Surface Treament - Tool Steel and Heat Treatment, Part 1

SURFACE TREATMENT

Tool Steel and Heat Treatment, Part 1

An introduction to heat treatment for the moldmaker.

By David Pye

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Metals have been used by mankind since man stopped using flint as a tool for scraping hides and killing animals for meat. Man knew about iron, but could not raise a melting temperature high enough to extract the iron from the ore. So easier metals were used such as copper. Man discovered how to alloy copper to make brasses and bronzes for many different applications, such as sword making, armor, shields, spears and many other applications.

It was approximately 4,500 years ago that the Chinese discovered how to raise the temperature of iron to such a temperature that it would melt. It was through a simple reciprocating bellows system that kept a continuous stream of air to the fire and made it hot enough to melt the iron. Iron was then used for tools for scraping, raking, hoeing, tilling, and so on. So toolmaking really started to use iron approximately 4,500 years ago. Carbon was introduced into the iron when the iron was being forged. The carbon source was from the forge fire when wood/charcoal was being burned.

Figure 1: Schematic tree of metal grouping. Figures courtesy of Pye Metallurgical.

There is evidence of martensite being created by the ancient Chinese and also in India with Wootz iron. The ancients were not able to identify the martensite that was formed, only that they knew if you heated the iron and cooled it down, it became hard. It was with the advent and use of the metallurgical microscope that martensite (and other structures) were able to be identified and named.

There is reference in Homer’s Odyssey to the use of iron and the iron being tempered. It was around the mid-nineteenth century that the effects of adding other metals into the molten iron was experimented with., and then the effects of alloying the steel were investigated. Alloying of steel (iron plus carbon) using different alloying elements really came into its own at the turn of the 20th century.

What Is Heat Treatment?
So now that you’ve had a brief introduction into the history of tool steels, let’s get to the basics of heat treatment. Heat treatment of metal is a method of manipulating the metal (in terms of its physical properties) to achieve the operational condition of the metal—both for machining and then for operation. Most metals can be made to be soft or hard, or tough or wear-resistant or both tough and wear-resistant. They also can be made to reduce the possibility of corrosion or they can be made to be soft. This is all done with the application of heat to the metal.

There are many definitions of metals:

Metals are a group of elements in individual form such as tin, gold, silver, iron, aluminum, chromium, vanadium, etc.
Metals usually are very dense and good conductors of heat.
Metals are usually malleable and easily manipulated. They can easily be polished to a shiny surface finish.

In our case metals are usually alloyed with other elements to change the physical condition of the metal, such as, iron with carbon to make simple steels and copper with zinc to make simple brass. There are many combinations of mixtures of metal elements to give equally, many different characteristics. The metals groups can be divided into two distinct categories (see Figure 1).

Figure 2: Steel categorizations.

It can be seen from the tree that we have focused only on steels (ferrous) and other metals (non ferrous). In this series we will be focusing on the ferrous group of metals (steels).

What Is Steel?
Steel is simply an alloy of iron plus carbon. Even a very small amount of carbon will significantly change the steels characteristics of hardness, tensile strength, impact strength and compressive strength. Steels can be categorized as shown in Figure 2.

The tree is a very simplified method of both identifying and categorizing steels. A similar tree is done for the non-ferrous metals that illustrates how the non-ferrous metal groups are distinguished from each other.

Figure 3: Simple schematic of heat up and cool down for heat treatment.

The applications section of Figure 2 is an extension of the tool steel category, which shows the second group of tool steels. Tool steels by quench method and tool steels by application methods are shown in the schematic tree.

Simple Heat Treatment Metallurgy
The heat treatment of any steel simply means that you will apply heat to the steel to raise it to a required temperature and then cool it down in an appropriate manner. Figure 3 demonstrates the heat treatment process. The schematic illustrates a simple process of heat up, soak at temperature and cool down, which represents the basic schematic for every type of heat treatment that one can think of—including surface treatment processes. Heat treatment is used to make the steel soft for machining and manipulation, and then into the final metallurgy necessary for the steel to function in its particular environment.

The next article will continue the thermal process technology definition and also will continue with tool steel classification.

Terminology

It is necessary to define some of the important terminology of heat treatment so that one can begin to understand the language of metallurgy.

Figure A: A typical annealing cycle.

Annealing: To make the metal soft for machining and fabrication/forming/bending. Accomplished by raising the temperature into what is known as the austenite temperature region, equalizing and followed by very slow cooling down to almost room temperature (see Figure A).

Austenite: That phase which is created by raising the steels temperature. Austenite is a created phase which comprises of 14 atoms (forming a molecule). The Austenite phase has its atoms oriented one on each corner of a cube-like structure, plus one on each face of the cube.

Air hard: This is a group of steels that requires only an air cool after the austenitizing treatment. However the problem will be that the steel will both oxidize and decarburize. So one must austenitize the steel under a protective atmosphere—such as vacuum or other atmospheres.

Atmosphere: An atmosphere can be as simple as air. However, oxidation and decarburization will occur. The atmosphere can be a vacuum (vacuum is an atmosphere at a low pressure). The atmosphere also can be nitrogen, argon, or other types of gases up to hydrocarbon gases—such as methane, butane or other hydrocarbon gases.

Bainite: A phase that is formed by raising the steel to its austenitizing temperature (hardening temperature) followed by quenching down to a temperature that is above the start of the formation of the martensite phase. The steel is then held for a period of time (depending on the cross-section of the steel) to allow the bainitic structure to form. Bainite will produce a hardness value in the region of 55HRC (depending on the selected quench medium temperature). The steel will not require tempering after the quench operation. Be sure that the steel is held long enough for the bainite phase to form.

Figure B: Ferrite structure. Body centered cubic latic.

Body centered cubic lattice structure: This refers to the molecular structure of iron at room temperature and can be known (depending on the carbon content) as ferrite and cementite. It is a cubic structure with 9 atoms present—one on each corner and one in the center of the cube (see Figure B).

Boronizing: A surface treatment process that will form very hard alloy borides in the immediate surface. This also includes also boron nitrides. Boronizing can produce hardness values in the region of 1800 Hardness Vickers. It can be accomplished either from a gaseous environment, a paste or a pack cementation method.

Carbides: Can be formed as simply as an iron carbide. The carbon will react with carbide forming elements such as iron, chromium, vanadium, molybdenum, tungsten.

Carbon: The element that is present in steel. It takes only a small amount to vary the mechanical and metallurgical properties of steel. It is also the ingredient to turn iron into steel.

Carburize: A surface treatment process that requires carbon to be added into the surface of the steel. The steel is usually a low carbon steel. It is used to create a high surface hardness and a tough core. Known also as case hardening. The process can be accomplished either by pack cementation methods, gas atmosphere with a hydrocarbon enrichment gas, low-pressure carburizing (vacuum) molten salt or plasma conditions.

Decarburization: This is when carbon leaves the surface of the steel due to a heat treatment procedure. If the atmosphere is weaker in carbon than that of the steel, then the steel will give out its surface carbon. If the atmosphere is richer in carbon than that of the steel, the steel will carburize.

Dead soft: A full anneal process usually applied to non-ferrous metals. However, it can be applied to the steels, which means exactly the same thing. The steel is in its softest condition.

Dew point: This means the temperature and pressure of the atmosphere at which a gas begins to condense (precipitate) into a liquid from the gaseous vapor. This term is usually applied to carbon’s potential control of a heat treatment processing atmosphere.

Distortion: Caused by induced stresses form machining, it can also be caused by phase changes as the steel is heated up and cooled down. Distortion from phase changes will occur. You cannot stop that. However steps can be taken to minimize the distortion. Distortion can occur from how the steel is positioned in the furnace. There are two types of distortion: size and shape.

Double temper: Applied generally to the high alloy tool steels, such as the H series, D series and the high speed steel series. It is not confined to these steels, but is generally applied to them. The purpose of the double temper is first to thermally decompose any retained austenite that may be present. Second, to precipitate out the secondary alloy carbides in order to give the steel its hardness. The aforementioned steels also are known as secondary hardening steels. The steels will increase slightly in hardness as a result of the carbide precipitation and the thermal decomposition of the retained austenite. More tempers can be given if necessary.

Figure C: Iron carbon equilibrium diagram.

Equilibrium phase diagram: Diagrams that relate temperature to composition and what is happening at particular temperatures. For example, one can make a phase diagram of salt in water. In our case it is a phase diagram of the relationship of carbon in iron in relation to temperature. It is known as the iron carbon equilibrium diagram, or more accurately the iron cementite equilibrium diagram (see Figure C).

Eutectoid line: The transition line seen on the iron carbon equilibrium diagram that separates and divides ferrite from cementite.

Face centered cubic lattice structure: A molecular structure that is cubic with an atom on each corner of the cube, plus one atom on each face of the cube (14 atoms). This structure is known as austenite, which can only be created in steel. It cannot exist without the application of heat. The austenite creation temperature is that temperature which is above the upper critical line on the iron carbon equilibrium transformation temperature line. In other words, the austenitizing temperature (hardening temperature) is determined by the carbon content of the steel that is being treated.

Ferrite: This is the term that is used to describe a steel with a carbon content below 0.70 percent. The ferrite molecular structure is that of a body centered cubic lattice structure (BCC). The structure has an atom on each face and one in the center of the structure(9 atoms).

Fluidized bed furnace: A thermal heat transfer system that will transfer its heating energy into aluminum oxide particles. The particles are agitated by the passing of a process gas through the bed and each of the particles separate and transfers heat by collision with each other.

Full anneal: When a steel is heated into the upper critical region (austenite) and cooled very slowly under controlled conditions.

Grain growth: A function of grain growth caused by too high an austenitization temperature as well as too long at the austenitize temperature.