1 Scope of application

This international standard describes a method for testing the hardness of steel by the end destruction method (Jomini test), in which the test piece is 25 mm in diameter and 100 mm in length.

Note: In accordance with the agreement and in order to define the scope of application, the tests described in the international Standard may be replaced by calculations of the Jomini curve based on accepted mathematical models (see Annex C). In case of dispute, the test shall be carried out.

2 Standard reference

The following standard documents include the relevant provisions referenced in this International standard. Changes or revisions to relevant publications do not apply to this document. However, we encourage parties to agreements based on the international standard to investigate the possibility of using the latest versions of the following standard documents. For undated references, the latest version of the standard document in question may apply. Iso and members of the International Electrotechnical Commission keep records of international standards currently in force.

ISO 6507-1, Metallic materials -Vickers Hardness tests - Part 1: Test methods.

ISO 6508-1, Metallic Materials -Rockwell Hardness tests - Part 1: Test methods (classes A, B, C, D, E, F, G, H, K, N and T).

ISO 6508-2, Metallic Materials -Rockwell Hardness Tests - Part 2: Verification and calibration of test machines (classes A, B, C, D, E, F, G, H, K, N and T).

ISO 6508-3, Metallic Materials -Rockwell Hardness tests - Part 3: Calibration of contrast blocks (grades A, B, C, D, E, F, G, H, K, N and T).

3 Principle

The test included:

a) Heat a cylindrical row test piece to a specific temperature within the range of austenite for a period of time;    

b) Under specific conditions, quench one end of it with water;

c) The hardness is detected at a specific point on the longitudinal plane of the test piece to determine the hardness of the steel by changing the hardness.

4 Sign and meaning

The form of test pieces with their preparation

5.1 Sampling

In the absence of specific requirements in the product standard and, unless otherwise specified in the order, regardless of the thickness (or diameter) of the product, sample samples may be taken by:

- Hot rolling or forging of test pieces between 30 mm and 32 mm in diameter;

- Machining of products with a diameter of mm and a shaft surface of at least mm (see Figure 1).

If the product is produced by continuous forging, it is recommended to consider a loss ratio of at least 8:1 before sampling.

In all forming processes before processing the test piece, the deformation caused by the product from all aspects should be as uniform as possible.

For independently forged pairs, the original transverse interface before deformation must be at least three times the corresponding required 30 mm to 32 mm transverse interface.

If special provisions are made, test pieces can be obtained through the appropriate forging procedure and tested under conditions similar to forging.

The axis of the plane of the test piece should be at the same position as it is from the surface of the product (see Figure 1). For this purpose, the test pieces will be labeled so that the position on the cylindrical list can be clearly identified.

5.2 Dimension

5.2.1 The test piece is a circular table with a diameter of 25 mm and a length of 100 mm.

5.2.2 The end diameter of the test piece without quenching should be 30 mm to 32 mm, depending on the line of the end. Shown in Figure 2 are two examples: a test piece with a flange or notch (to quickly find its center with proper support and place it in the position of the quenching operation).

5.2.3 If necessary, the test piece will be marked (on the opposite end of the end to be quenched) so that its position corresponding to the original product can be found.

5.3 Heat Treatment

Unless otherwise agreed, the specimen will be normalized prior to processing and quenching. The normalization process will be carried out within the average temperature range specified in the material standard. If the material standard does not specify a specific temperature for normalization, the normalization temperature will be specified by a special agreement or selected by the testing department. The residence time in the normalized temperature is about minutes.

Heat treatment is carried out in each test to ensure that there is no trace of decarbonization of the processed test piece.

5.4 Process

The cylindrical surface of the test piece should be processed using high-quality turning; The surface of the end of the test piece should be polished and finely polished without burrs (see Figure 2).

6 Instrument

An instrument is a device used to quench test pieces.

6.2 The position of the end of the water supply pipe relative to the test piece holder should ensure that the distance between the end of the water supply pipe and the end of the test piece to be quenched is (12,5 ± 0,5) mm (see Figure 3).  

6.3 The test piece support shall ensure that the test piece is precisely positioned in the center of the end of the supply pipe and that the test piece is firmly held during water spraying. When the test piece is placed on it should be dry, and the test piece should avoid splashing water when placed and before the actual quenching and during the end quenching.

6.4 When the test piece is not placed in the proper position, the height of the water column from the water supply pipe should be (65 ± 10) mm (see Figure 4).

The temperature of the water in the supply pipe should be (20 ± 5) °C.

If a comparison test is carried out, the water temperature used during the test should be the same as the above temperature.

6.5 The test piece should avoid being blown by the wind during heating and quenching.

Heating and quenching of test pieces

7.1 Heat

7.1.1 The test piece shall be uniformly heated to the temperature specified in the relevant product description or special protocol for 20 minutes, and then at the specified temperature for another minute. For special types of furnaces, the time may be determined by the time it takes for the center of the specimen to reach an ideal temperature as previously experienced (this temperature is measured by placing a thermocouple in a hole drilled along the central axis of the specimen head end).

7.1.2 Take care to minimize the decarbonization of the specimen and prevent oxidation and the appearance of scales.

7.2 Quench

7.2.1 The time interval between removing the specimen from the stove and starting to spray water shall not exceed 5 seconds.

During the period between removing the test piece from the furnace and placing the clamp in between, the test piece can only be held by tongs on the flange side or the notched side of the unquenched end.

7.2.2 Spray time is at least 10 minutes. After this, it can be immersed in cold water to complete the cooling work of the test piece.

2 Preparation and measurement of hardness after quenching

8.1 On the surface of the test piece, create two 180-degree planes for measuring hardness, both of which are parallel to the central axis of the test piece. If the test piece is made by machining, the two planes should be the same distance from the product surface (see Figure 1) and should be 0.4 mm to 0.5 mm deep. These surfaces should be machined with sufficient coolant and high-quality grinding wheels to avoid overheating, as the heat may change the microstructure of the test piece.

8.2 To ensure that the specimen does not become soft during sanding, dip the specimen in 5% (V/V) nitric acid solution until it is completely black. The color of the test piece should be uniform.

If spots appear, it proves that soft spots have appeared on the test piece, and the two planes should be rebuilt with an Angle of 90 degrees, and then the above method is used to prove that the two planes are acceptable. Here, the hardness measurement is carried out on the second set of planes and recorded in the test report.

8.3 Care should be taken to ensure that the specimen is firm and firmly held during hardness measurement.

The device that moves the test piece on the hardness measuring machine should be able to accurately locate the center of the plane and ensure that the gap between the notches is ± 0,1 mm. The latter is drilled on the axis of the plane according to ISO 6508-1, ISO 6508-2 and ISO 6508-3.

8.3.1 The Rockwell hardness measurement can be replaced by the Vickers Hardness HV30 measurement according to ISO 6507-1 if special agreement is made.

8.3.2 It is necessary to ensure that the edge of the hardness notch on the first plane does not affect the measurement on the second plane.

8.4 The location of the measuring point should ensure that one of the following two decisions can be made:

a) Draw a curve representing the change in hardness (see 8.4.1);

b) The hardness of one or more specific points is measured (see 8.4.2).

8.4.1 Draw a curve representing the change in hardness

8.4.1.1 In general, the distance between the first 8 points and the quenched end (expressed in millimeters) is as follows (see Figure 5) :

1,5 – 3 – 5 – 7 – 9 – 11 – 13 – 15

There should be a 5 mm gap between the subsequent points.

8.4.1.2 If the hardness of the steel is low, the distance between the first measuring point and the quenched end should be 1.0 mm; Then the distance between the second point and the quenched end shall be 11 mm, and the other points shall be 1 mm apart from each other. The last 5 points should be 13 mm, 15 mm, 20 mm, 25 mm and 30 mm from the quenched end.

Note: The distance between the hardness notch in 8.4.1.1 and 8.4.1.2 does not always conform to the minimum distance specified in ISO6508. However, according to this international standard, the hardness value obtained is generally accurate enough.  

8.4.2 Specific point hardness measurement

The hardness may be measured at one or more points at a specific distance from the quenched end, which may include/exclude the points in 8.4.1.1 and 8.4.1.2.

3 Expression of result

9.1 Hardness of any point

For any distance d, the average hardness of the two planes specified in 8.1 at that distance d will be recorded, with values in principle accurate to 0,5 HRC or 10 HV.

9.2 Hardness curve drawing

The distance d is plotted on the horizontal coordinate, and the corresponding hardness value is plotted on the vertical coordinate. The following ratio is recommended:

- On the horizontal coordinate, or 10 mm represents 5 mm distance, or 10 mm represents 1 mm distance on low-hardness steel;

- On the ordinate, 10 mm represents 5HRC or 50HV.

Note: If a computer aided device is used to draw a Jomini curve, the computer program automatically corresponds to the scale of the axes.

Description of the hardness characteristics of a specific steel

Use one of the following methods:

a) Draw the hardness curve;

b) The hardness of 3 points is given: one point is 1.5 mm from the quenched end (1 mm for low-hardness steel), and the other two points are determined according to the specific convention;

c) The hardness of two points is given, and the distance between these two points is determined by specific convention;

d) The hardness of a point at a specific distance from the quenched end is given;

e) Give the hardness table corresponding to the distance.

9.4 Code representation of measurement results

The measurement result can be expressed in the following form: the letter "J" followed by two numbers, such as

Jxx-d,

Where

xx refers to hardness, expressed in HRC or HV 30;

d refers to the distance between the measuring point and the quenched end, expressed in millimeters.  

Example

J35-15 indicates that the hardness of a point 15 mm from the quenched end is 35RHC; (See Figure 6),

JHV 450-10 indicates a hardness of 450 HV 30 at a point 10 mm from the quenched end.

Note: Code Jd = xx may also be used, see A.2.4.

4 Test results

The test report should include the following information:

a) Refer to this international standard, ISO 642;

b) Grade of steel;

c) Casting number;

d) Chemical composition;

e) Sampling method;

f) the conditions of normalization treatment and the conditions of heating of the test piece;

g) The hardness test method used;

h) Test results.

Note: It is recommended to indicate water temperature for comparison of test results.

Annex A (Standardization)

Product specifications

A.1 Method

Use one of the following methods:

a) Use the following curve to indicate the hardness depth value of the end quenching (Jomini test) :

1) The limit curve, above which the end quenching (Jomini test) hardness depth curve of the steel is drawn; Or

2) Limit curve, draw the end quenching (Jomini test) hardness depth curve of the steel below the curve; Or

3) The upper end quenching and the lower end quenching (Jomini test) curve, in the middle of this curve draw the end quenching (Jomini test) hardness depth curve of the steel; (see figure A.1);

b) Give specific points on the end quenching (Jomini test) curve:

- Upper limit, or;

- Lower limit, or

- Range between upper and lower limits:

i) by indicating the distance between a point with a certain hardness and the quenched end; Or

ii) Indicate the hardness of a point at a specific distance from the quenched end.

A.2 Description Hard

Hardness can also be described as follows:

A.2.1j 45-6/18 indicates A hardness of 45 HRC at a point between two points 6 mm from the quenched end and 18 mm from the quenched end (see Figure A2).

A.2.2j 35/48-15 indicates A hardness between 35HRC and 48HRC at a point 15 mm from the quenched end (see e.g. A3).

A.2.3 JHV 340/490-15 indicates A hardness between HV 340 and HV 490 at a point 15 mm from the quenched end.

A.2.4 In some countries, the following codes may also be used:

J 15 = 35/45 indicates an HRC value between 35 and 45 at a point 15 mm from the quenched end.

Annex B (Standardization)

Additional information on Article 10

In many cases, it is useful to know the cooling rate of the test surface.

The quenching process conditions specified in Section 6 of 7.2 permit the cooling rate at the end of the test piece to be regarded as a certain value.

Considering that in most cases it is easy to ignore the heat generated by the change of the steel structure during the cooling process and that the thermal conductivity of different steels is different from that of the standard test piece, the temperature fluctuation of the test piece can be expressed in various ways. The following methods are for reference only:

a) Figure B.1: Curve net with velocity U/UA as a function of time, where:

UA austenitizing temperature;

U refers to the temperature at a point on the surface at a certain distance from the quenched end.

Figure B.2: Variation in cooling rate of the surface of a test specimen performing a Jomini test at nearly 700 ° C, expressed in ° C /s. Here the velocity change is a function of the distance between each point and the quenched end.