# Introduction

n engineering similar metals are required to possess very strange combination of properties when they are subjected to different conditions of working. They may have to be subjected to, twisting, impact loading, as well as to withstand various stresses like tensile, compressive and shear in different places of their utility. Moreover for using them or their alloys as a tool material, they may require hardness, toughness, along with softer shank. In order to induce certain desirable properties in the metals, Heat treatment operations are applied to the material. For this purpose the metals may have to be heated to different temperatures, cooled and reheated in different media. The properties of metals can be improved, graded or altered practically by controlled heating and cooling i.e. by heat treatment [1].

Microscopic examination (microanalysis) is the study of the structured materials under a microscope at large magnification. The structure observed is called microstructure. A definite though only qualitative relationship exists between the structure of a metal observed in an optical microscope and certain properties of that metal. In many cases microanalysis shows that the variations in alloy properties are due to variations in chemical composition and conditions of treatment [1].

Polishing of specimen surface for microscopic study is done to prepare a smooth, deformation or distortion free surface for giving a clear two-dimensional view of the microstructures present. But, the polished surface of a uniform specimen appears bright without any detail under the metallurgical microscope. To make its structure apparent under the microscope it is necessary to impart unlike appearances to the constituents. This is accomplished by selectively corroding or etching the polished surface [1].

Transformation temperature is a function of chemical composition and heat treatment and quenching processes done on these alloys. In heat treatment process, the rate of quenching, exposure time and heat treatment temperature control the forward and reverse transformation temperatures of austenite to martensite and martensite to austenite [2].

Hardening is the heat treatment process which increases the hardness of a steel piece by heating it to a certain high temperature and then cooling it rapidly to room temperature. In this process a piece of steel is heated to a temperature of 30°C to 50°C above the upper critical temperature for hypo-eutectoid steels and by the same temperature above lower critical temperature for hyper-eutectoid steels. Depending upon its thickness, it is held at this temperature for a specified time and then cooled in a suitable cooling medium (quenching bath) like water, brine, oil or current of air from blower or compressor [3]. However, water is corrosive with steel, and the rapid cooling can sometimes cause distortion or cracking [4].

Annealing is that heat treatment process which softens an already hardened steel piece by heating it to a certain high temperature and then cooling it very slowly to room temperature. This process refines grain structures, softens the steel, improves its machinability I and restores its ductility by reducing hardness. It also removes internal stresses [1].

The tensile strength and hardness of steel produced by annealing are less than that produced by normalizing [5].

Normalizing is a heat treatment process to make steel moderate hard from soften steel. The normalizing of steel is carried out by heating approximately 100'F above the upper critical temperature line and then cooling in air to room temperature. It does not soften the steel to the extent it is done by annealing and also it does not restore ductility as much as done by annealing [2].
II.

# Various Microstructures

Prediction of microstructure transformations is prerequisite for successful prediction of mechanical properties after a heat treatment and of generation of stresses and strains during a heat treatment. Phase transformation modeling is one of the main challenges in modeling of heat treatment [6]. During annealing, softening processes are under way in the microstructure and, in some cases, recovery and recrystallization take place as well. Naturally, the morphology of carbides changes as well [7].  The following steps were carried out in our experimental investigation: ? Samples of AISI 1020 steel were prepared for hardness test in Brinell Hardness Tester machine. ? After that the following specimens were heat treated in the furnace for reaching the austenization temperature (850-900°C) of the following specimens. ? Then the specific heat treatment operation like hardening, annealing and normalizing had been done. ? For specific heat treated specimen, the hardness test was done for assessing the change in hardness. ? Metallographic tests were carried out to observe the changes in microstructures after heat treatment.


# a) For Annealing

In this case the specimen was put in the furnace for 850°C and we kept it in this situation for approximately 35 minutes. After that it was cooled in a heap of ashes so that it was cooled down at a very slow rate.

The function of annealing is to restore ductility and also removes internal stresses but its Brinell Hardness Number is less than hardening because here carbon get more time to react with oxygen in the atmosphere for slow cooling rate.


# b) For Hardening

In this case the specimen was put in the furnace for 850°C and we kept it in this situation for approximately 10 minutes. After that it was cooled in water so that it was cooled down very quickly.

The function of hardening is to increase the hardness of the specimen and so its Brinell hardness number is larger than annealing and normalizing because here carbon cannot get more time to react with oxygen (for quick cooling rate), so carbon is trapped with the specimen and formed martensite.


# c) For Normalizing

In this case the specimen was put in the furnace for 850°C and we kept it in this situation for approximately 45 minutes. After that it was cooled in room temperature.

Normalizing does not soften the steel to the extent it is done by annealing and also it does not restore ductility as much as is done by annealing. there is a significant change in hardness number of the hardened specimen comparing with normalizing and annealing. It is happened because of self-locking of carbon particles in the hardened specimen. In the microstructures of these specimens we have indicated the carbon-saturated region with arrow marks (Figure 4.1, Figure 4.2, Figure 4.3) and these help to find out the differences among the microstructures of annealed, normalized and hardened specimens comparatively. As slow cooling is done in annealing so it transforms austenite to soft pearlite and also mixed with ferrite or cementite and this cementite increases the brittleness of the steel. Normalizing converts soft steel to moderate hard steel. In this case cooling rate is faster than annealing and for this reason, when the specimen is cooled in room temperature then ferrite and cementite are formed but their quantity is less. So the specimen is enhanced with considerable ductility by reducing its brittleness. In hardening process austenite structure is directly formed into martensite structure for fast cooling.

Actually the rapid cooling converts most of the austenite into martensite which is a hard constituent and more stable than austenite at ordinary temperatures.   


# Conclusion

Here we have tried to show that the different means of cooling rate are responsible to provide significant change or effect on the microhardness of steels depending on the carbon content of steel. The microhardness increases with the increasing cooling rate and carbon content due to solid solution hardening and formation of the martensite phase. Thus heat treatment is used to obtain desired properties of steels such as improving the toughness, ductility or removing the residual stresses.
31![Figure 3.1 : Muffle furnace (manual)](image-2.png "Figure 3 . 1 :")
3234![Figure 3.2 : Muffle furnace (automatic)](image-3.png "Figure 3 . 2 :Figure 3 . 4 :")
![Case Study of Heat Treatment on AISI 1020 Steel Issue V Version I](image-4.png "A")
4![Figure 4.1 :](image-5.png "Figure 4")
4243![Figure 4.2 :](image-6.png "Figure 4 . 2 :Figure 4 . 3 :")
2Its
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		Materials and Metallurgy
		
			VKManchanda
		
		
			GB SNarang
		
		
			2005
			Khanna Publishers
		
	
	6th Edition




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		Materials Science and Engineering" A
		
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			SidneyHAvner
		
		
			2010
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		Present Accomplishments and Future Challenges of Quenching Technology
		
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				the 6th International Federation of Heat treatment and Surface EngineeringCongress, IFHTSE, Kyongju, Korea
		
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