Effect of ageing on the strain rate sensitivity and serrated flow of a type 316 stainless steel at 923 K

Scripta METALLURGICA et MATERIALIA

Vol. 26, pp. 685-689, 1992 Printed in the U.S.A.

Pergamon Press plc All rights reserved

EFFECT OF A G E I N G ON THE STRAIN R A T E S E N S I T I V I T Y A N D S E R R A T E D FLOW OF A TYPE 316 S T A I N L E S S STEEL AT 923 K

K.G. Samuel, S.L. Mannan and P. Rodriguez Metallurgy Programme Indira Gandhi Centre for Atomic Research Kalpakkam 603 102, INDIA (Received October 17, 1991) (Revised December 9, 1991) Introduction It has been noted previously that negative strain rate sensitivity is a characteristic f e a t u r e of m a t e r i a l s d e f o r m e d u n d e r c o n d i t i o n s p r o d u c i n g s e r r a t e d flow []-3]. Serrated flow in austenitic stainless steels is attributed to dynamic strain ageing (DSA) [4,5], which arises due to the diffusion of solute atoms to the moving dislocations. It has been shown that serrated flow at a temperature can be suppressed in Mg-AI [6], AI-Mg-Si alloy [7], 6063 alloy [8] and type 316 stainless steel [9] by ageing at a suitable time temperature combination where the depletion of the solute species responsible for DSA occurs through precipitation. Ageing of type 316 stainless steel in the temperature range 773-1123 K results in chromium carbide precipitation and thus the depletion of chromium and carbon, which are responsible for dynamic strain ageing, from grain boundary regions which are preferential sites for dynamic strain ageing [5]. It is of interest to see if the presence of solutes causing DSA is accompanied by negative strain rate sensitivity and if precipitation which depletes the solutes makes the strain rate sensitivity positive. In the present investigation the strain rate sensitivity and serrated flow at 923 K in a type 316 austenitic stainless steel is examined after prolonged ageing at 923 K and compared with the behaviour at 923 K without ageing. Experimental

The chemical composition (wt%) of type 316 stainless steel used in the present investigation was: C: 0.06, Cr: 16.02, Ni: 11.95, Mo: 2.21, Mn: 1.60, Si: 0.54, S: 0.02, P: 0.036. Tension tests were carried out in the temperature range 300-1123 K in the unaged material and at 923 K after ageing at 923 K for 5000 h at a base strain rate (~b) of 3X10 -4 s -I in a floor model Instron 1195 universal testing machine. Strain rate sensitivity was examined by strain rate jump tests where in one case strain rate lJumps were made between the base strain rate and one decade higher (~h = 3x10 ~ s- ) and in another case between the base strain rate and one decade lower (et = 3x10 "Ns -t ) using a push button speed selector switch and a proportional cbart drive mode. The load scale was magnified using a ten step zero suppression facility and the accuracy of load measurement was ± N. Results Consistent the temperature

irregularities range 523-923

and D i s c u s s i o n

in the load-elongation curves were observed in K for unaged material at the base strain rate

685 0036-9748/92 $5.00 + .00 Copyright (c) 1992 Pergamon Press plc

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3xlO -4 s-! . At 523 K a major portion of the load-elongation curve was smooth, instability setting in only towards the end. At higher temperatures the serrations developed into well defined patterns classified as type A, B or C [lO,ll]. Type A serrations are characterised by an abrupt rise followed by a drop to below the general level of the stress-strain curve. Type B serrat~ons are oscillations that occur in quick succession about the general level of the stress-strain curve while Type C serrations are stress drops that occur below the general level of the flow curve. At 623 K and 723 K predominantly type A serrations were observed. The magnitude of yield drop was found to be higher at higher temperatures. At 823 K serrated flow began as type A and later developed into A+B type. The interval between type A serration at this temperature was less than that at 623 K or 723 K. Type C serrations were observed at 923 K and serrations abruptly ended after about 15-18% plastic strain. At 1023 K and above the entire load-elongation curve was smooth. The results relating to the serrated flow behaviour of this material have been detailed elsewhere [12J. Figure i shows the typical plot of load elongation curves at 923 K of as received material during the strain rate change tests. The strain rate sensitivity of the material was examined in the re$ion which showed pronounced serrations (strain upto ~--15-18%) and as well as in the region which showed smooth behaviour at the base strain r a t e . A downward change in the strain rate (strain r a t e change between ~5 and ~& , curve (a)) showed a near zero strain rate sensitivity (as evidenced by the same flow stress at lower strain rate) in the region which showed pronounced serrations at the base strain rate. The flow curve at the lower strain r a t e was free from serrations. In the high strain region (i.e. after the abrupt end of serrations at the base strain rate) a downward change in the strain rate showed clearly positive strain rate sensitivity. An upward change in the strain rate (strain rate change between ~b and ~k , curve (b)) at 923 K showed negative strain rate sensitivity and serrated flow at both low and high strain regions. J

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,

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EXTENSION,ram

FIG.I.

Typical plot of load elongation curves at 923 K during strain rate change tests.

The ductility of the material decreased with decreasing strain rate at 923 K as shown in Fig. 2. This is in accordance with our earlier results in a type 316 austenitic stainless steel at 873 K [13]. Since the material exhibited low ductility at low strain rates, the number of strain rate jumps that could be carried out was limited. Typical plots of load elongation curves during strain rate change tests at 923 K on specimens aged at 923 K for 5000 h are shown in fig.3. The material was free from serrations at the base strain rate. On a downward change in strain rate (curve (a)), the material showed a positive strain rate sensitivity and no

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sign of serrations at the lower strain rate whereas an upward strain rate (Curve (b)) resulted in a negative strain rate serrations at the higher strain rate.

687

change in sensitivity

the and

6O AISI 316, 923 K

Totol~ D

5 0 -QEK]E20Uniform e~,

nnnn~

[]

~40

FIG.2. Strain rate dependence of ductility 316 stainless steel at 923 K. (As received)

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,

-'

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I I IIIIII

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FIG.3. Typical plots of load elongation curves at 923 K during strain rate change test (Aged at 923 K/5000 h)

... '

EXTENSION,ram

Serrated flow is one of the manifestations of dynamic strain ageing, the other manifestations being a peak in the work hardening rate, peak or plateau in the flow stress or a ductility minimum in their variation with temperature [4]. There have been other observations which suggest that the operation of a dynamic strain ageing mechanism does not necessarily lead to serrated flow [14,15]. Though Type C serrations prevailed at the base strain rate in the as received material, serrations of a different type are observed at the higher strain rate in both as received and aged conditions. At higher strain rate mostly type B serrations were observed during the initial stages of deformation and then changing over to a new pattern of type A + type C with the load level decreasing during the C part of serration following a stair case pattern (Fig ib and Fig 3b). In the present investigation it is observed that in the as received material a serration free stress strain curve develops after a downward change in strain rate at 923 K, while an upward change in strain rate, even after the

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disappearance of serrations off the end of flow curve, leads to the development of serrations. This could be explained based on the strain rate-temperature map for serrated yielding developed for the as received material as shown in F~g.4. At 923 K during a downward change in strain rate the material enters In a serration free region. During the upward change it enters in the serrated flow region. The behaviour of the aged material with respect to strain rate change can also be understood in terms of a strain rate - temperature map for serrated yielding. The boundary between serrated and smooth flow in the strain rate temperature plot has been shown to shift to a lower temperature region with the depletion of elements which cause dynamic strain ageing [16,1?].

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00

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FIG.4.

Strain rate temperature serrated flow.

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TIWPERATURE

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The absence of serrations at 923 K on the aged specimen clearly implies that the process of precipitation influences the serrated flow. Hayes and Hayes [18] observed complete disappearance of serrated flow in 2 i/4Cr-iMo steel quenched and tempered at 866 K. They have proposed that this is due to the formation of complex carbides. The effect of ageing on the serrated flow behaviour was studied in other alloy systems [6-8 ] and attributed to decrease in solute content due to ageing. In austenitic stainless steels the grain boundaries are preferred sites for carbide precipitation. It has been proposed by Thorvaldsson and Dunlop [19] that the grain boundary acts as a collection plate, and large areas around the grain boundary become depleted in chromium through volume and grain boundary diffusion to the growing M ~ c a r b i d e s . Measurement with the STEM-EDS technique have confirmed the exlstence of narrow chromium depleted zones around the grain boundaries in austenitic stainless steel [20,21]. The presence of chromium or carbon has been found to be necessary in austenitic alloys for serrated flow to occur [16]. Hayes and Hayes [18] have proposed that disappearance of serrated flow could occur either by a progressively longer strain to the onset of serrations (a strain delay mechanism) or by a progressively smaller strain to the disappearance of serration (disappearance off the end of the flow curve). In either case disappearance was proposed to be the result of a precipitation reaction during the course of the test. In order that serrations persist at any strain, a minimum critical carbon concentration at the arrested dislocation has to be achieved through the combination of (i) carbon atmosphere build up on the dislocation line, (ii) carbon depletion to precipitate sinks and (iii) carbon depletion from dislocation by precipitate reaction on the dislocation line. The rate at which carbon atmosphere builds up on arrested dislocations during straining may be lower in a prior aged material compared to the as received material since the available carbon in the matrix is lower due to prior precipitation. With prior precipitation there will be more sinks in the matrix available for carbon depletion off the dislocation line. As a result carbon will be depleted more rapidly. Carbon depletion from dislocations by reaction on the

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dislocation line will be slower in the aged material because of the lower rate at which carbon builds on the dislocation line. Due to the interplay of the above, the critical carbon concentration on the arrested dislocation may not reach if the material is aged for a sufficient time. This could explain the absence of serrations in the aged material and the abrupt disappearance or apppearance at a certain time temperature combination of ageing. At high rates, dislocations moving at higher speeds through the matrix would collect higher concentrations of carbon and could acquire critical concentration for serrated flow to develop. Conclusions The phenomenon of serrated yielding at 923 K has been studied in a type 316 stainless steel in the as received condition and after ageing at 923 K for 5000 h. Ageing at 923 K for 5000 h is found to eliminate serrated flow which otherwise would have been present at 923 K. From the strain rate jump tests, negative strain rate sensitivity is always found to be associated with serrated flow. The observation of serrated flow at a particular strain rate could be correlated with the strain rate - temperature plots for occurrence/disappearance of serrated flow. The absence of serrated flow in the aged material at 923 K is explained as due to the difference in (i) carbon build up on the dislocation line, (ii) the presence of the number of precipitate sinks and (iii) precipitate reaction on the dislocation line between as received and aged materials. References

i. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

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