Research on the Effect of Pouring Temperature on Hot-Tear Susceptibility of A206 Alloy by Simulation
- PDF / 2,032,657 Bytes
- 7 Pages / 593.972 x 792 pts Page_size
- 29 Downloads / 207 Views
INTRODUCTION
BEING the subject of metallurgical researches since 1940s, hot-tear defects still impose huge expenses to casting industries. Both ferrous and nonferrous alloys, especially those having a wide mushy zone, are susceptible to these defects.[1–3] Hot tearing, also referred to as hot cracking, hot shortness, super solidus cracking, and shrinkage brittleness, is a tear which occurs during solidification of molten alloys while there is still some remaining melt between dendrites.[4] Hot tearing occurs in the last stages of solidification, in areas without sufficient feed metal. When the solid fraction is close to unity, tensile strains or contraction stresses arise in the ‘‘mushy zone’’ of the casting.[5,6] In order to study the hot tearing of cast alloys in most methods, including the Ring test and the Dog bone test,[7–9] visual observation of castings followed by measuring the dimensions of the tears after solidification is necessary. However, none of these methods are accurate and reliable regarding their possible errors in detection and measurement of the lengths of the tears. They cannot also distinguish between the influences of different features on the occurrence of hot tearing. Recently, developed methods use load-measuring equipments,[10,11] including ultrasonic waves, X-ray microtomography, and in situ observations, to evaluate the hot tearing.[4,12–15] However, analysis of their complex output data is a difficult task. Different factors, including the alloy composition, casting geometry and design, mold and core material, core design, and superheat, can influence the hot-tear susceptibility.[16] Although the proposed criteria are all quantified, they can only assess hot-tear susceptibility in limited (experimental) conditions. Moreover, some of the proposed methods require a special sample.
MOHAMAD REZA NASRESFAHANI, Lecturer, and MOHAMAD JAVAD RAJABLOO, Student, are with the Department of Materials Engineering, Najafabad Branch, Islamic Azad University, Najafabad, Iran. Contact e-mail: [email protected] Manuscript submitted January 16, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
For example, the hot tearing criterion proposed by Clyne and Davies[17] is based on the assumption that at the last stage of freezing it is difficult for the liquid to move freely, so liquid mass feeding cannot accommodate the strains developed during this stage. The last stage of freezing is considered as the most susceptible to hot tearing in this criterion. The tearing susceptibility coefficient is defined by the ratio of the vulnerable time period where hot tearing may develop, (t0.99 t0.9), and time available for the stress relief process where mass feeding and liquid feeding occur, (t0.9 t0.4). Where t0.99 is the time when the volume fraction of solid, fs, is 0.99, t0.9 is the time when fs is 0.9, and t0.4 is the time when fs is 0.4. Therefore, the cracking susceptibility coefficient HCS reads HCS ¼
t0:99 t0:9 t0:9 t0:4
½1
For using this criterion, we must determine solid fraction and its time by thermal ana
Data Loading...
