Carburizing and quenching process of low speed and heavy duty gear

1.Technical Requirements

20grni2moa steel material, module M = 6, number of teeth z = 38, width of teeth H = 8.2 mm, the technical requirements are.

1) The hardness of gear forging blank after normalizing is HB < 240;

2) After carburizing and quenching, the hardness of the tooth surface is hrc57-61, and the hardness of the center is hrc40-45;

3) The depth of carburized layer is 1.4-1.8 mm;

4) After carburizing and quenching, the structure of tooth surface is cryptocrystalline or fine needle martensite. Acicular or network carbides and a large amount of retained austenite are not allowed. Carbides below grade 3 are qualified. Martensite plus retained austenite below grade 3 are qualified. After quenching in the center, it is low-carbon martensite, and the ferrite in the center is not more than grade 2;

5) After carburizing, the hardness gradient is gentle and the carbon concentration is gentle.

2. Test Process

In the carburizing process, excessive acetone is injected into the furnace to make the atmosphere in the furnace supersaturated and non-equilibrium state. The CO content in the furnace gas is constantly changing. In order to prevent the influence of the change of CO content in the furnace gas on the carbon potential control, the actual analysis value of CO content in the furnace gas is input at the same time of inputting the M2y value of temperature and oxygen probe to the microcomputer, so as to ensure the accuracy of carbon potential control.

The test material is 20grni2moa, and the equipment is a box type multi-purpose furnace with controllable atmosphere, which is controlled by computer. The atmosphere used is direct generating atmosphere, and the carburizing medium is acetone. The temperature sensor, CO analyzer and special oxygen probe are used to accurately control the furnace temperature and carbon potential in the furnace.

Before the test, the carburizing process was simulated by car-o-prof software to check whether the parameters were reasonable.

As shown in the figure is the carbon concentration gradient distribution map based on the simulated value and the actual test results. From the figure, the carbon concentration gradient of the workpiece is relatively gentle, which shows that its mechanical properties are relatively good. The results of computer simulation and actual test are basically consistent. The simulated carbon concentration gradient has a certain reference and guiding significance for the actual production, especially for the process of hard tooth surface machining after carburizing, it can also be seen how much machining allowance the workpiece should have in advance.

3. Metallographic Examination

Carburizing furnace sample, the sample state is carburizing, cooling, direct quenching and tempering. The sample block is grinded manually on the grinding wheel, and then polished with sandpaper for relevant detection. According to the test requirements, the depth of carburized hardened layer, microstructure and microhardness of carburized layer in different states were tested. The inspection is carried out according to the mechanical industry standard JB / T 6141.2-92 heavy duty gear and carburizing quality inspection and JB / T 6141.3-92 heavy duty gear and carburizing metallographic inspection.

Testing equipment: hardness test, hrc-150a Rockwell hardness tester; effective hardening depth test, fw-700 microhardness tester; metallographic test, xb-3 ordinary metallographic microscope.

Metallographic examination results.

1) The structure and grade of carburized layer are martensite and retained austenite, grade 2; carbide grade 2; lath martensite + a little free ferrite grade 2 in the core.

2) Microhardness test of carburized layer.

According to the standard, the effective hardening depth of carburized layer is 1.7mm, and the hardness of the core of the test sample is hrc43.