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The effect of test environment on tensile properties of 316L austenitic stainless steel part I: Serrated flow characteristics
Scripta Metdhrgicn et Mataialia, Vol. 33. No. 9, pp. 14S9-14921995 Elm&r Sci~lce Ltd cowright 0 1995 Acta Metallwca Inc. PrintedintheUSAAllri&tsresemed 09%-716x/95 $9.50+ .oo
Pergamon
0956-7162[(95)00423-8
THE EFFECT OF TEST ENVIRONMENT ON TENSILE PROPERTIES OF 316L AUSTENITIC STAINLESS STEEL PART I: SERRATED FLOW CHARACTERISTICS A.A. Abduluyahed, K. Roiniatowski and K.J. Kurzydlowski Institute of Materials Science and Engineering Warsaw University of Technology 02-524 Warsaw, Narbutta 85, POLAND (Received June 8,1995) (Revised July 10,1995) Introduction
The stmss-straincurves of iron based alloys deformed over a wide temperature range exhibit a serrated flow associated with the Portevin-LeChatelier (PLC) effect (l-4). This type of behavior, which depends on both temperature and strain rate has been observed in both substitutional and interstitial alloy systems (5-7). Serrated yielding is usually explained by dynamic strain aging, which arises due to the di.Rusionof solute atomic to the moving dislocations. Serrated flow in the austenitic stainless steels has also been attributed to precipitation of t:hromium carbide and thus depletion of chromium and carbon (8-12). Pmperties ofmaterials in general depend not only on the test temperature but also on the test environment. The effect of test atmosphere on mechanical properties of intermetallics has been recently confirmed in (13). From that point &view it should be pointed out that most of the studies in the past on PLC effect were carried out in standard air atmosphere. In the present study the combined effect of temperature and test atmosphere on the mechauical properties of 3 16L in the range of serrated flow was investigated. Two basic test conditions were employed: straining in air and vacuum. ExDerimental
The experiments were performed on a commercial 3 16L stainless steel. The type 3 16L stainless steel investigated has the composition given in Table 1. The tensilespecimenswere pmduced acxoding to DIN specitications (14). The specimens were annealed at 900 “Cfor one :hourthenwafer quenched Tensiontests were carried out in the temperature range from 500 to 600 OCat strain rate 2~10~ s-l. The test atmospheres employed were air and vacuum of 1.33x1@’ Pa. The load-extension curves in each case were characterized in terms of number of serrations per interval of strain The frequency is plotted againstthe percent of strain in Figure 1. It is clear from the data shown there that the Ikequency is systematically higher in air than in vacuum. The normalized flow stress (a/E) dependence on temperature is shown in Figure 2. It can be noted that the flow stress increases with the increasing test temperature when tested in air, while it decreases with the 1489
Vol. 33, No. 9
SERRATED FLOW OF STAINLESS STEEL
1490
TABLE 1 Chemical Composition of the 3 16L Stainless Steel 1Elem,,tp-~pqYq~I(~~~~~~~~
increasing temperature when tested in vacuum. The figure depicts that the normalized flow stress is shitled to a higher level in vacuum. The variation with temperature of the ultimate tensile strength (UTS) for different atmospheres is also shown in Figure 2. It can be seen that the UTS in vacuum is systematically higher than in air. The difference between the UTS values in air and vacuum decreases as the test temperature increases. X-ray micro-probe examinationshave been carried out on the specimens strained in vacuum and air. These studies proved that, as one could expect, the specimen strained in air exhibits a much more thicker layer of oxides on its outer surface (see Fig.3).
The present results clearly demonstrate a measurable environmental effect on the mechanical properties of 3 16L stainless steel in the range of temperatures of serrated flow. An assumption has been made in the present work that it can be explained in terms of the intluence of oxidation layer on the tensile properties of the material. It should be noted first that the infhrenceof surface layers on tensile properties of austenitic stainless steels is negligible in most cases of technical importance. This is due to their: (a) high resistance to corrosion and (b) remarkable capacity for plastic deformation. (On the other hand an important exception from this rule is stress indwzed cracking-see for example (15). The e&et of surface layer on the tensile properties of materials is particularly reduced when they deform in a uniGormas do austenitic stainless steels at low temperatures. In this case the intluence of surface layer is proportional to thickness-to-radius ratio. On the other hand it should be pointed out that the plastic deformation of austenitic stainless steels in the temperature range of serrated flow is much less uniform. Thus it is
0
0 2
4 STRAIN
6 %
s
2
4
6 STRAIN
8
%
Figure 1. Plot of the number of serrations per unit strain againstthe strain and test kmpemtue tested in: (a) air and @) vacuum.
Vol. 33, No. 9
SERRATEDFLOWOF STAINLESSSTEEL
1491
-I a)
Em
wo
520
540
580
560
TEMPERATURE
600
C
Figure 2. The variation of normalized flow stress (a) and ultimate tensile stren& (b) tested in air and in vacuum as fimctim of testing temperature.
suggested here that the sutlhce layers contributed to the properties of 3 16L steel in the experimental conditions adopted in the present work. The effect of the surface layers on the serration frequency, flow stress and ultimate tensile strength to some extent can be rationalized in terms of the oxide layers cracking. Since cracks on the surface act as stress concentrators a thicker oxide layers developed on the specimens strained in air can be used to explain a higher frequency of serration. The same argument can be used to link a thicker layer of oxides with a lower ultimate tensile strength of specimens strained in air. The explanation of the enviromnental effect on the flow stress seems to be more complex as one should take into account a potential difhrsionof inter&it&r into the strained specimen. The importance of this phenomenon is supported by the observed dependence of the flow stress on the temperature. In the temperature range from 500 to 600°C the flow stress of the specimens strained in vacuum decreases with the increasing temperature and the opposite trend is observed for specimens strained in air. Further studies of this effect are in progress. Concludiw
Remarks
One of the major observations of this work is that the test enviromnent intluences the properties of an austenitic stainless steel in the conditions routinely used in mechanical testing. It is suggested here that to a large extent it can be accounted for in terms of the oxide layer impact on the mechanical properties of the specimens strained in air and vacuum. 600
4
HO-
W
500. 450 400. 8
350. 300.
‘
250. 200. 150. 100 soO-
Figwz 3. Results of X+ay miauph investigation of the surf&e of the specken: (a) strained in the air and (b) shined arrow inch&s position of the oxygen peak).
in vacuum (the
1492
SERRATED
FLOW
OF STAINLESS
STEEL
Vol. 33, No. 9
Acknowledgements
This work has been supported by a grant from the Polish State Commit& for Scientific Research. Special thanks are to Dr J.J.Bucki and K.Sikorski for their valuable contribution to the experimental part. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
KJ. Kmzydlowski,RA Varin and K. Tangri, Zeitshrift fhr Metallkunde, @, 469 (1989). K. G. Samuel S. I,. Mannan and P. Rodrigueq Ser. Melall., & 685(1992). S. VW S. Veuugopal,P. V. Sivaprasad and P. Rodriguez Materials Tram., JIM, & 1040 (1992). S. L. Mannan, K. G. Samuel and P. Rodriguez, Mat. Sci. Eng., al43 (1984-1985). M.C. Chaturredi and D.J. Liody, Mater. Sci.Eng. z 107 (1983). Van deu Beukel, Acta Metall., a 965 (1980). E. Pink and A Grinberg, Mater. Sci. Eng., al (1981). J.T.Barnby, J.ofthe Iron end Steel Institute, && 392 (1965). K. G. Samuel, S. L. Mannan end P. Rodriguez, Acta Metal. ,s 2323 (1988). S. N. Monteiro, I. LeMay and L. H. DeAlmeida, Ser. Metall., & 581(1981). RAMulfordandU.F.Kocks,ActaMet_,21,1125(1979). S. Ve&&san, C. P&j, P. V. Sivaprasad aad P. Rodriguez, Acta Metall. &, 569 (1992). C. T. Liu, Ser. MetaIl., z 25 (1992). G.E. Dieter, Mechanical Metallurgy 3ti ed,. pp. 295, McGraw-Hill Book Company (1986). T. Magnh, ISIJ htemationaI, M 223 (1995).
Pergamon
0956-7162[(95)00423-8
THE EFFECT OF TEST ENVIRONMENT ON TENSILE PROPERTIES OF 316L AUSTENITIC STAINLESS STEEL PART I: SERRATED FLOW CHARACTERISTICS A.A. Abduluyahed, K. Roiniatowski and K.J. Kurzydlowski Institute of Materials Science and Engineering Warsaw University of Technology 02-524 Warsaw, Narbutta 85, POLAND (Received June 8,1995) (Revised July 10,1995) Introduction
The stmss-straincurves of iron based alloys deformed over a wide temperature range exhibit a serrated flow associated with the Portevin-LeChatelier (PLC) effect (l-4). This type of behavior, which depends on both temperature and strain rate has been observed in both substitutional and interstitial alloy systems (5-7). Serrated yielding is usually explained by dynamic strain aging, which arises due to the di.Rusionof solute atomic to the moving dislocations. Serrated flow in the austenitic stainless steels has also been attributed to precipitation of t:hromium carbide and thus depletion of chromium and carbon (8-12). Pmperties ofmaterials in general depend not only on the test temperature but also on the test environment. The effect of test atmosphere on mechanical properties of intermetallics has been recently confirmed in (13). From that point &view it should be pointed out that most of the studies in the past on PLC effect were carried out in standard air atmosphere. In the present study the combined effect of temperature and test atmosphere on the mechauical properties of 3 16L in the range of serrated flow was investigated. Two basic test conditions were employed: straining in air and vacuum. ExDerimental
The experiments were performed on a commercial 3 16L stainless steel. The type 3 16L stainless steel investigated has the composition given in Table 1. The tensilespecimenswere pmduced acxoding to DIN specitications (14). The specimens were annealed at 900 “Cfor one :hourthenwafer quenched Tensiontests were carried out in the temperature range from 500 to 600 OCat strain rate 2~10~ s-l. The test atmospheres employed were air and vacuum of 1.33x1@’ Pa. The load-extension curves in each case were characterized in terms of number of serrations per interval of strain The frequency is plotted againstthe percent of strain in Figure 1. It is clear from the data shown there that the Ikequency is systematically higher in air than in vacuum. The normalized flow stress (a/E) dependence on temperature is shown in Figure 2. It can be noted that the flow stress increases with the increasing test temperature when tested in air, while it decreases with the 1489
Vol. 33, No. 9
SERRATED FLOW OF STAINLESS STEEL
1490
TABLE 1 Chemical Composition of the 3 16L Stainless Steel 1Elem,,tp-~pqYq~I(~~~~~~~~
increasing temperature when tested in vacuum. The figure depicts that the normalized flow stress is shitled to a higher level in vacuum. The variation with temperature of the ultimate tensile strength (UTS) for different atmospheres is also shown in Figure 2. It can be seen that the UTS in vacuum is systematically higher than in air. The difference between the UTS values in air and vacuum decreases as the test temperature increases. X-ray micro-probe examinationshave been carried out on the specimens strained in vacuum and air. These studies proved that, as one could expect, the specimen strained in air exhibits a much more thicker layer of oxides on its outer surface (see Fig.3).
The present results clearly demonstrate a measurable environmental effect on the mechanical properties of 3 16L stainless steel in the range of temperatures of serrated flow. An assumption has been made in the present work that it can be explained in terms of the intluence of oxidation layer on the tensile properties of the material. It should be noted first that the infhrenceof surface layers on tensile properties of austenitic stainless steels is negligible in most cases of technical importance. This is due to their: (a) high resistance to corrosion and (b) remarkable capacity for plastic deformation. (On the other hand an important exception from this rule is stress indwzed cracking-see for example (15). The e&et of surface layer on the tensile properties of materials is particularly reduced when they deform in a uniGormas do austenitic stainless steels at low temperatures. In this case the intluence of surface layer is proportional to thickness-to-radius ratio. On the other hand it should be pointed out that the plastic deformation of austenitic stainless steels in the temperature range of serrated flow is much less uniform. Thus it is
0
0 2
4 STRAIN
6 %
s
2
4
6 STRAIN
8
%
Figure 1. Plot of the number of serrations per unit strain againstthe strain and test kmpemtue tested in: (a) air and @) vacuum.
Vol. 33, No. 9
SERRATEDFLOWOF STAINLESSSTEEL
1491
-I a)
Em
wo
520
540
580
560
TEMPERATURE
600
C
Figure 2. The variation of normalized flow stress (a) and ultimate tensile stren& (b) tested in air and in vacuum as fimctim of testing temperature.
suggested here that the sutlhce layers contributed to the properties of 3 16L steel in the experimental conditions adopted in the present work. The effect of the surface layers on the serration frequency, flow stress and ultimate tensile strength to some extent can be rationalized in terms of the oxide layers cracking. Since cracks on the surface act as stress concentrators a thicker oxide layers developed on the specimens strained in air can be used to explain a higher frequency of serration. The same argument can be used to link a thicker layer of oxides with a lower ultimate tensile strength of specimens strained in air. The explanation of the enviromnental effect on the flow stress seems to be more complex as one should take into account a potential difhrsionof inter&it&r into the strained specimen. The importance of this phenomenon is supported by the observed dependence of the flow stress on the temperature. In the temperature range from 500 to 600°C the flow stress of the specimens strained in vacuum decreases with the increasing temperature and the opposite trend is observed for specimens strained in air. Further studies of this effect are in progress. Concludiw
Remarks
One of the major observations of this work is that the test enviromnent intluences the properties of an austenitic stainless steel in the conditions routinely used in mechanical testing. It is suggested here that to a large extent it can be accounted for in terms of the oxide layer impact on the mechanical properties of the specimens strained in air and vacuum. 600
4
HO-
W
500. 450 400. 8
350. 300.
‘
250. 200. 150. 100 soO-
Figwz 3. Results of X+ay miauph investigation of the surf&e of the specken: (a) strained in the air and (b) shined arrow inch&s position of the oxygen peak).
in vacuum (the
1492
SERRATED
FLOW
OF STAINLESS
STEEL
Vol. 33, No. 9
Acknowledgements
This work has been supported by a grant from the Polish State Commit& for Scientific Research. Special thanks are to Dr J.J.Bucki and K.Sikorski for their valuable contribution to the experimental part. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
KJ. Kmzydlowski,RA Varin and K. Tangri, Zeitshrift fhr Metallkunde, @, 469 (1989). K. G. Samuel S. I,. Mannan and P. Rodrigueq Ser. Melall., & 685(1992). S. VW S. Veuugopal,P. V. Sivaprasad and P. Rodriguez Materials Tram., JIM, & 1040 (1992). S. L. Mannan, K. G. Samuel and P. Rodriguez, Mat. Sci. Eng., al43 (1984-1985). M.C. Chaturredi and D.J. Liody, Mater. Sci.Eng. z 107 (1983). Van deu Beukel, Acta Metall., a 965 (1980). E. Pink and A Grinberg, Mater. Sci. Eng., al (1981). J.T.Barnby, J.ofthe Iron end Steel Institute, && 392 (1965). K. G. Samuel, S. L. Mannan end P. Rodriguez, Acta Metal. ,s 2323 (1988). S. N. Monteiro, I. LeMay and L. H. DeAlmeida, Ser. Metall., & 581(1981). RAMulfordandU.F.Kocks,ActaMet_,21,1125(1979). S. Ve&&san, C. P&j, P. V. Sivaprasad aad P. Rodriguez, Acta Metall. &, 569 (1992). C. T. Liu, Ser. MetaIl., z 25 (1992). G.E. Dieter, Mechanical Metallurgy 3ti ed,. pp. 295, McGraw-Hill Book Company (1986). T. Magnh, ISIJ htemationaI, M 223 (1995).