Delubrication

The delubing of stainless steel compacts influences oxygen and carbon balance, which can affect corrosion resistance of the sintered part by precipitation of chromium carbides. Carbide precipitation (sensitization) depletes chromium from grain boundaries.

Sensitization. For a number of austenitic stainless steels, the maximum carbon content allowable to avoid sensitization is 0.03%. Higher carbon contents are tolerable only if the cooling rate after sintering is high enough to prevent sensitization and if the parts are not subjected to elevated temperatures again such as in welding. Critical cooling rates can be estimated from time-temperature-sensitization curves for wrought austenitic stainless steels. Carbide precipitates can be readily shown with optical microscopy. A properly sintered low-carbon austenitic structure (Fig. 12a) is free of carbide precipitates and has clean and thin grain boundaries with evidence of twinning. Higher carbon structures show necklace type (Fig. 12b) and continuous precipitates (Fig. 12c) of chromium-rich carbides in the grain boundaries.

Fig. 12 Microstructures of type 316L stainless steel sintered in hydrogen at 115 °C (1200 °F) (Glyceregia). (a) C is 0.015%, clean and thin grain boundaries. (b) C is 0.07%, necklace type chromium-rich carbide precipitates in grain boundaries. (c) C is 0.11%, continuous chromium-rich carbide precipitates in grain boundaries.

For ferritic stainless steels, the maximum amounts of C + N allowable to avoid sensitization are much lower, namely — 100 to 150 ppm (Ref 14). Because such low contents of interstitials are difficult to obtain in practice, niobium and/or titanium are often used as stabilizers in wrought ferritics. For P/M versions of ferritic stainless steels, niobium is the stabilizer of choice as niobium becomes less oxidized during water atomization than titanium. More information on sensitization conditions and corrosion is discussed in the article "Corrosion-Resistant Powder Metallurgy Alloys" in this Volume.

Atmospheres. In the early years of stainless steel parts production, much of the sintering was done in dissociated ammonia at 2050 to 2100 °F (1120 to 1150 °C). Under these conditions, lubricant removal in the preheat zone of the furnace occurs under partial decomposition such that residual carbon contents typically exceed 0.03%. As this residual carbon is not sufficiently removed during low temperature (<1205 °C, or 2200 °F) sintering, the sintered parts have low-

corrosion resistance due to the formation of chromium-rich carbides upon cooling. Delubing was, therefore, often done separately in air, which resulted in lower-carbon contents because of the more complete combustion of the lubricant. While air delubing reduced the carbon content to more acceptable levels, particularly for austenitic stainless steels, it also was at the expense of increased oxidation. Oxidation begins before complete lubricant removal (Fig. 13). It appears impossible to obtain maximum carbon removal without additional oxidation. The oxides formed during delubing are not always removed during sintering and can impair the corrosion resistance of the sintered parts.

Temperature.

0.06

0.04

0.02

Temperature.

i

6.0 g/cm3

i sy * *

In air

6.6 gfcm3

Temperature, °C 400 500

300 900

Temperature. DF

In DA

™*m

In air

1000 700

600 900

Temperature, "F

In DA

\ V t \ <i \

■ ^c

In DA

Fig. 13 Effect of delubrication temperature on oxygen and carbon contents, and weight loss of 316LSC transverse rupture strength compacts of two densities--6.0 g/cm3, dashed lines, and 6.6 g/cm3, solid lines-lubricated with Acrawax and delubed for 30 min in (a) air and (b) dissociated ammonia

With higher sintering temperature (>1205 °C, or 2200 °F), the reaction between residual oxygen and carbon is more complete, and delubing is, therefore, preferably completed in a reducing atmosphere. As Fig. 14(a) shows for several stainless steels, lubricated with 1% Acrawax and pressed to green densities of 6.5 to 6.7 g/cm3, delubing in dissociated ammonia prevents any significant oxidation up 510 to 538 °C (900 to 1000 °F). Although carbon removal under these conditions is not yet at its maximum, and still >0.03% (Fig. 14b), a higher sintering temperature effectively lowers the carbon content to below 0.03%. It is partly because of these relationships that stainless steel parts sintered at high temperatures often exhibit better corrosion resistance than those sintered at lower temperatures.

Fig. 14 Effect of delubing temperature on (a) oxygen content and (b) carbon content of stainless steel transverse rupture strength bars (6.5 to 6.7 g/cm3) lubricated with 1% Acrawax and delubed for 30 min in dissociated NH3

For high-temperature vacuum sintering, the reaction between residual carbon and oxygen is even greater so that carbon can even be added, in the form of graphite, to some stainless steels to achieve lower-oxygen contents at acceptable carbon contents (Ref 9, 15, 16). Lower-oxygen contents are beneficial to both corrosion resistance (Ref 7, 9) and dynamic mechanical properties.

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