Why Tool Steels need a double Tempering Heat Treatment?

 Tempering of Tool Steels

Tempering modifies the properties of quench-hardened tool steels to produce a more desirable combination of strength, hardness, and toughness than obtained in the quenched steel. The as quenched structure of tool steel is a heterogeneous mixture of retained austenite, un tempered martensite, and carbides.

More than one tempering cycle may be necessary to produce an optimum structure. It is normally desirable to transform all retained austenite to ensure complete hardness, improve toughness, and minimize distortion during service. This can be more nearly accomplished by two or more shorter time tempering cycles than by a single and longer cycle.

In the higher-alloy tool steels, a small amount of un tempered martensite is formed from retained austenite during the cool down from the first tempering cycle. It is good practice to double temper to ensure more nearly complete transformation of retained austenite and to temper freshly formed martensite. For some highly alloyed grades of tool steel, triple or quadruple tempering is recommended.

The changes that take place in the microstructure during tempering of hardened tool steels are time-temperature dependent. Time at tempering temperature should not be less than 1 h for any given cycle.

Most manufacturers of high-speed steels recommend multiple tempers of 2 h or more each to attain the desired microstructure and properties. Maintaining recommended tempering times, temperatures, and number of tempers (a minimum of two) ensure attainment of consistent tempered martensitic structures and overcomes uncertainties caused by variations in the amount of retained austenite in the as-quenched condition. These variances are functions of differences in heat chemistry, prior thermal history, hardening temperatures, and quenching conditions. Other factors that influence the tempering requirements of high-speed steels are:

· Increasing the free (matrix) carbon content increases the amount of retained austenite in the as-quenched condition

· The amount of retained austenite significantly affects the rate of transformation, particularly for short tempering cycles. Multiple tempering is more important to attain an acceptable structure if short tempering times are used.

· Cobalt in alloys such as M42 reduces the amount of retained austenite in the as-quenched condition and accelerates the transformation of the retained austenite during tempering.

Enough time should be allowed during tempering for the temperature to be distributed uniformly throughout the tools before time at temperature is counted. This is especially true for low tempering temperatures and for tools that have large sections.

Reference

(ASM hand book of Heat Treating, 1991)