2020 Volume 61 Issue 1 Pages 169-175
The influence of alloying elements on continuous cooling transformation (CCT) behavior was investigated for 16 mass% chromium cast irons containing 0.9 to 4 mass% Mo, V and Nb separately using a transformation measurement apparatus with subzero cooling function. On CCT curves, pearlite and martensite transformations appeared irrespective of the type of alloying element. When Mo was added by more than 1 mass%, bainite transformation was observed in addition to the two transformations above. Regardless of the type of alloying elements, pearlite transformation delayed when the amount of each alloying element was increased. It was found that Mo addition was most effective for delaying pearlite transformation in these alloying elements. MS and Mf temperatures increased slightly by increasing V and Nb contents. Both temperatures decreased with the addition of Mo up to 2 mass% and increased with the addition of Mo over 2 mass%. Regardless of the type and amount of alloying elements added, pearlite transformation delayed, and MS and Mf temperatures decreased with increasing austenitizing temperature.
This Paper was Originally Published in Japanese in J. JFS 90 (2018) 373–380. Figures 2–4, 12 and 15 were slightly changed.
High Cr cast irons containing Cr from 10 to 30 mass% (hereafter indicate as “%”) widely used for abrasive wear resistant materials such as rolls of hot strip and mineral pulverizing mills because M7C3 carbide with high hardness and excellent wear resistance precipitates during solidification.
When the high Cr cast irons are used as parts and components of machines in the industry, some alloying elements like Mo, Ni and Cu are usually added to improve the heat treatment behavior. The alloying elements dissolve not only in carbide but also some of them are distributed to matrix. Therefore, the alloying elements in the matrix affect the transformation behaviors. In order to obtain the appropriate properties as materials for target, the transformation behaviors of the cast iron must be understood and the adequate heat treatments must be also given. For the sake of it, it is important to clarify the relationship between alloying elements and behavior of continuous cooling transformation (CCT) which gives the basics of practical heat treatment.
In the previous paper, CCT diagrams of plain high Cr cast irons varying the Cr/C value from 2 to 15 were investigated and the relationship between Cr/C value and behavior of CCT curve was made clear.1) There, it was found that pearlite nose time (tP-n) and MS, Mf temperatures increased with an increase in Cr/C value regardless of Cr and C contents.
In this study, therefore, strong carbide formers such as Mo, V and Nb were added separately to 16%Cr cast iron and the effect of each alloying element on continuous cooling transformation was clarified.
The plain 16%Cr cast irons with the Cr/C values ranging from 4 to 6 and 16% Cr cast irons with 0.9 to 4% of each Mo, V and Nb were prepared to reveal their transformation behaviors.
Charge materials of 1.3 kg were melted using a high frequency introduction furnace and poured from 1773 K into CO2 sand mold in Y-block shape with cavity size of 10 × 52 × 90 mm. The chemical compositions of specimens are shown in Table 1. After the cast ingots were annealed at 1223 K for 18 ks, the test pieces with a size of ϕ4 × 10 mm for Formastor were made by means of a wire-cutting machine.
The CCT diagrams or CCT curves of specimens were constructed based on thermal expansion curves obtained using an automatic measuring apparatus with subzero function (Formastor-F and F-2, Fuji Electric Industrial Co., Ltd.). The test piece was heated up to two levels of austenitizing temperatures, 1273 K and 1323 K for 600 s. After holding it at each austenitizing temperature for 1.8 ks, it was cooled keeping a constant rate in various cooling. The temperatures at start and finish of pearlite (PS, Pf), bainite (BS, Bf) and martensite (MS, Mf) transformations were determined from the inflection points on cooling curve.
How to determine the start and the finish points of each transformation are illustrated in Fig. 1. A tangent line was drawn on the straight portion of cooling curve just before expansion or contraction appeared in the curve, and then, the points at which the cooling curve was apart from the tangent line were adopted as the start and/or the end of transformations.
Method of determining transformation point on CCT curve.
Nose temperature (TP-n) and nose time (tP-n) of P transformations were determined in CCT diagrams of specimens cooled from each austenitizing temperature. The AC1 temperature shown in each CCT diagram adopted the average value of transformation temperatures obtained when the same kinds of specimens were heated for austenitizing. MS and Mf temperatures adopted those measured when the test piece was cooled at the largest cooling rate of 10 K/s from each austenitizing temperature.
2.3 Hardness measurementMacro vickers hardness were measured at a load of 196 N (20 kgf) for all the specimens which finished transformation and the relation of hardness vs. half cooling time which shows the criterion of hardenability was discussed.
CCT diagrams of plain 16%Cr cast iron cooled from 1273 K and 1323 K austenitizing and those of 16%Cr cast iron with 0.92% and 2.96%Mo cooled from 1273 K austenitizing are shown in Fig. 2(a) and (b), respectively. Regardless of Mo addition, P, MS and Mf transformations appeared around 900, 350 and 150 K, respectively. Additionally, the B transformation appeared in the specimen with 2.96%Mo. P nose was shifted greatly to the long time side by addition of Mo. The temperatures of MS and Mf lowered a little compared with plain 16% Cr cast iron.
CCT diagrams of plain 16%Cr cast irons cooled from 1273 K and 1323 K austenitizing (a) and 16%Cr cast irons with 0.92 and 2.96%Mo cooled from 1273 K austenitizing (b).
The CCT diagrams of specimens with 0.89 and 2.89%V and those with 1.06 and 2.71% Nb cooled from 1273 K austenitizing are shown in Fig. 3 and Fig. 4, respectively. In all of specimens, P, MS and Mf transformations occurred, while B transformation did not appear. In same kind of specimens, the tP-n moved slightly to the long time side and the temperatures of MS and Mf rose a little in comparison with the case of plain 16% Cr cast iron.
CCT diagrams of 16%Cr cast irons with V.
CCT diagrams of 16%Cr cast irons with Nb.
To discuss the effect of alloying elements on the behaviors of continuous cooling transformation, the relationship between TP-n, AC1 and amount of alloying elements was summarized in the specimens cooled from 1273 K austenitizing and is shown in Fig. 5. In case of the specimen with Mo, AC1 temperatures did not change, while TP-n increased with an increase in Mo content. When V or Nb content was increased, on the other hand, the AC1 and TP-n rose at very small increasing rate. As for CCT diagrams cooled from 1323 K austenitizing, the effects of their alloying elements on the TP-n was almost same as those cooled from 1273 K austenitizing.
Effect of alloying elements on AC1 and Pearlite nose temperature (TP-n) of 16%Cr cast irons.
It is well known that the driving force of P transformation (ΔG) is expressed by the following equation as a function of the degree of supercooling (ΔT).2)
| \begin{equation} (\Delta G = (\Delta H/T_{\text{E}})\cdot\Delta T) \end{equation} | (1) |
The trends in the increasing of AC1 and TP-n are almost same level even if Mo, V or Nb was added to the cast iron. Therefore, it can be said that the supercooling affected little on P transformation.
3.3 Effects of alloying elements on pearlite nose time (tP-n)The relationship between tP-n and amount of alloying elements are shown in Fig. 6(a) in the case of specimen cooled from 1273 K and (b) in the case of cooling from 1323 K, respectively. It is found that the logarithm of tP-n increases in proportion to an increase in Mo content. When V or Nb was added, on the other hand, the tP-n moved to the long time side up to 1% each and it kept constant.
Effect of alloying elements on Pearlite nose time (tP-n) of 16%Cr cast irons. (a) Austenitizing: 1273 K, (b) Austenitizing: 1323 K.
In the case of austenitizing at 1323 K shown in (b), the tP-n shifted more to the long time side than the case of 1273 K austenitizing.
In the individual austenitizing temperature, the relationships between tP-n and Mo content were expressed as following eqs. (2) and (3).
| \begin{equation} t_{\text{P-n}}\ (\text{s})\ (1273\,\text{K}\gamma) = 231\exp(\text{1.1x%Mo})\quad (\mathrm{R} = 0.93) \end{equation} | (2) |
| \begin{equation} t_{\text{P-n}}\ (\text{s})\ (1323\,\text{K}\gamma) = 461\exp(\text{1.1x%Mo})\quad (\mathrm{R} = 0.94) \end{equation} | (3) |
In order to make the precipitation of secondary carbide as little as possible during cooling, the test pieces were quenched into subzero region at highest cooling rate after cooling from 1273 K austenitizing. With respect to the test pieces, the concentrations of alloying elements in matrix were measured using EDS and the results are shown in Fig. 7. Mo concentration increased linearly as the Mo content in specimen increased up to 3%. When Mo content got over 3%, Mo concentration in matrix rose remarkably. On the other hand, V concentration in matrix increased gradually up to 3% of V content in the specimen. However, Nb concentration was unchanged even if the Nb content in specimen was increased. These results agree well with the relationship between tP-n and amount of alloying element in the specimens.
Relationship between concentrations of alloying elements in matrix and those of specimen.
As shown in Fig. 2(b), B transformation did not appear in the specimen with 0.98%Mo but appeared in specimen with 2.96%Mo. The B transformation was also found in Fe–Cr–C alloy with Cr/C value of 11.1) Therefore, it is considered that the B transformation can be related to the Mo content and the Cr/C values of specimen. The effects of Mo content and Cr/C value on precipitation of B are shown by mapping in Fig. 8. The data from previous papers and references (Maratray)6) are added in this figure. It is found that B transformation occurs in the region with Cr/C value from 2 to 7 and Mo content more than 1%, and that with Cr/C value over 10 and Mo free.
Effect of Mo content and Cr/C value on precipitation of bainite in 16%Cr cast irons.
The relationship between TB-n and Mo content in the specimens is shown in Fig. 9. TB-n was around 600 K up to 3% Mo in specimen and then it lowered to 520 K at 3.66%Mo regardless of the austenitizing temperatures. On the other hand, as shown in Fig. 10, the tB-n rose to 7000 s at 2%Mo in specimen and over 2%, it decreased gradually with an increase in Mo content. This could be due to the formation of eutectic M2C carbide mainly composed of Mo during solidification. Resultantly, the concentrations of Mo and C in matrix were decreased. Pattyn7) was reported that tB-n of 17%Cr cast iron with Mo shifted to the long time side with increasing of Mo content and the results showed similar trend to those of this study.
Effect of Mo content in specimen on bainite nose temperature (TB-n) of 16%Cr cast iron.
Effect of Mo content in specimen on bainite nose time (tB-n) of 16%Cr cast iron.
The relationships between alloy contents in specimens and MS and Mf temperatures are shown in Fig. 11. The MS and Mf temperatures decreased as the Mo content rose to 2% and then, went up. As for V or Nb addition, they continued to rise proportionally in a small rate with an increase in both the elements. In the case of the specimens austenitized at 1323 K, both MS and Mf temperatures also rose lineally with an increase in contents of elements. On the contrary, MS ad Mf temperatures were 30 to 60 K and 16 to 31 K lower than those austenitized at 1273 K, respectively.
Effect of alloying elements on MS and Mf temperature in 16%Cr cast irons.
The reason of a rise in MS ad Mf temperatures by increasing in the Mo content is explained as follows. Mo dissolved in γ makes the matrix strengthen by solid solution hardening due to lattice distortion. It becomes an obstacle to shearing necessary for martensite transformation. Then, the large driving force for the M transformation is needed and as the result, MS ad Mf temperatures lowered.8)
The reason why MS and Mf temperatures rose due to a decrease in C concentration in γ by the eutectic M2C carbide.
The reason why MS ad Mf temperatures went up with an increase in V and Nb contents, on the other hand, is that C content in γ decreased by crystallization of primary NbC during solidification and precipitation of secondary carbides (MC) during annealing and austenitizing. Moreover, both of MS and Mf temperatures of the specimens cooled from 1323 K austenitizing were lower than those cooled from 1273 K. This is because an increase in C and alloying element in γ makes γ stable.
3.6 Relationship between hardness after finishing transformation and alloying elementsThe macro-hardness was measured for the specimens after finishing the continuous cooling transformation. The relationship between hardness and half cooling time of the specimen with 2% Mo, V or Nb is shown in Fig. 12. The solid and dashed lines show macro-hardness of specimens without subzero treatment and the broken line shows that with subzero treatment. In the specimens with subzero treatment, the hardness gradually decreased with an increase in the half cooling time regardless of kinds of alloying elements and austenitizing temperatures. In the specimens without subzero treatment, on the other hand, the macro-hardness rose as the half cooling time increased up to 300 s and over 300 s, they decreased. The low hardness of specimens at a small half cooling time is due to more existence of retained austenite and the increase in hardness up to 300 s is due to a rise of MS temperature by a decrease in C concentration in γ accompanying on additional precipitation of secondary carbides during cooling. The reason why the hardness decreased gradually over 300 s is due to the fact that P or B transformation took place before M transformation and the γ was stabilized by C enrichment.
Relationship between hardness after finishing continuous cooling and half cooling time of 16%Cr cast iron with 2%Mo, V and Nb.
Figure 13 shows the effect of Mo content on maximum hardness (Hmax) of completely cooled specimens. The Hmax increased to the highest values of 900 to 1000HV20 when Mo content went up to 1%. As Mo content increased over 1%, Hmax of the specimens cooled from 1273 K austenitizing were kept highest value, while those from 1323 K austenitizing reduced gradually. When compared the effect of austenitizing temperature, the Hmax of specimens cooled form 1323 K were lower than those cooled from 1273 K. The reason can be discussed in relation to the difference in the amount of retained γ in the specimens. The specimens cooled from higher austenitizing temperature of 1373 K contain more retained γ than those from 1273 K. Therefore, the MS and Mf temperatures are lower in 1373 K austenitizing. In this condition, it is possible that some retained austenite still existed compared with that in specimens cooled from lower austenitizing temperature of 1273 K.
Effect of Mo content on maximum hardness (Hmax) after finishing continuous cooling of 16%Cr cast iron.
The relationship between Hmax and V content is shown in Fig. 14. The Hmax rose linearly as V content increased and that of specimens with subzero treatment were 200HV20 higher than those without subzero treatment. This reason is explained by that as V content increase, more precipitation secondary carbides increased during annealing and austenitizing and then the MS temperature rose due to a reduction of C in matrix and the amount of retained γ was decreased. The subzero treatment could make the latter effect increase.
Effect of V content on maximum hardness (Hmax) after finishing continuous cooling of 16%Cr cast iron.
In comparison with the effect of austenitizing temperature, Hmax of specimens cooled from 1323 K were approximately 150 HV lower than those cooled from 1273 K.
The relationship between Hmax and Nb content is shown in Fig. 15. In addition of Nb, the subzero treatment was given to all the specimens. The Hmax values showed 900HV20 irrespective of Nb content. This reason should be that Nb was consumed to precipitate primary MC (NbC) carbide and therefore, an increase in hardness caused by precipitation of primary carbides was well balanced well with a decrease in hardness of martensite caused by the reduction of C.
Effect of Nb content on maximum hardness (Hmax) after finishing continuous cooling of 16%Cr cast iron.
The effects of alloying elements on continuous cooling transformation behavior of high Cr cast irons with Cr/C value from 4 to 6 were investigated. The results obtained are summarized as follows.
| \begin{equation*} t_{\text{P-n}}\ (\text{s}) = 231\exp(\text{1.1(%Mo)})\ \text{at 1273$\,$K austenitizing} \end{equation*} |
| \begin{equation*} t_{\text{P-n}}\ (\text{s}) = 461\exp(\text{1.1(%Mo)})\ \text{at 1323$\,$K austenitizing} \end{equation*} |
As Mo content increased, B nose time (tB-n) sifted up to 2% and then, changed over to short time side.
