PART 3
PART 1, PART 2, PART 3, PART 4, PART 5, PART 6, PART 7
According to one hypothesis, if the layer of a material covers graphitic pores [17] stresses accumulate between the metal matrix and graphite, which finally causes cracks to appear. At some stages it is possible to consider the process of material deterioration during friction in hydrogen as the process of appearing and developing of cracks. The main crack can propagate between grains of аustenite in cold-work manganese steels (Fig.5.). Fig. 5. Propagation of the main crack along the grains of аustenite in manganese cold-work steel However, in alloys which have inclusions of graphite, the propagation of the main crack can be much more complicated. Therefore, the appearance and propagation of the main crack to be near the inclusion of graphite is possible (Fig. 6а). The main crack can fade inside carbide and phosphide lattices (Fig. 6b.). Here are a lot of inclusions of graphite which cover a large area, and the destruction can progress when some cracks are formed. The cracks can be spread between metal matrix and the inclusions of graphite [4]. a) b) c) d) Fig. 6. Conditional schemes of crack propagation in graphitized steel or grey-iron (1 - main crack; 2 - metal matrix; 3 - carbides; 4 - phosphides; 5 - inclusions of graphite) Therefore, even in a microsection of a manufactured object the part of graphite is parted from the metal matrix (Fig.7 a ) judging by the size of cavities, so the cracking process can take place along the structural components. Our results were obtained from the research carried out by testing graphitized cast-alloys on the friction machine SMC-2 using a roller-pin set. The disc was made of cast-iron, and the counter-body (a roller) was made of steel 45G2. Metallographic investigations were carried out using modern optical equipment. Strengthening during friction is one the obvious advantages that raise the wear resistance of both steel and grey-iron. Fig. 7b indicates that the change of microhardness is dependent on the distance of the friction zone (pin edge) [20].
a) b)
Fig. 7. Microstructure of grey-iron which does not contain graphite × 200 (a), after hardening in the friction processes (1 – direction of steps; 2 – pin end) (b).
