Precautions for quenching and tempering and quality inspection

Precautions for quenching and tempering and quality inspection

1. Since the heating and quenching temperature of quenching and tempering is relatively high, it is necessary to prevent oxidative decarburization.

2. The cross-section of the workpiece changes greatly, and the arc R should be increased as much as possible at the sharp corners, and the quenching heating temperature should be the middle and lower limit to prevent quenching cracking.

3. The quenched parts should not be stacked in the cooling tank to prevent large distortion or even cracking caused by uneven cooling.

4. After quenching, it should be tempered in time, and the interval is generally not more than 4h. It is easy to crack and temper in time.

5. When the furnace is installed by high-temperature tempering, the furnace temperature shall not exceed the maximum temperature required by the process; after the furnace is released, the workpieces shall not be stacked on the wet ground to cool. The second type of temper brittleness should be cooled quickly.

6. After bending and straightening, the quenched and tempered part must be subjected to 500~550℃×3~4h stress relief tempering.

7. The quenched and tempered parts should be quenched completely, and the hardness should be inspected after quenching to determine the tempering temperature.

8. The temperature of the coolant should be strictly controlled, and no over-temperature quenching is allowed.

The quality inspection of quenched and tempered parts mainly includes the following points.

1. Metallographic inspection. Hardness is only a superficial phenomenon, and the metallographic structure is the essential thing. Many units do not check the metallographic structure of quenched and tempered parts. This is a misunderstanding. After quenching and tempering, the mechanical properties are unqualified, and the problem often lies in the metallographic structure.

2. Hardness inspection. Before checking the hardness, the surface of the workpiece should be cleaned to remove the oxide scale, decarburized layer and burrs that are not suitable for the hardness check.

The part to be inspected for hardness shall be determined by the inspector according to the process documents. At least 3 points shall be inspected at the hardened part, and the average value shall be taken as the indicated value. The quenched parts are tested with Rockwell hardness tester. After high temperature tempering, Brinell hardness tester is allowed. For larger size workpieces, they can be tested with hand hammer hardness tester.

3. Appearance inspection. After quenching and tempering, the surface of the workpiece should be observed with the naked eye to be free from cracks, corrosion, burns and other macro defects.

4. Deformation inspection. The shaft parts are supported by top or V-shaped blocks at both ends, and the radial runout is measured with a dial indicator. There is no center hole and small shaft parts, which can be inspected on a flat plate with a feeler gauge.

The deformation of the workpiece should be less than 1/3 to 1/2 of its machining allowance to ensure that the subsequent processing surface is smooth and free of scale.