Volume 13 - Corrosion Part 12 potx

absorption follows Sievert's law; that is, absorption is proportional to the square root of the partial pressure of nitrogen in the sintering atmosphere This nitrogen absorption provides significant strengthening (Fig 3) Upon completion of sintering, when the part enters the cooling zone of the furnace, the solubility of nitrogen decreases sharply with temperature (Fig 12) As a result, Cr2N begins to precipitate at the temperature at which the nitrogen content crosses the solubility limits More important, below about 1150 °C (2100 °F), additional nitrogen is absorbed from the sintering atmosphere, leading to more Cr2N precipitation and chromium depletion along the grain boundaries The net result is inferior corrosion resistance due to grain-boundary corrosion Fig 12 Solubility of nitrogen in austenitic stainless steel in equilibrium with gaseous nitrogen or Cr2N Source: Ref 9 The rate of this detrimental nitrogen absorption increases with decreasing part density and with decreasing dew point A high dew point, however, leads to the problem of excessive oxidation The basic relationship of this phenomenon is shown in Fig 13 The data in Fig 13, which were developed for the bright annealing of stainless steel in dissociated NH3 atmospheres, show the extent of nitrogen and oxygen absorption as a function of dew point At high dew points (higher than about -37 °C, or -35 °F, depending on part size), the rate of oxidation is severe enough to produce a dull surface At dew points of about -45 °C (-50 °F) or lower, nitrogen absorption increases so much that the corrosion resistance deteriorates because of excessive Cr2N formation Thus, optimum bright annealing of austenitic stainless steels must be done within a narrow dew-point range Although the authors (Ref 14) caution against applying these findings to sintered stainless steels based on the unexplained higher nitrogen contents found for their parts sintered in dissociated NH3, it should be noted that such higher nitrogen contents are expected on the basis of known solubility data for nitrogen in type 316L (Fig 12) considering the differing methods of nitrogen analysis used Fig 13 Safe operating parameters with respect to dew point can be developed for a specific set of operating conditions and quality requirements The safe zone here is for sintering in an atmosphere of 30% H2-70% N2 at 1035 °C (1900 °F) Source: Ref 14 Chromium nitride sensitization may in some cases be limited to a very shallow surface depth of the part With very slow cooling, however, absorption and precipitation proceed toward the interior of the porous part Figure 14 shows Cr2N precipitates in the grain boundaries of parts that were sintered under conditions that produced nitrogen contents from 55 to 6650 ppm Increasing nitrogen content correlates with increasing amounts of precipitation and increasing localized corrosion (Fig 14) Figure 15 shows the microstructure of a type 316L part that was sintered in dissociated NH3 and cooled very slowly Slow cooling produced a lamellar structure of Cr2N and low-chromium austenite of very poor corrosion resistance Fig 14 Scanning electron micrographs of type 316L stainless steel (a) Sintered 45 min in 100% H2 at 1350 °C (2460 °F); 66 ppm N (b) Sintered 45 min in 75% H2 at 1350 °C (2460 °F); 3100 ppm N (c) Sintered 45 min in 25% H2 at 1350 °C (2460 °F); 4300 ppm N (d) Sintered 45 min in 25% H2 at 1150 °C (2100 °F); 6650 ppm N The amount of intergranular precipitate increases with nitrogen content Source: Ref 13 Fig 15 Micrograph showing the lamellar structure of Cr2N and low-chromium austenite in sintered type 316L that was slowly cooled in dissociated NH3 Etched with Marble's reagent 700× Source: Ref 9 Corrosion resistance data for sintered types 304L and 316L in NaCl solutions and in 10% HNO3, reflecting the effect of Cr2N precipitation, are shown in Fig 8, 16, and 17 Figures 8 and 16 show that a higher sintering temperature, fast cooling rates (75 °C/min, or 135 °F/min, versus 8 °C/min, or 14 °F/min), and the use of type 316L rather than type 304L provide better corrosion resistance That these measures are beneficial follows directly from the austenite-nitrogen phase diagram (Fig 12) Fig 16 Effect of composition, cooling rate, and sintering temperature on corrosion resistance of type 304L and tin-modified type 304L P/M stainless steels (sintered density: 6.5 g/cm3; sintering atmosphere: dissociated NH3) in 5% aqueous NaCl B rating indicates