The Critical Aspects of Preparing Tool Steels Through Heat Treatment

Overview

The heat treatment of tool steel is one of the most important aspects of the final tool. Without proper heat treatment, the quality and functionality of the tool is degraded to the point where it becomes defective and unusable. A correctly designed heat treating process ensures that the final product, the tool itself, functions according to design and intent, and that it will meet all promulgated performance specifications.

A Process of Molecular Modification

Without delving into the complex metallurgical chemistry of the heat treating process, it’s important to understand the basic principles of why heat treating is so important. There are three fundamental phases that tool steel typically progresses through during a heat treatment protocol: annealed, austenite, and martensite.

First, the steel itself is an alloy created by combining carbon with iron. Other elements can be added to the mix as well to give the final product different characteristics based on tool performance requirements. For example, the addition of the carbon to iron makes the final product, steel, stronger. If chromium is added to the mix, the resulting metal, called stainless steel, does not oxidize the same way iron does, making the final tool product easier to clean and maintain.

The process of molecular modification is extremely critical to the quality—and ultimate value—of the final product. In order to obtain the high quality and valuable tool steel, the heat treating process must be accomplished with an exceptional amount of precision and uniformity during every step and cycle.

Tool Steel Microstructure

The phases that define the process of heat treating tool steel alter the microstructure of the steel itself. Observable under a microscope, heat treatment rearranges the atoms of the iron, carbon, and any other metal components, which serves to give the final material specifically desired properties.

Here are explanations of the three heat treatment phases of the tool steel heat treatment process. Once again, the speed at which the tool steel reaches the desired phase and the duration of the phase itself has a significant impact on the overall effectiveness of the heat treating process and the quality of the final tool steel.

Annealed Phase

Before heat treatment, tool steel is typically supplied in an annealed state. Annealing actually reduces the hardness of the tool steel making it easier to work with. Annealing requires heating the tool steel alloy to a precise temperature for a specific period of time. The precision of this process of heating and cooling is consistent throughout all aspects of the heat treating process. The various durations of the heating and cooling cycles, as well as the temperatures at which the steel is treated, must be extremely precise and closely controlled.

Austenite Phase

Austenite, also known as gamma-phase iron, is the result of a micro atomic process where high heat alters the crystal structure of ferrite. The process of creating austenite, called austenitization, is the first step in an overall heat treating process. Austenization is important because in its altered state, austenite can absorb more carbon into its molecular structure. Based on further heat treating processes and how those processes are carried out, the metal takes on additional desired properties, such as increased hardness or tensile strength, to name two. Altering—and improving—the mechanical properties of the final tool steel product is an important step in the manufacturing of any final products that use the altered steel.

Austenite takes its name from Sir William Chandler Roberts-Austen, who pioneered the process of austenitization.

Martensite Phase

Technically speaking, martensite refers to any crystalline structure that results from a process that does not displace large numbers of atoms, called displacive transformation. Instead, martensite is formed through a diffusionless process that creates miniscule manipulations of the atomic structure of the atoms to create different properties in the material. The process of creating martensite is called a martensitic transformation.

A martensitic transformation occurs when heated steel is cooled very rapidly, thereby preventing the atomic structure from slowly rearranging into equilibrium positions. The end result of a martensitic transformation is an exceptionally hard steel.

Although very hard, the atomic structure of tool steel in martensite form causes the material to be extremely brittle and therefore unusable for tools. The additional steps of the overall heat treating process serve to eliminate this characteristic.

The process of martensitic transformation was named after Adolf Martens, a prominent 19th century German metallurgist.

Basic steps of Heat Treating Tool Steel

There are four basic steps in the process of heat treating tool steel: Preheating, Heating (also caused austenitizing), Quenching, and Tempering. Depending on the tool steel being treated and the ultimate applications for which it is intended, other steps can be added to the process as well.

The temperature of the treatment, the duration of the treatment, and the frequency of the treatment (for example, if a certain step must be done multiple times) are all dependent on the type of tool steel that is being treated, as well as the end product that the tool steel will be used for.

Pre-Heating

Heating tool steel rapidly from room temperature to the point where the atomic structure changes to austenite can significantly degrade or completely destroy the product. Depending on the type of tool steel in process, this target temperature can range anywhere from 1400° to 2400° Fahrenheit. Transforming tool steel from the annealed phase to the austenite phase alters the volume of the steel.

Rapidly heating tool steel to these temperatures can cause thermal shock, which in turn causes the tool steel to crack. Additionally, depending on the shape and configuration of the tool steel, rapid changes in volume can cause it to warp to a point where it is unusable.

These problems can be avoided by a thorough pre-heating process that takes the tool steel from room temperature to a point just below the target austenitization point. The duration of the preheating process must be sufficient to ensure that the tool is heated uniformly throughout. Once the preheating process is completed and the tool steel is stable, austenitization can commence.

Heating (Austenitization)

The transformation of ferrite to austenite occurs at various temperatures, depending on the component content of the alloy being treated. For example, in basic carbon steel, austenitization occurs at around 1,350º Fahrenheit.

When an alloy reaches the critical austenitization temperature, the micro atomic structure opens so that it can absorb more carbon from the already present iron carbides. It is extremely critical that this process be precisely controlled both in terms of process temperature and duration. Incomplete initial austenitization can leave undissolved carbides in the atomic matrix.

Metallurgical engineers determine the optimum time and temperature for heating based on many factors, such as the tools steel being treated and the desired end results. For example, generally speaking a lower austenitizing temperature increases the toughness of the end product, whereas higher temperatures will increase the hardness of it.

Quenching

Quenching is the process of rapidly cooling the hot austenite into the much harder, desired endstate martensite micro atomic structure. As with the heating process, the duration and process methodology used for quenching are configured based on the desired final product.

For low alloy tool steel that must be quenched quickly in order to preserve the martensite structure, oil is typically the medium that provides the best results. For higher alloy tool steel, air cooling is the most effective approach. Additionally, for certain types of steel, a water quenching process is recommended.

As with all of the steps in the tool steel hardening process, quenching must be meticulously measured, managed, and controlled. Depending on the configuration, size, and shape of the product that is quenched, even rapid oil quenching (often referred to as “drastic quenching”) can be uneven throughout the finished product. This lack of uniformity can distort the finished shape or cause cracking.

Tempering

Tempering tool steel makes the newly formed martensite less brittle. Without proper tempering, martensite will crack—or even shatter—very easily. Proper tempering is an essential step in the overall tool steel heat treating process.

With that said, the precision required for proper austenitization is much less critical during the tempering step, although the rapid heating of the tool steel should be avoided. The heat intensity is typically determined by the hardness required for the finished material—a higher tempering temperature yields a harder product. Instead of a precise value, most alloys have a relatively wide range of acceptable tempering temperatures.

The key to effective tempering is patience. Depending on the tool steel and final application, multiple tempering steps may be required. A tempering step should include about an hour of heating for every inch of thickness, but in any event never less than 2 hours for each step, regardless of the size. The material should be cooled to room temperature—warm to the touch, about 75°—before the cycle is repeated.

Other Treatment Steps

Depending on the composition of the tool steel, there are cases where quenching alone is not sufficient for the complete conversion of austenite to martensite. This result is an end product that has not hardened completely and that might be brittle. One way to get around this deficiency is to cryogenically freeze the tool steel to a temperature below 0° Fahrenheit. There is a risk of cracking during a cryogenic freezing treatment, so for that reason the deep freeze cycle is conducted after the first tempering treatment.

Factors Affecting The Final Product

In a properly executed heat treatment process, tool steel will expand due to the changes in atomic structure. Although there are many factors that cause this, typically the expansion of tool steel after heat treating is between .002” and .0005”. Depending on the final application (for an example a slight expansion of the tool steel is more critical in a scalpel than a hammer), although nominal, this expansion must be taken into account.

On the other hand, if the heat treating process is not precisely controlled and depending on the exact composition of the tool steel, the process can actually result in shrinkage of the material. Typically resulting from improper regulation of temperature (too high or too low) or time (too long or not enough), the austenite does not fully convert into martensite. In addition to material shrinkage, this scenario can also have adverse impacts on other mechanical properties of the tool steel. Generally speaking, if shrinkage occurs, cryogenic cooling will complete the conversion process and revert the tool steel back to its desired state.

Table of Heat Treating Specifications by Tool Steel Type

The following table provides general recommendations for the appropriate hardening and tempering temperatures based on steel type, as well as the recommended type of quench process.

Type of Steel Harden °F Temper °F Quench
Medium Alloy (A2) A2 1700-1800 350-1000 Air
High Carbon A6 1800-1875 400-1000 Air
High Carbon D2 1800-1875 400-1000 Air
Oil Harden O1 1450-1500 350-500 Oil
Water Harden W1 1400-1550 350-650 Water
Shock Resisting S7 1650-1750 400-1200 Oil
Hot Work Alloy H13 1825-1875 1000-1200 Air / Oil
Molybdenum High Speed M2 2150-2250 1000-1200 Air / Oil / Salt
Tungsten High Speed T2 2300-2375 1000-1100 Air / Oil / Salt
Medium Carbon 1040 1550 1550 Water
Alloy Steel 4130 1600 0 Oil
Medium Carbon 4140 1575 400-1200 Oil

Precision + Uniformity = Value

Heat treating tool steel does more than adding significant value to the treated material—it makes the use of the tool steel possible. Without properly applied heat treating, tools simply wouldn’t work or couldn’t even be made. Modern metallurgical engineering is essential to the production and manufacturing of tool steel and all of its applications.

Heat treating is a process of critical tolerances, however. It’s not something that can be figured out on the fly and then done haphazardly. Heat treating not only requires human expertise, but it also requires highly engineered, state-of-the-art equipment that can ensure precision and uniformity throughout the entire process.