SUB-OBJECTIVE. Heat treatment can be used in three ways to tailor the properties of a metal to a particular use. These three ways are:

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1 LESSON 4 LECTURE HEAT TREATMENT SUB-OBJECTIVE At the end of the Lesson, the Trainees will be able to demonstrate an understanding of the ways that heat treatment affects the properties of metals. 1.0 INTRODUCTION Heat treatment is one means that can be employed to control certain properties of metals. A metal can be made tougher and stronger by heat treatment. It can also be made more workable and easier to machine. Heat treatment can be used in three ways to tailor the properties of a metal to a particular use. These three ways are: 1. Hardening 2. Tempering and 3. Annealing. The process by which these treatments are carried out vary from one metal to another, but all involve heating and controlled cooling of the metal being treated. 2.0 HARDENING Hardening metal affects properties other than hardness alone. Hardening also makes a metal stronger, and generally makes it much more brittle. A typical use of hardening is to strengthen tools and other working implements. Knife blades, for example, must be hardened to retain a sharp cutting edge, and this is true of cutting tools in general. Hardening is also used to treat the shafts of rotating machinery in order to prevent wear on bearing surfaces and to strengthen the shaft. Fig Hardening and annealing of plain carbon steels. LESSON 4. PAGE 1

2 2.1 QUENCH HARDENING PLAIN CARBON STEELS Fig summarizes those methods by which the structure, and the properties, of plain carbon steels can be changed by heat treatment processes. Fig Heat treatment of plain carbon steels. The temperature to which the steel must be heated to wipe out the initial structure and properties will depend upon the carbon content of the steel. Fig shows the temperatures from which plain carbon steels should be quenched hardened. It will be seen that these are the same temperatures as for annealing. The only difference being in the rate of cooling Fig The degree of hardness steel achieves is solely dependent upon: 1. The carbon content. 2. The rate of cooling. LESSON 4. PAGE 2

3 2.2 EFFECT OF CARBON CONTENT Fig Rapid cooling increases hardness. There must be sufficient carbon present to form the hard iron carbides in the steel when it is heated and quenched. The effect of carbon content when steel is heated and quenched is shown in Table Table Effect of carbon content. EFFECT OF HEATING AND QUENCHING (RAPID COOLING) TYPE OF STEEL Below 0.25 Negligible Becomes tougher Becomes hard Becomes very hard Mild Medium carbon High carbon LESSON 4. PAGE 3

4 2.3 RATE OF COOLING The rapid cooling necessary to harden steel is known as QUENCHING. The liquid into which the steel is dipped to cause this rapid cooling is called the QUENCHING BATH. In the workshop, the quenching bath will contain either 1. Water 2. Quenching oil (on NO account use lubricating oil) The more rapidly a plain carbon steel is cooled the harder it becomes. Unfortunately, rapid cooling can lead to CRACKING and DISTORTION, therefore the work piece should not be cooled more rapidly than is required to give the desired degree of hardness. For plain carbon steels, the cooling rates shown in Table are recommended. Table Rate of cooling. REQUIRED TREATMENT Toughening Toughening Hardening Hardening QUENCHING BATH Oil Oil Water Oil CARBON CONTENT % TEMPERING Hardened plain carbon steel is very brittle and unsuitable for immediate use. A further process known as TEMPERING must be carried out to greatly increase the toughness of steel at the expense of some harness. Tempering consists of re heating the steel to a suitable temperature and quenching again. The temperature to which the steel is heated depends upon the use to which the component is going to be put. Table gives suitable temperatures for tempering components made from plain carbon steel. LESSON 4. PAGE 4

5 Table Suitable temperatures for tempering components. TEMPER COLOR COMPONENT Pale straw Medium straw Dark straw Brown Brownish-purple Purple Blue Edge tools Turning tools Twist drills Taps Press tools Cold chisels Spring Toughening (crankshafts) In the workshop, the tempering temperature is usually judged by the color of the oxide film that appears on a freshly polished surface of the steel when it is heated. Some tools, such as chisels, only require the cutting edge hardened, the shank being left tough to withstand the hammer blows. 4.0 OVERHEATING PLAIN CARBON STEELS It is a common mistake to overheat steel with the hope that it will become harder. As already stated, the hardness only depends upon the carbon content of the steel and the rate of cooling. Once the correct hardening temperature has been reached, any further increase in temperature only slows up the rate of cooling and tends to reduce the hardness. Further overheating also cause crystal growth resulting in a weak and defective component. On the other hand, if the hardening temperature is not reached the component will not harden no matter how quickly it is cooled. 5.0 EXAMPLES OF HARDENING PLAIN CARBON STEELS To harden and temper a cold chisel made from octagonal chisel steel having a carbon content of 0.6/0.7 percent. From Fig it will be seen that the correct hardening temperature for a 0.6/0.7 per cent plain carbon steel lies between 820 carbon content is fairly low, and a chisel is a simple shape, it can be safely quenched in water to achieve maximum hardness. To temper the chisel, first polish the cutting end so that the temper colors caused by the oxide film can be observed. The shank is then heated as shown in Fig. LESSON 4. PAGE 5

6 and the polished end is watched as the temper colors travel towards it. Fig Tempering a cold chisel. If the chisel is to cut metal, the cutting edge should be a brownish purple when it is quenched. If the chisel is to cut brick or concrete the cutting edge should be purple to give it greater impact resistance. Letting the heat travel down from the shank ensures that the shank is left in a tough rather than hard condition so that it will not shatter when hit by a hammer. 6.0 ANNEALING Annealing is done to metals for two purposes: 1. To soften a metal and make it easier to work and machine, and 2. To relieve internal stresses and strains. Annealing a metal that has been heat treated or worked in some way will serve to return the metal to properties very close to those of that metal's untreated state. In the shop, annealing is often used to renew copper gaskets and to soften copper sheet to make it easier to cut and to work. Annealing also relieves stresses in freshly welded fabrications and makes the pieces more resistant to fatigue. Sometimes annealing must be avoided, such as when drill bits or chisels are sharpened by grinding. In this instance, the tool is quenched repeatedly so that it will not lose its hardness from accidental annealing. LESSON 4. PAGE 6

7 To anneal a hardened or tempered steel object, in order to make it easier to work or machine, the metal must be heated again to its critical range. In this instance, the object is cooled very slowly, either in air or in a furnace until it is well below the critical temperature. This returns the metal to its original "normal" state. 7.0 CASE HARDENING Case Hardening is a special type of heat treatment. It is used to produce a very hard surface while the inside of an object stays tough and ductile. Case hardening is accomplished by heating an object that is packed in s substance with a high carbon content. The carbon is absorbed into the surface of the metal more and more the longer the object is heated at high temperature. The more carbon absorbed the deeper the hardening of the surface. LESSON 4. PAGE 7