Study On The Hardening Behavior, Mechanism And Their Influence On Abrasion Resistance Of High Chromium White Iron Subjected To Sub-critical Treatment

High-chromium white irons have long been considered the candidate materials for wear-resistance components in mining and materials industry due to their excellent abrasion resistance imparted by the hard alloy eutectic carbides present in the microstructure. For many applications castings are heat treated prior to service for improving abrasion resistance. The heat treatment used commonly is destabilization treatment. However, deforming and fissuring easily happen to many big castings using destabilization treatment. And the temperature of destabilization treatment is high and the energy resource consumption of it also is very big. Thereby, it is very important to excogitate new heat treatments saving on energy resource, in particular, at present that the energy resource gradually reduces. Study results show that the alloys which have abundant austenite in as-cast structure have favorable secondary hardening behavior and abrasion resistance by sub-critical treatment. Compared with destabilization heat treatment, adopting sub-critical treatment canavoid deforming and fissuring of casts, reduce cost, save on energy resource and improve working conditions of works.High chromium white irons in used today are high alloying materials and there are many factors to influence its structure and abrasion resistance. It is required that a sound understanding of the many metallurgical processes taking place in their processing for sake of achieving appropriate heat treatment. But the phase transition, secondary hardening mechanism and abrasion resistance of high chromium white cast iron after sub-critical treatment lacks a full-scale and systemic study. Thereby, it is crucial to study on hardening behavior and its hardening mechanism of high chromium white iron subjected to sub-critical heat treatment for popularization of this heat treatment and abrasion resistance improving of high chromium white iron.Based on a review of application, development and progress of high chromium white iron, the solid-state phase transition, hardening behavior and their influence on abrasion resistance of high chromium white iron subjected to sub-critical heat treatment are lucubrated systematacially recur to diffusion and phase transition in solid metal, EET et al theories and by means of various analytical techniques such as XRD, SEM, TEM, EPMA and DTA.The investigation for the microstructure of high chromium white iron indicates the as-cast microstructure consists of the retained austenite, martensite and eutectic carbides, and it has no influence on that the alloying system is different. The sub-critical treatment does not change the content, distribution and configuration of eutectic carbides and can reduce austenite content to a lower level (<5%), even eliminate austenite. The matrix with different austenite content can be achieved by controlling the temperature and holding time of sub-critical heat treatment.The solid-state phase transition was studied by TEM. Results show (Fe,Cr)₂₃C₆ precipitates in sub-critical heat treatment firstly. The (Fe,Cr)₂₃C₆ precipitated previously would aggregate, grow up and in situ transform to M3C carbide. And, at the same time, ε-carbide precipitates from the supersaturated martensite obtained in solidifying process. Coupled with (Fe,Cr)₂₃C₆ and s-carbide have transformed into M3C completely, the pearlitic matrix forms. Addition of Mo and Cu would suspendprecipitation and transformation of secondary carbides due to Mo and Cu can reduce activity of C in austenite. And Mo and Cu precipitate as MoC or Fe2MoC at boundary of (Fe,Cr)23C6 in sub-critical heat treatment.The hardening behavior of three alloys subjected to sub-critical heat treatment was studied. The results indicate that abundant austenite is required in as-cast structure of high chromium white iron to achieve secondary hardening. The secondary hardening does not take place if austenite content is lower. And there are about 10-20% austenite in the matrix when alloys achieve maximal effect of secondary hardening. The hardening mechanism of high chromium white iron subjected to sub-critical heat treatment is not single but synthetic action of two sets of strengthening and softening actions. Alloys achieve best secondary hardening effect when strengthening and softening actions come to balance. There is another secondary hardening peak occurring when Mo and Cu was added due to precipitation ofMoCandFe2MoC.Furthermore, the hardening kinetic model of high chromium white iron subjected to sub-critical heat treatment was constructed by introducing equivalent hardness concept and summing up analysis and discussion of solid-state phase transition of it. The relational expression between equivalent hardness {Heq) and temperature (T) and holding time (/) of sub-critical heat treatment was obtained. The hardening kinetic model proved to be reasonable and effective by comparing calculational hardening curve and experimental hardening curve.Results of abrasion wear tests show the abrasion resistance of high chromium white iron can be improved by sub-critical heat treatment. The abrasion resistance achieves the best when austenite content reaches a certain level and decreases when austenite content lower than that. This attributes to that the retained austenite in the matrix can inhibit micro fracture on wear subsurface from forming and extending. And the work hardening due to the strain induced martensite formation is favorable to improve abrasion resistance under high stress abrasion conditions. There is a good coincidence relation between abrasion resistance and hardness of high chromium white irons.The empirical electron theory of solids and molecules (EET) was induced to high chromium white iron. The performance and phase transition characteristic of alloys were revealed from electronic level. And some performances of alloys were also forecasted combining phase diagram calculation and experimental results.The valence electron structure of austenite of Fe-C-Cr high chromium cast iron was studied based on equilibrium phase diagram of Fe-C-Cr system. Results show that the C-Cr bonds are the strongest bonds of all bonds in alloying austenite in composition range of industrial application and, thereby, cause partial aggregation of C-Cr atomic groups. The weighting of partial aggregation of C-Cr atomic groups would be increased greatly and more austenite would be reserved to room temperature when Cr/C>6. The Fe-C-Cr high chromium cast irons achieve best mechanical property when Cr/C=5.5-6.5.The valence electron structure of Fe-C-Cr-Mn alloying austenite was analyzed. Results show Mn has marked influence on structure of alloys when Mn content is in the range of 2⁴%. And the effect becomes weak when Mn content lower 2% or over 4%. The relationship between Mn content and VT/Va of structure was forecasted by combining analysis of weighting of structural units, and the results accord with that of experiment magnificently.