Hardenability and Jominy End Quench Test

The influence of alloy composition on the ability of a steel alloy to transform to martensite for a particular quenching treatment is related to a parameter called hardenability. For every different steel alloy there is a specific relationship between the mechanical properties and the cooling rate. Hardenability is used to describe the ability of an alloy to be hardened by the formation of martensite as a result of a given heat treatment. One standard procedure that is widely utilized to determine hardenability is the Jominy end quench test. The heating and cooling treatment of the steel specimens have a great effect on the phase of the microstructure of the steel specimen.

The addition of alloys or coarsening of the austenitic grain structure increase the hardenability of steel. Any steel that has low critical cooling rate will harden deeper than one that has a high cooling rate of quenching. The size of the part that is being quenched has a direct effect upon the hardenability of the material.

The objective of the experiment is to take readings in the Rockwell C scale along the flat surface of the Jominy specimen and to plot Hardness versus distance from quenched end.

Equipment:

Electric Furnace

Jominy End Quench Test Fixture

Jominy Specimens (Made as per ASTM standard)

Rockwell Hardness Testing Machine

Procedure:

1. Preheat the furnace to 1700 oF.

2. Place the Jominy specimen in the furnace and soak for one hour.

3. Turn the water on at the Jominy sink. Adjust the free water column to about 2.5 in. Swivel the baffle plate to block the water column so that there is no contact between water and the test specimen when the test specimen is initially placed on the fixture.

4. Remove the Jominy specimen from the furnace and place it in the fixture as shown in Figure 1. Swivel the baffle out of position so that the water impinges on the bottom of the specimen without wetting the sides of the specimen. Leave water running for about 15 minutes.

Figure 1. Jominy end quench fixture

5. Remove the Jominy specimen from the fixture and grind a flat on the side of the specimen.

6. Mark points on the ground surface at an interval of 1/16 in. up to 2 in. distance from the quenched end as shown in Figure 2.

Figure 2.

7. Take readings at an interval of 1/16 in. by measuring the Rockwell C hardness at each point marked in the previous step.

8. Plot the data for Rockwell Hardness versus Distance from quenched end as shown in Figure 3.

Figure 3

Determining the Hardenability of a Material:

In the Jominy test the quenching medium is eliminated from consideration by standardizing the effect of the quenching medium on a steel sample regardless of the medium. This is designated by the ratio of the heat transfer factor F to the thermal conductivity K of the material. This ratio is called H-factor which indicates the severity of a cooling rate.

H = F/K

F= Heat transfer factor

H= Thermal conductivity

H-Factor

Quench Conditions

Agitation

0.20

Poor oil quench

None

0.70

Strong oil quench

Violent

1.00

Poor water quench

None

1.50

Very good water quench

Strong

2.00

Brine quench

None

5.00

Brine quench

Violent

Table 1. H-Factors

Table 1 shows the H-values for various quench conditions and type of agitation. For H=0.20 the least severe cooling rate is observed. As H -factor increases the severity of the cooling rate also increases.

Figure 4

Figure 5

Figure 4 indicates the hardenability property of a certain type of steel regardless of the quenching medium. In order to find the effect of a certain type of quenching on the hardenability of a steel, additional graphs (Figure 5) that relate the size and the quenching medium must accompany the Jominy hardenability curves (Figure 4). Figure 5 represents the case where the hardness at the center of a bar with a specific size is to be calculated.

If a specific hardness is required for the center of a round part, first define the type of quench and the diameter of the part. Using Figure 5 draw a horizontal line from the diameter axis and intersect it with the F-Factor curve. Then, draw a vertical line from the intersection point and read the distance from the quenched end. Use the dis distance in Figure 4 to determine the hardness at the center of the part.

Example: a 1.5 in diameter steel part is quenched in oil with no agitation. (H=0.20). If we use a 3140 steel, what would be the hardness number at the center of the part ?

First go to Figure 5 and draw a horizontal line from 1.5 in. diamater and intersect it with H=0.20 . Draw a vertical line from the intersection point and read distance from quenched end, which is 3/4 in.

Figure 6

3/4 in is the same as 12/16 in. Go to Figure 4 and locate 12/16 in. Draw a vertical line from 12/16 in distance from quenched end until it intersects the hardenability curve for 3140 Steel. Read the hardness.

Figure 7

Jominy Number:

The location of a certain hardness can be described by a Jominy number by Jx = y, where x = hardness number (Rockwell C scale) and y = distance from quenched end (expressed as y/16 in ).

A Rockwell hardness of 35 Rc 5/16 in. from the quenched end is written as a Jominy number, where x = 35 and y = 5:

J35 = 5

A Rockwell hardness of 30 Rc 1/2 in. from the quenched end is written as a Jominy number where x = 30 and y = 8:

J30= 8

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Last Update: November 16, 1999

By: Serdar Z. Elgun