Influence Of Process Conditions On The Wear Resistance Of Bainite Ductile Iron

Bainite ductile irons(austempered ductile iron(ADI), as-cast bainite ductile iron) have extensive application prospect for sheet metal forming molds because of their excellent strength, toughness, fatigue resistance, and abrasion wear resistance. At present, the research of bainite ductile iron is focused on the effect of composition and microstructure on the tensile properties, fatigue properties and fracture toughness. However, little literatures are reported on the wear performance and wear mechanism of bainite ductile iron, which restricts the development and application of bainite ductile iron molds. Therefore, it has a very important significance to study the wear performance and wear mechanism of bainite ductile iron, for production process development, prolong service life prolonging, and production costs reduction of bainite ductile iron molds.In this paper, the austempering process was used to prepare ADI, the influence of austempering temperature and holding time on the microstructure and wear-resistance property of ADI was investigated. The wear mechanism of ADI was discussed by analyzing the changes of the amount of the residual austenite and hardness of ADI before and after abrasive wear tests, and the abrasive surface morphology and abrasive dusts composition. On the other hand, the as-cast bainite ductile irons were prepared by adding Cr ontent. The effect of Cr content and annealing temperature on microstructure and abrasive wear performance of as-cast bainite ductile irons was investigated. The following results were obtained.When holding time was of 1.5h, the matrix of ADI was consist of acicular lower bainite and small content of residual austenite at austempering temperature of 300?, ADI showed the best wear-resistance performance. With the increase of austempering temperature, the bainite became coarser gradually and form feathery upper bainite at austempering temperature of 400?. The wear loss of ADI increased with the increase of austempering temperature. When the austempering temperature was of 300?, with the increase of the holding time, the bainite became coarser, the residual austenite decomposed into ferrite and carbide if the holding time was longer than 4.5h. As the abrasive test loads were less than 160 N, ADI with holding time of 3.0h showed the best wear-resistance performance. But, ADI with holding time of 1.5h showed the best wear-resistance performance if the load was higher than 160 N, then decreased gradually with the holding time. For the same austempering process condition, the wear loss of ADI increased with the abrasive test load.The changes of the amount of residual austenite and the hardness of ADI before and after abrasive wear tests were investigated. It showed that, the amount of residual austenite reduced after the abrasive tests, and the hardness increased. When the austempering temperature was of 300?and holding time was of 1.5h, the amount of residual austenite decreased from 23.0% to 19.1%, while the hardness increased from 503 HV to 713 HV.When the matrix of ADI consists of acicular lower bainite and little residual austenite, the wear mechanism was mainly the oxidative exfoliation wear and relatively less fatigue wear and adhesive wear; when the matrix of ADI was feathery upper bainite and residual austenite, the wear mechanism was preferentially fatigue wear and adhesive wear, the oxidative exfoliation wear became weak relatively. When the austempering temperature was of 300?, the holding time was of 7.0h or 9.0h, there were carbides in the matrix of ADI, and the wear mechanism was fatigue wear, abrasive wear and less adhesive wear.As-cast bainite ductile irons were prepared by Cr alloying. The bainite content in matrix increased, and the wear-resistance of as-cast bainite ductile iron was increased with the Cr content. When the Cr content was of 0.4%, the were-resistance was the best; the wear-resistance began to decrease while the Cr content was increased to 0.6%.When the annealing temperature was of 500?, the residual austenite decomposed, while the bainite still kept the as-cast morphology. With the annealing temperature raised, both the residual austenite and bainite decomposed; the matrix decomposed to lamellar pearlite and ferrite if the annealing temperature was up to 800?, the wear-resistance was lower than the as-cast bainite ductile iron. Hardness of as-cast and annealed bainite ductile irons became higher than that before testing.