UNIT 11394: SELECT AND SPECIFY MATERIALS FOR MECHANICAL ENGINEERING APPLICATIONS

LAB 2: JOMINY END QUENCH

Object: To show hardenability in a material.

Inroduction:

The Jominy end quench test is used to measure the hardenability of a steel, which is a measure of the capacity of the steel to harden in depth under a given set of conditions.

Knowledge about the hardenability of steels is necessary to be able to select the appropriate combination of alloy steel and heat treatment to manufacture components of different size to minimize thermal stresses and distortion. The Jominy end quench test is the standard method for measuring the hardenability of steels. This describes the ability of the steel to be hardened in depth by quenching.

Hardenability depends on the chemical composition of the steel and also be can affected by prior processing conditions, such as the austenitizing temperature.

Hardenability:

Hardenability is the ability of a steel to partially or completely transform from austenite to some fraction of martensite at a given depth below the surface, when cooled under a given condition. For example, a steel of a high hardenability can transform to a high fraction of martensite to depths of several millimetres under relatively slow cooling, such as an oil quench, whereas a steel of low hardenability may only form a high fraction of martensite to a depth of less than a millimetre, even under rapid cooling such as a water quench. Hardenability therefore describes the capacity of the steel to harden in depth under a given set of conditions.

Steels with high hardenability are needed for large high strength components, such as large extruder screws for injection moulding of polymers, pistons for rock breakers, mine shaft supports, aircraft undercarriages, and also for small high precision components such as die-casting moulds, drills and presses for stamping coins. High hardenability allows slower quenches to be used (e.g. oil quench), which reduces the distortion and residual stress from thermal gradients.

Steels with low hardenability may be used for smaller components, such as chisels and shears, or for surface hardened components such as gears.

Test specimens and their preparation:

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 normalised to eliminate differences in microstructure due to previous forging, and then austenitised. 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.

Apparatus for hardening the test piece:

The specimen is suspended from a wire and held in a furnace to austenitise the microstructure at around 900ºC. It is then carefully and quickly moved to the quenching machine and positioned above a water jet. The water jet is started and sprayed onto the bottom of the specimen until the specimen is cool.

Quenching the sample:

As the water jet sprays onto the end of the hot, glowing specimen, a cold dark region spreads up the specimen. The cold region has transformed from austenite to a mixture of martensite, ferrite and

pearlite. The proportions of the phases at any position depend on the cooling rate, with more martensite formed where the cooling rate is fastest. Ferrite and pearlite are formed where the cooling rate is slower.

Test procedures:

Heating the test piece . The test piece shall be heated to the hardening temperature of about 800 to 900oc and held at the temperature for about 30 minutes. Precautions should be taken so as to avoid the carburization and decarburization and to keep scaling to a minimum.

Hardening the test piece. The water flow shall be adjusted so that the jet of water rises to a free height of 65mm above the 12.5mm orifice without the test piece in position. Splashing of the sides of the test piece during quenching should be avoided. The time between the removal of the test piece from the furnace and the beginning of the quench shall be not more than 5 seconds. The temperature of the water should be between 5 to 30oC and the jet shall be directed against the bottom face of the test specimen for not less than 15 minutes.

Preparation of surface for hardness testing. The surface of the test piece shall be prepared for the measuring the hardness by grinding flats along the full length of the hardened test piece to depth of 0.4mm below its original diameter.

Measurement of hardness after hardening

The Rockwell hardness test should be carried out in accordance with BS891. Hardness values shall be determined at positions of 1.5mm, 3mm, 5mm, 7mm, 9mm, 11mm, 13mm and 15mm and at 5mm intervals thereafter from the water- quenched end of the test piece using the Rockwell C test.

0 2 4 6 8 10 12 14 160.05.0

10.015.020.025.030.035.040.045.050.055.060.065.0

Hardenability chart (HRC/mm)

Distance(m m)

1.5 3 5 7 9 11 13 15 20 25 30 35 40 45 50

HRC 56.9 55.5

54.8 54 53 50.8

47 43.8

38.9 35.7 33.45 32 31.5

31.2 31.0

Results: By comparing the above graph with the graphs given in the handout, it can be concluded that the given test material is AISI 4140.

Test piece diameter 25.03mm

Chemical composition: C- 0.4%, Mn-0.8%, Cr-0.95% and Mo-0.25%

Conclusion:

As, we go further away from the quenched end of the test specimen the hardness decreases which is a result of microstructural variation which arises since the cooling rate decreases with distance from the quenched end. The cooling rate along the Jominy test specimen varies from about 225 °C s-1 to 2 °C s-1. This happens because when we heat up the specimen up to a temperature of about 800 to 900oC the steel gets austenitised which then transforms into fractions of martensite if the rate of cooling is relatively high otherwise, it transforms into bainite or ferrite/pearlite microstructures if the cooling rate is slow.

(Mehul Bansal)