Estimation of Hardenability by Jominy End Quench Test

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Author: Michael Smetana

Objective

                The objective of this experiment is determine the effects of cooling rates on 4140 steels by using the Jominy End Quench Test.

Materials Used

                The materials used were a mounted specimen of 4140 Alloy Steel of dimensions 25mm diameter and 100 mm in length, file, Vice, Ruler, permanent marker, and Rockwell C Hardness testing machine. 

                In general this experiment would also consist of an 880 degree furnace, water spout, and a mount for the specimen, but because lab time isn't long enough to heat a sample for two hours then cool it at a uniform these materials were not used.

Background

                Hardenability

                The ability of an alloy steel to transform to Martensite for a particular quenching treatment.

                Bainite

                A fine non-lamellar structure consisting of Ferrite & Cementite along with Carbites.

                Jominy end quench test

The test sample is a cylinder with a length of 102 mm (4 inches) and a diameter of 25.4 mm (1 inch).   The steel sample is normalized to eliminate differences in microstructure due to previous forging, and then austenitized. This is usually at a temperature of 800 to 900°C. The test sample is quickly transferred to the test machine, where it is held vertically and sprayed with a controlled flow of water onto one end of the sample. This cools the specimen from one end, simulating the effect of quenching a larger steel component in water.

The cooling rate varies along the length of the sample from very rapid at the quenched end, to rates equivalent to air cooling at the other end.

The round specimen is then ground flat along its length to a depth of 0.38 mm (15 thousandths of an inch) to remove decarburised material. The hardness is measured at intervals from the quenched end. The interval is typically 1.5 mm for alloy steels and 0.75 mm for carbon steels.

High hardness occurs where high volume fractions of martensite develop. Lower hardness indicates transformation to bainite or ferrite/pearlite microstructures.[i]

Procedure

                The first step of this project would consist of heating the sample to 880 Degrees Celsius for two hours then cooling it according to the Jominy Test, but since the amount of time allotted for this experiment was not sufficient enough these steps were completed at an earlier time and our experiment started at the Filing Stage.

                First thing to do after completing the Jominy Test was to place the sample in a Vice, Clamping it down securely and then Filing of a Flat grove about 3-5 mm wide all the way down the specimen.  After the grove has been successfully grinded out approximately 25 marks were made at every two millimeters started from the cooled end.

                The next step is to remove the sample from the vice and place it on the Rockwell Hardness testing machine.  Run the machine 25 times at each 2 mm increment as marked, then record all the values.

Results

            The Pure Data is defined by the Blue Line while the Polynomial interpolation is defined by the Red Line.

Discussion

                Noticing that the results didn't quite look like the projected graph of the Jominy End Quench Test Graph, it is still easy to see how a material strength can be reduced by the cooling rates and the distance from the cooling end.

                To put this into comparison to general quenching practices in the USA of submerging the entire sample in Water at one time, it may show that even when using this type of quenching technique the outer edge of the material may be harder then the inner portion of the specimen. 

                Since each type of material has its own cooling rates and hardness structure it is difficult to say how a ceramic specimen would act as compared to these metallic specimens.  From past experiences rapid cooling of ceramic materials leads to a cracks in the material as it cannot release the stress build up.

Conclusion

                The results show that the materials strength is affected by the rate of cooling on the sample.  Using these results a specific type of cooling can be used to achieve desired results or desired metallic structures such as the formation of Bainite.

                In this experiment I have learned that certain portions of the metal will exhibit different properties based on the rate of cooling and the way cooling takes place.  This means to me that just testing one section of the metal will not always be a global hardness for this metal, then to get more accurate results it would be required to take more tests or to know the quenching method.