Qualitative study of carbides in liquid phase sintered M3:2 high speed steel

The microstructure of liquid phase sintered M3:2 high speed steel and the effect of adding carbon and silicon on the microstructure was characterized by scanning electron microscopy, dispersive spectrometry, and X-ray diffraction. Various types of carbides were formed depending on the added carbon and/or silicon, the sintering atmosphere and the cooling rate. The microstructure of sintered M3:2 high speed steel samples in vacuum conditions without the addition of Si and graphite is composed mainly of MC and M6C carbides inside cells and primarily at cell boundaries. The M2C eutectic carbides in these samples were formed at cell boundaries and their amounts and morphology depends on the cooling rate. Sintered samples in N2 atmosphere with added carbon and silicon, M6C eutectic and M7C3 eutectic carbides were dominantly formed while carbonitrides were formed in smaller amounts.


Introduction and literature review
High speed steels (HSSs) are complex iron-base alloys with high content of C (0.5 %-1.65 %) and carbide forming elements like Mo, W, Cr and V [1,2].The microstructure and properties of the HSS are due to the synergic effect of these alloying elements and heat treatment.The microstructure of HSS consists of a martensitic matrix with a dispersion of three types of high hardness and wear resistant carbides.These carbides are called primary, eutectic and secondary carbides.Primary and eutectic carbides solidify directly from the melt.Eutectic carbides which are coarse and unevenly distributed are formed due to the segregation of the alloying elements during solidification of the liquid phase [3,4].These eutectic carbides have deteriorating effects on the mechanical properties [5].Therefore, the control of the morphology, size and amount of these eutectic carbides is an important precondition for improving the properties of HSS.
The main types of carbides found in high-speed steels are the MC carbides (Nb and V-rich) which can be found as large blocky particles termed primary carbides because they form directly from the melt.The MC carbide has a higher V level and lower Fe and Cr levels in comparison to other carbides as M 6 C and M 2 C. The M 6 C carbide is rich in the heavy elements Mo and W which are normally present in M 6 C at the same level as they are in total composition in the HSS [5].The formation of M 6 C is favored by high levels of C and Mo.For a given amount of V, increasing the amount of W to higher levels favors the formation of M 6 C at the expense of MC type whereas a high amount of Mo has the opposite effect [6].The addition of Si to HSS favors the formation of pearlite in the matrix, suppressing the formation of martensite in the matrix and the formation of grain boundary carbides [7,8].The main compositional difference between MC, M 6 C and M 2 C carbides is that M 6 C contains the highest Fe and MC contains the lowest Fe content [9][10][11].
The M 2 C carbide (Mo/W-rich) forms during rapid cooling or at high carbon content.The morphology of M 2 C can be classified into two types, namely the lamellar shape and the rod-like shape [7,8].Lamellar-like morphology is formed at low cooling rates or high V content while rod-like morphology is favored by high cooling rates or minor contents of N and Ca [10,11].The Cr rich M 7 C 3 carbide is a metastable carbide and is mainly distributed along grain boundaries [11].
The MC, M 2 C, M 6 C, and M 7 C 3 carbides can be solidified as primary and/or eutectic carbides directly from the melt [12,13].During solidification the liquid phase solidifies through different eutectic reactions leading to the formation of up to four eutectics: (γ + MC), (γ + M 6 C) or (γ + M 2 C) and (γ + M 7 C 3 ) [14].The morphology, size, distribution, and composition of these carbides are affected by the cooling rate, sintering atmosphere and the addition of alloying elements.
The present work aims to identify and characterize the different carbides present in liquid phase sintered M3:2 HSS using different techniques also to study the effect of the sintering cycle and the effect of the alloying elements Si and C on the morphology and distribution of carbides in as sintered M3:2 high speed steel.The effect of alloying elements Si and graphite on HSS is to promote the formation of unstable carbides like M3C and M2C carbides after sintering.These unstable carbides when HSS is heat treated will form fine secondary carbides like MC, M2C, M 6 C that are beneficial to the mechanical properties.

Material
The base powder used in the experiments was spherical gas-atomized M3:2(ASP 2023) grade HSS supplied by Erasteel Kloster AB, Söderfors, Sweden.The chemical composition of the M3:2 HSS powder is shown in Table 1 with a particle size range of 50-150 μm.
These values are taken from Erasteel Kloster AB.
Loose powder mixtures were prepared from M3:2 as base powder, elemental Si and graphite.The elemental powders added were in the micrometer range.The different samples prepared are shown in Table 2.The sample compositions and the different thermal conditions are designed to study their effect on the resulting microstructure.Sintering was carried out in a pilot furnace under high vacuum (10 − 5 mbar) with a heating rate of 20 • C/min to 1000 • C followed by10 • C/min to the sintering temperature 1270 • C and 10 min holding time followed by subsequent cooling either furnace cooling where the furnace is shut down after 10 min at the sintering temperature or using a cooling rate of 5 • C/min.Samples 1-3 were sintered in vacuum and sample 4 was sintered in a tubular furnace under N 2 gas flow.A fully dense material made of the M3:2 grade powder was achieved by sintering at around 1270 • C.

Characterization and testing
To determine the type of carbide, present in the microstructure of as sintered M3:2 several identification techniques were used in this work as shown in Table 3.Besides the techniques mentioned in Table 3, X-ray diffraction was used (Bruker D8 Theta Advance instrument with Cr Kα radiation with a wavelength λ = 2.29 Å and with a scanning range of 20-160 • ) on sample 1, to decide whether or not M 2 C carbide is present in the as sintered material.The results are shown in Fig. 1.The obtained diffraction signals indicate clearly the presence of MC, M 6 C and M 2 C carbides.The intensity coming from M 2 C carbides is weak indicating a low content of M 2 C in sample 1.This is also confirmed by the microstructure of sample 1 which clearly shows M 2 C as seen in Fig. 2.
Density in the sintered compacts was measured using the Archimedes principle by weighing the samples dry, immersed and in wet conditions.The theoretical density was calculated by adding the phase constituents.

X-ray microanalysis (EDS)
The metallographic samples were analyzed by OM and SEM equipped with EDS, after grinding, polishing and etching with Nital (6 % nitric acid alcohol).The chemical composition of the elements was determined by energy-dispersive X-ray spectroscopy (EDS) analysis using a LEO 1550 Germini instrument.Knowing that the X-ray signals come from a volume larger than the individual carbides, three levels of accelerating voltages were used (20, 15 and 10 kV) to decrease the signals coming from the bulk material.The M 6 C carbide is quite rich in Fe compared to MC, which is low in Fe and Cr but rich in V. EDS analysis was done at two locations point 1 and point 2 of sample 1 as shown in Figs. 3 and 4.
The different carbides were analyzed using SEM images BSE mode and SE mode.Images of carbides contain different contrasts, carbides containing elements with higher atomic numbers (Z) appear brighter and dark carbides have lower Z.Carbides that contain elements with an atomic number lower than Fe like MC carbides (V-rich) appear darker than the matrix.However, carbides that contain elements with higher atomic numbers than Fe like M 6 C (Mo rich) carbides appear brighter than the matrix.Also the morphology of carbides was examined using Murakami etchant (3 g K3Fe (CN)6 + 10 g NaOH+100 ml H2O), in which M2C were selectively etched but not the matrix and MC carbides.By using the program Thermo-Calc the theoretical amount of M6C carbide, at different temperatures and at different Si content was predicted using the database TCFE of the software.From Fig. 5 it can be seen that the weight fraction of M6C decreases with the addition of Si.Furthermore, as temperature decreases there is a sharp decrease in the amount of M6C at about 850 • C, 768 • C and 955 • C.This decrease can be attributed to the formation of other types of carbides like M23C6 and M7C3.

Results and discussion
Figs. 2 and 6 are SEM micrographs of the microstructure of sample 1.The microstructures consist of martensite matrix and carbides that are mainly distributed at the martensite lath and at the grain boundaries.Figs. 3 and 4 show the EDS spectra of points 1 and 2 depicted in Fig. 2. From the EDS analysis at point 1 comparing the Fe Lα peaks from spectrum 1 (20 kV) with spectrum 2 (10 kV) as

Table 2
The thermal parameters for sintering the four samples.W. Khraisat and W. Abu Jadayil microstructure.Fig. 7 shows the microstructure of sample 1 after polishing and etching using Murakami's Etching solution at room temperature for longer periods of more than 5 min.The Murakami solution etched selectively the matrix and M 2 C carbides in sample 1 and completely etched out the M 2 C carbide.The MC carbides protruded from the matrix and the M 2 C carbides indented in the matrix.
However, in sample 2 the M 2 C carbides are present in higher amounts compared to sample 1 and the M 2 C carbide are present in the microstructure of sample 2 having a rod-like morphology as seen in Fig. 8 and a lamellar morphology as seen in Fig. 9. Examining the microstructures shown in Figs. 8 and 9 one can observe that MC carbide is in the interior of the lamellar M 2 C and in the case of the rodlike morphology MC carbide is located at the M 2 C/matrix interface.
Examining Figs.8-12 compound carbides (mixed carbide cluster) of MC, M 2 C and M 6 C are present at grain boundaries.This means that these carbides are not independent of each other, because the solute atoms which are rejected by one carbide are usually needed for the growth of the other.This can be explained by the fact that with the formation of MC carbides, V is consumed from the residual liquid while elements like Mo (the lowest solubility at MC carbides) and Cr contents increase in the residual liquid.Meanwhile, the eutectic M 6 C carbides need plenty of alloying elements like Mo and W during precipitation.This will consume high amounts of V, Mo and W elements while enriching the liquid phase with Cr.
The importance of the alloying elements in high-speed steel and the thermal parameters like temperature and cooling rate result is   W. Khraisat and W. Abu Jadayil and M 23 C 6 carbides.Mechanical properties of HSS depend mainly on the presence of carbides especially those obtained during solidification and that are known as primary or eutectic carbides or carbides obtained during subsequent heat treatments performed at temperatures high enough to allow either the homogenization of the matrix or the transformation of some primary carbides to secondary carbides.This heat treatment stage is required to produce tailored microstructures that can lead to improved mechanical properties.During subsequent heat treatments of HSS after sintering the M 3 C carbide transforms to other carbide, like M 2 C, MC, M 23 C 6 and M 7 C 3 .These carbides are called secondary carbides and the MC and M 2 C formation proceed through the dissolution of cementite [15].The M 2 C  W. Khraisat and W. Abu Jadayil carbide is thermodynamically unstable and will decompose into fine M 6 C and MC carbides therefore, the precipitation of M 2 C carbide promotes the refinement of carbide precipitates thus benefiting the mechanical properties of high-speed steels.
Si interacts with C in a repulsive manner resulting in higher carbon activity when these two elements are present in the bulk.This interaction causes carbon to segregate to the grain boundary due to the higher free energy change between the bulk and the grain.This can be best illustrated using the Wagner formalism [16]: where, X γ i is the mole fraction of element i in austenite and ∈ j i is the interaction coefficient between element i and j (Wagner interaction parameter).The interaction coefficient between C and Si is found in Ref. [17] as: This means more C will segregate towards the grain boundary in the ternary Fe-C-Si system compared to the binary Fe-C system.This is true in the case when the material has been completely melted however in the case when adding Si to HSS powder and then sintering the mixture using liquid phase sintering (solid phase and liquid phase) the repulsive interaction between the slow diffusing Si and the fast-diffusing C will promote the diffusion of C to the interior of the solid particles.This will increase the Si amount at grain boundaries and lower the C amount.The C-rich carbides like MC and M 3 C will form in the cell interior and the low C carbides like the M 7 C 3 will form along cell boundaries.
Adding 0.5 wt% C and 3 wt% Si to M3:2 HSS (Sample 4) affects the type and morphology, and amount of the carbides present in the microstructure of steel.According to the calculation done using Thermo-calc it is found that with an increase of C, the amount of Mo and C in liquid are increased while other elements like V are decreased.This will favor the solidification of the M 6 C eutectic (seen in Fig. 11) as the solidification proceeds the Mo and W content of the residual liquid is lowered meanwhile the Cr to be enriched this will allow other carbide types such as M 2 C and M 7 C 3 , which are rich in Cr, to precipitate.By examining the microstructure of sample 4 both eutectic M 7 C 3 and M 6 C carbides are present in the microstructure.The M 7 C 3 carbide is favored by high Cr and C contents [18].The microstructure of sample 4 consists of block-like primary M 7 C 3 and eutectic M 7 C 3 carbides.The primary M 7 C 3 carbides are non-closed hollow hexagon-shaped and the eutectic M 7 C 3 eutectic carbide grow as rods and blades, with the growth direction perpendicular to the image plane, and form a continuous network confined within each eutectic colony.
Three types of eutectic carbide morphologies can be seen in Fig. 11.These are the M 6 C eutectic, having a fishbone morphology, and two M 7 C 3 eutectic colonies.The first M 7 C 3 eutectic colony is rod-like consisting of hexagonally shaped rods at the center and becoming coarser as they join with increased distance from the center (Fig. 12).The rod-like morphology is surrounded by blocky primary M 7 C 3 carbides.The primary M 7 C 3 has two main features.The first feature is hollows observed in the center of bulky carbides, which are a representative feature of primary M7C3 carbides [19].Several joints among different parts of the carbides are also frequently observed.This agrees with the findings of reference [20].The second M 7 C 3 eutectic colony is blade-like carbides, consisting of rods joining together to form straight blades, which are often described as lamellar (Fig. 11).
Besides the homogeneous nucleation, carbides sometimes nucleate heterogeneously on inclusions like MnS and M (C, N) during solidification [21,22].From the microstructural examination of sintered samples (Figs. 9, 10 and 12) none of the MnS and M (C, N) inclusions are seen to exist alone; they are all located on primary and eutectic carbides.According to Aguirre et al. [23] sintering of HSS in the N2 atmosphere leads to the formation of carbonitrides M (C, N).W. Khraisat and W. Abu Jadayil

Fig. 2 .
Fig. 2. SEM micrograph of sample 1 showing martensitic matrix with M 6 C milky white contrast and MC along cell boundaries.Also can be seen one M 2 C eutectic colony (upper left).

Fig. 5 . 3 Fig. 6 .
Fig. 5. Weight fraction of the M 6 C carbide at different temperatures and at different wt% Si.

Fig. 7 .
Fig. 7. Microstructure of sintered M2 high-speed steel etched with Murakami's etchant at room temperature for long periods showing the MC carbides protruded from the matrix and the M2C carbides indented in the matrix (upper left).

Fig. 8 .
Fig. 8. SEM micrograph of sample 2 showing a cluster of M 2 C rod-like morphology (light grey) and MC carbide (grey) and M 6 C (milky white).Also, MnS inclusion is seen located at M 2 C carbide in the middle of the image (dark grey).

Fig. 9 .
Fig. 9. SEM micrograph of sample 2 showing a cluster of M 2 C lamellar morphology (light grey) and MC carbide (grey).Also evident in the microstructure MnS (dark grey) at the carbide/matrix interface.

Fig. 11 .
Fig. 11.SEM micrograph of sample 4 showing martensitic matrix and three eutectic carbides.The fish bone morphology (milky white), rod-like colony(left-middle) and blade-like colony (upper right and upper left).

Table 3
Carbide identification methods.