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Isothermal formation of martensite in a 12Cr-9Ni-4Mo maraging stainless steel
Scripta Met&r@
Pergamon
et Mat&h,
Vol. 33, No. 9, pp. 1367-1373,1995 Elsevia Science Ltd Copright 0 1995 Acta Metallurgica Inc. PnntedintheUSA.AUri&ts-ed 0955716xm $9.50 + .cn
0956-716X(95)00386-X
ISOTHERMAL FORMATION OF MARTENSITE IN A 112Cr-9Ni-4Mo MARAGING STAINLESS STEEL M. Holrnquist’, J.-O. Nilsson and A. Hultin Stigenberg LDepartmentof Welding Research Department of Physical Metallurgy ‘Department of Wire Research Steel Research and Development AB Sandvik Steel S-81 1 81 Sandviken, Sweden (Received Jannary 26,1995) Introduction
The unusuaI combination of ultrahigh strength and ductility of maraging steels makes them very attractive in a large variety of applications (1,2). The maraging steel nsed in the present investigation, termed lRK9 1, is employed in the medical industry in products such as surgical needles. It is exceptional in the sense that agehardening is performed at 475 “C by thermodynamically stable qnasicrystalline precipitates of icosahedral symmetry, whereby a strength as high as 3000 MPa may be achieved (3,4). The present paper is concerned with the nature of the martensite,which provides the basis for the maraging treatment. Rather than forming mauensite during cooling, lRK9 1 develops martensite when held at a constant temperatum in a mnge ti-omroom temperature and below. The fact that the volume fraction of martensite can increase with time at a constant temperature was first observed by Kurdjumov and M&&nova in 1948 (5). To distingnishit from the more common cooling-inducedso called athermal martensite, this type of martensite has been termed isrotheunalmartensite (6). Generally, the isothermal reaction is observed to occur below the martensitic start temperature, N, but there are examples of martensite forming isothermally also above Iv&. While athermal martensite usually forms by the growth of already existing plates, isothermal martensite develops by the continual nucleation of new plates (7). Isothermal martensite formation is characterized by C-cnrve kinetics, the nose of which is usually at subzero temperatures. This has been verified in a rather wide variety of alloys from 1.6 % C steel with a nose temperature at -100°C (5) to more highly alloyed steels such as Fe-14.4% Cr-9% Ni with the nose at -30°C (8) and in Fe-23%INi-3.6%Mn with the nose at - 140°C (9). The purpose of the present investigation was to assess the kinetics of isothermal martensite formation in the maraging steel lRK91 and to determine the influence of physical parameters such as solution heat treatment and cooling rate. ExDerimental Procedure
Sandvik IRK91 was produced as wire ti-oma firll scale 7 ton melt and cold drawn from 5.5 to 4 mm diameter. The production was made according to normal production practice. The chemical composition of the steel grade is given in Table 1 below. Standardexperimental material was produced by anstenitizing at 1050°C for 1367
Vol. 33, No. 9
ISOTHERMAL MARTENSlTE
1368
TABLE 1 Chemical Composition of lFX91 (wt?h) Steel
C+N
Cr
h4n
Fe
Ni
MO
Ti
Al
Si
CU
Sandvik lRK91
co.05
12.0
0.3
bal.
9.0
4.0
0.9
0.30
0.15
2.0
5 min followed by quenching in water prior to testing. To study the infhtence of grain size, larger grains were produced by austenitizingat 1lOO”C,1150°C and 1200°C. The volume fraction of martensite was measured using two different techniques. One of these was the magnetic balance, which was used as a reference. This technique is based on the measurement of the force exerted on the specimen in an inhomogeneous magnetic field (10). However, the magnetic balance operates only at room temperature. To be able to measure n&en&e also at other temperatures the electric resistivity change was used as a measure of the evolution of martensite. This technique was made quantitative by calibrating against the magnetic balance at room temperature. The resistivity was measured from the potential drop along a 400 mm wire using a current of 1.OOOA, which produced no detectable heating. Isothermal transformation experiments at an arbitrary temperature in the range from room temperature down to - 100 “C were conducted in an insulated bath containing a mixture of liquid nitrogen and ethylene. The temperature was controlled to within *2 “C. The temperature dependence of resistivity was compensated for through the relation p(T)=p,(l+a(T-T,,)}, where p(T) and p0 are resistivities at temperature T and T, and ELis the temperature coefftcient experimentally determined in this investigation to be 7.923e10d K-r. The relative change in resistivity was assumed to be proportional to the volume fraction of magnetic phase in accordance with previous investigators (e.g. 8). The close correspondence between these two curves shown in Fig 1 confirms that this is a reasonable assumption. Room temperature hardness of material transformed in the range -100°C up to room temperature was measured using a Vickers type of testing at a load of 2 kg in a hardness testing machine of type Zwick 32 12. Tensile testing of 250 mm wire specimens was performed in an INSTRON 4505,100 kN testing machine. The interruption of the tran&brmationcaused by the very short time required to perform the hardness test was judged to be of no significance on the hardness results. Light optical microscopy (LOM) was performed in a Nikon Optiphot 66 to study isothermal development of surface marten&. Specimens for LOM were solutionheat treated in evacuated capsules to avoid disturbing
_Rel.
1
Figure 1. Volume percentage of m&en&e methods.
chmgc
of rcsirtivily
10
as a iimction of time indicating good agreement between magnetic balance and rcsistivity
ISOTHERMALMARTENSITE
Vol. 33, No. 9
IO
I
IO’
Iooo
100
1369
Time [min]
Figure 2. TlTdiagmmfor lRK91 au&n&edat 40% aud 60% by vohune of marteusite.
1050°C for 5 min and subsequently quenched in water showing TIT-curves
for 20%,
oxidation and subsequently air cooled to room temperature. Despite this, the surface was covered by a very thin oxide layer, which was removed by a gentle electrolytic etching prior to examination. Results
Both magnetic and resistivity measurements were found to give highly reproducible values as verified in repeated measurements. No effect of the magnetic field in the magnetic balance on the transformation behaviour was noted. Only isothermalmartensite was observed, no athermal martensite being observed when quenching to temperatures as low as -196°C. Isothermal martensite formation was investigated at the following temperatures: RT, 0°C -25”C, -40°C -60°C and -100°C. The volume fraction ofmartensite as a function of time was measumd and replotted in a TTT-diagram as shown in Fig 2, from which it is seen that the nose temperature is about -40°C. At this temperature 20% is formed during the Iirst 3 min, while the saturation percentage of 72% is reached afkr about 4h. For comparison, the saturation level at room temperature, 61?/0,requires a hold time of 24h.
0.1
I
IO
loo
loo0
loom
iouoooloooooo
Time [min]
Figure 3. Evolution of martemi teatroomtempemturea
attuxtionofaustenitizing~.
1370
Vol. 33, No. 9
ISOTHF,RMAL MARTENSITE
1
10
100
1000
Time to 40% magnetic phase [min] Figure 4. The time to reach 40% of martensite at room temperature for 4 dikkrent grain sizes.
Thegrainsizeswerefoundtobe23~,54~, 128pmand215pmafter5minheattreatmentat 105O”C, 1 lOO”C, 1150°C and 1200°C respectively. The evolution of martensite as a function of austenitizing heat treatment was measured at room temperature. These results are shown in Fig 3, where it can be seen that the transformation rate increases rapidly with grain size i.e. austenitizing temperature. The saturation level of marten& is vi&tally the same for all 4 heat treatments. If the results in Fig 3 are replotted, the time to attain 40% of martensite as a function of grain size can be presented as in Fig 4. The effect is so strong that the time required to form 40% martensite is found to diEer by more than one order of magnitude for the extreme values of grain size. There is also a pronounced effect of cooling rate. As shown in Fig 5 cooling from 1200 oC in air and water respectively resulted in significantdi&rences in transformation kinetics. While less than 10 min was required to form 40% of marknsite in the water-quenched condition, more than 100 min was required in the air cooled condition. An almost equally strong effect of cooling rate was found also after austenitizing at 1050 ’ C. Hardness values were also plotted as in a TTT-diagram (see Fig 6), the curves corresponding to isohardness values of 2 10,240,270 and 300 HV. As in the case of martensite transformation rate the nose temperature is about -40°C. Tensile testing of room temperature transformed material showed an evolution with time similar to hardness and volume traction of martensite as shown in Table 2. The elongation was inversely related to strength. It was possible to follow the isothermalformation of martensite through continual observations in the LOM as illustratedin Figs 7 ad. However, it should be pointed out that the amount of surface martensite was much
0
20 0
IO 0.1
0
.
12OO*CI5 minRI20
0
12WUS
minhir
d I
10
loo
low
loo00
IcoooOIowwa
Time [min]
Figure 5. Evolution of martensite at room tenperature obtained after heat treatment at 1200 “C for water and air cooling.
ISOTHERMALMARTENSITE
Vol. 33, No. 9
-200
,-fl 0.1
1371
n
10 Time [m$
I
Icoo
IO”
evolution refle.ctedas hardness in terms of isohardness curves ranging from 2 10 HV to 300HV.
Figure 6. hktensite
higher than expected horn the magnetic measurements of the bulk material, implying that several attempts were made before imaging of martensite events became successful.
The fact that the volume fraction of martensite obtained by the magnetic balance. can be translated to an electric signal enables us to quantify martensite transformation contimtally also at other temperatures than room temperature as verified in Fig 1. An important conclusion to be drawn corn the experiments is that, if there is an M, temperature, it must be below -196°C. Hence, above this temperature there is no athermal m&en&e that obscures the interpretation of isothermal martensite transformation. The nose temperature, i.e. the temperature at which the transformation kinetics is fastest, is shown to be about -40°C. As shown in Fig 2 the tran&oimationbehaviour follows C-curve kinetics, as expected Tom previous work (9). It is interesting that this transformation is reflected in terms of iso-hardness curves resembling the C-curves in Fig 2 and a nose temperature at -40°C. This, in combination with mechanical results in Table 2, shows that there is an intimate relation. between isothermal martensite formation and mechanical properties.
TABLE 2 Results from Tensile Testing Time (tin>
3
8
26
50
80
120
145
210
216
300
311
4185
10172
Yield
222
228
238
249
274
272
311
328
373
364
333
492
528
Tensile strength @@a)
719
725
726
734
756
738
780
767
792
806
796
893
895
Elongation (%)
27
28
27
26
24
23
24
21
22
20
19
15
15
gay
ISOTHERMAL, MARTENSITE
1372
Figure 7. a-d: h4im min.
ofbthemal
Vol. 33, No. 9
ma&mite shown by arrows formed succes.sively at room temperature at 10,70,130
and 190
As shown in Fig 3 there is a pronounced effect of austenitizing temperature on the kinetics of martensite transformation. A crucial question is whether this is caused by the physical size of the grains themselves or if the higher temperature to obtain the larger grain size results in a higher concentration of quenched-in stresses, presumably in the form of supersaturation of vacancies that provide suitable sites for the nuclation of martensite embryos. Assuming an energy of formation of 2.1 eV quoted for Fe, it can be shown that the concentration of vacancies in equilibrium at 1200°C is more than 5 times higher than at 1050°C (11). The role of quenched-in vacancies can in fact be appreciated from Fig 5, where it is shown that, for a given grain size of 215 pm, water-quenchingenhances the kinetics of martensite transformation relative to air-cooling by at least one order of magnitude. Because of the difticulty to quantify the vacancy concentration in quenched material in non-equilibrium at room temperature it was not possible to estimate the relative contributions of grain size and vacancies. However, the proposed me&an&m by which isothermal martensite embryos nucleate continually rather than by growth of already transformed martensite plates (12), emphasizes the importance of vacancies. This is also consistent with the view that the isothermal nucleation process is activated by thermal fluctuations superimposed on regions of a very high strain (12,13) and the observation that grain boundaries do not act as dominant nucleation sites (14). In view of the observed role of cooling rate on the transg&on kinetics it is proposed in the present investigation that the vacancy clusters or loops produced during quenching provide such suitable strain embryos. This effect can be expected to be more pronounced in coarse-grained material and at higher cooling rates. A final remark relates to the observation by LOM of a much higher fraction of surface martensite than expected Ii-om bulk measurements. This occured despite the fact that the specimens were only lightly electrolyticallyetched and not subjected to any plastic deformation in terms of grinding or polishing. It can be
Vol. 33, No. 9
IsoTHERMALMARTENslTE
1373
inferred from this observation that the specimen surface can be of importance in nucleating isothermal martensite. The faster cooling rate of the surface region may have an additional contribution to this effect. Conclusions
1. Isothermal marknsite formation showing C-curve kinetics was found to occur in the maraging steel 1RK9 1, the nose temperature being about -40°C. The kinetics was found to be enhanced for higher austenitizing
treatment temperatures, presumably through a combination of larger grain size and a larger number of quenched in nuclei for isothermal martensite transformation. 2. Experiments involving ditfercnt cooling rates showed that fast cooling enhanced the transformation kinetics. Based on this observation it is suggested that quenched-in vacancy clusters provide suitable strain embryos for isothermal martensite nucleation. Acknowledgements
This paper is published by permission of AB Sandvik Steel. Dr B. Berglund is thanked for provision of laboratory facilities. Valuable discussions with Mr A. Wilson and the technical assistance of colleagues at Sandvik Steel R&D Centre are gratefully acknowledged. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
S. Florem, Met. Rev., 13. p. 115 (1968). R. F. Decker end S. Floreen, Proc. Co& “Maraging steels-Recentdevelopmentsand applications”, Phomix, USA TMWAIME, p. l(1988). P. Liu, A H&in Stigenbergand J.-O. Nilsson, Scripta Metall. et Mater., 31, No. 3, p. 249 (1994). J.-O. Nilsson, A Hultin Stigenbergand P. Liu, Metall. and Mater. Trans. A 254 p. 2225 (1994). G. V. Kunljumov and 0. P. Maksimova, Dokl. Akad., Nauk SSSR, 61. p. 83 (1948). N. N. Tbadani and M. A Meyers, Prog. Mater. Sci., 30, p. l(l986). S. Kajiwara, Acta Metall., 32, p. 407 (1984). S. A Kulin and G. R Speich, Trans. AIME, 194, p. 258 (1952). C. H. Shih, B. L. Averbachand M. Cohen, Trans. AlME, 203, p. 183 (1955). 0. Hedebmt, JemkontoreisAM., 138 (lo), p. 643 (1954). R E. Smallman, Modem Physical Metallurgy, Butterwortbs,London, (1985). E. S. Ma&Iii and M. Cohen, Tram. AIME, 194, p. 489 (1952). S. Kajiwam, hlater. Tram., JIM, 33, p. 1027 (1992). S. R Pati and M. Cohen, Acta Metall., 17, p. 189 (1969).
Pergamon
et Mat&h,
Vol. 33, No. 9, pp. 1367-1373,1995 Elsevia Science Ltd Copright 0 1995 Acta Metallurgica Inc. PnntedintheUSA.AUri&ts-ed 0955716xm $9.50 + .cn
0956-716X(95)00386-X
ISOTHERMAL FORMATION OF MARTENSITE IN A 112Cr-9Ni-4Mo MARAGING STAINLESS STEEL M. Holrnquist’, J.-O. Nilsson and A. Hultin Stigenberg LDepartmentof Welding Research Department of Physical Metallurgy ‘Department of Wire Research Steel Research and Development AB Sandvik Steel S-81 1 81 Sandviken, Sweden (Received Jannary 26,1995) Introduction
The unusuaI combination of ultrahigh strength and ductility of maraging steels makes them very attractive in a large variety of applications (1,2). The maraging steel nsed in the present investigation, termed lRK9 1, is employed in the medical industry in products such as surgical needles. It is exceptional in the sense that agehardening is performed at 475 “C by thermodynamically stable qnasicrystalline precipitates of icosahedral symmetry, whereby a strength as high as 3000 MPa may be achieved (3,4). The present paper is concerned with the nature of the martensite,which provides the basis for the maraging treatment. Rather than forming mauensite during cooling, lRK9 1 develops martensite when held at a constant temperatum in a mnge ti-omroom temperature and below. The fact that the volume fraction of martensite can increase with time at a constant temperature was first observed by Kurdjumov and M&&nova in 1948 (5). To distingnishit from the more common cooling-inducedso called athermal martensite, this type of martensite has been termed isrotheunalmartensite (6). Generally, the isothermal reaction is observed to occur below the martensitic start temperature, N, but there are examples of martensite forming isothermally also above Iv&. While athermal martensite usually forms by the growth of already existing plates, isothermal martensite develops by the continual nucleation of new plates (7). Isothermal martensite formation is characterized by C-cnrve kinetics, the nose of which is usually at subzero temperatures. This has been verified in a rather wide variety of alloys from 1.6 % C steel with a nose temperature at -100°C (5) to more highly alloyed steels such as Fe-14.4% Cr-9% Ni with the nose at -30°C (8) and in Fe-23%INi-3.6%Mn with the nose at - 140°C (9). The purpose of the present investigation was to assess the kinetics of isothermal martensite formation in the maraging steel lRK91 and to determine the influence of physical parameters such as solution heat treatment and cooling rate. ExDerimental Procedure
Sandvik IRK91 was produced as wire ti-oma firll scale 7 ton melt and cold drawn from 5.5 to 4 mm diameter. The production was made according to normal production practice. The chemical composition of the steel grade is given in Table 1 below. Standardexperimental material was produced by anstenitizing at 1050°C for 1367
Vol. 33, No. 9
ISOTHERMAL MARTENSlTE
1368
TABLE 1 Chemical Composition of lFX91 (wt?h) Steel
C+N
Cr
h4n
Fe
Ni
MO
Ti
Al
Si
CU
Sandvik lRK91
co.05
12.0
0.3
bal.
9.0
4.0
0.9
0.30
0.15
2.0
5 min followed by quenching in water prior to testing. To study the infhtence of grain size, larger grains were produced by austenitizingat 1lOO”C,1150°C and 1200°C. The volume fraction of martensite was measured using two different techniques. One of these was the magnetic balance, which was used as a reference. This technique is based on the measurement of the force exerted on the specimen in an inhomogeneous magnetic field (10). However, the magnetic balance operates only at room temperature. To be able to measure n&en&e also at other temperatures the electric resistivity change was used as a measure of the evolution of martensite. This technique was made quantitative by calibrating against the magnetic balance at room temperature. The resistivity was measured from the potential drop along a 400 mm wire using a current of 1.OOOA, which produced no detectable heating. Isothermal transformation experiments at an arbitrary temperature in the range from room temperature down to - 100 “C were conducted in an insulated bath containing a mixture of liquid nitrogen and ethylene. The temperature was controlled to within *2 “C. The temperature dependence of resistivity was compensated for through the relation p(T)=p,(l+a(T-T,,)}, where p(T) and p0 are resistivities at temperature T and T, and ELis the temperature coefftcient experimentally determined in this investigation to be 7.923e10d K-r. The relative change in resistivity was assumed to be proportional to the volume fraction of magnetic phase in accordance with previous investigators (e.g. 8). The close correspondence between these two curves shown in Fig 1 confirms that this is a reasonable assumption. Room temperature hardness of material transformed in the range -100°C up to room temperature was measured using a Vickers type of testing at a load of 2 kg in a hardness testing machine of type Zwick 32 12. Tensile testing of 250 mm wire specimens was performed in an INSTRON 4505,100 kN testing machine. The interruption of the tran&brmationcaused by the very short time required to perform the hardness test was judged to be of no significance on the hardness results. Light optical microscopy (LOM) was performed in a Nikon Optiphot 66 to study isothermal development of surface marten&. Specimens for LOM were solutionheat treated in evacuated capsules to avoid disturbing
_Rel.
1
Figure 1. Volume percentage of m&en&e methods.
chmgc
of rcsirtivily
10
as a iimction of time indicating good agreement between magnetic balance and rcsistivity
ISOTHERMALMARTENSITE
Vol. 33, No. 9
IO
I
IO’
Iooo
100
1369
Time [min]
Figure 2. TlTdiagmmfor lRK91 au&n&edat 40% aud 60% by vohune of marteusite.
1050°C for 5 min and subsequently quenched in water showing TIT-curves
for 20%,
oxidation and subsequently air cooled to room temperature. Despite this, the surface was covered by a very thin oxide layer, which was removed by a gentle electrolytic etching prior to examination. Results
Both magnetic and resistivity measurements were found to give highly reproducible values as verified in repeated measurements. No effect of the magnetic field in the magnetic balance on the transformation behaviour was noted. Only isothermalmartensite was observed, no athermal martensite being observed when quenching to temperatures as low as -196°C. Isothermal martensite formation was investigated at the following temperatures: RT, 0°C -25”C, -40°C -60°C and -100°C. The volume fraction ofmartensite as a function of time was measumd and replotted in a TTT-diagram as shown in Fig 2, from which it is seen that the nose temperature is about -40°C. At this temperature 20% is formed during the Iirst 3 min, while the saturation percentage of 72% is reached afkr about 4h. For comparison, the saturation level at room temperature, 61?/0,requires a hold time of 24h.
0.1
I
IO
loo
loo0
loom
iouoooloooooo
Time [min]
Figure 3. Evolution of martemi teatroomtempemturea
attuxtionofaustenitizing~.
1370
Vol. 33, No. 9
ISOTHF,RMAL MARTENSITE
1
10
100
1000
Time to 40% magnetic phase [min] Figure 4. The time to reach 40% of martensite at room temperature for 4 dikkrent grain sizes.
Thegrainsizeswerefoundtobe23~,54~, 128pmand215pmafter5minheattreatmentat 105O”C, 1 lOO”C, 1150°C and 1200°C respectively. The evolution of martensite as a function of austenitizing heat treatment was measured at room temperature. These results are shown in Fig 3, where it can be seen that the transformation rate increases rapidly with grain size i.e. austenitizing temperature. The saturation level of marten& is vi&tally the same for all 4 heat treatments. If the results in Fig 3 are replotted, the time to attain 40% of martensite as a function of grain size can be presented as in Fig 4. The effect is so strong that the time required to form 40% martensite is found to diEer by more than one order of magnitude for the extreme values of grain size. There is also a pronounced effect of cooling rate. As shown in Fig 5 cooling from 1200 oC in air and water respectively resulted in significantdi&rences in transformation kinetics. While less than 10 min was required to form 40% of marknsite in the water-quenched condition, more than 100 min was required in the air cooled condition. An almost equally strong effect of cooling rate was found also after austenitizing at 1050 ’ C. Hardness values were also plotted as in a TTT-diagram (see Fig 6), the curves corresponding to isohardness values of 2 10,240,270 and 300 HV. As in the case of martensite transformation rate the nose temperature is about -40°C. Tensile testing of room temperature transformed material showed an evolution with time similar to hardness and volume traction of martensite as shown in Table 2. The elongation was inversely related to strength. It was possible to follow the isothermalformation of martensite through continual observations in the LOM as illustratedin Figs 7 ad. However, it should be pointed out that the amount of surface martensite was much
0
20 0
IO 0.1
0
.
12OO*CI5 minRI20
0
12WUS
minhir
d I
10
loo
low
loo00
IcoooOIowwa
Time [min]
Figure 5. Evolution of martensite at room tenperature obtained after heat treatment at 1200 “C for water and air cooling.
ISOTHERMALMARTENSITE
Vol. 33, No. 9
-200
,-fl 0.1
1371
n
10 Time [m$
I
Icoo
IO”
evolution refle.ctedas hardness in terms of isohardness curves ranging from 2 10 HV to 300HV.
Figure 6. hktensite
higher than expected horn the magnetic measurements of the bulk material, implying that several attempts were made before imaging of martensite events became successful.
The fact that the volume fraction of martensite obtained by the magnetic balance. can be translated to an electric signal enables us to quantify martensite transformation contimtally also at other temperatures than room temperature as verified in Fig 1. An important conclusion to be drawn corn the experiments is that, if there is an M, temperature, it must be below -196°C. Hence, above this temperature there is no athermal m&en&e that obscures the interpretation of isothermal martensite transformation. The nose temperature, i.e. the temperature at which the transformation kinetics is fastest, is shown to be about -40°C. As shown in Fig 2 the tran&oimationbehaviour follows C-curve kinetics, as expected Tom previous work (9). It is interesting that this transformation is reflected in terms of iso-hardness curves resembling the C-curves in Fig 2 and a nose temperature at -40°C. This, in combination with mechanical results in Table 2, shows that there is an intimate relation. between isothermal martensite formation and mechanical properties.
TABLE 2 Results from Tensile Testing Time (tin>
3
8
26
50
80
120
145
210
216
300
311
4185
10172
Yield
222
228
238
249
274
272
311
328
373
364
333
492
528
Tensile strength @@a)
719
725
726
734
756
738
780
767
792
806
796
893
895
Elongation (%)
27
28
27
26
24
23
24
21
22
20
19
15
15
gay
ISOTHERMAL, MARTENSITE
1372
Figure 7. a-d: h4im min.
ofbthemal
Vol. 33, No. 9
ma&mite shown by arrows formed succes.sively at room temperature at 10,70,130
and 190
As shown in Fig 3 there is a pronounced effect of austenitizing temperature on the kinetics of martensite transformation. A crucial question is whether this is caused by the physical size of the grains themselves or if the higher temperature to obtain the larger grain size results in a higher concentration of quenched-in stresses, presumably in the form of supersaturation of vacancies that provide suitable sites for the nuclation of martensite embryos. Assuming an energy of formation of 2.1 eV quoted for Fe, it can be shown that the concentration of vacancies in equilibrium at 1200°C is more than 5 times higher than at 1050°C (11). The role of quenched-in vacancies can in fact be appreciated from Fig 5, where it is shown that, for a given grain size of 215 pm, water-quenchingenhances the kinetics of martensite transformation relative to air-cooling by at least one order of magnitude. Because of the difticulty to quantify the vacancy concentration in quenched material in non-equilibrium at room temperature it was not possible to estimate the relative contributions of grain size and vacancies. However, the proposed me&an&m by which isothermal martensite embryos nucleate continually rather than by growth of already transformed martensite plates (12), emphasizes the importance of vacancies. This is also consistent with the view that the isothermal nucleation process is activated by thermal fluctuations superimposed on regions of a very high strain (12,13) and the observation that grain boundaries do not act as dominant nucleation sites (14). In view of the observed role of cooling rate on the transg&on kinetics it is proposed in the present investigation that the vacancy clusters or loops produced during quenching provide such suitable strain embryos. This effect can be expected to be more pronounced in coarse-grained material and at higher cooling rates. A final remark relates to the observation by LOM of a much higher fraction of surface martensite than expected Ii-om bulk measurements. This occured despite the fact that the specimens were only lightly electrolyticallyetched and not subjected to any plastic deformation in terms of grinding or polishing. It can be
Vol. 33, No. 9
IsoTHERMALMARTENslTE
1373
inferred from this observation that the specimen surface can be of importance in nucleating isothermal martensite. The faster cooling rate of the surface region may have an additional contribution to this effect. Conclusions
1. Isothermal marknsite formation showing C-curve kinetics was found to occur in the maraging steel 1RK9 1, the nose temperature being about -40°C. The kinetics was found to be enhanced for higher austenitizing
treatment temperatures, presumably through a combination of larger grain size and a larger number of quenched in nuclei for isothermal martensite transformation. 2. Experiments involving ditfercnt cooling rates showed that fast cooling enhanced the transformation kinetics. Based on this observation it is suggested that quenched-in vacancy clusters provide suitable strain embryos for isothermal martensite nucleation. Acknowledgements
This paper is published by permission of AB Sandvik Steel. Dr B. Berglund is thanked for provision of laboratory facilities. Valuable discussions with Mr A. Wilson and the technical assistance of colleagues at Sandvik Steel R&D Centre are gratefully acknowledged. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
S. Florem, Met. Rev., 13. p. 115 (1968). R. F. Decker end S. Floreen, Proc. Co& “Maraging steels-Recentdevelopmentsand applications”, Phomix, USA TMWAIME, p. l(1988). P. Liu, A H&in Stigenbergand J.-O. Nilsson, Scripta Metall. et Mater., 31, No. 3, p. 249 (1994). J.-O. Nilsson, A Hultin Stigenbergand P. Liu, Metall. and Mater. Trans. A 254 p. 2225 (1994). G. V. Kunljumov and 0. P. Maksimova, Dokl. Akad., Nauk SSSR, 61. p. 83 (1948). N. N. Tbadani and M. A Meyers, Prog. Mater. Sci., 30, p. l(l986). S. Kajiwara, Acta Metall., 32, p. 407 (1984). S. A Kulin and G. R Speich, Trans. AIME, 194, p. 258 (1952). C. H. Shih, B. L. Averbachand M. Cohen, Trans. AlME, 203, p. 183 (1955). 0. Hedebmt, JemkontoreisAM., 138 (lo), p. 643 (1954). R E. Smallman, Modem Physical Metallurgy, Butterwortbs,London, (1985). E. S. Ma&Iii and M. Cohen, Tram. AIME, 194, p. 489 (1952). S. Kajiwam, hlater. Tram., JIM, 33, p. 1027 (1992). S. R Pati and M. Cohen, Acta Metall., 17, p. 189 (1969).