Preservation of Geometrical Integrity of Supersolidus-Liquid-Phase-Sintered SKD11 Tool Steels Prepared with Powder Injec
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POWDER Injection Molding is often applied in fabricating small components that have complicated shapes and high hardness, which make them difficult or costly to machine. Among the various PIM materials, tool steels are widely used for parts that require good wear resistance.[1–3] One example is SKD11, used in the rotating cams in the hinges of notebook computers and mobile phones. It has a composition similar to that of D2 tool steels, as shown in Table I. Although SKD 11 has been widely used in the the PIM industry, complexshaped parts made of SKD11 have suffered serious problems in dimensional control or even distortions.[4–6] The main reason is that SKD11 compacts are generally consolidated through the supersolidus liquid phase sintering (SLPS) process, i.e., sintering above the solidus line in the L + c + carbide or L + c region, as shown in Figure 1, where c is the austenite, carbide is the M7C3, and L is the liquid. To attain high sintered densities, the amount of liquid must be well controlled; if too much liquid is formed, then gravitational forces cause distortion or even slumping, while too little of the liquid amount will not densify the compact.[7–9] Assuming that a successful sintering without distortion can be obtained with a liquid amount between 5 and 15 wt pct, this assumption leads to a very narrow temperature range of 1515 to 1523 K (1242 to 1250 °C), as shown in Figure 2. This small temperature range suggests that the amount of liquid is very sensitive to the temperature variation, K.H. CHUANG, Graduate Student, and K.S. HWANG, Professor, are with the Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan, Republic of China. Contact e-mail: [email protected] Manuscript submitted July 1, 2010. Article published online December 22, 2010 1896—VOLUME 42A, JULY 2011
or the slope of the liquid fraction curve is steep, at about 1.25 wt pct/K. Thus, if SKD11 is sintered at 1520 K (1247 °C) with 10 wt pct liquid using a standard industrial sintering furnace with a guaranteed temperature uniformity of ±10 K, the density will be too low for parts sintered in the solid state at 1510 K (1237 °C), while distortion will occur for parts sintered at 1530 K (1257 °C) because 18 wt pct liquid is present. The above example demonstrates that when the solidus and liquidus lines in a phase diagram are flat and the temperature difference between these two lines is small, as shown in Figure 1 for SKD11, the amount of liquid becomes very difficult to control, for the lever rule indicates that a small temperature variation will result in large changes in the amount of the liquid phase. Figure 1 also indicates that higher carbon content decreases the optimal sintering temperature and enlarges the L + c + carbide phase region, which is beneficial for SLPS.[10,11] However, the method of adding carbon is usually not practical because SKD11 has a specification for carbon, between 1.4 and 1.6 wt pct. To resolve the shape retention problem caused by the temperature sensitivity of the li
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