STEEL FOUNDERS' SOCIETY OF AMERCA TECHNICAL SERVICE REPORT #101

Transcription

1 STEEL FOUNDERS' SOCIETY OF AMERCA TECHNICAL SERVICE REPORT #101 OBSERVATIONS ON THE EFFECT OF NITROGEN ON AUSTENITE STABILITY IN CF-3 AND CF-8 STAINLESS STEEL Published by the STEEL FOUNDERS' SOCIETY OF AMERICA Mr. Malcolm Blair Technical and Research Director March 1990

2 OBSERVATIONS ON THE EFFECT OF NITROGEN ON AUSTENITE STABILITY IN CF-3 AND CF-8 STAINLESS STEEL C.E. Bates 1, M. Blair 2, and D. Callihan 3 ABSTRACT Nitrogen is used in both the cast and the wrought steel industries in concentrations up to about 0.20% as a nickel replacement and an interstitial austenite strengthener. If the nitrogen content is unusually low, around 0.03% or less, austenite may not be completely stable in CF-3 and CF-8. Deformation of unstable austenite may cause decomposition to martensite with a substantial increase in ultimate strength. This technical note on effects of nitrogen on austenite stability is based on prior SFSA research and some recent tensile results obtained by an SFSA member foundry. 1 C. E. Bates is the Head of the Metals Section for Southern Research Institute. 2 M. Blair is the Technical and Research Director for Steel Founders Society of America. 3 D. Callihan is the Director of Technical Services for CMI-Quaker Alloy, Inc.

3 OBSERVATIONS ON THE EFFECT OF NITROGEN ON AUSTENITE STABILITY IN CF-3 AND CF-8 STAINLESS STEEL It has been recognized for about 50 years that additions of nitrogen to wrought austenitic stainless steels can be used to increase the tensile strength as well as the creep and stress-rupture properties.(1,2) Nitrogen, like carbon, interstitially strengthens austenite but without the deleterious intergranular carbide precipitation associated with carbon. Thus, the addition of nitrogen to low carbon stainless steels can improve room and elevated temperature properties without increasing the tendency to sensitization. Many references document the significant strengthening effects with slight reductions in elongation.(3-15) A laboratory study was conducted in 1981 under the direction of the Steel Founders' Society to examine the effects of nitrogen on the strength and microstructure of CF-3 and CF-8 stainless steel. Experimental heats of CF-3 and CF-8 stainless steel containing nitrogen were produced in an induction furnace for property and microstructural evaluations. In the first series of CF-8 heats, a nearly constant base composition and chromium ratio were maintained, and nitrogen additions were made to determine nitrogen effects on the ferrite volume and tensile properties. The nitrogen concentration was varied over the range of 0.05 to 0.25%. The nominal composition in this series of castings is given below. CF-8 Nominal Composition (Series I) Element (%) Concentration (%) Nitrogen Range Carbon 0.07 Chromium 21.0 Nickel 8.5 Silicon 1.25 Manganese 0.50 Molybdenum 0.05 Calculated Ferrite Range Test castings were produced as nominal 2 inch thick Y-blocks weighing approximately 80 lb. Molds were produced from AFS GFN 64 silica sand bonded with sodium silicate and cured with 1% ester catalyst. Each mold was coated with an alcohol-based zircon wash and dried in a circulating air oven at 280 F for approximately 24 hours prior to pouring. Each casting was solution heat treated in an air furnace at 2000 F and held for one hour at temperature. The castings were quenched in agitated room temperature water. The effects of nitrogen on the yield and ultimate strength and elastic modulus are illustrated in Figure 1. The yield strength increased about 10 ksi and the ultimate strength increased by about 4 ksi as the nitrogen concentration increased from 0.02 to 0.20%. The yield strength appeared to increase more rapidly with low nitrogen additions, but more slowly at higher nitrogen 1

4 concentrations. The ultimate tensile strength changed very little with increasing nitrogen additions. The second series of CF-8 castings was produced to evaluate the effects of nitrogen at an approximately constant ferrite content. The chromium concentration was adjusted as the nitrogen concentration was increased to maintain the composition ratio near 1.10 and produce a ferrite volume fraction of about 10%. The nitrogen concentration was varied from about 0.02 to 0.25%, and the nominal composition was as follows: CF-8 Nominal Composition (Series II) Element (%) Nitrogen Range Concentration (%) Carbon Chromium Range Nickel 7.0 Silicon 1.25 Manganese Molybdenum Calculated Ferrite Content 9.0 The effects of nitrogen on the tensile strength, yield strength and elastic modulus of CF-8 containing about 10% ferrite are illustrated in Figure 2. The yield strength increased by about 20 ksi as the nitrogen concentration increased from 0.02 to 0.20%. The rate of strength increase was about 1.5 ksi per 0.01 nitrogen below 0.10%N and about 0.5% ksi per 0.01% nitrogen above O.10%N. The ultimate tensile strength decreased initially with nitrogen additions and then increased slightly with nitrogen concentrations between 0.10 and 0.25%. It is noted that the highest ultimate strength values were obtained at nitrogen concentrations below about 0.05%. The third series of castings was similar to the second, except the carbon content was reduced to about 0.012% to produce CF-3 stainless steel. Nitrogen additions were made in the range of 0.02 to 0.25% at an approximately constant calculated ferrite content of 16 to 17%. The nominal composition for this series is given below. CF-3 Concentration Range (Series III) Element (%) Concentration (%) Nitrogen Range Carbon Chromium Range Nickel 7.5 Silicon 1.30 Manganese 0.50 Calculated Ferrite Range The tensile, yield and modulus values are graphically illustrated in Figure 3. The yield strength of the low nitrogen CF-3 casting was about 30,000 psi and progressively increased to about 56,000 psi as the nitrogen content increased to 0.23%. The ultimate tensile strength of these castings ranged widely. The ultimate tensile strength was over 120 ksi in the low nitrogen alloy because 2

5 the austenite was unstable and partially transformed to martensite during testing. However, the austenite was stabilized at increasing nitrogen concentrations which eliminated this strengthening mechanism. The ultimate strength increased as the nitrogen concentration increased from 0.16 to 0.23%, probably as a result of interstitial strengthening. RECENT OBSERVATIONS Recently a heat of AOD refined A351 was processed by a member of the Steel Founders' Society and cast to produce a large valve disk. The casting weighed about 350 lbs and had sections up to 4 inches thick. An integrally cast 1x1x6 inch test bar was produced and in the process of qualifying the casting, tensile specimens were pulled from both the test bar and a four inch thick section. The tensile results obtained from the cast on bar and from the 4 inch thick section are presented in Table 1. The yield strengths were about 35 ksi in both specimens and reductions of area were about 51-53%. The ultimate strength, however, was 93.5 ksi in specimen #3 removed from the cast on bar compared to 74 ksi in a specimen cut from the 4-inch thick section of the casting. Magne-Gage readings on specimen #1 indicated an as-cast ferrite content of about 12% in the casting and a value of about 2% in the cast attachment. A lower ferrite content in the attachment was consistent with the faster solidification and cooling of the bar. Typical microstructures from the 4 inch section and cast-on bars are.illustrated in Figure 4 at 100x after etching with glyceregia. The four inch thick section had about 11-14% ferrite in an austenite matrix as indicated by the Mane-Gage. The attached test bar casting had a much lower ferrite content, about 3%, and the ferrite was much more dispersed as might be expected in the more rapidly solidified attachment. The microstructures in the tensile gage sections of each specimen are illustrated in Figures 5. Some tensile strain induced transformation of austenite was evident in both specimens, but it was especially pronounced in the low ferrite samples. Magnetic measurements also provide evidence of the strain induced transformation. A magnet placed near the head of the tensile specimen was only mildly attracted corresponding to about 3% ferrite content. A magnet held near the gage was much more strongly attracted with Magne-Gage readings greater than 25% indicating substantial martensitic formation during testing. CONCLUSIONS Nitrogen is an austenite stabilizer and is being used in both the wrought and cast steel industries as an effective nickel replacement. It is soluble in CF-3 and CF-8 steels up to concentrations of about 0.2% and is now commercially used in concentrations up to about 0.15% as an effective interstitial strengthener and austenite stabilizer. Assuming a constant microstructure 3

6 (austenite and ferrite content), each 0.01% addition of nitrogen will contribute about 1000 psi in increased yield strength. At very low nitrogen contents such as sometimes experienced in AOD refined 18Cr-8N steels, the austenite may not be stable during mechanical deformation. Deformation may induce transformation of austenite to martensite with progressive increases in ultimate strength as more martensite forms. Yield strengths appear to be affected very little by this phenomena. 4

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13 REFERENCES Adcock, F., Journal Iron and Steel Institute, II(1926). Franks, R., "Chromium Steels Improved by Nitrogen," Iron Age, September 7, Rassbach, H.P., Saunders, E.R., and Harbrecht, W.L., "Electric Furnace Steel Proceedings of the American Institute of Mining and Metallurgical Engineers, 11, Kadlecek, Philip, "Elevated Temperature Properties as Influenced by Nitrogen Additions to Types 304 and 316 Austenitic Stainless Steels," ASTM STP 522, AM Soc. for Testing & Materials, 1973, pp Cullen, T.M. and Davis, M.W., "Influence of Nitrogen on the Creep- Rupture Properties of Type 316 Steel," ASTM STP 522, American Society for Testing & Materials, 1973, pp Domis, W.F., "Creep and Creep-Rupture Properties of Types 304N and 316N Stainless Steels," ASTM STP 522, American Society for Testing & Materials, 1973, pp Heger, J.J., "Mechanical Properties and Corrosion Resistance of a High-Strength Chromium-Manganese-Nitrogen Stainless Steel," ASTM STP 369, American Society for Testing & Materials, 1965, pp Spaeder, C.E., Comis, W.F., and Brickner, K.G., "High Nitrogen Austenitic Stainless Steels," ASTM STP 522, American Society for Testing & Materials, Goodell, P.D. and Freeman, J.W., "Elevated Temperature Properties of Nitrogen-Containing Type 304L Austenitic Stainless Steel," ASTM STP 522, American Society for Testing & Materials, Beck, F.H., Schoefer, E.A., Flowers, J.W., and Fontana, M.G., "New Cast High Strength Alloy Grades by Structure Control," ASTM STP 369, American Society for Testing &Materials. Wellner, P., "Austenitic Cast Steel Containing Nitrogen," J. of Steel Casting Research, 85, Dec. 1978, abridged from Sulzer Technical Review, 60, Research No. 1978, p Gunia, R.B. and Woodrow, G.R., Journal of Materials, 5, No. 2, (1970). Ferry, B.N. and Eckel, J.F., Journal of Materials, 5, No. 1, March Haefner, K., Lahr, A.F., Meinhart, W.L., and Kanter, J.J., Proceedings of the American Society for Testing & Materials, 59 (1959). 11

14 15. Kasak, A., Hsiao, C.M., and Dulis, E.J., Proceedings of the American Society for Testing Materials, 59 (1959). 12