Study On Plasma Arc Re-melting Technology And Mechanism On White Cast Iron Roll Teeth Surface

Roll is the main part in flour production equipment, its wear resistance is good or bad will directly affect the quality and grade of flour and productivity. Currently, the common material for grinding roller in the world's flour milling industry is white cast iron, because it contains large amounts of free cementite which has the nature of brittle hard and good wear resistant. Some studies have shown cast iron with moderate thermal conductivity and micro-structure of thin was very suitable for re-melting process, and after re-melting performance it would be improved. In this research, hypo-eutectic white cast iron roll teeth surface were re-melted by high energy density plasma arc and the re-melting process was explained with metal science theory. Finite element analysis software was used to simulate the melting temperature field, and the re-melting composition, microstructure, wear and other aspects of the wear-resistant layer were analyzed systematically. This paper also studied the influencing factors and patterns of the wear-resistant layer properties.According to principles of metal, plasma arc re-melting is a process of metal melting, solidification, crystallization and phase transformation. Roll teeth absorbed heat during the plasma arc scanning process, and the temperature on top reached melting point in a short time and re-melting occurred. Temperature inside rose to phase transition point, so phase transition occurred. As the iron-carbon phase diagram has a dual character, the carbon may be in the form of cementite or graphite, depending on the material of heating and cooling rate. Plasma arc re-melting of acute thermal quench change and thermodynamic equilibrium is difficult to achieve, so it prevents the diffusion of alloying elements in the materials and graphite-based processes, resulting in the change of organization state and structure, which affects the various properties of materials.With necessary and reasonable assumptions, the geometric model was established and the plasma arc re-melting process was simulated using finite element software. Simulation results showed that the melting process completed in a very short period of time, and the temperature changed from 25℃to 1200℃then cooled to 808.3℃in 1.5s. Re-melting and phase transition regional width was about 1.5mm, and the width of the melting area is not wide. Increasing the current up to 7.5A in the same conditions, the re-melting and phase transition region increased, and the melting area was too large losing the re-melting's practical significance. When the current was 7.0A and the distance between the spray gun and roll teeth top was 5.0mm, it was suitable for re-melting processing, and the top of the teeth don't serious melted.The carbon content was significantly increased after the roll teeth re-melting. The carbon content was greater than or equal to 4.3%, reaching the composition point of eutectic and hypereutectic transition. The highest carbon content point of the teeth appeared in the distance about 0.2-0.4mm. Ledeburite number increased significantly and pearlite can be refined, and the carbide morphology has also changed after re-melting. Hypoeutectic white cast iron transformed to eutectic and hyper-eutectic mixture organizations, which contained cementite, Ledeburite and a small amount of pearlite. The results were consistent with the composition analysis. Organizations grew toward the side of the roll teeth.The orthogonal test results and analysis determined that the relationship between various factors and wear layers depth and hardness was not a single linear, and the current is the main factors of wear-resistant layer hardness, while the distance between roll teeth top and nozzle affected the depth mainly. The re-melting parameters of white cast iron roll teeth surface with plasma arc were as follows:current values of 7.0A, distance between plasma spray gun and roll teeth of 5mm, moving speed 2.0mm/s, working gas pressure of 6.5MPa.After the plasma arc re-melting, friction coefficient decreased from 1.7-1.8 to 1.5-1.6, hardness values increased from 58HRC-60HRC to 64HRC-65HRC, and the average hardness value of the top reached 64.5HRC. Friction and wear form have changed. a large number of pits existed on worn surface of the sample before re-melting, while after re-melting wear debris which generated by the friction pair surface scratches increased. Abrasive wear was the main form after re-melting. After re-melting cementite and pearlite became thinner and lager than before, and in the same pearlite crystal orientation of the lamella tended to be uniform. The inter-lamellar spacing decreased which can effectively hinder dislocation slip. Composition change, the organization's refinement and change, the improvement of hardness are of great significance to improving the wear resistance.