ISIJ International
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Transformations and Microstructures
Fabrication of Ultrafine Grained High Speed Steel with Satisfactory Carbide Dissolution by Electropulsing Treatment
Jiatao Zhang Hongli ZhaoQiuyue ShiDonwei MaJianxin Sun
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2019 Volume 59 Issue 11 Pages 2126-2129

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Abstract

Microstructure of M2 high speed steel (HSS) after electropulsing treatment (EPT) was studied. The secondary carbides precipitated during pre-tempering were dissolved completely during EPT. But the secondary carbides in the starting microstructure cannot be dissolved completely upon conventional reheating to the temperature same as that during EPT. The austenite grain size was ultra-refined by EPT. This study provides a new way to refine austenite grain size in HSS without diminishing the dissolution amount of carbides.

1. Introduction

High speed steels (HSSs) are complex alloy tool steels with high carbon and high alloying elements, such as W, Mo, Cr, V, or Co.1,2) Because of their high wear resistance, good red hardness and reasonable toughness when treated correctly,3) HSSs grab over 30% of the global cutting tool market.1) The mechanical properties of HSSs are greatly influenced by carbides in this material, such as carbide type, volume percent, shape, and size and distribution.4) Primary (eutectic) carbides are primarily responsible for the wear resistance of HSSs. Secondary carbides influence primarily the hardness and red hardness of HSSs.1) Nano sized secondary carbides can precipitate from the quenched matrix during tempering, providing considerable secondary hardening. But inadequate carbide dissolution during austenitization will reduce the supersaturation of as-hardened martensite, reducing the secondary hardening during tempering.

Heat treatment not only influences the carbide morphologies, but also actually determines the grain size in HSSs. Grain refinement can be expected to improve the toughness. Low-temperature austenitization benefits to obtain fine grain size, but the supersaturated carbon and alloying elements in solution in the quenched matrix could be reduced due to the decreasing of dissolved amount of carbides, diminishing the secondary strengthening, and resulting in secondary carbides coarsening during tempering, because undissolved carbides can act as nucleation sites.5) Higher austenitization temperature promotes more sufficient carbide dissolution, resulting in better precipitation hardening during triple tempering.5) However, the toughness could be markedly reduced because of the coarsening of prior austenite grain.5) So, it seems difficult to refine grain size without sacrificing carbide dissolution amount during conventional heat treatment (CHT). However, in this study, a new process will be developed to solve this problem by using electropulsing treatment (EPT).

EPT enjoys intense studies by material scientists and engineers due to its great potential in modifying microstructure and improving mechanical properties of metalic materials.6,7,8,9,10) However, it is wondered if the microstructure of HSS can be modified by EPT, because the dissolution temperatures of most alloy carbides are higher than those precipitates in light weight alloys. In this work, the microstructure of HSS after EPT has been studied.

2. Experimental Procedure

Commercial M2 HSS (0.85wt.%C, 5.68W, 5.05Mo, 1.75V, 4.07Cr, 0.42Si, 0.35Mn, 0.007S, 0.019P, balance Fe) was used as the tested material in this study. The as-received (annealed) M2 steel samples were separated into two groups for EPT and CHT respectively. One group of samples for CHT was austenitized at 1180°C for 10 min in the vacuum tube furnace, followed by oil-quenching. Then those quenched samples were tempered at 550°C for 1 hour. The tempered samples were once again separated into two groups for EPT and CHT, respectively. For EPT, the sample was quenched by oil immediately at the end of discharging. The temperature of the specimen was measured by infrared thermometer (Fig. 1(a)). The pulse current parameters are annotated in Fig. 1(b). For comparison, another group of tempered samples, subjected to CHT, were heated at the heating rate of 10°C/min to 1138°C, held for 1 min and 30 min, respectively, and then quenched by oil.

Fig. 1.

(a) Electric circuit diagram of electropulsing equipment, (b) Timing chart of pulse current and thermal history of electropulsing treatment, and (c) Position of sample for evaluation of hardness test and microstructure observation and XRD analyses. (Online version in color.)

The microstructure was observed by SEM. The hardness test, microstructure observation and XRD analysis were carried out at the position of the specimen shown in Fig. 1(c). The grain size and carbide volume fraction were measured by Image-pro plus 6 software. The final result of each sample was taken from more than 20 SEM images observed at different fields with a magnification of 1000.

3. Results and Discussion

The microstructure of the as-received M2 steel consisted of coarse eutectic carbides and spheroidal pearlite (Figs. 2(a) and 2(c)). A1 and Acm temperatures of M2 HSS are 795°C and 821°C respectively.11) When the sample with the annealed structures was treated by EPT to the peak temperature of 1181°C, plenty of carbides in the starting structures was dissolved during discharging. The hardness of the sample was improved to 56.5 in HRC after quenching (Figs. 2(b) and 2(d)). This mean that the matrix had underwent austenite transformation upon heating during discharging. But it can still be seen that there were some globular small carbides distributed in the matrix, besides those coarse primary carbides (Fig. 2(d)). Those small globular carbides should be un-dissolved carbides of spheroidized pearlite in the starting annealed microstructure.

Fig. 2.

Microstructures of M2 samples: (a) annealed (as-received), (b) A+EPT at 1181°C, and (c) and (d) high magnification images of (a) and (b), respectively.

Although lots of carbides in the annealed structures can be dissolved during EPT, the amount of dissolved carbides was still insufficient due to the very short heating time during discharging. In order to increase the amount of dissolved carbides, the annealed M2 steel should be austenitized conventionally. After austenitizing at 1180°C for 10 min in the furnace and subsequent quenching, the microstructure of M2 steel consisted of martensite and remnant coarse carbides (Fig. 3(a)). Small carbides in the annealed structure were dissolved completely (Fig. 3(b)). The hardness was improved to 64.7 in HRC. XRD spectrum reveal that the residual coarse carbides in CHT-1180°C sample are MC and M6C (Fig. 4). Coarse MC carbides rich in V, while coarse M6C carbides rich in Mo and W.2) M6C and MC carbides can be dissolved partially into matrix during austenitization at temperature as high as 1070°C, but large carbides could not dissolve completely, because coarse carbides are more stable thermodynamically.12) In essence, primary MC and M6C carbides precipitate at 1294 and 1288°C in equilibrium, respectively, much higher than the austenitizing temperature in most cases.13) So, coarse primary carbides are hardly dissolved completely during heat treatment. But quenching from the recommended austenitizing temperature of 1180°C was enough to ensure satisfactory dissolution amount of carbides.

Fig. 3.

Microstructure of M2 samples after CHT: (a) CHT-1180°C, (b) high magnification images of (a), and (c) tempered after CHT.

Fig. 4.

XRD spectrum of M2 samples subjected to the heat treatments or EPT: ① Tempered, ② Temper + CHT at 1138°C, ③ CHT at 1180°C, and ④ Temper + EPT at 1138°C. (Online version in color.)

Austenite preferentially nucleates at high angle grain boundary and carbide/ferrite interface.14,15) Precipitation of carbides during tempering will produce lots of carbide/ferrite interfaces, providing more potential sites for austenite nucleation, which should be good for grain refinement. When the conventional quenched M2 steel was tempered at 550°C for 1 hour, numerous tiny globule and rodlike carbides with the length in the range of 50–700 nm precipitated from the quenched matrix (Fig. 3(c)). During tempering of M2 HSS, fine carbide particles can precipitate both on the internal twin boundaries of plate martensite and in the matrix between the twin boundaries. Types of carbides precipitated during tempering could be M23C6, M6C, M2C and MC, depending on detailed alloy composition and heat treatment.2,16)

Figure 5 shows an effect of electropulsing temperature on microstructure of M2 samples. When the tempered sample was treated by EPT and quenched from 1095°C, it consisted of coarse primary carbides and a bimodal grain size matrix (Figs. 5(a) and 5(d)). And its hardness was 65.1 in HRC. This indicated the starting tempered matrix had been through austenite transformation. But the austenite transformation had not completed. Austenite nucleus nucleated preferentially at grain boundary. So, newly formed fine austenite grains distributed along grain boundaries of the starting microstructure before the completion of austenite transformation (Fig. 5(d)). However, a few new austenite grains, nucleated inside the parent grain boundary, can still be found (Fig. 5(d)). When the peak temperature was increased to 1138°C during EPT, the austenite transformation completed, obtaining uniform grain size (Figs. 5(b) and 5(e)). And the average grain size was ultra-refined to 4 μm. When further increasing the peak temperature to 1183°C, grains grew coarsening (Figs. 5(c) and 5(f)). But the grain size was still less than that of CHT-1180°C sample.

Fig. 5.

Microstructure of pre-tempered M2 samples after EPT: (a) EPT at 1095°C, (b) EPT at 1138°C, (c) EPT at 1185°C, and (d)–(f) high magnification images of (a)–(c), respectively.

EPT demonstrates dramatic grain refinement ability, which comes from the effect of pulse current on the austenite transformation, but not due to cyclic transformation or low austenitization temperature. In order to clarify the difference between above effects, the pre-tempered sample was reheated to 1138°C by CHT for comparison. During reheating by CHT, the matrix of the pre-tempered sample was austenitized again. The average grain size was 10 μm. But the sample still consisted some sphere tiny carbides with the diameter in the range of 50–200 nm, besides those coarse primary carbides (Fig. 6). Even the soaking time was prolonged to 30 min, the secondary carbides cannot still be dissolved completely at the austenitizing temperature of 1138°C (Fig. 6(d)). And the grain size increased drastically. If in order to dissolve secondary carbides completely during CHT, the austenitizing temperature must be increased. This would result in grains coarsening further.

Fig. 6.

Effect of soaking time at 1138°C during CHT on microstructure of tempered samples, (a) soaking for 1 min, (b) soaking for 30 min, and (c) and (d) high magnification images of (a) and (b), respectively.

The statistical results about grain size and carbide content indicate that the hybrid treatment process of T (tempering)+EPT for M2 steel, namely prior conventional solution treatment and tempering combined with EPT, can obtain ultrafine grain size and higher degree of carbide dissolution, compared with CHT at the same austenitization temperature (Fig. 7). Secondary carbides can be dissolved completely when heated to 1138°C during EPT . However, secondary carbides cannot be fully dissolved when heated to 1138°C by CHT. This suggests that the dissolution of carbide in HSS can be promoted by EPT. But heterogeneous distribution of coarse primary carbides, formed originally due to the segregation of eutectic carbides in the cast microstructure and arranged in bands parallel to the mechanical deformation direction,17) cannot be modified neither by EPT nor CHT.

Fig. 7.

(a) Changes in both number and volume fractions of grains as a function of grain size, and (b) Changes in average grain size and carbides volume% in each treatment. (Online version in color.)

The matrix of M2 sample quenched from 1180°C was till supersaturated in carbon and alloy elements even after 1 h tempering at 550°C. When the pre-tempered M2 sample was reheated, new carbides continued to precipitate. Types of carbides precipitated during reheating could be M23C6, M6C, M2C and MC, depending on the temperature.2,16) M23C6 could start to precipitate in the range of 150–426°C, then dissolved completely at 980°C.18) M2C-type carbide was a metastable phase that can decompose into M6C and MC types of carbides from 897.2 to 1221.5°C.11,19) New precipitated carbides could nucleate on the pre-existed secondary carbide,5) increasing the carbide size and thermodynamical stability. However, the supersaturated matrix will not decompose during rapid heating.20)

It was thought that longer heating time was helpful for carbide dissolution. But secondary carbides were fully dissolved in very short time during EPT. The rapid dissolution behavior of carbide during EPT is attributed to the athermal effect of high density pulse current. When electric current is applied to metals, the microstructure tends to transform into a new configuration with higher electrical conductivity.21,22) Before and after the microstructure transformation during EPT, the electrical conductivity of local or whole metal changes from one value to another, causing the change of current distribution from j 0 to j 1 . The related free energy change ΔGe can be expressed as:21)   

Δ G e = μ 8π j 0 (r) j 0 ( r )- j 1 (r) j 1 ( r ) | r- r | d 3 r d 3 r (1)
where μ is the magnetic permeability, r and r′ are two different position in metal, respectively. The sub-index 0 and 1 denote the states before and after transformation. When electric current passes through the metal, the thermodynamics of microstructure transformation tends to generate the most negative ΔGe.21) The electrical resistivity of carbides is higher than the metal matrix. With the assistance of high density pulse current, high electrical resistivity carbide phase dissolve, the Gibbs free energy of the system drops. Consequently, when the pulse current was applied to M2 steel at the annealed or tempered state, containing more carbides than the solution state, the pulse current produced an extra driving force to promote the dissolution of carbides.

The grain size of M2 steel after hybrid T+EPT treatment was ultra-refined to 4 μm when quenched from 1138°C (Figs. 5 and 7). The total number and volume fractions of grains with the size less than 5 μm are 88% and 51% respectively. Grain size of conventional cast HSSs ranges from 20–200 μm.23) Depending on austenitization temperature, grain size of HSSs obtained via casting, powder metallurgy or spray forming after forging ranges from 6–60 μm.5,24,25) While the grain size of laser additive manufactured or laser treated HSSs ranges from 2–15 μm.26,27) However, formation of network eutectic carbides and coarse dendrites is inevitable during the solidification of laser melted zone, probably deteriorating the toughness and ductility of HSSs. Thus, EPT is a promising way to ultra-refine the grain size of HSSs unaffected by reticulated eutectic carbides.

EPT induced grain refinement comes from the coupling of thermal and athermal effects of high density pulse current. The heating rate during EPT in this study was about 6.2×103°C/s. A large overheating was obtained. High overheating could decrease the austenite nucleation barrier, resulting in the increase of nucleation rate.

Another important factor for EPT induced grain refinement is the athermal effect of pulse current itself. The free-energy change of a current carrying system is ΔW= ΔW0We, where ΔW0 and ΔWe are the changes of free-energy in a current-free and current-carrying systems, respectively.6) ΔWe can be given as:28)   

Δ W e = μ 0 g( a,   b ) ξ( σ 1 ,    σ 2 ) j 2 ΔV (2)
where μ0 is the magnetic susceptibility in vacuum, g(a, b) is the geometric factor, ξ(σ1, σ2) is a factor that depends on the electrical properties of the nucleus and the matrix, j is the electric current density, ΔV is the volume of a nucleus. ξ(σ1, σ2) can be written as:6)   
ξ( σ 1 ,    σ 2 ) =( σ 2 - σ 1 ) /( σ 1 +2 σ 2 ) (3)
In the temperature of phase transformation from α to γ, (σ1=σγ)>(σ2=σα),6) where σγ and σα is the conductivity of γ and α phase respectively. Thus ξ(σ1, σ2)<0.7,29) Generally as b»a, so g(a, b) is positive.28) The sign of ΔWe is determined by ξ(σ1, σ2), that is ΔWe<0. This implies that electropulsing can decrease the thermodynamic energy barrier of αγ phase transformation.

Due to ΔWe<0, so the nucleation rate Ie=I0exp(−ΔWek−1T−1) in the current carrying system is greater than I0,8) where I0 is the nucleation rate in a current-free system, k the Boltzmann constant, T the temperature. Thus, the nucleation rate of γ phase can be improved by EPT.

4. Conclusion

A new process has been developed to fabricate ultrafine grained HSS with satisfying amount of carbide dissolution. This new process combines the advantages of conventional heat treatment and EPT. Firstly, sufficient dissolution of primary carbide can be obtained by conventional austenitization at suitable temperature. Then the austenite grain size can be ultra-refined by EPT. Comparing with CHT, EPT can significantly promote the carbides dissolution in HSS.

Acknowledgements

Fund supports: The Science Foundation of Hubei Province (No. 2017CFB293); The PhD Start-up Fund of HUAT (No. BK201605).

References
 
© 2019 by The Iron and Steel Institute of Japan
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