Abstract
The present work has investigated the decomposition behaviour of M2C eutectic carbides with different morphologies in high speed steels. Both plate-like and fibrous M2C carbides decompose into M6C and MC at high temperatures, the decomposition of which is controlled by the diffusion of alloying elements in carbides. However, they are different in the decomposition process and thermal stability. Compared with plate-like M2C, fibrous M2C carbides are less stable, depending on the decomposition process. MC carbides are nucleated at the interface of fibrous M2C/matrix, whereas they are nucleated inside plate-like M2C carbides. The matrix plays different roles in the decomposition. It provides W, Mo and V for the decomposition of fibrous M2C and Fe for the plate-like M2C.
Introduction
High speed steels are widely used in making high speed cutting tools, which require high hardness and wear resistance at elevated temperatures. They are ferrous based alloys of the Fe–C–X multicomponent system, where X represents different alloying elements, such as molybdenum, tungsten, vanadium, chromium, etc., thus resulting in the formation of a great amount of carbides. The mechanical properties of high speed steels are determined by different microstructural parameters, particularly by the dimensions and distributions of carbides, which are closely related to the networks of eutectic carbides.1,2
The solidification of high speed steels gives rise to several different types of eutectic carbides, mainly M2C, M6C and MC, depending on chemical compositions and solidification conditions.3,4 As for the most popular M2 high speed steel, the predominant type is M2C eutectic carbide with the hexagonal close packed structure.5 Previous studies have shown that the morphologies of M2C carbides can be classified into two types, the plate-like shape and the fibrous one.6 Plate-like M2C is favoured by low cooling rates or high vanadium, whereas fibrous M2C is promoted by high cooling rates or nitrogen.7,8
M2C eutectic carbides generally exist in coarse networks during the solidification of high speed steels. Such distributions and dimensions of carbides are unfavourable. Different methods have been applied to solve this problem, including modification, forging and heat treatment.9–11 Among them, heat treatment is an effective way because M2C carbides are metastable and decompose into M6C and MC during heating, which improves the microstructural homogeneity of high speed steels.12 Previous work has shown that fibrous M2C carbides are more inclined to get separated and spheroidised after heating, obtaining more favourable distributions and dimensions of carbides, compared with that of plate-like M2C.13 In order to clarify the reason, it is necessary to know more about the decomposition process of M2C. However, in previous studies, particular stress has been laid on the decomposition behaviour of plate-like M2C.12,14 Few papers were published reporting the decomposition behaviour of fibrous M2C, and the differences between them are also rarely known. The present work was undertaken to provide such information.
Experimental
AISI M2 high speed steel was used in the present work. Its chemical composition is Fe–0·85C–0·35Si–0·23Mn–5·92W–4·74Mo–3·86Cr–1·81V (wt-). The steel was remelted by a non-oxidation process and deoxidised by silicon. Molten steel was cast in sand and iron moulds, the dimensions of which are Φ60×150 and Φ25×150 mm respectively.
In order to investigate the decomposition process of M2C carbides, samples from the ingots were heated up to 1100°C for 30, 60 and 120 min respectively. The morphological evolution of eutectic carbides was observed using an FEI Sirion 400 scanning electron microscope (SEM). Further observation of the three-dimensional morphology of carbides after heating was also performed in specimens deeply etched with 5 mL HF+100 mL H2O2. The chemical compositions of carbides and the matrix were quantitatively analysed by Genesis 60S energy dispersive spectroscopy linked to SEM. They were measured at 10 different positions and then averaged.
X-ray diffraction (XRD) was employed to study the phase transition during carbide decomposition using a Bruker D8 diffractometer with Cu Kα radiation. The specimens for XRD were carbide powders extracted from the ingots. The extraction of carbides was performed using electrolysis, operating at 40 V, 0°C. The electrolyte contained 7 g citric acid, 20 mL hydrochloric acid and 250 mL methanol. Moreover, the fractions of decomposed M2C carbides were also quantitatively examined. Based on the XRD profiles of the carbide powders extracted from the ingots before and after heating, the areas of diffraction peak of M2C, including 
, 
and 
, were calculated respectively. Given the fixed mass of carbide powders, the fraction of decomposed M2C was approximately equal to the ratio of M2C peak areas of heated samples to those before heating.
Results and discussion
Decomposition process of M2C eutectic carbides
Figure 1 shows two typical morphologies of M2C eutectic carbides in M2 high speed steel solidified under different conditions. M2C carbides present the plate-like shape in the sand mould ingot, whereas they present the fibrous shape in the iron mould ingot, exhibiting different morphological characteristics. The morphology change of M2C is caused by the transition of the liquid/solid interface structure from faced to non-faceted due to higher cooling rates of liquid steel solidified in the iron mould.15
Typical morphologies of M2C eutectic carbides in high speed steels
It has been demonstrated that plate-like M2C carbides are metastable and decompose at high temperatures.11 The decomposition also occurs in fibrous M2C carbides. Figures 2 and 3 show that both plate-like and fibrous M2C carbides decompose into M6C and MC during heating. However, the decomposition process is different for these two carbides. After the decomposition of plate-like M2C carbides, three phases with different contrasts form inside the plates, being M6C, MC and remained M2C from the outside to the inside. M6C carbides are distributed at the interface of M2C/matrix, whereas MC carbides are distributed inside M6C or at the interface of M6C/M2C. It suggests that the formation of M6C precedes the nucleation of MC. For the fibrous M2C carbides, however, the nucleation sites of MC are obviously different and more numerous. Both MC and M6C carbides are distributed at the interface of the M2C/matrix, indicating that they are almost simultaneously precipitated.
Morphology of M2C eutectic carbides after heating at 1100°C for 30 min
X-ray diffraction profiles of carbide powders extracted from ingots after heating at 1100°C for 30 min
The chemical compositions are obviously different between M2C and precipitated carbides, M6C and MC (Table 1). It is found that MC carbides are much richer in V, whereas M6C carbides are a little richer in other elements, such as W, Mo and Cr, compared with those in M2C carbides. It suggests that M6C and MC need different alloying elements from M2C carbides, and the decomposition process is accompanied by the element diffusion. The process of element diffusion during decomposition of M2C will be discussed in detail later.
Chemical compositions of different carbides in ingots after heating at 1100°C for 30 min/wt-
As seen in Fig. 2a, the precipitated MC carbides appear to be isolated from each other, suggesting that the growth of M6C and MC is independent of each other.12 Actually, that is not true. Figure 4 shows the three-dimensional morphology of decomposed M2C carbides in which the outer M6C carbides are partly removed and MC carbides are revealed by deeply etching the samples. Figure 4a shows that MC carbides are not isolated particles indeed. They present the dendritic structure and branch heavily during the decomposition of plate-like M2C. It indicates that MC carbides grow together with M6C carbides, although the growth rate of MC is relatively slower than that of M6C, thus MC being surrounded by M6C. In contrast, the dendritic structure of MC carbides, generated by the decomposition of fibrous M2C, is less obvious, and the dimensions of MC are much smaller. It could be attributed to the fact that much more MC carbides are almost simultaneously nucleated at the interface of fibrous M2C/matrix, compared with that of plate-like M2C.
Three-dimensional morphology of M2C eutectic carbides after heating at 1100°C for 60 min
With increasing heating time, the decomposition of M2C carbides proceeds, and more M6C and MC carbides form (Fig. 5). Meanwhile, M6C and MC carbides start to dissolve into the matrix. For the plate-like M2C, the outer M6C carbides are dissolved first, which leaves the inner MC carbides outside and favours their dissolution subsequently. It is noted that many M2C carbides still remain undecomposed and keep the original coarse plate-like shape after heating for 1 h. In contrast, few fibrous M2C carbides could be observed. Owing to the dissolution of M6C and MC, fibrous carbides start to get separated and spheroidised, refining the dimensions of carbides.
Morphology of M2C eutectic carbides after heating at 1100°C for 60 min
Thermal stability of M2C eutectic carbides
Besides the decomposition process, it is also interesting to note that the thermal stability is different for these two carbides. As illustrated in Fig. 5, many plate-like M2C carbides remain undecomposed, while fibrous M2C carbides are rarely observed after heating for 1 h at 1100°C.
In order to characterise the thermal stability of M2C, XRD was used to examine the decomposition process. Figure 6 shows the carbide types in M2 ingots before and after heating. It is noted that a few plate-like M2C carbides still remain undecomposed even after heating for 2 h. In contrast, the amount of fibrous M2C carbides decreases remarkably after heating for 1 h, and they almost decompose completely after 2 h. The results are consistent with the SEM observation.
X-ray diffraction profiles of carbide powders extracted from M2 ingots under different heat treatment states
The decomposed fractions of M2C carbides were quantitatively examined based on the XRD profiles, as shown in Fig. 7. More fibrous M2C carbides decompose than plate-like M2C carbides after the same heating time, showing the lower thermal stability of fibrous M2C. At the initial stage of reaction, M2C carbides decompose rapidly into M6C and MC. With increasing heating time, the decomposition rates decline gradually. The fractions of decomposed M2C carbides obey approximately a parabolic relationship with the heating time and a linear relationship with the square root of heating time. It indicates that the decomposition of M2C is mainly a diffusion controlled process.
Fraction of decomposed M2C eutectic carbides as function of heating time and square root of heating time at 1100°C
Discussion of different decomposition behaviours between plate-like and fibrous M2C
The results above illustrate that both plate-like and fibrous M2C carbides are metastable and decompose into M6C and MC carbides at high temperatures. However, the two carbides have different decomposition process and thermal stability. The reasons are discussed in the following.
As shown in Table 1, vanadium is enriched in MC carbides, while other alloying elements are enriched in M6C carbides, indicating that the element diffusion is very necessary for the decomposition of M2C. Fredriksson et al. have investigated the decomposition of plate-like M2C, showing that M2C mainly provides V for MC, while W and Mo for M6C. The matrix also participates in the decomposition and provides Fe for M6C.14 Thus, it is conceivable that the changes in compositions of M2C and matrix should influence the decomposition process of M2C.
Tables 1 and 2 illustrate the chemical compositions of M2C eutectic carbides and the matrix between adjacent M2C. Compared with the plate-like M2C, the fibrous M2C is rich in Fe while poor in other alloying elements. It is known that M2C carbides are generated by the eutectic reaction, L→M2C+austenite, the process of which is accompanied by the element diffusion in the liquid steel.6 With increasing cooling rates, the element diffusion is insufficient, thus decreasing the concentrations of alloying elements in M2C while increasing those in the matrix.
Chemical compositions of matrix between adjacent M2C carbides in primary ingots/wt-
As shown in Table 2, the concentrations of alloying elements are much higher in the matrix between fibrous M2C. One might wonder whether the matrix provides other elements for the decomposition of fibrous M2C, such as V, W and Mo, due to the fact that the concentrations of alloying elements are low in fibrous carbides. This inference is supported by the variation in chemical compositions of the matrix during heating (Fig. 8). As shown in Fig. 8a, the contents of W, Mo and V in the matrix change little or even increase during the decomposition of plate-like M2C, confirming that the matrix rarely provides these elements but mainly Fe for M6C.14 Therefore, for the decomposition of plate-like M2C, M6C carbides form first at the interface of M2C/matrix because the composition of M6C is more similar to that of M2C. They obtain Fe from the matrix and other elements from M2C carbides. After the formation of M6C, the adjacent M2C carbides are richer in V, which favours the formation of MC at the interface of M2C/M6C.
Variation of chemical compositions of matrix between adjacent M2C carbides during heating at 1100°C
In contrast, the fibrous M2C carbides are relatively poor in alloying elements except Fe and may not provide enough elements for the formation of M6C and MC. As shown in Fig. 8b, the contents of W, Mo and V in the matrix decrease during the decomposition of fibrous M2C, indicating that the matrix provides these elements for the decomposition. It is believed that both fibrous M2C and the matrix provide V for MC and other elements for M6C. Therefore, MC forms at the interface of fibrous M2C/matrix together with M6C. As illustrated in Table 1, it is interesting to note that the concentration of Fe is higher in the fibrous M2C than that in M6C and MC. It is probable that Fe that is necessary for M6C and MC is supplied by the fibrous M2C rather than the matrix, which is different from the decomposition of plate-like M2C.
After the nucleation of M6C and MC, a carbide layer forms between M2C and the matrix, hindering the decomposition reaction. During the subsequent decomposition process, alloying elements necessary for M6C and MC have to diffuse across the whole layer to the interface of M2C/M6C or M2C/MC. Thus, the decomposition process of M2C is controlled by the element diffusion in carbides. As the carbide layer grows thicker, the decomposition rates decrease gradually (Fig. 7).
Compared with the plate-like M2C, the fibrous M2C decomposes more rapidly during heating. The reason could be attributed to two aspects. First, MC carbides form at the interface of carbide/matrix during the decomposition of fibrous M2C, whereas they form inside the carbides during the decomposition of plate-like M2C. As the diffusion rates of elements are much higher at the interface than inside the carbides, the decomposition rate of fibrous M2C would be higher than that of plate-like M2C. Second, the thickness of fibrous carbides is refined, which shortens the diffusion distance of elements during the decomposition and accelerates the decomposition process of fibrous M2C. The lower thermal stability of fibrous M2C promotes the separation and spheroidisation of carbide networks, favouring the refinement and homogeneous distribution of carbides in high speed steels.13
Conclusions
In summary, M2C eutectic carbides are metastable and decompose into M6C and MC carbides at high temperatures. The decomposition process of M2C is controlled by the element diffusion in carbides. Compared with the plate-like M2C, the fibrous M2C is less stable and easier to spheroidise during heating, depending on the decomposition process of M2C. During the decomposition of fibrous M2C, the MC and M6C carbides are both precipitated at the interface of M2C/matrix and obtain W, Mo and V from the matrix. In contrast, MC carbides form inside the plate-like M2C, and the matrix mainly provides Fe for the decomposition of plate-like M2C.
Footnotes
Acknowledgements
The present research was supported by the Fund of Transformation of Scientific and Technological Achievements from Jiangsu Province (grant no.BA2010139) and the Opening Project of Jiangsu Key Laboratory of Advanced Metallic Materials.