1. INTRODUCTION
 

The extensive use of the experimental work particularly in design and construction of a new mechanical and structural elements, and the use of testing procedures for the controlling processes of manufacturing and construction are important and well recognized forms of our technical development.

All the branches of engineering specially those dealing with structures and machines, mostly concerned with metals. The properties of rnetatlic alloys are usually determined by the tests.

Steels, for example are used in multitude of applications involving their ability to withstand service loadings without permanently deforming and rupturing. On the other hand the ability of metals undergoes large permanent deformation, thus permitting the formation of different shapes under the applications of proper forces [2],

It is necessary for an engineer engaged in testing work to have a general understanding of the common methods of testing properties of the metals. The mechanical properties are the most important requirements of the metals from the engineering point of view in selecting them for design purposes. Mechanical properties and microstructure of metals describes their behaviour under mechanical and physical usage [3,5].

The methods of measurement of these properties and evaluating their significant form, the subject matter of this research work. 'The Effect of Heat Treatment on Microstructure and Mechanical properties of low carbon Steel", which was selected for the investigation.

Low carbon steel SAE 3010 steel is extensively used for deep drawing of motor car bodies, motor cycle parts, and other domestic applications, must processes the desired mechanical properties. The required properties can be altered by method of manufacturing process and heat treatment [6]. Therefore, the present work was planned to investigate the relationship between mechanical properties, microstructure and heat treatment for intelligent selection of manufacturing processes, properties, and application for particular purpose.

2. EXPERIMENTAL PROCEDURES

2.1 Material and specimen preparation.

Low carbon steel sheet was received from local market having the following chemical compositions:

O0.12%, Si-0.23%, Mn=0.49% and Al=0.05% by weight.

This was confirmed from Central Testing Laboratories at Pakistan Steel Mills.

Ten standard specimens were prepared for tensile tests as shown at Fig. 1. Each group contains two specimens has given abbreviation as listed in Table. 1.

The various heat treatment operations such as full annealing, normalizing, recrystallizing annealing and hardening operations were carried out on specimens using muffle furnace, ranging heating temperature upto HOOoC . The purpose of various heat treatment operations were to obtain the variation in mechanical properties and microstructure. The detail of such heat treatment operations are given in Table. 2.

The tensile tests were carried out on BSI standard specimens using Tensometer in the Department of Mechanical Engineering, Mehran University of Engineering & Technology Jamshoro. Load vs extension diagram of each specimen was plotted automatically on the graph paper as shown at Fig. 2. The tensile strength and ductility of each specimen were calculated [1J and the values are given in Table. 3.

The hardness of each specimen was also checked using micro vicker hardness tester at the local of 500 gram in the Department of Metallurgical Engineering, Mehran University of Engineering & Technology Jamshoro. The results are given in Table. 4.

2.4 Metallography

The metallographic investigations were also carried out on specimens using inverted metallurgical microscope. The specimens were metallographically prepared and investigated after and before heat treatment operations. The microstructure of each specimen is given at Figs. 3-7.

3 RESULTS AND DISCUSSIONS

The microstructure of low carbon steel SAE 1010 consists fine grains of ferrite and small amount of pearlitc Fig.3. Tensile strength of specimen as received AR1 is 41.4 Kg/mm² and hardness VHN 116, elongation 30% and reduction in area 10.1%. Such mechanical properties of specimen AR1 was due to prior plastic deformation during rolling operation. Hardness and tensile strength of steel increases due to mechanical working while the ductility, elongation and reduction in area decreases [1,4].

FIG I. TENSOMETER TEST  SPECIMEN

TABLE 1. ABBREVAT1ON OF SPECIMENS

TABLE 2. VARIOUS HEAT TREATMENT OPERATIONS

TABLE. 3. TENSILE PROPERTIES OF SPECIMENS

TABLE 4. VICKER HARDNESS OF SPECIMENS

By comparing the mechanical properties of specimen RE2 with AR1 sample RE2 shows the lowest tensile strength (37.64 Kg/mm²) and hardness (VHN 102) reduction in area 16% and elongation 24%. This decrease in T.S and hardness were due to recrystallizing annealing at 690°C. Recrystallizing annealing reduces the strain hardening effect, refine the grains and reform the deformed grains [9]. By considering the experimental result given in Table 3. The specimen AN3 which was annealed at 910°C for 5 minutes. Result shows that full annealed specimen has higher tensile strength, hardness and higher ductility as compared to specimen RE2. Such increases in properties was due to proper reaustenitizing temperature of steel. At this temperature the deformed structure is fully homogenized and during the slow cooling form austenitic range to room temperature final microstructure consists fine ferrite grain in which the pearlite is more uniformally distributed throughout the matrix of ferrite Fig. 5. It was suggested [8,9] that full annealing increases the tensile strength and hardness by refining of grains size and uniform distribution of the phases. In such treatment, the steel is more suitable for further manufacturing process.

The mechanical properties of the normalized specimen NR4 showing the higher values as compared to AR1 , RE2 and AN3 The increase in tensile strength and hardness as compared other annealed and as received specimen was due to proper austenitizing temperature 910°C and higher rate of cooling. It was suggested that a normalized steel has higher tensile strength and hardness due to the fast rate of cooling which reduces the space between ferrite and cementite plates (peartlited). Normalized steel has fine pearlite which increases the tensile strength and hardness.

The highest tensile strength 75.29 Kg/mm² and highest hardness (VHN 160) were obtained in WQ5 specimen. This specimen was autenitized at 910°C for 5 minutes and then water quenched. This treatment increases the tensile strength and hardness but decreases the elongation and reduction in area 18% and 4.8% respectively. When low carbon steels are rapidly quenched from its austenilic range to room temperature, the austenite will decomposed into a mixture of same low carbon martensite and finer pearlite Fig. 7. As a result of this microstructure which is hard. increases the tensile strength and hardness and reduces the ductility [9]. The load vs extension diagram (Fig. 2), of this specimen shows the behaviour of brittle material.

4.                    CONCLUSIONS

(i)         i. Tensile strength and hardness of low carbon
SAE1010 steel increases with plastic deformation
and ductility decreases due to strain hardening
effect.

(ii)        ii. Recrystallizing annealing reduces the tensile strength and hardness. This treatment increases the ductility by refining the deformed structure.

(iii)       iii. Full annealing resulted the best combination in mechanical properties such as high tensile strength, hardness and ductility. This treatment is recommended for further manufacturing process and also to be considered as final heat treatment after manufacturing process.

Normalizing treatment has also resulted the higher tensile strength and hardness then annealed specimen. This treatment is also recommended as final treatment after manufacturing.

iv. Water quenching of low carbon steel specimen has the highest tensile strength and hardness. However this treatment gives the lowest ductility compared to other treatments, it is recommended when the strength and hardness is the prime factor in design.

The microstructure of steel consists the matrix of ferrite and small amount of pearlite. The fines of pearlite increases by increasing in the rate of cooling.

The various heat treatment operations effects has also the microstructure and mechanical properties of low carbon steel. Thus by correlating these parameters i.e. heat treatment micro structure and mechanical properties would result more economical design and manufacturing of product.

ACKNOWLEDGMENT

The author is very much thankful to the authorities of Mehran University of Engineering and Technology, Jamshoro and General Manager Central Testing Laboratories, Pakistan Steel Mills, Karachi, for the provision of facilities for experimental work.

REFERENCES

[]]            Monsanto, "Instruction Manual of Tensometer Type W", Monsanto Ltd., England, 1962.

[2]           Lak, T.Y., "Engineering Physical Metallurgy and Heat Treatment", Mir Publication, 1979.

[3]                   Avner; "Introduction to Physical Metallurgy", Second Edition, McGrawHill. 1979.

[4]                   Holzcr, A.J., and Wrighl, P.K., "A Comparison of Results from Tensile Test Mechanical Testing and Machining", Jr. of Material Science Engineering, Vol: 51, p.p. 81-92, 1981.

[5]           Honey Combe; "Microstructure and Properties", Adward Arnold, 1982. *

[6]           David, D., and Oclmar, "Structure Properties and Heat Treatment", Pitman, 1983.

[7]           Zialkin,