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Investigate the structural changes, which take place when steel is cooled at different rates and the effect these changes have on the material's properties.

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Introduction

Heat Treatment of Steel Name: Andrew Holmes Student Number: 2103086 Course: BSc Forensic Science Year: 1 Unit: Material Science Date: 21/02/02 Introduction The purpose of this experiment is to investigate the structural changes, which take place when steel is cooled at different rates and the effect these changes have on the material's properties. The aims and objectives of the experiment were as follows: * To investigate the structural changes that occur when steel is cooled at different rates. * To investigate the effect of these structural changes on the material's properties by testing the hardness of the steel specimens using the Vickers Hardness Test. * To examine each specimen under an optical microscope. * To analyse the metallurgical structure of the steel specimens in each case and compare the rates each specimen was cooled with its hardness. Theory Hardness can be defined as the ability of a material to resist surface abrasion. A phase can be defined as a homogenous body of matter exiting in some prescribed physical form. A grain is a region of orderly packed atoms within a metal. The surfaces that divide the different regions of orderly packed atoms are termed as grain boundaries. ...read more.

Middle

Each specimen is prepared for microscopic examination by grinding and polishing. A mirror like surface free from any visually observed scratches is needed for this experiment. This is to ensure when looking at the surface with an optical microscope, that the pyramid shape is easily recognisable. The hardness is then tested using the Vickers Hardness Machine. The Vickers pyramid hardness test uses a square based diamond pyramid as the indentor. In this test, the diagonal length of the square impression is measured by means of a microscope which has a variable slit built into the eyepiece. The width of the slit is adjusted so that its edges coincide with the corners of the impression, and the relative diagonal length of the impression is then obtained from a small instrument geared to the movement of the slit, and working on the principle of a revolving counter. The ocular reading thus obtained is converted to Vickers Pyramid Hardness Number (VPN) by reference to tables. Results Attempt Number Quenching Media 1st 2nd 3rd Average Hardness Ocular Hardness Ocular Hardness Ocular Hardness Water 443 283 449 276 N/A N/A 279.5 Oil 527 200 527 200 N/A N/A 200 Air (Annealed) ...read more.

Conclusion

This process of heating then slowly cooling causes the steel to be softer and therefore more machinable. Whilst the tensile strength is little affected by this treatment, ductility is increased. This specimen took the longest to cool because there was less heat loss from the steel into the surrounding environment, and the warm air in the furnace prolonged the heat insulation of the steel. Its metallurgy should then follow that it was the least hard. This is shown in the results of the experiment. The fourth specimen was normalised. This is done to refine the grain of steel, which improves its ductility and toughness. After a short period of 'soaking', the steel is removed from the furnace and cooled in still air when the fine-grained austenite will transform to fine-grained ferrite and pearlite. This cooling process takes a long time because although the initial heat loss from the steel specimen is high on removal from the furnace, the subsequent heat loss decreases gradually over time; thus the cooling rate is slow. Conclusion In summary, the effects that different rates of cooling have on steel are shown to increase the hardness of steel by rapid cooling in water (1st specimen) or decrease the hardness by softening it through annealing (3rd specimen). Refernces Higgins, R.A. (1994) Properties of Engineering Materials 2nd edition. London: Arnold ...read more.

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