Basics of Tempering - compilation of news

Quenched steel is hard and brittle. To enhance its plastic properties and relieve internal stresses, tempering is performed. This heat treatment process involves heating the hardened steel to a temperature below A_c1, holding it at that temperature, and then cooling it.

Tempering is not required after hardening with an isothermal transformation.

Low, Medium and High-temperature Tempering

Tempering is classified into three types based on temperature: low-temperature tempering, medium-temperature tempering, and high-temperature tempering.

Low-temperature tempering takes place between 150 and 250°C. It primarily relieves quenching stresses while preserving high hardness and wear resistance. This process is used for tools, gauges, some springs, carburized parts.

Medium-temperature tempering takes place between 250 and 500°C. It improves tensile strength and elasticity while maintaining adequate impact resistance, though with a significant reduction in hardness. It is commonly applied to hardened springs, dies, leaf springs, automotive parts, and impact tools.

High-temperature tempering takes place above 500°C but below A_C1. It enhances impact resistance while maintaining sufficient tensile strength and elasticity. Hardness is reduced to facilitate machining. The combination of hardening and high tempering is referred to as toughening, which will be discussed later in this article.

Tempering duration in practice ranges from several minutes to a few hours. Longer tempering times ensure uniform properties across the material.

Cooling

Carbon steels are typically air-cooled, while certain alloy steels containing Cr, Ni, or Mn require water or oil cooling to prevent tempering brittleness. However, cooling in water or oil after tempering above 400°C may introduce internal stresses. At temperatures below 300°C, the cooling rate has minimal significance.

Temper Embrittlement and Tempered Martensite Embrittlement

Some steels are susceptible to temper embrittlement, which results in a decline in mechanical properties along with reduced impact resistance and hardness. It is classified into two types:

• Tempered Martensite Embrittlement ("TME") typically occurs in low-alloy steels tempered within the temperature range of 250–450°C. This type of brittleness cannot be prevented, so steels prone to it are usually tempered at lower temperatures.

• Temper Embrittlement ("TE") is most common in chromium and nickel-chromium steels tempered between 450–550°C. It can be partially mitigated by accelerating the cooling process by using oil.

Some steels are vulnerable to both types of tempering brittleness simultaneously.

Toughening

Toughening is a process combining quenching and high-temperature tempering. After hardening, the material is cooled to the tempering stage. This method optimizes mechanical properties while maintaining machinability. It is commonly used for processing structural steels (e.g., 30H, 45H, 50H, 30HM, 35HM), tool steels, and high-speed steels. Toughening generally preserves the mechanical property balance of normalised steel while slightly enhancing them for practical applications.

The effectiveness of heat treatment is measured by the ratio of R_e (yield strength) to R_m (tensile strength).

Example of toughening: the hardening and tempering guidelines for cobalt-molybdenum high-speed steel SK8M / 1.3247:

Hardening involves three heating stages. First, the workpiece is heated to approximately 580°C. In the second stage, the temperature is increased to 850-950°C. Finally, the material undergoes austenitization by soaking at 1170-1210°C. Cooling is performed at 550-570°C in an SH430 salt bath. The first tempering process occurs at 560°C immediately after hardening, followed by a second tempering at 530°C. Each stage is completed by slow air cooling.

Austempering, martempering

Austempering and martempering are described in my article on quenching and hardening.

Bibliography

Mikołaj Scelina, Atlas Metalograficzny struktur, Wydawnictwa naukowo-techniczne, 1964

Leszek Adam Dobrzański, Metaloznawstwo i obróbka cieplna, Wydawnictwa szkolne i pedagogiczne, 1986

Kornel Wesołowski, Metaloznawstwo i obróbka cieplna, Państwowe wydawnictwa szkolnictwa zawodowego