Effect of Surfactant Tellurium on the Microstructure and Mechanical Properties of M42 High-Speed Steel

Abstract

The microstructure and mechanical properties of high-speed steels are sensitive to some surface-active additives. In this work, four M42 high-speed steel ingots containing tellurium (Te) in concentrations ranging from 0 to 500 ppm were prepared with a vacuum induction furnace, and their microstructure and mechanical properties were systematically investigated. Te combines with Mn and possibly forms MnTe phase located in the interdendritic regions together with eutectic carbides. Te obviously refines the primary dendrite stem and decreases the secondary dendrite arm spacing of as-cast M42 steel. Te hardly affects the fraction of eutectic mixtures, but greatly refines the size of eutectic mixtures. The presence of 163 ppm Te strongly promotes the formation of M₆C eutectic carbides. This trend, however, is reversed at higher Te contents. 163 ppm Te increases the red hardness of M42 steel from 58.51 HRC to 60.8 HRC by suppressing the dissolution of secondary carbides. Further increasing Te content has no significant effect on the red hardness. Moreover, Te slightly deteriorates the tensile strength of as-cast M42 steel, while substantially improves the compressive and bending strength of tempered M42 steel. The reasons why Te affects the formation tendency of M₆C eutectic carbides as well as the mechanical properties of M42 steel were discussed.

1. Introduction

High-speed steels contain high amounts of carbon (C) and strong carbide-forming elements, such as Cr, Mo, W and V.¹⁾ Therefore, high-speed steels are featured with a high level of carbide precipitates.²⁾ These carbides give high-speed steels high wear resistance and red hardness, which are critical for high-speed steels. AISI M42 steel, a Mo series high-speed steel, exhibits superior performance over many conventional high-speed steels,³⁾ and is widely used as cutting tool materials for difficult-to-cut metals.

The characteristics of carbide precipitates, such as size, amount, morphology, type and spatial distribution, greatly affects the wear resistance, red hardness, and other mechanical properties of high-speed steels.⁴⁾ Most of these carbides are formed via eutectic reaction at the end of solidification, known as eutectic carbides including M₂C carbides, M₆C carbides, MC carbides, etc. Here, M and C represent metallic elements (Fe, Mo, W, Co, V and Cr) and carbon, respectively. Like many other high-speed steels, M42 steel mainly contains M₂C and M₆C eutectic carbides distributed in the interdendritic regions in the form of a continuous network.^5,6,7,8) M₂C carbides are easier to be broken down during hot working.^2,9,10) Moreover, M₂C carbides are thermodynamically unstable and will decompose into fine M₆C and MC carbides during high-temperature solution treatment.^2,4,11) Therefore, formation of M₂C eutectic carbides is desired to improve the performance of high-speed steels.

Formation of M₂C and M₆C eutectic carbides compete inevitably during the solidification of high-speed steels.²⁾ Therefore, various influencing factors on the type of eutectic carbides have been extensively investigated in the past few decades, such as major alloying elements, cooling rates, and surface-active additives. Formation of M₂C carbides is favored with respect to M₆C by high contents of C^12,13) and V,^9,10,13,14) and a high Mo/W ratio.^12,14,15) Moreover, high cooling rates favor the formation of M₂C carbides.^4,5,10) The effects of surface-active additives, such as RE (rare earth metals), N, Mg, Ca, Bi and so on, on the type of eutectic carbides, solidification microstructure, and mechanical properties of high-speed steels have been explored, revealing very complex phenomena. Chaus¹⁶⁾ reported that RE-based additives (ferrocerium, silicomishmetal and alumoyttrium) decrease the size of the primary grains of cast high-speed steels R6M5K5 and R6M5, whereas increase the amount of eutectic carbides. Moreover, the impact toughness, red hardness and wear resistance are improved or deteriorated, depending on the type and amount of the RE-based additives. Jiao et al.⁸⁾ observed that Ce decreases the amount of eutectic carbides in M42 steel, and promotes the formation of M₆C eutectic carbides. Boccalini Jr et al.¹⁷⁾ found that RE (mischmetal) not only favors the formation of M₆C eutectic carbides in M2 high-speed steel, but also promotes the formation of duplex M₂C/MC and M₆C/MC eutectics. Boccalini et al.,²⁾ Jiao et al.,⁶⁾ and Luo et al.¹⁸⁾ found that N promotes the formation of M₂C eutectic carbides. The reasons have not been well understood. Boccalini et al.²⁾ proposed that N increases the supercooling for the formation of M₂C eutectic carbides. Moreover, Jiao et al.⁶⁾ reported that high N addition decreases the hardness and compression strength of as-cast M42 steel, whereas increases the ductility. Jiao et al.⁷⁾ observed that Mg promotes the formation of M₂C carbides, but decreases the amount of eutectic carbides in M42 steel. Chaus¹⁹⁾ revealed that Bi refines both as-cast microstructure and eutectic carbides in M2 and T30 high-speed steels. Moreover, Bi increases their wear resistance, whereas significantly decreases their red hardness. Ca²⁰⁾ and K/Na²¹⁾ can refine the eutectic carbides in high-speed steels, but have negligible effect on the type of the carbides.

Except Bi, all the aforementioned surface-active additives have extremely high chemical activity but low solubility in high-speed steels, which makes the application of these additives in high-speed steels very difficult. Moreover, their effects on the mechanical properties of high-speed steel are still not satisfying. Therefore, the aim of this work is to explore a new surface-active additive, so as to improve all-round performance of M42 high-speed steel. Tellurium (Te) is identified due to the below reasons. First, Te is the strongest non-metallic surface-active element in steel.²²⁾ Second, Te has been reported to benefit steel properties. Te, even present in a minute amount, can significantly increase the removal rate of non-metallic inclusions,^23,24,25) and globularize MnS inclusions in sulfur-containing free-machining steels.^26,27) Moreover, Narihiro et al.²⁸⁾ reported that Te not only refines the as-cast microstructure of a low carbon steel, but also inhibits the growth of austenite grains during solution treatment. Arkhurst et al.²⁹⁾ found that addition of Te results in superior yield strength and ultimate tensile strength of an oxide-dispersion-strengthened steel by refining the grains. Therefore, in this work, four M42 steel ingots containing different amounts of Te were prepared, and their microstructure and mechanical properties were systematically investigated.

2. Experimental

2.1. Material Preparation

Four Te-bearing M42 steel ingots each weighing 8 kg were produced by melting high purity base materials in a vacuum induction furnace at around 1823 K under argon atmosphere. The liquid steel was deoxidized by adding 0.02% Al at 12 minutes before casting. Pure Te particles wrapped in iron foil were added into molten M42 steel at 2 minutes before casting. The amounts of Te added were 0, 0.03 wt.%, 0.06 wt.% and 0.10 wt.%, respectively. Then, the molten steel was cast into a cylindrical mould made of cast-iron. When naturally cooled to room temperature, the ingots were taken from the chamber of the furnace. The diameters of the top and bottom bases of the cylinder-like ingots were about 92 mm and 76 mm, respectively. The height of the ingots was about 160 mm. The chemical composition of the M42 steel is listed in Table 1.

Table 1. Chemical composition of M42 steel, wt.%.

C	Si	Mn	V	Cr	Co	Mo	W	Fe
1.10	0.31	0.30	1.15	3.75	8.00	9.50	1.50	Bal.

Due to shrinkage cavity and impurity, the upper and lower parts of the ingots with respective heights of 45 mm and 10 mm were cut off. The remaining middle part was cut into two halves along the longitudinal axis, and one half for each ingot was forged at around 1373 K into a plate with dimensions of 100 mm × 110 mm × 22 mm.

2.2. Heat Treatment

After forging, M42 steel was subjected to heat treatment. The typical heat treatment process for M42 steel is annealing → quenching → tempering. For annealing, the forged M42 plates were loaded into a muffle furnace at room temperature and heated up to 1153 K at 360 K/h. After holding there for 6 hours, the furnace was cooled to 873 K at 20 K/h, and then naturally cooled to room temperature with power off. Specimens with dimensions of 10 mm × 10 mm × 6 mm were cut from the annealed M42 plates for subsequent quenching and tempering treatment. The steel specimens were put into the muffle furnace at 1453 K for 30 min, followed by oil quenching. Thereafter, the quenched specimens were subjected to triple tempering at 823 K and 923 K for 1 h each time. Note that to prevent oxidation during heating, the surface of the M42 steel was coated with oxidation resistant refractory.

2.3. Compositional and Microstructural Analysis

The oxygen and nitrogen contents of the M42 steel were analyzed with the oxygen/nitrogen analyzer (LECO, TC500). The carbon and sulfur contents were analyzed with the carbon/sulfur analyzer (ELTRA, CS3000). The Te contents were analyzed with the inductively coupled plasma mass spectrometer (FPI, EXPEC7000). To reduce background interference, an equal amount of Te-free M42 steel was added into the standard solutions.

To observe macroscopic distribution of M₆C eutectic carbides in each M42 steel ingot, a 1/4 disc was cut from the ingot at 10 mm from the bottom. After mechanical grinding, the 1/4 disc was etched in Nital solution (8% nitric acid in alcohol) for 20 minutes at room temperature. Then, images on the etched disc were taken with a high-resolution digital camera.

To reveal the dendrite structure of each M42 steel ingot, a 10 mm × 10 mm × 6 mm steel specimen was taken at 1/2 R position of a disc (6 mm in thickness), which was cut from the ingot at 105 mm from the bottom. After mechanical grinding and polishing, the specimen was etched with Groesbeck reagent (4% K₂MnO₄ + 4% NaOH + 92% H₂O) for 3–4 seconds at room temperature, and then observed with an optical microscope (Olympus, DSX510). Thereafter, the secondary dendrite arm spacing (SDAS) was measured with the aid of an image analyzer (Image J). To ensure measurement accuracy, 15 dendrites with at least 5 secondary dendrite arms were measured for each specimen. Moreover, identical specimens were taken from the same position of the M42 steel ingots for microstructural and compositional analysis using an electron probe micro-analyzer (FEG EPMA JXA-8530 F) equipped with energy dispersive spectrometers (EDS) and wavelength dispersive spectrometers (WDS).

2.4. Measurement of Mechanical Properties

The macro-hardness of as-cast and tempered M42 steel at room temperature was measured with a digital Rock-well hardness tester (HRS-150D, China). For red-hardness measurement, specimens tempered at 823 K were reheated at 893 K for 4 hours, followed by air cooling to room temperature. For each specimen, at least 10 measurements at different positions were performed and averaged.

The tensile test, three-point bending test and compression test for M42 steel were performed at room temperature using a Shimadzu universal testing machine (AG-Xplus 100 kN) with a constant loading rate of 0.005 mm/s. The specimens for tensile test were machined in the transverse direction from the lower part of as-cast M42 steel ingots. The specimens for bending and compression tests were machined in the longitudinal direction from tempered M42 plates.

3. Results and Discussion

3.1. As-cast Microstructure of M42 Steel

Table 2 shows the contents of Te, S, O and N in as-cast M42 steel. A clear trend was observed that the O contents in 3# and 4# ingots are obviously much lower than that in the Te-free ingot (1#). This is consistent with previous finding that Te promotes the flotation of non-metallic inclusions.^23,24,25) Due to the low boiling point of Te (988°C), Te yield is not high, around 50%.

Table 2. Contents of Te, S, O and N in as-cast M42 steel as well as Te yields.

No.	S (ppm)	O (ppm)	N (ppm)	Te (ppm)	Te yield (%)
1#	43	12	21	0	/
2#	44	16	20	163	54.3
3#	35	8	30	272	45.3
4#	39	5	13	500	50.0

Figure 1 shows the effect of Te on the microstructure of as-cast M42 steel observed at R/2 position. All ingots have typical dendritic microstructure, with eutectic carbides in black color distributed in the interdendritic regions. Obviously, the Te-free M42 steel has not only the coarsest primary dendrite stem, but also the largest eutectic carbides. Usually, the as-cast microstructure is quantitatively described with the secondary dendrite arm spacing (SDAS). Therefore, the SDAS values at different Te contents were measured and presented in Fig. 2. With the increase of Te content, the SDAS value decreases linearly. Specifically, the SDAS value is 24.9 μm in the Te-free M42 steel, and is decreased to 21.9 μm at 500 ppm Te. Therefore, Te addition can significantly refine the solidification microstructure of M42 steel. Such a phenomenon is consistent with the report by Narihiro et al.²⁸⁾ that Te refines the solidification microstructure of a low carbon steel 0.10%C-0.45%Si-0.34%Mn-0.30%Cu-Fe.

Fig. 1. The microstructure of as-cast M42 steel with different Te contents. (a) 0 ppm; (b) 163 ppm; (c) 272 ppm; (d) 500 ppm.

Fig. 2. Effect of Te content on the SDAS.

As well known, the SDAS of a given alloy is mainly affected by cooling rate. Increasing cooling rate can results in smaller SDAS.²⁾ The presence of solutes also strongly affects the SDAS of high-speed steel, as observed in this work and in previous studies by Jiao et al.^7,8) on Ce and Mg. Feurer and Wunderlin, as described by Kurz and Fisher,³⁰⁾ developed a secondary dendrite coarsening model applicable to all solidifying metals, which can be written as:

  

$$\mathrm{S}\mathrm{D}\mathrm{A}\mathrm{S}={\left\{\frac{M{D}_{\mathrm{l}}{\gamma }_{\mathrm{s}\mathrm{l}}{T}_{\mathrm{l}}}{{\rho }_{\mathrm{l}}{L}_{\mathrm{f}}{m}_{\mathrm{l}}(k_{\mathrm{p}}-1)}\cdot \frac{ln\left(c_{\mathrm{e}}/c_0\right)}{c_{\mathrm{e}}-c_0}\cdot t_{\mathrm{f}}\right\}}^{1/3}$$(1)

where M is a material-specific constant, D_l is the solute diffusivity, γ_sl is the liquid-solid interfacial energy, T_l is the liquidus temperature, c_e is the eutectic composition, c_o is the alloy composition, ρ_l is the liquid density, L_f is the latent heat of fusion, m_l is the liquidus slope, k_p is the partition coefficient, and t_f is the local solidification time. Clearly, the presence of Te decreases the SDAS from many aspects. The main reason may be that Te increases the constitutional undercooling. The partition coefficient of Te is currently unknown. Te has very similar chemical properties to S, which has a very low partition coefficient of about 0.05. Thus, strong accumulation of Te in front of the advancing solid-liquid interface due to segregation is expected. In addition, the liquid-solid interfacial energy γ_sl can be largely decreased by the presence of surface-active solutes,³¹⁾ also favoring the decrease in SDAS. Furthermore, strong adsorption of surface-active solutes at the liquid-solid interfaces can kinetically retard the growth of the dendrites, as observed from the refinement of the primary dendrite stem (Fig. 1).

Figure 3 shows the area fraction and size of eutectic mixtures distributed in the interdendritic regions. Here, eutectic mixtures refer to the eutectic carbides and the austenite phase. The area fraction slightly fluctuates with increasing Te content, suggesting that Te does not inhibit the formation of the eutectic carbides. However, Te significantly refines the size of eutectic mixtures. Note that eutectic mixtures are in the form of continuous network, which could be seen more clearly in the below figures. The traditional expression of equivalent diameter for discrete particles is not suitable anymore to describe the size of eutectic mixtures. To the best knowledge of the authors, currently there are no reliable methods to accurately characterize the size of eutectic mixtures. Hence, a new expression of equivalent diameter (D_e) is proposed in this work, as defined in Eq. (2), where S and P are total area and perimeter of eutectic mixtures on polished section, respectively. The values of S and P were measured with the aid of Image J from 15 SEM micrographs randomly taken at the magnification of 500x. The total observed area was 6.7 mm². Note that for the measurement of P value, only the outline between primary austenite phase and eutectic mixtures was counted in. As seen in Fig. 3, 163 ppm Te substantially decreases the equivalent diameter of eutectic mixtures from 14.7 μm to 12.9 μm. Further increase of Te content only leads to slight fluctuation of the equivalent diameter. Since the area fraction of eutectic mixtures is almost irrelevant to Te content, the smaller equivalent diameter is attributed to the increased perimeter of eutectic mixtures. Theoretically, the perimeter of eutectic mixtures is equal to that of primary austenite dendrites. The refined primary austenite dendrites by Te addition provides more locations for the formation of eutectic mixtures, thus leading to smaller equivalent diameter of eutectic mixtures.

  

$$D_{\mathrm{e}}=\frac{4S}{P}$$(2)

Fig. 3. Effect of Te content on the area fraction and size of eutectic mixtures. (Online version in color.)

3.2. Characteristics of Eutectic Carbides

Figure 4 shows the micro-distribution of M₂C and M₆C eutectic carbides as well as elemental mappings of the carbides in as-cast M42 steel containing 163 ppm Te. M₂C and M₆C eutectic carbides never mix with each other,²⁾ as observed in Fig. 4(a). M₆C eutectic carbides occupy the upper left region of the figure, while M₂C eutectic carbides occupy the lower right region. Moreover, M₆C eutectic carbides are much brighter than M₂C carbides in the BSE (backscattered electrons) image, indicating that M₆C eutectic carbides contain more heavy elements. This phenomenon is more directly revealed in Figs. 4(b)–4(i), which show the elemental mappings of the eutectic carbides. Obviously, M₂C eutectic carbides contain more V, while M₆C eutectic carbides are richer in Mo and W. The two types of carbides have insignificant difference in Cr and Co contents. In Figs. 4(h) and 4(i), the Mn-rich bright spots overlap with the Te-rich spots, suggesting the formation of MnTe phase.^27,32) Moreover, the MnTe phase is distributed in the interdendritic regions, and coexists with the eutectic carbides, as indicated with the circles in Fig. 4(a). Thus, Te is first enriched in the residual liquid due to segregation upon cooling of M42 melt, and then combined with Mn to form MnTe phase. Enrichment of soluble Te in the residual liquid not only favors the refinement of the as-cast microstructure of M42 steel, but also increases the effect of Te on the characteristics of the eutectic carbides, such as carbide size (Fig. 3) and carbide type to be described in the below section.

Fig. 4. Micro-distribution of M₂C and M₆C eutectic carbides as well as elemental mappings of the carbides in as-cast M42 steel containing 163 ppm Te. (Online version in color.)

Figure 5 presents the morphologies of typical M₂C and M₆C eutectic carbides in the M42 steel with different Te contents. M₂C eutectic carbides shown in Fig. 5(a) are in lamellar-like morphology, and no midplanes were observed in the carbides. Both rod-like and plate-like M₂C carbides were frequently observed. The rod-like M₂C carbides, which corresponds to broken plate-like M₂C carbides, are usually related to high cooling rates.^5,13) Mostly, the interface between metal matrix and M₂C eutectic carbides is clear and well outlined, also indicating fast cooling.²⁾ M₆C eutectic carbides shown in Fig. 5(b) have characteristic central platelets.²⁾ The compositional analysis listed in Table 3 (+2 spot) also indicates that the eutectic carbides are M₆C. The secondary platelets are coarsened toward the end of freezing of the eutectic melt, and connected with neighboring platelets, forming a near-continuous “wall” in most cases. Such a phenomenon is well consistent with the report by previous researchers.^2,5,8) M₂C and M₆C eutectic carbides shown in Figs. 5(c) and 5(d) respectively were taken from the M42 steel containing 272 ppm Te. No obvious difference was identified, compared with the eutectic carbides in the Te-free M42 steel (Figs. 5(a) and 5(b)).

Fig. 5. Microstructure of representative M₂C and M₆C eutectic carbides in as-cast M42 steel. (a) and (b) 0 ppm Te; (c) and (d) 272 ppm Te. (Online version in color.)

Table 3. Chemical compositions of MC and M₆C eutectic carbides, at.%.

Spots	C	V	Cr	Fe	Co	Mo	W	M/C ratio
+1	40.37	7.04	9.21	5.18	0.35	36.17	1.68	1.48
+2	13.20	3.42	4.05	41.98	4.40	30.47	2.50	6.58

Moreover, coarse and blocky carbides were frequently found at the interface between metal matrix and M₂C eutectic carbides, as indicated with the dashed-line arrows in Fig. 5(a). Table 3 lists the chemical composition of a representative blocky carbide (+1 spot). The atomic ratio M/C (1.48) indicates that the blocky carbide may be monocarbide MC. Note that the relatively lower carbon measurement in the blocky carbide may be attributed to inaccurate compositional analysis via the EPMA-WDS due to its small size. According to Boccalini et al.,²⁾ the blocky MC carbides arises from divorced eutectic reaction.

3.3. Effect of Te on the Formation of M₆C Eutectic Carbides

During solidification of M42 steel, mainly M₂C eutectic carbides are formed. M₆C eutectic carbides only appear locally, especially in the center of the ingot where the cooling rate is relatively slow. Figure 6 shows the optical macrographs of a 1/4 disc cut from the M42 steel after etching in Nital solution for 20 minutes, displaying the macro-distribution of M₆C eutectic carbides. According to Jiao et al.,^7,8) M₆C eutectic carbides appear bright after etching, as indicated with the arrow in Fig. 6(a), while M₂C eutectic carbides and metal matrix appear dark. Therefore, the bright spots mainly consist of M₆C eutectic carbides. Clearly, the relative amount of M₆C eutectic carbides gradually increases from the edge to the center of the ingots, which is in good agreement with previous reports.^2,5,7,8) Presence of Te significantly affects the relatively amount of M₆C eutectic carbides. Specifically, without Te addition, M₆C eutectic carbides are only occasionally observed. When Te content is increased to 163 ppm, formation of M₆C eutectic carbides is substantially promoted. This trend, however, is reversed when more Te is present. At 272 ppm and 500 ppm Te, M₆C eutectic carbides become much less, and their amounts are approximately equal. The effect of Te on the formation of M₆C carbides differs from other surface-active elements, such as RE^8,17) and Mg,⁷⁾ which monotonically increases or decreases the formation of M₆C eutectic carbides. The underlying reasons will be explained below.

Fig. 6. Macro-distribution of M₆C carbides in as-cast M42 steel with different Te contents. (a) 0 ppm; (6) 163 ppm; (c) 272 ppm; (d) 500 ppm. (Online version in color.)

M₂C and M₆C eutectic carbides compete inevitably during the solidification of high-speed steels.²⁾ Besides cooling rate, the type and content of alloying elements also significantly affect their relative amounts. Regarding carbide-forming elements, it has been reported that the formation of M₆C eutectic carbides is favored with respect to M₂C by low contents of C^12,13) and V,^9,10,13,14) and a low Mo/W ratio.^12,14,15) Table 4 gives the chemical composition of M₂C and M₆C eutectic mixtures, which are transformed from the molten pool before eutectic reaction. Note that the compositional analysis was performed over circular regions, whose areas were as large as possible but still within eutectic mixtures, as indicated with circles in Figs. 5(a) and 5(b). As seen, in both M42 steel ingots with or without Te addition, C, V, and Cr are significantly more enriched in M₂C eutectic mixtures than in M₆C eutectic mixtures, which is in good agreement with previous finding that high C and V contents favor the formation of M₂C eutectic carbides.^{9,10,12,13,14)} On the contrary, M₆C eutectic mixtures contain higher contents of Mo and W, as reported by Jiao et al.⁷⁾ in M42 and by Kheirandish³³⁾ in M7, which is similar in composition to M42. Thus, high Mo and W may favor the formation of M₆C eutectic carbides at least in M42. The above experimental results are consistent with the thermodynamic calculations in a previous study by the current authors.³⁴⁾ As has been widely reported, surface-active elements can affect the segregation of alloying elements.^{7,27,35,36,37,38,39)} Regarding high-speed steels, Jiao et al.⁷⁾ reported that Mg increases the segregation of C in the residual melt in M42 but decreases segregation of Mo. Yin et al.³⁸⁾ and Qu et al.³⁹⁾ found that RE decreases the segregation of C, Mo and V in the residual melt in M2. Therefore, it is highly possible that Te affects the chemical composition of residual melt before eutectic reactions, which then affects the formation tendency of M₆C eutectic carbides.

Table 4. Chemical composition of representative M₂C and M₆C eutectic mixtures, wt.%.

Ingots	C	V	Cr	Co	Mo	W	Fe	Note
1# (0 ppm Te)	1.92	2.35	4.27	5.98	19.31	3.32	Bal.	M2C eutectic
	1.61	1.79	3.43	6.07	22.38	4.33	Bal.	M6C eutectic
2# (163 ppm Te)	2.16	2.14	4.40	6.07	18.37	3.30	Bal.	M2C eutectic
	1.80	1.52	3.54	6.13	20.45	4.32	Bal.	M6C eutectic

To reveal how Te affects the segregation of the main alloying elements in M42, their segregation coefficients were measured and presented in Fig. 7. Here the segregation coefficient of element i is defined as w_Ei/w_Mi, where w_Ei and w_Mi are mass fraction of element i in eutectic mixtures and adjacent primary austenite dendrites, respectively. Note that the compositional analysis of eutectic mixtures was performed using the WDS spectrometers in scan mode (circle) over circular regions, whose areas were as large as possible but still within eutectic mixtures, as indicated with circles in Figs. 5(a) and 5(b). The compositional analysis of adjacent primary austenite dendrites was performed using the WDS spectrometers in spot mode in the center of the dendrites, as indicated with the red spot in Fig. 5(a). For a given element, its segregation coefficient in M₂C eutectic mixtures is slightly different from that in M₆C eutectic mixtures. This may be attributed to different local solidification conditions, which affect the degree of segregation of the element. However, it is evident that presence of Te significantly inhibits the segregation of C, V, Mo and W, while slightly promotes the segregation of Cr. Therefore, compared with the Te-free M42 steel, C, V, Mo and W in the residual melt before eutectic reaction may become relatively lower for the Te-bearing M42 steel. As has been revealed above, decreasing C and V favors the formation of M₆C eutectic carbides. On the contrary, lower Mo and W contents promote the formation of M₂C eutectic carbides. This may be the root cause why Te addition first promotes the formation of M₆C eutectic carbides, and then inhibits them.

Fig. 7. Segregation coefficient of alloying elements between eutectic mixtures and metal matrix. (Online version in color.)

3.4. Mechanical Properties of M42 Steel

3.4.1. Hardness and Red Hardness

Figure 8 shows the hardness of M42 steel tempered at 823 K and 923 K, as well as the red hardness. Note that the optimal tempering temperature is around 823 K for many high-speed steels including M42 steel. The tempered hardness at 823 K is over 67 HRC, which is consistent with the reports by other researchers.^40,41) The addition of Te slightly increases the tempered hardness at 823 K. At 500 ppm Te, the hardness is increased by 0.3 HRC. When increasing the tempering temperature to 923 K, the hardness of Te-free M42 steel is lowered to 54.1 HRC. However, presence of Te significantly retains the hardness. At 163 ppm Te, the hardness is up to 57.7 HRC. Further increasing Te content has no significant effect on the tempered hardness.

Fig. 8. Effect of Te content on the macro-hardness of as-cast and tempered M42 steel as well as on the red hardness. (Online version in color.)

The presence of 163 ppm Te increases the red hardness from 58.51 HRC to 60.8 HRC. Similarly, the red hardness does not significantly increase anymore with further raising Te content. Chaus¹⁹⁾ reported that surfactant Bi significantly decreases the red hardness of M2 and T30 high-speed steels. Te shows completely different function. As observed in Fig. 8, the effect of Te on the tempered hardness is more obvious when raising tempering temperature from 823 K to 923 K. Therefore, it is expected that the effect of Te on the red hardness may become more significant when the red hardness is characterized at higher temperatures.

To reveal the reasons why Te significantly retains the tempered hardness at 923 K, the microstructure of the M42 steel tempered at 823 K and 923 K was analyzed and presented in Fig. 9. The microstructure is featured by primary and secondary carbides. Here the primary carbides refer to the carbides transformed from initial eutectic carbides, and the secondary carbides, indicated with dashed-line in Fig. 9(d), refer to the carbides precipitated in the primary austenite dendrite during forging and subsequent heat treatment. The size of the secondary carbides is much smaller, around 0.5 μm. At 823 K, no obvious difference in terms of carbide characteristics, such as number density of the secondary carbides, was observed in the M42 steel containing 272 ppm Te (Fig. 9(b)), compared with the Te-free M42 steel (Fig. 9(a)). The secondary carbides are approximately uniformly distributed in the metal matrix for both steels. However, substantial dissolution of the secondary carbides occurs in the Te-free M42 steel tempered at 923 K. As seen in the region bounded by circles of dashed-line (Fig. 9(c)), no secondary carbides were observed anymore. On the contrary, no significant dissolution of the secondary carbides was observed in the M42 ingot containing 272 ppm Te (Fig. 9(d)). Clearly, Te significantly increases the tempered hardness at 923 K by suppressing the dissolution of the secondary carbides. Possibly, Te increases the red hardness at 893 K via the same manner. According to Ostwald ripening,⁴²⁾ large carbides will grow while small carbides will dissolve during high-temperature heating. Clearly, Te, as a strong surface-active element, significantly slows down the process of Ostwald ripening. Similar phenomena related to surface-active elements P and Sb have also been observed on other kinds of precipitates by Sakuma et al.⁴³⁾

Fig. 9. The microstructure of tempered M42 steel. (a) 823 K, 0 pm Te; (b) 823 K, 272 ppm Te; (c) 923 K, 0 pm Te; (d) 923 K, 272 ppm Te. (Online version in color.)

3.4.2. Tensile Strength and Elongation of As-cast M42 Steel

Figure 10 shows the tensile strength and elongation at break of as-cast M42 steel at room temperature versus Te content. Approximately, Te linearly decreases the tensile strength from 1011 MPa to 924 MPa, when increasing Te content from 0 to 500 ppm. No clear trend was observed in terms of the elongation at break. With increasing Te content, the elongation slightly decreases and then increases. The elongation at 500 ppm Te is 3.87%, which is very close to the value of the Te-free M42 steel.

Fig. 10. Effect of Te content on the tensile strength and elongation of as-cast M42 steel. (Online version in color.)

3.4.3. Compressive and Bending Strength of Tempered M42 Steel

Figure 11 presents the effect of Te on the compressive and bending strength of tempered M42 steel. The presence of 163 ppm Te significantly increases the compressive strength by 113 MPa from 3886 MPa. However, further increasing Te content has no obvious effect on the compressive strength. On the contrary, the bending strength increases approximately linearly with the increase of Te content. Specifically, the bending strength is increased from initial 1604 MPa to 1851 MPa at 500 ppm Te.

Fig. 11. Effect of Te content on the compressive and bending strength of M42 steel tempered at 823 K. (Online version in color.)

In this work, we observed that Te slightly deteriorates the tensile strength of as-cast M42 steel, while substantially improves the compressive and bending strength of tempered M42 steel. Te may affect the mechanical properties of M42 steel in two different ways. On the one hand, Te refines both the dendrite structure of M42 steel and the eutectic carbides (Figs. 1 and 2), which theoretically is beneficial to the mechanical properties of M42 steel. On the other hand, presence of Te is detrimental to steel mechanical properties,⁴⁴⁾ as Te, which has similar chemical properties to sulfur, is regarded as an impurity element in steel, and forms large-sized MnTe precipitates (Fig. 4). For as-cast M42 steel, the harm of Te may outweigh its benefit. Therefore, the tensile strength of as-cast M42 steel is deteriorated by Te addition, and the elongation shows no clear trend in response to Te addition. On the contrary, for tempered M42 steel, the harmful effect of Te is largely weakened after heat treatment due to Te redistribution. During solidification of M42 steel, Te, due to strong segregation, is enriched in the residual liquid, where MnTe precipitates are formed (Fig. 4). Kawamura et al.³²⁾ suggested that MnTe precipitates may dissolve into metal matrix during heat treatment, and then re-precipitate during quenching, leading to uniform distribution of MnTe precipitates. Similarly, dissolution and redistribution of MnS precipitates during heat treatment of sulfur-containing free-cutting steel have been reported by some researchers.⁴⁵⁾ In this work, it is quite difficult to detect the redistribution of the MnTe precipitates, as both MnTe precipitates and carbides are white in color when imaged in BSE mode of the SEM. The solidus temperature of M42 steel is about 1475 K.⁵⁾ M42 steel was heated at 1453 K for solution treatment in this work. Such a high temperature is very favorable in kinetics for the dissolution of MnTe precipitates.

4. Conclusions

The effect of surfactant Te on the microstructure and mechanical properties of M42 steel was investigated experimentally. The following conclusions may be drawn from this work.

(1) Te significantly decreases the secondary dendrite arm spacing of as-cast M42 steel mainly by increasing constitutional undercooling. Te hardly affects the fraction of eutectic mixtures, but greatly refines the size of eutectic mixtures.

(2) The composition of M₂C and M₆C eutectic mixtures in M42 steel differs slightly. Compared with M₆C eutectic mixtures, M₂C eutectic mixtures are richer in C, V and Cr, but poorer in Mo and W, suggesting high C, V and Cr contents favor the formation of M₂C eutectic carbides, while high Mo and W contents promote the formation of M₆C eutectic carbides.

(3) The presence of 163 ppm Te strongly promotes the formation of M₆C eutectic carbides. This trend, however, is reversed when more Te is present. Te inhibits the segregation of C, V, Mo and W during solidification. Therefore, Te may change C, V, Mo and W relative contents in the residual melt before eutectic reaction, thus changing the formation tendency of M₆C eutectic carbides.

(4) Te has a very limited beneficial effect on the hardness of M42 steel tempered at 823 K. However, Te greatly increases the red hardness characterized at 893 K from 58.51 HRC to 60.8 HRC by suppressing the dissolution of secondary carbides. This effect is expected to be more significant when the red hardness is characterized at higher temperatures. Moreover, Te slightly deteriorates the tensile strength of as-cast M42 steel, while substantially improves the compressive and bending strength of tempered M42 steel. The underlying reasons may be related to the facts that Te refines the microstructure of M42 steel on the one hand, and that Te behaves as an impurity element on the other hand. The harmful effect of Te is largely weakened after heat treatment due to Te redistribution.

Acknowledgements

This work was supported by the National Natural Science Foundation of China [51904067, 52174309], and by the Program of Introducing Talents of Discipline to Universities (No. B21001).

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