Boronizing, which involves diffusion of boron atoms into steel substrate to form iron borides, is a well-known diffusion coating process and numerous studies have demonstrated the outstanding tribological properties of boronized steel vis-a-vis carburized or nitrided steels However, the high cost of the boronizing process has severely limited its applications One way to bring down the cost of the boronizing process is to reduce the thickness of the boronizing mixture to be packed around the component (called pack thickness) to the minimum required level without compromising on the properties of the boride coating The present study attempts to estimate the optimum pack thickness required to form boride coating of adequate thickness and property in the case of a low carbon steel boronized at 940°C for 2 h Low carbon steel samples have been boronized with varying pack thickness in the range 2-25 mm and the resulting boride coatings have been examined for thickness, microstructure, microhardness profile and abrasion resistance An analysis of the results obtained indicated that a pack thickness of 10 mm is sufficient to obtain boride coatings of adequate thickness and optimum properties

Influence of the pack thickness of the boronizing mixture on the boriding of steel

Abrasive wear behavior of boronized AISI 8620 steel

Steels provided with boronized diffusion coatings are known to exhibit excellent wear resistance at low sliding speeds However, the sliding wear behaviour of boronized coatings at high sliding speed is not known In the present study, the high speed sliding wear and friction behaviour of a boronized medium carbon steel has been characterized using a friction dynamometer The boronized samples were tested against a hardened tool steel disc The sliding tests were carried out at velocities up to 8 m s −1 and normal loads up to 2 MPa These tests indicated that the wear rate of the steel can be reduced by about an order of magnitude as a result of boronizing The coefficient of friction of the boronized steel was higher than that of the unboronized steels especially at higher sliding speeds

The high speed sliding wear behaviour of boronized medium carbon steel

Borided steels are known to exhibit excellent wear resistance at room temperature. However, the sliding wear behaviour of borided steels at high temperatures is not known. In the present study, AISI 440C and 52100 bearing steels which are extensively used in industry, were borided by pack method at 950 °C for 2 h. X-ray diffraction analysis of boride layers on the surface of steels revealed various peaks of FeB, Fe 2 B and CrB. The thickness and hardness of boride layers on the 52100 and 440C steels were 56 ± 6 and 47 ± 4 μm and 1970 and 2160 HK, respectively. Dry sliding wear tests of these borided steels were performed against Si 3 N 4 bearing ball at a constant sliding speed and load at elevated temperatures. The temperature changed between room temperature and 600 °C. These tests indicated that the wear rates of unborided and borided steels increase with temperature and borided 52100 and 440C steels exhibit considerably lower wear rate at all temperatures, compared with unborided steels. At temperature of 600 °C, borided 52100 and 440C steels have a wear resistance of about 3 and 2.5 times higher than that of unborided steels, respectively. Examination of the worn surface of borided steels showed that, worn surfaces were covered with a discontinuous compact layer especially above temperature of 300 °C.

Tribological behaviour of borided bearing steels at elevated temperatures

In this study the wear resistance, corrosion resistance, and oxidation resistance of boronized metallic alloys were investigated. Thermochemical treatment was performed by powder pack boronizing process at temperature 850-950 °C for 4 h. Saw-tooth morphology and smooth interface microstructures were observed with an optical microscope; microhardness was measured across the coating depth. The phases present in the boron coatings depend on the substrate material. High-temperature oxidation resistance was investigated and it was found that boron coating on ferrous alloys can resist temperatures up to 800 °C. The corrosion resistance of the boronized samples was improved and the corrosion rate was calculated for boronized and plain specimens. Wear testing was conducted by following the procedures of ASTM G99, ASTM D2526, and ASTM D4060. The obtained experimental results revealed that boronizing significantly improves the wear-resistance, corrosion-resistance, and oxidation resistance of metallic alloys.

K.-H. Habig

Papers

Wear behaviour of boride layers on alloyed steels