IPTEK, The Journal for Technology and Science, Vol.19, No. 1, February 2008 7 Development of Ultra-Carbon Steel Prepared by Mechanical Alloying and Subsequent Hot Pressing Abstract⎯It is known that the most effective way to im- prove mechanical properties of steels is by microstructural refinement. Iron and graphite powders mixture with hypo- eutectoid composition were mechanically alloyed (MAed) and subsequently hot pressed (HPed) to provide steels with fine structure. Mechanical Alloying (MA) was carried out using a planetary ball mill for 100, 200 and 300 h, while HP at 41 MPa at various temperatures: 610ºC (below), 730ºC (near) and 800ºC (above the eutectoid transformation point A 1 ). The mechanically alloyed (MAed) powders were cha- racterized by X-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM). While, the HPed compacts were characterized by SEM, Vickers hardness and tensile test. During MA, refinement of crystallite, formations of super-saturated iron solid solution and Fe/C amorphous phase occured first. With further MA time, these phases might begin to transform to more stable phases such as carbides. In the case of HP at 610ºC (below A 1 ), very fine cementites were precipitated in fine ferrite grain with sub-micron meter in the size. However, the mechanical properties cannot be attained because of low sinterability. At 730ºC (near A 1 ), the strength reaches the maximum value. With further temperature increase (at 800ºC), the sintering progressed well and the coarsening occured, resulting the decrease in strength. However, the fracture strain increased significantly. The steels obtained in the present study had mechanical properties comparable to those of standard (JIS) steels through the welle stablished heat treatment such as normalizing and thermal refining. Keywords⎯Mechanical alloying, Ultra-carbon steel, Fine grain ferrite, Fine cementite, Hhot pressing, Heat treatment. I. INTRODUCTION t is well known that mechanical properties of steels such as strength and toughness usually can be impro- ved by grain refinement through thermal refining and addition of alloying elements. However, the grain refine- ment through thermal refining has a limitation up to only several ten micrometers. On the other hand, in ecomate- rial perspective, the addition of alloying elements (such as special elements that become trumped elements etc.) is not desirable due to the problem in recycling process and resource depletion. Recently, mechanical alloying (MA) was utilized to produce non-equilibrium phases such as amorphous [1], Manuscript received August 31, 2006; revised May 22, 2007 1 Nurul Taufiqu R. is with Research Center for Physics, Indonesian Institute of Sciences, INDONESIA 2 H. Sueyoshi is with Department of Nano Structured and Advanced Materials, Kagoshima University, JAPAN [2], super-saturated solid solution [2] and nano-crystal- line phases [3], [4]. There were reported [5], [6], [7], [8] that these phases also could be obtained in hyper-eutec- toid Fe-C systems. The alloying mechanism during MA also had been discussed [8], [9]. During MA, amorphous Fe/C phase and super-saturated iron solid solution transform to metastable carbides such as Fe 3 C and Fe 7 C 3 [9], [10], [11], [12]. It is supposed that if the MAed Fe-C powders were consolidated, very fine-grained carbon steels could be attained. Reffering to [7], [8] had succeeded to produce Fe-high C system hard alloys from the MAed powders. In order to make a comparative study, in the present study hypo-eutectoid Fe-C powders were MA-HPed and their mechanical properties were then compared with the standard (JIS) carbon steels through the well-established heat treatment. II. EXPERIMENTAL PROCEDURE Elemental powders of iron (99.5 mass %, 5 µm) and graphite (99.9 mass %, <78 µm) with hypo-eutectoid composition (Fe-0.4 mass % C and Fe-0.6 mass % C) were put into a cylindrical pot (SKD, 450 cm 3 ) under an argon atmosphere with two types of stainless balls (SUS 304, µ12 mm: µ 7 mm = 1:1). The ball-to-powder mass ratio was 8:1. MA was carried out at 120 rpm for 100, 200 and 300 h by using a planetary ball mill (PM400, Retsch Industry Inc.). The MAed powders were characterized by X-ray diffraction analysis (XRD), scan- ning electron microscopy (SEM) and differential thermal analysis (TG-DTA).TG-DTA was performed under argon atmosphere with heating rate of 0.33 °C/ s. After MA for 200 and 300 h, the Fe/C alloying powders were HPed at various temperatures (A: below (610°C), B: near (730°C) and C: above (800°C) the eutectoid transformation point A 1 ) at 41 MPa under nitrogen gas atmosphere. The HPed compacts were examined by tensile test at a crosshead speed on 0.5 mm/ min, microstructural observation by using SEM and Vickers hardness measurement. III. RESULT AND DISCUSSION Figure 1 shows SEM images of as-received iron and graphite powders. While Figure 2 shows Fe-0.4C and Fe- 0.6C powders mixtures MAed for 300 h. The morphology of graphite powders before MA is irregular. While, iron powders is almost spherical (see Figure 1). As shown, after MA for 300 h, the extremely deformed powder par- ticles are observed. The relatively coarsened particles are Nurul Taufiqu Rochman 1 and H. Sueyoshi 2 I