to twice. However, with intermediate annealing, although not unlimited, multiple repetitions are possible. As the number of repetitions increases, the ease of quenching also increases, requiring greater attention to quenching cracks and deformation.

For high-speed steel, repeated quenching without annealing results in coarse and brittle grains, leading to a fish scale-like pattern on the cross-section, known as “fish scaling”. This pattern may also resemble marble, hence the term “marble fracture”. While the hardness of high-speed steel with fish scaling may not change significantly, it generally becomes brittle, a drawback that must be avoided.

Repeated quenching often indicates inadequate quenching techniques. It is advisable to increase the tempering process and ensure successful quenching in a single attempt.

A typical repeated tempering process involves the following steps:

  1. Austenitizing: Heating the steel workpiece to its austenitizing temperature.
  2. Isothermal Transformation: Quenching the workpiece in a heat bath maintained at 200°C or slightly higher until the entire workpiece reaches a uniform temperature.
  3. Tempering: Immediately transferring the workpiece to a higher temperature heat bath to temper the bainite formed during the isothermal transformation.

This two-step tempering process, also known as austempering, is characterized by a lower temperature and faster cooling rate in the isothermal transformation bath, making it suitable for larger workpieces. Additionally, the resulting tempered bainite microstructure offers superior mechanical properties.

During tempering, attention must be paid to the tempering temperature. A step-wise increase in tempering temperature, known as step tempering, can be employed to achieve the desired hardness.