Study On Microstructure And Properties Of High Chromium Cast Iron For Engineering Ships

A lot of equipments on engineering ships are working in the condition of abrasive wear.High chromium cast iron is widely used in the wear-resistant parts on engineering ships becauseof its excellent abrasion resistance. The microstructure of as-cast high chromium cast iron isaustenite matrix and eutectic (Fe,Cr)7C3carbides network which has high hardness. Although ithas certain abrasion resistance, as-cast high chromium cast iron exhibits drawbacks, such as lowtoughness, which limits its use areas. Heat treatment is one of the important ways to improve themicrostructure and properties of high chromium cast iron.In this paper, based on the analysis for the working requirements and wear failuremechanism of dredger mud pump blade, the heat treatment rule of high chromium cast iron bladewas studied by metallography and properties tests. The optimum heat treatment processparameters was achieved which could improve the wear resistance of high chromium cast iron.It is shown that the chemical composition of blade is similar to the KmTBCr20Mo cast ironin chinese standard GB/T8263-1999. The as-cast microstructure is austenitic matrix and unevenblocky eutectic (Fe,Cr)7C3carbides which show radial distribution partly. The hardness of as-casthigh chromium cast iron is57.5HRC, and the impact absorbing energy K is3J.The heat treatment process parameters of high chromium cast iron for impeller blade wasstudied. The results show that when the high chromium cast iron is austenitized at800℃,900℃,1000℃and1100℃for3h and cooled in air, the hardness first rises and then falls with theaustenitizing temperature increasing, it reaches the highest hardness at1000℃. After holding for1h,2h,3h,4h at1000℃respectively, it is indicated that the cast iron is up to maximumhardness when holding for2h. When the cast iron air-quenched at1000℃for2h and temperedat250℃and450℃for2h separately, it is found that the hardness both increase slightly, butthe impact absorbing energy is improved significantly to4.13J by tempering at250℃. Theoptimal heat treatment process for the blade is1000℃×2h air quenching+250℃×2htempering. The microstructure after heat treatment is dispersed (Fe,Cr)7C3carbides+secondarycarbides+tempered martensite and a small amount of retained austenite.Through sliding friction-wear test and simulation condition wear test with sand, it is shownthat the main wear mechanism of impeller blade is abrasive wear, together with fatigue wear.Martensitic transformation in austenite matrix and morphology and distribution improvement of eutectic carbides enhance the hardness and toughness of blade materials, which would affect thewear resistance significantly. After1000℃×2h air quenching+250℃×2h tempering,the sand abrasion resistance capacity of blade material is increased by34.41%in slidingfriction-wear test, and the impact abrasion resistance is increased by37.1%in simulationcondition wear test.