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Increase of martensite content in cold rolled AISI 304 steel produced by annealing at 400°C
Volume IO,number I,2
MATERNAL LETTERS
September 1990
Increase of martensite content in cold rolled AISI 304 steel produced by annealing at 400°C R. Montanari D~partimento di Ingegneria Meccanica, II Universitci di Rama, Tar Vergata. ??a. E. Carnevale, 00173 Rome, Italy
Received 2 May 1990; in final form 14 June 1990
X-ray measurements were used to investigate the phenomenon of the increase of ferromagnetic phase in cold roiled AISI 304 steel induced by annealing at 400°C. The role played in the microstructure evolution by the recovery of defective structures (dislocations and stacking faults), the precipitation of carbides and the a’ -by reversion is discussed with reference to the two stages of transformation evidenced by experimental results. The effects due to a different initial content of a’ phase and to the presence of e martensite are also considered.
1. induction An increase of ferromagnetic phase takes place in austenitic stainless steels with biphasic structure (a’ + y ) due to heating at a temperature of about 400” C [ 11. This phenomenon, which occurs in materials where a’ martensite was induced by both cooling and deformation [ 2 1, produces an increase of yield and tensile strengths depending on the amount of the initial a’ phase content and on the time of annealing. In order to explain the martensite formation during stress-relief treatment, Chu~leb and Martynov [ 31 advanced the hypothesis that carbides precipitate in austenitic phase and then new a’ crystals form and grow around them. Extensive TEM observations of Mangonon and Thomas [4] evidenced that the new a’ crystals do not contain precipitates and have a lenticular shape, as martensite in Fe-C and Fe-Ni-C alloys, different from the lath shape characteristic of strain-induced martensite. Moreover new and old a’ crystals have different orientations. Consequently these authors ruled out the possibility that new a’ crystals can grow around old a’ crystals or carbides present in y phase as proposed by Chukhleb and Martynov. Smith and West [ 51 ascribed the “magnetic phase peak” at 400°C to the formation of a ferrite as an
intermediate stage of the a’ +y reversion. Results of magnetic measurements, performed directly at the annealing temperature, did not show evidence of an increase of ferromagnetic phase [ 61. On this basis Harries [ 7 ] argued that the thermal treatment at 4OO”C, producing a fine carbide precipitation, lowers the content of the alloying elements in solid solution and thus increases M,(a’ ) and concluded that the new ferromagnetic phase forms during cooling to room temperature. The present work was undertaken to attain more detailed information about the microstructural evolution of AISI 304 stainless steel during isothermal annealing at 400°C for times up to 7.2 X lo4 s.
2. Experimental In this research investigations were carried out on AISI 304 stainless steel with the following chemical composition: C 0.06, Ni 8.60, Cr 18.25, Mn 0.82, MO 5.16, S 0.0 15, P 0.02 1, Si 0.56, Ti, Cu, V traces and the balance Fe (wt.Oh). All the samples were homogenized for 1.8~ lo3 s at 1555°C and then water quenched. The martensitic structures were induced by cold rolling at room temperature; three sets of samples with a different
0167-577x/90/% 03.50 0 1990 - Elsevier Science Publishers B.V. (North-Holland)
57
content of a’ phase were prepared, varying the total amount of plastic deformation. The samples were then annealed up to 7.2 x 1O4 s at 400°C in argon atmosphere. The microstructure evolution was monitored by means of X-ray diffraction (XRD) measurements, performed at room temperature after subsequent times of heat treatment. To avoid effects due to macroscopic inhomogeneities the specimens were always analyzed in the same zones. The cooling rate from the temperature of annealing was controlled and maintained nearly constant for all the tests. X-ray diffraction patterns were recorded over a 28 range from 15” to 75” using a Philips PW 1729 diffractometer with MO Ku radiation. High precision profiles of the XRD lines were obtained by step-scanning with 28 angular intervals of 0.005 ’ and counting times of 10 s for each step. XRD profiles were then analyzed by means of the Warren-Averbach technique [ 81 to separate the contribution of internal strains and coherently diffracting domain size to line broadening. The dislocation densities p were derived substituting the values of the mean quadratic strains (e’), obtained from W-A analysis, in the expression given by Williamson and Smallman [ 91: p= (25/Fb*)
,
(e*)
(1)
where b is the modulus of the Burgers vector and F is a parameter depending on dislocation interactions (Fz 1). The stacking fault probability (Y,which represents the fraction of slip planes affected by stacking faults, was determined from the peak shifts of (200) and (220) y reflections after cold working, using the relation [lo]: cr=A(2@22,,-2@200)o(7c2/90~)
x (ttan@,,, with (2@220
A(29220
+ jtant9,,,)-’ -
2@200)~
(2) =
W220
-
2@2,L.
Table 1 Volume fraction of the phases present in the three sets of AISI 304 stainless-steel samples after cold rolling at room temperature Set
y austenite
a’ martensite
E martensite
A B C
0.61 0.37 0.17
0.36 0.63 0.83
0.03
of analyzed reported.
samples
(indicated
as A, B and C) are
3. Results and discussion As shown in fig. 1, the curves representing the volume fraction of a’ martensite versus annealing time show two well distinct maxima: the first one after x 4 x 1O3s and the second one after = 3.5 x 1O4s. The curves have a decreasing background: after 7.2 x 1O4 s of annealing the amount of martensite is about two thirds of the initial value. It is clear that the continuous decrease of the background is due to a’ +y reversion, whereas the origin of maxima 1 and 2 is more difficult to explain. Their occurrence at different times of annealing suggests that they are caused by different phenomena, which characterize two distinct transformation stages (indicated as stage 1 and 2). In fig. 2 the trends of the half height widths p of the (220) y reflections as a function of the time of thermal treatment are shown. The values of BZo decrease markedly in stage 1 whereas in stage 2 they present a broad maximum. The hypothesis put forward by Harries [ 71 about the formation of new a’ phase following annealing at 400°C can explain stage 2 of the transformation. The temperature M,( a’ ) is related to the composition of austenitic steels by the following empiric expression [ 12 ] :
-
-2@*ooxz”n..
The volume fraction of the y, a’ and c phases was determined on the basis of the integrated intensities Ihkl of the XRD lines using the method of Dickinson
1111. In table 1 the volume fractions of the phases present before heat treatments at 400°C in the three sets 58
September 1990
MATERIALS LETTERS
Volume 10, number 1,2
M,(u’)=1305-61.1%Ni-41.7%Cr -33.3%
Mn-27.8%
Si-
1667%(C+N)
.
(3)
Although the temperature of annealing (400°C) is low, fine carbides can nucleate in particular along a’ / y interfaces; the consequent compositional fluctuations and removal of the alloying elements from solid
Volume 10. number 1.2
MATERIAL
0.8
September 1990
LETTERS
T= 400°C
STAGE
6
-.-yc
2
v-,.&.”
*...A . . . .y.
/-*
“\
\
L
_--*-... . .._* --.,
..K”
0
‘---*__.
-..._ “..&
.-...._
A.....* .‘A . . . . . __..
I
I 2
3 ANNEALING
I 4 TIME
I 5 tsec 1. 1O-4
A .A
I 7
6
Fig. 1. AISI 304 steel sample Sets A, B, C (see text): volume fraction of a’ martensite versus annealing time at 400°C. Stages 1and 2 are indicated in the figure.
T= 400°C a12
; STAGE
1
STAGE
2
g 0.08 : i
0.06
E I LL 2 a04 L
---._.., *“-II..._ ‘“I..
I 2 3 ANNEALING
I
I
I
4 TIME
Fig. 2. AISI 304 steel sample sets A, B, C (see text): half height widthflof data are corrected for instrumental broadening.
(see).
5 1O-4
I 6
A --..
.*
7
(220) y XRD reflections versus anneaiing time at 400°C. The
59
Volume 10, number I,2
MATERIALS LETTERS
solution, can determine in some zones processes of martensitic transformation when the samples are cooled down to room temperature. The trends of the PI20 (fig. 2 ) confirm this explanation. Recalling that the main factors, which cause broadening of XRD profiles, are internal strains and coherently diffracting domain sizes and considering that the average grain dimension of the austenite increases steadily during stage 2, the rising part of the &, maxima has to be ascribed to higher stresses in the y phase. This is consistent with the hypothesis of the martensitic transformation since the volume expansion of those zones, which transform, produces states of compression with elastic and plastic deformation in the surrounding austenitic matrix. The origin of the maximum 1 in the curves of a’ volume fraction versus time of annealing cannot be connected with carbide precipitation processes since in this case the times involved are too short. On the other hand, this is confirmed also by TEM observations of Mangonon and Thomas [4] carried out on samples annealed 5.6 x lo3 s at 400°C. The possibility that the increase of a’ phase (maximum 1) can be due to the E+U’ transformation has been also considered, but there are good reasons to rule out this hypothesis. First of all, in samples of set A the ( 10 1) t reflection disappears after an annealingof2x103sat400”Cwhilethe (lll)ylinebecomes more intense and the ( 110) a’ line is practically unaffected (fig. 3), indicating that the transformation, which occurs, is e-y and not ~+a’. It is worth recalling that y*e is a reversible transformation based on dissociation of dislocations into partials ( y-+ E) and on the recombination of partials into whole dislocations (e+y), as shown by Mangonon and Thomas [ 41. This kind of mechanism, which was observed also in AI-Ag alloys [ 13 1, is often indicated in the literature as a faulting-unfaulting mechanism. The e-+y reversion occurs during heating by unfaulting mechanism since the stacking fault energy of AISI 304 steel, low at room temperature (r= 22 mJ/ m’), increases with temperature. Lecroisey and Pineau [ 141 determined for an alloy of similar composition at room temperature a value W/aTx
8~ 10e2 mJ/m’“C
Furthermore 60
maximum
. 1 is observed
also in sam-
September 1990
9
10
11
12
8 (deg)
Fig. 3. XRD patterns of cold rolled AISI 304 steel (set A) before and after an annealing of 2 X 1O3s at 400°C.
ples of sets B and C, where the hexagonal phase t is absent or at least present in quantities not detectable by X-ray precision measurements. Recovery of austenite during stage 1 is well documented by the decreasing trends of dislocation density p and stacking fault probability (Y, obtained using eqs. ( 1) and (2) and represented in fig. 4. On the other hand, it is noteworthy that j3220of the y phase decreases markedly whereas pin,, &,, and p2i 1of the a’ phase do not change appreciabily. This result suggests that the annealing treatment for the times involved in stage 1, relaxes the stresses more in austenite than in martensite and causes non-equilibrium conditions at the a’ /y interfaces. The dislocations, forming the boundary between a’ and y phases, are therefore forced to move with the consequence of a growth of the laths and an increase of martensite volume fraction.
Volume IO, number 1,2
MATERIALS LETTERS
September 1990 -4
I
0 0
0.5 ANNEALING
I TIME
(set ). IO-’
0
1
Fig. 4. Stage 1:stacking fault probability (Yand dislocation density p of austenite versus annealing time at 400” C for samples of set A.
4. Conclusions
Acknowledgement
X-ray measurements of the a’ amount in cold rolied AISI 304 steel after repeated annealing at 400 oC evidenced two stages of transformation, characterized by maxima 1 and 2, occurring respectively after x4x103sand ~53.5~10~~. The origin of maximum 1 is attributed to the recovery of defective structures (dislocations and stacking faults) in the y phase. The growth of preexisting martensitic laths can occur when the stresses of the surrounding austenitic matrix relax. The increase of martensite content corresponding to maximum 2 is connected to processes of carbide precipitation. Lowering of the alloying elements in solid solution and compositional fluctuations, produced by fine carbide fo~ation, increase the M,(a’ ) temperature and thus new ma~ensitic phase can form during cooling to room temperature. The a’ -+y reversion, the other important process influencing the evolution of the biphasic structure of cold rolled 304 steel, is active in both stage 1 and 2 and its effects become dominant after relatively long times of thermal treatment at 400°C. These results are common to samples with different initial contents of martensitic phase.
The author is grateful to Mr. Franc0 Barbieri, Physics Department, Bologna University, for his help in the research. References [ 1] P, Marshall, Austcnitic stainless steels- microstructure and mechanical properties (Elsevier, New York, 1984) p. 30. [2] K.B. Guy, E.P. Butler and D.R.F. West, Met. Sci. 17 (1983) 167. [ 31 A.N. Chukhleb and V.P. Martynov, Phys. Met. Metall. IO (1960) 80. [ 4] P.L. Mangonon Jr. and G. Thomas, Metall. Trans. I ( 1970) 1587. [5] H. Smith and D.R.F. West, Met. Tech. 1 (1974) 259. [6 ] M.W. Bowkett, Ph.D. Thesis, University College, Cardiff (1980). [ 71 D.R. Harries, International Conference on Mechanical Behaviour and Nuclear Applications of Stainless Steels at Elevated Temperatures, Varese, May ( 198 I 1. [S] B.E. Warren and B.L. Averbach, J. Appl. Phys. 21 (1950) 595. [9] G.K. Williamson and R.A. Smallman, Phil. Mag. I ( 1956) 34. [lo] M.S. Paterson, J. Appl. Phys. 23 (1952) 805. [ 111 M.J. Dickinson, J. Appl. Cryst. 2 ( 1969) 176. [ 121 G.J. Eicheimann and F.C. Hull, Trans. ASM 45 (1953) 77. [ 131 J.A. Hren and G. Thomas, Trans. TMS-AIME 227 ( 1963) 308. [ 141 F. Lecroisey and A. Pineau, Metali. Trans. 3 ( 1972) 387.
61
MATERNAL LETTERS
September 1990
Increase of martensite content in cold rolled AISI 304 steel produced by annealing at 400°C R. Montanari D~partimento di Ingegneria Meccanica, II Universitci di Rama, Tar Vergata. ??a. E. Carnevale, 00173 Rome, Italy
Received 2 May 1990; in final form 14 June 1990
X-ray measurements were used to investigate the phenomenon of the increase of ferromagnetic phase in cold roiled AISI 304 steel induced by annealing at 400°C. The role played in the microstructure evolution by the recovery of defective structures (dislocations and stacking faults), the precipitation of carbides and the a’ -by reversion is discussed with reference to the two stages of transformation evidenced by experimental results. The effects due to a different initial content of a’ phase and to the presence of e martensite are also considered.
1. induction An increase of ferromagnetic phase takes place in austenitic stainless steels with biphasic structure (a’ + y ) due to heating at a temperature of about 400” C [ 11. This phenomenon, which occurs in materials where a’ martensite was induced by both cooling and deformation [ 2 1, produces an increase of yield and tensile strengths depending on the amount of the initial a’ phase content and on the time of annealing. In order to explain the martensite formation during stress-relief treatment, Chu~leb and Martynov [ 31 advanced the hypothesis that carbides precipitate in austenitic phase and then new a’ crystals form and grow around them. Extensive TEM observations of Mangonon and Thomas [4] evidenced that the new a’ crystals do not contain precipitates and have a lenticular shape, as martensite in Fe-C and Fe-Ni-C alloys, different from the lath shape characteristic of strain-induced martensite. Moreover new and old a’ crystals have different orientations. Consequently these authors ruled out the possibility that new a’ crystals can grow around old a’ crystals or carbides present in y phase as proposed by Chukhleb and Martynov. Smith and West [ 51 ascribed the “magnetic phase peak” at 400°C to the formation of a ferrite as an
intermediate stage of the a’ +y reversion. Results of magnetic measurements, performed directly at the annealing temperature, did not show evidence of an increase of ferromagnetic phase [ 61. On this basis Harries [ 7 ] argued that the thermal treatment at 4OO”C, producing a fine carbide precipitation, lowers the content of the alloying elements in solid solution and thus increases M,(a’ ) and concluded that the new ferromagnetic phase forms during cooling to room temperature. The present work was undertaken to attain more detailed information about the microstructural evolution of AISI 304 stainless steel during isothermal annealing at 400°C for times up to 7.2 X lo4 s.
2. Experimental In this research investigations were carried out on AISI 304 stainless steel with the following chemical composition: C 0.06, Ni 8.60, Cr 18.25, Mn 0.82, MO 5.16, S 0.0 15, P 0.02 1, Si 0.56, Ti, Cu, V traces and the balance Fe (wt.Oh). All the samples were homogenized for 1.8~ lo3 s at 1555°C and then water quenched. The martensitic structures were induced by cold rolling at room temperature; three sets of samples with a different
0167-577x/90/% 03.50 0 1990 - Elsevier Science Publishers B.V. (North-Holland)
57
content of a’ phase were prepared, varying the total amount of plastic deformation. The samples were then annealed up to 7.2 x 1O4 s at 400°C in argon atmosphere. The microstructure evolution was monitored by means of X-ray diffraction (XRD) measurements, performed at room temperature after subsequent times of heat treatment. To avoid effects due to macroscopic inhomogeneities the specimens were always analyzed in the same zones. The cooling rate from the temperature of annealing was controlled and maintained nearly constant for all the tests. X-ray diffraction patterns were recorded over a 28 range from 15” to 75” using a Philips PW 1729 diffractometer with MO Ku radiation. High precision profiles of the XRD lines were obtained by step-scanning with 28 angular intervals of 0.005 ’ and counting times of 10 s for each step. XRD profiles were then analyzed by means of the Warren-Averbach technique [ 81 to separate the contribution of internal strains and coherently diffracting domain size to line broadening. The dislocation densities p were derived substituting the values of the mean quadratic strains (e’), obtained from W-A analysis, in the expression given by Williamson and Smallman [ 91: p= (25/Fb*)
,
(e*)
(1)
where b is the modulus of the Burgers vector and F is a parameter depending on dislocation interactions (Fz 1). The stacking fault probability (Y,which represents the fraction of slip planes affected by stacking faults, was determined from the peak shifts of (200) and (220) y reflections after cold working, using the relation [lo]: cr=A(2@22,,-2@200)o(7c2/90~)
x (ttan@,,, with (2@220
A(29220
+ jtant9,,,)-’ -
2@200)~
(2) =
W220
-
2@2,L.
Table 1 Volume fraction of the phases present in the three sets of AISI 304 stainless-steel samples after cold rolling at room temperature Set
y austenite
a’ martensite
E martensite
A B C
0.61 0.37 0.17
0.36 0.63 0.83
0.03
of analyzed reported.
samples
(indicated
as A, B and C) are
3. Results and discussion As shown in fig. 1, the curves representing the volume fraction of a’ martensite versus annealing time show two well distinct maxima: the first one after x 4 x 1O3s and the second one after = 3.5 x 1O4s. The curves have a decreasing background: after 7.2 x 1O4 s of annealing the amount of martensite is about two thirds of the initial value. It is clear that the continuous decrease of the background is due to a’ +y reversion, whereas the origin of maxima 1 and 2 is more difficult to explain. Their occurrence at different times of annealing suggests that they are caused by different phenomena, which characterize two distinct transformation stages (indicated as stage 1 and 2). In fig. 2 the trends of the half height widths p of the (220) y reflections as a function of the time of thermal treatment are shown. The values of BZo decrease markedly in stage 1 whereas in stage 2 they present a broad maximum. The hypothesis put forward by Harries [ 71 about the formation of new a’ phase following annealing at 400°C can explain stage 2 of the transformation. The temperature M,( a’ ) is related to the composition of austenitic steels by the following empiric expression [ 12 ] :
-
-2@*ooxz”n..
The volume fraction of the y, a’ and c phases was determined on the basis of the integrated intensities Ihkl of the XRD lines using the method of Dickinson
1111. In table 1 the volume fractions of the phases present before heat treatments at 400°C in the three sets 58
September 1990
MATERIALS LETTERS
Volume 10, number 1,2
M,(u’)=1305-61.1%Ni-41.7%Cr -33.3%
Mn-27.8%
Si-
1667%(C+N)
.
(3)
Although the temperature of annealing (400°C) is low, fine carbides can nucleate in particular along a’ / y interfaces; the consequent compositional fluctuations and removal of the alloying elements from solid
Volume 10. number 1.2
MATERIAL
0.8
September 1990
LETTERS
T= 400°C
STAGE
6
-.-yc
2
v-,.&.”
*...A . . . .y.
/-*
“\
\
L
_--*-... . .._* --.,
..K”
0
‘---*__.
-..._ “..&
.-...._
A.....* .‘A . . . . . __..
I
I 2
3 ANNEALING
I 4 TIME
I 5 tsec 1. 1O-4
A .A
I 7
6
Fig. 1. AISI 304 steel sample Sets A, B, C (see text): volume fraction of a’ martensite versus annealing time at 400°C. Stages 1and 2 are indicated in the figure.
T= 400°C a12
; STAGE
1
STAGE
2
g 0.08 : i
0.06
E I LL 2 a04 L
---._.., *“-II..._ ‘“I..
I 2 3 ANNEALING
I
I
I
4 TIME
Fig. 2. AISI 304 steel sample sets A, B, C (see text): half height widthflof data are corrected for instrumental broadening.
(see).
5 1O-4
I 6
A --..
.*
7
(220) y XRD reflections versus anneaiing time at 400°C. The
59
Volume 10, number I,2
MATERIALS LETTERS
solution, can determine in some zones processes of martensitic transformation when the samples are cooled down to room temperature. The trends of the PI20 (fig. 2 ) confirm this explanation. Recalling that the main factors, which cause broadening of XRD profiles, are internal strains and coherently diffracting domain sizes and considering that the average grain dimension of the austenite increases steadily during stage 2, the rising part of the &, maxima has to be ascribed to higher stresses in the y phase. This is consistent with the hypothesis of the martensitic transformation since the volume expansion of those zones, which transform, produces states of compression with elastic and plastic deformation in the surrounding austenitic matrix. The origin of the maximum 1 in the curves of a’ volume fraction versus time of annealing cannot be connected with carbide precipitation processes since in this case the times involved are too short. On the other hand, this is confirmed also by TEM observations of Mangonon and Thomas [4] carried out on samples annealed 5.6 x lo3 s at 400°C. The possibility that the increase of a’ phase (maximum 1) can be due to the E+U’ transformation has been also considered, but there are good reasons to rule out this hypothesis. First of all, in samples of set A the ( 10 1) t reflection disappears after an annealingof2x103sat400”Cwhilethe (lll)ylinebecomes more intense and the ( 110) a’ line is practically unaffected (fig. 3), indicating that the transformation, which occurs, is e-y and not ~+a’. It is worth recalling that y*e is a reversible transformation based on dissociation of dislocations into partials ( y-+ E) and on the recombination of partials into whole dislocations (e+y), as shown by Mangonon and Thomas [ 41. This kind of mechanism, which was observed also in AI-Ag alloys [ 13 1, is often indicated in the literature as a faulting-unfaulting mechanism. The e-+y reversion occurs during heating by unfaulting mechanism since the stacking fault energy of AISI 304 steel, low at room temperature (r= 22 mJ/ m’), increases with temperature. Lecroisey and Pineau [ 141 determined for an alloy of similar composition at room temperature a value W/aTx
8~ 10e2 mJ/m’“C
Furthermore 60
maximum
. 1 is observed
also in sam-
September 1990
9
10
11
12
8 (deg)
Fig. 3. XRD patterns of cold rolled AISI 304 steel (set A) before and after an annealing of 2 X 1O3s at 400°C.
ples of sets B and C, where the hexagonal phase t is absent or at least present in quantities not detectable by X-ray precision measurements. Recovery of austenite during stage 1 is well documented by the decreasing trends of dislocation density p and stacking fault probability (Y, obtained using eqs. ( 1) and (2) and represented in fig. 4. On the other hand, it is noteworthy that j3220of the y phase decreases markedly whereas pin,, &,, and p2i 1of the a’ phase do not change appreciabily. This result suggests that the annealing treatment for the times involved in stage 1, relaxes the stresses more in austenite than in martensite and causes non-equilibrium conditions at the a’ /y interfaces. The dislocations, forming the boundary between a’ and y phases, are therefore forced to move with the consequence of a growth of the laths and an increase of martensite volume fraction.
Volume IO, number 1,2
MATERIALS LETTERS
September 1990 -4
I
0 0
0.5 ANNEALING
I TIME
(set ). IO-’
0
1
Fig. 4. Stage 1:stacking fault probability (Yand dislocation density p of austenite versus annealing time at 400” C for samples of set A.
4. Conclusions
Acknowledgement
X-ray measurements of the a’ amount in cold rolied AISI 304 steel after repeated annealing at 400 oC evidenced two stages of transformation, characterized by maxima 1 and 2, occurring respectively after x4x103sand ~53.5~10~~. The origin of maximum 1 is attributed to the recovery of defective structures (dislocations and stacking faults) in the y phase. The growth of preexisting martensitic laths can occur when the stresses of the surrounding austenitic matrix relax. The increase of martensite content corresponding to maximum 2 is connected to processes of carbide precipitation. Lowering of the alloying elements in solid solution and compositional fluctuations, produced by fine carbide fo~ation, increase the M,(a’ ) temperature and thus new ma~ensitic phase can form during cooling to room temperature. The a’ -+y reversion, the other important process influencing the evolution of the biphasic structure of cold rolled 304 steel, is active in both stage 1 and 2 and its effects become dominant after relatively long times of thermal treatment at 400°C. These results are common to samples with different initial contents of martensitic phase.
The author is grateful to Mr. Franc0 Barbieri, Physics Department, Bologna University, for his help in the research. References [ 1] P, Marshall, Austcnitic stainless steels- microstructure and mechanical properties (Elsevier, New York, 1984) p. 30. [2] K.B. Guy, E.P. Butler and D.R.F. West, Met. Sci. 17 (1983) 167. [ 31 A.N. Chukhleb and V.P. Martynov, Phys. Met. Metall. IO (1960) 80. [ 4] P.L. Mangonon Jr. and G. Thomas, Metall. Trans. I ( 1970) 1587. [5] H. Smith and D.R.F. West, Met. Tech. 1 (1974) 259. [6 ] M.W. Bowkett, Ph.D. Thesis, University College, Cardiff (1980). [ 71 D.R. Harries, International Conference on Mechanical Behaviour and Nuclear Applications of Stainless Steels at Elevated Temperatures, Varese, May ( 198 I 1. [S] B.E. Warren and B.L. Averbach, J. Appl. Phys. 21 (1950) 595. [9] G.K. Williamson and R.A. Smallman, Phil. Mag. I ( 1956) 34. [lo] M.S. Paterson, J. Appl. Phys. 23 (1952) 805. [ 111 M.J. Dickinson, J. Appl. Cryst. 2 ( 1969) 176. [ 121 G.J. Eicheimann and F.C. Hull, Trans. ASM 45 (1953) 77. [ 131 J.A. Hren and G. Thomas, Trans. TMS-AIME 227 ( 1963) 308. [ 141 F. Lecroisey and A. Pineau, Metali. Trans. 3 ( 1972) 387.
61