High Strength Iron Compacts Containing Carbon, Copper and Tin

Abstract

The addition of five percent of copper + tin in various ratios to iron powder compacts produced a liquid phase at sintering temperatures of 900-950°C; considerably below the temperatures normally employed for sintering iron-copper compacts. The mechanisms of liquid phase sintering in this system have been studied. Electron probe microanalysis of sintering models showed that the addition of tin to the iron-copper system progressively increased the solubility of iron in the liquid phase, and decreased the rate of copper diffusion into the iron. Likewise, the presence of copper during sintering decreased the rate of tin diffusion into the iron. This interaction between copper and tin results in the presence of a low melting-point liquid phase throughout the sintering process. No evidence of stabilisation of the α -iron phase due to tin solution at 950°C was detected either metallographically or by electron probe microanalysis at Cu:Sn ratios of 3:2 to 15:2. The addition of carbon to iron compacts containing five percent of copper + tin gave greatly improved mechanical properties when sintered in argon or carbon monoxide. As carbon is a ∀ -stabiliser, it is suggested that the major sintering mechanism is due to the enhanced iron solution rate in the liquid enabling dissolution and re-precipitation processes to occur, and not to any increased sintering rate due to stabilisation of the α-phase. The sintering atmosphere employed was found to control the final mechanical properties of the sintered compacts. Compacts with low Cu:Sn ratios (high tin content) gave high strengths only when sintered in pure hydrogen. Compacts with high Cu:Sn ratios yielded high strengths when sintered in various atmospheres.  Heat treatment of Fe-Cu-Sn or Fe-Cu-Sn-C compacts did not improve the mechanical properties to the same extent as did judicious choice of the optimum combination of copper-tin ratio, carbon content, and furnace atmosphere.