Heat treatment of steel

Heat treatment of steel

Heat treatment of steel involves the different stages in which there is producing martensitic steel using water, oil and air. The successful heat treatment of steel to produce a predominantly martensitic microstructure throughout the cross-section depends mainly on three factors:

1. The composition of alloy
2. The types and character of quenching medium
3. Size and shape of specimen

The influence of this factor involves different properties of steel:-

• Hardenability

The influence of alloy composition on the ability of steel alloy to martensite for a particular quenching treatment is related to a parameter called Hardenability. Hardenability is a term used to describe the ability of an alloy to be harden by the formation of martensitic microstructure. As a result of heat treatment. Hardenability is not hardness which is a resistance to indentation but it is qualitative measure of rate at which hardness drops off with distance into the interior of the specimen as a result of diminished martensitic content. The steel is said to be occupied by martensitic.

• Hardness test

One standard method that is widely used to determined hardenability is the ‘Jominy End Quenched Test’.

In this procedure the specimen in the form of cylinder having diameter 25.4 mm (1”) and length 100 nm (\(\sim\)4”) is Austenized at a prescribed temperature for a prescribed time i.e. the material is heated up to the temperature of austenite. The material is removed from the furnace and mounted in a fixture as shown in figure. The lower end of material is quenched by a jet of water of specified flow rate of water of specified flow rate and temperature.

The cooling rate is maximum at the quenched end and diminished with the position along the length of specimen. After the piece has cooled to a room temperature shallow flat 0.4 mm ground along the length of specimen and Rockwell hardness test are made for the first 50 mm long each flat. A hardenability curve is produced when hardness is plot against function of position of quenched end. Typical hardenability curve is shown in figure in which the quenched end is cooled must rapidly. And exhibit maximum hardness.

• Hardenability curve

1. The cooling decreases with the distance form quenched end. Hardness also decreases with distance.A steel that is highly hardenable with retained large hardness value for relative long distance. Each steel has its one unique hardenability curve. There are 4 different types of steels:
2. The quenched end is cooled must rapidly and it exhibits the maximum hardness as shown in hardness distance curve. 100 % martensite is the product of this position for most steel.

i.Martensite ii. Martensite and pearlite iii. Fine pearlite iv. Pearlite

3. The correlation between position along the specimen and continuous cooling transformation. For iron-carbon alloy is shown in the diagram. Superimpose with cooling curve at four different Jominy-position and corresponding microstructure.4.With

4.With increase in temperature of steel the hardness for all composition decreases. Under colling the hardness increases.

5.All five alloy represented by weight % composition of carbon 40% as 1040, 5140, 8640, 4140, 4340 has identical hardness at the quenched end i.e. 57 HRC. Here hardness is function of carbon content only which is same for this entire alloy.

6.Hardness profiles of different composition are indicative of influence of cooling rate of the microstructure. At the quenched end that is approximately \(600^\circ c\). 100% martensite is present for five alloys. However, microstructure of 4 alloy consist primary of mixture of martensite, martensite and bainite or cooling the composition of bainite increases.

7.The disparity un hardenbility behavior of steel is explain by the presence of nickel, chromium, molybdenum in the alloy.

8.Hardenabilty curve also depends upon the carbon content. As the carbon content increases the hardness initially increases.

9.During the production of steel there is always a slight unavoidable variables in composition and overlarge grain size.

10.Bainite contain increases with decrease with cooling rate.

11.Alloying elements delay the bainite reaction.

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