Test methods for external thread fasteners (3)

Test methods for external threaded fasteners (3):

1. Thread decarburization layer: ASTM F2328 and ISO 8981, the requirements for thread decarburization layer and inspection methods are completely consistent. The decarburization test is to determine whether the thread surface of the fastener is decarburized after heat treatment and to determine the depth of the decarburized layer. There are two methods for thread decarburization detection: metallographic method and hardness method. They are introduced as follows:

1. Metallographic method: The metallographic method can measure the completely decarburized layer G (G in the American standard), and can detect the height of the metal substrate (incomplete decarburized layer) E (N in the American standard). Suitable for bolts, screws and studs of all performance levels and all diameter specifications.

2. Hardness method: The hardness method is suitable for bolts, screws and studs with a pitch P≥1.25mm (US standard ≥28 teeth per inch). The height of the metal substrate (the height of the incomplete decarburization layer) E (N in the American standard) can be measured, and the partially decarburized layer (or called the incomplete decarburization layer) can be detected by the microhardness method. It is suitable for arbitration inspection of incomplete decarbonization.

The specific process of these two test methods is as follows: (Guide: How to choose the correct quenching medium in the heat treatment of fasteners)

1. Sample preparation: The first step of the two test methods is the same, that is, sample preparation should be carried out. First, the surface of the sample is cleaned, the surface coating is removed, and then the sample is cut. The sample is cut along the transverse and longitudinal sections of the thread axis at about 1 times the thread diameter from the end of the thread. And inlaid in plastic inlay powder. After the sample block is heated and fixed, the sample block is ground and polished. After grinding and polishing, the sample block was immersed in 3% nitric acid for corrosion in order to fully show the metallographic structure. After etching, grinding and polishing are performed again to make the sample meet the test requirements.

(Test methods for external threaded fasteners (3))

2. Sample detection: The second step of the two methods is: metallographic method put the sample under the metallographic microscope (generally magnify 100 letters), use the glass display screen with scale on the microscope, and use the eyepiece The maximum thread decarburization depth (G in the US standard) and the base metal height (the minimum height of the incomplete decarburization layer) E (N in the US standard) are measured. In the hardness method, a Vickers hardness tester with a test force of 300g is used to test the hardness value of the sample block, and a total of three points are tested. The first point is the reference point. If HV(2)≥HV(1)-30, it indicates decarburization; if HV(3)≤HV(1)+30, it indicates carburization. If the maximum depth of the fully decarburized layer is greater than or equal to 0.015 mm (ISO) or 0.0006 in (SAE), the hardness method cannot be used.

For the minimum height E (or N) of the incomplete decarburization layer, the level 5 of the U.S. standard should be 6H (equivalent to ISO 8.8); the level 8 of the U.S. standard should be 2/3H (equivalent to 10.9 of ISO) level).

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