Practice of carburizing of mesh belt furnace for screws

When carbonitriding, the flow rate of ammonia gas is 3~5L/h. When the temperature in the furnace is stable at 910, the carbon potential distribution of each section in the furnace is set to be 1.15%. The N-660-QF mesh belt furnace is divided into 3 zones, 1 heating zone and 2 heat preservation zones. The metallographic test is carried out in three areas with fixed carbon sheets (original carbon content <0.08%). The area is organized into ferrite and pearlite, the middle section of the insulation zone is reticulated carbide, and the latter section is pearlite. The front part of the carbon potential measured by the weighing method is complicated for the distribution of the atmosphere in the mesh belt furnace. The carbon potential is controlled by only one point to control the atmosphere inside the mesh belt furnace, and the carbon potential of each section in the furnace cannot be accurately reflected. The productivity test shows that although the carbon potential of the carbon controller is close to the measured value of the fixed carbon sheet, the carbon potential in the middle section is always high, and the carbon potential in the heating zone near the front of the furnace is low; the carbon potential in the latter section Stable, slightly rising.

Carburizing or carbonitriding in mesh belt furnace, for low carbon steel 20Mn, SWRCH22A, 20MnTiB or medium carbon, medium carbon alloy steel SWRCH35K, SCM435, SCR435 steel screws, carburizing layer should be selected according to screw specifications, for alloy The steel can be directly quenched by carburizing, and the carbon steel should be air-cooled after being carburized, and then quenched by secondary heating. The test process is shown in Table 1. Table 1 Self-tapping locking screw heat treatment process Steel carburizing layer Carburizing temperature Quenching temperature Tempering temperature Carbon potential setting time f Observed by metallographic micromirrors, the workpiece carbide is small or granular. Distribution (矣2), martensite is a fine needle-like distribution (矣3).

As we all know, there are many main factors affecting the carburizing process, such as temperature and time. When the material of the screw, the carburizing temperature and the carbon potential are determined, the carburizing time will be determined according to the depth of the carburizing layer. Generally, the shallow carburizing time is about 1.5h, the active atoms produced by various carburizing agents or carburizing gases at high temperatures are +. The carburizing carbon potential in the continuous mesh belt furnace is set to 1.0%-1.2%. The alloying elements in the steel have a great influence on the carbon absorbing ability of the steel and the internal diffusion of carbon. Mn, B, Ti carbide forming elements can increase the carbon content of the surface of the layer. Under the condition of high toughness and proper hardenability, self-tapping screws are commonly made of 20Mn, 20MnTiB, SCM435, SCR435 and SWRCH35K steel. .

The carburizing process of mesh belt furnace is shown in Table 2. We have produced screws for many years and the product quality is stable and reliable. However, it must be noted that the oil and gas of the unwashed screws have a significant influence on the furnace atmosphere. If the small cleaning T net directly affects the furnace atmosphere and control accuracy, it is also necessary to pay attention to whether there is residual yellow powder on the mesh belt during the empty furnace, such as There is a need to take care of the cleaning of the handling screws. Since the mesh belt furnace + has air-cooling conditions after carburizing, the low-alloy steel or carbon steel carburizing direct quenching process should be used as much as possible.

Table 2 mesh belt furnace carburizing process list steel type carburizing M deep / mm area plus ID area insulation area / X, carbon potential set time hydrogen brittleness is a prominent quality hazard of self-tapping screws. In the direct quenching of hydrocarbons, the problem of hydrogen embrittlement is much more prominent than that of carburized parts. Hydrogen gas generated during pickling, electroplating, and hydrogen in the surrounding medium (generally up to 60% to 70%) can be dissolved in steel. When the cathode current efficiency of the electroplating and phosphating process is less than 100%, a large amount of hydrogen atoms are generated to adhere to the surface of the screw, thereby causing hydrogen permeation. This behavior of hydrogen has an important effect on the hydrogen embrittlement damage of steel.

Self-tapping screw flooding time is 6 ~ 8h. Since the temperature can increase the diffusion coefficient of hydrogen in steel, 150~200T insulation can eliminate hydrogen embrittlement, but the temperature is too high, for example, above 4001, hydrogen dissolves in steel also increases. Hydrogen no longer automatically escapes from the steel and does not achieve hydrogen removal. The hydrogen flooding temperature is selected according to the production conditions such as core hardness, surface roughness plating time, plating thickness, pickling time, acid concentration, etc., and the hydrogen flooding time should be determined before the passivation treatment and just after the plating (within 4 hours). .

The mesh belt carburizing furnace is especially suitable for occasions with large variations in variety, quantity and process, especially for large-scale screw carburizing in the fastener industry. Only by continuously improving and perfecting materials, heat treatment and testing can it be produced. Market competitive products. (20051006)

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