Research On The Effect Of Heat Treatment Process Of Low Nickel And Molybdenum Ductile Iron

Ductile iron samples containing 0.25% molybdenum and 0.4%/0.6%/0.8% nickel were austempered and annealed at low graphitization temperature in the study. The microstructure and fracture morphology of the samples were observed before and after the heat treatment. The effect of different heat treatment process on the ductile iron samples’ mechanical properties was analyzed. Then the optimum heat treatment parameters were obtained by statistical analysis. The content of retained austenite was analyzed by X-ray diffraction(XRD). The influence of molybdenum and nickel on the sensitivity of heat treatment process was analyzed statistically. The main conclusions of the thesis were as follows:(1) There are graphite ball, ferrite, pearlite and little cementite in the microstructure of as-cast ductile iron. The content of ferrite increased and the content of pearlite decreased after adding molybdenum and nickel. The hardness and tensile strength decreased while the impact toughness increased. Some river pattern and tear edges have been found in the SEM morphology of the impact fracture surface of the as-cast samples. The form of the fracture is quasi-cleavage fracture which characterized by river pattern and tear edges. There are more river pattern and less tear edges on the fracture surface after annealed. Some small dimples were found on the tear edges. The fracture appearance showed that quasi-cleavage fracture and ductile fracture happened on the annealed samples during tests.(2) The tensile strength, impact toughness and hardness of the austempered samples were improved by different degrees. Some tear ridges were found in the SEM morphology of the fracture surface of the austempered samples with 0.25% molybdenum and 0.6% nickel. The tear ridges showed to crossing distribution and can prevent the crack from diffusing. There are some small dimples distributed on the tear ridges and some river pattern was found diffusely distributed among the tear ridges. The fracture appearance showed that quasi-cleavage fracture and ductile fracture happened on the austempered samples during tests.(3) The content of the residual austenite in the samples with 0.25% molybdenum and 0.6% nickel decreased with the quenching temperature increasing when austenitizing at 920℃. There are more residual austenite in the samples when austenitizing at 930 ℃than austenitizing at 920℃. The content of the residual austenite in the samples which containing 0.25% molybdenum and 0.4% nickle is less than in the samples without nickel when the samples were austenitized at 920℃ and quenched at 370 ℃, while the molybdenum improved the hardenability. Austenite became more stable and the content of the residual austenite increased after austempered with the increase of the content of nickel.(4) There are some acicular lower bainite in the samples which containing 0.25% molybdenum and 0.6% nickel when austenitized at 920℃. The lower bainite became thicker with the quenching temperature increasing. When the samples austenitized at 920℃ and quenched at 350℃ some thicker lower bainite appeared and some feathered upper bainite appeared when quenched at 370 ℃and 390℃. Feathered upper bainite was found in the samples in which there is on molybdenum or nickel when austenitized at 920 and quench℃ ed at 370℃, due to the molybdenum and nickel improved the hardenability.(5) The variance of the mechanical property value of the samples containing molybdenum and nickel are smaller than the samples without molybdenum or nickel and the values are more stable after graphitizing annealed at low temperature. So adding appropriate amount of molybdenum and nickel is good for us to produce as-annealed ductile iron with stable mechanical property. The variance of the tensile strength value of samples with 0.4% /0.8%nickel are bigger than the samples without molybdenum or nickel, while the variance of the samples containing 0.25% molybdenum and 0.6% nickel increased significantly. The variance of the value of elongation and impact absorbing energy and brillnell hardness increased after austempered under different heat treatment processing. So it is important to strictly control the austempering processing to produce mechanical properties qualified ductile iron when the sample contains molybdenum and nickel.