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Tempering of AISI 403 stainless steel
Materials Science attd Engineering, A 171 ( 1993 ) 13-19
13
Tempering of AISI 403 stainless steel L. C. Lim*, M. O. Lai and J. Ma l)epartment o]"~lechanical and Production Engineering, National University of Singapore, Kent Ridge, Singapore 0511 ¢Singapore)
D. O. Northwood and Baihe Miao l)epartment (ifMechanical Engineering, Univer~'it3' of Windsor, Windsor, Ont. NgB 3P4 (Canada) Received l)ccembcr 7. 1~-~t.~2:m revised torm May 18. 19931
Abstract The tempering behaviour of AISI 403 martensitic stainless steel over the temperature range from 480 to 720 °C has been investigated, with particular anention being paid to the secondary hardening and sensitization effects produced by the tormation of alloy carbides during tempering. The secondary hardening effect was determined by means of a standard hardness test. The extent of sensitization and its effect on the mechanical properties of the tempered steel were assessed by means of a modified Strauss test and a low speed tensile test in brine respectively. The results show that the following four distinctive processes occurred during tempering of AISI 403 martensitic stainless steel: annealing, secondary hardening, sensitization and healing. The tempering conditions over which each of these fimr processes dominates are presented in the form of a tempering map for easy reference.
I. Introduction Martensitic stainless steels are extensively used in the chemical and power industries and as compressor blades in modern aircraft engines I l, 2]. T h e y are generally heat treated to provide moderate corrosion resistance and a good combination of mechanical properties. This often involves an austenitizing heat treatment followed by air or fan quenching. It is well known that tempering of the as-quenched martensitic steel can bring about secondary hardening when the softening effect due to annealing is offset by the precipitation of alloy carbides in the material [3, 4]. However, for most stainless steels, tempering in the range from 450 to 540 °C also leads to poor impact properties and poor corrosion resistance [3, 5, 6]. T h e loss in corrosion resistance in tempered stainless steels has been explained by the widely accepted "chromium depletion theory", which has been described in some detail in a number of monographs [7-10]. For austenitic stainless steels, chromium carbides precipitate along the grain boundaries during tempering. As the carbides grow, carbon diffuses faster *Author to whom correspondence should be addrcssed. Present address: GINTIC Institute of Manufacturing Technology, Nanyang Technological University, Nanyang Avenue, Singaporc 2263. Singapore. 11921-5093/93,/$6.0~1
than chromium from the matrix to the grain boundaries. In so doing, chromium in the vicinity of the grain boundaries is drawn to form the carbides, resulting in the formation of chromium depletion zones. T h e above p h e n o m e n o n is more commonly referred to as "sensitization of stainless steel". Since chromium is one of the major elements providing resistance to corrosion, the depletion regions will be susceptible to corrosion attack. However, if the heating time is prolonged, the sensitization effect will eventually disappear as a result of chromium diffusion from the matrix to the depletion zone, a p h e n o m e n o n known as healing. In the case of martensitic stainless steels, carbides precipitate at as low as 300 °C during tempering. T h e speed of sensitization in stainless steels containing martensite is generally much more rapid, because the carbides form rapidly within the martensitic laths and along the lath boundaries, and the resulting corrosion can be transgranular, intergranular or mixed [11-13]. Healing of the chromium depletion zone in these steels also occurs more rapidly and is attributed to the faster chromium diffusion in b.c.c, martensite than in f.c.c. austenite [ 13]. Since secondary hardening and sensitization of stainless steels both involve the formation of alloy carbides, which for AISI 403 and 410 martensitic stainless steels are largely the chromium carbides, it is of interest to know whether these two processes occur © 1993 ElsevierSequoia. All rights reserved
14
L. C. Lirn et al.
/
Tempering of AlS1403 stainless steel
TABLE 1. Chemicalcompositionof AIS1403 steel (wt.%) C
Cr
Si
Mn
Ni
Mo
P
S
N
AI
Cu
Fe
0.099
11.68
0.37
0.40
0.40
<0.01
0.01
<0.005
0.0095
<0.01
0.03
Bal.
simultaneously or in sequence during tempering of the steels. In the present study the various phenomena occurring during tempering of AISI 403 martensitic stainless steel will be investigated. As will be shown later, these two phenomena appear in sequence and each dominates over a certain range of tempering conditions. The results, including other related phenomena, are represented in the form of a tempering map for easy reference.
2. Experimental details 27
2.1. Material a n d heat treatments
The starting material was AISI 403 martensitic stainless steel (AISI 403 MSS) in the form of forged blades. Table 1 gives the composition analysis of the steel used. Two types of specimens were prepared: metallographic and tensile test specimens. The tensile specimens, the dimensions of which are shown in Fig. 1, were machined from the "fir tree" region of the blades by means of electrodischarge machining (EDM). The remaining portions were sectioned into smaller sizes for hardness and metallographic studies. All specimens were austenitized at 925 °C for 40 min followed by oil quenching. They were wrapped in steel foils during austenitizing to prevent oxidation at the surfaces. The as-quenched specimens, which registered a Rockwell hardness value of 38 HRC, were then subjected to various tempering conditions. Five tempering temperatures were used: 480, 540, 600, 660 and 720 °C. The tempering time was varied from 1 to 3h. 2.2. Hardness test Hardness tests were conducted on the specimens by using a Rockwell hardness tester. This was done to determine the optimum time for peak hardness at a given tempering temperature. Before the test the top and bottom faces of the specimens were ground to remove the scales formed during the heat treatments and also to ensure fiat surfaces for measurement. Three data points were taken per specimen and the average value was recorded. 2.3. Sensitization test
A modified Strauss test procedure [14] was used in the present work for assessing the extent of sensitiza-
THICKNESS
- 0.Smm
Fig. 1. Dimensions of tensile specimens and locations on blade from which they were cut. tion in the steel after tempering. The optimum etching time was determined through a series of trials which showed that 5 s of etching in boiling concentrated sulphuric acid was effective in revealing the chromium depletion zone adjacent to grain boundaries without severe bulk corrosion. An etching time of 5 s was thus chosen for all subsequent tests. 2.4. Tensile test in brine
The brine used was prepared by adding 30 ml of saturated salt solution to 970 ml of distilled water. Before the tensile tests the specimens were ground on both sides to 0.3 mm from the original thickness of 0.5 mm to remove the re-cast layer produced by EDM and the scale formed during the earlier heat treatments. Then the specimen was mounted on a screw-driven universal testing machine. Plasticine wsa attached to the lower grip section of the specimen and shaped into the form of a cup. The cup was the filled with brine until the entire specimen gauge was immersed in it. The top of the cup was left opened to ensure that the brine was always aerated.
L. ('. Lira et el.
/
Tempering of A IS144)3 stainless steel
All specimens were pulled to fracture at a nominal strain rate of 2.4 x 10 - 5 s- 1 under ambient conditions. T h e various tensile properties, namely 0.2% yield strength (YS), ultimate tensile strength (UTS) and reduction in area (R.A), were measured from the load-displacement curves recorded and the fractured specimens. T h e fracture surfaces were examined by means of scanning electron microscopy.
15
for 4 h and 540 °C for 2 h respectively. It can be seen that the material is relatively unattacked (Fig. 3a) or only slightly attached (Fig. 3b) by the reagent. On the other hand, for the specimen tempered at 660 °C for 3 h (Fig. 3c), preferential attack along the prior austenite boundaries by the reagent becomes evident. For the steel tempered at 660 °C fo 2 h (Fig. 3d), the attack is most evident in that both the grain matrix and the prior austenite boundaries are equally severely affected.
3. Results 3. l. Hardness m e a s u r e m e n t s
T h e hardness values of the steel after various tempers are given in column 2 of Table 2. Each value is the average of three measurements made on a single sample. For easy reference the hardness values are plotted in Fig. 2 against the tempering temperature and time. Also highlighted in the figure is a band of tempering conditions corresponding to maximum secondary hardening. Note that the maximum hardness of 41 H R C was attained after tempering the as-quenched steel at 480 °C for 3 h. 3.2. Modified Strauss tests
Examination of the specimens after the modified Strauss test showed that the extent of sensitization varied considerably depending on the tempering conditions used, as can be seen from Fig. 3. Figures 3a and 3b are taken from the specimens tempered at 720 °C
~ U °v ELi cr D b< rr w n Ud
720
20
~8
17
12
660
25
22
20
t5
600
30
26
23
20
I-
480
I
2
3
TIME,
(hr).
5~o
Fig. 2. Hardness after various tempering conditions.
TABLE 2. Tensile properties in brine Tempering time (h) and temperature (c(.
Hardness :' (HRC)
UTS (MPa)
0.2% (MPa)
Elongation (%)
(%)
As-quenched !air) (brine)
38 38
1427 1250
1
480 540 61)0 660 7211
28 37 30 25 20
2
4811 540 600 660 720
3
480 540 600 660 720
RAb
691) 703
10.0 8. I
35.3 34.1
1350 1290 859 838 645
1028 1127 774 701 586
7.9 8.0 7.0 9.1 12.7
45.8 45.4 50.2 39.6 (l) 60.0
27 33 26 22 18
1300 1241 714 620 550
600 790 601 550 351
18.0 9.5 9.4 7.5 10.9
49.2 53.8 39.5 (I) 36.6 (|) 72.3
41 28 23 20 17
1064 1000 690 686 380
743 639 542 438 303
9.2 10.9 8.8 11.1 11.0
55.8 41.0 (I) 45.0 (I) 49.4 75.2
~'_+2 HRC. ~The letter I within parentheses indicates that the fracture mode under the stipulated tempering condition is largely intergranular, as opposed to the generally dimpled fracture surface found under other tempering conditions.
16
L. C. Lira et al.
/
Tempering of AlS1403 stainless steel
0
Fig. 3. Micrographs of specimens after modified Strauss tcst: a, unsensitizcd (degree 1), tempered at 720°C for 4 h; b. slightly sensitized (degree 2), tempered at 540 °C for 2 h; c, severely sensitized (degree 3), tcmpcrcd at 660 °C for 3 h; d, severely sensitized (degree 3'), tempered at 660 °C for 2 h.
TABLE 3. Classification of degree of sensitization Degree of sensitization
3'
Description Sensitization not detectable (Fig. 3a) Slight sensitized (Fig. 3b) Severely sensitized but largely limited to prior austenite boundaries (Fig. 3c) Severely sensitized both trans- and intergranularly (Fig. 3d)
On the basis of the results of the modified Strauss tests, it is possible to classify the degree of sensitization as in Table 3 (see Fig. 3 for corresponding micrographs). Figure 4 summarizes the results of the modified Strauss tests for specimens tempered under various
conditions. The region where severe sensitization had occurred (i.e. degrees 3 and 3') is highlighted, which shows that sensitization of AISI 403 MSS is most severe when tempered between 540 and 660 °C. 3.3. Tensile tests in brine
Table 2 gives the tensile propertes of the steel tested in brine. Only one specimen was tested for each tempering condition. For comparison with the results described in Sections 3.1 and 3.2, the tensile ductilities (expressed in terms of reduction in area) of specimens tempered under various conditions are plotted in Fig. 5. The region where specimens suffered a considerable loss in tensile ductility is highlighted. Also indicated below each data point in Fig. 5 is the mode of fracture of the specimen concerned determined by means of scanning electron microscopy. In general,
L. ('. Lirn et al.
720
2
2
2
1
t
1
1
1
2
3
4
/
Tempering of AIS140,? stainless steel
17
L.J
uJ 660 frD
~< FtI,i
60o
13._ b40 5ILl I-- 480
TIME, (hr).
Fig. 4. Degree of sensitization after various tempering conditions.
60
t
7%
72
k...J
o v
660
6OO
~
NmY/'///4:D//,
Fig. 6. Scanning electron fractographs showing typical (a) intergranular and (b) ductile fracture appearances.
340 I--
480
M3 ~L]:
I
.~9 !7;)
b6 O)
?
3
TIME, (hr).
Fig. 5. Tensile ductility in brine after various tempering conditions. The fracture mode is indicated in parentheses below the data point: I, intergranular; D, ductile. the appearance of the fracture surfaces of the specimens can broadly be classified as either largely intergranular (I) or ductile (D). Figure 6 shows typical examples of the fracture appearances. In the case when the specimens failed in a largely intergranular manner, an annular ring of intergranular fracture was observed around the outside of the fracture surface. Under the test conditions used, the intergranular fracture region was often conspicuous and occupied up to one-half to two-thirds of the fracture area across the specimen thickness. It should be mentioned, however, that despite the above, the load-displacement curves did not seem to correlate with the degree of sensitization of the specimen but were to a greater extent controlled by the plasticity of the matrix and hence by the hardness of the specimen after tempering.
It is evident from Fig. 5 that specimens exhibiting a low tensile ductility always failed in an intergranular manner, while those exhibiting a reasonable tensile ductility produced a ductile fracture appearance. Interestingly, tempering conditions which gave rise to low tensile ductility and an intergranular fracture mode corresponded fairly well with those when severe sensitization of the steel was detected.
4. Discussion 4.1. Secondary hardening during tempering The hardness measurement results compiled in Fig. 2 show that for a given tempering time the hardness of AISI 403 MSS attains a peak value when tempered in the range from 480 to 540 °C before it drops significantly at higher tempering temperatures. This finding is consistent with the observations of Barker [3] on AISI 420 (0.22% C) MSS. The highlighted band in Fig. 2 indicates the tempering conditions for maximum hardness attainable by secondary hardening. Over the range of tempering conditions investigated, peak hardness due to secondary
18
L. C Lira et al.
/
Temperingof AIS1403 stainless steel
hardening occurs in a shorter time for tempering at a higher temperature. It is interesting to note that the values of peak hardness registered are generally comparable with, if not higher than, the as-quenched value of 38 HRC. However, on both sides of the band of peak hardness the hardness values drop fairly abruptly, The present observations illustrate that when properly engineered, the secondary hardening effect can be used as a means for further strength improvement for martensitic stainless steel. This improvement in strength is brought about by the precipitation of fine Cr23C 6 heterogeneously in the martensitic matrix. Evidence for the above has been obtained by transmission electron microscopy, the results of which are reported in a companion paper [15].
4.2. Effect of tempering on sensitization The results of the modified Strauss test given in Fig. 4 show that when AISI 403 MSS is tempered in the temperature range from 500 to 680°C, sensitization occurs. However, with increasing holding time the specimens recover from sensitization, i.e. healing occurs. These observations agree with the chromium depletion theory described in Section 1. It is interesting to note that in this temperature range, sensitization generally takes a longer time to occur when tempering is carried out at lower temperatures, e.g. 540 °C. In the case of tempering at 720 ° the sensitization effect is hardly detected, mainly because it occurs and heals in a relatively short time (less than 1 h). On the other hand, at 480 °C the sensitization effect remains insignificant even after 4 h. Figure 4 further suggests that AISI 403 MSS is relatively immune to sensitization at 480 °C or below. 4.3. Fracture characteristics in brine
If a tempered steel were severely sensitized, it would be intergranularly attacked by brine during the tensile test and would fracture intergranularly and possess a low ductility. Comparison between Figs. 3 and 5 shows that this is indeed the case: specimens exhibiting a low tensile ductility and an intergranular fracture mode are also those which have suffered from severe sensitization, while those exhibiting a reasonable tensile ductility and a ductile fracture appearance show either no or insignificant sensitization. Therefore the fracture appearance exhibited by the specimen in low rate tensile tests in brine can be a useful means to gauge whether AISI 403 MSS is sensitized following a given heat treatment. This is true despite the fact that to a greater extent the load-displacement curves obtained from low speed tensile tests in brine are controlled by the plasticity of the matrix and hence by the hardness of the specimen after a given tempering treatment.
HEALING
~ U -._. u_i cr D t< tY m a_ ym ~-
720 660
~/~ SEVERE ~
SENSITIZATION
600
54o • 74 ,90,~. "
480
ANNEALINe~/J~ 1
2
3
TIME, (hr).
Fig. 7. Tempering map for AISI 403 MSS. The regions over which each of the phenomena of annealing, secondary hardening, sensitization and healing dominates are demarcated. The cross-hatched regions represent the transitional regions. 4.4. Overall behaviour
It is evident from Figs. 2, 4 and 5 that the following four phenomena occur during tempering of AISI 403 MSS: annealing, secondary hardening, sensitization and healing. These results are compiled in Fig. 7 in the form of a tempering map showing the range of tempering conditions over which each of these phenomena dominates. Steels tempered in the annealing zone will suffer a decrease in hardness but a gain in ductility. When tempered in the secondary hardening zone, the strength of the steel will be maintained or even further impoved owing to the carbide precipitation effect. However, if the steel is tempered in the sensitization zone, its resistance to intergranular corrosion will be impaired, rendering it susceptible to intergranular attack when used in a corrosive environment. In the healing zone the steel is expected to have recovered from sensitization but to suffer a considerable loss in strength owing to the high temperature tempering effects. It can also be seen from Fig. 7 that annealing predominates at low tempering temperatures. With increasing tempering temperature, secondary hardening then becomes dominant, followed by sensitization and finally by healing. More importantly, Fig. 7 shows that the various zones do not overlap. It therefore allows one to make use of the information contained to advantage in selecting proper tempering conditions to suit a particular application. 5. Conclusions The tempering conditions for secondary hardening and for sensitization of AISI 403 martensitic stainless
L. ('. l, irn et al.
/
Tempering of AIS1403 stainless steel
steel have been investigated. The results show that the following phenomena occur during tempering of AISI 403 MSS: annealing, secondary hardening, sensitization and healing. The tempering conditions over which each of these four processes dominates are presented in the form of a tempering map for easy reference.
Acknowledgments The authors gratefully acknowledge the technical help received from T. Tan and S. K. Tung. The material used in the present work was kindly supplied by Singapore Aerospace Manufacturing Pte. Ltd.
References 1 Metals Handbook, Vol. 3, Iron-base Heat-resistant Alloys, ASM, Metals Park. OH, 9th edn., 1980, p. 196.
19
2 Metals Handbook, Vol. 9, Wrought Stainles Steels, ASM, Metals Park, OH, 9th edn., 1980, p. 285. 3 R. Barker, Metallurgia (1967) 49. 4 K. J. Irvine, D. J. Crowe and E B. Pickering, J. Iron Steel Inst., 187(1960) 386. 5 J. Gordon Parr and A. Hanson, Art Introduction to Stainless Steel, ASM, Metals Park, OH, 1965, p. 32. 6 R. A. Luda, Stainless Steel, ASM, Metals Park, OH, 1986, p. 33. 7 V. Cihal, lntergranular Corrosion of Steels and Alloys, Elsevier, Amsterdam, 1984, p. 79. 8 L. Colombier and J. Hoehmann, Stainless and Heat Resisting Steels, Edward Arnold, Norwich, 1967, p. 145. 9 A.J. Sedriks, Corrosion of Stainless Steels. Wiley, New York, 1979, p. 110. 10 C. I,. Briant, Metallurgical Aspects of Environmental t"aih~res, Elsevier, Amsterdam, 1985, pp. 113, 127. 11 J.E. Truman, Br. Corros. J., 11 (2)( 1976192. 12 C. I,. Briant and A. M. Ritter, Scr. Metall.. 13 (1979) 177. 13 C. L. Briant and A. M. Ritter. Metall. lrans. A, II (1980) 2009. 14 J. B. Lee, Corrosion, 37 ( 1981 ) 437. 15 B. Miao, D. O. Northwood, L. C. Lira and M. O. Lai, Mater. Sci. Eng., AI71 (1993)21.
13
Tempering of AISI 403 stainless steel L. C. Lim*, M. O. Lai and J. Ma l)epartment o]"~lechanical and Production Engineering, National University of Singapore, Kent Ridge, Singapore 0511 ¢Singapore)
D. O. Northwood and Baihe Miao l)epartment (ifMechanical Engineering, Univer~'it3' of Windsor, Windsor, Ont. NgB 3P4 (Canada) Received l)ccembcr 7. 1~-~t.~2:m revised torm May 18. 19931
Abstract The tempering behaviour of AISI 403 martensitic stainless steel over the temperature range from 480 to 720 °C has been investigated, with particular anention being paid to the secondary hardening and sensitization effects produced by the tormation of alloy carbides during tempering. The secondary hardening effect was determined by means of a standard hardness test. The extent of sensitization and its effect on the mechanical properties of the tempered steel were assessed by means of a modified Strauss test and a low speed tensile test in brine respectively. The results show that the following four distinctive processes occurred during tempering of AISI 403 martensitic stainless steel: annealing, secondary hardening, sensitization and healing. The tempering conditions over which each of these fimr processes dominates are presented in the form of a tempering map for easy reference.
I. Introduction Martensitic stainless steels are extensively used in the chemical and power industries and as compressor blades in modern aircraft engines I l, 2]. T h e y are generally heat treated to provide moderate corrosion resistance and a good combination of mechanical properties. This often involves an austenitizing heat treatment followed by air or fan quenching. It is well known that tempering of the as-quenched martensitic steel can bring about secondary hardening when the softening effect due to annealing is offset by the precipitation of alloy carbides in the material [3, 4]. However, for most stainless steels, tempering in the range from 450 to 540 °C also leads to poor impact properties and poor corrosion resistance [3, 5, 6]. T h e loss in corrosion resistance in tempered stainless steels has been explained by the widely accepted "chromium depletion theory", which has been described in some detail in a number of monographs [7-10]. For austenitic stainless steels, chromium carbides precipitate along the grain boundaries during tempering. As the carbides grow, carbon diffuses faster *Author to whom correspondence should be addrcssed. Present address: GINTIC Institute of Manufacturing Technology, Nanyang Technological University, Nanyang Avenue, Singaporc 2263. Singapore. 11921-5093/93,/$6.0~1
than chromium from the matrix to the grain boundaries. In so doing, chromium in the vicinity of the grain boundaries is drawn to form the carbides, resulting in the formation of chromium depletion zones. T h e above p h e n o m e n o n is more commonly referred to as "sensitization of stainless steel". Since chromium is one of the major elements providing resistance to corrosion, the depletion regions will be susceptible to corrosion attack. However, if the heating time is prolonged, the sensitization effect will eventually disappear as a result of chromium diffusion from the matrix to the depletion zone, a p h e n o m e n o n known as healing. In the case of martensitic stainless steels, carbides precipitate at as low as 300 °C during tempering. T h e speed of sensitization in stainless steels containing martensite is generally much more rapid, because the carbides form rapidly within the martensitic laths and along the lath boundaries, and the resulting corrosion can be transgranular, intergranular or mixed [11-13]. Healing of the chromium depletion zone in these steels also occurs more rapidly and is attributed to the faster chromium diffusion in b.c.c, martensite than in f.c.c. austenite [ 13]. Since secondary hardening and sensitization of stainless steels both involve the formation of alloy carbides, which for AISI 403 and 410 martensitic stainless steels are largely the chromium carbides, it is of interest to know whether these two processes occur © 1993 ElsevierSequoia. All rights reserved
14
L. C. Lirn et al.
/
Tempering of AlS1403 stainless steel
TABLE 1. Chemicalcompositionof AIS1403 steel (wt.%) C
Cr
Si
Mn
Ni
Mo
P
S
N
AI
Cu
Fe
0.099
11.68
0.37
0.40
0.40
<0.01
0.01
<0.005
0.0095
<0.01
0.03
Bal.
simultaneously or in sequence during tempering of the steels. In the present study the various phenomena occurring during tempering of AISI 403 martensitic stainless steel will be investigated. As will be shown later, these two phenomena appear in sequence and each dominates over a certain range of tempering conditions. The results, including other related phenomena, are represented in the form of a tempering map for easy reference.
2. Experimental details 27
2.1. Material a n d heat treatments
The starting material was AISI 403 martensitic stainless steel (AISI 403 MSS) in the form of forged blades. Table 1 gives the composition analysis of the steel used. Two types of specimens were prepared: metallographic and tensile test specimens. The tensile specimens, the dimensions of which are shown in Fig. 1, were machined from the "fir tree" region of the blades by means of electrodischarge machining (EDM). The remaining portions were sectioned into smaller sizes for hardness and metallographic studies. All specimens were austenitized at 925 °C for 40 min followed by oil quenching. They were wrapped in steel foils during austenitizing to prevent oxidation at the surfaces. The as-quenched specimens, which registered a Rockwell hardness value of 38 HRC, were then subjected to various tempering conditions. Five tempering temperatures were used: 480, 540, 600, 660 and 720 °C. The tempering time was varied from 1 to 3h. 2.2. Hardness test Hardness tests were conducted on the specimens by using a Rockwell hardness tester. This was done to determine the optimum time for peak hardness at a given tempering temperature. Before the test the top and bottom faces of the specimens were ground to remove the scales formed during the heat treatments and also to ensure fiat surfaces for measurement. Three data points were taken per specimen and the average value was recorded. 2.3. Sensitization test
A modified Strauss test procedure [14] was used in the present work for assessing the extent of sensitiza-
THICKNESS
- 0.Smm
Fig. 1. Dimensions of tensile specimens and locations on blade from which they were cut. tion in the steel after tempering. The optimum etching time was determined through a series of trials which showed that 5 s of etching in boiling concentrated sulphuric acid was effective in revealing the chromium depletion zone adjacent to grain boundaries without severe bulk corrosion. An etching time of 5 s was thus chosen for all subsequent tests. 2.4. Tensile test in brine
The brine used was prepared by adding 30 ml of saturated salt solution to 970 ml of distilled water. Before the tensile tests the specimens were ground on both sides to 0.3 mm from the original thickness of 0.5 mm to remove the re-cast layer produced by EDM and the scale formed during the earlier heat treatments. Then the specimen was mounted on a screw-driven universal testing machine. Plasticine wsa attached to the lower grip section of the specimen and shaped into the form of a cup. The cup was the filled with brine until the entire specimen gauge was immersed in it. The top of the cup was left opened to ensure that the brine was always aerated.
L. ('. Lira et el.
/
Tempering of A IS144)3 stainless steel
All specimens were pulled to fracture at a nominal strain rate of 2.4 x 10 - 5 s- 1 under ambient conditions. T h e various tensile properties, namely 0.2% yield strength (YS), ultimate tensile strength (UTS) and reduction in area (R.A), were measured from the load-displacement curves recorded and the fractured specimens. T h e fracture surfaces were examined by means of scanning electron microscopy.
15
for 4 h and 540 °C for 2 h respectively. It can be seen that the material is relatively unattacked (Fig. 3a) or only slightly attached (Fig. 3b) by the reagent. On the other hand, for the specimen tempered at 660 °C for 3 h (Fig. 3c), preferential attack along the prior austenite boundaries by the reagent becomes evident. For the steel tempered at 660 °C fo 2 h (Fig. 3d), the attack is most evident in that both the grain matrix and the prior austenite boundaries are equally severely affected.
3. Results 3. l. Hardness m e a s u r e m e n t s
T h e hardness values of the steel after various tempers are given in column 2 of Table 2. Each value is the average of three measurements made on a single sample. For easy reference the hardness values are plotted in Fig. 2 against the tempering temperature and time. Also highlighted in the figure is a band of tempering conditions corresponding to maximum secondary hardening. Note that the maximum hardness of 41 H R C was attained after tempering the as-quenched steel at 480 °C for 3 h. 3.2. Modified Strauss tests
Examination of the specimens after the modified Strauss test showed that the extent of sensitization varied considerably depending on the tempering conditions used, as can be seen from Fig. 3. Figures 3a and 3b are taken from the specimens tempered at 720 °C
~ U °v ELi cr D b< rr w n Ud
720
20
~8
17
12
660
25
22
20
t5
600
30
26
23
20
I-
480
I
2
3
TIME,
(hr).
5~o
Fig. 2. Hardness after various tempering conditions.
TABLE 2. Tensile properties in brine Tempering time (h) and temperature (c(.
Hardness :' (HRC)
UTS (MPa)
0.2% (MPa)
Elongation (%)
(%)
As-quenched !air) (brine)
38 38
1427 1250
1
480 540 61)0 660 7211
28 37 30 25 20
2
4811 540 600 660 720
3
480 540 600 660 720
RAb
691) 703
10.0 8. I
35.3 34.1
1350 1290 859 838 645
1028 1127 774 701 586
7.9 8.0 7.0 9.1 12.7
45.8 45.4 50.2 39.6 (l) 60.0
27 33 26 22 18
1300 1241 714 620 550
600 790 601 550 351
18.0 9.5 9.4 7.5 10.9
49.2 53.8 39.5 (I) 36.6 (|) 72.3
41 28 23 20 17
1064 1000 690 686 380
743 639 542 438 303
9.2 10.9 8.8 11.1 11.0
55.8 41.0 (I) 45.0 (I) 49.4 75.2
~'_+2 HRC. ~The letter I within parentheses indicates that the fracture mode under the stipulated tempering condition is largely intergranular, as opposed to the generally dimpled fracture surface found under other tempering conditions.
16
L. C. Lira et al.
/
Tempering of AlS1403 stainless steel
0
Fig. 3. Micrographs of specimens after modified Strauss tcst: a, unsensitizcd (degree 1), tempered at 720°C for 4 h; b. slightly sensitized (degree 2), tempered at 540 °C for 2 h; c, severely sensitized (degree 3), tcmpcrcd at 660 °C for 3 h; d, severely sensitized (degree 3'), tempered at 660 °C for 2 h.
TABLE 3. Classification of degree of sensitization Degree of sensitization
3'
Description Sensitization not detectable (Fig. 3a) Slight sensitized (Fig. 3b) Severely sensitized but largely limited to prior austenite boundaries (Fig. 3c) Severely sensitized both trans- and intergranularly (Fig. 3d)
On the basis of the results of the modified Strauss tests, it is possible to classify the degree of sensitization as in Table 3 (see Fig. 3 for corresponding micrographs). Figure 4 summarizes the results of the modified Strauss tests for specimens tempered under various
conditions. The region where severe sensitization had occurred (i.e. degrees 3 and 3') is highlighted, which shows that sensitization of AISI 403 MSS is most severe when tempered between 540 and 660 °C. 3.3. Tensile tests in brine
Table 2 gives the tensile propertes of the steel tested in brine. Only one specimen was tested for each tempering condition. For comparison with the results described in Sections 3.1 and 3.2, the tensile ductilities (expressed in terms of reduction in area) of specimens tempered under various conditions are plotted in Fig. 5. The region where specimens suffered a considerable loss in tensile ductility is highlighted. Also indicated below each data point in Fig. 5 is the mode of fracture of the specimen concerned determined by means of scanning electron microscopy. In general,
L. ('. Lirn et al.
720
2
2
2
1
t
1
1
1
2
3
4
/
Tempering of AIS140,? stainless steel
17
L.J
uJ 660 frD
~< FtI,i
60o
13._ b40 5ILl I-- 480
TIME, (hr).
Fig. 4. Degree of sensitization after various tempering conditions.
60
t
7%
72
k...J
o v
660
6OO
~
NmY/'///4:D//,
Fig. 6. Scanning electron fractographs showing typical (a) intergranular and (b) ductile fracture appearances.
340 I--
480
M3 ~L]:
I
.~9 !7;)
b6 O)
?
3
TIME, (hr).
Fig. 5. Tensile ductility in brine after various tempering conditions. The fracture mode is indicated in parentheses below the data point: I, intergranular; D, ductile. the appearance of the fracture surfaces of the specimens can broadly be classified as either largely intergranular (I) or ductile (D). Figure 6 shows typical examples of the fracture appearances. In the case when the specimens failed in a largely intergranular manner, an annular ring of intergranular fracture was observed around the outside of the fracture surface. Under the test conditions used, the intergranular fracture region was often conspicuous and occupied up to one-half to two-thirds of the fracture area across the specimen thickness. It should be mentioned, however, that despite the above, the load-displacement curves did not seem to correlate with the degree of sensitization of the specimen but were to a greater extent controlled by the plasticity of the matrix and hence by the hardness of the specimen after tempering.
It is evident from Fig. 5 that specimens exhibiting a low tensile ductility always failed in an intergranular manner, while those exhibiting a reasonable tensile ductility produced a ductile fracture appearance. Interestingly, tempering conditions which gave rise to low tensile ductility and an intergranular fracture mode corresponded fairly well with those when severe sensitization of the steel was detected.
4. Discussion 4.1. Secondary hardening during tempering The hardness measurement results compiled in Fig. 2 show that for a given tempering time the hardness of AISI 403 MSS attains a peak value when tempered in the range from 480 to 540 °C before it drops significantly at higher tempering temperatures. This finding is consistent with the observations of Barker [3] on AISI 420 (0.22% C) MSS. The highlighted band in Fig. 2 indicates the tempering conditions for maximum hardness attainable by secondary hardening. Over the range of tempering conditions investigated, peak hardness due to secondary
18
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Temperingof AIS1403 stainless steel
hardening occurs in a shorter time for tempering at a higher temperature. It is interesting to note that the values of peak hardness registered are generally comparable with, if not higher than, the as-quenched value of 38 HRC. However, on both sides of the band of peak hardness the hardness values drop fairly abruptly, The present observations illustrate that when properly engineered, the secondary hardening effect can be used as a means for further strength improvement for martensitic stainless steel. This improvement in strength is brought about by the precipitation of fine Cr23C 6 heterogeneously in the martensitic matrix. Evidence for the above has been obtained by transmission electron microscopy, the results of which are reported in a companion paper [15].
4.2. Effect of tempering on sensitization The results of the modified Strauss test given in Fig. 4 show that when AISI 403 MSS is tempered in the temperature range from 500 to 680°C, sensitization occurs. However, with increasing holding time the specimens recover from sensitization, i.e. healing occurs. These observations agree with the chromium depletion theory described in Section 1. It is interesting to note that in this temperature range, sensitization generally takes a longer time to occur when tempering is carried out at lower temperatures, e.g. 540 °C. In the case of tempering at 720 ° the sensitization effect is hardly detected, mainly because it occurs and heals in a relatively short time (less than 1 h). On the other hand, at 480 °C the sensitization effect remains insignificant even after 4 h. Figure 4 further suggests that AISI 403 MSS is relatively immune to sensitization at 480 °C or below. 4.3. Fracture characteristics in brine
If a tempered steel were severely sensitized, it would be intergranularly attacked by brine during the tensile test and would fracture intergranularly and possess a low ductility. Comparison between Figs. 3 and 5 shows that this is indeed the case: specimens exhibiting a low tensile ductility and an intergranular fracture mode are also those which have suffered from severe sensitization, while those exhibiting a reasonable tensile ductility and a ductile fracture appearance show either no or insignificant sensitization. Therefore the fracture appearance exhibited by the specimen in low rate tensile tests in brine can be a useful means to gauge whether AISI 403 MSS is sensitized following a given heat treatment. This is true despite the fact that to a greater extent the load-displacement curves obtained from low speed tensile tests in brine are controlled by the plasticity of the matrix and hence by the hardness of the specimen after a given tempering treatment.
HEALING
~ U -._. u_i cr D t< tY m a_ ym ~-
720 660
~/~ SEVERE ~
SENSITIZATION
600
54o • 74 ,90,~. "
480
ANNEALINe~/J~ 1
2
3
TIME, (hr).
Fig. 7. Tempering map for AISI 403 MSS. The regions over which each of the phenomena of annealing, secondary hardening, sensitization and healing dominates are demarcated. The cross-hatched regions represent the transitional regions. 4.4. Overall behaviour
It is evident from Figs. 2, 4 and 5 that the following four phenomena occur during tempering of AISI 403 MSS: annealing, secondary hardening, sensitization and healing. These results are compiled in Fig. 7 in the form of a tempering map showing the range of tempering conditions over which each of these phenomena dominates. Steels tempered in the annealing zone will suffer a decrease in hardness but a gain in ductility. When tempered in the secondary hardening zone, the strength of the steel will be maintained or even further impoved owing to the carbide precipitation effect. However, if the steel is tempered in the sensitization zone, its resistance to intergranular corrosion will be impaired, rendering it susceptible to intergranular attack when used in a corrosive environment. In the healing zone the steel is expected to have recovered from sensitization but to suffer a considerable loss in strength owing to the high temperature tempering effects. It can also be seen from Fig. 7 that annealing predominates at low tempering temperatures. With increasing tempering temperature, secondary hardening then becomes dominant, followed by sensitization and finally by healing. More importantly, Fig. 7 shows that the various zones do not overlap. It therefore allows one to make use of the information contained to advantage in selecting proper tempering conditions to suit a particular application. 5. Conclusions The tempering conditions for secondary hardening and for sensitization of AISI 403 martensitic stainless
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Tempering of AIS1403 stainless steel
steel have been investigated. The results show that the following phenomena occur during tempering of AISI 403 MSS: annealing, secondary hardening, sensitization and healing. The tempering conditions over which each of these four processes dominates are presented in the form of a tempering map for easy reference.
Acknowledgments The authors gratefully acknowledge the technical help received from T. Tan and S. K. Tung. The material used in the present work was kindly supplied by Singapore Aerospace Manufacturing Pte. Ltd.
References 1 Metals Handbook, Vol. 3, Iron-base Heat-resistant Alloys, ASM, Metals Park. OH, 9th edn., 1980, p. 196.
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2 Metals Handbook, Vol. 9, Wrought Stainles Steels, ASM, Metals Park, OH, 9th edn., 1980, p. 285. 3 R. Barker, Metallurgia (1967) 49. 4 K. J. Irvine, D. J. Crowe and E B. Pickering, J. Iron Steel Inst., 187(1960) 386. 5 J. Gordon Parr and A. Hanson, Art Introduction to Stainless Steel, ASM, Metals Park, OH, 1965, p. 32. 6 R. A. Luda, Stainless Steel, ASM, Metals Park, OH, 1986, p. 33. 7 V. Cihal, lntergranular Corrosion of Steels and Alloys, Elsevier, Amsterdam, 1984, p. 79. 8 L. Colombier and J. Hoehmann, Stainless and Heat Resisting Steels, Edward Arnold, Norwich, 1967, p. 145. 9 A.J. Sedriks, Corrosion of Stainless Steels. Wiley, New York, 1979, p. 110. 10 C. I,. Briant, Metallurgical Aspects of Environmental t"aih~res, Elsevier, Amsterdam, 1985, pp. 113, 127. 11 J.E. Truman, Br. Corros. J., 11 (2)( 1976192. 12 C. I,. Briant and A. M. Ritter, Scr. Metall.. 13 (1979) 177. 13 C. L. Briant and A. M. Ritter. Metall. lrans. A, II (1980) 2009. 14 J. B. Lee, Corrosion, 37 ( 1981 ) 437. 15 B. Miao, D. O. Northwood, L. C. Lira and M. O. Lai, Mater. Sci. Eng., AI71 (1993)21.