Obtaining Martensitic Structures during Thixoforming of Hypoeutectic Gray Cast Iron

Lucas Bertolino Ragazzo,Eugênio José Zoqui,Davi Munhoz Benati,Rodolfo Lopez Nadal

doi:10.1155/2015/170562

Abstract

The control of parameters such as liquid fraction, holding time, and cooling rate during thixoforming can help control the final microstructure of the thixoformed part, thus improving its mechanical properties. This study intended to investigate conditions required to obtain martensite in hypoeutectic gray cast iron at 3.1% CE (carbon equivalent) deformed in the semisolid state. Samples heated up to 1130, 1135, and 1145°C (liquid fractions of 10, 30, and 45%) were compressed into platens without any holding time (0 s). If a sample presented a martensitic structure for 0 s holding time, new samples were retested at the same temperature for 30, 60, and 90 s holding times. The die casting process was simulated by allowing the platens to become locked after hot compression. Samples that cooled in the locked platens were submitted to higher cooling rates than samples that cooled with the platens open and presented martensite instead of the conventional ferrite and pearlite. Thus, the factor that had the greatest influence on the formation of martensite was the cooling rate rather than stress. The thixoforming process presented good morphological stability, which is highly desirable for industrial applications.

Highlights

Thixoforming involves heating a material until it reaches the semisolid state followed by forming
Controlling some of the process parameters, such as liquid fraction, holding time, deformation, and cooling rate, can help control the final microstructure and promote martensitic transformation, improving the mechanical properties of the thixoformed parts [1, 2]
As described in Experimental Procedure, when samples subjected to a holding time of 0 s were found to contain martensite, additional samples were tested at the same temperature for holding times of 30, 60, and 90 s

Summary

Introduction

Thixoforming involves heating a material until it reaches the semisolid state followed by forming. The mechanical properties of the thixoformed product are determined by the final microstructure of the material used. Steel and cast iron have been widely researched as potential raw materials for semisolid processing and have been shown to have a wide range of beneficial properties, such as good fluidity and stability in the semisolid state [3,4,5,6,7]. Some cast irons, including hypoeutectic gray cast iron, have been successfully used as raw material for semisolid processing [9,10,11,12]

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Conclusion