GB/T 3098.23-2020Mechanical properties of fasteners - M42~M72 bolts, screws and studs (English PDF)

8.5 Purchaser's control

The purchaser may select the appropriate test series from 8.6 to control the quality of the delivered fasteners according to the test methods in Chapter 9.

In case of dispute, the test methods specified in Chapter 9 shall be followed.

8.6

Tests that can be performed on fasteners and machined test pieces

8.6.1 General

According to the test methods specified in Chapter 9, Tables 7 ~ 9 specify the applicability of FF1 ~ FF2 and MP1.

Tables 7 and 8 provide FF1 and FF2 test series for the finished fastener tests.

FF1: Used to determine the properties of finished bolts and screws with standard heads and standard or thin rods (full load capacity), i.e., ds > d2 or ds~d2, see Table 7.

FF2: Used to determine the properties of finished studs with standard or thin rods (full load capacity), i.e., ds > d2 or ds~d2, see Table 8.

Table 9 provides the MP1 test series for fastener material performance tests and/or improved process tests. FF1 and FF2 can also be used for this purpose.

MP1: Tests for determining fastener material properties and/or process improvements on machined specimens, see Table 9.

8.6.2 Applicability

The applicability of various test methods to fasteners is as specified in Tables 7 ~ 9.

8.6.3 Delivery of test results

When the purchaser requires the delivery of a report including test results (special order), they shall follow the provisions of Chapter 9 and select the test method from

Tables 7 ~ 9. Special tests specified by the purchaser shall be agreed upon at the time of ordering.

- The firmness of the junction between the head and the unthreaded shank or the threaded part.

9.1.2 Scope of application

This test is applicable to bolts and screws that meet the following requirements:

- Flat bearing surface or serrated surface;

- The head has a stronger bearing capacity than the threaded shank;

- The head has a stronger bearing capacity than the unthreaded shank;

- The unthreaded shank diameter ds > d2 or ds~d2;

- Nominal length l >= 2.5d;

- Thread length b >= 1.5d;

- Thread length b < 2d of bolted structure;

- 42 mm <= d <= 72 mm.

9.1.3 Equipment

The tensile testing machine shall comply with the requirements of GB/T 16825.1.

Automatic centering devices shall not be used because they have a greater influence on the wedge pad angle specified in Figure 1 and Table 12.

9.1.4 Test device

The fixture, wedge and thread fixture shall comply with the following provisions:

- Hardness: >= 45 HRC;

- Thread of internal thread fixture: As specified in Table 10;

- Through hole diameter dh: As specified in Table 11;

- Wedge: As specified in Figure 1, Table 11, Table 12.

The method for calculating the tensile strength is shown in 9.1.6.1.

9.2.6.2 Technical requirements

The fastener shall break within the length of the unthreaded thread.

For fully threaded screws, if the break begins within the length of the unthreaded thread, it is allowed to extend or expand to the junction of the head and the thread, or enter the head before breaking.

The tensile strength (Rm) shall comply with the provisions of Table 2. The minimum tensile load (Fm,min) shall comply with the provisions of Table 3 or Table 5.

9.3 Proof load test of finished fasteners

9.3.1 General

The proof load test includes two steps:

- Implement the specified proof load (see Figure 3);

- Measure the permanent elongation caused by the proof load.

9.3.2 Scope of application

This test is applicable to fasteners that meet the following requirements:

- Bolts and screws with stronger head bearing capacity than threaded shank;

- Bolts and screws with stronger head bearing capacity than unthreaded shank;

- Unthreaded shank diameter ds > d2 or ds~d2;

- Nominal length l >= 2.5d of bolts and screws;

- Thread length b >= 2.0d;

- Total length lt >= 3.0d of studs;

- 42 mm <= d <= 72 mm.

9.3.3 Equipment

The tensile testing machine shall comply with the provisions of GB/T 16825.1. When clamping fasteners, oblique pulling shall be avoided, an automatic centering device can be used.

9.3.4 Test device

Each end of the test piece shall be properly processed as shown in Figure 3. To measure the length (before and after the load is applied), the fastener shall be placed in a bench- type measuring instrument with a spherical probe (or other appropriate method). Gloves or pliers shall be used to minimize the measurement error caused by temperature.

Measure the total length of the fastener before the load is applied (l0).

Screw the fastener test piece into the thread fixture as shown in Figure 3. Two thread fixtures shall be used for studs. The effective thread engagement length shall be at least

1d. The length of the unthreaded thread (lth) shall be 1d.

Note: To meet the requirement of lth = 1d, it is recommended to first tighten the thread fixture to the end of the thread; then, unscrew the fixture according to the number of threads equivalent to 1d.

Apply the proof load specified in Table 4 or Table 6 to the fastener axially.

The separation rate of the testing machine chuck shall not exceed 3 mm/min. The proof load shall be maintained for 15 s.

After unloading, measure the total length of the fastener (l1).

9.3.6 Test results

After unloading, the total length l1 of the fastener shall be the same as l0 before loading (with a tolerance of ±12.5 µm as the allowable measurement error). Certain uncertainties, such as straightness, thread centering, measurement errors, may cause the fastener to elongate significantly when the proof load is first applied. In this case, a load

3% larger than the value specified in Table 4 and Table 6 can be used and the test can be repeated according to 9.3.5. If the length (l2) after the second unloading is the same as the length (l1) before loading (with a tolerance of ±12.5 µm as the allowable measurement error), it shall be considered to meet the requirements of this test.

9.4 Tensile test of machined specimens

9.4.1 General

This test can determine:

- Tensile strength (Rm);

- 0.2% non-proportional extension stress (Rp0.2);

- Elongation at break of machined specimens (A);

- Sectional reduction of machined specimens (Z).

9.4.2 Scope of application

This test is applicable to fasteners that meet the following requirements:

a) Machined specimens made of bolts and screws:

- 42 mm <= d <= 72 mm;

- Thread length bt >= 1d;

- Determination A: Total length lt >= 6d0 + 2r + 2bt (see Figure 4);

- Determination Z: Total length lt >= 4d0 + 2r + 2bt (see Figure 4).

b) Machined test pieces made from studs:

- 42 mm <= d <= 72 mm;

- Thread length bt >= 1d;

- Measurement A: Total length lt >= 6d0 + 2r + 2bt (see Figure 4);

- Measurement Z: Total length lt >= 4d0 + 2r + 2bt (see Figure 4).

9.4.3 Equipment

The tensile testing machine shall comply with the provisions of GB/T 16825.1. When clamping fasteners, avoid oblique pulling and use an automatic centering device.

9.4.4 Test device

The fixture and thread fixture shall comply with the following provisions:

- Hardness: >= 45 HRC;

- Through hole diameter dh: As specified in Table 11;

- Thread of internal thread fixture: As specified in Table 10.

9.4.5 Machined test pieces

Machined test pieces shall be made from bolts and studs that have passed the size inspection, etc.; test pieces (eccentric test pieces) shall be made at 1/4 d of the bolt and stud diameter, wherein the test rod diameter shall be 3/8 d. The test method shall be in accordance with the provisions of GB/T 228.1. Where there is a difference between the test results of the actual bolts and studs and the test pieces, the actual test results shall prevail. See Figure 4.

fastener;

- For fasteners that can be subjected to tensile test (see 9.1, 9.2, 9.4): Determine the maximum hardness of the fastener.

Note: There may be no direct conversion relationship between hardness and tensile strength. In addition to considering the theoretical maximum tensile strength, the maximum hardness value is also determined by other factors (such as avoiding brittle fracture).

Hardness can be measured on appropriate surfaces or thread cross sections.

9.5.2 Scope of application

This test is applicable to fasteners that meet the following requirements:

- All specifications;

- All performance grades.

9.5.3 Test method

Hardness can be determined by Vickers, Brinell or Rockwell hardness test.

a) Vickers hardness test

The Vickers hardness test shall be carried out in accordance with the provisions of GB/T 4340.1.

b) Brinell hardness test

The Brinell hardness test shall be carried out in accordance with the provisions of GB/T 231.1.

c) Rockwell hardness test

The Rockwell hardness test shall be carried out in accordance with the provisions of GB/T 230.1.

9.5.4 Test procedure

9.5.4.1 General

The hardness test shall be carried out on fasteners that have passed the dimensional inspection.

9.5.4.2 Determination of hardness on the cross section of the thread

Take a cross section 1d from the end of the thread and it shall be properly treated.

E of the non-decarburized layer of the thread shall comply with the technical requirements specified in Table 13.

When the total decarburization reaches the maximum value specified in Table 2, the hardness measurement method shall not be used.

9.7 Carburization test

9.7.1 General

This test is applicable to determine whether carburization is formed on the surface of quenched and tempered fasteners during the heat treatment process. For the assessment of the carburization state of the surface layer, the difference between the base metal hardness and the surface hardness is the decisive indicator.

Note: Carburization is harmful because an increase in surface hardness can cause brittle fracture or reduce fatigue resistance. Care shall be taken to distinguish whether the increase in hardness is caused by carburization or heat treatment or surface cold working hardening, such as rolling threads after heat treatment.

The carburization test may be carried out by one of the following methods:

- Determination of hardness on a longitudinal section;

- Determination of hardness on the surface.

In case of dispute, the hardness test specified in 9.7.2 is the arbitration test method.

9.7.2 Determination of hardness on a longitudinal section

9.7.2.1 Scope of application

This method is applicable to fasteners that meet the following provisions:

- All specifications;

- Grades 8.8 and 10.9.

9.7.2.2 Preparation of test pieces

The test pieces shall be prepared in accordance with 9.6.2.2, but etching and removal of surface coating are not required.

9.7.2.3 Test procedure

The Vickers hardness at points 1 and 3 shall be measured as shown in Figure 7. The test force is: 2.942 N (Vickers hardness test HV0.3).

If the test piece has been used in the test according to 9.6.3.3, the hardness at point 3 shall be on the thread pitch line and measured on the tooth adjacent to the hardness at points 1 and 2.

9.7.2.4 Technical requirements

The Vickers hardness value HV(3) at point 3 shall be equal to or less than the Vickers hardness value HV(1) at point 1 plus 30 Vickers units.

More than 30 Vickers units indicates carbon addition. See Table 2 (No.10 and footnotes d, e) for hardness requirements for grades 8.8 and 10.9.

9.7.3 Determination of hardness on the surface

9.7.3.1 Scope of application

This method is applicable to fasteners that meet the following requirements:

- All specifications;

- Grades 8.8 and 10.9.

9.7.3.2 Preparation of test pieces

Prepare an appropriate surface by grinding or polishing at the head or end of the fastener, to ensure the reproduction and maintenance of the original surface characteristics of the material.

Take a cross section 1d from the end of the thread and prepare it appropriately.

9.7.3.3 Test procedure

Determine the Vickers hardness on the prepared surface as specified in 9.5.4.2.

The base metal hardness shall be measured on the cross section.

The test force used in measuring the above hardness is: 2.942 N (Vickers hardness test

HV0.3).

9.7.3.4 Technical requirements

The surface hardness value shall be equal to or less than the base metal hardness value plus 30 Vickers units.

The surface hardness value exceeds 30 Vickers units at the 1/2 radius, indicating that it has been carbonized. See Table 2 (No.10 and footnotes d, e) for the maximum surface hardness of 8.8 and 10.9.

9.8 Re-tempering test