The University of Michigan * Office of Research Administration Ann Arbor, Michigan CREEP PROPERTIES AT 1100~ AND 1200~F FOR. TYPE 304L AUSTENITIC STEEL WITH CONTROLLED RESIDUAL ELEMENTS by R. Jackowski J. W. Freeman Project 04603-46-T May 29, 1964 Bay 29A 1964 Report 24..0 THE TIMKEN ROLLER BEARING COMPANY STEEL AND TUBE DIVISION CANTON, OHIO

CREEP PROPERTIES AT 1100~ AND 1200~F FOR TYPE 304L AUSTENITIC STEEL WITH CONTROLLED RESIDUAL ELEMENTS The properties of Type 304L austenitic steel under creep conditions have been subject to considerable uncertainty. The creep resistance of Type 304L steel is generally considered to be lower than for Type 304H with a minimum carbon content of 0. 04 percent. There are some data indicating equal strengths for the "L" grade as made in production. In addition, The International Nickel Company Inc. has published data indicating that small amounts of certain elements in Type 304L steel result in creep-rupture strengths as high as those for Type 304H..:rhese elements could be present unknown as residuals from the srin the scrap sed the charge or be purposefully added. The investigation covered by this report involved the determination of the creep resistance of air melted steel with 0. 04% Mo, 0. 10% V and 0. 074% Cb as "controlled" residuals. The same:stock was also remelted in a consumable arc vacuum furnace to check the effect of melting in vacuum. The V and Cb content of the vacuum melted material was not reported. The heat treatment used was 15 minutes at 1900~F, water quenched. CONCLUSIONS The Type 304L steel with "controlled" residual elements had creep strengths at 1100~ and 1200~F close to the average for Type 304H steel. Evidently the "residual" elements brought the creep resistance up. There was very little difference in the creep strengths of the original stock from the air arc furnace heat and the same material consumable electrode vacuum remelted. The creep rates of the vacuum melted 1

material started out lower and changed less with increasing times of testing than did those from the air melted condition. Both the air and the vacuum melted materials underwent very little change in tensile and impact properties at room temperature as a result of creep testing. Both had exceptionally high ductility. EXPERIMENTAL MATERIAL Standard 0. 505-inch diameter tensile specimens were supplied from heat treated bar stock. Part of the specimens were from an air melted heat. Stock from the air melted heat was remelted in a consumable electrode vacuum furnace. The Timken Roller Bearing Company made the heats, supplied machined specimens and reported the following chemical compositions for the two materials: Chemical Composition (percent) Heat No. C Mn P S Si Cr Ni Mo V Cb 15356.023 1.25.014.013.72 17.94 11.26.04.10.074 IV3814.020 1.19.012.015.76 17.95 11.30.05 Bar stock from each heat was solution treated for 15 minutes at 1900~F and water quenched. The hardness of the air melted heat was 174 Brinell and 162 for the consumable electrode vacuum melted heat. RESULTS Creep tests up to 5, 100 hours duration at 11000 and 1200~F (Figures 1 through 3) indicated the following: (1) The tests at 9,600 psi and 1100~F were discontinued at 2, 859 hours when the creep rates were down to 0. 0035% per hour. The measurement uncertainty of such creep rates together with the rates being so 2

far below the 0. 01% per hour rate showed that further testing would not be useful. (2) There was very little difference in creep rate between the air melted and vacuum melted materials for the tests at 9, 600 psi at 11000F and 6, 000 psi at 1200~F. There was a slight difference in the higher stress tests. The air melted material started with faster rates but the creep rate decreased more with time of testing. (3) Log-log stress-creep rate curves were used to estimate the stresses for creep rates of 0. 01% per 1000 hours (Figure 4): Temperature 0.01% per 1000 hour (~F) creep strength (psi) 1100 13,000 1200 7,700 The differences in minimum creep rates were sufficiently small that there seems to be no reason to report a difference in creep strength for the air versus vacuum melted materials. Presumably continuing the 9, 600 psi creep tests to longer time periods at 11000F would have resulted in lower creep rates than those reported. This, however, would have little effect on the reported 0. 01% per 1000 hour creep strength. The rates at 14, 500 psi are sufficiently close to 0. 01% per 1000 hours so that the creep strength would not be appreciably changed by lower creep rates at 9,600 psi. Tensile and Impact Tests at room temperature after creep testing The lower stress specimens from the creep tests are compared with the original material on the basis of tensile and impact tests at room temperature in Table I. The following comments are based on this data: (1) The samples from the air melted heat had tensile strengths about 5, 000 psi higher than the vacuum melted heat. Both showed about the same increase in tensile strength as a result of creep testing. 3

(2) The same comments apply to yield strengths except that the differential was greater between the two heats. (3) There was little difference in ductility for either material between heats or as a result of testing. (4) Impact strengths from the modified specimens hardly changed during testing and were about the same for both materials. Microstructural Examination The air melted material had a uniform fine grain size of ASTM 7-8. The vacuum melted stock had a mixed grain of ASTM 2-3 and 7-8. (See Plates 1 through 6). Creep testing at 1100~F hardly changed the microstructure. Some precipitation occurred in the grain boundaries. Testing at 1200~F did result in some precipitation in the grain boundaries. The precipitate particles were small and scattered along the grain boundaries. Clear structure adjacent to the grain boundaries probably resulted from depletion of elements from the grains to form the precipitates, which were probably carbides with some sigma. This effect was most pronounced at stringers of particles carried over from ingot compositional segregation. It should be noted that the structures of the original materials and the creep specimens from the 11000F tests had to be etched far more than those tested at 1200~F to develop the structures shown. The precipitation at 1200~F resulted in the grains being outlined with far less etching. DISCUSSION The creep strengths for 0. 01% per 1000 hours were slightly higher than the "average" for Type 304H steel reported by George Smith in reviewing the ASME Boiler Code Stresses. Presumably the small amounts of V and CB brought the stress up from that considered char4

acteristic of Type 304L steel. The strengths are also comparable to those for Type 304 steel in previous investigations for Timken (Table II). There was some slight tendency for the "L" heats to be slightly lower at 1200 Fo Comparison with the more extensive ASME data showed that Heats 15356 and IV3814 had strengths similar to Type 304 steel and it therefore appears that factors other than carbon content controlled the level of properties, Table II also shows the far higher creep strengths for all carbon levels when the tests were longer than 2, 000 hours. The decrease in creep rate with testing time especially for air melted heats certainly increases the indicated "creep strengths" in the more prolonged tests. There are certain features of the data which are not understood. Vacuum melting would be expected to remove nitrogen, which ought to have reduced creep resistance compared with air melted steel, Evidently some other factor was involved that is not understood. It is probably the result of interactions between trace elements. The creep data seem to indicate that the low carbon material with controlled "trace" elements can have creep strengths equal to Type 304H steel. Furthermore such material retained ductility and impact strength at room temperature to a remarkable degree after creep testing. The information supplied did not indicate whether the specimens were made from hot-finished or cold-worked material. It is therefore uncertain whether or not the grain size differences between the air and vacuum melted material were due to differences in the conditions of making the bar stock, or to differences resulting from air and vacuum melting. It is quite common for vacuum melting to lower grain growth temperature. 5

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