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Machine-induced phase transformation in a maraging steel
Materials Science and Engineering, A159 (1992) 261-265
261
Machine-induced phase transformation in a maraging steel F. Habiby, T. N. Siddiqui, H. Hussain, M. A. Khan, A. ul H a q a n d A. Q. K h a n Metallurgy Division, Dr. A. Q. Khan Research Laboratories, Kahuta, P.O. Box 502, Rawalpindi (Pakistan) (Received June 4, 1992)
Abstract Phase transformation, i.e. martensite to austenite (a-7) reversion, is detected in grade 350 maraging steel as a result of machining processes. The thermal stability of machine-induced austenite is found to be higher than that of the thermally produced austenite. The machined samples, when ausaged with and without prior solution annealing, exhibit different amounts of austenite formation at same ausaging temperatures. The austenite start temperature for machined samples is found to be lower than for the unmachined samples. Texture measurements of machine-induced and thermally produced austenites are also reported.
1. Introduction One of the many advantages of maraging steel is its very low dimensional changes during age hardening which makes machining possible to the designed finish tolerances in the annealed condition. During the manufacturing of precision engineering components, a series of complex machining processes are involved. These processes include considerable plastic deformation and friction between surfaces. This may not only increase the defect density but also alter the structural morphology and may produce new crystallographic phases, thereby influencing its physical and mechanical properties [1, 2]. In hysteresis of motor components such as rotors, the electrical and magnetic properties are of great significance. We are considering 18% Ni maraging steel, grade 350, as a potential material for such an application. This steel, when ausaged in the temperature range between 570 and 750 °C, exhibits the formation of austenite phase up to 50% by volume [3], and correspondingly the coercive force increases from 29 to 140 Oe depending on the amount of austenite present [4]. Thus, by suitable heat treatment, relative amounts of austenite and martensite phases can be varied to achieve the desired electrical and magnetic properties in this material. We have earlier reported machine-induced austenite for the first time in grade 350 18% Ni maraging steel [2]. The presence of machine-induced austenite may obviously be expected to alter the electrical and magnetic properties significantly. Eddy current values give qualitative information about the structural changes in 0921-5093/92/$5.00
a given sample. These values could be determined quickly and therefore may be used as a first qualitative check for austenite phase determination before quantitative X-ray analysis is undertaken. In the presence of machine-induced austenite, an important and critical aspect is the heat treatment of such components. It must be emphasized that, in the presence of machineinduced austenite, the heat treatment (to obtain the desired ratio of austenite and martensite phases) has to be modified compared with the usual heat treatment, which is no longer applicable and may lead to rejection of the components. It is therefore of practical importance to investigate the structural phase transitions induced in maraging steels by the operation of machining and plastic deformation. In this paper we report reverse martensite to austenite phase transformation in 18% Ni maraging steel, grade 350, due to machining. The effect of the heat treatment and thermal stability of machineinduced austenite is also discussed. Textures of machine-induced and thermally produced austenite are also measured.
2. Materialand methods The material used in this investigation is 18% Ni maraging steel, grade 350, whose chemical composition is given in Table 1. The as-received material was solution annealed in the form of extruded bars. In the manufacture of rotors, 10 mm thick pieces were cut from the as-received bars, on heavy duty high speed cutting machines. Rough machining of semifinished © 1992 - Elsevier Sequoia. All rights reserved
F. Habiby et al.
262
/
Phase transformation in a maraging steel
TABLE 1. Chemical composition of 18% Ni maraging steel, grade 350 Ni 18.14
Co 12.5
Mo 3.96
Ti 1.65
C
S
0.02
0.01
AI 0.1
Fe Balance
shape was carried out on a lath machine and final machining was performed on a CNC machine to obtain the finished product which was 4.5 mm thick. In both cases, machining parameters were as follows: cutting speed, 22 m min-t; feed, 0.2 mm; depth of cut, 0.2 mm; rake angle, 5°; coolant, cutting oil soluble in water. The finished components were checked by eddy current against a standard. This standard is a rotor made from same material in the 100% martensitic structure. For eddy current measurements Dr. Foster's D E F E C T O S C O P type S-2830 equipment was used [2]. Heat treatment was carried out in a vacuum furnace and controlled cooling was carried out under an argon atmosphere. Quantitative analysis of constituent phases was carried out by X-ray diffractometry using Co K a radiation. The distribution of machineinduced and thermally produced austenite is compared with the help of texture measurements. For this reason, the orientation density of the austenite phase was studied by pole figure measurements up to 70 °, obtained from the planes of the specimens recorded at 5° spiral pitch. The Vickers hardness was measured at a 30 kgf load.
3.
Results
and discussion
Figure 1, spectrum a, shows the X-ray diffraction of the as-received sample indicating only the b.c.c, martensite structure before machining. The same samples after the above-mentioned machining operations exhibit 15% austenite on the surface (Fig. 1, spectrum b). The amounts of austenite on the surface and in the mid-thickness were the same. The formation of machine-induced austenite is supposed to be a combined effect of machining parameters such as cutting speed, tool geometry, tool material, and flow rate of coolant during machining which may alter the microstructure. Research continues to calculate the stress levels in the materials at each step of machining to pinpoint the exact conditions under which transformation from b.c.c, to f.c.c, takes place. However, it has been checked to a great degree of accuracy by closely monitoring the structural state of components at every step of the machining process that the phase transformation from b.c.c, to f.c.c, during machining does occur in this
•-II
+
+
+
v.
~
~.
• 44
t
,
|IA
M.F
20 ~o.)
Fig. I. X-ray diffractograms of grade 350 maraging steel in the annealed condition before machining (spectrum a), after machining showing 15% austenite (spectrum b) and after annealing at 820 °C for 1 h, after which some austenite is still present in the sample (spectrum c).
material. Furthermore, the reproducibility of the results has also been confirmed by repeated experiments. It is worth mentioning that although the machined samples contain 15% austenite their hardness value was of the order of 340 HV, i.e. almost the same value as before they were machined. The samples in which 15% austenite is produced by thermal treatment possess a hardness value of 600 HV [4]. Thus it may be concluded that the anstenite produced by machining is not due to increase in temperature• Furthermore, it may also be concluded that both austenite and martensite phases in this material have the same order of hardness. These findings are in confirmation of our previous work [4]. The eddy current values of machined components were found to vary in the range from 40 to 80 arbitrary units. X-ray diffraction analysis revealed that the eddy current values increased as the amount of machineinduced austenite decreased and, at a value of 80, no austenite was detected and the structure contained 100% martensite. Similarly, 20 vol.% austenite determined quantitatively by X-ray diffraction corresponds to eddy current values of 40 arbitrary units. On the basis of results for thousands of such components, the values of eddy current are related to the amount of austenite as depicted in Fig. 2. The trend plotted in Fig. 2 is a result of a large number of observations which have shown that the electrical and magnetic properties of maraging steel are greatly influenced by the presence of a dual phase structure [4]. The present authors are not aware of any published work describing martensitic reverse transformation in maraging steels due to machining or any other mode of
F. Habiby et al.
/
263
Phase transformation in a maraging steel
cold working. However, a number of workers have shown that the b.c.c, a-Fe transforms to a close-packed f.c.c, phase after deformation under pressure [5, 6]. Also, there is evidence [7] that deformation of Fe-Ni alloys under pressure leads to the transformation of their b.c.c, phase into an f.c.c, phase. The abovementioned phase transformations from b.c.c, to f.c.c, in a-Fe and Fe-Ni alloys are caused by quasi-hydrostatic pressure which involves a shear mechanism and produces a typical microstructure of small fragments of irregular austenite plates [8]. Bowden and Kelly [9] have suggested that, in pressure-induced conditions, the a-martensite in Fe-Ni alloys undergoes a transformation to the 7 phase on the basis of the resultant shape strain values. A diffusion-free b.c.c, to f.c.c, shear transformation can be realized by means of hydrostatic pressure which stabilizes the f.c.c, phase [10-12]. However, in contrast to the above examples, in 18% Ni maraging steel, grade 350, the retained austenite reported in the literature is formed under high temperature aging at 550-750 °C by precipitate reversion and matrix reversion mechanisms [13-15]. It is highly unlikely that machine-induced austenite observed in this study is due to precipitate or matrix reversion mechanisms as the working temperature during the machining processes adopted in this study never approaches critical values where phase transformation is expected. It may therefore be speculated that machine-induced austenite forms by a shear mechanism similar to what is observed in pressureinduced transformation in Fe-Ni [7] and a-Fe [5, 6]. It is therefore possible to have two types of austenite, i.e. machine-induced austenite and thermally produced austenite [16]. It is also of practical interest to study the crystallographic nature and its orientation relationship with the martensite matrix and its mechanical and thermal stability as it will have a pronounced effect on the electrical and magnetic properties of the material. The thermal stability of machine-induced austenite is also found to be different from that of the thermally produced austenite [4]. The former was found to be stable up to 850 °C while the latter is dissolved at 790 °C. The X-ray diffractogram (Fig. 1, spectrum c) revealed retention of machine-induced austenite even after annealing at 820 °C. A higher annealing temperature of the order of 850 °C was needed to obtain 100% martensite at room temperature. The austenite start (AS) temperature in machined samples after annealing at 850 °C also shifts to a lower temperature, i.e. 450 °C as compared with the usually observed value of 570 °C. Rohde and Graham [17] also observed a decrease in AS temperature in Fe-28.4at.%Ni-0.4at.%C in pressure-induced samples. Their interpretation was based on a thermodynamical model wherein the hydrostatic pressure interacted with the volume change that
100
!o 4O
I
0
I
!
10 AUSTENITE
I
I
11 %
20
26
Fig. 2. Machine-induced austenite as a function of eddy current. 800 700
80
HARDNESS
o
7O
4-
Z I00
~
eo~e
600
7~400
otu
Q
•I - 2oo IO0
10 I
I
I
I
I
I
I
I
410 450 490 630 570 810 Q60 690 7 3 0
I
I
I
0 1'7'0 810 850 890
TEMPERATURE *C
Fig. 3. Vickers' hardness and austenite percentage as a function of ausaging temperature in the as-machined condition.
accompanied the martensite-to-austenite transition. Kaufman et al. [18] also observed a decrease in AS temperature with hydrostatic pressure in Fe-9.Sat.%Ni alloy which could be predicted from a similar thermodynamic model. The ausaging behaviour of machined samples without prior annealing is shown in Fig. 3. This figure depicts the changes in austenite percentage and hardness values as a function of ausaging temperature. The amount of austenite appears to increase above 15% in machined samples as the ausaging temperature is increased above 500 °C and attains a value of 70% around 700 °C. A further increase in the ausaging temperature leads to a decrease in the austenite percentage and no austenite is detected after annealing at 850 °C. It is interesting to mention that the aging temperature of the machined samples has also shifted towards the higher side and the peak hardness was obtained at 550 °C. As mentioned above, the austenite in the machined samples may be removed by solution annealing at 850 °C. The ausaging behaviour of such a sample is shown in Fig. 4. In this case austenite formation starts
264
F. Habiby et al.
soo
/
Phase transformation in a maraging steel eo
L
D
coo
~'200
-.
•
o ' percentage o Fig. 4. Vickers' hardness and austenite as a function
~~., ~
of ausaging temperature after annealing at 850 °C for 1 h in the as-machined condition.
(a)
above 450 °C and increases very slowly up to 610 °C and registers a value of 15% at this level. A further increase in ausaging temperature leads to a rapid increase in austenite. At 690 °C a maximum value of 55% is obtained. Further increases in ausaging temperature decrease the austenite percentage. However, a hump or secondary peak is clearly visible near 750 °C in machined samples when ausaged after solution annealing (Fig. 4) which is not observed in machined plus ausaged samples (Fig. 3). The observation of a secondary peak is in agreement with previously published results [19] which have shown that the highest obtainable coercive force coincides with the secondary peak. Further ausaging leads to a gradual decrease in austenite percentage and, at 850 °C, no austenite is detected. The hardness in this material increases with increase in percentage austenite up to 540 °C (Figs. 3 and 4). This increase in hardness is mainly due to the hardening precipitates that form during aging. The drop of hardness corresponds to the dissolution of these precipitates, and the lowest hardness corresponds to the maximum austenite formed as a result of precipitate reversion. This is in agreement with other published work [13, 14]. The textures of machine-induced austenite and similar amounts of austenite produced by thermal treatment were compared by pole figure measurements. Both the austenites exhibit similar texture (Fig. 5). On the basis of texture data it can be said that thermally produced austenite forms on the same sites as the machine-induced austenite. Although no published data on texture of this nature are available, there is evidence that the reverted austenite grain boundaries exactly correspond to the prior austenite grain boundaries [16]. The texture results presented (Fig. 5) support the findings of Maki et al. [20]. However, it cannot be ruled out that the machine-induced austenite and
,tj I
--"
410 460 490 630 670 1110 660 690 730 T?O 810 850 8gO
TEMPERATURE °C
,.,
~'~ ~.'f
,'~,, ' "-~'/-~
L D ~ , - - ' - " - - - ] ~
:/~))~vff" ° ~
~ .
'
Fig. 5. (200) pole figures of (a) machine-induced austenite and (b) thermally produced austenite. thermally produced austenite have a different orientation relationship with the parent martensitic matrix, which is a subject of continuing research.
4. Conclusions The thermal stability of machine-induced austenite is higher (850 °C) than that of the thermally produced austenite (790 °C). The AS and austenite finish temperatures are also affected by the machining process. Similar textures are obtained in the case of machineinduced austenite and thermally produced austenite, indicating that the thermally produced austenite forms on similar sites to the machine-induced austenite. The reverse martensite-to-austenite transformation in 18% Ni maraging steel is most probably a result of the complex stress-strain configuration during machining processes. However, further investigations are
F. Habiby et al.
/
Phase transformation in a maraging steel
u n d e r w a y to p i n p o i n t t h e e x a c t m a c h i n i n g c o n d i t i o n s for such a transformation.
References
8
9 10 ll
1 M. A. Smirnov, V. G. Pushin, R. R. Romanova, N. T. Kaareva, L. V. Smirnov, D. P. Rodionov and E. S. Atroschchenko, Phys. Met. Metall., 51 (1981) 121. 2 T. N. Siddiqui, M. A. Khan, A. ul Haq and A. Q. Khan, Proc. Ist Int. Syrup. on Advanced Materials, Pakistan, 1989, A. Q. Khan Res. Lab., Rawalpindi, 1990, p. 198. 3 A. ul Haq, E H. Hashmi and A. Q. Khan, Met. Mater., I (1985)22. 4 M. Farooq, F. Habiby, A. ul Haq, F. H. Hashmi and A. Q. Khan, Proc. ICOMA T, Japan, 1986, Jpn. Inst. Metals, 1986, p. 72. 5 T. Takahasi and W. A. Bassett, Science, 145 (3631)(1964) 483. 6 B. A. Bilby and J. W. Christian, The Mechanism of Phase Transformation in Metals, Institute of Metals, London, 1956, p. 121. 7 V. A. Teploc, V. E Pilyugin, R. I. Kuznetsov, D. I. Tupitsa,
12 13 14 15 16 17 18 19 20
265
V. A. Shabashov and V. M. Gundyrov, Phys. Met. Metall., 64 (1987)83. V.D. Sadovskiy, V. A. Teplov, D. I. Tupitsa, K. M. Demchuk, R. I. Kuznetsov, A. N. Martemyanov and L. N. Romashev, Phys. Met. Metall., 56 (1983) 775. H.G. Bowden and E M. Kelly, Acta Metall., 15 (1967) 1489. E M. Kelly, Metallurgical Developments in High Alloy Steels, ISlSpec. Rep. No. 86, 1964, p. 146 (Iron and Steel Institute). E A. Allen and C. C. Early, J. Iron Steel Inst., 166 (1950) 281. L.E. Pope and L. R. Edwards, Acta Metall., 21 (1973) 2 8 l . . R. E Decker, J. T. Eash and A. J. Goldman, Trans. ASM, 55 (1962) 58. S. Floreen, Met. Rev., 13(1968) 115. F. Habiby, A. ul Haq, E H. Hashmi and A. Q. Khan, Proc. ICOMA T, Japan, 1986, p. 560. E Habiby, A. ul Haq and A. Q. Khan, unpublished work, 1991. R. W. Rohde and R. A. Graham, Trans. Metall. Soc. AIME, 245 (1969) 2441. L. Kaufman, A. Legenaar and J. S. Harvey, Progress in Very High Pressure Research, Wiley, New York, 1960, p. 90. M. Ahmed, M. Farooq, E H. Hashmi and A. Q. Khan, Proc. 4th Asia Pacific Conf. on EM, July 1988, to be published. T. Maki, H. Morimoto and I. Tamura, Trans. Iron Steel Inst. Jpn., 20 (1980) 700.
261
Machine-induced phase transformation in a maraging steel F. Habiby, T. N. Siddiqui, H. Hussain, M. A. Khan, A. ul H a q a n d A. Q. K h a n Metallurgy Division, Dr. A. Q. Khan Research Laboratories, Kahuta, P.O. Box 502, Rawalpindi (Pakistan) (Received June 4, 1992)
Abstract Phase transformation, i.e. martensite to austenite (a-7) reversion, is detected in grade 350 maraging steel as a result of machining processes. The thermal stability of machine-induced austenite is found to be higher than that of the thermally produced austenite. The machined samples, when ausaged with and without prior solution annealing, exhibit different amounts of austenite formation at same ausaging temperatures. The austenite start temperature for machined samples is found to be lower than for the unmachined samples. Texture measurements of machine-induced and thermally produced austenites are also reported.
1. Introduction One of the many advantages of maraging steel is its very low dimensional changes during age hardening which makes machining possible to the designed finish tolerances in the annealed condition. During the manufacturing of precision engineering components, a series of complex machining processes are involved. These processes include considerable plastic deformation and friction between surfaces. This may not only increase the defect density but also alter the structural morphology and may produce new crystallographic phases, thereby influencing its physical and mechanical properties [1, 2]. In hysteresis of motor components such as rotors, the electrical and magnetic properties are of great significance. We are considering 18% Ni maraging steel, grade 350, as a potential material for such an application. This steel, when ausaged in the temperature range between 570 and 750 °C, exhibits the formation of austenite phase up to 50% by volume [3], and correspondingly the coercive force increases from 29 to 140 Oe depending on the amount of austenite present [4]. Thus, by suitable heat treatment, relative amounts of austenite and martensite phases can be varied to achieve the desired electrical and magnetic properties in this material. We have earlier reported machine-induced austenite for the first time in grade 350 18% Ni maraging steel [2]. The presence of machine-induced austenite may obviously be expected to alter the electrical and magnetic properties significantly. Eddy current values give qualitative information about the structural changes in 0921-5093/92/$5.00
a given sample. These values could be determined quickly and therefore may be used as a first qualitative check for austenite phase determination before quantitative X-ray analysis is undertaken. In the presence of machine-induced austenite, an important and critical aspect is the heat treatment of such components. It must be emphasized that, in the presence of machineinduced austenite, the heat treatment (to obtain the desired ratio of austenite and martensite phases) has to be modified compared with the usual heat treatment, which is no longer applicable and may lead to rejection of the components. It is therefore of practical importance to investigate the structural phase transitions induced in maraging steels by the operation of machining and plastic deformation. In this paper we report reverse martensite to austenite phase transformation in 18% Ni maraging steel, grade 350, due to machining. The effect of the heat treatment and thermal stability of machineinduced austenite is also discussed. Textures of machine-induced and thermally produced austenite are also measured.
2. Materialand methods The material used in this investigation is 18% Ni maraging steel, grade 350, whose chemical composition is given in Table 1. The as-received material was solution annealed in the form of extruded bars. In the manufacture of rotors, 10 mm thick pieces were cut from the as-received bars, on heavy duty high speed cutting machines. Rough machining of semifinished © 1992 - Elsevier Sequoia. All rights reserved
F. Habiby et al.
262
/
Phase transformation in a maraging steel
TABLE 1. Chemical composition of 18% Ni maraging steel, grade 350 Ni 18.14
Co 12.5
Mo 3.96
Ti 1.65
C
S
0.02
0.01
AI 0.1
Fe Balance
shape was carried out on a lath machine and final machining was performed on a CNC machine to obtain the finished product which was 4.5 mm thick. In both cases, machining parameters were as follows: cutting speed, 22 m min-t; feed, 0.2 mm; depth of cut, 0.2 mm; rake angle, 5°; coolant, cutting oil soluble in water. The finished components were checked by eddy current against a standard. This standard is a rotor made from same material in the 100% martensitic structure. For eddy current measurements Dr. Foster's D E F E C T O S C O P type S-2830 equipment was used [2]. Heat treatment was carried out in a vacuum furnace and controlled cooling was carried out under an argon atmosphere. Quantitative analysis of constituent phases was carried out by X-ray diffractometry using Co K a radiation. The distribution of machineinduced and thermally produced austenite is compared with the help of texture measurements. For this reason, the orientation density of the austenite phase was studied by pole figure measurements up to 70 °, obtained from the planes of the specimens recorded at 5° spiral pitch. The Vickers hardness was measured at a 30 kgf load.
3.
Results
and discussion
Figure 1, spectrum a, shows the X-ray diffraction of the as-received sample indicating only the b.c.c, martensite structure before machining. The same samples after the above-mentioned machining operations exhibit 15% austenite on the surface (Fig. 1, spectrum b). The amounts of austenite on the surface and in the mid-thickness were the same. The formation of machine-induced austenite is supposed to be a combined effect of machining parameters such as cutting speed, tool geometry, tool material, and flow rate of coolant during machining which may alter the microstructure. Research continues to calculate the stress levels in the materials at each step of machining to pinpoint the exact conditions under which transformation from b.c.c, to f.c.c, takes place. However, it has been checked to a great degree of accuracy by closely monitoring the structural state of components at every step of the machining process that the phase transformation from b.c.c, to f.c.c, during machining does occur in this
•-II
+
+
+
v.
~
~.
• 44
t
,
|IA
M.F
20 ~o.)
Fig. I. X-ray diffractograms of grade 350 maraging steel in the annealed condition before machining (spectrum a), after machining showing 15% austenite (spectrum b) and after annealing at 820 °C for 1 h, after which some austenite is still present in the sample (spectrum c).
material. Furthermore, the reproducibility of the results has also been confirmed by repeated experiments. It is worth mentioning that although the machined samples contain 15% austenite their hardness value was of the order of 340 HV, i.e. almost the same value as before they were machined. The samples in which 15% austenite is produced by thermal treatment possess a hardness value of 600 HV [4]. Thus it may be concluded that the anstenite produced by machining is not due to increase in temperature• Furthermore, it may also be concluded that both austenite and martensite phases in this material have the same order of hardness. These findings are in confirmation of our previous work [4]. The eddy current values of machined components were found to vary in the range from 40 to 80 arbitrary units. X-ray diffraction analysis revealed that the eddy current values increased as the amount of machineinduced austenite decreased and, at a value of 80, no austenite was detected and the structure contained 100% martensite. Similarly, 20 vol.% austenite determined quantitatively by X-ray diffraction corresponds to eddy current values of 40 arbitrary units. On the basis of results for thousands of such components, the values of eddy current are related to the amount of austenite as depicted in Fig. 2. The trend plotted in Fig. 2 is a result of a large number of observations which have shown that the electrical and magnetic properties of maraging steel are greatly influenced by the presence of a dual phase structure [4]. The present authors are not aware of any published work describing martensitic reverse transformation in maraging steels due to machining or any other mode of
F. Habiby et al.
/
263
Phase transformation in a maraging steel
cold working. However, a number of workers have shown that the b.c.c, a-Fe transforms to a close-packed f.c.c, phase after deformation under pressure [5, 6]. Also, there is evidence [7] that deformation of Fe-Ni alloys under pressure leads to the transformation of their b.c.c, phase into an f.c.c, phase. The abovementioned phase transformations from b.c.c, to f.c.c, in a-Fe and Fe-Ni alloys are caused by quasi-hydrostatic pressure which involves a shear mechanism and produces a typical microstructure of small fragments of irregular austenite plates [8]. Bowden and Kelly [9] have suggested that, in pressure-induced conditions, the a-martensite in Fe-Ni alloys undergoes a transformation to the 7 phase on the basis of the resultant shape strain values. A diffusion-free b.c.c, to f.c.c, shear transformation can be realized by means of hydrostatic pressure which stabilizes the f.c.c, phase [10-12]. However, in contrast to the above examples, in 18% Ni maraging steel, grade 350, the retained austenite reported in the literature is formed under high temperature aging at 550-750 °C by precipitate reversion and matrix reversion mechanisms [13-15]. It is highly unlikely that machine-induced austenite observed in this study is due to precipitate or matrix reversion mechanisms as the working temperature during the machining processes adopted in this study never approaches critical values where phase transformation is expected. It may therefore be speculated that machine-induced austenite forms by a shear mechanism similar to what is observed in pressureinduced transformation in Fe-Ni [7] and a-Fe [5, 6]. It is therefore possible to have two types of austenite, i.e. machine-induced austenite and thermally produced austenite [16]. It is also of practical interest to study the crystallographic nature and its orientation relationship with the martensite matrix and its mechanical and thermal stability as it will have a pronounced effect on the electrical and magnetic properties of the material. The thermal stability of machine-induced austenite is also found to be different from that of the thermally produced austenite [4]. The former was found to be stable up to 850 °C while the latter is dissolved at 790 °C. The X-ray diffractogram (Fig. 1, spectrum c) revealed retention of machine-induced austenite even after annealing at 820 °C. A higher annealing temperature of the order of 850 °C was needed to obtain 100% martensite at room temperature. The austenite start (AS) temperature in machined samples after annealing at 850 °C also shifts to a lower temperature, i.e. 450 °C as compared with the usually observed value of 570 °C. Rohde and Graham [17] also observed a decrease in AS temperature in Fe-28.4at.%Ni-0.4at.%C in pressure-induced samples. Their interpretation was based on a thermodynamical model wherein the hydrostatic pressure interacted with the volume change that
100
!o 4O
I
0
I
!
10 AUSTENITE
I
I
11 %
20
26
Fig. 2. Machine-induced austenite as a function of eddy current. 800 700
80
HARDNESS
o
7O
4-
Z I00
~
eo~e
600
7~400
otu
Q
•I - 2oo IO0
10 I
I
I
I
I
I
I
I
410 450 490 630 570 810 Q60 690 7 3 0
I
I
I
0 1'7'0 810 850 890
TEMPERATURE *C
Fig. 3. Vickers' hardness and austenite percentage as a function of ausaging temperature in the as-machined condition.
accompanied the martensite-to-austenite transition. Kaufman et al. [18] also observed a decrease in AS temperature with hydrostatic pressure in Fe-9.Sat.%Ni alloy which could be predicted from a similar thermodynamic model. The ausaging behaviour of machined samples without prior annealing is shown in Fig. 3. This figure depicts the changes in austenite percentage and hardness values as a function of ausaging temperature. The amount of austenite appears to increase above 15% in machined samples as the ausaging temperature is increased above 500 °C and attains a value of 70% around 700 °C. A further increase in the ausaging temperature leads to a decrease in the austenite percentage and no austenite is detected after annealing at 850 °C. It is interesting to mention that the aging temperature of the machined samples has also shifted towards the higher side and the peak hardness was obtained at 550 °C. As mentioned above, the austenite in the machined samples may be removed by solution annealing at 850 °C. The ausaging behaviour of such a sample is shown in Fig. 4. In this case austenite formation starts
264
F. Habiby et al.
soo
/
Phase transformation in a maraging steel eo
L
D
coo
~'200
-.
•
o ' percentage o Fig. 4. Vickers' hardness and austenite as a function
~~., ~
of ausaging temperature after annealing at 850 °C for 1 h in the as-machined condition.
(a)
above 450 °C and increases very slowly up to 610 °C and registers a value of 15% at this level. A further increase in ausaging temperature leads to a rapid increase in austenite. At 690 °C a maximum value of 55% is obtained. Further increases in ausaging temperature decrease the austenite percentage. However, a hump or secondary peak is clearly visible near 750 °C in machined samples when ausaged after solution annealing (Fig. 4) which is not observed in machined plus ausaged samples (Fig. 3). The observation of a secondary peak is in agreement with previously published results [19] which have shown that the highest obtainable coercive force coincides with the secondary peak. Further ausaging leads to a gradual decrease in austenite percentage and, at 850 °C, no austenite is detected. The hardness in this material increases with increase in percentage austenite up to 540 °C (Figs. 3 and 4). This increase in hardness is mainly due to the hardening precipitates that form during aging. The drop of hardness corresponds to the dissolution of these precipitates, and the lowest hardness corresponds to the maximum austenite formed as a result of precipitate reversion. This is in agreement with other published work [13, 14]. The textures of machine-induced austenite and similar amounts of austenite produced by thermal treatment were compared by pole figure measurements. Both the austenites exhibit similar texture (Fig. 5). On the basis of texture data it can be said that thermally produced austenite forms on the same sites as the machine-induced austenite. Although no published data on texture of this nature are available, there is evidence that the reverted austenite grain boundaries exactly correspond to the prior austenite grain boundaries [16]. The texture results presented (Fig. 5) support the findings of Maki et al. [20]. However, it cannot be ruled out that the machine-induced austenite and
,tj I
--"
410 460 490 630 670 1110 660 690 730 T?O 810 850 8gO
TEMPERATURE °C
,.,
~'~ ~.'f
,'~,, ' "-~'/-~
L D ~ , - - ' - " - - - ] ~
:/~))~vff" ° ~
~ .
'
Fig. 5. (200) pole figures of (a) machine-induced austenite and (b) thermally produced austenite. thermally produced austenite have a different orientation relationship with the parent martensitic matrix, which is a subject of continuing research.
4. Conclusions The thermal stability of machine-induced austenite is higher (850 °C) than that of the thermally produced austenite (790 °C). The AS and austenite finish temperatures are also affected by the machining process. Similar textures are obtained in the case of machineinduced austenite and thermally produced austenite, indicating that the thermally produced austenite forms on similar sites to the machine-induced austenite. The reverse martensite-to-austenite transformation in 18% Ni maraging steel is most probably a result of the complex stress-strain configuration during machining processes. However, further investigations are
F. Habiby et al.
/
Phase transformation in a maraging steel
u n d e r w a y to p i n p o i n t t h e e x a c t m a c h i n i n g c o n d i t i o n s for such a transformation.
References
8
9 10 ll
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