Evolution of Carbides in H13 Steel in Heat Treatment Process
2017 Volume 58 Issue 2 Pages 152-156
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2017 Volume 58 Issue 2 Pages 152-156
In the present work, the carbides in H13 steel were investigated with scanning electron microscope (SEM), energy dispersive spectrum (EDS), X-ray diffraction (XRD), and quantitative analysis method. The experimental results were compared with the calculation results by Thermo-calc software. The results show that the dendritic segregation exists generally in H13 ingots, primary M(C, N), M6(C, N) and a small amount of secondary carbides M7C3 precipitate in the segregation area. The composition segregation is improved after annealing and forging process. A large amount of secondary carbides M7C3 precipitate in the segregation area after annealing process. Primary carbide M6(C, N) are almost dissolved and M(C, N) are partially dissolved in the forged and annealed H13 steel. Most of carbides in the quenched and tempered H13 steel are fine secondary M6(C, N), M23C6 and M(C, N), besides a small amount of primary M(C, N) with smaller size. The evolution mechanism of carbides in heat treatment process was clarified by the calculated results.
H13 steel is one of the most widely used hot work die steel, which is widely used as die-casting mold, die forging and hot-extrusion die. It has several outstanding advantages such as high hardenability, abrasive resistance and toughness1). Carbide is important as a second phase in H13 steel. The type, quantity, size, morphology and distribution of carbide have important effect on the performance of steels2). Researches on several aspects had been made to interpret the effect of carbides in H13 die steel. Hu et al.3) studied the effect of Nb on the coursing behavior of M23C6 in H13 steel under different aging time, and found that addition of Nb can retard the coursing of M23C6. Ma et al.4,5) studied the effect of high temperature homogenizing treatment and hot forging treatment to carbide segregation existed in H13 steel, they reported that carbide segregation were eliminated effectually and improvement of the isotropy. The morphology and distribution of carbides in H13 steel had been improved obviously after cryogenic treatment6). Several efforts had been made to illustrate the improvement of carbides in H13 steel7–9), however, few studies focoused on the precipitation and evolution of carbides in H13 steel during each stage of heat treatment. Ning10) reported that the types of precipitates of H13 are mainly Cr-rich M23C6 and V-rich MC during annealing process. Xue11) reported that spheroidal carbides MC，M6C，M7C3 and M23C6 existed in H13 steel after the spheroidizing annealing treatment, and the alloying elements are different in different spheroidal carbides. Cai12) reported that M7C3 and M2C were found to have precipitated at temperatures over 500℃. Inoue and Masumoto13) studied the transformation behavior and mechanism of M3C → M7C3 → M23C6 → M6C and the crystallographic relationship between these carbides. In current work, the characteristic of carbides in H13 steel during each stage of heat treatment process was analyzed by experiment and thermodynamic calculation.
The experimental steel was melted by a medium-frequency induction furnace and then forged into a consumable electrode. The consumable electrode was refined through electroslag remelting process to produce remelted ingot with 180 mm in diameter. The top part of the ingot was cut as the as-cast steel samples. Subsequently, the remained ingot was annealed, forged plus re-annealed, and quenched plus tempered. The heat treatment processes in detail are listed in Table 1. Steel at every stage of the heat treatment process are coded in Table 2. The compositions of H13 ingot are listed in Table 3.
| Heat treatment process |
Process in detail |
|---|---|
| Forging process |
Forged between 860℃ to 1150℃ and forged into Φ325 mm in diameter. |
| Annealing process |
Heated to 760℃ for 2 h → reheated to 860℃ for 8 h → furnace cooled to 500℃ → air cooled to room temperature. |
| Quenching process |
Heated to 850 for 1 h → reheated to 1050℃ for 100 min → oil quenched. |
| Tempering process |
Heated to 590℃ for 4 h → air cooled to room temperature. |
| Ingot | Annealed | Forged plus annealed | Quenched plus tempered |
|---|---|---|---|
| A1 | A2 | A3 | A4 |
| C | Si | Mn | Cr | Mo | V | Al | P | S | O | Fe |
|---|---|---|---|---|---|---|---|---|---|---|
| 0.41 | 0.99 | 0.29 | 5.01 | 1.22 | 0.93 | 0.020 | 0.023 | 0.0060 | 0.0058 | Bal. |
The carbides in H13 steel with regard to temperature was calculated by Thermo-calc software containing TCFE6 database. Steel samples with 12 × 12 × 12 (mm) in size were cut at the position of 1/2 in length and 1/4 in diameter of ingot and every heat treated steel. Steel samples were grinded, polished and then etched with 4% nital. The morphologies and compositions of typical carbides in steel samples were analyzed by SEM-EDS. The carbides for XRD and chemical quantitative analysis are electrochemical extracted from steel samples (Φ12 mm × 85 mm) which are obtained at the same position of each heat treated steel ingots.
The microstructure of H13 steel after different heat treatment process was observed by SEM, as shown in Fig. 1. In Fig. 1(a) (sample A1), the dendritic segregation was widely distributed in H13 steel. The interdendritic space presents a net-shaped microstructure, where elements with portion coefficient less than 1 are concentrated and secondary phases precipitate out when solute elements are supersaturated. Figure 1(e) presents the coexisted primary V-rich and Mo-rich carbides in the interdendritic space. In fact, the amount of primary V-rich carbides is far more than that rich in Mo and there are no Cr-rich primary carbides in the as-cast ingot. After annealing, a large amount of secondary carbides precipitate in the interdendritic space and the white net-shaped carbide segregation area is composed, as shown in Fig. 1(b) (sample A2). Figure 1(f) shows the carbides in carbide segregation area where most of the massive secondary carbides with fine size are detected to be rich in Cr and the primary carbides are not decomposed. However, the carbide segregation was strongly decreased after the hot forging and re-annealing process, as shown in Fig. 1(c) (sample A3). The carbide segregation is weakened sharply, most of Mo-rich and part of V-rich primary carbides are decomposed (Fig. 1(g)). Further, the carbide segregation was not detected after the quenching and tempering process and only a small fraction of primary V-rich carbides remained in steel, as shown in Fig. 1(d) (sample A4). Moreover, secondary carbides get finer and more dispersive gradually from sample A2 to sample A4 by comparing Fig. 1 (f), (g) and (h).
Morphologies of different heat treated H13 steel: Steel sample of (a) and (e) A1; (b) and (f) A2; (e) and (g) A3; (d) and (h) A4.EDS spectrums of point (i) 1; (j) 2 and (k) 3 in Fig. (f).
The carbide types in H13 steel after each stage of heat treatment process were determined by XRD. The XRD spectrums are documented in Fig. 2. It should be illustrated that the first peak shown in Fig. 2 (a) is residual organics of electrolyte.
XRD spectrums of carbides in different heat treated H13 steel: Carbides types in (a) A1; (b) A2; (c) A4.
Due to the little difference between XRD spectrums of carbides in sample A2 and A3, the XRD spectrum of carbides in steel sample A2 is only presented for concision. As can be seen in Fig. 2(a), the carbides in sample A1 are MC, M6C and less M7C3. The observed results confirmed that the MC and M6C are usually in the forms of primary carbides, and M7C3 as secondary carbide. Carbides in sample A2 and A3 are M7C3, MC and less M6C, which was in accordance with that a large amount of secondary M7C3 carbides formed during annealing process shown in Fig. 1(b) and (e). Most of primary M6C and minority of primary MC carbides are decomposed and may be replaced by a small part of precipitated secondary M6C and MC in A3. Carbides in A4 are MC, M23C6, M7C3 and M6C, which reveals that the MC and M6C re-precipitated out after carbides dissolution. M23C6 carbides can be formed by the in-situ transformation of M7C314,15), so M23C6 carbides may be formed as a new phase by the transformation of some M7C3 carbides during the tempering process.
3.2 Thermodynamic calculation of carbide phases in heat treated H13 steelThe relationship between carbides and temperature was calculated by Thermo-calc software as plotted in Fig. 3(a), and the related data is listed in Table 4. The carbides formed in the temperature range of 800–900℃ is shown in Fig. 3(b). It is considered that the segregation of solute elements promote the formation of primary carbides during the solidification process, so the mass fraction of non-metallic elements was assumed to enlarge two times and the metallic elements enlarge three times in the segregation area except for Cr (enlarging two time as well) due to its better diffusion ability. The assumed compositions were calculated by Thermo-calc software to simulate the carbide formation process in the segregation area. The result is shown in Fig. 3(c).
Calculation results of carbide phases in the H13 steel. (a) 400～1600℃, (b) 800～900℃, (c) Simulated situation in segregation area.
| MC | M7C3 | M23C6 | M6C | M2C | |
|---|---|---|---|---|---|
| Dominated elements | V | Cr | Cr | Mo | Mo |
| Temperature intervals/℃ | −1328 | 759–907 | −795 | 861–867 | 862–882 |
Based on equilibrium thermodynamic calculation, carbides in H13 steel are secondary MC, M7C3, M2C, M6C and M23C6 according to Fig. 3(a), whereas M7C3, M6C and M2C exist only in a narrow temperature range. However, in non-equilibrium state, the temperature range for M7C3, M6C are largely extended due to the element segregation, as shown in Fig. 3(c). The solidification process of H13 steel can be summarized as follows according to Fig. 3:
1) MC began to precipitate at 1328℃ after the liquid phase of H13 steel disappeared (1346℃), the austenite formed completely, and then existed in the steel all through the heat treatment process;
2) M7C3 began to precipitate when the temperature is reduced to 907℃. M2C began to precipitate at 882℃ afterwards, and hence the precipitation rate of M7C3 decreased;
3) M2C is unstable and easily decomposed as M6C and MC16). The mass percent of M6C reached a maximum 0.11% at 862℃ and decomposed rapidly with the decreasing of temperature, the precipitation of M7C3 began to accelerate till the content of M7C3 reach the maximum;
4) M7C3 transformed to M23C6 at 795℃ and then reach the flat peak.
It can be inferred from Fig. 3(c) that primary carbides MC and M6C are formed directly in liquid phase. No primary M7C3 forms in this calculation of assumed composition. The calculation interprets the primary carbides forming process reasonably. On the other hand, the calculation also reveals that the increase of bulk contents of Cr, Mo, V and C promotes the dissolution temperatures of carbides and the possibility of the formation of primary carbides.
Both thermodynamic and kinetic conditions are needed to be met for the precipitation and decomposing of carbides. Therefore, the formation of carbides in H13 steel at different heat treatment stage can be interpreted by combining the thermodynamic condition, kinetic condition and the heat treatment process.
It indicates that M23C6 carbides exist in the equilibrium state of H13 ESR ingot, as shown in Fig. 3 (a) and (c). But the XRD result (Fig. 2 (a)) is contradictory to the calculated results by Thermo-calc, since the rapid cooling rate of ESR process and lead to the insufficient of kinetic conditions for the formation of M23C6. Owing to both thermodynamic condition and kinetic condition were met in the liquid phase, MC, M6C and M7C3 were detected in ESR ingot. M7C3, MC and few M6C were found in the annealed H13 steel, which is accordance with the carbides in H13 ESR ingot. There is a large difference in the content of each type of carbide, this is because a large amount of secondary M7C3 precipitated, M6C dissolved and MC remained unchanged under the sufficient thermodynamic and kinetic conditions during annealing process. M2C is difficult to generate during annealing, because the conditions of both thermodynamics and kinetics of M6C + MC → M2C are poor16). The carbides in quenched-tempered H13 steel are MC, M23C6 and a small amount of M7C3 and M6C. The evolution of carbides in H13 steel can be speculated according to the generating curves of three types of carbides in Fig. 3 (a) and (c): primary carbide MC cannot be dissolved completely in H13 steel. The reason is that the temperature range of forging and annealing thermodynamically ensure the inevitability of its existence. But the harmful effect of primary carbide MC can be reduced by forging and high temperature insulation for a long time to change its morphology in the dynamic equilibrium. With the improvement of composition segregation, the temperature range of M7C3 and M6C precipitation in the segregation zone narrowed and carbides remained partially during the quenching and tempering process. M23C6 precipitated easily during tempering process with a long time.
3.3 Quantitative chemical analyses of carbides in different heat treated H13 steelTable 5 shows the contents of carbides in H13 steel at different heat treatment stage. A large amount of primary carbides precipitated after the ESR process, and secondary carbides precipitated little or no. The content of carbides in sample A2 is the maximum while sample A1 is the minimum, this is because the severe segregation in the ESR ingot, a large amount of secondary carbides precipitated and primary carbides cannot be eliminated after annealing. A large amount of primary carbides were broken and dissolved partially into the matrix after forging and annealing. The segregation is improved. But a large amount of secondary carbides M7C3 still precipitated, so the content of carbides in sample A3 is next to that of sample A2. The segregation was further eliminated after high-temperature quenched. Since the precipitated amount of secondary carbides was few, and which size distribution was tiny and diffused, the content of carbides in sample A4 was the least. The working temperature of H13 die steel is high, which is equivalent to multiple tempering treatments. As the carbides precipitated and grew continuously, the content of carbides was increased gradually. The precipitation and transformation of carbides during the heat treatment require an incubation period, similarly, the growth of carbides takes time. It indicates that each phase of carbides could not reach equilibrium state both on temperature and time during the heat treatment process, resulting in the large difference between the calculatedvalue of content of carbides and the measured results.
| A1 | A2 | A3 | A4 | |
|---|---|---|---|---|
| Calculated values | 6.5 | 2.2 | 2.2 | 6.0 |
| Experimental values | 3.2 | 4.5 | 4.1 | 2.9 |
Note: the calculated values are the sum of carbide contents at different heat treatment temperatures according to Fig. 3(a).
The size of carbides in H13 die steel was decreased effectively after heat treatment. The alloy elements diffused rapidly and each phase in carbides was close to equilibrium under the condition of a high temperature in heat treatment, but the equilibrium required a long time to be reached or hard to be reached. With sample A2, for example, analyzed compositions of carbides is listed in Table 6. W appears as a residual element. MC and M6C are designated as M(C, N) and M6(C, N) respectively as trace nitrogen exists. Thermo-calc software was employed to calculate the composition of three types of carbides in sample A2 at 860℃, and then the ratios of major elements in carbides was calculated. The values of major elements come from Table 6.
| Cr | Fe | Mn | Mo | W | V | C* | N | ∑ | |
|---|---|---|---|---|---|---|---|---|---|
| M7C3 | 1.495 | 0.832 | 0.018 | 0.098 | 0.165 | 0.248 | 2.856 | ||
| M6(C,N) | 0.049 | 0.173 | 0.010 | 0.368 | 0.013 | 0.028 | 0.015 | 0.003 | 0.659 |
| M(C,N) | 0.084 | 0.076 | 0.004 | 0.562 | 0.137 | 0.028 | 0.891 | ||
| ∑ | 1.628 | 1.005 | 0.028 | 0.542 | 0.017 | 0.755 | 0.400 | 0.031 | 4.406 |
Note: the carbon contents are calculated.
The Cr-rich M7C3 is the dominant carbide which occupies a mass fraction of 64.82% (2.856/4.406) of total carbides according to Table 6. It is believed that the precipitate M7C3 is formed by the transformation of M3C17), which is usually replaced by M7C3 when the temperature increases. One or more types of transitional carbides may form during annealing before the stability is achieved8). The thermodynamic stability region of M6(C, N) in this steel is narrow, which cause the mass fraction of M6(C, N) is less than that of M7C3 and M(C, N). The lattice constant of the face-centered cubic M(C,N) is far less than that of M6(C, N) and M7C318). The secondary M(C, N) precipitated with high stability, small size and dispersive distribution, which present as a precipitation strengthening in steel19). Both the ratios of major elements of measured values and calculated values are listed in Table 7. The measured value is close to the calculatedvalue, but there still exist a certain difference. It can be inferred that each phase of carbides in H13 die steel could not reach equilibrium during heat treatment. Actually, it takes a long time for each phase to reach equilibrium. The composition of alloy elements of carbides approximated calculatedvalue with the prolonging of heat treatment time in Ref. 20). The evolution process of carbides in H13 die steel can be interpreted reasonably according to the results calculated by Thermo-calc software and measured results, but the listed evolution mechanism of each phase in carbides in H13 steel need further research.
| MC (V/(Cr + Mo)) |
M6C (Mo/Fe) |
M7C3 (Cr/Fe) |
|
|---|---|---|---|
| Calculated values | 4.67 | 1.78 | 1.69 |
| Experimental values | 3.51 | 2.13 | 1.80 |
Note: the calculated values are the sum of carbide contents at different heat treatment temperatures according to Fig. 3(a).
(1) The dendritic segregation widely distributed in H13 ESR ingot. MC and M6C which were the forms of primary carbides formed in the segregation area. The majority of primary M6C and minority of primary MC carbides are decomposed during the heat treatment process, but few primary MC remained in the final state of heat treatment.
(2) Carbides in annealed or forged H13 die steel were M7C3, MC and less M6C. After the quenching process, carbides in H13 die steel were MC, M23C6, M7C3 and MC. Carbides segregation were eliminated efficiently. Carbides were modified to tiny and diffused after the heat treatment process.
(3) Carbides remained partially with the improvement of composition segregation during the quenching and tempering process, as the temperature range of precipitation of M7C3 and M6C in the segregation area become narrow. M23C6 precipitated easily in the case of tempering process with a long time.
(4) Each phase of carbides in H13 steel could not reach equilibrium state both on temperature and time during the heat treatment process, which resulted from the large difference between the calculated value of carbides content and the measured value.
This work was financially supported by the National Natural Science Foundation of China (Grant No. 51374022 and No. 51504019) and the Project of High-tech Ships of Ministry of Industry and Information Technology of the Peoples republic of China (Grant No. [2014]508).
