Vulcanization treatment of MIM products

Vulcanization treatment of MIM products

Purpose of vulcanization treatment:

When vulcanization is used as anti-friction material in powder metallurgy products, iron-based oil-impregnated bearings are the most widely used. Sintered oil-impregnated bearings (with graphite content of 1%-4%) have simple manufacturing process and low cost. In the case of PV<18-25 kg·m/cm 2·sec, it can replace bronze, babbitt alloy and other anti-friction materials. However, under heavy working conditions, such as high sliding speed on the friction surface and large unit load, the wear resistance and life of the sintered parts will rapidly decrease. In order to improve the anti-friction performance of porous iron-based anti-friction parts, reduce the coefficient of friction, and increase the working temperature to expand its use range, vulcanization treatment is a method worthy of promotion.

Sulfur and most sulfides have certain lubricating properties. Iron sulfide is a good solid lubricant, especially under dry friction conditions, the presence of iron sulfide has good seizure resistance.

Powder metallurgy iron-based products, using its capillary pores can be impregnated with a considerable amount of sulfur. After heating, the sulfur and the iron on the surface of the pores can generate iron sulfide, which is evenly distributed throughout the product and plays a good lubrication on the friction surface And can improve the cutting performance. After vulcanization, the friction and cutting surfaces of the products are very smooth.

After the porous sintered iron is vulcanized, the most prominent function is to have good dry friction properties. It is a satisfactory self-lubricating material under oil-free working conditions (that is, no oil or no oil is allowed), and it has good seizure resistance and reduces the phenomenon of shaft gnawing. In addition, the friction characteristics of this material are different from general anti-friction materials. Generally, as the specific pressure increases, the friction coefficient does not change much. When the specific pressure exceeds a certain value, the friction coefficient increases sharply. However, the friction coefficient of porous sintered iron after vulcanization treatment decreases with the increase of its specific pressure in a large specific pressure range. This is a valuable feature of anti-friction materials.

The sintered iron-based oil-impregnated bearing after vulcanization can work smoothly below 250°C.


Vulcanization process:

The process of vulcanization treatment is relatively simple and does not require special equipment. The process is as follows: put sulfur in a crucible and heat it to melt. When the temperature is controlled at 120-130℃, the fluidity of sulfur is better at this time. If the temperature is too high, Not conducive to impregnation. The sintered product to be impregnated is preheated to 100-150°C, and then the product is immersed in the molten sulfur solution for 3-20 minutes, and the unpreheated product is immersed for 25-30 minutes. Depending on the density of the product, the wall thickness and the amount of immersion required to determine the immersion time. The immersion time for low density and thin wall thickness is less; vice versa. After leaching, the product is taken out, and the remaining sulfur is drained. Finally, put the impregnated product into the furnace, protect it with hydrogen or charcoal, and heat it to 700-720°C for 0.5 to 1 hour. At this time, the immersed sulfur reacts with iron to produce iron sulfide. For products with a density of 6 to 6.2 g/cm3, the sulfur content is about 35 to 4% (weight percentage). Heating and roasting is to make the sulfur immersed in the pores of the part form iron sulfide.

The sintered product after vulcanization can be treated with oil immersion and finishing.


Application examples of vulcanization treatment:

1. Flour mill shaft sleeves The shaft sleeves are installed at both ends of the two rolls, a total of four sets. The pressure of the roll is 280 kg, and the speed is 700-1000 rpm (P=10 kg/cm2, V=2 m/sec). The original tin bronze bushing was lubricated with oil slinger. Now it is replaced by porous sintered iron with density of 5.8 g/cm3 and S content of 6.8%. The original lubrication device can be used instead of the original lubrication device. Just drop a few drops of oil before driving and work continuously for 40 hours. The sleeve temperature is only about 40°C. ; Grinding 12,000 kg of flour, the bushing is still working normally.

2. The roller cone drill is an important tool for oil drilling. There is a sliding shaft sleeve on the top of the drill oil, which is under great pressure (pressure P=500 kgf/cm2, speed V=0.15m/sec. ), and there are strong vibrations and shocks.

Post time: Jul-12-2021