Morphology of graphite in hot-compressed nodular iron

Bulk material processed by the severe plastic deformation (SPD) technique were found possessing unique physical and mechanical properties [1–3]. SPD material were either nonferrous metal or soft steel. Recently, some researchers attempted to apply this technique to hard-to-work materials [4]. The present authors developed a new SPD process named Cylinder Covered Compression (CCC) and applied it to nodular iron [5]. Our previous work showed that graphite nodules became small with two extended tails along the direction transverse to the compressing force during hot-compression. After severe deformation, a large number of graphite flakes formed a lamellar structure with the metal matrix. This study aimed to build a deformation model of graphite in hot-compressed nodular iron, and then the model was used to explain why graphite area fraction increased on the axial sections of the specimen after more than 80% reduction. The nodular iron was obtained from the China Railway Shanhaiguan Bridge Co. Ltd, as 22.6 mm thick plates with a chemical composition (mass%) of 3.57C, 2.55Si, 0.22Mn, 0.021P, 0.013S and the balance in Fe. A new SPD process, named Cylinder Covered Compression (CCC), was developed by the present authors. The schematic illustration of the CCC process is shown in Fig. 1. Specimens, 8 mm in diameter with a 20 mm height, were machined from the cast plates. Cylinders of an 8 mm inner diameter and 10 mm outer diameter were made of medium carbon steel (0.45%C). The specimens enclosed by the cylinders were compressed on a Gleeble 3500 Machine. The details of the thermomechanical treatments are listed in Table I. The 30, 50, 65 and 80% deformed specimens were prepared by one pass of hot-compression. The 80% deformed specimens were cut into pieces, had surface layers ground away, were then stacked, embedded in a cylinder, and hotcompressed again. The 94% deformed specimen was prepared by two passes of about 80% hot-compressions.