Heat Treatment of Steel

Heat treatment is a process that involves the heating and cooling of metals of alloys in their solid-state to enhance specific desirable properties without changing the composition. The process is carried out to change the grain size, relieve stress, modify the structure and prepare the material after hot or cold working.

Other reasons for heat treatment of metals like steel include;

• To improve machinability

• Improve electrical and magnetic properties

• Increase resistance to corrosion, wear and heat and others.

There are various types of heat-treatment processes. The ideal method depends on the type of metal and the desired properties after the heat treatment process. The four primary heat treatment methods for stell include;

Annealing

Annealing is a heat treatment process that relieves stress and softens steel to improve grain structure and increase ductility.

Without the necessary pre-heating stage, processes like welding can leave the metal with uneven temperatures and some molten spots next to areas at room temperature. Such results can weaken steel, and as these areas around the weld get cooler, internal stresses develop around the brittle and hard spots. Annealing helps to fix this problem by relieving internal stress.

Annealing Process

The annealing process must be conducted carefully and procedurally to produce the highest level of ductility for steel and other ferrous metals. First, you must heat the metal slowly to the appropriate temperature, followed by soaking and then allowing it to cool by turning off the furnace and letting the furnace and the steel cool together or by burying it in an insulating material.

How long the metal soaks depends on its type and its mass. If it’s a low carbon steel, it requires the highest possible annealing temperature. As the carbon content increases, the annealing temperature decreases.

Normalising

Normalising is another steel heat treatment process that helps to relieve internal stress. This process removes internal stress caused by machining, forging, forming, casting, heat treatment or casting. Uncontrolled stress can cause the metal to fail. Normalising before any hardening can help ensure the success of the project.

Normalising Vs Annealing

Normalising and annealing sound like two processes that achieve the same results. However, there are several differences between the two.

Normalising only applies to ferrous materials like steel. Also, when normalising, the metal is typically heated to a higher temperature than when annealing. The metal is also air-cooled after removal from the furnace. Generally, normalised steel is stronger than annealed steel. Both options produce metals with high strength and ductility, but normalisation produces tougher steel than annealed steel. Normalisation is more recommended than annealing if the metal part in question is required to withstand impact or have maximum toughness to resist internal stresses.

Because normalised steel is air cool, the mass of the metal is critical in determining the cooling rate and the part's resulting hardness level. When normalising, small and thinner parts cool faster and become harder than thicker pieces. But with the furnace cooling of the annealing process, both thick and thin parts have comparable hardness.  

Hardening

The purpose of hardening is simply hardening steel and making it stronger. Unfortunately, although hardening increases strength, it also decreases ductility making the metal more brittle. You can remove some of the brittleness by tempering the metal after hardening.

When hardening the metal, you need to slowly heat it and soak it for a specified time to achieve a uniform temperature, then rapidly cool the metal by plunging it into water, brine or oil. This last process of plunging is called quenching. However, there are a few instances where hardening can also be done by air-cooling.

When you add alloys to steel to increase its hardness, you also increase the carbon’s ability to strengthen and harden. Therefore, the carbon content needed to produce the highest hardness level reduces in alloyed steels compared to plain carbon steels.

When hardening carbon steel, it must be cooled to under 537.77⁰C in less than one second. After adding alloys to the steel, which increases carbon’s effectiveness, you should increase the time limit beyond one second. You can also select a slower quenching medium to get the specified hardness.

Most carbon steels are quenched in water or brine. Alloy steels are mostly quenched in oil. The downside to quenching is that it produces high internal stress. To relieve the stress, you can opt to temper the steel before it becomes cold.

Tempering

When you harden a metal, regardless of the process, you introduce internal stresses after the rapid cooling process. After the hardening process, the steel is often harder than needed and too brittle. You can temper the steel to reduce the brittleness, relieve some of the internal stress or remove it completely.

When tempering;

• You start by heating the steel to a set temperature below its hardening temperature.

• Keep the steel at that temperature for a specified period.

• Then cool it, usually in the air.

The process sounds familiar because it is almost identical to the three heat treatment stages. It only differs in the temperature of tempering and its effect on steel’s strength, hardness and ductility.

The purpose of tempering steel is to reduce the hardness caused by the hardening process. It also helps to develop specific physical properties. Tempering follows hardening. And while it reduces brittleness, it also makes steel soft. You can control the level of hardness lost by managing the temperature during tempering.

Unlike annealing, hardening and tempering, which occur at a temperature higher than the metal’s upper critical point, tempering is always done at temperatures lower than the upper critical point.  

Tempering begins at 100 ⁰C after reheating steel that has already been hardened. You can continue tempering until you approach the low-critical point. You can pre-set the tempering temperature to select the hardness and strength you prefer. The least time for tempering is 60 minutes for parts less than one inch thick. If it’s more, you can add another hour for every additional inch of thickness. After removing the steel from the tempering furnace, cool it in the air the same way as the normalising process.

thyssenkrupp Steel Europe AG offers a variety of tailored heat treatment processes, including tempering, normalising, annealing and tempering, making it possible for end-users to purchase application-ready materials with the best and most desirable features.