To determine the hardenability of plain carbon steel. Theory Hardenability is the ability of a steel to be hardened and form martensite during quenching. Hardenability indicates the depth of hardness which is obtained from quenching process, and it is very important to the components of machine especially tool steel. One of the methods to determine the hardenability of steels is Jominy End-Quench Test. Hardening usually involves quenching where the steel is heated to austenite phase and fast cooling in the quench medium such as water, oil, salt solution, or air to produce microstructure of martensite.
The martensite gives hard and brittle properties of steel. Usually, for heavy steel components, the hardness decreases at the core of component because the microstructure formed is ferrite and pearlite. While at the surface of the component have higher hardness. The difference of the hardness between the core and surface can be explained by Continuous Cooling Transformation or CCT diagram of the steel. If the cooling rate of steel does not cross the transformation curve of the diagram, then the hardness on the whole steel can be obtained.
CCT diagrams allow prediction of the final microstructure of the steel taking into account the continuous nature of the process during cooling of austenite. Other than that, CCT diagrams take into consideration continuous cooling during quenching; the cooling curve assumes a constant cooling rate. As we can see from the diagram, martensite form at the temperature below than 200? C, it mean that the specimen with faster cooling rate will able to form more martensite and higher hardness. Continuous Cooling Transformation diagram | |
The above diagram show the time-temperature isothermal transformation diagram for the plain carbon steel Apparatus Specimen of plain carbon steel, furnace, Jominy End-Quench Unit, and Vickers Harness Tester. Experimental Procedure (a) Heat the specimen to temperature of 850°C and soak at that temperature for 20 minutes.
After that, its remain almost constant while the distance increase. In our experiment, there are some test errors where we are having small rise in our reading of the hardenability value which suppose decrease continuously. This is due to the uneven surface of the tested specimen or cause by the unbalanced cooling rate throughout the specimen. (b) Gives your opinion on the correlation between cooling rate of specimen and the graph obtained. The cooling rate at the end of the plain carbon steel are much higher compare to the.
Thus, in the graph it shows the highest hardness at the early stage and then slowly decreases when approaching to the core of the steel. We also can say that, at the end of the steel has the highest hardness which cause by the high content of martensite. However, the content of the martensite and hardness will drop when approaching to the centre of the steel. We can conclude that the cooling rate decreases from the end-quench to the centre of the steel, resulting in decreasing of the hardness.