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Changes in Mechanical Properties During Tempering Heat Treatment

Tempering heat treatment is a heat treatment operation in which the workpiece is heated to a temperature lower than the critical point after quenching, held for a certain period of time, and then cooled to room temperature. The purpose of tempering heat treatment is to obtain the required stable structure and performance, and to eliminate or reduce quenching internal stress.

1. Changes in Hardness and Strength of Tempering Heat tTreatment

Various carbon steels have similar changes in hardness and strength during tempering. The general trend is that as the tempering temperature increases, the hardness and strength decrease. Adding alloying elements to steel can reduce the tendency of hardness and strength.

2. The Change of Tempering Heat Treatment on Plasticity and Toughness

During tempering heat treatment of quenched steel, as the tempering temperature increases, the internal stress and microstructure change. While the hardness and strength continue to decline, the plasticity continues to rise. However, not all steel grades increase in impact toughness as the tempering temperature increases, and many steels may experience a decrease in toughness in the two temperature ranges. This phenomenon in which the impact toughness decreases with the increase of tempering temperature is called “tempering brittleness”.

3. Tempering Brittleness of Steel

The first type of temper brittleness: temper brittleness that occurs between 250-400 ℃, also known as low temperature temper brittleness. Almost all steels have temper brittleness. The methods to avoid are: reduce the content of impurity elements in steel; add Al to deoxidize or add Mo, W and other alloying elements that can reduce the first type of temper brittleness; use the heat treatment process of austempering instead of the heat treatment process of quenching and tempering.

The second type of temper brittleness: temper brittleness that occurs between 450-600 ℃, also known as high temperature temper brittleness. The second type of temper brittleness is affected by the chemical composition of the steel and the heat treatment process, and is also closely related to the cooling rate after tempering. Slow cooling increases the brittleness, and the lower the cooling rate, the greater the degree of embrittlement. The rapid cooling can eliminate or reduce the second type of temper brittleness.