A New Criterion for Prediction of Hot Tearing Susceptibility of Cast Alloys
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tearing is a common defect in cast alloys, especially in those with wide freezing ranges.[1–3] Most commonly used methods to study the hot tearing of cast alloys have been traditionally based on visual observation of the castings and measuring the dimensions of the tears incurred.[4–6] However, due to the possible errors in detection and measurement of the tears, as well as their inability to distinguish the influence of different parameters on the occurrence of hot tearing, they are not very accurate and reliable. Newer methods have been developed that use load measurement equipments, ultrasonic waves, or X-ray microtomographic observations to evaluate the hot tearing tendency.[7–10] However, most of these methods are expensive and their output data are complex. One of the established hot tearing criteria is that of Feurer.[11,12] Feurer considers two terms, i.e., SRG and SPV, which are abbreviations for Schrumpfungs Geschwindigkeit (solidification rate) and Speisungsvermo¨gen (supply assets) in German, respectively. SRG and SPV functions, shown in Eqs. [1] and [2], denote the volumetric rate of solidification shrinkage and the maximum rate of volumetric feeding through a dendritic network for a given mushy zone structure and hydrostatic pressure, respectively
MOHAMAD REZA NASRESFAHANI, Ph.D. Student, and BEHZAD NIROUMAND, Associate Professor, are with the Department of Materials Engineering, Isfahan University of Technology, 84156-83111 Isfahan, Iran. Contact e-mail: [email protected] Manuscript submitted October 25, 2013. Article published online May 21, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
SRG ¼
1 dV 1 dq ¼ V dt q dt
½1
f2L k22 ps ; 24pc3 gL2
½2
SPV ¼
where dV and dq are the changes in volume and density of the alloy by solidification, and fL, k2 (m), Ps (Pa), L (m), c, and g (Pa s) are liquid volume fraction, secondary dendrite arm spacing (or grain size), effective feeding pressure, length of porous network, tortuosity constant of the dendritic network, and viscosity of the liquid phase, respectively. According to this criterion, hot tearing occurs when transfer of the molten metal between the dendrite networks is unable to balance the volume reduction due to solidification shrinkage. In other words, hot tearing becomes likely if SRG>SPV. Feurer’s criterion uses only metallurgical parameters of the structural elements in the mushy zone to predict the hot tearing. It disregards the contraction-induced tensile stresses developed by the elements both inside and outside the mushy zone. These stresses are the main cause of hot tearing and significantly increase its occurrence. Some researchers have measured the contraction loads developed during solidification and tried to analyze the features of the recorded load–time graphs to predict the occurrence of hot tearing.[8,9,13–15] However, these methods also suffer from not considering the metallurgical parameters affecting the hot tearing occurrence. Another well-known problem in hot tearing is the random nature of its tests results. The scatter in the re
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