The hardenability of three type of steels Essay
The hardenability of three type of steels
The purpose of the Jominy test in this experiment is to determine the hardenability of three types of steels, namely EN 8, EN 16 and EN 24.
This report will try to illustrate how the hardenability of these types of steels vary and why. This will be done by using the Jominy end quench test on each of the test pieces and then testing the hardness of the particular steel from the quenched end, to the end of the test piece. Taking these different points will enable me to find out how the hardness varies throughout the test piece considering the cooling rate of the metal and thus illustrate the hardenability of the steel. I will also analyse how adding different alloying elements to the steel can improve its hardenability.
Theoretically what should happen is that as the distance increases from the quenched end of the test piece the hardness should decrease. This is because when the metal is heated red hot (above its A3 temperature) and then rapidly cooled at one end, in this case by means of water at a temperature of 250C, the carbon doesn’t get enough time to precipitate out and is trapped which makes the metal harder. But the test piece will only be quenched at one end and so the steel will have different hardness relative to the distance from the quenched end.
Three test pieces EN 8, EN 16 and EN 24 were heated above their A3 temperature to create a homogeneous austenitic structure and then end quenched by means of water at 250C. Taking safety in to consideration it should be noted that the test piece should not be handled until the whole test piece has cooled down. After this the test piece was mounted in the Vickers hardness testing machine and then the hardness determined throughout the test piece. The measurements were taken from the quenched end at intervals of 1mm until 5mm and then the intervals of 2mm until 15mm and then intervals of 3mm until the end of the test specimen. The results were then tabulated (see fig 1.0) and a graph drawn (see fig 1.1).
The data table in fig 1.0 indicates that En 8 has got very poor hardenability, En 16 has got poor hardenability but is better than En 8 and En 24 has got good hardenability. This can be suggested from the fact that the highest hardness value for En 8 at the quenched end (701) is relatively high compared with the lowest hardness value at the end of the specimen (214). For En16, the highest hardness value is 683 and the lowest hardness value is 369 at the end of the specimen. For En 24, the highest hardness value is 788 and the lowest hardness value is 660 at the end of the specimen. As you can see the difference of the hardness of the quenched end of the test specimen En 24 is not too large and so the hardness values throughout the test piece are similar thus good hardenability. This is illustrated much better on fig 1.1.
On the graph, the curve of En 24 is more or less constant and consisting of no significant decreases in the gradient, thus implying a good hardenability. En 8 on the other hand, has got the worst hardenability out of the tree test pieces and is clearly shown by the distinguished fall in the gradient. Initially the hardness value is 701 at the quenched end then suddenly at 3mm from the quenched end the gradient falls significantly until the hardness value is 306 and the distance is 7mm. after this the gradient is relatively constant. There appear to be no outliers in the data and so it can be assumed to be reliable. If there are any errors, they might be due to the fact that it is very difficult to control the temperature of the water and so the cooling rate varies accordingly.
The factors which could affect the hardenability of the steel are:
The chemical composition
Austenitic grain size prior to cooling.
The chemical composition or what the material is made up of is a big factor which affects the hardenability. Fig 1.2 shows what each type of steel contains. For example it can be seen from the table that only En 24 has got some nickel and chromium elements. The presence of nickel in steels results in increased strength by grain refinement so that the dislocations are trapped. The presence of chromium in a metal improves the susceptibility to heat treatment, thus giving it good hardenability properties. Also adding molybdenum to the metal reduces grain growth and has the ability to keep its hardness at high temperatures.
Austenitic grain size is related to the hardenability by the fact that as the austenitic grain size increases the hardenability increases because the grain boundary area is decreasing and so the area for ferrite transformation decreases. In other words, if you have large austenite grain size it will take longer for the ferrite transformation to take place and so the cooling rate will be made to slow down thus improving the hardenability. This is shown on the iron carbon phase diagram Fig 1.3.
Also when the microstructure of the metal is in austenite form, that is to say it is face centred cubic, the carbon is absorbed more as the interstitial gaps are bigger and so when it is quenched it traps the carbon by giving it no time to precipitate out of the metal when it transforms to body centred cubic. This makes the metal extremely hard and might even make the metal crack. So the hardenability depends on how you can control the grain growth of austenite and adding alloying elements to the metal both of which prolong the cooling time giving the metal good hardenability.
I have investigated the aim of this experiment which was to determine the hardenability of three types of steels, namely En 8, En 16 and En 24. From carrying out the Jominy test it can be concluded that En 24 had the greatest hardenability property En 8 had the poorest hardenability and En 16 was in between the two. This is shown on fig 1.1. En 24 had good hardenability because it had extra alloying elements such as nickel and chromium, which enhance the hardenability property by grain refinement. The key to good hardenability is controlling the ferrite transformation from austenite and to do this it is necessary to produce large austenite grains. Ferrite transformation can also be controlled by adding alloying elements which reduces grain growth.