2017 Volume 58 Issue 4 Pages 655-662
The effect of the Molybdenum (Mo) content on the heat treatment behavior of multi-alloyed white cast iron was investigated. The cast iron with varying Mo contents from 0.12 to 7.66% under the basic alloy composition of 5% Cr, W and V each was prepared. After annealing at 1223 K for 18 ks, the test specimens were austenitized at 1373 K for 3.6 ks in a vacuum furnace and subsequently hardened by a jet-spray of liquid nitrogen. The tempering was carried out at temperatures from 673 to 873 K at 50 K intervals for 12 ks. It was found that the hardness in the as-hardened state was increased progressively with an increase in the Mo content. The volume fraction of the retained austenite (Vγ) decreased markedly when the Mo content was increased over 1.17%. In the tempered state, the hardness curve showed clear secondary hardening due to the precipitation of fine secondary carbides and a reduction of the Vγ. The Vγ value in each specimen decreased gradually as the tempering temperature was elevated, but reduced greatly when tempered at 748 to 823 K. The maximum tempered hardness (HTmax) was obtained in the specimen tempered at 798 K where the Vγ was less than 10%. The HTmax increased first, and then subsequently decreased with an increase in the Mo content. The highest HTmax value, 946 HV30, was obtained in the specimen with 4.98%Mo. It was found that the 15–37%Vγ in the as-hardened state was necessary to get the hardness over 900 HV30 by tempering. The degree of secondary hardening (ΔHs) increased as the Mo content rose from 0.12 to 4.98% where the difference between the Vγ in the as-hardened state and that at HTmax (ΔVγ) was 22–23%.
Multi-alloyed white cast iron is a new type of alloyed white cast iron with multi-components that contains several kinds of strong carbide forming elements, such as chromium (Cr), molybdenum (Mo), vanadium (V) and tungsten (W).1–4) This white cast iron has been preferably applied to work roll materials in hot strip mills in the steel-making industry as well as some parts and components of pulverizing mills in the cement industry. In the typical microstructure of the multi-alloyed white cast iron, several kinds of complex carbides, such as MC, M2C, M6C and M7C3, precipitate as an eutectic and the matrix consists of austenite, bainite and/or martensite together with secondary carbides.1,2) It is known that the type and amount of carbides and the matrix structure are determined by the chemical composition of the cast iron and its heat treatment.5–7) Compared to conventionally rolled materials, such as Ni-hard cast iron and high Cr cast iron, the multi-alloyed white cast iron provides superior abrasive wear resistance, higher quality and longer service life in spite of a lower volume fraction of eutectic carbides.8)
The basic alloy composition is 5 mass% (as shown by %) of each carbide forming element, Cr, Mo, W and V, and with 2%C.1,2) Cobalt (Co) is not a carbide former, but it is added to improve the corrosion resistance, high temperature strength and thermal stability.1,2,9) The mechanical and wear properties of multi-alloyed white cast iron depends on the kind and amount of carbides and the matrix structure, which differs due to the heat treatment. To obtain desirable properties, therefore, a heat treatment must be introduced to this multi-alloyed white cast iron.
Generally, the hardening and tempering are applied to the multi-alloyed white cast iron in the same way as steel and general cast iron.8–10) While the alloyed cast iron is held at a high temperature, the retained austenite is destabilized by the precipitation of secondary carbides. Subsequently, the destabilized austenite transforms to bainite and/or martensite during the cooling for the hardening. During the tempering, there is not only the precipitation of the secondary carbides from martensite but also the decomposition of the retained austenite in the as-hardened state, and the residual austenite transforms into martensite post cooling. When the tempering temperature is increased to 773 K, the carbide reaction begins in-situ to form special carbide with extremely high hardness.
Mo is a strong carbide former and forms M2C eutectic carbide with a higher hardness than chromium carbide of the M7C3 type. In addition, a certain amount of Mo is distributed into the austenite and improves the hardenability of the cast iron. It is known that Mo promotes the precipitation of secondary carbides during the heat treatment.10) Therefore, an addition of Mo is very useful for multi-alloyed white cast iron.
Looking back over the last 30 years, the research and development on multi-alloyed or multi-component white cast iron has been carried out on the solidification sequence5–7) and phase transformation during heat treatment.9,11–14) However, systematic research on the behavior during the heat treatment, i.e., the behaviors of the hardness and the retained austenite during the heat treatment of the cast iron and variations due to the alloy content were quite few.11,12) Therefore, the effect of the Mo content on the heat treatment behavior of multi-alloyed white cast iron with a basic composition was investigated in this work.
Charge calculations were carried out to obtain the target chemical compositions of the test specimens using raw materials, such as steel scrap, pig iron, ferro-alloys and pure metals. The charge materials (30 kg) were melted in a high-frequency induction furnace. The melt was superheated to 1853 K. After being held for 600 to 900 s, the melt was poured at 1773–1793 K into preheated CO2 bonded sand molds in a round bar shape with a cavity size of 25 mm in diameter and 65 mm in length. A schematic drawing of mold is shown in Fig. 1. After pouring, the riser was covered instantly by dry exothermic powder to prevent the melt from cooling too fast. The chemical compositions and types of precipitated eutectic carbide of the specimens are summarized in Table 1. A disk shaped test piece with a 7 mm thickness was cut from the round bar with a wire-cutting machine, as shown in Fig. 1.
Schematic drawing of CO2 mold for round bar specimen and method to cut test pieces from speimens.
| Specimen | Element (mass%) | Type of eutectic carbide | |||||||
|---|---|---|---|---|---|---|---|---|---|
| C | Si | Mn | Cr | Mo | W | V | Co | ||
| No.1 | 2.05 | 0.51 | 0.48 | 5.13 | 0.12 | 4.95 | 5.09 | 1.99 | MC + M7C3 |
| No.2 | 2.08 | 0.47 | 0.48 | 5.09 | 1.17 | 4.92 | 5.03 | 2.01 | MC + M7C3 |
| No.3 | 2.09 | 0.52 | 0.50 | 5.11 | 3.02 | 5.06 | 5.10 | 2.01 | MC + M2C |
| No.4 | 2.00 | 0.53 | 0.49 | 4.96 | 4.98 | 4.98 | 5.01 | 2.03 | MC + M2C |
| No.5 | 2.06 | 0.50 | 0.47 | 5.00 | 7.66 | 4.98 | 5.01 | 1.98 | MC + M2C |
The test pieces were coated with an anti-oxidation solution to prevent them from oxidizing and decarburizing while being annealed. In the annealing process, the specimens were heated to 1223 K in an electric furnace and held there for 18 ks and then cooled to room temperature in the furnace. For the hardening, the annealed test pieces were austenitized at 1373 K for 3.6 ks in a vacuum furnace in which the pressure was controlled at about 0.05–0.07 Pa. While the specimens were held at the austenitizing temperature, the pressure was maintained at about 250 Pa. After austenitizing, the test pieces were quenched by nitrogen gas and cooled with a cold evaporator that stored the liquid nitrogen at an extremely low temperature. The evaporated nitrogen gas was jet-sprayed on to the test pieces under a pressure of 400 kPa. The hardened test pieces were tempered for 12 ks at 673–873 K at 50 K intervals in an electric furnace and cooled in still air.
2.3 Investigation of microstructureThe microstructures of the test piece were observed by optical microscope (OM) and scanning electron microscope (SEM). The heat-treated test pieces were polished using SiC emery papers and buffed with alumina powder with a grain size of 0.3 microns. Murakami's reagent was used to identify the type of carbide and Villella's reagent was used to reveal the matrix microstructure.
2.4 Measurement of hardnessThe macro-hardness of the test pieces was measured using a Vickers hardness tester with a load of 294.2 N (30 kgf), and the micro-hardness of the matrix was determined using a Micro-Vickers hardness tester with a load of 0.98 N (0.1 kgf). The hardness measurement was carried out randomly at five points on the surface of a test piece and the values were averaged.
2.5 Measurement of volume fraction of retained austeniteThe X-ray diffraction method was used to quantitatively measure the volume fraction of the retained austenite (Vγ). A special goniometer with sample stage to enable the test piece to rotate and swing simultaneously was used to cancel the preferred orientation of the austenite.15) The Mo-Kα characteristic line with a wave length of 0.007 nm (0.0711 A°) filtered by Zr foil was utilized as the source of the X-ray beam. The mirror polished specimen was attached to the sample stage and the goniometer was scanned from 24 to 44 degrees by 2θ. Four diffraction peaks on the chart, which were independent and without interference from the peaks of the unnecessary phases, were selected for the Vγ calculation. The diffraction peaks adopted were the (200) and (220) planes for ferrite (α) or martensite (M), and the (220) and (311) planes for austenite (γ). The Vγ was obtained by averaging the values calculated from the three combinations of the peaks, α200 - γ311, α200 -∑γ(220,311) and ∑α(200,220) - γ311.
The typical as-cast microstructures of the test specimens with different Mo contents are shown in Fig. 2. It was found that the microstructures of all the specimens showed a hypoeutectic composition consisting of a primary austenite dendrite and eutectic structures of (γ + eutectic carbide). The eutectic structures crystallized in the liquid region among the primary austenite dendrites, and the types of eutectic carbides crystallizing in this series of multi-alloyed white cast irons were MC, M2C and M7C3. The eutectic carbides in the 0.12% and 1.17%Mo specimens were MC and M7C3 types and those in the specimens with Mo contents more than 3% were MC and M2C types. These results agree well with those reported by Wu et al.5–7) The matrix structures of all the specimens were mostly austenitic together with some martensite. This is because the Mo dissolved in the matrix suppresses the pearlite transformation. However, the martensite cannot be distinguished clearly from these microphotographs due to the low magnification.
Microstructures of as-cast specimens with different Mo contents (by OM).
According to the as-cast microstructures in Fig. 2, it is found that the area fraction of austenite dendrite and eutectic structure seems to be much different by increasing the Mo content. Therefore, the area fractions were measured in each specimen using an image analyzer. It was found that there was little difference in the primary dendrite within the range of 46.8–48.2% even the Mo content increased. As for the amount of eutectic carbides, the M2C eutectic carbide increased with an increase in Mo content but MC carbide decreased. Resultantly, the total amount of eutectics in each specimen was almost the same. The solubility of Mo in the primary austenite dendrite of multi-alloyed white cast iron was reported by previous paper.13) The partition coefficient of Mo to primary austenite varied depending on the cooling rate of melt during freezing and an average value was 0.4. The dissolved Mo concentration increases until the saturation value and affected greatly on the matrix transformation.
The effect of the Mo content on the macro-hardness, micro-hardness and Vγ value in the as-cast state is shown in Fig. 3. The macro-hardness and micro-hardness increased gradually to the maximum values at 3%Mo, and then decreased as the Mo content increased. An increase of the hardness in the former stage of the Mo content lower than 3% could be due to the Mo being dissolved in the matrix and strengthening the matrix as well as the possible formation of a small amount martensite, raising the amount of M2C eutectic carbide which has higher hardness than M7C3 eutectic carbide as well as a decrease in the M7C3 eutectic carbide finally to disappear. Their effects raised the hardness with an increase in the Mo content. The hardness in the latter stage for over 3.0%Mo decreased due to an increase in the Mo decreases in the amount of MC eutectic carbide which has higher hardness than M2C carbide.
Effect of Mo content on macro-hardness and micro-hardness and volume fraction of retained austenite (Vγ) in as-cast state.
The Vγ in the as-cast state increased first and then decreased as the Mo content rose. An increase in the Vγ was due to an increase in the Mo concentration in the austenite that lowered the Ms temperature. Alternatively, a decrease in the Vγ in the specimens with more than 3%Mo was caused by more C and Mo being consumed to form M2C eutectic carbides during solidification. In other words, a reduction in the C and Mo contents in the austenite makes the Ms temperature rise that causes more martensite to be produced. Although the amount of martensite increased, the hardness decreased gradually due to an increase in the martensite could not compensate a decrease in hardness by the reduction of MC eutectic carbide. This is because more C and Mo consumed to form M2C eutectic carbides at the end of solidification. The reduction of C and Mo in primary austenite formed martensite with lower C concentration and has lower the hardness.
3.2 As-hardened stateThe microstructures from the matrices in all the as-hardened specimens are shown in Fig. 4. It is well-known that the morphologies of eutectic carbides change little during heat treatment, except for that of the M2C carbide. Considering the microstructures, the matrices are composed of fine secondary carbides (SC), martensite (M) and retained austenite (γR). It is clear that the precipitation of secondary carbides occurs during the austenitizing, and they are clearly revealed and seem to increase in number with an increase in Mo content. Then, it was found that eutectic M2C carbides decomposed by reacting with the surrounding austenite. It was reported that the secondary carbides were mostly of the MC and M6C types.11,15) The martensite that was transformed from the destabilized austenite could be discovered throughout the matrix, but the variation in the amount could not be clarified from these microphotographs. It is clear that the microstructures in the as-hardened state are quite different from those in the as-cast state. This demonstrates that the austenite in the as-cast condition was destabilized by the precipitation of fine carbides during the austenitizing, and the resultant residual and unstable austenite transformed into martensite during post-cooling.
Typical matrix microstructures of as-hardened specimens with different Mo contents (by SEM).
The effect of the Mo content on the hardness and Vγ in the as-hardened state is shown in Fig. 5. The macro-hardness and micro-hardness showed similar behaviors. The macro-hardness, which is expressed by the total hardness of the eutectic carbides and matrices, increased progressively as the Mo content rose. An increase in the macro-hardness is due to an increase in the M2C eutectic and the special secondary carbides together with more martensite in the matrix.
Effect of Mo content on macro-hardness and micro-hardness and volume fraction of retained austenite (Vγ) in as-hardened state.
The Vγ continued to decrease with a increase in the Mo content. This suggests that the amount of martensite increased as more secondary carbides precipitated. Since the precipitation of secondary carbides results in the reduction of C and the alloying of the elements in austenite, the Ms temperature rose and the Vγ was reduced.
3.3 Tempered stateDuring austenitizing after annealing, the precipitated secondary carbides in annealing stage dissolved because of higher temperature in austenitizing than that in annealing. In the tempered state, martensite dissolved or was tempered by precipitating carbides and additionally, strong carbide forming elements like Mo and V could form each pure carbide with much higher hardness according to the carbide reactions. Due to tempering, the stability of austenite was reduced and allowed the transformation of austenite to martensite during the post-cooling. Therefore, tempered martensite, newly transformed martensite and mixed secondary carbides formed during hardening and tempering exist in the matrix of tempered specimens. It is natural that such phases strengthen matrix structure and improve mechanical properties such as tensile strength, compression strength and wear resistance.
After hardening due to austenitizing at 1373 K for 3.6 ks, the test pieces were tempered at several temperatures from 673 to 873 K. The relationship between the macro-hardness, Vγ and tempering temperature is shown in Fig. 6. The data of the hardness and Vγ values in the as-hardened state were plotted in each figure for a better understanding of the tempering behavior.
Relationship between macro-hardness, volume fraction of retained austenite (Vγ) and tempering temperature of specimens with different Mo contents.
In each tempering curve, the hardness dropped greatly from the as-hardened state when the test pieces were tempered at 673 K. After that, the hardness began to increase to the maximum value before reducing as the tempering temperature increased. In other words, the tempered hardness curve showed evident secondary hardening. It is believed that this variation in the hardness is related to the phase transformation of the matrix. The increase in the hardness was due to the precipitation of the secondary carbides from the austenite during the tempering, and the rest of the austenite transformed to martensite during the post-cooling. Additionally, a large decrease in the amount of retained austenite due to the tempering is another reason. The micro-hardness showed similar behavior to the macro-hardness in all the specimens. The mutual relations between the amount of martensite and the decrease in the retained austenite as well as the precipitation ratio of the secondary carbides should determine the tempered hardness.
The maximum tempered hardness (HTmax) was obtained at a 798 K tempering temperature regardless of the Mo content. Near the HTmax, it can be said that the carbide reaction occurred in-situ. When the specimen was tempered to over 798 K at the HTmax, the hardness decreased greatly due to over-tempering. The degree of secondary hardening (ΔHs), which is defined as the difference in the hardness between the maximum tempered hardness (HTmax) and the hardness at which the secondary hardening begins, increased with an increase in the Mo content up to 5.0%. However, the ΔHs in the specimen with 7.66%Mo seemed to be smaller than the other specimens (e). The highest HTmax value, 948 HV30, was obtained in the specimen with around 5% Mo.
The Vγ in the tempered state decreased gradually as the tempering temperature increased to 748 K. After that, it decreased abruptly to 823 K. This means that the transformation rate of the austenite to martensite is high when tempering at 748–823 K. It can be seen that an increase in the amount of precipitated carbide reduced the stability of the retained austenite and encouraged the austenite to transform into martensite. Conversely, when the tempering temperature was over 823 K, the austenite transformed to pearlite or ferrite, and therefore, the Vγ decreased so it was close to nil. The Vγ values at the HTmax were less than 10% in all the specimens.
The relationship between macro-hardness and Vγ in all the tempered specimens is shown in Fig. 7. It was found that the hardness varied within a range against the Vγ value. The hardness increased to a maximum value and then decreased as the Vγ increased. The maximum hardness was obtained at around 5% Vγ. In the region with a very low Vγ, the hardness was low because the test pieces were over-tempered. When the Vγ passed the maximum hardness, the hardness decreased gradually with an increase in the Vγ. This was because the excessive soft retained austenite reduced as the matrix hardness increased while there was a decrease in the amount of martensite. The reason why the hardness values were scattered over a wide range of Vγ values less than 5% was due to the fact that the hardness was not influenced so much by such a small amount of Vγ but much more by the other phases, such as martensite, pearlite or bainite.
Relationship between macro-hardness and volume fraction of retained austenite (Vγ) of tempered specimens.
The effects of the Mo content on the HTmax and Vγ at HTmax are shown in Fig. 8. The HTmax values of the macro-hardness and micro-hardness showed similar variations corresponding to the Mo content of the specimens. Both of the hardness values increased to the greatest at around 5%Mo and decreased a little as the Mo content increased further. The Vγ value at the HTmax was slightly increased at 1.2%Mo, and then continued to decrease proportionally as the Mo content rose. It was found that the Vγ value at the HTmax for each specimen was generally less than 10%. This suggests that even if the test piece showed the maximum hardness in its tempered state, a certain amount of austenite still remained.
Effects of Mo content on maximum tempered hardness (HTmax) and volume fraction of retained austenite (Vγ) at HTmax of specimens.
In general, it is difficult to identify the type and morphology of precipitated secondary carbides during different heat treatment from microphotographs including the carbide reaction from microphotographs by OM or SEM. However, there was a paper14) to investigate the identification of precipitated carbides using XRD and TEM. It was reported that the MC, M6C and M7C3 carbides precipitated as secondary carbide in annealed, as-hardened and tempered states. The secondary carbides are very small in size but the type of carbides and when they formed cannot be distinguished. However, the size of precipitated secondary carbide formed by tempering and the carbide reaction should finer than those by annealing. A paper11) reported that the precipitated secondary carbides in the tempered state are mainly MC and M6C types. The carbides formed by carbide reaction in tempering can be expressed as following example for Mo;16)
| \[ \begin{split} {\it Matensite}\ ({\it Fe},{\it Mo},C) & {}\rightarrow M\ ({\it FeMo})_{2-3}C \rightarrow M\ ({\it FeMo})_6C \\ & {}\rightarrow Mo_6C\ {\it at}\ {\it around}\ 773K \end{split} \] |
To clarify how the Vγ value in the as-hardened state affected the hardness in the tempered state, the HTmax values were connected to the Vγ in the as-hardened state. The relationship is shown in Fig. 9. High values for the HTmax were obtained in the range of Vγ values lower than 40%. The HTmax decreased by over 30%Vγ in the as-hardened state because the excessive retained austenite remained after the tempering, and it reduced the matrix hardness greater than the secondary hardening that took place due to the precipitation of the secondary carbides and the transformation of the martensite. It is clear that 15–37%Vγ in the as-hardened state was necessary to obtain the high HTmax values of over 900 HV30 in the tempered state. This demonstrates that the Vγ in the as-hardened state contributed more to the secondary hardening.
Relationship between maximum tempered hardness (HTmax) and volume fraction of retained austenite (Vγ) in as-hardened state.
As shown previously in Fig. 6, the tempered hardness curve of each specimen displayed secondary hardening. The Mo content should affect the degree of secondary hardening (ΔHs). The relationship between ΔHs and Mo content is shown in Fig. 10. As the Mo content increased, the ΔHs increased gradually to the largest ΔHs and then decreased remarkably. It should be noted that the ΔHs of the micro-hardness was overall higher than that of the macro-hardness. The largest ΔHs for the micro-hardness was 150 HV0.1 and it was obtained at about 5%Mo. From these results, it can be said that the secondary hardening is accomplished by a change in the matrix structure that is determined by the precipitation of the secondary carbides, including the formation of pure carbide produced by the carbide reaction. The transformation of the martensite from the rest of the austenite is an additional reason.
Effect of Mo content on degree of secondary hardening (ΔHs).
In addition, it should be considered that the ΔHs was influenced by the Vγ value in the as-hardened state. Therefore, the difference between the Vγ in the as-hardened state and the Vγ at HTmax (ΔVγ), which relates to the amount of decomposed austenite during tempering, was related to the ΔHs. The relation of the ΔHs vs. ΔVγ is shown in Fig. 11. The highest ΔHs was obtained at 23% ΔVγ with specimen No.4 with about 5%Mo. When the ΔVγ was less than 23%, the ΔHs was reduced greatly as the ΔVγ decreased. Even if the ΔVγ increased to around 30%, the ΔHs remained over 100 HV0.1. This suggests that a relatively large amount of ΔVγ is needed to get a great degree of secondary hardening. Since the Vγ in the as-hardened state was determined by the amount of Mo dissolved in the austenitic matrix, the ΔHs was indirectly influenced by the Mo content of the specimen. Therefore, it can be ultimately said that the Mo promoted the secondary hardening due to the precipitation of the special Mo carbide with higher hardness.
Relationship between difference of Vγ in as-hardened state and Vγ at HTmax (ΔVγ) and degree of secondary hardening (ΔHs).
It has been reported11,12) that carbon balance (Cbal) is an important factor to determine the residual C content in the matrix and it influences the phase transformation. Therefore, the Cbal should be considered when discussing the HTmax.
The Cbal is shown by eq. (1)11).
| \[{\rm C}_{\rm bal} = \%{\rm C} - \%{\rm C}_{\rm stoich}\] | (1) |
In the multi-alloyed white cast iron, some of the MC, M2C and M7C3 carbides precipitated as their eutectics. In the cast iron containing the basic alloy composition of 5%Cr, 5%Mo, 5%W, 5%V, the MC and M2C carbides precipitated5), and therefore the %Cstoich is calculated by eq. (2).
| \[\%{\rm C}_{\rm stoich} = 0.06\%{\rm Cr} + 0.063\%{\rm Mo} + 0.033\%{\rm W} + 0.235\%{\rm V}\] | (2) |
| \[\%{\rm C}_{\rm stoich} = 0.099\%{\rm Cr} + 0.02\%{\rm Mo} + 0.011\%{\rm W} + 0.235\%{\rm V}\] | (3) |
| \[\%{\rm C}_{\rm stoich} = 0.099\%{\rm Cr} + 0.063\%{\rm Mo} + 0.033\%{\rm W} + 0.235\%{\rm V}\] | (4) |
Here, the relationships between HTmax and Cbal together with the Vγ of the specimens at HTmax are shown in Fig. 12. It was found that the HTmax increased first and then decreased with an increase in the Cbal. However, the results for specimens No. 1 and No.2 were a lot lower than the trend line. This may be due to a difference in the type of eutectic carbide crystallized. The MC and M7C3 eutectic carbides existed in specimens No.1 and No.2, while the MC and M2C carbides crystallized in specimens No.3, No.4 and No.5. Similar results were obtained for the HTmax vs. Vγ, as shown in Fig. 9. It is known that the hardness of M7C3 is quite low when compared with that of M2C carbide. The macro-hardness is the sum of matrix hardness and eutectic carbides. Since the type and amount of eutectic carbides change little during tempering because of low temperature, the tempering affects merely on the matrix hardness or micro-hardness which shows similar tendency to the macro-hardness. The difference between the macro-hardness and micro-hardness arises from the effect of eutectic carbide. In Fig. 12, the Cbal value at the highest HTmax was 0.05% when the Vγ was about 5%. Since the concentration of Mo in the matrix was increased by increasing the Mo content in the specimen, it was natural that the Mo promoted the secondary hardening greatly. In the general heat treatment, therefore, it can be said that a low Cbal, near zero, is enough to maintain the Vγ at a necessary level for obtaining the maximum hardness.
Relationship between maximum tempered hardness (HTmax), volume fraction of retained austenite (Vγ) at the HTmax and carbon balance (Cbal).
The effect of the Mo content on the heat treatment behavior of multi-alloyed white cast iron with a basic composition of other carbide forming elements was investigated using specimens with varying Mo content from 0.12 to 7.66%. The as-cast specimens were annealed at 1223 K for 18 ks. Annealed specimens were austenitized at 1373 K for 3.6 ks in a vacuum furnace and hardened by quenching with a jet-spray of liquid nitrogen. The tempering was done at temperatures between 673 K and 873 K, in 50 K intervals, for 12 ks. Then, the correlations among the hardness, volume fraction of retained austenite (Vγ), tempering temperature and Mo content were clarified. The obtained results can be summarized as follows:
As cast stateThe authors appreciate Chulalongkorn University for its financial support via the fund of “Chulalongkorn University to commemorate the 72nd anniversary of his Majesty King Bhumibala Aduladeja”. In addition, we would like to give thanks to the Manufacturing and Metallurgical Engineering Research Unit, Faculty of Engineering, Mahasarakham University and the Cast Metals Laboratory of the National Institute of Technology–Kurume College, Japan for the use of experimental devices.
