Editor’s Note: Throughout the years, information on the Jominy test has been highly searched on our website by Industrial Heating readers. In this issue, you are treated to two articles discussing this test, its technique and application.
The Jominy, or end-quench test, determines the capacity of steel to harden under defined heating and cooling parameters. ASTM A255 (Standard Test Methods for Determining Hardenability of Steels) provides the guidelines/procedures for performing the Jominy test.
Product Evaluation Systems, Inc. (PES) is an independent testing lab that provides chemical, mechanical, metallurgical and nondestructive testing for a wide variety of manufacturers. The Jominy test is one of the many standard tests that are performed at PES on a weekly basis.
A standard Jominy test consists of:
1. Normalize the wrought product for one hour based on ASTM A255 procedures.
2. Machine a 1-inch-diameter bar x 4-inch-long test specimen.
3. Austenitize the test bar for 30 minutes in a heated furnace.
4. Remove the specimen from the furnace and place in a quenching apparatus that holds the specimen vertical. Water is immediately applied to the bottom end only for cooling, per ASTM A255 procedures. The quenching must be a minimum of 10 minutes.
5. Remove the specimen from quench and grind two flats 180 degrees apart along the length of the bar to allow for hardness testing.
6. Perform Rockwell hardness across flat side, starting at the bottom (cooled fastest) in 1/16-inch increments toward top (cooled slowest).
7. Plot data: Y axis – Rockwell hardness number, X axis – distance from quenched end in inches.
As with any heat-treatment test, careful preparation of test specimens and following procedures is mandatory to obtain accurate results. The following are helpful hints based on ASTM A255 and our experiences at PES.
• Be sure that the furnace is at desired temperature prior to heating the Jominy bar.
• Monitor the temperature of the bar with a thermocouple. Drill a hole in the top of the bar axially to accept the thermocouple wire.
• Scale buildup on the surface can severely affect the quenching. The furnace atmosphere should be such to minimize scaling and decarburization. Heating in a nitrogen-atmosphere-controlled furnace will accomplish this. Another solution is to place the bar vertically in graphite “cups.”
• Transfer time from the furnace to start of quenching is critical – not more than five seconds. May want to practice with “trial” Jominy bars prior to actual run.
• The quenching support apparatus should be dry prior to accepting the Jominy bar.
• The water temperature should be maintained between 40-85°F. Place a thermometer in the tank during quenching to monitor temperature.
• Consideration of quench-tank capacity is necessary to maintain required temperature, especially if quenching sequential specimens. At PES, we use a 15-gallon tank.
• Make sure there is no additional cooling (e.g., a fan) that could affect specimen during quenching.
• Quenching should be maintained for a minimum of 10 minutes, after which the specimen can be removed from holder. Be careful because the specimen may still be warm. Quenching under water (immerse entire bar) is allowed at this point.
• Grinding of two flats – 180 degrees apart – the full length of the bar is required. The grinding must be performed so as not to affect the quenched structure of the bar. Use of a surface grinder with coolant is sufficient (Fig. 1). Do not heat the specimen in any way during grinding operation. A minimum of 0.015 inch (0.38 mm) depth should be removed to eliminate possible decarburized layer. PES has standardized the removal of approximately 0.050 inch (1.27 mm) per side, which also aids in stabilization during hardness testing.
• Etching the flat with 5% nitric or 50% hydrochloric acid will help detect tempering during grinding. The presence of lighter or darker areas indicates that hardness and structure may have been altered in grinding. Note: refer to ASTM A255 for etching details.
• Hardness (Rockwell C) impressions must be accurately spaced along the length of the flat surface (steps of 0.0625 inch/1.6 mm). Use of a fixture with a micrometer is recommended to achieve repeatable results (Fig. 2). It has been found that with reasonable operating care and a well-built fixture it is practical to locate the center of the first impression 0.0625 +/-0.004 inch (1.59 +/-0.010 mm) from the quenched end. The variations between spacings should be even smaller.
• One of the test-specimen flats must rest firmly on the hardness-machine anvil. It is important that no vertical movement be allowed when the load is applied.
• Checks against a standard test block (Rockwell C) should be performed periodically. Place the test block between the specimen and indenter to check the seating of the indenter and the specimen simultaneously (Fig. 3). Note: refer to ASTM E18 for use of test blocks and surface conditions.
• Readings shall be taken in steps of 0.0625 inch (1.6 mm) for the first 16 sixteenths (25.4 mm), then 18, 20, 22, 24, 28 and 32 sixteenths of an inch. Values below 20 HRC are not recorded because such values are not accurate.
• Hardness readings should preferably be made on two flats 180 degrees apart. Testing on two flats will assist in the detection of errors in specimen preparation and hardness measurement. If the two probes on opposite sides differ by more than 4 HRC points at any one position, the test should be repeated on new flats 90 degrees from the first two flats. If the retest also has greater than 4 HRC points spread, a new specimen should be tested.
• For reporting purposes, hardness readings should be recorded to the nearest integer, with 0.5 HRC values rounded to the next higher integer.
• PES’s typical test data (Table 1) and plotting of test results is shown in Figure 4.
This list of helpful hints is based on our experience at PES along with information gleaned from ASTM A255 and E18. IH
For more information: Contact Walt Moorhead, president, Product Evaluation Systems, Inc., 637 Donohoe Road, PO Box 722, Latrobe, PA 15650; tel: 724-834-8848; fax: 724-834-9151; e-mail: Walt@PES-Testing.com; web: www.PES-Testing.com
1. ASTM A255, Standard Test Methods for Determining Hardenability of Steels
2. ASTM E18, Standard Test Methods for Rockwell Hardness of Metallic Materials