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Microstructure—Strength relations in a duplex stainless steel
Materials Science and Engineering, 94 (1987) 175-181
175
Microstructure-Strength Relations in a Duplex Stainless Steel K. UNNIKRISHNAN* and A. K. MALLIKJ"
Department of Metallurgical Engineering, Indian Institute of Technology, Powai, Bombay 400076 (India) (Received June 11, 1986; in revised form January 28, 1987)
ABSTRACT
Metallographic studies were conducted on a duplex stainless steel to examine the phase relationships associated with cold rolling and annealing. The effect o f the metaUographic condition o f the alloy on the mechanical properties was investigated. Cold rolling and subsequent annealing in a temperature range o f 8 0 0 - 1 2 4 0 °C resulted in a sequence o f transformations. A t temperatures between 800 and 975 °C, the a phase is formed and at 1000 °C the alloy recrystallizes. A bo ve 1000 °C, the ~ and ~/ phases s h o w a banded appearance and the ferrite content increases with annealing temperature e x c e p t at 1240 °C, owing to the redistribution o f the alloying elements. The mechanical properties o f the material treated up to 1000 °C are controlled by the presence of the o phase and the lack o f recrystallization. A b o v e 1000 °C, the effect o f microstructure on the mechanical properties is dominated by the a m o u n t o f the ferrite phase, the associated microstructure and banding. A g o o d combination o f tensile properties is achieved by treatments which produce a ferrite c o n t e n t between 30% and 70%.
working in the two-phase region [3]. Cold working and annealing is a n o t h e r m e t h o d o f obtaining a duplex microstructure. The objective of this study was t o gain a bet t er understanding o f the d e v e l o p m e n t o f the microstructure of a commercially available duplex stainless steel as a result o f cold rolling and subsequent annealing, and to assess t he resultant mechanical properties.
2. EXPERIMENTAL DETAILS
2. I. Material and heat treatment The starting material was U50, one of t he commercially available duplex stainless steels f r o m Creusot Loire Steel C o m p a n y o f France, in the f o r m of a plate 3 m m thick. The chemical composition of t he alloy is shown in Table 1. The sheet material was cold rolled t o 0.65 m m thickness, and specimens for tensile and metallographic studies were prepared. Specimens were annealed in t he t e m p e r a t u r e range 8 0 0 - 1 2 4 0 °C for a fixed period of 1 h [1].
TABLE
1. INTRODUCTION Duplex stainless steels are becoming increasingly popular for their com bi na t i on of high strength and good corrosion resistance. T h e y represent an i m p o r t a n t expanding class o f stainless steels [1-6]. The pr i m a r y phases in duplex stainless steels, generally austenite and ferrite, are usually established by h o t *Present address: Radiometallurgy Division, Bhabha Atomic Research Centre, B o m b a y 400085, India. tPresent address: Director, Indian Instituteof Technology, Kanpur, India.
0025-5416/87/$3.50
1
Alloy composition
Element
Manufacturer's specification (wt.%)
Result of chemical analysis (wt.%)
Cr Ni Mo Cu Mn Si C S P Fe
20.97 6.77 2.5 1.55 1.66 0.50 0.028 0.008
21.16 6.96 2.41 1.54 1.65 0.50 0.028-0.029 --
0.024
Balance
--
Balance
© Elsevier Sequoia/Printed in The Netherlands
176 Purified argon gas was used as the furnace atmosphere.
2.2. Metallographic technique Specimens for metallographic examination were prepared by mechanical polishing and were etched electrolytically in an aqueous 25 wt.% solution of KOH. The microstructures were examined in an optical microscope and a Siemens Autoscan scanning electron microscope. Whereas austenite has bright contrast in both scanning electron microscopy (SEM) and optical microscopy, ferrite appears as a darker phase. The specimens were subjected to X-ray diffraction analysis with a diffractometer using Fe K s radiation. The elemental composition of the phases was determined by energy-dispersive X-ray microanalysis employing an EDAX 711 X-ray energy-dispersive spectrometer interfaced with the Siemens Autoscan scanning electron microscope. The areal fraction, equivalent diameter, mean chord and form factor of the ferrite phases were determined using a Leitz TAS plus automatic image analyser.
Fig. 1. Microstructure of the starting material.
2.3. Mechanical testing All treated specimens were tested for tensile properties and hardness at room temperature. Tensile tests were performed at a crosshead speed of 0.5 mm min -1, on duplicate sheet specimens, cut from the strip perpendicular to the rolling direction of the material with a gauge length of 12.5 mm a n d a width of 4 mm.
3. R E S U L T S
AND
DISCUSSION
3.1. Metallography and phase relationship The microstructure of the starting material (3 mm plate) is shown in Fig. 1, and is typified by elongated colonies of ferrite in the austenite matrix. The areal fraction of the ferrite phase in the material was 40%. X-ray diffraction analysis showed the presence of only the ~ and ~/phases in the starting material and in the cold-rolled material. The microstructures produced after cold rolling, annealing and water quenching are shown in Fig. 2. The ferrite phase which broke down because of cold rolling remained essentially unaffected by heat treatment at 800 °C. However, X-ray diffraction analysis
revealed the formation of the o phase in the material annealed from 800 to 950 °C, whereas annealing at 975 °C or higher temperatures does n o t give rise to the o phase. The appearance of o-phase peaks in X-ray diffraction was accompanied by a corresponding decrease in c~-phase peak intensity. The high content of chromium and m o l y b d e n u m in the ferrite phase presumably causes the formation of the o phase at the expense of ferrite [7]. Figure 2(a) shows the microstructure resulting from quenching the alloy from 1000 °C. It clearly reveals bands of newly formed ferrite grains in the rolling direction. The banding obviously arose because of heavy deformation by cold rolling before annealing. The microstructures produced from annealing at 1225 °C are shown in Figs. 2(b) and (c), and that from annealing at 1240 °C is shown in Fig. 2(d). The banding of the ferrite phase is seen in all specimens, and the ferrite content increases with increasing annealing temperature except after treatment at 1240 °C. The fairly straight, sharp and oriented ferrite-austenite boundaries in the microstructures indicate a crystallographic orientation relationship between the two phases.
177
i~ig. 2. Microstructures produced after annealing at (a) 1000 °C, (b) 1225 °C, (c) 1225 °C (SEM) and (d) 1240 °C. The ferrite c o n t e n t and its morphological parameters as related to annealing temperature are summarized in Table 2. The ferrite c o n t e n t in the material increases with increasing temperature, as indicated by the pseudobinary diagram (Fig. 3) of the Fe-Cr-Ni system [7]. In this diagram the composition of the afioy is displayed in terms of chromium and nickel equivalents, which relate to the relative power of the various alloying elements as a and 3' stabilizers. However, at 1240 °C, the ferrite content in the material was less than t h a t at 1225 °C. This could be
because of the redistribution of the alloying elements in the ferrite and austenite phases and the consequent influence on phase stability. Grobner and Biss [8] determined the ferrite phase boundaries in a stainless steel with 16 wt.% Cr, 1.5 wt.% Mo and 5 wt.% Ni at 1000 °C and above after treating the alloy for 2 h, and their results indicate that the phases in the present case may be in equilibrium at about 1200 °C. The ratios of the alloying elements in the ferrite and austenite phases were determined (Table 3) to improve the understanding of
178 TABLE 2 Relationship to the annealing temperature of the ferrite content and its morphological parameters
Annealing temperature (°C)
Areal fraction of ferrite (%)
Equivalent diameter
Mean chord
Form factor
Number of ferrite phase fields per 104pro 2
1000 1100 1175 1200 1225 1240
31.71 53.43 56.70 84.06 91.72 68.58
2.42 6.12 9.57 20.93 37.17 4.47
1.14 2.70 4.18 7.13 13.25 1,75
0.70 0.75 0.75 0.75 0.85 0.93
689 182 79 24 8 437
U50 1600 1400 -
1200
t "a+×
~
-
a /
- Y/
IOOO
2 1-
800 \
I
600 400
I
x.
I
~\
2OO \
0 %Ni 0 % Cr 55
5 50
I0 25
15 20
. . t _ _ ~ 20 25 50 15 I0 5
55 0
Fig. 3. Pseudobinary diagram for 65%Fe-Cr-Ni [7].
this phenomenon. Although it is believed that silicon, chromium and m o l y b d e n u m are ferrite stabilizers, and nickel, copper and manganese are austenite stabilizers, the results show that the phase-stabilizing capability of the elements depends on the temperature. Silicon, although considered a ferrite stabilizer, shows a tendency to stabilize austenite at 1240 °C by partitioning in favour of austenite, thereby raising serious d o u b t a b o u t the use of chromium and nickel equivalents for establishing the phase relationship in the alloy. The effect of annealing temperature on the equivalent diameter and mean chord (Table 2) shows an identical trend of variation. Since the ferrite bands were oriented perpendicular to the screen of the system during the image analysis, the ratio of the equivalent diameter to the mean chord indicates the extent of ferrite banding in the rolling direction. The coarsening effect of temperature on the ferrite phase is shown in Table 2 by the
TABLE 3 Ratio of alloying elements in the ferrite phase to those in the austenite phase after annealing at different temperatures
Annealing temperature
Si
Cr
Mn
Fe
Ni
Cu
Mo
2.92 3.97 4.17 1.60 1.39 0.88
0.78 1.04 1.11 1.18 1.18 1.08
0.51 0.87 0.71 0.74 0.90 0.71
1.45 0.94 0.94 0.96 0.99 0.96
1.17 1.08 0.99 0.79 0.74 1.05
0.89 1.02 0.79 0.84 0.84 0.99
1.00 1.07 2.37 1.41 1.61 2.02
(°c) 1000 1100 1175 1200 1225 1240
179
number of ferrite phase islands observed per 10 4 p m 2. With increasing temperature, the ferrite phase is coarsened n o t only by the transformation of austenite to ferrite, b u t also by the decrease in the number of ferrite phase fields. 3.2. Mechanical behaviour The tensile properties and hardness of the material in various annealed conditions are shown in Figs. 4 and 5 respectively. The data demonstrate the strong effect of heat treatment on mechanical properties. The tensile strength, yield strength and hardness decrease with increasing annealing temperature. The high levels of tensile strength, yield strength and hardness of the material and corresponding lower elongation o f the material treated below 1000 °C arise from the presence of the o phase [3, 7 , 9 ] and the lack of recrystallization. Above the recrystallization temperature (1000 °C), the microstructure dictates the properties of the material. The effect of microstructure on mechanical properties is dominated by the a m o u n t of ferrite present. The tensile strength of the material decreases with increasing ferrite con-
I100
tent (Fig. 6). Thus an increase in austenite content increases the tensile strength of the alloy, because of at least three factors [3, 10]. (i) The larger a m o u n t of austenite leads to the higher austenite work hardening which raises the tensile strength of the austenite-rich duplex steels relative to those of the alloys with less austenite. (ii) The extent of martensite formation, which greatly influences the amount of austenite work hardening, decreases as the proportion of austenite in the duplex steel is decreased. (iii) The higher ferrite content in the alloy reduces the ductility because of cleavage fractures. It is expected that the yield strength of the alloy will increase with increasing ferrite content [10]. However, in the present case, the values of the yield strength remained almost unchanged above the recrystallization temperature (Fig. 6). This is because of the banding of the ferrite and austenite phases. Austenite bands could yield, fairly independently of their content, and this results in the yielding of the material. The material exhibits maximum elongation when the ferrite c o n t e n t is 30% and it retains
40
400 I000
30
900
300
8OO
#-7oo ~E
20 %
I 0
600 200 I0
500
400 L~ -e Total elongation • -e* Uniform elongation 300
I
800
I
OlO
I
900 I 0 I100 Annealing temp. °C
I
1200
1300
Fig. 4. V a r i a t i o n in tensile properties after a n n e a l i n g at different t e m p e r a t u r e s .
I00
I
800
I
I
I
900 IO00 I100 Annealing temp. °C
I
1200
1300
Fig. 5. V a r i a t i o n in V i c k e r s ' hardness w i t h a n n e a l i n g temperature.
180 800
40 O-o-u
• -~2 700
0
0
30
600
g_ % 20 500
O-e
400 I0
300 0
I 20
I 40
I 60 % Ferrite
I 80
Total elongation
• - e * Uniform elongation
I I00
Fig. 6. E f f e c t o f ferrite c o n t e n t o n t h e tensile a n d yield s t r e n g t h s o f t h e material.
good ductility up to a ferrite content of 70% (Fig. 7). A sharp drop in elongation occurs at about 90% ferrite as the ferrite forms a nearcontinuous phase at this stage (Figs. 2(b) and 2(c)) which reduces the ductility. This is confirmed by the presence of cleavage fracture of ferrite and decohesions at the ferrite interface. Annealing at 1000,1100, 1175 and 1240 °C produced a ferrite content ranging from 30% to 70% which resulted in ductile failures exhibiting deep microvoids. The fracture surface formed from tensile testing of the material annealed at 1240 °C is shown in Fig. 8 and reveals ductile failure via void growth and coalescence at the austenite band. The hardness of the material decreases with increasing treatment temperature (Fig. 5). The abrupt fall in hardness resulting from treatment at 1000 °C is accompanied by recrystallization of the material, as revealed by the microstructure.
0 0
i 20
[ 40
~ 60 % Ferrite
t 80
t I00
Fig. 7. E f f e c t o f ferrite c o n t e n t o n t h e t o t a l a n d unif o r m e l o n g a t i o n s of t h e material.
4. C O N C L U S I O N S
(i) The recrystallization temperature for alloy U50 is about 1000 °C for a deformation of 78% and a heat treatment time of 1 h.
Fig. 8. S c a n n i n g e l e c t r o n f r a c t o g r a p h s h o w i n g m i c r o voids a t t h e a u s t e n i t e b a n d .
181 (ii) T h e ferrite c o n t e n t in t h e material increases with annealing t e m p e r a t u r e u p t o 1225 °C a n d decreases at 1 2 4 0 °C. This is caused b y t h e r e d i s t r i b u t i o n o f t h e alloying e l e m e n t s in the ferrite and a u s t e n i t e phases. (iii) T h e a phase is f o r m e d in the material because o f annealing f r o m 8 0 0 t o 9 5 0 °C, w h e r e a s annealing at o r above 975 °C does n o t give rise t o t h e a phase. (iv) T h e higher strength and l o w e r elongat i o n o f the material t r e a t e d u p t o 1 0 0 0 °C arise f r o m t h e p r e s e n c e o f the o phase and the lack o f recrystaUization. T h e variation in tensile p r o p e r t i e s o f t h e material a n n e a l e d above 1 0 0 0 °C is caused b y the change in t h e ferrite c o n t e n t , the associated m i c r o s t r u c t u r e a n d banding. A g o o d c o m b i n a t i o n o f properties is achieved b y t r e a t m e n t s w h i c h produce a ferrite c o n t e n t b e t w e e n 30% and 70%.
REFERENCES 1 K. Unnikrishnan, M.Tech. dissertation, Indian Institute of Technology, Bombay, 1985. 2 D.E. Nelson, W. A. Baslack III and J. C. Lippod, Metallography, 18 (1985) 215. 3 H. D. Solomon and T. M. Devine, Jr., Proc. Conf. on Duplex Stainless Steels, 1983, American Society for Metals, Metals Park, OH, 1983, p. 693. 4 N. Sridhar, J. Kolts and L. M. Flasche, J. Met., 31 (3) (1985) 31. 5 S. M. Wilhelm and R. D. Kane, Corrosion, 40 (1984) 431. 6 M. A. Streicher, Met. Prog., October (1985) 29. 7 H.D. Solomon, J. Heat Treating, 3 (1) (1983) 3. 8 P. J. Grobner and V. Biss, Metall. Trans. A, 15 (1985) 1379. 9 N. Sridhar, J. Kolts, S. K. Srivastava and A. I. Asphani, Proc. Conf. on Duplex Stainless Steels, 1983, American Society for Metals, Metals Park, OH, p. 481. 10 S. Floreen and H. W. Hayden, Trans. A m . Soc. Met., 61 (1968) 489.
175
Microstructure-Strength Relations in a Duplex Stainless Steel K. UNNIKRISHNAN* and A. K. MALLIKJ"
Department of Metallurgical Engineering, Indian Institute of Technology, Powai, Bombay 400076 (India) (Received June 11, 1986; in revised form January 28, 1987)
ABSTRACT
Metallographic studies were conducted on a duplex stainless steel to examine the phase relationships associated with cold rolling and annealing. The effect o f the metaUographic condition o f the alloy on the mechanical properties was investigated. Cold rolling and subsequent annealing in a temperature range o f 8 0 0 - 1 2 4 0 °C resulted in a sequence o f transformations. A t temperatures between 800 and 975 °C, the a phase is formed and at 1000 °C the alloy recrystallizes. A bo ve 1000 °C, the ~ and ~/ phases s h o w a banded appearance and the ferrite content increases with annealing temperature e x c e p t at 1240 °C, owing to the redistribution o f the alloying elements. The mechanical properties o f the material treated up to 1000 °C are controlled by the presence of the o phase and the lack o f recrystallization. A b o v e 1000 °C, the effect o f microstructure on the mechanical properties is dominated by the a m o u n t o f the ferrite phase, the associated microstructure and banding. A g o o d combination o f tensile properties is achieved by treatments which produce a ferrite c o n t e n t between 30% and 70%.
working in the two-phase region [3]. Cold working and annealing is a n o t h e r m e t h o d o f obtaining a duplex microstructure. The objective of this study was t o gain a bet t er understanding o f the d e v e l o p m e n t o f the microstructure of a commercially available duplex stainless steel as a result o f cold rolling and subsequent annealing, and to assess t he resultant mechanical properties.
2. EXPERIMENTAL DETAILS
2. I. Material and heat treatment The starting material was U50, one of t he commercially available duplex stainless steels f r o m Creusot Loire Steel C o m p a n y o f France, in the f o r m of a plate 3 m m thick. The chemical composition of t he alloy is shown in Table 1. The sheet material was cold rolled t o 0.65 m m thickness, and specimens for tensile and metallographic studies were prepared. Specimens were annealed in t he t e m p e r a t u r e range 8 0 0 - 1 2 4 0 °C for a fixed period of 1 h [1].
TABLE
1. INTRODUCTION Duplex stainless steels are becoming increasingly popular for their com bi na t i on of high strength and good corrosion resistance. T h e y represent an i m p o r t a n t expanding class o f stainless steels [1-6]. The pr i m a r y phases in duplex stainless steels, generally austenite and ferrite, are usually established by h o t *Present address: Radiometallurgy Division, Bhabha Atomic Research Centre, B o m b a y 400085, India. tPresent address: Director, Indian Instituteof Technology, Kanpur, India.
0025-5416/87/$3.50
1
Alloy composition
Element
Manufacturer's specification (wt.%)
Result of chemical analysis (wt.%)
Cr Ni Mo Cu Mn Si C S P Fe
20.97 6.77 2.5 1.55 1.66 0.50 0.028 0.008
21.16 6.96 2.41 1.54 1.65 0.50 0.028-0.029 --
0.024
Balance
--
Balance
© Elsevier Sequoia/Printed in The Netherlands
176 Purified argon gas was used as the furnace atmosphere.
2.2. Metallographic technique Specimens for metallographic examination were prepared by mechanical polishing and were etched electrolytically in an aqueous 25 wt.% solution of KOH. The microstructures were examined in an optical microscope and a Siemens Autoscan scanning electron microscope. Whereas austenite has bright contrast in both scanning electron microscopy (SEM) and optical microscopy, ferrite appears as a darker phase. The specimens were subjected to X-ray diffraction analysis with a diffractometer using Fe K s radiation. The elemental composition of the phases was determined by energy-dispersive X-ray microanalysis employing an EDAX 711 X-ray energy-dispersive spectrometer interfaced with the Siemens Autoscan scanning electron microscope. The areal fraction, equivalent diameter, mean chord and form factor of the ferrite phases were determined using a Leitz TAS plus automatic image analyser.
Fig. 1. Microstructure of the starting material.
2.3. Mechanical testing All treated specimens were tested for tensile properties and hardness at room temperature. Tensile tests were performed at a crosshead speed of 0.5 mm min -1, on duplicate sheet specimens, cut from the strip perpendicular to the rolling direction of the material with a gauge length of 12.5 mm a n d a width of 4 mm.
3. R E S U L T S
AND
DISCUSSION
3.1. Metallography and phase relationship The microstructure of the starting material (3 mm plate) is shown in Fig. 1, and is typified by elongated colonies of ferrite in the austenite matrix. The areal fraction of the ferrite phase in the material was 40%. X-ray diffraction analysis showed the presence of only the ~ and ~/phases in the starting material and in the cold-rolled material. The microstructures produced after cold rolling, annealing and water quenching are shown in Fig. 2. The ferrite phase which broke down because of cold rolling remained essentially unaffected by heat treatment at 800 °C. However, X-ray diffraction analysis
revealed the formation of the o phase in the material annealed from 800 to 950 °C, whereas annealing at 975 °C or higher temperatures does n o t give rise to the o phase. The appearance of o-phase peaks in X-ray diffraction was accompanied by a corresponding decrease in c~-phase peak intensity. The high content of chromium and m o l y b d e n u m in the ferrite phase presumably causes the formation of the o phase at the expense of ferrite [7]. Figure 2(a) shows the microstructure resulting from quenching the alloy from 1000 °C. It clearly reveals bands of newly formed ferrite grains in the rolling direction. The banding obviously arose because of heavy deformation by cold rolling before annealing. The microstructures produced from annealing at 1225 °C are shown in Figs. 2(b) and (c), and that from annealing at 1240 °C is shown in Fig. 2(d). The banding of the ferrite phase is seen in all specimens, and the ferrite content increases with increasing annealing temperature except after treatment at 1240 °C. The fairly straight, sharp and oriented ferrite-austenite boundaries in the microstructures indicate a crystallographic orientation relationship between the two phases.
177
i~ig. 2. Microstructures produced after annealing at (a) 1000 °C, (b) 1225 °C, (c) 1225 °C (SEM) and (d) 1240 °C. The ferrite c o n t e n t and its morphological parameters as related to annealing temperature are summarized in Table 2. The ferrite c o n t e n t in the material increases with increasing temperature, as indicated by the pseudobinary diagram (Fig. 3) of the Fe-Cr-Ni system [7]. In this diagram the composition of the afioy is displayed in terms of chromium and nickel equivalents, which relate to the relative power of the various alloying elements as a and 3' stabilizers. However, at 1240 °C, the ferrite content in the material was less than t h a t at 1225 °C. This could be
because of the redistribution of the alloying elements in the ferrite and austenite phases and the consequent influence on phase stability. Grobner and Biss [8] determined the ferrite phase boundaries in a stainless steel with 16 wt.% Cr, 1.5 wt.% Mo and 5 wt.% Ni at 1000 °C and above after treating the alloy for 2 h, and their results indicate that the phases in the present case may be in equilibrium at about 1200 °C. The ratios of the alloying elements in the ferrite and austenite phases were determined (Table 3) to improve the understanding of
178 TABLE 2 Relationship to the annealing temperature of the ferrite content and its morphological parameters
Annealing temperature (°C)
Areal fraction of ferrite (%)
Equivalent diameter
Mean chord
Form factor
Number of ferrite phase fields per 104pro 2
1000 1100 1175 1200 1225 1240
31.71 53.43 56.70 84.06 91.72 68.58
2.42 6.12 9.57 20.93 37.17 4.47
1.14 2.70 4.18 7.13 13.25 1,75
0.70 0.75 0.75 0.75 0.85 0.93
689 182 79 24 8 437
U50 1600 1400 -
1200
t "a+×
~
-
a /
- Y/
IOOO
2 1-
800 \
I
600 400
I
x.
I
~\
2OO \
0 %Ni 0 % Cr 55
5 50
I0 25
15 20
. . t _ _ ~ 20 25 50 15 I0 5
55 0
Fig. 3. Pseudobinary diagram for 65%Fe-Cr-Ni [7].
this phenomenon. Although it is believed that silicon, chromium and m o l y b d e n u m are ferrite stabilizers, and nickel, copper and manganese are austenite stabilizers, the results show that the phase-stabilizing capability of the elements depends on the temperature. Silicon, although considered a ferrite stabilizer, shows a tendency to stabilize austenite at 1240 °C by partitioning in favour of austenite, thereby raising serious d o u b t a b o u t the use of chromium and nickel equivalents for establishing the phase relationship in the alloy. The effect of annealing temperature on the equivalent diameter and mean chord (Table 2) shows an identical trend of variation. Since the ferrite bands were oriented perpendicular to the screen of the system during the image analysis, the ratio of the equivalent diameter to the mean chord indicates the extent of ferrite banding in the rolling direction. The coarsening effect of temperature on the ferrite phase is shown in Table 2 by the
TABLE 3 Ratio of alloying elements in the ferrite phase to those in the austenite phase after annealing at different temperatures
Annealing temperature
Si
Cr
Mn
Fe
Ni
Cu
Mo
2.92 3.97 4.17 1.60 1.39 0.88
0.78 1.04 1.11 1.18 1.18 1.08
0.51 0.87 0.71 0.74 0.90 0.71
1.45 0.94 0.94 0.96 0.99 0.96
1.17 1.08 0.99 0.79 0.74 1.05
0.89 1.02 0.79 0.84 0.84 0.99
1.00 1.07 2.37 1.41 1.61 2.02
(°c) 1000 1100 1175 1200 1225 1240
179
number of ferrite phase islands observed per 10 4 p m 2. With increasing temperature, the ferrite phase is coarsened n o t only by the transformation of austenite to ferrite, b u t also by the decrease in the number of ferrite phase fields. 3.2. Mechanical behaviour The tensile properties and hardness of the material in various annealed conditions are shown in Figs. 4 and 5 respectively. The data demonstrate the strong effect of heat treatment on mechanical properties. The tensile strength, yield strength and hardness decrease with increasing annealing temperature. The high levels of tensile strength, yield strength and hardness of the material and corresponding lower elongation o f the material treated below 1000 °C arise from the presence of the o phase [3, 7 , 9 ] and the lack of recrystallization. Above the recrystallization temperature (1000 °C), the microstructure dictates the properties of the material. The effect of microstructure on mechanical properties is dominated by the a m o u n t of ferrite present. The tensile strength of the material decreases with increasing ferrite con-
I100
tent (Fig. 6). Thus an increase in austenite content increases the tensile strength of the alloy, because of at least three factors [3, 10]. (i) The larger a m o u n t of austenite leads to the higher austenite work hardening which raises the tensile strength of the austenite-rich duplex steels relative to those of the alloys with less austenite. (ii) The extent of martensite formation, which greatly influences the amount of austenite work hardening, decreases as the proportion of austenite in the duplex steel is decreased. (iii) The higher ferrite content in the alloy reduces the ductility because of cleavage fractures. It is expected that the yield strength of the alloy will increase with increasing ferrite content [10]. However, in the present case, the values of the yield strength remained almost unchanged above the recrystallization temperature (Fig. 6). This is because of the banding of the ferrite and austenite phases. Austenite bands could yield, fairly independently of their content, and this results in the yielding of the material. The material exhibits maximum elongation when the ferrite c o n t e n t is 30% and it retains
40
400 I000
30
900
300
8OO
#-7oo ~E
20 %
I 0
600 200 I0
500
400 L~ -e Total elongation • -e* Uniform elongation 300
I
800
I
OlO
I
900 I 0 I100 Annealing temp. °C
I
1200
1300
Fig. 4. V a r i a t i o n in tensile properties after a n n e a l i n g at different t e m p e r a t u r e s .
I00
I
800
I
I
I
900 IO00 I100 Annealing temp. °C
I
1200
1300
Fig. 5. V a r i a t i o n in V i c k e r s ' hardness w i t h a n n e a l i n g temperature.
180 800
40 O-o-u
• -~2 700
0
0
30
600
g_ % 20 500
O-e
400 I0
300 0
I 20
I 40
I 60 % Ferrite
I 80
Total elongation
• - e * Uniform elongation
I I00
Fig. 6. E f f e c t o f ferrite c o n t e n t o n t h e tensile a n d yield s t r e n g t h s o f t h e material.
good ductility up to a ferrite content of 70% (Fig. 7). A sharp drop in elongation occurs at about 90% ferrite as the ferrite forms a nearcontinuous phase at this stage (Figs. 2(b) and 2(c)) which reduces the ductility. This is confirmed by the presence of cleavage fracture of ferrite and decohesions at the ferrite interface. Annealing at 1000,1100, 1175 and 1240 °C produced a ferrite content ranging from 30% to 70% which resulted in ductile failures exhibiting deep microvoids. The fracture surface formed from tensile testing of the material annealed at 1240 °C is shown in Fig. 8 and reveals ductile failure via void growth and coalescence at the austenite band. The hardness of the material decreases with increasing treatment temperature (Fig. 5). The abrupt fall in hardness resulting from treatment at 1000 °C is accompanied by recrystallization of the material, as revealed by the microstructure.
0 0
i 20
[ 40
~ 60 % Ferrite
t 80
t I00
Fig. 7. E f f e c t o f ferrite c o n t e n t o n t h e t o t a l a n d unif o r m e l o n g a t i o n s of t h e material.
4. C O N C L U S I O N S
(i) The recrystallization temperature for alloy U50 is about 1000 °C for a deformation of 78% and a heat treatment time of 1 h.
Fig. 8. S c a n n i n g e l e c t r o n f r a c t o g r a p h s h o w i n g m i c r o voids a t t h e a u s t e n i t e b a n d .
181 (ii) T h e ferrite c o n t e n t in t h e material increases with annealing t e m p e r a t u r e u p t o 1225 °C a n d decreases at 1 2 4 0 °C. This is caused b y t h e r e d i s t r i b u t i o n o f t h e alloying e l e m e n t s in the ferrite and a u s t e n i t e phases. (iii) T h e a phase is f o r m e d in the material because o f annealing f r o m 8 0 0 t o 9 5 0 °C, w h e r e a s annealing at o r above 975 °C does n o t give rise t o t h e a phase. (iv) T h e higher strength and l o w e r elongat i o n o f the material t r e a t e d u p t o 1 0 0 0 °C arise f r o m t h e p r e s e n c e o f the o phase and the lack o f recrystaUization. T h e variation in tensile p r o p e r t i e s o f t h e material a n n e a l e d above 1 0 0 0 °C is caused b y the change in t h e ferrite c o n t e n t , the associated m i c r o s t r u c t u r e a n d banding. A g o o d c o m b i n a t i o n o f properties is achieved b y t r e a t m e n t s w h i c h produce a ferrite c o n t e n t b e t w e e n 30% and 70%.
REFERENCES 1 K. Unnikrishnan, M.Tech. dissertation, Indian Institute of Technology, Bombay, 1985. 2 D.E. Nelson, W. A. Baslack III and J. C. Lippod, Metallography, 18 (1985) 215. 3 H. D. Solomon and T. M. Devine, Jr., Proc. Conf. on Duplex Stainless Steels, 1983, American Society for Metals, Metals Park, OH, 1983, p. 693. 4 N. Sridhar, J. Kolts and L. M. Flasche, J. Met., 31 (3) (1985) 31. 5 S. M. Wilhelm and R. D. Kane, Corrosion, 40 (1984) 431. 6 M. A. Streicher, Met. Prog., October (1985) 29. 7 H.D. Solomon, J. Heat Treating, 3 (1) (1983) 3. 8 P. J. Grobner and V. Biss, Metall. Trans. A, 15 (1985) 1379. 9 N. Sridhar, J. Kolts, S. K. Srivastava and A. I. Asphani, Proc. Conf. on Duplex Stainless Steels, 1983, American Society for Metals, Metals Park, OH, p. 481. 10 S. Floreen and H. W. Hayden, Trans. A m . Soc. Met., 61 (1968) 489.