2022 Volume 63 Issue 11 Pages 1557-1566
The relation between microstructure and creep property of austenitic heat-resistant cast steels with and without Nb addition under the condition of repeated vacuum carburizing and quenching was investigated. Cr-carbide scale is formed on the sample surface by a carburizing reaction, resulting in the depletion of Cr in the matrix adjacent to the scale. A carburized layer consisting of various fine carbides is observed below the Cr-depleted layer, and the carburized layer depth is suppressed by Nb addition. When process of vacuum carburizing and quenching is repeated, formation of voids caused by heating and rapid cooling is more remarkable in primary Cr carbides than in primary Nb carbides. As the carburized layer depth increases, creep rupture time of both cast steels shifts to the shorter time side; however, Nb addition is effective for extending creep rupture time at 1303 K.
This Paper was Originally Published in Japanese in J. JFS 94 (2022) 235–244.
In gas carburizing and quenching, the heating temperature ranges from 1173 K to 1223 K. Austenitic heat-resistant cast steels such as JIS SCH13, SCH21 and SCH22 are often used for loading jigs for parts undergoing this treatment.1,2) However, when heating at high temperature and rapid cooling are repeated for a long time, the jig life is remarkably reduced due to cracking or external deformation. Therefore the improvement of its heat resistance is desired. In general, it is known that the addition of nickel (Ni) is effective in improving heat resistance.3,4) But, adding a large Ni content is economically unfavorable because the cost of the raw material is relatively high and its price fluctuates considerably. For this reason, numerous studies have been actively conducted to improve heat-resistance such as high-temperature creep rupture strength or thermal shock resistance by adding a small amount of Nb, Ti, W, and Zr coupled with lowering the Ni content.5–7) Among these alloying elements, it has been reported that primary Nb carbide in Nb-added cast steels has an inhibitory effect on the crack initiation and growth thermal shock and creep environments in air.8,9)
On the other hand, the usage of vacuum carburizing has been expanding in the heat treatment industry in recent years, replacing traditional gas carburizing.10) The processing temperature is about 1273 ± 40 K, which is higher than that of gas carburizing, enabling a shorter carburizing time. As a result, the energy consumption and running cost for heat treatment can be reduced. Furthermore, vacuum carburizing is considered as an environmental-friendly surface treatment because of its low CO2 emissions. Many investigations on this process have been focused on the vacuum carburization behavior of carbon steel and low alloy steel, and on the difference in microstructure and mechanical properties between gas carburizing and vacuum carburizing processes.11–13) However, there are few reports on the high-temperature properties of heat-resistant cast steel for heat treatment jigs in a carburizing atmosphere, and the occurrence of cracks caused by creep as well as damage of jigs because of the higher processing temperature.
In this study, high-temperature vacuum carburizing and quenching treatments were repeatedly performed on the same industrial scale as the actual operating environment of the heat treatment jigs in order to investigate the influence of Nb addition on the microstructural change of heat-resistant cast steel and to examine its creep property as well.
JIS SCH21 steel (hereinafter referred to as “base steel”) and base steel containing 1.0% Nb (hereinafter referred to as “Nb-added steel”) were used as experimental alloys to be subjected to vacuum carburizing and quenching. These steels were melted in a 100 kg high-frequency induction furnace and poured into a sand mold for test coupon (JIS G0307 type a). As for the manufacturing conditions, a mixture of raw materials such as SUS304 scrap, electrolytic nickel, ferrochrome, ferrosilicon, etc. was melted in an air atmosphere. Each melt was then tapped into a ladle at around 1893 K followed by pouring into the mullite-based synthetic sand mold at the pouring temperature of 1793 K. The chemical compositions of ingots were analyzed by means of optical emission spectroscopy. The results are shown in Table 1.
The round bar with a diameter of 35 mm was first cut from the bottom of Y-shape ingot. Subsequently, as shown in Fig. 1(a) and Fig. 1(b), the specimens with a gauge length of 25 mm and diameter of 6 mm for assessment of creep property and ring-shaped specimens with an outer diameter of 25 mm and an inner diameter of 14 mm for microstructural observation were machined. The specimens were vacuum carburized and quenched using acetylene gas. In detail, a basket containing the specimens was placed in the chamber of a vacuum furnace of which the atmosphere was depressurized to 100 Pa. The heat treatment process is shown in Fig. 1(c). The specimens were soaked at 1253 K for 3.6 ks, carburized for 6.0 ks and followed by diffusion for 9.0 ks in the heating chamber. Afterward, the specimens were subjected to homogenization heat treatment in a gas cooling chamber at a quenching temperature of 1123 K for 3.6 ks. The carbon potential (CP) during carburizing was 1.4%, and the CP value for the diffusion stage and quenching was 0.8%. After heat treatment, the specimens were quenched in a 333 K oil bath and tempered at 443 K in air. Such a series of carburizing, quenching and tempering (hereinafter referred to as vacuum carburizing) were repeatedly conducted up to 10 cycles, and the properties were evaluated after 1, 5 and 10 cycles.
Shape and dimensions of specimens for creep rupture test (a) and microstructural analysis (b). Heat patterns of vacuum carburizing and quenching - tempering (c).
The microstructures were examined using a scanning electron microscope equipped with an X-ray energy dispersive spectrometer (SEM-EDS, JEOL, IT-100LA) after these cross-sections were wet-polished followed by buff polishing. In SEM observation, the backscattered electron mode that the contrast of primary carbide could be clearly observed was employed, and images were taken at an acceleration voltage of 20 kV. Carbon concentration profiles were measured using an electron probe micro analyzer (EPMA, Shimadzu, EPMA-1600) under the following measurement conditions: an acceleration voltage of 15 kV, an electron beam current of 0.2 µA, a beam spot of 1 µm and a scanning pitch of 6 µm, which were suitable for the detection of C-Kα radiation. The carbon mapping was performed on a 0.3 mm wide area from the surface of specimen to a depth of 3 mm. For carbon quantitative analysis, a series of standard specimens of Fe–C alloys with different carbon content were employed to measure the intensity of characteristic X-rays at each concentration. The relationship between the C Kα intensity and the carbon content were established to obtain the reference value. The average carbon concentration in the mapping region of the specimen was determined by comparing the carbon X-ray intensity of the mapping area with the reference value. The specimen hardness was measured from the surface to the depth using a micro-Vickers hardness tester (Shimadzu, HMV-G21FA) under a load of 2.98 kN and a dwell time of 15 seconds. Each value in the hardness profile was an average of five measurements.
An X-ray diffractometer (XRD, Malvern PANalytical, Empyrean) was used to identify the carbide phase on the specimen surface and cross-section. The XRD measurements were conducted between 30° and 85° using Cu-target with Ni filter and radiation operating at a voltage of 45 kV, a current value of 40 mA with a step scan of 0.013°.
After vacuum carburizing for 1, 5 and 10 cycles, one specimen was performed creep test under applied stress of 35 MPa at 1303 K in air. For comparison, both steels in as-cast state were also tested under the same condition. Creep tests were conducted at a constant temperature by three type R thermocouples, which were attached to the surface of specimen at the center and two shoulders of its gauge section. After testing, evaluation of microstructures near surface as well as the occurrence of micro-cracks was done by SEM-EDS.
The 10 mm thick specimen was cut from the chuck part of each specimen after vacuum carburizing, and the microstructure of the carburized area was observed at low magnification using SEM. The results are shown in Fig. 2. The appearance of the creep rupture specimens is also shown in the figure. First, visual observation of the entire specimen revealed that the surfaces of base steel and Nb-added steel had a silvery discoloration after 1-cycle carburizing. After up to 10 cycles, the surface color of base steel was changed little, whereas Nb-added steel showed a change to black at several locations. The cross-sectional microstructure shows a difference in color between the surface layer, which is presumed to have been carburized, and the center, which is thought to be non-carburized, as indicated by the white dotted line. Dense and fine carbides are recognized in the dark gray colored surface layer. This carbide layer spreads to the center of the specimen when the number of carburizing cycles increases. The carburized depth is defined as the region from the surface layer to the boundary of the dense carbide layer, indicated by the white dotted line in the figure. Comparison of the carburized depth of both steels revealed the carburized depth of Nb-added steel was smaller than that of base steel even when vacuum carburizing was performed under the same conditions. The difference was slight at low cycles, but it was noticeable at 5 or more cycles of carburizing. From this result, it is obvious that the addition of Nb suppresses the formation of a carburized layer during vacuum carburizing.
Appearance of test pieces after vacuum carburizing at 1253 K and their cross-sectional microstructures. Dotted line indicates carburized layer depth.
Figure 3 shows the hardness profiles in the cross-section of the carburized specimens. The upper figure is the results of base steel, and the lower figure is those of Nb-added steel. For reference, the hardness for as-cast state is also shown in each figure. Hardness of base steel and Nb-added steel in as-cast state (before vacuum carburizing) are almost the same, about 200 HV. When these specimens are vacuum carburized, the hardness shows a maximum value at the surface and gradually decreases toward the center. For both steels, the maximum hardness at surface increases with increasing the number of carburizing cycles, but the value of base steel tends to be higher than that of Nb-added steel in all carburizing cycles. On the other hand, as shown by vertical dotted lines in the figure, I, II and III represent the carburized depth of the specimens carburized for 1, 5 and 10 cycles, respectively. Regardless of test cycle, the hardness of the boundary between the carburized layer and the non-carburized internal region is almost same, about 290 HV. Thus, it is implied that Nb addition does not affect the hardness of the regions. Hardness of non-carburized internal region in both steels is around 200 HV and there is no significant change in hardness, as in as-cast state.
Hardness profiles for vacuum carburized experimental cast steels. (I: 1-cycle carburized depth, II: 5-cycle carburized depth, III: 10-cycle carburized depth)
For confirmation of the difference in the carbon concentration change through the depth direction in both steels due to the vacuum carburizing, 10-cycle carburized specimens were chosen to evaluate the carbon distribution by EPMA. Figure 4 shows the measured carbon concentration from the surface to a depth of 3 mm toward the center. The upper part of the figure is C Kα map and the lower part is the average carbon concentration for the corresponding analyzed area. As seen in the figure, there is little difference in the maximum surface carbon concentration of both steels, which is about 4.9 mass%. This result implies the carbon concentration at the extreme surface is independent of Nb addition. When the carbon concentration is less than 3 mass%, the carburized layer and the non-carburized matrix coexist. The carbon concentration for the non-carburized zone was less than 1 mass%, partly due to primary carbides, while the carbon concentration in the matrix-only region was about 0.2 mass%. Furthermore, a comparison of the carbon concentration corresponding to the same depth revealed that Nb-added steel is lower than base steel. Therefore, it is clear that the increment in carburized depth could be controlled by Nb addition. The reason can be explained as follows. For the carburized steel, the carburizing time was generally known to vary depending on the chemical compositions, especially carbon content (C%). In detail, to obtain the same carburized depth, the higher C% of the material, the shorter the carburizing time because the amount of carbon that is absorbed into the surface and diffuses into the core is lower. On the contrary, if the C% of the material is low, the carburizing time will be longer because the amount of carbon for penetration and diffusion is required much more.14) On the other hand, primary Cr-based carbide formed and coarsen during the solidification followed by cooling of heat-resistant cast steel, resulting in a high carbon amount in liquid phase and solid matrix being consumed. For this reason, when evaluating the carburized depth of heat-resistant cast steel, the amount of solute carbon in the matrix should be considered rather than C% in steel. In the case of Nb-added austenitic heat-resistant cast steel, apart from primary Cr-based carbide, a considerable amount of Nb, which has a very low solid solution limit in the matrix, reacts with carbon to form primary Nb-based carbide. As a result, the amount of consumed carbon is higher due to the formation of two kinds of primary Nb-based and Cr-based carbides, and the remained solute carbon in matrix is undoubtedly lower than in base steel. Therefore, during the diffusion stage of vacuum carburizing, the time required for carbide precipitation to reach the supersaturated state in matrix of Nb-added steel is longer than base steel, resulting in a thinner carburized depth for Nb-added steel. That is, Nb addition plays an important role in suppressing the growth of carburized depth through the formation of primary Nb carbides.
EPMA quantitative mapping and line scan of carbon concentration in 10-cycle vacuum carburized experimental cast steels.
Next, the microstructure of the carburized layer in both steels under each condition was observed by SEM. The observation results are shown in Fig. 5. The as-cast microstructures are also shown in the same figure for comparison. In as-cast state, it is observed that there are Nb carbides (white arrows) in addition to Cr carbides (black arrows) at the grain boundaries of Nb-added steel, unlike base steel. On the other hand, in any carburizing cycles, carbide scale was formed on the surface of specimen. For both steels, the thickness of the carbide scale increased with an increasing number of vacuum carburizing up to 5 cycles, but it seems to become thinner after 10 cycles. Also, many porosities were formed and coalesced at the carbide scale-matrix boundary as carburizing of specimens is repeated more than 5. Furthermore, with regard to the specimens carburized for 10 cycles, the occurrence of crevices was revealed at the boundary. Besides, the carbide-free zone (referred to as white layer) with a thickness of several µm is observed in the near-surface matrix adjacent to the carbide scale. Below the white layer, secondary carbides with various shapes such as lamellar or granular formed by carburization coexist with primary carbides. As shown in Fig. 6, the EDS element mapping revealed that the main constituent elements in the carbide scale are Cr and C. However, it was seen that Cr content in the white layer was almost depleted as compared with the carbide scale and matrix. The formation of the white layer is presumed as follows. When a considerable amount of carbon from carburizing atmosphere was absorbed into the steel during the carburization and diffusion stages, chromium dissolved in the near-surface matrix diffuses to the outermost surface and reacts with carbon to form a carbide scale. As a result, depletion of Cr and enrichment of Ni were observed in matrix near the surface adjacent to the carbide scale. In this study, silicon (Si) was almost distributed homogeneously in the matrix from the outermost layer toward the center. Enrichment of silicon was hardly detected at the surface. In addition, primary Nb carbides tend to be absent in the white layer. As for the cause of Nb carbide disappearance, Toudou et al., reported that solubility of Nb carbides increases with increasing carbon concentration in the high carbon region during the carburization stage of vacuum carburizing, and the volume fraction of Nb carbides in such the region decreases.15)
Change in near-surface microstructure due to vacuum carburizing at 1253 K.
EDS compositional mappings of near-surface microstructures of experimental cast steels after vacuum carburizing.
Here, to identify the phase of carbide formed by carburizing, the surfaces of the specimens carburized for 1, 5, and 10-cycles were examined by XRD. The results are displayed in Fig. 7. Generally, the depth detected by XRD is up to 100 µm from the surface, so the obtained XRD patterns in this study is of the region below the surface including carburized layer. The results of as-cast state are also shown in the same figure. Remarkably sharp diffraction peaks that are attributed to austenite are clearly observed in as-cast state of both steels, but the remained peaks considered to be primary carbides are extremely weak. Based on the analysis of the peak position of carbides, primary carbides existed in the austenitic matrix for base steel were identified as M7C3 (● mark) and M23C6 (○ mark), whereas NbC (◇ mark) and M23C6 were found to coexist in Nb-added steel. On the other hand, M3C peak (▼ mark) was detected in all carburized specimens, and the intensity of M23C6 and M7C3 peaks are higher than in as-cast state. In these carbides, M7C3 peak was relatively higher in intensity, and it was also observed that the intensity of the carbide peak increases with the number of carburizing cycles. Judging from the XRD result and the element mapping image shown in Fig. 6, Cr carbides precipitated and grown in the carburized layer due to the carburizing reaction may be M23C6, M7C3, and M3C. Apart from carbides in austenite in both steels, no diffraction peak of σ phase which causes high-temperature embrittlement was observed.
X-ray diffraction results of surfaces of experimental cast steels after vacuum carburizing at 1253 K.
The above results are discussed based on the equilibrium state diagram. In Fig. 8, the white circle mark in the left figure and the gray one in the right figure indicate the carbon content of base steel and Nb-added steel in as-cast state, respectively. The dashed arrows in the figure indicate the phase change during carburizing of both steels at 1253 K. First, looking at the phase diagram of base steel, M23C6 (M = Cr, Fe) type carbides exist in austenite (γ) in as-cast state (before vacuum carburizing) or at the initial stage of carburizing. The transformation from M23C6 to M7C3 occurs when the carbon concentration in matrix increases through the penetration and diffusion of carbon. In Nb-added steel, carbides in the austenitic matrix at 1253 K under thermodynamic equilibrium are MC (M = Nb) and M23C6. As carburization proceeds in this state, M23C6 transforms into the more stable M7C3 phase, as in base steel. When the carbon concentration increases above 4.7% for base steel and above 4.8% for Nb-added steel through carburizing, graphite (Gr) becomes a stable phase, and it coexists with other carbides. On the other hand, since M3C type carbide is a metastable phase, it does not appear on the equilibrium phase diagram. However, Morita et al. proposed that M3C should be included in the calculated phase diagram of multi-component systems, because the solid solution of alloying elements such as Cr in M3C reduces its formation energy.16) Considering this, crystallographic changes in carbides through carburizing at 1253 K in the specimens with and without Nb addition are presumed as follows.
Equilibrium phase diagrams of experimental cast steels calculated by Thermo-calc software. (Database: Fe-DATA Ver. 4)
Base steel: M23C6 ⇒ M23C6 + M7C3 + M3C ⇒ M7C3 + M3C ⇒ M7C3 + M3C + Gr
Nb-added steel: MC + M23C6 ⇒ MC + M23C6 + M7C3 + M3C ⇒ MC + M7C3 + M3C ⇒ M7C3 + M3C + MC + Gr
These calculated results are in good agreement with the transformation of carbides during carburizing except for Gr (seen in Fig. 7). Precipitation of Gr was not detected near the surface of both steels at the high carbon concentration side, even after 10 cycles of carburizing, as shown in Fig. 4. According to Ando et al., when Cr content in Fe–Cr–Si–C alloys is high, the formation rate of M3C on the iron surface is significantly faster than that of graphite under the carburizing atmosphere, so precipitation of graphite is suppressed.17) On the other hand, many researchers have reported the correlation between carbon concentration and carbide composition corresponding to the morphology of each secondary carbide precipitated in the carburized layer of heat-resistant cast steel during carburizing.18) For this reason, a detailed analysis of secondary carbide was omitted in this study.
3.2 Change in microstructure during vacuum carburizingIn general, it is well known that small cracks generated in heat-resistant cast steel are caused by deformation due to expansion and shrinking or by thermal stress due to temperature gradient in an environment where heating and rapid cooling are repeated. It has already been reported that the density and development of micro-cracks vary depending on the morphology, volume fraction, and dispersion of primary carbides existing in matrix of heat-resistant cast steel.8) In this study, as confirmed the external deformation of the ring-shaped specimens carburized for 10 cycles, the deformation rate of the outer diameter after carburizing is less than 1.0% as compared to that before carburizing for both cast steels. From this result, it is difficult to determine whether Nb addition causes a difference in thermal shock resistance after less than 10 cycles of carburizing. However, SEM observation of the ring-shaped specimens and the ends of creep specimens revealed that their microstructures were changed by the cyclic carburizing.
Here, Fig. 9 shows a representative example of the carburized layer and non-carburized zone of both steels observed at high magnification. First, focusing on the carburized layer, as indicated by the white and black arrows, micro-voids were appeared not only at the interface between Nb-based and Cr-based carbides and matrix but also inside primary carbides even in one cycle regardless of the presence or absence of Nb. Voids are also formed in carbides precipitated by carburizing, but there is a relatively higher volume fraction of voids in primary carbides. The void sites increase with repeated vacuum carburizing. Moreover, it was also observed that the coalescence of voids causes the occurrence of micro-cracks and fractures of carbides. Nevertheless, the linkage of these micro-cracks along the interface between matrix and carbide for the development of larger cracks was not found. On the other hand, in the non-carburized zone of the specimens, a large quantity of fine secondary carbides have precipitated around primary carbides. As in the carburized layer, the number of voids and micro-cracks in primary carbides existing in the non-carburized zone increased with increasing the number of cycles. It is difficult to measure the void size as well as void number, but observation of the entire specimens revealed that voids were larger and easier to coalesce in base steel, whereas voids were smaller but were numerous in Nb-added steel. Also, voids and cracks occur more remarkably in primary Cr carbide than in primary Nb carbide. Although primary Cr carbides in both steels have almost the same number of voids, these voids tend to be larger in size in base steel. As discussed in section 3.3, these voids are regarded as the starting points for crack initiation and lead to crack propagation in a high-temperature creep environment.
High magnification SEM images showing microstructures of carburized layer and non-carburized zone. Voids in primary Nb carbides (white arrows) and Cr carbides (black arrows).
Short-time creep rupture tests were carried out in air for specimens (Fig. 1(a)) that had been vacuum carburized up to 10 cycles. The purposes of these tests were to investigate the influence of the microstructure changed by vacuum carburizing, especially the carburized layer formed near the surface, on the creep property of the specimens. Figure 10 shows the creep rupture time of both steels tested at 1303 K with applied stress of 35 MPa versus the number of carburizing cycles. Also, creep strain for each condition is shown in the lower part of the figure. Nb addition to heat-resistant cast steel is found to be effective to prolong the rupture time in a high-temperature creep environment regardless of as-cast state or any carburizing conditions. Additionally, the creep property of both steels after 1 cycle is better than as-cast state. However, the creep rupture time tends to shorten with increasing number of vacuum carburizing cycles. Creep rupture time after carburizing for 5 cycles is almost the same level as that of as-cast state. As for the cycle of 10, Nb-added steel has a shorter rupture time than as-cast state. On the other hand, the creep strain of both steels is significantly reduced due to carburizing, is smaller for Nb-added steel than for base steel. These results imply that the advantage of Nb addition remains even after 10 cycles.
Comparison of short time creep properties of experimental cast steels after vacuum carburizing at 1253 K.
The specimens carburized for 10 cycles were selected to observe the cross-section of the ruptured section after the creep test. Figure 11 displays the low-magnification SEM images. For reference, the results of as-cast state are also shown in the same figure. The carburized layers formed near the surface of both steels did not disappear after high-temperature creep test. Observation of area near the fracture surface revealed that the sub-grains of Nb-added steel were deformed along the tensile direction less than base steel in as-cast state. Additionally, the sub-grains in the carburized specimen are less deformable than in as-cast state. This result well-corresponds to the creep strain shown in Fig. 10. On the other hand, judging from the crack morphology and crack growth direction, the internal cracks mainly occur at the boundary between the non-carburized zone and carburized layer in base steel, whereas cracks appear at the surface and propagate to the center through the carburized layer in Nb-added steel. Here, the surface layers (zone 1 and zone 3) and the internal regions (zone 2 and zone 4) indicated by the square frames were observed by SEM. The results are displayed in Fig. 12. The figure also shows SEM images of the enlarged microstructures of zone 1 in base steel and zone 3 in Nb-added steel and the corresponding EDS elemental maps. Since the creep test was conducted in air, chromium carbide scale on the extreme surface disappeared due to decarburization, and chromium oxide scale was formed instead. In the vicinity of surface, many voids exist at the boundary between matrix and the fine Cr carbide precipitated due to carburizing. SiO2 film was detected in these areas. Oxidation was also observed at the grain boundaries where creep cracks have already penetrated. Such a microstructure is similar to the typical microstructure on the surface of heat-resistant cast steel products exposed to an oxidizing atmosphere during service.19) Judging from this result coupled with the map of Si distribution shown in Fig. 6, SiO2 scale is difficult to appear in a vacuum carburizing atmosphere, unlike a gas carburizing atmosphere.20,21) Therefore, the effect of Si on carburization resistance is small under vacuum carburizing. On the other hand, the formation of voids in Nb-added steel is revealed to be more pronounced in coarse-grained primary Cr carbides than in small-grained primary Nb carbides. Cracks from the surface tend to propagate inward mainly along with the boundary of Cr carbide and matrix.
Low magnification SEM cross-sectional images of fractured tip of as cast state and 10-cycle vacuum carburized specimens after creep test at 1303 K in air. Zone 1, 3 for surface layers and zone 2, 4 for crack tips indicated by square frames were places for microstructural observation.
Microstructure of surface layer of 10-cycle vacuum carburized specimens after creep test. (Enlargement of Zone 1 for base steel and Zone 3 for Nb-added steel shown in Fig. 11)
Figure 13 shows the SEM images of the crack tip area propagated from the carburized layer of both steels that were vacuum carburized for 10 cycles after the creep test. In the upper and lower left figures, it was confirmed that cracks were formed in primary Cr carbide at the crack tip in zone 2 in base steel. As described above, thermal shock owing to repeated vacuum carburizing causes the formation of voids inside primary carbide and at the interface between matrix and carbide (Fig. 9). These voids are the crack initiation sites of carbide due to the applied stress in a creep environment. Also, it is thought that crack propagation will be promoted as the size and the density of voids increase. On the other hand, in zone 4 of Nb-added steel, although there are voids inside primary Nb carbide and micro-cracks at the interface between matrix and Nb carbide, the coalescence degree for the formation of larger crack is small, unlike primary Cr carbide. In other words, the presence of primary Nb carbides at the grain boundary is presumed to temporarily hinder the development of large cracks. Additionally, the SEM images suggest that secondary Cr carbides precipitated in the matrix may give little effect on suppressing the crack propagation.
Microstructure around crack tip in non-carburized zone of 10-cycle vacuum carburized specimens after creep test. (Enlargement of Zone 2 for base steel and Zone 4 for Nb-added steel shown in Fig. 11)
From an industrial view, the service life of the heat treatment jig made of austenitic heat-resistant cast steels is evaluated by the number of carburizing cycles. The life varies with the processing temperature and the total weight of parts loaded on the jig. In this case, creep strength and thermal shock resistance must be considered when designing heat treatment jigs. In this study, Nb addition was found to be effective to prevent carburization and improve the creep property of austenitic heat-resistant cast steel, but further investigation is needed to determine the difference in creep strength of steels with and without Nb addition.
The influence of Nb addition on microstructure and creep property of austenitic heat-resistant cast steels which were subjected to repeated vacuum carburizing, were investigated. The following findings were obtained.
Ngo Huynh Kinh Luan and Tetsuya Okuyama are equally contributed to this study.
In this study, vacuum carburizing was conducted at the plant of Okaya Heat Treatment Industry Co., Ltd. We would like to thank everyone involved.
