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Phone: 1 888 928 9927
1 843 871 2157


SERIES I: Introducing the Concept of Tool Steel Microstructure

SERIES II: Typical Failure Modes for Cold Work Tooling and Their Association with Microstructure

SERIES III: Basics of Heat Treatment • Part 1

SERIES III: Basics of Heat Treatment • Part 2

SERIES III: Basics of Heat Treatment • Part 3

SERIES III: Basics of Heat Treatment • Part 4

SERIES III: Basics of Heat Treatment • Part 5

SERIES III: Basics of Heat Treatment • Part 6


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SERIES III: Basics of Heat Treatment • Part 4

The first three issues of Zapp’s tool steel heat treatment series have examined the process primarily from a metallurgical point of view. We have shown how the heat treat process can be described in terms of the metallurgical transformations that take place relative to the microstructure of the material. We can now begin to look at the process in further detail by considering the practical aspects of what it takes to get the job done precisely in the commercial realm.

In theory, one could harden tool steel with a torch and a bucket of oil (and some might try in a pinch). However, unless performed by a magician, results by this method will no doubt be disappointing. Although sometimes viewed as a black art, the science of heat treating actually involves following carefully controlled heating and cooling steps in order to accomplish specific transformations within the material. Understanding and controlling the heat treatment process are paramount. In this scenario, the critical process variables consist of the following:

  1. Heating (ramp) rates
  2. Time and temperature of the parts at critical set points
  3. Cooling (quench) rates
  4. Atmosphere for surface protection

The importance of the thermal processing variables (items 1 thru 3) stands to reason (much like baking a cake). However, the need for atmosphere control needs some further discussion. This is critical because oxidation, scaling, and decarburization can occur if the work piece is exposed to air while at elevated temperature. The result is not only a rough open surface, but also a loss of hardness (decarburization) requiring significant grinding to remove the decarb layer to restore the proper surface hardness and integrity of the tooling. Evidence of decarb and scaling are readily apparent when looking at a microstructure as shown in Figure 1. In this type of view, one can directly measure the amount of removal that would be needed to get down to normal structure.

Figure 1

Figure 1

The degree of difficulty involved in the heat treatment of tool steels is very much related to the alloy content of the grade in question. Higher alloy grades tend to be much less forgiving, and require strict process control that can become more involved as demonstrated by the furnace chart shown in figure 2. Figure 2 is also an example of how a modern vacuum furnace can be utilized to manage the complete hardening cycle including vacuum/pressure, ramp rates, set points, and quench rates. Output from a load thermocouple was used to guarantee proper control of actual part time and temperature. The cycle consists of 4 preheat steps, a high heat soak, and a high speed quench including an isothermal hold to minimize distortion.

Figure 2

Figure 2: Furnace Chart for Processing High Alloy Tool Steel

An example of a typical load that has just been run through such a hardening cycle is shown in Figure 3. It is apparent that the parts are very clean with little in the way of appearance to indicate they have been hardened. The picture also reveals how the parts were fixtured to allow uniform heating and cooling. It is also apparent how a load thermocouple in a “dummy” slug was used to maintain accurate control of the actual load temperature

Figure 3

Figure 3: Load of HSS Tool Blanks after Hardening

Up to this point we have established the need for process control and have identified the elements that must be given consideration if the job is to be done correctly. Vacuum processing has in fact become the most common way of treating tool steels, but salt baths continue to play an important if somewhat limited role. There also continues to be examples of very basic “tool room” type processing that can be found in everyday use. In the next installment, we will continue looking at the equipment used and compare the pros and cons of the methodology required in each case.

Questions or comments may be sent to Gary Maddock at

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