How is the quality of large forgings inspected after surface quenching

How is the quality of large forgings inspected after surface quenching?

The surface quenching of large forgings is the same as the surface quenching of ordinary small and medium-sized parts. Routine quality inspection is required. The contents include:

1) surface hardness;

2) depth of hardened layer;

3) macroscopic inspection of appearance (crack, deformation, burning loss, etc.);

4) metallographic. But the condition of large forging is different, the inspection method is also different.

1. hardness test

Large forgings usually use portable shore hardness tester or after surface quenching's hardness tester with digital display to measure, because the two kinds of measuring hardness tester can be arbitrarily on the parts surface, no moving parts, as these two hardness measurement for measuring head bounce, and thus has high surface finish requirements, can use sand paper or sharpening stone polishing surface measurement. If the measuring surface can be put flat, the hardness tester vertical direction is better, reduce the error. Especially with shore hardness tester, must keep the vertical direction of the falling rod bounce, otherwise cause a large error. Under no circumstances shall surface hardness be measured with a hammer or brinell hardness tester.

During hardness measurement, 3-5 measuring points must be selected for uniform distribution in the full length direction and circumference direction of the quenching zone, so as to understand the uniformity of quenching hardness, especially the surface hardness of the roll.

For the tooth surface quenching of large gear, measure the hardness in the tooth surface with an arc part, if it is a fixed quenching, you can measure the hardness in the tooth top circle. In order to measure the hardness of gear tooth surface accurately, the gripper gear hardness tester (rockwell) can be used to measure.

Hardness can also be detected by using a file if the hardened surface of a narrow surface cannot be penetrated into the hardness meter. It is better to use a calibrated combination of several hardness standard files.

2. Distribution of hardened layer and depth of hardened layer

General large forgings surface hardening depth requirement is according to the appropriate frequency induction heating and process parameters to decide, exceed the scope of the hardening layer depth by heat conduction to achieve, the depth is not very accurate, also not checked, but for some large forgings surface quenching, especially the contact fatigue strength requirement of large forgings, because large radius of curvature of the surface contact fatigue life and fatigue strength and surface hardening layer transition area is close to the shear stress ratio. Therefore, the depth of the surface hardening layer directly affects the contact fatigue life of such large forgings. For example: large wheel tread, large heavy duty gear, prefer the surface hardness is lower, but the depth of the hardened layer is deeper. For cold roll, there is also a hardening layer depth requirements, cold roll surface quenching is usually deeper than the depth of conventional quenching.

In addition, the distribution shape of the surface hardening layer is closely related to the distribution of residual stress, which requires the surface quenching of large forgings with high fatigue strength.

Mass check the hardening layer depth and distribution of small parts damaged parts sampling can be used, namely, from a process specification in a large number of parts with nitric acid alcohol after cut out a polished section observation after corrosion, and large forgings can make specimens of similar shape, with the same cut section examination after surface quenching process specification, such as using a large axle shaft sleeve, make big big modulus gear using two identical modulus teeth for specimen examination.

The method to check the depth of the hardened layer can be in accordance with the ministerial standard :GB5617 -- 85 "determination of the effective hardened layer depth of steel after induction quenching or flame quenching". The color (black) layer thickness is usually measured after corrosion by nitrate alcohol.

3. Appearance inspection

Magnetic particle detection, fluorescence detection, ultrasonic detection and other methods to observe whether the quenching surface crack. Visually inspect the surface for burn marks, paying particular attention to the sharp corners, holes and edges of the end face, as these are prone to overheating, burns and even melting.

4. Metallographic examination

Large forgings surface quenching rarely metallographic examination, if the user has a requirement (such as cold roll inspect overheating in the organization), with a movable metallographic microscope to observe in the surface of parts, the surface hardening of metallographic examination generally just know a horse's body acupuncture thickness, because the rough acicular martensite damaging fatigue performance.

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