Fluidized-bed carburizing in chemically active powders of C22 steel

The paper presents the new method of fluidized-bed carburizing in chemically active micropowders. Basic principles of the thermo-chemical treatment method are described. Discussed in the article carburizing process combines the most important advantages of the box and atmospheric fluidized-bed methods. The source of carbon in this case is based on a special mixture of active carbon and chemically inert bed particles, which is also a heat accumulator ensuring the uniform temperature distribution in the treated part. Results of the exemplary carburizing processes carried out at temperature of 930°C for 4 hours are presented. Samples of C22 unalloyed carbon steel were carburized, quenched in oil (after cooling to hardening temperature of 870°C) and tempered at 180°C for 2 hours. Two types of carburizing mixtures with different percentage of active carbon were used. Performed carburizing processes allowed forming of layers with surface hardness in the range of 720÷770 HV. Cross-section hardness measurements showed that, depending on the active carbon content, it is possible to obtain effective carburized layer thickness of about 0.7 to 1.2 mm. Resistance to wear of the carburized steel was characterized by 3 rollers-cone method. Carried out friction test demonstrated the superior tribological properties of the hardened layers. Metallographic observations proved the correct carburized layer structure consisting of lath martensite and retained austenite. The microscopic observations were supplemented with X-ray diffraction (XRD) analyses. Diffraction patterns revealed the diffraction peaks derived from martensite, austenite and a very weak peaks from the Fe3C carbide phase. Internal stresses measurements showed beneficial from the point of view of fatigue properties state of compressive stress. It was stated that the obtained properties of carburized layers were comparable with properties of layers formed using conventional gas carburizing method.