2021 年 61 巻 5 号 p. 1660-1668
A 27 wt.% Cr white cast iron has been subjected to various destabilization heat treatments. The transformation of the matrix phase as well as the precipitatiopn of secondary carbides by destabilization heat treatments, which have been clearly determined in this paper. The results revealed that in the destabilization temperature (about 880°C), the matrix phase is enriched with elements such as C and Cr, due to dissolution of eutectic carbides at high temperature. The secondary carbides were precipitated along grain boundary of C, Cr- rich matrix, they grew up within the matrix phase. When the destabilization temperature increases up to 1000°C the number, volume and size of secondary carbides also increase, respectively (secondary carbide size up to 2,22 µm). At 1050°C/3 h, the size of secondary carbides reduce significantly with a high distribution density in the matrix phase (grain size reduce to below 0,8 µm). At 1100°C and holding time for 3 hours, secondary carbides were dissolved into the matrix, and therefore, reduce the number and grain size of secondary carbides. Effects of secondary carbides on corrosion of alloy were determined by polarization test of alloys in H2SO4 5 vol.% solution at room temperature, by the depth of corrosion layer and by microstructure analyzing of corroded surfaces of alloys. The 1050°C/3 h alloy is the best corrosion resistance between the tested alloys with a large amount of fine secondary carbides and uniformly distributed within the matrix.
Chromium white cast iron, also known as the Fe–Cr–C alloy system, has chromium content up to 40 wt.% and carbon content can be up to 4 wt.%. High chromium gives these irons good wear resistance, impact toughness and even good corrosion resistance, so they are widely used in many industrial applications such as mineral processing, cement manufacturing, slurry pumping, and pulp and paper manufacturing industries.1,2,3,4,5,6) High chromium white cast irons could be hypoeutectic, eutectic and hypereutectic based on the ratio of carbon and alloy elements added.2,3) The hypoeutectic high Cr irons solidify as primary austenite dendrites with a network of interdendritic eutectic carbides M7C3 filling the gap of those primary austenite dendrites.7,8,9) Above 12% Cr, the form of eutectic carbides changes to the discontinuous M7C3 type.10,11)
It has been suggested by several authors that the secondary carbides play important roles in determining the mechanical properties of high chromium white cast iron.12,13,14) The size, number, volume and distribution of secondary carbides in high chromium white cast iron depend on chemical composition, temperature and holding time of destabilization heat treatments.15)
The single phase microstructure materials are most suitable in resisting corrosion. However, the materials have two phase microstructure, therefore, the behavior corrosion will depend on the morphology of the second phase and the different in electrochemical potential of the two phases.12) On the high chromium white cast iron, there was a different microgalvanic coupling between the carbides and the matrix. It was found that the matrix acted as a sacrificial anode, since the corrosion potential of the carbides was believed to be more noble than that of the matrix.16) Therefore, in the corrosion of high chromium white cast iron, the matrix phase is very important because it is responsible for the corrosion behavior of the alloy. Corrosion of alloy involves the damage of surfaces by electrochemical metallic dissolution. Lu et al.17) studied the effects of the precipitation of the secondary phase on corrosion of Mg–Zn–Ca alloy and they suggested that the volume fraction of secondary phase and grain size are both key factors controlling the bio-corrosion rate of the Mg–Zn–Ca alloy. The type of secondary carbides depends on the alloy composition and destabilization temperature.18) Secondary carbides consist of M7C3 and M23C6 or both.19,20,21,22) Dudzinski et al.22) reported that in the case of Cr7C3, iron, molybdenum or vanadium can replace the chromium atoms in the lattice of this carbide to give (Fe,Cr)7C3 or M7C3 carbides.19)
The precipitation of the secondary carbides depends on the destabilization treatments of alloy. Thus, the matrix phase was also effected by the destabilization temperature. The destabilization temperature range of 800–1100°C for time of to 8 hours holding.15,31) This precipitation process always includes three different stages: nucleation, growth and coarsening. Almost all of the literatures reports concluded that precipitation occurred during the holding period. Furthermore, even at longer holding times and higher holding temperature cause a coarsening of the carbides and reduces the number of secondary carbides.19)
Many studies on the precipitation process of secondary carbides from destabilization heat treatments on high chromium white cast iron. However, these studies mainly evaluated the effect of temperature destabilization on formation and growth of secondary carbides and their shape that missed the change of the matrix phase with increasing temperature of the destabilization, nucleation sites for secondary carbide precipitation. When the destabilization temperature increases up to a certain temperature, the matrix phase is enriched with some elements such as C and Cr and since then started the formation and growth of secondary carbides. This paper analysed the effects of destabilization temperature on the matrix phase, the formation and growth of secondary carbides of 27 wt.% Chromium white cast iron, thereby evaluated their effects to the corrosion of 27 wt.% Cr white cast iron.
The alloys, which have the composition as indicated in Table 1, was melted in a medium-frequency induction furnace at 1550°C (± 50°C), cast in vacuum sand mold in the specimens φ30×300(mm), followed by destabilization heat treatments at from 850°C to 1100°C with diferent period time of the holding (up to 4 h holding time) and then cooled with the furnace. The alloys are cut to dimensions of 10 mm×10 mm×10 mm, sand with SiC paper, and polish with Cr2O3 power. These alloys were polished and color etching with solution (3 g Na2S2O5 plus 30 ml distilled water and 2 ml HNO3 96 vol.%) or by H2SO4 5% solution. The etching time can from some munites (color etching) to 20 mumites (in H2SO4 vol.% solution) before anslysing the microstructure of alloy.
| Alloy | C | Mn | Si | Cr | Ni | Mo | Fe |
|---|---|---|---|---|---|---|---|
| As-cast | 2.03 | 0.46 | 0.33 | 27.5 | 1.37 | 0.84 | Bal. |
The microstructure characteristics of alloys were analyzed by using optical microscope (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction analysis (XRD- with X’pert Pro instrument: K-alpha=1.54A°; Anode: Cu; Goniometer Radius: 173 mm). The volum fractions of eutectic carbides, secondary carbides size were analyzed using a Carl Zeiss digital image analyzer. The microhardness of carbides, matrix phase was measured with a Leica Vickers microindenter (with a load of 10 g or 100 g). Electrochemical corrosion tests were used to investigate the corrosion behavior of alloys in H2SO4 5 vol.% solution at room temperature. Alloys were epoxy coated with 1 mm2 area of electrode surface, sanded with SiC paper (1000 grit size), and polished before were tested by the galvanostatic. The evaluation method for corrosion depth of alloys by the digital optical microscope with a large depth-of-field, Keyence’s VHX-6000 Series. The experimental data: corrosion tests, image analyzing and microhardness, they all were obtained at room temperature.
Figure 1(a) showed the microstructure of as-cast alloy: primary austenite (γ) and eutectic carbide M7C3. The microstructure of the as-cast alloy basically consists of metastable austenitic dendrites and an eutectic mixture of austenite/M7C3. After heat treatment, they easily transforms into another type such as martensite (X-ray diffraction results show in Fig. 5). These results are common in many experimental efforts.23,24,25,26,27)
The change of primary austenite on destabilization heat treatments (all of alloys were eched by the same color etching). (Online version in color.)
Destabilization heat treatments at different temperature (from room temperature up to 900°C) and different holding time (up to 3 hours) indicated the change about microstructure of matrix phase (Figs. 1(b)–1(d)). Clearly, there had been a change in the primary matrix with increasing of destabilization temperature (850–900°C). At higher temperatures (900°C - 1h holding time), there was a sensitivity to color of etching solution within some special zones of matrix phase (arrows in Fig. 1(e)), this due to in this zones, secondary carbides were formed (authors used color etching to corrode the matrix for all of researching alloys). This changing of matrix phase increases from the 900°C to 1100°C (Figs. 1(f)–1(h)). Microstructure of matrix phase changes with increasing destabilization temperature. At 1100°C/3 h destabilization heat, the austenite phase was found in the matrix (Fig. 1(h) and x-ray diffraction results in Fig. 5).
Figure 2(a) showed the microhardness of matrix phase at 880°C/2 h holding time. The microhardness of matrix phase is 380 HV for the as-cast (Table 2), at 880°C/2 h holding time, microhardness of matrix phase increased to 778 HV, even is 1173 HV (Fig. 2(a)), It had been strongly increasing about microhardness of the austenite phase when destabilization treatments. Figures 2(b)–2(c) illustrates EDS of the matrix phase of the as-cast and the 880°C/2h alloy. The content of some elements such as C, Cr, Fe was difference between two alloys. In particular, the content of C, Cr of the alloy with the holding time for 2 hours at 880°C was higher than the ones of as-cast. So, destabilization heat treatments at a high temperature, the matrix phase is enriched with C, Cr, which has high microhardness. Figures 2(d)–2(e) showed some zones that eutectic carbides were expanded, even they can be melted. Eutectic carbides can be thermal expanded, this lead to decrease their microhardness as well as lead to the difference about microhardness at certain positions of the same particle. Figure 2(f) revealed that the microhardness in some regions of the eutectic carbide are significantly different. The microhardness at the center of a particle is 2149 HV0.01 but at the edge position of the same particle is only 449 HV0.01. Figure 3 showed the different hardness values at some positions (center and edge) in big eutectic carbide with the increase of destabilization heat treatment (from 900–1100°C). This big difference hardness values between some positions is highest at 900–950°C and at higher destabilization temperature, the difference hardness between positions tends to decrease. At 1100°C, this different hardness is only 10–15% (at center and at edge of particle). This results due to: at high destabilization heat treatment, the eutectic carbides were expanded first, thus led to decrease the hardness value. At higher temperature, they can be dissolved into the matrix, so at high temperature (1050–1100°C), the difference hardness value between some positions in the same particle is not much. Thermal expansion and the dissociation of eutectic carbides is also verified by the results of the volume fraction of eutectic carbides (Table 2). The volume fraction of eutectic carbides increases from as-cast to the heat treated at 950°C/3 h alloy (45%) and had 38 vol.% at 1100°C/3h destabilization temperature. This process leads to some elements such as C, Cr, Fe diffuse from dissolved eutectic carbides to the matrix. So, if destabilization temperature is high enough, matrix phase was enriched with C, Cr is formed. At high temperature, austenite can dissolve higher carbon content.21)
Optical micrographs (a, d, e, f) and EDS spectra from the matrix (b–c) show the microstructure changes of matrix phase upon destabilization heat treatments. (Online version in color.)
| Alloy | EC volume (vol.%) | Microhardness of matrix (HV) | Size (length) of SC (μm) |
|---|---|---|---|
| As-cast | 35,1 | 380 | – |
| 880°/3 h | 43 | 778 | – |
| 900°/3 h | 43,6 | 860 | 0,30 |
| 950°/3 h | 45 | 946 | 1,7 |
| 1000°/3 h | 42,1 | 1034 | 2,22 |
| 1050°/3 h | 40,3 | 1241 | 0,8 |
| 1100°/3 h | 38 | 1109 | 0,4 |
Diagram shows the different hardness values at some positions (center and edge) in the coarse eutectic carbide at different destabilization temperature (from 900–1100°C). (Online version in color.)
A number of studies on high-Speed Steel confirmed that there are solution of eutectic carbides by austenitizing heat treatments.28,29) When studied the effect of Austenitizing Conditions on the Microstructure of AISI M42 high-Speed Steel. Yiwa Luo et al.28) reported that the eutectic carbides dissolve into the metallic matrix and their continuous network distribution changed into the broken network and afterwards carbon and alloying elements dissolved into the austenite.
Table 2 showed the effect of destabilization heat treatments on the microhardness of the matrix and size of secondary carbides of the studying alloys. This was because of the expansion of eutectic carbides at high destabilization temperature. When eutectic carbides were dissolved, some elements such as C, Cr, Fe diffusus to the matrix, a Cr, C-rich matrix phase was formed. The secondary carbides also precipitated in there. The microhardness of matrix also changes due to the precipitation and growth of secondary carbides. The highest microhardness of matrix is at the 1050°C/3 h alloy (Table 2).
Figure 4 showed the formation and growth of secondary carbides during destabilization heat treatments. Figure 4(a) illustrates the structure of the matrix phase at 880°/3 h holding (the matrix phase was enriched with C, Cr) with the present of fine austenite grain. At 900°C/1 h, the fine secondary carbides precipitated firstly at along austenite grain boundaries and afterward secondary carbides grew up when increasing the holding time (for 2–3 hours –Figs. 4(c)–4(d)). Increase in the destabilization temperature to 1000°C causes further microstructural changes, the grain size of the secondary carbides increase significantly. The largest size particle can be up to 2,2 μm at 1000°C/3 h destabilization treatments alloy (Fig. 4(e)). At higher destabilization temperature (1050°C), the grain size of secondary carbides has reduced significantly (only 0,8 μm), and the morphology of secondary carbides precipitated uniform within matrix. At 1100°C for 3 h holding, the distribution and grain size of secondary carbides were shown in Fig. 4(g). Compared to the 1050°C/3 h alloy, it was found that the volume fraction and grain size of secondary carbides reduce. Figure 3(g) have found that the dissolution of secondary carbides within the matrix. Bedolla-Jacuinde et al.27) also reported that at 1150°C, secondary carbides volume fraction of 15 vol.% occurred at only 5 min; subsequently the volume fraction diminished to 2 vol.% at 1 h soaking. They reported that at 1150°C, initial amount of secondary carbides precipitated during heating to the holding temperature and these carbides dissolved during holding when the equilibrium composition of chromium and carbon in austenite was reached.
The precipitation and growth of secondary carbides during destabilization heat treatments. (Online version in color.)
Figure 5 showed the shape of eutectic carbides (in the casting alloy) and the secondary carbides of alloys after destabilization heat (at 900°C/3 h, 1000°C/3 h and 1050°C/3 h respectively). Eutectic carbides are long rod-like, hexagonal M7C3 carbides (Fig. 5(a)). At 900°C, fine secondary carbides were precipitated in the matrix (about 200−300 nm), their shape is rod-like. When the destabilization heat temperature increase from 900°C to 1000°C, grain size of secondary carbides increased (up to 2 μm) but they are not changed their shape (Figs. 5(b)–5(c)). At 1050°C/3 h, long rod-like secondary carbides become short rod-like carbides (the ratio between wide and length of a particle is about 0.8, if it will be perfect sphere if this value is 1-Fig. 5(d)). Besides, Karantzalis et al.7) and Powell et al.30) suggested that white cast iron with high Cr content, the secondary carbide stoichiometry followed the sequence M7C3, and M23C6. Their morphology varied from rod like, for M7C3, to fiber like for M23C6. The microstructures of the destabilized alloys after heat treatment at 900 and 1050°C for 3 hours also was illustrated by X-ray diffraction results (Figs. 5(e)–5(f)). The carbides received after the destabilization are only M7C3.
Morphology of eutectic carbides M7C3 and secondary carbides at difference destabilization heat treatment (a–d); X-ray diffraction results for heat treated alloys (e–f). (Online version in color.)
The C, Cr- rich matrix phase was formed due to the diffusion C, Cr from the dissolved eutectic carbides at high destabilization treatments. The secondary carbides precipitate along grain boundaries of the C, Cr- rich matrix phase, their growth, distribution and morphology depend on the time and temperature destabilization. The secondary carbides absorb C, Cr, Fe.., they grow in any directions with different sizes. If the secondary carbides grow in the same direction, they can mix together and grow into the larger particles (at 1000°C) or grow independently with a lot of directions to form fine particles (at 1050°C). The mount of the retained austenite can be presented at the matrix after this transformation (Fig. 4(h)).
3.2. Effect of the Morphology and Size Grain of Secondary Carbides on the Corrosion BehaviorThe alloys were soaked in H2SO4 5 vol.% in different periods of to estimate the depth of corrosion layer (Fig. 6(a)). There was a significant different in the depth corrosion between alloys. The alloy at 1050°C/3 h destabilization treatments, which had the lowest depth of corrosion, the depth corrosion was measured at the deepest zone of the alloy (heated at 1050°C/3 h) (Fig. 6(b)). This proved that 1050°C/3 h alloy, which had the highest corrosion resistance.
The effect of precipitation of secondary carbides on the depth corrosion (a) (the depth corrosion was measured at the deepest zone of the alloy in H2SO4 5 vol.%). (b- The obtained result of the 1050°C/3 h (1 hour in H2SO4 5 vol.%)). (Online version in color.)
To investigate the corrosion behavior of different alloys, the potential of alloys are carried out in H2SO4 5 vol.% solution at room temperature (25–30°C). Electrochemical corrosion method was used to investigate the corrosion behavior of alloys in H2SO4 5 vol.% solution at room temperature. The methods is the determination of polarization curves and extrapolation of linear parts of the polarization curves. The intersection point between the extrapolated Tafel regions, in other words the over-voltage curves, gives the corrosion current. And the other main electrochemical method for determination of corrosion rates is the (linear) polarization resistance method.
The polarization curves of alloys in 5% H2SO4 are shown in Fig. 7 with corrosion data obtained from the curves given in Table 3. The corrosion current density of the 1050°C/3 h had the lowest value (1,3639 mA/cm2), so the corrosion rate also had the lowest (15,848 mm/year). This is the best corrosion resistance between tested alloys. The 1100°C/3 h had the lowest corrosion potential but the corrosion current density is higher than 1050°C/3 h alloy so the corrosion rate is also higher. The 900°C/3 h alloy has the most negative corrosion potential and corrosion current density (−508,94 mV and 5,0314 mA/cm2, respectively), so the corrosion rate of this alloy has also the highest value (63,138 mm/year). Moreover, the 1050°C/3 h alloy was significantly reduced the current density at −200 mV potential. This due to in the corrosion process there was formed a passive film. When this film was broken down, the corrosion process will be continued. Therefore, based on the results obtained in the galvanostatic polarization method and corrosion depth determining method, they have no real concerns but they complement each other to confirm the same result.
Polarization curves of the alloys in H2SO4 5vol.% solution at room temperature. (Online version in color.)
| Alloy | Ecorr. (corrosion pontential -mV) | Jcorr. (corrosion current density mA/cm2) | Corrosion rate (mm/year) |
|---|---|---|---|
| as-cast (1) | −488,45 | 5,0314 | 58,465 |
| 900°/3 h (2) | −508,94 | 5,4336 | 63,138 |
| 950°/3 h (3) | −500,64 | 3,9473 | 45,866 |
| 1000°/3 h (4) | −470,85 | 3,6272 | 42,148 |
| 1050°/3 h (5) | −478,261 | 1,3639 | 15,848 |
| 1100°/3 h (6) | −469,71 | 3,4886 | 40,305 |
The corrosion resistance of different alloys was greatly influenced by surface morphologies, precipitation and grain size of secondary carbides. Figure 8 showed the surface morphology of alloys at different destabilization temperature and surface morphology after corrosion, respectively (alloys were soaked in H2SO4 5 vol.% solution in 5 hours). In the as-cast alloy (Fig. 8(a)) always defects in the eutectic carbides/matrix interface so the alloy was corroded, this boundaries are the high corrosion zones. The corrosion process also happened at austenite grain boundaries and formed holes within the matrix (arrow, Fig. 8(b)). Neville et al.31) reported that the corrosion attack is initiated around the M7C3 carbides (carbides/matrix interface). The corrosion solution will attack at these regions, where exist defects surface and so the surface of alloy was destroyed. Figures 8(d), 8(f), 8(k) illustrated that there was pits in the matrix and at the matrix/eutectic carbides interface. The effect of the corrosion solution on corroded surface of the 1050°C/3 h alloy was the lowest. This alloy had large amount of fine secondary carbides precipitated in the matrix so there was not much spaces between secondary carbides (Fig. 8(g)). Figure 8(h) indicated the corroded surface of the 1050°C/3 h alloy, the corroded surface of alloy is almost no effects by destroying of the corrosion and there was some small holes due to the corrosion process. The matrix was corroded firstly at the austenite grain boundary, when this grain boundary is filled by secondary carbides, the corrosion at grain boundary will become the corrosion between particles of secondary carbides, this distribution of secondary carbides can be seen as a shielding to protect phase matrix from the corrosion.
Effect of the distribution of secondary carbides on the corrosion behavior (alloys were tested in H2SO4 5 vol.% in 5 hours). (Online version in color.)
The matrix phase plays an important role of the corrosion process of high chromium white cast iron after destabilization. This corrosion happened at eutectic carbides/matrix inter-phase (defects at eutectic carbides/matrix interface). Neville et al.31) reported that there is the galvanic corrosion between two phases in high chromium white cast iron (eutectic carbides and matrix), when they have different microstructure phases. They formed pits and holes in the surface and destroy the surface of material when they were corroded. The formation of secondary carbides at the grain boundaries appeared to have a significant influence on the corrosion of the heat treatments alloys. Lu et al.17) reported in research “Effects of secondary phase and grain size on the corrosion of biodegradable Mg–Zn–Ca alloys” that the volume fraction of secondary phase and grain size are both key factors controlling the corrosion rate of alloy.
As a result of the above mentioned, secondary carbides formed and grew at 880°C to 1000°C within the matrix with size grain increase up to 2,22 μm, however the corrosion resistance of this alloy has not improved yet. But at higher destabilization temperatures (1050°C), the corrosion of alloy significant improved. There are a large amount of fine and near continuous secondary carbides (with the size is in the rang of less than 0,8 μm) within matrix. This distribution density of secondary carbides can be seen as a passive film to reduce the corrosion for alloy.
In general, the precipitation, distribution, grain size, morphology of secondary carbides have a significant effect on the corrosion of alloys. Secondary carbides are formed along austenite grain boundaries within matrix, so the defects can be easily formed in there. If the austenite grain size is fine so the nucleation of secondary carbides are created with large amount, they grow in many directions to form the fine secondary carbides and uniform. This reduces defects and improve the corrosion resistance for alloy. The best corrosion resistance is the 1050°C/3h destabilization heat treated alloy.
The precipitation process of secondary carbides by destabilization heat treatments and their effects on the corrosion of 27 wt.% chromium white cast iron were investigated.
The main conclusions obtained from this study are as follows:
(1) At high destabilization temperature, some eutectic carbides were expanded and dissolved in the matrix. This process created a Cr, C-rich matrix. Secondary carbides precipitated firstly at along grain boundaries of Cr, C - rich matrix phase at 900°C, afterward they grew up with increasing the destabilization heat treatments (include destabilization temperature and holding time). The biggest grain size of secondary carbides were up to 2,22 μm at 1000°C/3 hours treated alloy. The 1050°C/3 hours alloy, grain size of secondary carbides was below 0,8 μm and microstructure consist of large number of small secondary carbides dispersed and distributed with a high density within matrix.
(2) The corrosion behavior of 27 wt.% chromium white cast iron mainly depended on some zones such as the eutectic carbides/matrix interface and austenite grain boundaries. Two factors are controlling the corrosion rate of heat treated alloys: the volume fraction and grain size of secondary carbides.
(3) The 1050°C/3 hours destabilization alloy was the best corrosion resistance, which had large amount of fine particles and distribution in many directions in the matrix and when corroded, a high density of secondary carbides on the surface, which prevents the corrosion for this alloy.
This study was financially supported by Ha Noi University of Science and Technology. (T2018-PC-225). The authors would like to thank Heat Treatment Laboratory, School of Material Science and Engineering, Ha Noi University of Science and Technology for your support and for the help during the test.
