Blackening Treatment of Powder Metallurgy Automotive Parts

The steam treatment of automotive parts is mainly used for the surface treatment of iron based parts. Iron forms a dense Fe3O4 oxide film on the surface in high-temperature steam, which can resist further oxidation of iron and the penetration and damage of other acid and alkali ions. So steam treatment can make iron have antioxidant and corrosion resistance, and Fe3O4 is hard and smooth, which can also increase the surface hardness of iron-based parts and reduce the friction coefficient. Due to its large, simple, and low cost, steam treatment is widely used in the manufacturing of powder metallurgy parts.

The process of steam blackening treatment on the surface of powder metallurgy automotive shock absorber guide piston bottom valve includes cleaning, degreasing, steam treatment, and oiling. Iron based powder metallurgy parts are usually directly immersed in oil after steam treatment. Before steam treatment, the surface of the parts should be cleaned to remove surface oil and rust. Cleaning can be done using acid pickling, which is followed by immediate water cleaning. Most of the cleaning is done directly with oil using an ultrasonic cleaning machine. After cleaning, the surface is fresh and clean, making it easy for iron reaction to generate Fe3O4 during steam treatment.

Steam treatment is carried out in an electric furnace, and the processed parts are placed in a material frame and placed in the furnace. First, use nitrogen to exhaust the air inside the furnace, then raise the temperature to 260 ℃, and then introduce water vapor with a pressure of 40MPa. Continue to raise the temperature to 580 ℃, keep it warm for 2 hours, and then cut off the power to cool down to 420 ℃, and then cut off the gas to exit the furnace. Cooling in air, due to the protection of Fe3O4 film, will not be further oxidized. The surface of the parts after being fired is blue. If the parts need to be immersed in oil, otherwise the extracted parts can be directly immersed in the oil to obtain dark blue and nearly black oil soaked blackened parts.

The factors that affect the processing of powder metallurgy parts include part density, processing temperature, and processing time. The weight increase of oxygen in powder metallurgy parts increases with the decrease of part density. This is because the density is low, the number of pores is large, and the surface that reacts with water vapor increases, resulting in high oxidation weight gain of the parts. The temperature of steam treatment is generally below 570 ℃, usually between 510 ℃ and 560 ℃, and the result of steam treatment at this temperature range is Fe3O4. If it is too high, it will form FeO, while if it is too low, the reaction speed will be slow. The processing time is generally maintained at around 1 hour.