Carbon Control

Tight control of carbon levels in powder metal parts is one of the distinguishing attributes of the high-quality P/M producer. The carbon content affects many of the properties of the finished part such as dimensional tolerances and mechanical properties. The pick up of carbon also decreases the corrosion resistance of stainless steel.

Carbon control is achieved primarily through a fine tuning of the sintering atmosphere, as it relates to temperature, admixed carbon, carbon and oxygen content of the powder, and delubrication. The control of this atmosphere is greatly improved through the use of synthetic (N2H2) versus generated atmospheres.

The reactions relating to carbon control occur primarily in the hot zone and the slow-cool zone of the furnace (Fig. 7). If the atmosphere is very active—an endothermic atmosphere with 60% combustibles (hydrogen and carbon monoxide) at — 5 °C (40 °F) dew point, for example—a steel surface with —0.8% combined carbon is decarburized to —0.2% combined carbon (Fig. 10) in the hot zone (typically 1120 °C, or 2050 °F) to a depth of 0.5 to 2.5 mm (0.02 to 0.1 in.).

Fig. 10 Carbon equilibrium in endo gas at different dew points. Endo gas composition: 40% hydrogen, 20% carbon monoxide, and 40% nitrogen

The degree and depth of decarburization depend on the dew point of the active endothermic atmosphere, the processing time and temperature in the hot zone, and the porosity of the steel part. Higher values for these parameters lead to a greater degree of decarburization.

During slow cooling from —1120 to 815 °C (2050 to 1500 °F) under an active endothermic atmosphere, carbon is more or less restored to the surface of the part. Decarburization in the hot zone followed by restoration of carbon in the slow-cooling zone also occurs in other active atmospheres, such as nitrogen-diluted endothermic or nitrogen-diluted dissociated methanol, with a combustible content of 20% or more hydrogen and carbon monoxide. The slow-cooling section of the furnace is designed to aid carbon restoration.

If the atmosphere is neutral to carbon (a low dew point synthetic nitrogen-base atmosphere with <10% combustibles such as hydrogen, for example), decarburization in the hot zone and subsequent carbon restoration in the slow-cooling zone do not occur at all or occur to a much less extent than they do under active endothermic atmospheres. Carbon potential for neutral atmospheres is unrelated to temperature. Consequently, neutral atmospheres, unlike endothermic atmospheres, are suitable for high-temperature sintering of steel P/M parts.

Furthermore, parts with different carbon levels can be sintered in the same tray or in random tray sequence under neutral atmosphere without the need for furnace conditioning. Carbon content in the steel part depends on the amount of admixed graphite in the powder and the dew point of the neutral atmosphere. With a lower dew point, less carbon is lost.

In some specialty sintering applications, such as those used for magnetic and stainless steels and high-ductility iron, carbon must be reduced to the lowest possible level during sintering. In such applications, atmospheres with carbon-containing gases such as carbon monoxide should be avoided. A relatively high dew point is required to decarburize the part without adversely affecting the sintered quality of the part.

In some applications, an atmosphere that is slightly oxidizing to sintered parts during cooling at <540 °C (1000 °F) is desired to provide controlled oxidation for improved corrosion resistance and increased service life.

0 0

Post a comment