The influence of flow-forming parameters and microstructure on the quality of a D6ac steel
Introduction
Flow-forming is now a well established metal forming process for the production of cylinders, flanged components, axi-symmetric sheet metal parts, etc. The process has been studied both experimentally and theoretically by many workers. The mechanics of deformation in the shear spinning of cones and tubes has been studied by Hayama [1], Kalpakcioglu and Rlagopal [2], Kegg [3], Kobayashi and Thomsen [4], Kobayashi et al. [5], and Singhal et al. [6]. Using the concepts of localized deformation, the shear spinning of long tubes has been predicted and applied on hard-to-work materials such as stainless steel, titanium and different grades of superalloys. Although the analytical models are very helpful in predicting the required power for spinning, the influences of the critical mechanical parameters (such as the feed rate, the shape of the contact line, the roller angle and the percentage deformation), metallurgical parameters (chemical composition, cleanness of the alloy and state of the microstructure) and the spinnability of the worked material are not well documented. In fact, the mutual influence of mechanical and metallurgical parameters along with the complexity of the process can lead to various defects that result in the rejection of many finished products.
In this investigation, the influences of the above-mentioned spinning parameters on the extent of forming defects such as wave-like surfaces, microcracks and bore were studied. The variation in mechanical and metallurgical properties of the material under various forming schedules was investigated and the optimum conditions for the elimination of the defects were determined. Also, the influences of cold reduction and heat treatment on the microstructure and mechanical properties (tensile strength, charpy V-notch, KIC and hardness) of the specimens were studied. The variation of the microstructure as a function of percentage reduction and heat treatment variables was observed using optical and electron microscopy. Finally, fractography studies were carried out and the type and chemical composition of the inclusions for each experimental condition were determined.
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Materials and method
The chemical composition of the material used in this investigation is shown in Table 1. The microstructure of the as-received material consisted of ferrite and globular cementite. The preforms had thicknesses of 4.5, 2.5 and 2 mm and feed rates between 0.31 and 0.91 mm/rpm were employed. The experiments were carried out in a 3-roller spinning machine with roller angles of 15°, 25° and 30° and a percentage reduction of 28–67% was applied. The type and distribution of spinning defects were
The influence of the feed rate
The spinning force and power were evaluated using the strip method presented by Kobayashi and Thomsen [4]. As the axial movement of the mandrel (the feed rate) depends on the thickness and diameter of the preform [8], depending on these two parameters feed rates of 0.3, 0.4, 0.51, 0.68, 0.81 and 0.91 were applied to the three types of preforms (i.e. 4.5, 2.5 and 2 mm thickness). The results of the present experiments have shown that for feed rates lower than 0.4 mm/rpm, an increase in internal
Conclusions
- 1.
The value of S/L ratio is indicative of the type of defects appearing after flow-forming.
- 2.
The S/L ratio depends on the attack angle α and percentage reduction.
- 3.
Defect-free workpieces were spun with an attack angle of 30°.
- 4.
By controlling the heat-treatment parameters, it is possible to obtain the best combination of strength and toughness.
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