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TOOL STEEL
TECHNICAL LIBRARY

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
 

ZAPP HIGH-PERFORMANCE
STEEL GRADES:

TOOL STEELS
Z-TUFF PM
Z-WEAR PM
Z-A11 PM
Z-A11LV PM
Z-420 PM

HIGH SPEED STEELS
Z-M4 PM
Z-T15 PM
Z-M48 PM
Z-MAX PM
Z-M2
 

ZAPP PERFORMANCE PRODUCTS:
ZDM BLANKS
 

TOOL STEEL TECHNICAL TRAINING

SERIES III: Basics of Heat Treatment • Part 2

Tool steels are heat treatable because of the fact that iron has several allotropic forms (don’t panic — this gets pretty basic!). This has to do with the way the iron atoms arrange themselves in the metallic crystal structure. At room temperature‚ iron usually exists as a body centered cubic (bcc) arrangement of atoms known as alpha (α) iron (or ferrite). When heated, iron transforms to a face centered cubic arrangement of atoms known as gamma (γ) iron (or austenite). These low and elevated temperature forms of iron are shown in figure 1.

Alpha Gamma

The first step in the heat treatment of tool steels (which are alloys of iron‚ carbon and various other elements) involves heating beyond the temperature at which structure transforms to austenite (austenitizing temperature). Carbon plays an extremely critical role in this process. It is insoluble in ferrite and exists as part of a separate carbide phase in the annealed (or soft) condition. However‚ carbide will actually start to dissolve once the material becomes austenitized at high temperature. This is a reversible process and no hardening will occur provided the material is cooled slowly back to room temperature (austenite will transform back to ferrite plus carbide). However‚ rapid cooling (or quenching) from high temperature will result in iron atoms becoming forced into a new‚ highly distorted arrangement known as martensite. Fast cooling does not allow sufficient time for diffusion to occur and the dissolved carbon atoms essentially find themselves trapped between iron atoms which are attempting to transform back to the bcc structure. The crystal lattice becomes highly stressed in this condition resulting in a significant increase in hardness and strength of the material. Martensite must subsequently be tempered which helps to reduce brittleness while improving toughness and shock resistance.

Obviously there is much detail that has been left out of the discussion at this point‚ and it could be ascertained that the devil is in fact in the details. However‚ the above description provides a framework from which to examine a schematic of a typical tool steel harden and temper cycle as shown in figure 2. This depicts the series of heating‚ soaking‚ and cooling steps that accompany the basic microstructure transformations. The essential parts of the process can be broken down into preheating‚ high heat (or austenitizing)‚ quenching‚ and then the various intervals of heating and cooling that are involved in tempering (the schematic shows a double temper).

Schematic of Heat Treatment Cycle

It is relatively easy these days to program set points into the instrumentation of a modern heat treat equipment. However‚ the results ultimately depend on the actual time and temperature experienced by the parts being run, and herein lies much of the “art” involved in the process. Figure 3 shows real life data taken from thermocouples attached to a large cutting tool during salt bath hardening which provides an interesting comparison to the generic schematic shown above.

Salt Bath Hardening Cycle

The information presented in this installment is intended to provide a foundation for the ongoing “Tool Steel Heat Treatment” series. The next installment will begin to take a closer look at the individual parts of the process and will also include microstructure comparisons of samples in the annealed‚ as quenched‚ and tempered condition.

Questions or comments may be sent to Gary Maddock at gmaddock@zapp.com.

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