- Email: [email protected]
The eutectic carbides and creep rupture strength of 25Cr20Ni heat-resistant steel tubes centrifugally cast with different solidification conditions
Materials Science and Engineering A293 (2000) 252 – 260 www.elsevier.com/locate/msea
The eutectic carbides and creep rupture strength of 25Cr20Ni heat-resistant steel tubes centrifugally cast with different solidification conditions X.Q. Wu *, H.M. Jing, Y.G. Zheng, Z.M. Yao, W. Ke, Z.Q. Hu State Key Laboratory for Corrosion and Protection, Institute of Metal Research, South District, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110015, PR China Received 21 December 1999; received in revised form 3 May 2000
Abstract The eutectic carbides and creep rupture strength of 25Cr20Ni heat-resistant steel tubes centrifugally cast with different solidification conditions were investigated in detail. The results reveal that the eutectic carbides precipitated primarily at the dendrite and grain boundaries show various morphologies from the outer wall to the inner wall along radial direction of the cast tubes, consisting of the thin film-like carbides, the blocky carbides, the lamellar carbide clusters resembling the pearlite and the skeleton-like carbides. The initial solidification conditions have significant influences on the grain morphologies and the distribution of the eutectic carbides in the cast tubes. Increasing the cooling rate markedly promotes the development of the columnar grains and restrains the precipitation of the eutectic carbides, while an applied electromagnetic field during the centrifugal solidification induces a notable grain refining and a marked change of the precipitation zones of the eutectic carbides from the dendrite boundaries to the grain boundaries. The applied electromagnetic markedly improves the creep rupture strength of the centrifugal cast 25Cr20Ni heat-resistant steel tubes. The relationships between the initial solidification conditions and the solidification processes as well as the creep rupture strength were discussed at length. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Heat-resistant steel tubes; Centrifugal solidification; Electromagnetic field; Eutectic carbides; Creep rupture strength
1. Introduction In view of the cheaper costs and the better mechanical properties, centrifugally cast 25Cr20Ni heat-resistant steels were extensively used to fabricate the petrochemical components served at high temperature such as the steam-reformer tubes and the pyrolysis tubes since early 1960s [1 – 6]. The prominent advantages of the steel are the higher creep rupture strength and the better resistance to the high temperature oxygen-rich, carbon-rich and sulfur-rich atmospheres. In general, the strengthening of this steel is mainly dependent on two types of factors. One is a network of the eutectic carbides precipitated initially in the solidification process and distributed at the dendrite and grain * Corresponding author. Tel.: +86-24-23915895; fax: + 86-2423894149. E-mail address: [email protected] (X.Q. Wu).
boundaries. The other is a fine array of the secondary carbides precipitated heterogeneously within the dendrites and grains from as cast supersaturated matrix during service. The former is critical to prevent the grain boundary sliding and the latter is to restrict the dislocation motion at high temperature [7]. But at a higher temperature or during a long-term service, the secondary carbides tend to coarsen and coalescence and their strengthening effects diminish gradually. Clearly, this is not beneficial to the high temperature service of the engineering components fabricated using 25Cr20Ni heat-resistant steel. However, the eutectic carbides precipitated initially in this centrifugally cast steel still show a better stability and a sharp strengthening effect at high temperature such as 1323 K [8]. In this context, it is the initial eutectic carbides for the 25Cr20Ni heatresistant steel is a lasting strengthening factor at high temperature for long-term service.
0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 0 9 8 4 - 9
X.Q. Wu et al. / Materials Science and Engineering A293 (2000) 252–260
253
Table 1 Chemical compositions of typical tube materials Material
HK40 Mod. HK Hi-Si HK HP 36×S 36×A PG2535Nb KHR35CW 36×T PG28485
Amount (wt.%) of the following elements Ni
Cr
C
Nb
W
Si
Mn
Other
20.5 22.5 21.5 33.8 34.1 34.2 35.2 35.4 44.8 45.7
24.5 23.5 24.1 24.3 25.9 25.1 25 25 315.5 27.7
0.4 0.42 0.5 0.43 0.43 0.43 0.45 0.4 0.35 0.57
– – – 0.69 1.11 0.8 1.57 1.4 1.34 –
– – – – 1.52 – – 1.2 1.7 4.25
1.75 1.2 2.2 1.71 1.6 1.5 2.2 1.6 1.6 1.8
1.5 1.0 1.5 1.11 1.2 1.4 1.0 0.9 0.9 1.11
MoB0.5 – – – – 3.3A1 – 0.4Mo – –
In order to prolong the service life of the petrochemical components such as the steam-reformer tubes and the pyrolysis tubes served at high temperature, many efforts have been made to improve the high temperature properties of the 25Cr20Ni heat-resistant steel during last three decades. The most common way is to adjust or improve the chemical compositions of the steel. As a result, several improved heat-resistant steels were developed and applied in the industry production successfully and extensively [9 – 16]. The typical steels are listed in Table 1, from which it is clear that there are two main characteristics on the traditional development of the 25Cr20Ni steel. One is the gradual increase of the Ni content in the steel, the other is the increase of additive alloy elements such as W, Mo, Nb, Ti and Zr in the steel. Although the high temperature properties of the heat-resistant tube materials are improved to some extent by the measures stated above, some other defaults also ensue. For example, increasing the Ni content markedly weakens the coking resistance of the tube materials according to the experiment results of Shinohara et al. [17]. Moreover, increase of the Ni content and the alloy elements in the tube materials are unavoidable to give rise to a corresponding increase of the fabrication cost of the heat-resistant tubes, which does not coincide with the demands of the industrialized production. To further improve the high temperature properties of the heat-resistant tube materials, many researchers have devoted themselves to improving the fabrication process of the cracking tubes. In recent years, a project supported by the Sinopec Technology of China was carried out to develop a new tube-fabricated process that connects the centrifugal force field and the electromagnetic field [18 – 21]. As part of above project, present work primarily investigated the eutectic carbides and creep rupture strength of 25Cr20Ni heatresistant steel tubes centrifugally cast with different solidification conditions and discussed the relationships
between the initial solidification conditions (the cooling rate and the applied electromagnetic field) and the solidification processes as well as the creep rupture strength in detail.
2. Experimental procedures The compositions (wt.%) of the 25Cr20Ni heat-resistant steel used in present experiments are Cr 24.81, Ni 20.05, C 0.37, Mn 1.32, Si 1.27, PB 0.005, SB0.007 and balanced by Fe. The steel was remelted by heating up to 1923 K in a high frequency induction furnace and poured into a rotating mold fixed on the equipment for electromagnetic centrifugal casting schematically shown in Fig. 1. The rotating speed of the cast mold was 1700 rpm and the cooling conditions and the electromagnetic field intensity applied were listed in Table 2. After complete solidification, tubular samples with length being 120 min, outer diameter 70 mm and a wall thickness 15 mm were obtained. The metallographic specimens were cut from the middle region of each cast
Fig. 1. Schematic diagram of the electromagnetic centrifugal casting equipment.
X.Q. Wu et al. / Materials Science and Engineering A293 (2000) 252–260
254 Table 2 Casting process parameters Cast tube C0 C1 C2
Exciting current (A) 0 0 10
3. Experimental results Casting mol material
3.1. Macrostructures
Graphite Cast iron Cast iron
Fig. 3 shows macrostructures on the cross-sections of the centrifugally cast 25Cr20Ni heat-resistant steel tubes. The macrostructures on the cross-section of each cast tube can be clearly divided into three zones, which demonstrates typical structure of an outer layer of the chill crystals (zone A), then the columnar grains (zone B) and along the inner layer the equiaxed grains (zone C). The thickness fraction of each zone on the crosssections of the cast tubes is listed in Table 3. It is clear
Fig. 2. Sampling diagram and the sizes of the creep rupture test specimens.
tube, about 60 min from one end of the tube as shown in Fig. 2. The cross-sections for structure observation (i.e. the A–A’ section in Fig. 2) were prepared via standard metallography methods and etched with a 20 vol.% CuSO4, 40 vol.% HCl and H2O solution. Both an optical microscope and a JSM-6301F (JEOL USA, Peabody, MA) scanning electron microscopy (SEM) with an ISIS 300 series energy-dispersive X-ray analyzer (EDAX) were used in morphology observation and qualitative composition analysis for the eutectic carbides. Both the thickness fraction of macrostructures on the cross-sections of the cast tubes and the volume fraction of the eutectic carbides were determined by employing linear analyzing method [22]. The specimens for the creep rupture tests were machined from the center region of the wall section of each cast tube with the specimen axis parallel to the tube axis. The sampling diagram and the sizes of the specimens for the creep rupture tests were illustrated schematically in Fig. 2, too. The creep rupture tests were conducted at 1144 and 1223 K.
Fig. 3. Macrostructures on the cross-sections of the centrifugally cast 25Cr20Ni heat-resistant steel tubes: (a) tube C0; (b) tube C1; (c) tube C2.
X.Q. Wu et al. / Materials Science and Engineering A293 (2000) 252–260 Table 3 Thickness fraction of macrostructures Tube
Thickness fraction (%) Zone A
C0 C1 C2
4 3 3
Zone B 27 83 54
Zone C 69 14 43
that increasing the cooling rate during the centrifugal solidification markedly promotes the continuous growth of the coarse columnar grains. On the contrary, the introduction of an electromagnetic field during the centrifugal solidification promotes the development of the equiaxed grains. Moreover, both the columnar grains in the zone B and the equiaxed grains in the zone C are refined by the introduction of the electromagnetic field.
3.2. Eutectic carbides 3.2.1. Morphologies of eutectic carbides In spite of the different initial solidification conditions, the eutectic carbides in three cast tubes have similar morphology characteristics. Fig. 4 shows the typical morphologies of the eutectic carbides in the tube C0. It is clear that the eutectic carbides are primarily
255
precipitated at the grain and dendrite boundaries and show various morphologies along the radial direction of the cast tubes. The zone A (the chill zone) in the neighborhood of the outer wall of the cast tube consists of the fine and equiaxed grains. In this zone, the eutectic carbides exhibit a thin film-like morphology and are primarily distributed along the serrate grain boundaries as shown in Fig. 4a. The thickness of the film-like carbides varies along the grain boundaries and some discontinuous film-like carbides appear as thin as a wafer. The average thickness of this type of carbides is about 0.6 mm. In the zone B of the cast tube, except for the film-like carbides distributed along the grain boundaries, a few isolated blocky carbides are also precipitated at the grain and dendrite boundaries. Fig. 4b shows the morphologies of the eutectic carbides in the outer layer of the zone B (i.e. the layer adjacent to the zone A). It is interesting that some lamellar eutectic carbide clusters resembling the pearlite appear at the grain boundaries and partially at the dendrite boundaries. A few blocky carbides are wrapped in these lamellar carbide clusters or precipitated solely at the dendrite boundaries. Observed at high magnification, the lamellae and the wrapped blocky carbides are more apparent as illustrated in Fig. 4c. With increase of the distance from the zone A, the lamellar eutectic carbide clusters disappear gradually, instead the quantity of the isolated blocky
Fig. 4. Typical morphologies of the eutectic carbides in the tube C0: (a) the thin film-like carbides in the zone A; (b) the morphologies of the eutectic carbides in the outer layer of the zone B; (c) the high magnification of the lamellar carbide clusters shown in (b); (d) the skeleton-like carbides in the zone C.
X.Q. Wu et al. / Materials Science and Engineering A293 (2000) 252–260
256
Fig. 5. EDAX qualitative analysis of the eutectic carbides in the cast tubes. Table 4 Volume fraction of the eutectic carbides in the cast tubesa Tube
Zone B
Zone C
fraction of the eutectic carbides, especially the grain boundary eutectic carbides (see the tube C2). According to Vgb/Vt, it is clear that the ratio of the volume fraction of grain boundary carbides against total carbides in the tube C0 is slightly higher than that in the tube C1, but both of which are much less than that in the tube C2, especially in the zone C. This reveals that most of the eutectic carbides in the tube C2 are distributed at the grain boundaries, while the eutectic carbides in the tube C0 and the tube C1 are mainly distributed at the dendrite boundaries, which is consistent with the microstructure shown in Fig. 6, too. In addition, the relative difference in the volume fraction of the eutectic carbides between the zone B and the zone C, F, reduces successively from 29.5% for the tube C0 to 16.2% for the tube C1 and to 8.63% for the tube C2. This indicates that a higher cooling rate or an applied electromagnetic field during the centrifugal solidification gives rise to a more homogeneous distribution of the eutectic carbides along the radial direction of the cast tubes.
F.%
3.3. Creep rupture strength of centrifugally cast tubes C0 C1 C2
Vt.%
Vgb.%
Vgb/Vt
Vt.%
Vgb.%
Vgb/Vt
6.54 5.95 8.58
2.38 1.84 4.71
0.36 0.31 0.55
9.28 7.1 9.39
4.47 3.08 7.21
0.48 0.43 0.77
29.5 16.2 8.63
a Vt denotes volume fraction of total eutectic carbides;Vgb denotes volume fraction of grain boundary eutectic carbides; F denotes
Vt(zone C)−Vt(zone B) ×100%. Vt6 (zone C)
carbides increases and a few skeleton-like carbides begin to appear at the dendrite and grain boundaries. In the zone C of the cast tube, the eutectic carbides primarily consist of the coarse skeleton-like carbides distributed at the grain and dendrite boundaries as shown in Fig. 4d. The morphology of the isolated carbides distributed at the dendrite boundaries also changes from blocky to skeletal. In addition, in spite of the difference in morphology mentioned above, it is found by the EDAX qualitative analysis shown in Fig. 5 that all the eutectic carbides are similarly rich in Cr and contain some Fe and Ni, too.
3.2.2. Volume fraction of eutectic carbides The volume fraction of the eutectic carbides in each cast tube has been measured and is listed in Table 4. From Vt, and Vgb in the table, it can clearly be seen that the volume fraction of the eutectic carbides in both the zone B and the zone C decreases with increase of the cooling rate (see the tube C0 and the tube C1). But the application of an electromagnetic field during the centrifugal solidification markedly increases the volume
The creep rupture data obtained at 1144 K and 1223 K for the tube C0, the tube C1 and the tube C2 are plotted to give Larson–Miller rupture curves as shown in Fig. 7. It is clear that the tube C0 and the tube C1 almost display a similarity in the creep rupture strength, while the tube C2 shows a markedly higher creep rupture strength. With increase of the creep temperature and the creep time, the advantage in the creep rupture strength for the tube C2 tends to become more distinguished. This reveals that introducing an electromagnetic field during the solidification markedly improve the creep rupture strength of the centrifugally cast 25Cr20Ni heat-resistant tube.
4. Discussion
4.1. Macrostructures As described previously, the macrostructures of the centrifugally cast 25Cr20Ni heat-resistant steel tubes may be divided into three zones along the radial direction of the tubes and be considerably influenced by the initial solidification conditions. In process of the centrifugal casting, when the molten steel is poured into a rotating metal mold, the mold wall (at a much lower temperature than the melt) rapidly cools the zone of the melt in contact with it, which results in a considerable magnitude of thermal and constitutional supercooling. So, heterogeneous nucleation will occur relatively rapid and a fine and equiaxed zone of crystal will be obtained. The above process is too rapid to be influenced
X.Q. Wu et al. / Materials Science and Engineering A293 (2000) 252–260
257
by the solidification conditions. Therefore, a similar chill zone (the zone A) is formed on the outer wall for all three tubes (Fig. 3). Moreover, the above chill effect gives rise to a sharp temperature gradient rapidly built up at the front of the solidification interface, and
Fig. 7. Larson – Miller rupture curves for the centrifugally cast 25Cr20Ni heat-resistant steel tubes.
Fig. 6. Microstructures in the zone C of the cast tubes: (a) tube C0; (b) tube C1; (c) tube C2.
promotes the grains grow along a preferred orientation parallel to the heat flow direction. As a result, a columnar grain zone (the zone B) is developed adjacent to the zone A. Since the cast iron mold inherently has a higher cooling ability than the graphite mold, it causes a higher temperature gradient in front of the solidification interface. Furthermore, a higher cooling rate can effectively prevent the chill nuclei (induced by the chill effect) from drifting away the mold wall and restrain the equiaxed grains developed in the bulk melt in advance of the solidification interface. In this context, a higher cooling rate diminishes the barriers for the columnar grains to develop continuously along the radial direction of the cast tubes. So, the tube C1 shows the continuous and coarse columnar grains along its radial direction (Fig. 3b)(Table 2). However, the graphite mold results in a relatively lower temperature gradient at the solidification front. Moreover, the drifting of the chill nuclei from the graphite mold wall to the bulk melt is relatively common. Thus, with advance of the solidification interface, some new grains may be developed in the liquid phase before the solidification interface. The growth of these new grains will interrupt or retard the continuous growth of the columnar grains. Consequently, the shorter columnar grains are formed in the tube C0 (Fig. 3a) (Table 2). When an electromagnetic field is introduced during the centrifugal solidification, the interactions between the rotating melt and the magnetic field will give rise to an electromagnetic stirring effect (EMS), which strengthens the fluid flow in front of the solidification interface and accelerates the nucleation of new grains by way of the dendrite broken mechanism or localized remelting mechanism [23–25]. Moreover, the fluid flow induced by the EMS increases the thermal transfer in the bulk melt and results in more homogenous distribution of the temperature in the melt. As a result, the new grains
258
X.Q. Wu et al. / Materials Science and Engineering A293 (2000) 252–260
may possibly survive and grow into the equiaxed grains. Furthermore, the simultaneous nucleation may take place in the relatively cool bulk melt. All above effects will retard the continuous growth of the columnar grains and favor the nucleation and growth of the equiaxed grains in the bulk melt. Thus, substantial quantities of small equiaxed grains are produced in the tube C2 (Fig. 3c)(Table 2). In addition, since the nuclei of the columnar grains adjacent to the mold wall are also multiplied by the EMS during the initial stage of the centrifugal solidification, the columnar grains in the outer wall for the tube C2 are refined markedly, too.
4.2. Eutectic carbides In terms of the structural diagram drawn by Nishino and Kagawa [26], the solidification process of the 25Cr20Ni heat-resistant steel includes L g and L g+M7C3. With proceeding of the solidification, the solute atoms such as C and Cr are released into the bulk melt continuously from the solidification interface. Along with advance of the solidification interface and dropping of the solidification temperature, the solutes gradually concentrate in the residual melt, finally the melt reaches the eutectic compositions of the 25Cr20Ni steel and the eutectic transformation takes place. According to the solidification sequence, the dendrite and grain boundaries are the final solidification zones, at later stage of the solidification, the residual melt rich in solutes primarily congregates in these zones. Therefore, after complete solidification, the eutectic carbides predominantly appear at the dendrite and grain boundaries, in which Cr is rich as shown in Fig. 5. The morphology and distribution of the eutectic carbides (M7C3) in the cast tubes are closely related to the redistribution of the solutes and the growth conditions of the presolidified g grains during the centrifugal solidification. According to the solute distribution model developed by Burton et al. [27], the effective partition coefficient kE solute can be described as following k0 kE = (1) k0 +(1−k0) exp( − RdN/DL) where k0 denotes the equlibrium partition coefficient of solute; R the speed of solidification interface or the speed of solidification; dN the thickness of diffusion boundary layer in advance of the solidification interface; DL the diffusion coefficient of solute in liquid phase. In terms of the Scheil equation, the solute distribution in a definite volume is E − 1) CL = C0 f (k L
(2)
here CL denotes the solute concentration in the residual melt, C0 the initial solute concentration of the melt, fL the fraction of the residual melt. Clearly, the higher the kE, the lower the CL namely, the smaller the solute concentration in the residual melt.
4.2.1. Zone A In the zone A for each cast tube, the cooling rate is the highest since the chill effect caused by the cast mold wall. A large number of g grains are nucleated simultaneously and the growth of the pre-solidified g grains is very rapid in this zone. In terms of Fig. 1 and equation Fig. 2, kE tends to approach unity on account of the higher speed of solidification R and CL is smaller correspondingly. This reveals that the solute partition is suppressed to a large extent during the solidification in this zone. Therefore, with advance of the solidification interface, most of the solutes are trapped in the pre-solidified g grains and little amount of the solutes is rejected into the melt in front of the solidification interface. At later stage of the solidification in this zone, very little amount of the melt resided among the g grains can reach the eutectic compositions. So, after complete solidification, the thin film-like eutectic carbides shown in Fig. 4a are precipitated along the boundaries of the g grains. 4.2.2. Zone B In the zone B, the solidification process is similar to the directional solidification since the thermal flow is along the radial direction of the cast tubes and is opposite to the growth direction of the g grains. In the outer layer of the zone B (the layer adjacent to the zone A), the chill effect caused by the cast mold wall is weakened relatively. The nucleation and growth of the g grains also slow down. According to the Fig. 1 and Fig. 2, kE tends to change towards k0 on account of the decrease of R and CL is inclined to increase correspondingly. The solute partition in this layer during the solidification is relatively sufficient as compared with the zone A and more solutes are rejected into the bulk melt from the solidification front. At later stage of the solidification in this layer, relatively more melt resided at the dendrite and grain boundary zones may reach the eutectic compositions. As a result, some blocky eutectic carbides and lamellar eutectic carbide clusters resembling the pearlite appear in this zone (Fig. 4b). Since this layer is just close to the zone A, the cooling rate and the supercooling caused by the cast mold wall is still higher, the lamellar eutectic carbides are very thin (Fig. 4c). With increase of the distance from the cast mold wall, the cooling rate and the supercooling lower gradually, thus the blocky carbides increases and a few skeleton-like carbides begin to appear at the dendrite and grain boundaries. 4.2.3. Zone C In the zone C, since it is far from the cast mold wall, the thermal transfer and the growth of the g grains in this zone slower again as compared with the zone B. Analogously, kE, tends to increase towards k0, on account of the decrease of R and CL is inclined to increase according to Fig. 1 and Fig. 2. Therefore, after complete
X.Q. Wu et al. / Materials Science and Engineering A293 (2000) 252–260
solidification in this zone, the coarse and skeleton-like eutectic carbides are precipitated at the dendrite and grain boundaries as shown in Fig. 4d.
4.2.4. Influences of the solidification conditions It is clear from Table 3 and Fig. 6 that the initial solidification conditions (cooling rate and applied electromagnetic field) have marked influence on the distribution of the eutectic carbides in the centrifugally cast tubes. In general, since the thermal dissipation in the root zones of the dendrites and grains is slower, the growth rate in these zones is smaller and the solutes rejected from the solidification interface are primarily aggravated in these zones. So, after solidification, lots of the eutectic carbides are distributed at the dendrite and grain boundaries. A typical example is shown in Table 3 (the tube C0) and Fig. 6a. With increase of the cooling rate, the growth rate of the dendrites and grains increases. According to the Fig. 1 and Fig. 2, kE tends to increase towards unity on account of the increase of R and CL is inclined to decrease correspondingly. This reveals that the segregation of the solutes in the root zones of the dendrites and grains is restrained to some extent. Thus the eutectic carbides formed at the dendrite and grain boundaries decrease after solidification as shown in Table 3 (the tube C1) and Fig. 6b. As stated previously, an electromagnetic field introduced during the centrifugal solidification induces an EMS. Such an EMS enforces the convection in the bulk melt, decreases the solute enrichment ahead of the solidification front, and homogenizes the solute distribution in the bulk melt [25]. Thus, the distribution of the eutectic carbides precipitated in the tube C2 is more homogeneous along the radial direction. The forced convection above will decrease the thickness of the diffusion boundary layer in advance of the solidification interface and result in the decrease of dN. Moreover, as shown in Fig. 3c and Fig. 6c, the EMS not only promotes the development of the equiaxed grains, but also induces the refining of both the columnar grains and the equiaxed grains in the cast tube. The nucleation of these fine grains is unavoidable to release a large amount of latent heat and delay the solidification speed R. The influences of the EMS on dN and R will cause that kE, decreases towards k0 and CL increases correspondingly in terms of Fig. 1 and Fig. 2. As a result, more residual melt may reach the eutectic compositions at later stage of the solidification. Therefore, the volume fraction of the eutectic carbides markedly increases as shown in Table 3 (the tube C2). In addition, the EMS strongly promotes the grain refining, which causes most of initial dendrite boundaries are turned into the equiaxed grain boundaries, thus the eutectic carbides are primarily aggregated at the grain boundaries rather than the dendrite boundaries in the tube C2 (Table 3) (Fig. 6c).
259
4.3. Creep rupture strength In general point of view, the grain boundaries are the weakness in the deformation at high temperature for the pure metal or the single-phase alloy. Therefore, the finer the grains, the lower the deformation resistance, and the lower the creep rupture strength. However, For the 25Cr20Ni heat-resistant steel studied in present work, lots of the eutectic carbides are precipitated at the boundaries of the g grains and dendrites as described previously. Thus, the grain boundaries in the 25Cr20Ni heat-resistant steel must play a different role in the creep at high temperature as compared with the single-phase alloy. According to the creep model combined matrix/ boundary strengthening [28], the creep speed (or creep resistance) o can be expressed as o; = A
s− sb0 E
n
exp −
Qsd RT
(3)
Here sb0 denotes a back stress acting in the grain center zone due to intergranular carbide precipitation for a given applied stress s and can be expressed by
2KGbs sb0 = m d
1 2
(4)
Where m can be regarded as the intergranular carbide density, K is the constant, G is shear modulus, b is the length of the Burger’s vector and d is the mean grain diameter. Clearly, the carbides precipitated along the grain boundaries will improve the creep resistance. Moreover, the bigger the precipitated carbide density m along the grain boundaries and the smaller the mean grain diameter d, the larger the back stress abo, and thus the higher the creep resistance. From Fig. 3c and the Vgb/Vt in Table 3, it is clear that an applied electromagnetic during the centrifugal solidification results in a remarkable grain refining and causes more eutectic carbides precipitated along the grain boundaries, thus the tube C2 shows the highest creep rupture strength among the three cast tubes (Fig. 7). Although the total volume fraction of the eutectic carbides in the tube C0 is very similar to that in the tube C2, but most of the eutectic carbides in the tube C0 are distributed at the dendrite boundaries rather than the grain boundaries, so the creep rupture strength for the tube C0 is lower as compared with the tube C2. In spite of the least total volume fraction Vt, of the eutectic carbides in the tube C1, the Vgb/Vt is close to that in the tube C0, thus the creep rupture strength is similar for this two cast tubes (Fig. 7). In addition, according to the pile-up theory of dislocation [29], the concentration stress t at a distance r ahead of the pile-up can be expressed as
L t= t0 r
1 2
(5)
260
X.Q. Wu et al. / Materials Science and Engineering A293 (2000) 252–260
Here t0 is the applied stress, L is the pile-up length (or the average distance from the dislocation source to grain boundary) and can generally be taken as the grain radius d/2. Clearly, the finer the grains, the smaller the stress concentration at the grain boundaries, and the lower the drive force of the nucleation for the creep cavities or cracks. Moreover, in terms of the results obtained by Zhu et al. [30], the greater percentage of the eutectic carbides present in the material provides higher resistance to the linkage of the creep cavities or cracks. For the tube C2, the applied electromagnetic during the centrifugal solidification induces the remarkable grain refining and increase of the eutectic carbides precipitated along the grain boundaries, which is effective to increase the resistance to nucleation and linkage of the creep cavities or cracks. This effect may also be one of reasons that the tube C2 show the most excellent creep rupture strength among the three cast tubes.
5. Conclusions 1. The eutectic carbides in the centrifugally cast 25Cr20Ni heat-resistant steel tubes are primarily precipitated at the dendrite and grain boundaries and show various morphologies from the outer wall to the inner wall along the radial direction of the cast tubes, consisting of the thin film-like carbides, the blocky carbides, the lamellar carbide clusters resembling the pearlite and the skeleton-like carbides. 2. The initial solidification conditions have significant influences on the grain morphologies and the distribution of the eutectic carbides in the centrifugally cast 25Cr20Ni heatresistant steel tubes. Increasing the cooling rate markedly promotes the development of the columnar grains and restrains the precipitation of the eutectic carbides, while an applied electromagnetic field during the centrifugal solidification results in a notable grain refining and a marked change of the precipitation zones of the eutectic carbides from the dendrite boundaries to the grain boundaries. 3. The morphology and distribution of the eutectic carbides in the centrifugally cast 25Cr20Ni heat-resistant steel tubes are closely related to the solute redistribution and the growth conditions of the pre-solidified g grains during the solidification. 4. The introduction of an electromagnetic field during the solidification markedly improves the creep rupture strength of the centrifugally cast 25Cr20Ni heat-resistant steel tubes. The main reasons may be that the refined grains and the increased eutectic carbides along the grain boundaries caused by the EMS increases the creep resistance and retards the nucleation and linkage of the creep cavities or cracks. .
Acknowledgements The financial supports received from the Sinopec Technology and the Science and Technology Foundation of Liaoning are gratefully acknowledged.
References [1] D.J. Cox, D.E. Jordan, Materials Technology in Steam Reforming Processes, Pergamon Press, London, 1964, p. 121. [2] J.A. VanEcho, D.B. Roach, A.M. Hall, J. Basic Eng. Trans. ASME 89 (1967) 465. [3] S. lbarra, Met. Prog. 117 (2) (1980) 62. [4] R.H. Kane, Corrosion 37 (4) (1981) 187. [5] D.J. Hall, M.K. Hossain, R.F. Atkinson, High Temp. High Press. 14 (1982) 527. [6] S.J. Zhu, P.E. Li, J. Zhao, Z.B. Cao, Mater. Sci. Eng. A 114 (1989) 7. [7] E.A.A.G. Ribeiro, R. Papaleo, J.R.C. Guimaraes, Metall. Trans. A 17 (1986) 691. [8] M.B. Zaghloul, T. Shinoda, R. Tanaka, Trans. ISU 17 (1977) 28. [9] L.T. Shinoda, M.B. Zaghloul, Y. Kondo, R. Tanaka, Trans. ISU 18 (1978) 139. [10] H. Wen-Tai, R.W.K. Honeycombe, Mater. Sci. Tech. 1 (5) (1985) 385. [11] G.D. Barbabela, L.H. Almeida, T.L. Silveira, I. May, Mater. Char. 26 (1) (1991) 1. [12] G.D. Barbabela, L.H. Almeida, T.L. Silveira, I. May, Mater. Char. 26 (1) (1991) 193. [13] G.D.A. Soares, L.H. Almeida, T.L. Silveira, I. May, Mater. Char. 29 (4) (1992) 387. [14] C.W. Thomas, M. Borshevsky, A.N. Marshall, Mater. Sci. Tech. 8 (10) (1992) 855. [15] R.A.P. lbanez, G.D.A. Soares, L.H. Almeida, I. May, Mater. Char. 30 (4) (1993) 243. [16] C.W. Thomas, K.J. Stevens, M.J. Ryan, Mater. Sci. Tech. 12 (5) (1996) 469. [17] T. Shinohara, I. Kohchi, K. Shibata, J. Sugitani, K. Tsuchida, Werkst Korros. 37 (1986) 410. [18] Y Yang, Q. Liu, Y Jiao, Z. Hu, G. Jia, G. Zhang, Y Gao, J. Zhang, Proceedings of International Symposium on Electromagnetic Processing of Materials, Nagoya, Japan. [19] Y Yang, Q. Liu, Y Jiao, Z. Hu, J. Zhang, Y Gao, Proceedings of the 2nd Pacific Rim International Conference on Advanced Materials and Processing, Kyongju, Korea, the Korea Institute of Metals and Materials, June, 1995, pp. 149. [20] Y Yang, Q. Llu, Y Jiao, et al., ISU Int. 35 (4) (1995) 389. [21] W. Zhang, Y. Yang, Q. Liu, et al., Mater. Sci. Technol. 14 (4) (1998) 306. [22] H.L. Ren, Testing Methodologies for Metallography, Metallurgy, Industrial Publishing Company of China, Bejing, 1986, p. 161. [23] W.D. Griffiths, D.G. McCartney, Mater. Sci. Eng. A 216 (1996) 47. [24] A. Prodhan, D. Sanyal, J. Mater. Sci. Lett. 16 (1997) 958. [25] X.Q. Wu, Y.S. Yang, Y.F. Zhu, Z.Q. Hu, Z. Metall. 90 (7) (1999) 531. [26] K. Nishino, N. Kagawa, Tetsu-to-Hagane 58 (1) (1972) 107. [27] J.A. Burton, R.C. Prim, W.G. Slichter, J. Chem. Phys. 21 (1953) 1987. [28] J.S. Zhang, P.E. Li, J.Z. Jin, Acta Metal. Mater. 39 (12) (1991) 3063. [29] A.N. Stroh, Adv. Phys. 6 (1957) 418. [30] S.J. Zhu, Mater. Sci. Eng. A 127 (1990) L7.
The eutectic carbides and creep rupture strength of 25Cr20Ni heat-resistant steel tubes centrifugally cast with different solidification conditions X.Q. Wu *, H.M. Jing, Y.G. Zheng, Z.M. Yao, W. Ke, Z.Q. Hu State Key Laboratory for Corrosion and Protection, Institute of Metal Research, South District, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110015, PR China Received 21 December 1999; received in revised form 3 May 2000
Abstract The eutectic carbides and creep rupture strength of 25Cr20Ni heat-resistant steel tubes centrifugally cast with different solidification conditions were investigated in detail. The results reveal that the eutectic carbides precipitated primarily at the dendrite and grain boundaries show various morphologies from the outer wall to the inner wall along radial direction of the cast tubes, consisting of the thin film-like carbides, the blocky carbides, the lamellar carbide clusters resembling the pearlite and the skeleton-like carbides. The initial solidification conditions have significant influences on the grain morphologies and the distribution of the eutectic carbides in the cast tubes. Increasing the cooling rate markedly promotes the development of the columnar grains and restrains the precipitation of the eutectic carbides, while an applied electromagnetic field during the centrifugal solidification induces a notable grain refining and a marked change of the precipitation zones of the eutectic carbides from the dendrite boundaries to the grain boundaries. The applied electromagnetic markedly improves the creep rupture strength of the centrifugal cast 25Cr20Ni heat-resistant steel tubes. The relationships between the initial solidification conditions and the solidification processes as well as the creep rupture strength were discussed at length. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Heat-resistant steel tubes; Centrifugal solidification; Electromagnetic field; Eutectic carbides; Creep rupture strength
1. Introduction In view of the cheaper costs and the better mechanical properties, centrifugally cast 25Cr20Ni heat-resistant steels were extensively used to fabricate the petrochemical components served at high temperature such as the steam-reformer tubes and the pyrolysis tubes since early 1960s [1 – 6]. The prominent advantages of the steel are the higher creep rupture strength and the better resistance to the high temperature oxygen-rich, carbon-rich and sulfur-rich atmospheres. In general, the strengthening of this steel is mainly dependent on two types of factors. One is a network of the eutectic carbides precipitated initially in the solidification process and distributed at the dendrite and grain * Corresponding author. Tel.: +86-24-23915895; fax: + 86-2423894149. E-mail address: [email protected] (X.Q. Wu).
boundaries. The other is a fine array of the secondary carbides precipitated heterogeneously within the dendrites and grains from as cast supersaturated matrix during service. The former is critical to prevent the grain boundary sliding and the latter is to restrict the dislocation motion at high temperature [7]. But at a higher temperature or during a long-term service, the secondary carbides tend to coarsen and coalescence and their strengthening effects diminish gradually. Clearly, this is not beneficial to the high temperature service of the engineering components fabricated using 25Cr20Ni heat-resistant steel. However, the eutectic carbides precipitated initially in this centrifugally cast steel still show a better stability and a sharp strengthening effect at high temperature such as 1323 K [8]. In this context, it is the initial eutectic carbides for the 25Cr20Ni heatresistant steel is a lasting strengthening factor at high temperature for long-term service.
0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 0 9 8 4 - 9
X.Q. Wu et al. / Materials Science and Engineering A293 (2000) 252–260
253
Table 1 Chemical compositions of typical tube materials Material
HK40 Mod. HK Hi-Si HK HP 36×S 36×A PG2535Nb KHR35CW 36×T PG28485
Amount (wt.%) of the following elements Ni
Cr
C
Nb
W
Si
Mn
Other
20.5 22.5 21.5 33.8 34.1 34.2 35.2 35.4 44.8 45.7
24.5 23.5 24.1 24.3 25.9 25.1 25 25 315.5 27.7
0.4 0.42 0.5 0.43 0.43 0.43 0.45 0.4 0.35 0.57
– – – 0.69 1.11 0.8 1.57 1.4 1.34 –
– – – – 1.52 – – 1.2 1.7 4.25
1.75 1.2 2.2 1.71 1.6 1.5 2.2 1.6 1.6 1.8
1.5 1.0 1.5 1.11 1.2 1.4 1.0 0.9 0.9 1.11
MoB0.5 – – – – 3.3A1 – 0.4Mo – –
In order to prolong the service life of the petrochemical components such as the steam-reformer tubes and the pyrolysis tubes served at high temperature, many efforts have been made to improve the high temperature properties of the 25Cr20Ni heat-resistant steel during last three decades. The most common way is to adjust or improve the chemical compositions of the steel. As a result, several improved heat-resistant steels were developed and applied in the industry production successfully and extensively [9 – 16]. The typical steels are listed in Table 1, from which it is clear that there are two main characteristics on the traditional development of the 25Cr20Ni steel. One is the gradual increase of the Ni content in the steel, the other is the increase of additive alloy elements such as W, Mo, Nb, Ti and Zr in the steel. Although the high temperature properties of the heat-resistant tube materials are improved to some extent by the measures stated above, some other defaults also ensue. For example, increasing the Ni content markedly weakens the coking resistance of the tube materials according to the experiment results of Shinohara et al. [17]. Moreover, increase of the Ni content and the alloy elements in the tube materials are unavoidable to give rise to a corresponding increase of the fabrication cost of the heat-resistant tubes, which does not coincide with the demands of the industrialized production. To further improve the high temperature properties of the heat-resistant tube materials, many researchers have devoted themselves to improving the fabrication process of the cracking tubes. In recent years, a project supported by the Sinopec Technology of China was carried out to develop a new tube-fabricated process that connects the centrifugal force field and the electromagnetic field [18 – 21]. As part of above project, present work primarily investigated the eutectic carbides and creep rupture strength of 25Cr20Ni heatresistant steel tubes centrifugally cast with different solidification conditions and discussed the relationships
between the initial solidification conditions (the cooling rate and the applied electromagnetic field) and the solidification processes as well as the creep rupture strength in detail.
2. Experimental procedures The compositions (wt.%) of the 25Cr20Ni heat-resistant steel used in present experiments are Cr 24.81, Ni 20.05, C 0.37, Mn 1.32, Si 1.27, PB 0.005, SB0.007 and balanced by Fe. The steel was remelted by heating up to 1923 K in a high frequency induction furnace and poured into a rotating mold fixed on the equipment for electromagnetic centrifugal casting schematically shown in Fig. 1. The rotating speed of the cast mold was 1700 rpm and the cooling conditions and the electromagnetic field intensity applied were listed in Table 2. After complete solidification, tubular samples with length being 120 min, outer diameter 70 mm and a wall thickness 15 mm were obtained. The metallographic specimens were cut from the middle region of each cast
Fig. 1. Schematic diagram of the electromagnetic centrifugal casting equipment.
X.Q. Wu et al. / Materials Science and Engineering A293 (2000) 252–260
254 Table 2 Casting process parameters Cast tube C0 C1 C2
Exciting current (A) 0 0 10
3. Experimental results Casting mol material
3.1. Macrostructures
Graphite Cast iron Cast iron
Fig. 3 shows macrostructures on the cross-sections of the centrifugally cast 25Cr20Ni heat-resistant steel tubes. The macrostructures on the cross-section of each cast tube can be clearly divided into three zones, which demonstrates typical structure of an outer layer of the chill crystals (zone A), then the columnar grains (zone B) and along the inner layer the equiaxed grains (zone C). The thickness fraction of each zone on the crosssections of the cast tubes is listed in Table 3. It is clear
Fig. 2. Sampling diagram and the sizes of the creep rupture test specimens.
tube, about 60 min from one end of the tube as shown in Fig. 2. The cross-sections for structure observation (i.e. the A–A’ section in Fig. 2) were prepared via standard metallography methods and etched with a 20 vol.% CuSO4, 40 vol.% HCl and H2O solution. Both an optical microscope and a JSM-6301F (JEOL USA, Peabody, MA) scanning electron microscopy (SEM) with an ISIS 300 series energy-dispersive X-ray analyzer (EDAX) were used in morphology observation and qualitative composition analysis for the eutectic carbides. Both the thickness fraction of macrostructures on the cross-sections of the cast tubes and the volume fraction of the eutectic carbides were determined by employing linear analyzing method [22]. The specimens for the creep rupture tests were machined from the center region of the wall section of each cast tube with the specimen axis parallel to the tube axis. The sampling diagram and the sizes of the specimens for the creep rupture tests were illustrated schematically in Fig. 2, too. The creep rupture tests were conducted at 1144 and 1223 K.
Fig. 3. Macrostructures on the cross-sections of the centrifugally cast 25Cr20Ni heat-resistant steel tubes: (a) tube C0; (b) tube C1; (c) tube C2.
X.Q. Wu et al. / Materials Science and Engineering A293 (2000) 252–260 Table 3 Thickness fraction of macrostructures Tube
Thickness fraction (%) Zone A
C0 C1 C2
4 3 3
Zone B 27 83 54
Zone C 69 14 43
that increasing the cooling rate during the centrifugal solidification markedly promotes the continuous growth of the coarse columnar grains. On the contrary, the introduction of an electromagnetic field during the centrifugal solidification promotes the development of the equiaxed grains. Moreover, both the columnar grains in the zone B and the equiaxed grains in the zone C are refined by the introduction of the electromagnetic field.
3.2. Eutectic carbides 3.2.1. Morphologies of eutectic carbides In spite of the different initial solidification conditions, the eutectic carbides in three cast tubes have similar morphology characteristics. Fig. 4 shows the typical morphologies of the eutectic carbides in the tube C0. It is clear that the eutectic carbides are primarily
255
precipitated at the grain and dendrite boundaries and show various morphologies along the radial direction of the cast tubes. The zone A (the chill zone) in the neighborhood of the outer wall of the cast tube consists of the fine and equiaxed grains. In this zone, the eutectic carbides exhibit a thin film-like morphology and are primarily distributed along the serrate grain boundaries as shown in Fig. 4a. The thickness of the film-like carbides varies along the grain boundaries and some discontinuous film-like carbides appear as thin as a wafer. The average thickness of this type of carbides is about 0.6 mm. In the zone B of the cast tube, except for the film-like carbides distributed along the grain boundaries, a few isolated blocky carbides are also precipitated at the grain and dendrite boundaries. Fig. 4b shows the morphologies of the eutectic carbides in the outer layer of the zone B (i.e. the layer adjacent to the zone A). It is interesting that some lamellar eutectic carbide clusters resembling the pearlite appear at the grain boundaries and partially at the dendrite boundaries. A few blocky carbides are wrapped in these lamellar carbide clusters or precipitated solely at the dendrite boundaries. Observed at high magnification, the lamellae and the wrapped blocky carbides are more apparent as illustrated in Fig. 4c. With increase of the distance from the zone A, the lamellar eutectic carbide clusters disappear gradually, instead the quantity of the isolated blocky
Fig. 4. Typical morphologies of the eutectic carbides in the tube C0: (a) the thin film-like carbides in the zone A; (b) the morphologies of the eutectic carbides in the outer layer of the zone B; (c) the high magnification of the lamellar carbide clusters shown in (b); (d) the skeleton-like carbides in the zone C.
X.Q. Wu et al. / Materials Science and Engineering A293 (2000) 252–260
256
Fig. 5. EDAX qualitative analysis of the eutectic carbides in the cast tubes. Table 4 Volume fraction of the eutectic carbides in the cast tubesa Tube
Zone B
Zone C
fraction of the eutectic carbides, especially the grain boundary eutectic carbides (see the tube C2). According to Vgb/Vt, it is clear that the ratio of the volume fraction of grain boundary carbides against total carbides in the tube C0 is slightly higher than that in the tube C1, but both of which are much less than that in the tube C2, especially in the zone C. This reveals that most of the eutectic carbides in the tube C2 are distributed at the grain boundaries, while the eutectic carbides in the tube C0 and the tube C1 are mainly distributed at the dendrite boundaries, which is consistent with the microstructure shown in Fig. 6, too. In addition, the relative difference in the volume fraction of the eutectic carbides between the zone B and the zone C, F, reduces successively from 29.5% for the tube C0 to 16.2% for the tube C1 and to 8.63% for the tube C2. This indicates that a higher cooling rate or an applied electromagnetic field during the centrifugal solidification gives rise to a more homogeneous distribution of the eutectic carbides along the radial direction of the cast tubes.
F.%
3.3. Creep rupture strength of centrifugally cast tubes C0 C1 C2
Vt.%
Vgb.%
Vgb/Vt
Vt.%
Vgb.%
Vgb/Vt
6.54 5.95 8.58
2.38 1.84 4.71
0.36 0.31 0.55
9.28 7.1 9.39
4.47 3.08 7.21
0.48 0.43 0.77
29.5 16.2 8.63
a Vt denotes volume fraction of total eutectic carbides;Vgb denotes volume fraction of grain boundary eutectic carbides; F denotes
Vt(zone C)−Vt(zone B) ×100%. Vt6 (zone C)
carbides increases and a few skeleton-like carbides begin to appear at the dendrite and grain boundaries. In the zone C of the cast tube, the eutectic carbides primarily consist of the coarse skeleton-like carbides distributed at the grain and dendrite boundaries as shown in Fig. 4d. The morphology of the isolated carbides distributed at the dendrite boundaries also changes from blocky to skeletal. In addition, in spite of the difference in morphology mentioned above, it is found by the EDAX qualitative analysis shown in Fig. 5 that all the eutectic carbides are similarly rich in Cr and contain some Fe and Ni, too.
3.2.2. Volume fraction of eutectic carbides The volume fraction of the eutectic carbides in each cast tube has been measured and is listed in Table 4. From Vt, and Vgb in the table, it can clearly be seen that the volume fraction of the eutectic carbides in both the zone B and the zone C decreases with increase of the cooling rate (see the tube C0 and the tube C1). But the application of an electromagnetic field during the centrifugal solidification markedly increases the volume
The creep rupture data obtained at 1144 K and 1223 K for the tube C0, the tube C1 and the tube C2 are plotted to give Larson–Miller rupture curves as shown in Fig. 7. It is clear that the tube C0 and the tube C1 almost display a similarity in the creep rupture strength, while the tube C2 shows a markedly higher creep rupture strength. With increase of the creep temperature and the creep time, the advantage in the creep rupture strength for the tube C2 tends to become more distinguished. This reveals that introducing an electromagnetic field during the solidification markedly improve the creep rupture strength of the centrifugally cast 25Cr20Ni heat-resistant tube.
4. Discussion
4.1. Macrostructures As described previously, the macrostructures of the centrifugally cast 25Cr20Ni heat-resistant steel tubes may be divided into three zones along the radial direction of the tubes and be considerably influenced by the initial solidification conditions. In process of the centrifugal casting, when the molten steel is poured into a rotating metal mold, the mold wall (at a much lower temperature than the melt) rapidly cools the zone of the melt in contact with it, which results in a considerable magnitude of thermal and constitutional supercooling. So, heterogeneous nucleation will occur relatively rapid and a fine and equiaxed zone of crystal will be obtained. The above process is too rapid to be influenced
X.Q. Wu et al. / Materials Science and Engineering A293 (2000) 252–260
257
by the solidification conditions. Therefore, a similar chill zone (the zone A) is formed on the outer wall for all three tubes (Fig. 3). Moreover, the above chill effect gives rise to a sharp temperature gradient rapidly built up at the front of the solidification interface, and
Fig. 7. Larson – Miller rupture curves for the centrifugally cast 25Cr20Ni heat-resistant steel tubes.
Fig. 6. Microstructures in the zone C of the cast tubes: (a) tube C0; (b) tube C1; (c) tube C2.
promotes the grains grow along a preferred orientation parallel to the heat flow direction. As a result, a columnar grain zone (the zone B) is developed adjacent to the zone A. Since the cast iron mold inherently has a higher cooling ability than the graphite mold, it causes a higher temperature gradient in front of the solidification interface. Furthermore, a higher cooling rate can effectively prevent the chill nuclei (induced by the chill effect) from drifting away the mold wall and restrain the equiaxed grains developed in the bulk melt in advance of the solidification interface. In this context, a higher cooling rate diminishes the barriers for the columnar grains to develop continuously along the radial direction of the cast tubes. So, the tube C1 shows the continuous and coarse columnar grains along its radial direction (Fig. 3b)(Table 2). However, the graphite mold results in a relatively lower temperature gradient at the solidification front. Moreover, the drifting of the chill nuclei from the graphite mold wall to the bulk melt is relatively common. Thus, with advance of the solidification interface, some new grains may be developed in the liquid phase before the solidification interface. The growth of these new grains will interrupt or retard the continuous growth of the columnar grains. Consequently, the shorter columnar grains are formed in the tube C0 (Fig. 3a) (Table 2). When an electromagnetic field is introduced during the centrifugal solidification, the interactions between the rotating melt and the magnetic field will give rise to an electromagnetic stirring effect (EMS), which strengthens the fluid flow in front of the solidification interface and accelerates the nucleation of new grains by way of the dendrite broken mechanism or localized remelting mechanism [23–25]. Moreover, the fluid flow induced by the EMS increases the thermal transfer in the bulk melt and results in more homogenous distribution of the temperature in the melt. As a result, the new grains
258
X.Q. Wu et al. / Materials Science and Engineering A293 (2000) 252–260
may possibly survive and grow into the equiaxed grains. Furthermore, the simultaneous nucleation may take place in the relatively cool bulk melt. All above effects will retard the continuous growth of the columnar grains and favor the nucleation and growth of the equiaxed grains in the bulk melt. Thus, substantial quantities of small equiaxed grains are produced in the tube C2 (Fig. 3c)(Table 2). In addition, since the nuclei of the columnar grains adjacent to the mold wall are also multiplied by the EMS during the initial stage of the centrifugal solidification, the columnar grains in the outer wall for the tube C2 are refined markedly, too.
4.2. Eutectic carbides In terms of the structural diagram drawn by Nishino and Kagawa [26], the solidification process of the 25Cr20Ni heat-resistant steel includes L g and L g+M7C3. With proceeding of the solidification, the solute atoms such as C and Cr are released into the bulk melt continuously from the solidification interface. Along with advance of the solidification interface and dropping of the solidification temperature, the solutes gradually concentrate in the residual melt, finally the melt reaches the eutectic compositions of the 25Cr20Ni steel and the eutectic transformation takes place. According to the solidification sequence, the dendrite and grain boundaries are the final solidification zones, at later stage of the solidification, the residual melt rich in solutes primarily congregates in these zones. Therefore, after complete solidification, the eutectic carbides predominantly appear at the dendrite and grain boundaries, in which Cr is rich as shown in Fig. 5. The morphology and distribution of the eutectic carbides (M7C3) in the cast tubes are closely related to the redistribution of the solutes and the growth conditions of the presolidified g grains during the centrifugal solidification. According to the solute distribution model developed by Burton et al. [27], the effective partition coefficient kE solute can be described as following k0 kE = (1) k0 +(1−k0) exp( − RdN/DL) where k0 denotes the equlibrium partition coefficient of solute; R the speed of solidification interface or the speed of solidification; dN the thickness of diffusion boundary layer in advance of the solidification interface; DL the diffusion coefficient of solute in liquid phase. In terms of the Scheil equation, the solute distribution in a definite volume is E − 1) CL = C0 f (k L
(2)
here CL denotes the solute concentration in the residual melt, C0 the initial solute concentration of the melt, fL the fraction of the residual melt. Clearly, the higher the kE, the lower the CL namely, the smaller the solute concentration in the residual melt.
4.2.1. Zone A In the zone A for each cast tube, the cooling rate is the highest since the chill effect caused by the cast mold wall. A large number of g grains are nucleated simultaneously and the growth of the pre-solidified g grains is very rapid in this zone. In terms of Fig. 1 and equation Fig. 2, kE tends to approach unity on account of the higher speed of solidification R and CL is smaller correspondingly. This reveals that the solute partition is suppressed to a large extent during the solidification in this zone. Therefore, with advance of the solidification interface, most of the solutes are trapped in the pre-solidified g grains and little amount of the solutes is rejected into the melt in front of the solidification interface. At later stage of the solidification in this zone, very little amount of the melt resided among the g grains can reach the eutectic compositions. So, after complete solidification, the thin film-like eutectic carbides shown in Fig. 4a are precipitated along the boundaries of the g grains. 4.2.2. Zone B In the zone B, the solidification process is similar to the directional solidification since the thermal flow is along the radial direction of the cast tubes and is opposite to the growth direction of the g grains. In the outer layer of the zone B (the layer adjacent to the zone A), the chill effect caused by the cast mold wall is weakened relatively. The nucleation and growth of the g grains also slow down. According to the Fig. 1 and Fig. 2, kE tends to change towards k0 on account of the decrease of R and CL is inclined to increase correspondingly. The solute partition in this layer during the solidification is relatively sufficient as compared with the zone A and more solutes are rejected into the bulk melt from the solidification front. At later stage of the solidification in this layer, relatively more melt resided at the dendrite and grain boundary zones may reach the eutectic compositions. As a result, some blocky eutectic carbides and lamellar eutectic carbide clusters resembling the pearlite appear in this zone (Fig. 4b). Since this layer is just close to the zone A, the cooling rate and the supercooling caused by the cast mold wall is still higher, the lamellar eutectic carbides are very thin (Fig. 4c). With increase of the distance from the cast mold wall, the cooling rate and the supercooling lower gradually, thus the blocky carbides increases and a few skeleton-like carbides begin to appear at the dendrite and grain boundaries. 4.2.3. Zone C In the zone C, since it is far from the cast mold wall, the thermal transfer and the growth of the g grains in this zone slower again as compared with the zone B. Analogously, kE, tends to increase towards k0, on account of the decrease of R and CL is inclined to increase according to Fig. 1 and Fig. 2. Therefore, after complete
X.Q. Wu et al. / Materials Science and Engineering A293 (2000) 252–260
solidification in this zone, the coarse and skeleton-like eutectic carbides are precipitated at the dendrite and grain boundaries as shown in Fig. 4d.
4.2.4. Influences of the solidification conditions It is clear from Table 3 and Fig. 6 that the initial solidification conditions (cooling rate and applied electromagnetic field) have marked influence on the distribution of the eutectic carbides in the centrifugally cast tubes. In general, since the thermal dissipation in the root zones of the dendrites and grains is slower, the growth rate in these zones is smaller and the solutes rejected from the solidification interface are primarily aggravated in these zones. So, after solidification, lots of the eutectic carbides are distributed at the dendrite and grain boundaries. A typical example is shown in Table 3 (the tube C0) and Fig. 6a. With increase of the cooling rate, the growth rate of the dendrites and grains increases. According to the Fig. 1 and Fig. 2, kE tends to increase towards unity on account of the increase of R and CL is inclined to decrease correspondingly. This reveals that the segregation of the solutes in the root zones of the dendrites and grains is restrained to some extent. Thus the eutectic carbides formed at the dendrite and grain boundaries decrease after solidification as shown in Table 3 (the tube C1) and Fig. 6b. As stated previously, an electromagnetic field introduced during the centrifugal solidification induces an EMS. Such an EMS enforces the convection in the bulk melt, decreases the solute enrichment ahead of the solidification front, and homogenizes the solute distribution in the bulk melt [25]. Thus, the distribution of the eutectic carbides precipitated in the tube C2 is more homogeneous along the radial direction. The forced convection above will decrease the thickness of the diffusion boundary layer in advance of the solidification interface and result in the decrease of dN. Moreover, as shown in Fig. 3c and Fig. 6c, the EMS not only promotes the development of the equiaxed grains, but also induces the refining of both the columnar grains and the equiaxed grains in the cast tube. The nucleation of these fine grains is unavoidable to release a large amount of latent heat and delay the solidification speed R. The influences of the EMS on dN and R will cause that kE, decreases towards k0 and CL increases correspondingly in terms of Fig. 1 and Fig. 2. As a result, more residual melt may reach the eutectic compositions at later stage of the solidification. Therefore, the volume fraction of the eutectic carbides markedly increases as shown in Table 3 (the tube C2). In addition, the EMS strongly promotes the grain refining, which causes most of initial dendrite boundaries are turned into the equiaxed grain boundaries, thus the eutectic carbides are primarily aggregated at the grain boundaries rather than the dendrite boundaries in the tube C2 (Table 3) (Fig. 6c).
259
4.3. Creep rupture strength In general point of view, the grain boundaries are the weakness in the deformation at high temperature for the pure metal or the single-phase alloy. Therefore, the finer the grains, the lower the deformation resistance, and the lower the creep rupture strength. However, For the 25Cr20Ni heat-resistant steel studied in present work, lots of the eutectic carbides are precipitated at the boundaries of the g grains and dendrites as described previously. Thus, the grain boundaries in the 25Cr20Ni heat-resistant steel must play a different role in the creep at high temperature as compared with the single-phase alloy. According to the creep model combined matrix/ boundary strengthening [28], the creep speed (or creep resistance) o can be expressed as o; = A
s− sb0 E
n
exp −
Qsd RT
(3)
Here sb0 denotes a back stress acting in the grain center zone due to intergranular carbide precipitation for a given applied stress s and can be expressed by
2KGbs sb0 = m d
1 2
(4)
Where m can be regarded as the intergranular carbide density, K is the constant, G is shear modulus, b is the length of the Burger’s vector and d is the mean grain diameter. Clearly, the carbides precipitated along the grain boundaries will improve the creep resistance. Moreover, the bigger the precipitated carbide density m along the grain boundaries and the smaller the mean grain diameter d, the larger the back stress abo, and thus the higher the creep resistance. From Fig. 3c and the Vgb/Vt in Table 3, it is clear that an applied electromagnetic during the centrifugal solidification results in a remarkable grain refining and causes more eutectic carbides precipitated along the grain boundaries, thus the tube C2 shows the highest creep rupture strength among the three cast tubes (Fig. 7). Although the total volume fraction of the eutectic carbides in the tube C0 is very similar to that in the tube C2, but most of the eutectic carbides in the tube C0 are distributed at the dendrite boundaries rather than the grain boundaries, so the creep rupture strength for the tube C0 is lower as compared with the tube C2. In spite of the least total volume fraction Vt, of the eutectic carbides in the tube C1, the Vgb/Vt is close to that in the tube C0, thus the creep rupture strength is similar for this two cast tubes (Fig. 7). In addition, according to the pile-up theory of dislocation [29], the concentration stress t at a distance r ahead of the pile-up can be expressed as
L t= t0 r
1 2
(5)
260
X.Q. Wu et al. / Materials Science and Engineering A293 (2000) 252–260
Here t0 is the applied stress, L is the pile-up length (or the average distance from the dislocation source to grain boundary) and can generally be taken as the grain radius d/2. Clearly, the finer the grains, the smaller the stress concentration at the grain boundaries, and the lower the drive force of the nucleation for the creep cavities or cracks. Moreover, in terms of the results obtained by Zhu et al. [30], the greater percentage of the eutectic carbides present in the material provides higher resistance to the linkage of the creep cavities or cracks. For the tube C2, the applied electromagnetic during the centrifugal solidification induces the remarkable grain refining and increase of the eutectic carbides precipitated along the grain boundaries, which is effective to increase the resistance to nucleation and linkage of the creep cavities or cracks. This effect may also be one of reasons that the tube C2 show the most excellent creep rupture strength among the three cast tubes.
5. Conclusions 1. The eutectic carbides in the centrifugally cast 25Cr20Ni heat-resistant steel tubes are primarily precipitated at the dendrite and grain boundaries and show various morphologies from the outer wall to the inner wall along the radial direction of the cast tubes, consisting of the thin film-like carbides, the blocky carbides, the lamellar carbide clusters resembling the pearlite and the skeleton-like carbides. 2. The initial solidification conditions have significant influences on the grain morphologies and the distribution of the eutectic carbides in the centrifugally cast 25Cr20Ni heatresistant steel tubes. Increasing the cooling rate markedly promotes the development of the columnar grains and restrains the precipitation of the eutectic carbides, while an applied electromagnetic field during the centrifugal solidification results in a notable grain refining and a marked change of the precipitation zones of the eutectic carbides from the dendrite boundaries to the grain boundaries. 3. The morphology and distribution of the eutectic carbides in the centrifugally cast 25Cr20Ni heat-resistant steel tubes are closely related to the solute redistribution and the growth conditions of the pre-solidified g grains during the solidification. 4. The introduction of an electromagnetic field during the solidification markedly improves the creep rupture strength of the centrifugally cast 25Cr20Ni heat-resistant steel tubes. The main reasons may be that the refined grains and the increased eutectic carbides along the grain boundaries caused by the EMS increases the creep resistance and retards the nucleation and linkage of the creep cavities or cracks. .
Acknowledgements The financial supports received from the Sinopec Technology and the Science and Technology Foundation of Liaoning are gratefully acknowledged.
References [1] D.J. Cox, D.E. Jordan, Materials Technology in Steam Reforming Processes, Pergamon Press, London, 1964, p. 121. [2] J.A. VanEcho, D.B. Roach, A.M. Hall, J. Basic Eng. Trans. ASME 89 (1967) 465. [3] S. lbarra, Met. Prog. 117 (2) (1980) 62. [4] R.H. Kane, Corrosion 37 (4) (1981) 187. [5] D.J. Hall, M.K. Hossain, R.F. Atkinson, High Temp. High Press. 14 (1982) 527. [6] S.J. Zhu, P.E. Li, J. Zhao, Z.B. Cao, Mater. Sci. Eng. A 114 (1989) 7. [7] E.A.A.G. Ribeiro, R. Papaleo, J.R.C. Guimaraes, Metall. Trans. A 17 (1986) 691. [8] M.B. Zaghloul, T. Shinoda, R. Tanaka, Trans. ISU 17 (1977) 28. [9] L.T. Shinoda, M.B. Zaghloul, Y. Kondo, R. Tanaka, Trans. ISU 18 (1978) 139. [10] H. Wen-Tai, R.W.K. Honeycombe, Mater. Sci. Tech. 1 (5) (1985) 385. [11] G.D. Barbabela, L.H. Almeida, T.L. Silveira, I. May, Mater. Char. 26 (1) (1991) 1. [12] G.D. Barbabela, L.H. Almeida, T.L. Silveira, I. May, Mater. Char. 26 (1) (1991) 193. [13] G.D.A. Soares, L.H. Almeida, T.L. Silveira, I. May, Mater. Char. 29 (4) (1992) 387. [14] C.W. Thomas, M. Borshevsky, A.N. Marshall, Mater. Sci. Tech. 8 (10) (1992) 855. [15] R.A.P. lbanez, G.D.A. Soares, L.H. Almeida, I. May, Mater. Char. 30 (4) (1993) 243. [16] C.W. Thomas, K.J. Stevens, M.J. Ryan, Mater. Sci. Tech. 12 (5) (1996) 469. [17] T. Shinohara, I. Kohchi, K. Shibata, J. Sugitani, K. Tsuchida, Werkst Korros. 37 (1986) 410. [18] Y Yang, Q. Liu, Y Jiao, Z. Hu, G. Jia, G. Zhang, Y Gao, J. Zhang, Proceedings of International Symposium on Electromagnetic Processing of Materials, Nagoya, Japan. [19] Y Yang, Q. Liu, Y Jiao, Z. Hu, J. Zhang, Y Gao, Proceedings of the 2nd Pacific Rim International Conference on Advanced Materials and Processing, Kyongju, Korea, the Korea Institute of Metals and Materials, June, 1995, pp. 149. [20] Y Yang, Q. Llu, Y Jiao, et al., ISU Int. 35 (4) (1995) 389. [21] W. Zhang, Y. Yang, Q. Liu, et al., Mater. Sci. Technol. 14 (4) (1998) 306. [22] H.L. Ren, Testing Methodologies for Metallography, Metallurgy, Industrial Publishing Company of China, Bejing, 1986, p. 161. [23] W.D. Griffiths, D.G. McCartney, Mater. Sci. Eng. A 216 (1996) 47. [24] A. Prodhan, D. Sanyal, J. Mater. Sci. Lett. 16 (1997) 958. [25] X.Q. Wu, Y.S. Yang, Y.F. Zhu, Z.Q. Hu, Z. Metall. 90 (7) (1999) 531. [26] K. Nishino, N. Kagawa, Tetsu-to-Hagane 58 (1) (1972) 107. [27] J.A. Burton, R.C. Prim, W.G. Slichter, J. Chem. Phys. 21 (1953) 1987. [28] J.S. Zhang, P.E. Li, J.Z. Jin, Acta Metal. Mater. 39 (12) (1991) 3063. [29] A.N. Stroh, Adv. Phys. 6 (1957) 418. [30] S.J. Zhu, Mater. Sci. Eng. A 127 (1990) L7.