HEAT TREATMENT- WHAT, WHY AND HOW?

Hardness and Toughness. Two words that seem similar in meaning but actually, are words whose meanings are poles apart. When you consider a structural material, say a steel rod, hardness refers to the energy that it takes to deform (stretch, compress, bend) the rod whereas toughness refers to the energy that it takes to completely break the rod. The harder a metal is, the more brittle it becomes and less ductile it is. In contrast, the more ductile a metal is, the tougher it becomes. Hard metals are not able to absorb energy beyond a certain extent and eventually break up without any warning when a small crack appears on its surface. Tough metals absorb way more energy and hence, are less prone to cracks and are more flexible. You may have heard about or seen a stress vs strain curve of metals (those of you pursuing your majors in materials science, you probably are so used to it). If not, below are two graphs that depict a hard metal and a tough metal. (The crosses denote the fracture point)

Now, when it comes to building huge steel structures, you neither can use very hard steel nor very tough steel. Very hard also means very brittle, so the structure will collapse even due to small cracks whereas very tough means the plastic region will easily be attained without the steel regaining its original shape, not a good choice even though the structure won’t break up easily. For engineering purposes, you can not count on a part to be sound anymore once it crosses the yield point. Hence, you need to adjust the level of hardness and toughness that will result in a robust structure, neither easily developing cracks and breaking down nor attaining the plastic region easily. There are various methods to achieve just a good mixture of the two. One such method is heat treatment.

Heat treatment is basically used to increase the strength and hardness of a metal along with other subsequent processes to lessen the brittleness of the hardened metal. The inside crystalline structure of a metal changes when it undergoes heat treatment.

A bit of detail would be worth mentioning here. Cast steels and iron are made up of uniform pearlitic grain structure. Pearlite is quite soft and hence not suitable for many common applications. So what’s done next is that this steel is heated above its transition temperature (~727°C) which results in pearlite changing to austenite. The rate of austenizing depends on the carbon content of the steel. Here, it should be ensured that the temperature is high enough so that the core also transforms into austenite. Now, this heated steel is rapidly cooled in a quenching fluid. Water is the most efficient fluid for this purpose when maximum hardness is desired. If hardness can be sacrificed, other fluids may be used. On cooling, austenite changes to martensite, a much harder crystal. Steels with martensite structure are often used as the cutting edge of blades in machining processes. This hardening process is popularly known as ‘’hardening by quenching’’.

After quenching, the metal becomes very hard and brittle due to an overabundance of martensite. Moreover, it becomes highly stressed, prone to cracks and failures. So what we now need to do is to relieve the metal of these stresses and lessen its brittle nature. Also, the toughness needs to be increased. One way to do so is by warming it up. Though doing so will cause the hardness to decrease at the cost of increasing toughness, the temperature at which it is warmed is carefully regulated so as to get just the right mixture of toughness and hardness in the metal. The process incorporated at this stage is what we call ‘’tempering’’.

Tempering means reheating the quenched metal for an adequate period of time to achieve the desired level of hardness and strength. Adequate tempering temperature needs to be selected. Low tempering temperature will result in more hardness and low ductility whereas high tempering temperature will result in less hardness and high ductility. We need a temperature between the two extremes. When warmed up, the crystalline structure of quenched metal changes from martensite to tempered martensite. The level of hardness starts decreasing and that of toughness starts increasing, ultimately creating a balance. Metals processed by tempering are more suitable for engineering applications.

Other heat treatment processes that are used are ‘’annealing’’ and ‘’normalizing’’. The only difference between the two is the method by which the heated metal is cooled. In the former, the metal is cooled very slowly by putting in an oven or special furnaces whereas it is simply cooled in air in the latter (cools relatively faster). Annealing results in a tough and ductile metal whereas normalizing makes the metal hard and strong. Annealing can increase toughness and alleviate internal stresses that contribute to brittleness. In fact, the speed of cooling is a critical difference between annealing and tempering.

Now in order to practically test the effectiveness of the various heat treatment processes described above, a simple experiment was setup. A number of ⅛ in. diameter steel rods (1% carbon content by weight and each made by a different process) with their two ends kept on fixed supports on a wall were loaded with weights and the ultimate loads at which they broke were noted. The following results were obtained:

Steel rod made by	Maximum load sustained (kg)
Quenching	6
Normalizing	16
Tempering	55

Clearly, one of the best quality steel components is obtained by tempering process.

One important noteworthy point is that not all types of steel can undergo the heat treatment process. Mild or low-carbon steels (~0.05-0.25% carbon by weight) are generally hardened by carburization (case-hardening) whereas high-carbon steels (~0.30-1.70% carbon by weight) are the ones suitable for heat treatment. A probable reason for this being that heating low-carbon steels to temperatures of about 727°C may cause the carbon content to be lost and make the steel structurally weaker. Case-hardening in such cases increases the carbon content in the steel.

At home, you could easily carry out a simple experiment to test the strength of different types of processed steels. You would need a bunsen burner, a small bucket of water, a pair of pliers and most importantly, four to five bobby pins (aka hairpins). Bobby pins are actually high-carbon steels and can easily undergo heat-treatment. In its normal state, it has a springy nature (as it’s used to hold hair together). Following table shows the tests performed along with their outcomes. (Note: Use a different pin for each process)

Process	Action	Outcome (upon pulling the two ends apart using pliers)
Annealing	Heat the pin until red-hot and cool in air	Springiness lost but pin doesn’t break, still flexible
Quenching	Heat the pin until red-hot and immediately immerse in water	Springiness lost, breaks on pulling the two ends apart
Tempering	After quenching, gently heat the pin again for about a minute	Original springy nature retained

Heat treatment is therefore, a process that finds applications in various industries. During manufacturing, some materials might need to be involved in procedures that alter their crystalline nature. Heat treatment is a way of controlled heating and cooling procedures to change metal’s physical properties and improve them to be used in a large range of industries to aid various manufacturing steps. Even half-shaft splines used in Formula Student race cars are manufactured by case-hardening process. Thus, when it comes to finding processes that strengthen automotive and other components, heat treatment plays a vital role in it, enhancing the strength and durability of metals along with their quality and appearance.