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Phone: 1 888 928 9927
1 843 871 2157
ztasales@zapp.com

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 3

Tool steels are generally supplied in the fully annealed condition. In the annealed (soft) condition, the material has the best machinability which facilitates the “soft” tool making operations such as cutting, drilling and milling. The hardness of annealed tool steels varies by grade but typically runs in the range of 229 to 285 BHN with low alloyed steel grades at the lower annealed hardness range and higher alloyed grades at the upper end of the range.

Figure 1 below shows the microstructure of annealed Z-Wear PM material. You can be observe how its ferritic matrix contains a large number of both large (primary) and small (secondary) carbides. These appear as large and small globular shapes outlined in dark gray. In fact most of the alloy and carbon are in the form of carbide when the grade is in this condition.

Figure 1

Figure 1: Annealed Z-Wear PM (ferrite and carbide)

Let’s take a step by step look at the process of transforming the soft, annealed tool steel into hardened and tempered material ready to be finish ground and put into application. Each step in this process has a specific purpose, and the final outcome (a properly hardened tool with an appropriate hardness and tempered microstructure) is dependent on the correct execution of each individual step.

STEP 1. PREHEATING

In general, the heat treatment of tool steels involves a batch process performed in either a vacuum furnace or salt bath. After fixturing and loading into baskets, the tools are ramped up to a preheat set point. This is typically done at around 1500 to 1600°F which is near to the point where the material transforms from the ferrite and carbide structure to austenite. Preheating minimizes the surface to center temperature differential for uniform heating and, therefore, reduces residual stresses to minimize distortion in tools. Furnaces with programmable controls may be set to have multiple preheats which further aids in uniform heating, as well as, monitoring time and temperature control throughout the load.

STEP 2. AUSTENITIZING TEMPERATURE (HIGH HEAT)

During austentizing, the material arrives at the prescribed high heat or hardening temperature. Austenitizing is a critical step in the heat treatment process. It is critical to achieve both the correct temperature level and the necessary time at this temperature. At this point, the tool steel is fully austentitized and begins to dissolve into solution the necessary amount of carbon and alloy to subsequently harden. Insufficient time and temperature can result in low hardness while excessive time and temperature will result in serious damage to the material due to overheating. The more simple tool steels have relatively straight forward recipes for hardening while the high speed grades and higher alloy PM grades require a bit of a selection process to determine the optimum parameters given the desired hardness. The heat treatment of the PM grades can actually be tailored to suit specific applications.

STEP 3. QUENCHING

If austenitizing is akin to putting the “bullets into the gun”, then it is quenching that “pulls the trigger” on the hardening process. Quenching is the process of rapidly cooling from the high heat such that the material does not have time to return to its original soft, annealed condition. Because the carbon and alloy dissolved at high temperature essentially become “trapped”, the quenched material is forced into a martensitic matrix which has very high hardness and strength. The actual transformation of the material from austenite to martensite begins at around 400 to 500°F and continues progressively as temperature is further decreased. The ideal quench rate is dependent upon the composition of the tool steel. Some tool steels require oil quenching or even water quenching, but the most common alloys today are said to be air hardening. In any case, strong positive cooling is critical to the development of an optimum heat treated structure.

Figure 2 shows the structure of Z-Wear PM in the as-quenched condition after vacuum hardening from an austenitizing temperature of 1900°F. In comparison to the annealed structure in Figure 1, most of the small carbides are no longer visible because these carbides were dissolved and have been trapped into solution. Figure 2 also shows some of the austenite grain boundaries with fine spacing which is an indication of proper hardening and good tool steel quality.

Figure 2

Figure 2: As-Quenched Z-Wear PM (martensite plus retained austenite)


STEP 4. TEMPERING

The final steps in the heat treat process are tempering. Although not as complex as hardening, tempering is equally critical. Tempering specifically improves as-quenched structure which is highly stressed and inherently brittle. During tempering, the material is heated to an intermediate temperature which is grade and hardness dependent. This temperature is typically in the range of 400 to 1100°F and held for several hours before cooling back to room temperature. The goal of tempering is to arrive at a structure that consists entirely of well tempered martensite (tougher structure). This sounds simple, but can involve challenges especially in regard to the high alloy tool steels and high speed grades. High alloy tool steels retain a percentage of the austenite after quenching which can subsequently transform to untempered martensite during the temper process. As a result, these alloy types require multiple tempers (sometimes referred to as having a double or triple draw) in order to achieve a stable, fully tempered structure.

Figure 3 show the microstructures of hardened Z-Wear PM following a single and double temper, respectively, at 975°F. The degree of darkening within the matrix provides a good indicator of the progress made due to the fact that the specimen will more readily etch in the tempered condition (while as-quenched structure does not etch much at all). The image on the right reflects a proper final heat treated structure.

Figure 3

Figure 3: Tempered Z-Wear PM


This series has thus far provided an overview of the changes that occur during the heat treatment of tool steels, as well as, why each step is critical to the final performance in tooling applications. Future series installments will take a more thorough look at each of the heat treat steps. Along the way we will cover some of the equipment considerations, provide a few hints and tips, and also consider some of the areas that can go wrong.

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

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