Hardening: Introductory Consideration

Hardening of tool steels, as shown in Fig. 5-2, is accomplished by three heat-treating steps: austenitizing , cooling or quenching for martensite formation, and tempering. Each heat treatment step produces major changes in microstructure, which when properly controlled lead to the final properties required for a given application. However, the microstructural changes contribute to and are accomplished within a framework of dimensional changes. These changes must also be understood and controlled in order to minimize distortion, residual stresses, and possibly cracking during the final stages of heat treatment.

Schematic diagram of tool steel heat treatment steps for final hardening.

Thermal expansion occurs on heating to the A_c1 temperature for the steel. At that point, contraction occurs as austenite, with its close-packed crystal structure, replaces ferrite, with its more open crystal structure. When the ferrite is completely replaced by austenite, thermal expansion again occurs with continued heating of austenite and any residual carbides.

On cooling, the austenite and carbide microstructure contracts. If the cooling rate is low, as is the case for annealing, the austenite starts to transform to ferrite and additional carbides at the A_r1 temperature for the steel. The formation of the ferrite-carbide microstructure causes expansion, as shown by the dashed line in Fig. 5-8, and when austenite transformation to ferrite and cementite is complete, thermal contraction again occurs.

5-8- Dilation of a 1% C steel on heating and either slow cooling (dashed line) or quenching (solid line).

Eventually, at the M_s temperature for the steel, martensite forms, and significant low-temperature expansion occurs as the bet crystal structure of martensite replaces austenite. The densities and thermal expansion characteristics of tool steels are alloy dependent, and Table 5-2 lists these properties for selected tool steels.

The great variations of chemical composition of the various types of tool steels cause the microstructural changes to be accomplished at a variety of temperature ranges and heating and cooling conditions; the specific heat treatment parameters recommended for each class of tool steels are given in Table 5-3 .

Austenitizing for Hardening

Austenitizing for hardening is accomplished by heating the spheroidized carbide-ferrite microstructures of tool steels to an appropriate austenitizing temperature. The use of the Fe-C diagram for alloyed tool steels is only approximate, and the austenite-carbide two-phase field may be considerably expanded to lower carbon contents and higher temperatures by the strong carbide-forming elements commonly used for tool steels.

The very first consideration regarding the austenitizing of tool steels is the need for preheating many of the highly alloyed grades. an annealed tool steel microstructure will thermally expand on heating to the austenitizing temperature and will contract during austenite formation .If the temperature of a part is not uniform, as a result of temperature differences between thr surface and interior, the dimensional changes occur at different times as a function of position, and localized volume changes may cause stresses to be generated. For example, rapidly expanding regions, by virtue of position and higher temperature, will cause tensile stresses to be exerted on regions that have not expanded as rapidly. These stresses may cause plastic flow or distortion and, if high enough, may cause cracking, especially in highly alloyed tool steels with high carbide contents and low hot ductility and fracture resistance. Preheating establishes uniform temperature throughout a workpiece prior to heating to the final austenitizing temperature and minimizes the thermal shock and localized dimensional changes that would develop on heating a cold workpiece directly to the hardening temperature. Not all tools require preheating. For example, small parts in which temperature gradients on heating are minimal, and parts with simple geometries that would minimize stress concentrations due to section changes, may not require preheating. In contrast, high-speed tool steels hardened in salt baths may be subjected to two preheating steps. one of the preheat steps is performed below the lower critical temperature, prior to austenite formation, and the other is performed after austenite has formed, prior to heating to the final austenitizing temperature, which for high-speed steels is quite high. Preheating establishes uniform temperature throughout a workpiece prior to heating to the final austenitizing temperature and minimizes the thermal shock and localized dimensional changes. Preheating and final austenitizing are commonly accomplished in adjacent salt baths or furnaces set at the respective temperatures, but depending on production requirements and grade of steel, may also be performed in a single furnace.

 Austenitizing for hardening must accomplish several critical functions for the subsequent quenching and tempering heat treatment stages:

• Establish the volume fraction of undissolved alloy carbides that will contribute to wear resistance
• Adjust the chemistry of the austenite to provide the required Ms temperature, and thus a reasonable balance of martensite and austenite after quenching
• Adjust the chemistry of the austenite to provide the required hardenability in order to maximize the amount of martensite formed on quenching or cooling from the austenitizing temperature
• dissolve sufficient alloying elements, which on quenching will be placed in supersaturation in the martensite and thus be available for precipitation and secondary hardening during tempering
• Control austenitic grain size to prevent coarsening and associated impaired fracture resistance
• Remarkably, all of these very critical functions are accomplished during a single heating in the temperature ranges recommended for hardening the specific grades of tool steels. The key to successful austenitizing is to control the extent of alloy carbide dissolution as the tool steel composition approaches equilibrium at high temperatures. As alloy carbides dissolve, the alloying elements and carbon, which once made up the carbides, partition into the austenite.

Fig 5 – 1 0 Schematic diagram of high-speed tool steel hardening heat treatment steps. Two stages of preheating are shown.

Table (5-2) Density and thermal expansion of tool steel

Table 5-3 Hardening and tempering temperatures and procedures for tool steels

Fig. 5-9 Fe-C phase diagram showing maximum temperatures for austenitizing for hardening carbon tool steels