Abstract
Carbide precipitates in Thin Wall Ductile Iron (TWDI) used for automotive applications needs to be eliminated or reduced for improved strength, ductility, crack propagation resistance and good machinability. Ductile iron thin section profiles (≤3 mm) present danger of massive carbide precipitations in the as-cast sample. Precipitated carbide phase is brittle and negatively affects the mechanical properties of the iron matrix. The suppression of carbide formation is associated with the nucleating properties of the nodularizer and innoculant alloys. This treatment is vital in ensuring that carbide precipitation, flake graphite structure and non-nodular graphite phases are reduced or completely eliminated in the TWDI castings. Therefore, the temperature and technique of treatment would influence the yield of the process, and ultimately the mechanical properties. In this study, the effect of nodularization and inoculation treatment temperature on the microstructure and mechanical properties of TWDI castings is examined. The results indicate that good nodularity and nodule count with better percent elongations are achieved using low treatment temperatures in descending order of 1490°C, 1470°C and 1450°C, but have negative effect at lower treatment temperature of 1430°C. However, TWDI castings have superior properties in terms of nodule counts and nodularity at 1450°C. Treatment temperature does not produce significant influence on ultimate tensile strength (UTS) and hardness of TWDI castings. TWDI castings show poor nodularity, nodule count and ductility at higher inoculation treatment temperatures of 1550°C, 1530°C and 1510°C.
Highlights
In the production of sound Thin Wall Ductile Iron (TWDI) castings, adequate melt treatment involving efficient inoculation and nodularization are imperative
This same procedure is repeated for treatment temperatures of 1530 ̊C tapped at 1535 ̊C, 1510 ̊C tapped at 1515 ̊C, 1490 ̊C tapped at 1495 ̊C, 1470 ̊C tapped at 1475 ̊C, 1450 ̊C tapped at 1455 ̊C and 1430 ̊C tapped at 1435 ̊C
The hardness trend of TWDI with treatment temperatures used is shown in Table 3 and Figure 2
Summary
In the production of sound Thin Wall Ductile Iron (TWDI) castings, adequate melt treatment involving efficient inoculation and nodularization are imperative. There should be synergy between these two important treatments for the production of good nodularity, high nodule count and carbide free TWDI. Nodularization treatment is a modification treatment where the shape of the graphite is changed from flake to spheroid or nodules This shape is responsible for the peculiar properties of ductile iron. Ductile Iron application for lightweight components is being limited by inability to produce as-cast carbide free thin (≤3 mm) wall parts [2] This inability results from the formation of large volumes of carbide precipitates, poor nodularity and nodule count, presence of non-nodular graphite structures and undesired matrix types in the cast TWDI part. This study presents a molten metal processing technique for treatment temperature that eliminates primary carbides and non-nodular graphite structures in pearlitic-ferritic ductile iron castings useful for automotive applications
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