Sintering

Most commercial sintering of stainless steel parts is completed in belt, pusher, walking beam, and vacuum furnaces. A wide range of processing parameters is common. Typical sintering atmospheres include hydrogen, dissociated ammonia, H2-N2 mixtures, and all of "low" dew point, as well as vacuum. Sintering temperatures range from 1120 to 1344 °C (2050 to 2450 °F), and sintering times range from 20 to 60 minutes. Insufficient sintering, either too short a time or at too low a temperature, will result in sintered parts showing insufficient bonding, original particle boundaries, and sharp, angular pores as shown in Fig. 15 for a 316L stainless steel part sintered for 30 min at 1093 °C (2000 °F) in H2. Such sintering produces parts with a high concentration of interstitials (carbon and oxygen), low-corrosion resistance, and inferior mechanical properties.

Fig. 15 Insufficiently sintered 316L showing original particle boundaries and sharp, angular pores. As-polished cross section

The significance of the various sintering parameters is discussed in the following paragraphs.

Sintering in Hydrogen. In spite of higher cost, the use of hydrogen for the commercial sintering of stainless steel parts has been increasing at the expense of dissociated ammonia. The main reasons for this change are that with present sintering furnaces, it is very difficult to obtain good corrosion resistance in dissociated ammonia, and the increasing use of ferritic stainless steels in magnetic applications also requires the absence of nitrogen in the sintered part. Nevertheless, sintering in hydrogen also requires careful control to obtain maximum corrosion resistance.

Figure 16 shows the redox curves for chromium and silicon in 316L in H2 of various dew points at atmospheric pressure (Ref 17). Chromium and silicon are the two constituents in 316L with the highest oxygen affinities. Sintering with dew-point sintering temperature combinations to the left of the two curves of Fig. 16 will result in selective oxidation of chromium and silicon, respectively, and produce parts of inferior corrosion resistance. Thus, for the lowest sintering temperatures employed in commercial sintering of stainless steels (1120 °C, or 2050 °F), the hydrogen should have a dew point not higher than approximately -40 °C (-40 °F) to ensure reduction of chromium oxides in 316L stainless steel (Fig. 16). A similar relation is shown in Fig. 2(c) for pure chromium.

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Sintering temperature, "F 1400 1600 1800 2000 £200

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700 S00 900 1000 1100 1200 1300 Sintering temperature, °C

Fig. 16 Redox curves for chromium and silicon in 316L in H2 at atmospheric pressure. Source: Ref 17

What happens after sintering, that is, during cooling, is even more important. Figure 16 shows two scenarios. For both scenarios, the sintering temperature is 1200 °C (2192 °F). In the first scenario, the dew point of the sintering atmosphere is -40 °C (-40 °F); in the second scenario, it is -60 °C (-76 °F). In the first scenario, the stainless steel part crosses the Si02/Si redox curve (Fig. 16) upon cooling at —1070 °C (1960 °F). At this temperature, the rate of oxidation of silicon is quite rapid and, therefore, rapid cooling is necessary to prevent or minimize, the formation of silicon oxides on the surface of the stainless steel part. Figure 17 shows a scanning electron microscopy (SEM) of such oxide precipitates for 316L. These precipitates do not cause the part to discolor, and they are visible only under a microscope. When tested in aqueous FeCl3, in accordance with ASTM G 46 (20 °C), such parts exhibit inferior corrosion resistance due to pitting. Higher-alloyed stainless steel powder such as SS-100 (20Cr-17Ni-0.8Si-5Mo) appear to be more immune to this type of corrosion (Ref 10).

Fig. 17 Spheroidal silicon oxide particles formed on 316L part on cooling

With the second scenario in Fig. 16, the lower dew point of -60 °C (-76 °F) causes the parts to cross the SiO2/Si redox curve in the cooling zone of the furnace at the much lower temperature of —890 °C (1632 °F). At that temperature, the rate of silicon oxidation either is very slow, or any oxides formed at that temperature have little effect on the corrosion resistance of the part. Figure 18 shows tentative critical cooling rate-dew point combinations as a function of dew point for three hydrogen sintered austenitic stainless steels. Upper critical cooling temperatures, that is, the lowest high temperatures where rapid cooling is to commence, are shown in Fig. 19.

Fig. 18 Effect of cooling rate and dew point upon corrosion resistance (5% aqueous NaCl) of hydrogen sintered stainless steels. Dashed curves representing maximum corrosion resistance are tentative. Corrosion resistances shown in parentheses are percentages of maximum corrosion resistance for given grade and density.

Fig. 19 Upper critical cooling temperature and iso-corrosion resistance curves (5% aqueous NaCl) for H2 sintered 316L (schematic)

Processing to the right of the cooling rate-dew point curves produces maximum corrosion resistance. Processing to the left results in rapid deterioration of corrosion resistance as shown schematically in Fig. 19 for 316L. It is clear from these relationships that for maximum corrosion, resistance, sintering in hydrogen requires very low dew points and/or rapid cooling after sintering. Mechanical properties of hydrogen sintered stainless steels are given in the article "Powder Metallurgy Stainless Steels" in this Volume.

Sintering in Vacuum. In the early years of commercial sintering of stainless steel parts, vacuum furnaces were said to be good alternatives to other types of sintering because of their low consumption of gas. After years of experience, parts producers learned that in addition to high initial capital cost, vacuum furnaces also were costly to maintain. Nevertheless, it is clear from the previous section on sintering in hydrogen that with the typical furnaces (belt, pusher, and walking beam) used presently in the industry, the number one property of stainless steel, superior corrosion resistance, is for most presently used P/M stainless steels not attainable to a sufficient degree. Therefore, future P/M opportunities that require excellent corrosion resistance cannot be realized until furnace manufacturers construct furnaces that are capable of lower dew points and parts producers equip their furnaces with (already available) rapid cooling devices. This is where vacuum furnaces are used. With a state of the art vacuum furnace, it is much easier to maintain a low dew point and to obtain rapid cooling than it is with a typical atmosphere furnace. Nevertheless, certain precautions are necessary.

For maximum corrosion resistance of vacuum sintered stainless steel, surface depletion of chromium due to high-vapor pressure and the presence of original surface oxides must be minimized. Sintering under a partial pressure of nitrogen or argon of —1500 /'m of mercury effectively reduces chromium losses. Reference 9 shows that after high temperature sintering (>1205 °C, or 2200 °F), where chromium losses are more severe, holding the parts prior to cooling for a short time at a lower temperature, or by increasing the partial pressure of argon to 1 at the lower temperature, significantly improves corrosion resistance. Both measures allow the parts to replenish (from the interior) surface chromium that was lost at the high-sintering temperature.

In spite of the high-oxygen contents of water atomized stainless steel powders, vacuum sintered stainless steel parts usually are bright. This is because some of the surface oxides, during sintering, diffuse into the interior.

In the absence of an external-reducing gas atmosphere, vacuum-sintered stainless steel parts have relatively low carbon and oxygen contents due to the reaction between carbon and oxygen (or oxides) particularly at high-sintering temperatures, to form carbon monoxide (Fig. 20) (Ref 18 and 19). There is evidence, however, that the typical amounts of carbon present in a stainless steel part after delubrication are insufficient for removing most of the original oxide particles present on the outer surfaces of a part. These unreduced original oxide particles give rise to pitting corrosion. Admixing small amounts of graphite to the stainless steel powder, overall oxide reduction, particularly at the higher-sintering temperatures, is greatly enhanced and is also sufficient to reduce surface oxides. Alternatively, a small partial pressure of H2 should accomplish the same.

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When making graphite additions to a stainless steel powder, it should be kept in mind that the carbon content of the sintered part will increase. Thus, the optimum graphite addition is the maximum addition that produces no chromium carbide precipitates in the cooling zone of the sintering furnace. It depends, among other factors, on the composition of the stainless steel, the oxygen content of powder, the sintering temperature, and the cooling rate employed.

Vacuum-sintered stainless steels should be rapidly cooled in a non-oxidizing gas to prevent the formation of deleterious surface oxides. Cooling in nitrogen will result in the formation and precipitation of chromium nitrides on the surfaces of the parts. The attendant chromium depletion will cause the parts to have very low-corrosion resistance. Mechanical properties of vacuum sintered stainless steels are shown in the article "Powder Metallurgy Stainless Steels" in this Volume.

Sintering in H2-N2 Gas Mixtures. Sintering at 1120 °C (2050 °F) in dissociated ammonia was the most widely used method for sintering stainless steels in the 1960s and 1970s. Dissociated NH3 was not only less expensive than H2, but it also increased the strength of the sintered parts, although at some reduction in ductility, to levels comparable to wrought stainless steels of the same composition, at densities of —85 to 90% of theoretical. The strengthening is the result of nitrogen absorption during sintering. The amount of nitrogen absorbed follows known phase equilibria in accordance with Sievert's law, that is, nitrogen absorption is proportional to the square root of the partial pressure of nitrogen in the sintering atmosphere. (See the article "Corrosion-Resistant Powder Metallurgy Alloys.") The relationships for 304L are shown in Fig. 21 (Ref 20), showing both the amount of nitrogen absorbed as a function of sintering temperature and sintering atmosphere (dissociated ammonia and nitrogen) and the strength increase due to nitrogen absorption.

Fig. 21 (a) Effect of nitrogen content on ultimate tensile strength and elongation of 304L stainless steel. (b) Effect of sintering temperature on amount of absorbed nitrogen for 304L. Source: Ref 20

The problems with nitrogen absorption and chromium nitride (Cr2N) precipitation during cooling (after sintering), and sensitization and loss of corrosion resistance, when sintering is done in dissociated ammonia, are described in Ref 21. They were not appreciated for many years, in part because corrosion resistance demands were modest and/or corrosion resistance was not assessed. Later, with increasing demands for improved corrosion resistance, and as more quantitative information on the effect of sintering in dissociated ammonia became available, recommendations were made to limit nitrogen absorption to some 3000 ppm in austenitic stainless steels. While this limitation seemed to satisfy some corrosion resistance requirements in an acidic environment, it was unsatisfactory for parts tested in aqueous NaCl. Even with small amounts of Cr2N precipitates, rust spots would form in a short time. Figure 22 shows examples of Cr2N precipitates in sintered austenitic stainless steels.

Fig. 22 Chromium nitride precipitates in 316L (a) sintered at 1150 °C (2100 °F) in dissociated NH3; 4500 ppm N2; Cr2N precipitates along grain boundaries (1), and within grains (2). (b) sintered at 1120 °C (2050 °F) in dissociated NH3 and slowly cooled; 6500 ppm N2; Cr2N in lamellar form near surface (1) and as grain boundary precipitates in the interior (2).

Fig. 22 Chromium nitride precipitates in 316L (a) sintered at 1150 °C (2100 °F) in dissociated NH3; 4500 ppm N2; Cr2N precipitates along grain boundaries (1), and within grains (2). (b) sintered at 1120 °C (2050 °F) in dissociated NH3 and slowly cooled; 6500 ppm N2; Cr2N in lamellar form near surface (1) and as grain boundary precipitates in the interior (2).

In an early study, Sands et al. (Ref 22) pointed out that 316L sintered in dissociated NH3 required a cooling rate of 200 °C/min for preventing nitrogen absorption and precipitation of Cr2N. More recently, Frisk et al. (Ref 23) determined in a laboratory study that sintering of 316L in dissociated NH3 at 1250 °C (2280 °F) required cooling rates of >450 °C/min (Fig. 23). The higher critical cooling rate of Frisk et al. can probably be ascribed to their much lower dew point (-100 °C versus -40 to -60 °C for Sands), which allows for more rapid nitrogen absorption during cooling as illustrated in Fig. 24 (Ref 24) for the bright annealing of stainless steel strip. The deleterious reactions of increasing nitrogen absorption with decreasing dew point, and of increasing oxidation with increasing dew point, leave a relatively narrow dew point window for sintering in dissociated ammonia.

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Fig. 24 Effect of dew point on nitrogen absorption and oxidation of 316L shim, disk, and bar stock annealed for 15 min at 1038 °C (1900 °F) in 30%H2-70%N2. It took 2.3; 2.8; and 4.7 min respectively to cool the three materials from 1038 °C (1900 °F) to 538 °C (1000 °F). Source: Ref 24

These cooling rate requirements are even higher than those for sintering in hydrogen to prevent oxidation during cooling, and the cooling-rate dew point relationship appears to be reversed. It is, therefore, not surprising that stainless steel parts producers are increasingly shifting towards hydrogen sintering at the expense of sintering in dissociated NH3.

Sintering in an atmosphere of 10%N2-90%H2 may be a more practicable compromise that greatly reduces the high-cooling rate requirements of dissociated NH3 to more manageable levels, while still benefiting substantially from the solid solution strengthening obtainable with the lower nitrogen concentration. Good corrosion resistance for such conditions were reported by Larsen (Ref 25) and Mathiesen (Ref 26).

The positive effect of nitrogen on corrosion resistance, as documented for wrought stainless steels, is expected to apply equally to sintered stainless steels. However, this beneficial effect has not yet been well documented for. sintered stainless steels, probably because of the overshadowing negative effect of excessive nitrogen absorption on the surface of parts from the sintering atmosphere during cooling. Mechanical and other properties for 316L and 434L sintered in dissociated NH3 are given in the article "Powder Metallurgy Stainless Steels" in this Volume.

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