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Unformatted text preview: v,” V" German - Jordan University E . . Experiment No. 4 "Hardening Steel Using Different Quenching Media" Theoretical Course by: Dr. Naseem Haddad —— Director of NID&TC -RSS Practice preformed by: Eng. Abeer G. Daood. Objectives 1— Heating steel to appropriate temperature, and then quenching it using different quenching media. - 2- Measuring the hardness of steel after quenching by different media. 3— Studying the changes in hardness of steel due to the changes of the quenching media. Introduction Plain carbon steels and alloy steels are among relatively few engineering materials, which can be usefully heat-treated in order to vary their mechanical properties. Steels are heat treated, because of the structural changes that can take place within solid iron carbon alloys. The various heat—treatment processes appropriate to plain carbon steels are: a— Full annealing. b— Normalizing. 0- Process annealing. d— Hardening. e- Tempering. In all above process the steel is heated slowly to the appropriate temperature according to its carbon content and then cooled, Figs. 4.1 and 4.2. 0 Composition (Atom % C) 25 100 % Fe Composition (Weight % C) 100 % FegC Fig. 4.1 Fe—C phase diagram. ‘16 "' :3. PM Failigfiw Refine -. ‘ ‘7' u -' “we!” _.. 3H; ._ .v' Kg‘mpemrwe. m "f gang-«aims: Ali‘s-radii 55in“ Jammie +fle~3¥re ' x +37g‘ie'm" '22: . mimawamgmmmumwum (a: ' W. M an M {Tandem perv rant“. Fig. 4.2 Heat treatment temperatures for plain carbon steels. It is the rate of cooling which determines the ultimate structure and properties that the steel will have at the end of each of the above mentioned heat treatment process. Figure 4.2 shows the temperature band from which plain carbon steels are cooled, when they are quench hardened. It can be seen that this temperature band is not continued below approximately 0.3%, as no appreciable hardening takes place when steel is heated. The process of hardening involves rapidly quenching the steel from appropriate temperature into quenching media. Hypereutectoid steels are heated to 30~SO°C above their upper critical temperature (A3) prior to quenching. There is no particular 'advantage in heating hyper-eutectoid steels above their Acm when hardening them and, in practice; the hardening temperature normally used is just 30—50°C above their lower critical temperature (A1). Quenching hypereutectoid steels from this lower temperature helps to prevent cracking and distortion. If the steel is cooled-quickly (quenched) from its hardening temperature, the equilibrium transformation into Pearlite and Ferrite or Pearlite andCernentite do not have time to take place. Instead a structure called Martensite is formed. This is due to the allotropic transformation of the face—centered Austenite, 'which eau dissolve a high percentage of carbon (see Fig. 4.1), to the body—centered Ferrite, which eau only dissolve rather a little percentage of carbon. As the steel is cooled quickly (quenched), then there will not be enough time for equilibrium transformation, and the diffusion of the carbon atoms during this transformation. Accordingly the unit cells formed will be distorted, due to the super saturation of these body centered unit cells with carbon atoms (Matensite). This distortion of the unit cells Cause' slip Virtually impossible to occur, and the steel becomes very hard and brittle. The most commonly used quenching media in order of severity are: > a- Compressed air blast. b- Oil. 0- Water. d- 10% Brine (water +10% salt) ‘17 , The choice of quenching media depends upon the type of steel being treated and the resultant properties required. The highest cooling rate, from the quenching media mentioned above, can be" achieved by the 10 % Brine, while water can provide a cooling rate lower than the 10% Brine, but higher than oil. The lowest cooling rate eau be obtained by quenching the heated steel with compressed air blast. Procedure 1. 2. 8"?!“ Cut three specimens with similar dimensions from a steel containing more than 0.3%C bars. Heat the furnace to the hardening temperature of the specimens, which depends on the percentage of carbon of the'steel bar used. Then put these three specimens close each other inside the heated fumanelnmdertdpmtect these four from oxidation, it is better to cover them with sand inside a steel or cast iron container. ' Leave the specimens inside the furnace for 30—60 minutes. Withdraw one specimen from the furnace using a nickel tongs.- Quench it in one of the four quenching media, and stir it in the liquid during quenching, until it becomes cold. . Repeat step 4 and 5 for the other three specimens, using a different quenching medium for each steel specimen. Choose a surface of each four specimens, and grind it properly to prepare it for - hardness test. Measure the hardness of the ground surface of each specimen in five. positions. starting from the core towards the outer surface. Take the average of these measured five hardness readings for each specimen. Compare between the results, and discuss the effect of each quenching medium On the hardness of the steel specimens. 18 ...
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