5 Ways the Tool and Die Sector Uses Heat Treating

What determine the key mechanical properties of metals – hardness/softness and strength/brittleness and toughness? How does heat treatment of metals work to create the hardest and most long-lasting parts? What guarantees the quality of metal products in production? Let’s take a closer look.

Introduction

In the minds of the average person without special education, metallurgy consists of a simple list of sequential actions, such as mining various metals, fusing them in certain proportions, and casting the alloys into molds, followed by machining the hardened billets. Looking at the periodic table of elements, which contains almost a hundred different metals, it is easy to confirm this concept of metallurgy. After all, the variety of alloys used in technology and the variability of their properties could seemingly be explained by the combination of these metal elements in different proportions. However, this view is not only inaccurate, it is wrong. So what exactly is going on in reality?

Metal Alloys: a Brief Overview

First of all, not all metals can be used to create structural alloys and products made from these alloys. In fact, the list of common engineering metals is quite small and includes only eight elements: iron, aluminum, copper, chromium, nickel, titanium, zinc, and magnesium. Other metals are not as common, but are still important in various industries. These include mercury, tungsten, cobalt, tantalum, and rhenium. Precious metals, gold, silver, platinum, and palladium, are also used not only in financial and jewelry businesses, but also in technology due to their unique properties.

It should be noted that there are some industries that use specific metals for specific purposes. For example, depleted uranium is used in the military to create armor-piercing projectiles due to its uniquely high mass density, which gives the projectile increased kinetic energy. This property also makes the metal more resistant to impact and suitable for making armor. For example, the latest modifications of Abrams tanks produced after 1998 contain depleted uranium armor elements in the front hull and turret.

Lead is also used to protect against ionizing radiation due to its high density, which contributes to efficient radiation absorption. Rhodium has extremely high corrosion and chemical resistance and very low electrical resistance, so it is valued as a material for electrical contacts and electroplating. It is also a material for neutron flux detectors in nuclear reactors.

Secondly, not all of these metals can be mixed in the liquid state to form alloys: some metals do not dissolve in each other, or separate from each other when cooled, forming a solid mixture of metals.

Thirdly, metals are the main components of alloys, but nonmetals are also extremely important for them. For example, carbon is contained in steel (usually up to 2%) and in cast iron (from 2 to 4.5%). The alloying process should also be considered in this context. Alloying in metallurgy is the introduction of elemental impurities into metal alloys to help produce materials of a given chemical composition and structure with the required physical, chemical and mechanical properties. In fact, alloying is the process that is most responsible for the creation of the entire variety of alloys. These are mostly non-metallic elements: chromium, nickel, manganese, tungsten, titanium, tantalum, aluminum, vanadium, copper, cobalt, niobium, zirconium, and rare earth metals. However, the nonmetals silicon, boron, selenium, and nitrogen can be used as alloying elements.

Finally, the properties of alloys depend not only on their composition, but also on the way the metal is processed. Here we are getting closer to the heat treatment process. But before it, we need to understand what the structure of an alloy is, because it changes under the influence of heat treatment, ultimately changing the properties of the alloy.

Structure of Alloys

There are three types of alloys: solid solutions, chemical compounds, and mixtures:

– Solid solution alloys are characterized by the fact that during their formation, one component (the solvent) retains its crystal structure, and the atoms of the other one (the dissolved component) are arranged in the crystal phase of the solvent. In such alloys, the atoms of the dissolved component are arranged in a disordered fashion.

– Chemical compounds are formed when a chemical reaction occurs between alloy components to form a new compound. These can be combinations of metals with nonmetals, as well as compounds containing two or more metals. In their pure form, they have a single-phase structure with an ordered arrangement of component atoms.

– Mixtures are formed by components that do not dissolve in each other in the solid state and are also incapable of chemical interaction to form a compound. In mixture alloys, each component retains its own crystal lattice. The structure of the mixture is heterogeneous, consisting of two or more phases. The mechanical properties of mixtures depend on the quantitative ratio of the components, as well as on the size and shape of the phases (grains).

In addition, there are components that can form several different phases depending on the conditions of heating and cooling of the alloy.

In practice, things are still much more complicated. After all, real alloys, such as the most common steel and cast iron, are actually complex mixtures of many phases that are formed depending on their composition, melt cooling conditions, and subsequent mechanical and heat treatment of billets and products.

Heat treatment results in changes in phase composition, grain size reduction or enlargement, redistribution of impurities, and surface modification. All these processes change the properties of the final material.

For the manufacture of tool parts and stamping dies, the most common material is hard and durable steel grades. This is due not only to its specified mechanical properties, but also to its availability. However, other materials such as hard metals, carbides or special alloys can be used in certain cases, depending on the specific material requirements and application conditions.

Steel vs. Cast Iron in Tool & Die Sector

As we have already mentioned, steel and cast iron are two different types of iron alloys, and they have different compositions, properties, and uses. It is widely believed that the main difference between these alloys is the carbon content. Cast iron contains more than 2% carbon, while steel contains less than 2%. This is largely true, but there are steel grades that have a carbon content of up to 3%. But that doesn’t make it cast iron. So how can one clearly distinguish between cast iron and steel?

The main feature that helps to distinguish between cast iron and steel is the microstructure of the material. In cast iron, the microstructure usually consists of a large number of carbon inclusions (graphite and iron carbide phases), which gives it its brittleness. Steel, on the other hand, has a much more homogeneous microstructure with no visible carbon inclusions, and it usually has better mechanical properties such as ductility and strength.

Appearance can also help distinguish cast iron from steel. Graphite is black and iron carbide is white, so cast iron is usually gray to white. Cast iron can be made black by special heat treatment of its surface. The surface of cast iron is rougher, rougher, also because of its grain structure. Steel usually has a smoother surface and can have a variety of colors, including silver, blue, or even black, also depending on the treatment and coating.

For technical applications, the most important difference in mechanical properties of cast iron and steel is the following:

Cast iron has a lower hardness than steel, it is a softer material that is less resistant to scratches and wear.
Steel has higher strength compared to cast iron. It can withstand higher loads and provide better resistance to deflection and tearing.
Steel has a higher yield strength and tensile strength than cast iron. This means that steel can deform more before breaking and has better ductility.
Cast iron has a higher stiffness (lower flexibility) than steel. This means that it is less flexible and less able to absorb shocks.