Influence of Alloying Elements on the Carburizing Behavior in Acetylene Atmosphere

Abstract

Three steel types (AISI 1020, AISI 8620, AISI 4120) with similar carbon content and different Cr content were used as test specimens to closely examine the effect of alloying elements for carbon penetration and diffusion on the steel surface during vacuum carburizing. The carbon mass gain according to the carburizing time was measured using a microbalance, and the average carbon flux, which is an indicator of the carbon penetration rate, was calculated using the measured weight as a variable. The outermost surface of the carburized specimen was observed by scanning electron microscopy (SEM) and Raman spectroscopy (RS), and the reason for the change in carburization rate according to the steel type was identified in relation to the equilibrium carbon contents calculated from Thermo-Calc. The overall carbon distribution and distribution of alloy elements on the outermost surface were quantitatively analyzed using an electron probe microanalyzer (EPMA). On the surfaces of the AISI 1020 and AISI 4120 carburized specimens, graphite layers and grain boundary carbide were formed during the carburizing process, which hindered the carburization rate, while no abnormal layer was observed on the surface of the AISI 8620 carburized specimens, so the overall carburization results were excellent.

1. Introduction

The carburizing process is one of the surface hardening methods for the penetration and diffusion of carbon into the surface of low-carbon steel and maintaining core hardness while improving strength and wear resistance on the surface of the steel material. Gas carburizing is mostly used in the heat treatment industry because the carburizing reaction occurs in an equilibrium state, and precise carbon potential control is possible [1,2]. The carburizing source used in gas carburizing uses methane and propane as saturated hydrocarbons, and since it takes a long time to reach the equilibrium phase, soot is easily formed [3,4,5]. On the other hand, the vacuum carburizing process uses acetylene, an unsaturated hydrocarbon, in a low-pressure (1–10 Torr) atmosphere to rapidly decompose iron as a catalyst, causing a fast carburizing reaction in a non-equilibrium state. It is a process that is in the spotlight in terms of energy, quality, and environment because there is almost no process [6,7]. The carburizing behavior is largely classified into decomposition, adsorption, and diffusion stages, and the amount of carbon that penetrates the surface in unit time and cross-sectional area when the carburizing gas is initially decomposed can be defined as the surface carbon flux [8]. In vacuum carburizing, the carbon flux has a value of 10–70 mg/cm²h, which is higher than that of gas carburizing (4–8 mg/cm²h), but it is difficult to measure the carbon potential [9,10]. While the carbon concentration on the steel surface is high, if carburization is not accurately controlled, abnormal layers such as cementite or carbon-saturated layers are formed, which not only reduce carburization efficiency but can also cause damage to the inner wall of the furnace or pump. Moreover, since the surface carbon concentration and diffusion coefficient vary during the carburizing process depending on the composition of the product, it is necessary to study the carburizing behavior according to the alloy composition.

Various studies have been conducted on the effect of carburizing properties according to alloy composition. Tobie et al. [11] conducted a study to improve tempering resistance by increasing the molybdenum content (from 0.2 to 0.7 wt.%) in low alloy steel containing 0.2 wt.% carbon. Saito et al. [12] studied the changes in precipitates and grain sizes by carburizing steel containing 0.018 wt.% Al and steel containing 0.032 wt.% and 0.035 wt.% Nb and Al, respectively, at 1050 °C. Wada et al. [13] conducted a study on the change in carbon activity and diffusivity according to the content of alloying elements such as Mo, Cr, and V under various temperatures in methane and hydrogen atmospheres. Murai et al. [14] studied the effect of hardenability and equilibrium carbon concentration according to the alloy composition during the cooling process after the carburizing process. Hwang et al. [15] predicted from the numerical sum of the effects of each alloying element on the activity of carbon in ternary Fe-X-C austenite (X = Mn, Ni, Cr, and Mo). Rowan et al. [16] derived carbon transfer coefficient and diffusivity from experimental values through the direct flux integration method to study the effects of alloying elements through gas carburizing of AISI 1018, 4820, 5210, and 8620 steel. Liu et al. [17] calculated the carbon diffusivity considering the Neumann value (q), which is a carbon activity control factor according to the alloy composition. However, most of the studies on the effect of alloying elements on carburizing behavior have been considered from the experimental results of the gas carburizing process, and the research cases from the vacuum carburizing process using acetylene gas are incomplete. In addition, although the measured hardness profile can partially represent the degree of carbon diffusion in the steel, it is difficult to compare quantitative carbon diffusion based on hardness data due to differences in hardenability depending on the alloy elements contained. Therefore, in this study, three representative steel types (AISI 1020, AISI 8620, and AISI 4120 steel) were selected as carburizing test specimens, and the effect of the alloying element Cr content on the carburizing behavior was evaluated in terms of carbon mass gain, microstructure, carbon flux, carbon concentration.

2. Experimental Procedures

In order to investigate the effect of the alloying elements on the carburizing behavior of test pieces (AISI 1020, AISI 8620, and AISI 4120 steel) were introduced for comparison. Experimental steels were conducted into 30 mm × 10 mm by electric discharge wire cutting. Each steel type has a distinct difference in chromium content compared to other alloy elements. The chemical composition of each test piece is shown in

Table 1. Chemical compositions of test samples (wt.%).

Chemical Composition	C	Si	Mn	P	S	Ni	Cr	Mo
AISI 1020	0.192	0.234	0.312	0.034	0.025	-	0.108	-
AISI 8620	0.213	0.256	0.754	0.040	0.018	0.452	0.459	0.163
AISI 4120	0.197	0.219	0.752	0.046	0.036	-	1.053	0.221

, and the carbon concentration and other alloy components were analyzed using a CS analyzer (CS-800, ELTRA GMBH, Haan, Germany) and an emission spectrometer (QSN-750, OBLF Company, Witten, Germany), respectively.

In the carburizing process, as shown in Figure 1, the test piece was loaded into a horizontal quartz furnace, evacuated to 0.01 Torr, and then the temperature in the carburizing chamber was raised to 950 °C and maintained for 30 min. After injecting acetylene gas (C₂H₂, 99.99%) at a flow rate of 30 sccm for six carburizing times (30, 60, 120, 180, 300, 600 s) while controlling the pressure in the carburizing chamber to 10 Torr, it was quenched for 10 min. To measure the amount of carbon entered during the carburizing process, the weight change before and after carburization in each specimen was measured up to 0.0001 g with a microbalance (GR-200, A&D, Tokyo, Japan), and the average carbon per steel type according to the carburizing time was calculated by using Equation (2).

After polishing the carburized specimen using SiC abrasive paper and 1 μm diamond suspension, it was corroded in 3% Nital solution for about 1 min, and the cross-sectional microstructure and carburized thickness were observed using an optical microscope (HRM-300, Huvitz, Anyang-si, Republic of Korea). An electron probe microanalyzer (JXA-8500F, JEOL, Tokyo, Japan) was used to measure the concentration gradient of alloying elements and the overall carbon concentration gradient of the carburized layer up to 20 μm from the surface of the carburized specimen and the current, voltage, and focus size of the electron beam used were each 20 nA, 15 kV, 1 μm² were set. In order to accurately analyze the carbon concentration within the range that carburized products can allow, some specimens were AISI 4120 (0.192 wt.%C), AISI 4140 (0.409 wt.%C), AISI 1055 (0.563 wt.%C), and AISI 52100 (0.987 wt.%C) was selected and Kα intensity was measured according to carbon contents. The carbon concentration at each location of the carburized specimen was quantitatively analyzed based on the calibration curve. A micro Vickers hardness tester (HM-210B, Mitutoyo, Kanagawa, Japan) was used to measure the hardness distribution in the depth direction of the carburized specimen. The measured load was 100 kgf, and the load time was 10 s, measured five times. The hardness value was calculated by taking the arithmetic average of the remaining values, excluding the maximum and minimum values. In addition, a field emission scanning electron microscope (NNS-450, FEI Hong Kong Company, Hong Kong, China) and a Raman analyzer (LabRAM HR, Jobin Yvon, Kyoto, Japan) were used to closely analyze the uppermost surface of the carburized specimen. X-ray photoelectron spectroscopy analysis (Axis Ultra DLD, Kratos analytical, Manchester, UK) of the carburized layer was performed, and the analysis parameters were as follows: Al anode target, energy resolution 0.440 eV.

The mass gain and carbon concentration profile of the carburized specimen can be predicted by applying the process conditions, carbon diffusivity, and boundary conditions to the thin film solution as input data. In particular, it is important to use the carbon diffusivity considering the alloy composition and reasonable boundary conditions in order to more accurately predict the carburizing behavior.

$${\overline{J}}_c=\frac{{\rho }_{Fe}}{100}\frac{S_c}{t}$$

(1)

Equation (1) means the average carbon flux entering the iron surface during the entire carburizing time in a vacuum carburizing reaction, ρ_Fe is the density of iron, S_c is the mass gain of carbon during the carburizing process, and t is the boosting time when acetylene is injected [18].

$${\overline{J}}_c=\frac{{\rho }_{Fe}}{100}\sqrt{\frac{D_C}{\pi t}}\left(C_{sat}-C_s\right)$$

(2)

The average carbon flux equation can be re-expressed as a function of the carbon diffusivity (D_c) and the carbon concentration difference (ΔC), as shown in Equation (2), because S_c is equal to the integral of the carbon concentration profile in the carburized steel [19,20].

$$C\left(x,t\right)=C_s+∆C erf\left(\frac{x}{2\sqrt{D_{c}t}}\right)$$

(3)

Since the carbon flux in the vacuum carburizing reaction is very large, it can be considered that the surface carbon concentration reaches saturation in an instant at the beginning of carburizing. Considering the local equilibrium assumption, the carbon concentration-depth profiles in the boosting stage can be calculated by applying Equation (3). C_s means the surface carbon concentration reached saturation, and x means the specific distance from the steel surface.

$$D_c^0=\frac{1.43\mathit{exp}\left(-\frac{\mathrm{19,700}}{T}\right)exp\left[0.00242\mathit{exp}\left(\frac{6790}{T}\right)\left(\%C\right)\right]}{1-0.232\left(\%C\right)}$$

(4)

Here, $D_c^0$ denotes the carbon diffusivity in the Fe-C binary system derived by Collin [21] in Equation (4), R is the gas constant, and T is the temperature in Kelvin degrees. Neumann and Person [8] presented the q value as a function of the alloying elements to calculate the carbon diffusivity, including the effect of alloying elements

q = 1 + [%Si](0.15 + 0.033[%Si]) − [%Mn] × 0.0365 − [%Cr](0.13 − 0.0055[%Cr]) + [%Ni](0.03 − 0.03365[%Ni]) − [%Mo](0.025 − 0.01[%Mo]) − [%Al](0.03 − 0.02[%Al]) − [%Cu](0.016 + 0.0014[%Cu] − [%V}(0.22 − 0.01[%V])

(5)

where the amount of alloying elements means the weight percentage.

3. Results and Discussion

Figure 2 is a phase diagram of Fe-C-Cr calculated under the condition of 950 °C using Thermo-Calc. The carbon concentration of the initial base material is C₀, and the equilibrium carbon concentration is expressed as C_γ/θ, C_γ/G, or C_eq according to the upper interface, and the equilibrium carbon concentration according to the upper interface is determined during the carburizing process by the Cr concentration contained in the steel. The equilibrium carbon concentrations of the three steel types used in this study are shown in

Table 2. The equilibrium carbon content (wt.%).

wt.%C	AISI 1020	AISI 8620	AISI 4120
C0	0.192	0.213	0.197
Cγ/θ	1.347	1.291	1.211
Cγ/G	1.323	1.337	1.435
Ceq	1.356	2.426	5.022

based on the Cr concentration, and the saturated carbon concentration (C_sat) for calculating the carbon flux is γ/(γ + G) with a carbon activity of 1. The equilibrium carbon concentration of the phase boundary (C_γ/G) was used. In the case of AISI 4120 steel, the initial carbon concentration was 0.197 wt.%, and C_γ/θ, C_γ/G, and C_eq were 1.211, 1.435, and 5.022 wt.%, respectively [22].

Figure 3 shows the mass gain curve by the infiltrated carbon according to the boosting time under the conditions of a carburizing temperature of 950 °C and 10 Torr. The dotted line denotes the mass gain curve calculated through Equation (3) using the difference between the equilibrium carbon concentration (C_γ/G) of the γ/γ + G phase boundary and the initial carbon concentration (C₀) and the carbon diffusivity (D_c) as variables. The carbon diffusivity (D_c) was calculated by applying the q value derived by substituting in Equation (5) (1.54 × 10⁻⁷ cm²/s (AISI 1020), 1.71 × 10⁻⁷ cm²/s (AISI 8620), 1.62 × 10⁻⁷ cm²/s (AISI 4120)). The points are the mass gain measured using a microbalance through carburizing experiments. In all steel types, the mass gain tends to increase as the boosting time increases. At the initial boosting time (under 2 min), AISI 4120, AISI 8620, and AISI 1020 are in the same carburizing time order. Since the three types of steel used in the experiment have similar initial carbon concentrations, there is no difference in carbon diffusivity, but the calculated mass gain curve differs depending on the type of steel because the concentration gradient (ΔC) differs according to the Cr content. In particular, the measured mass gain of the AISI 1020, AISI 8620, and AISI 4120 steels falls below the calculated value at a specific boosting time. The calculated mass gain curve was calculated assuming that no abnormal layers, such as carbon and carbide, were formed on the surface of the carburized specimen. Therefore, it is judged that the measured mass gain was lower than the calculated value because the abnormal layer present on the surface of the carburized specimen inhibited carbon penetration.

Figure 4 is a microstructure observed using an optical microscope after etching the cross-section of a specimen carburized for 5 min according to the steel type with nital etchant. Since all carburized specimens have a high carbon concentration on the surface, the microstructure that forms the carburized layer is mostly composed of martensite and some retained austenite, which has the effect of improving hardenability. Since the AISI 1020 specimen contains almost no alloying elements that improve hardenability, martensite and some ferrite were observed in the cross-sectional structure despite the high surface carbon concentration due to the carburizing process, and an abnormal layer with a thickness of about 0.5 μm was observed on the outermost surface. Referring to the Fe-Cr-C phase diagram (Figure 2), AISI 1020 steel is dominated by graphite formation through carburization, so there is no thermodynamically stable region for carbide. Most of the martensitic structure was observed in the cross-section of the AISI 8620 specimen, and no abnormal layer was observed on the outermost surface. On the other hand, in the AISI 4120 specimen, grain boundary carbide was observed up to a depth of 12 μm from the surface. This is because the surface carbon concentration already exceeded the equilibrium carbon concentration (C_γ/θ: 1.211 wt.%C) of AISI 4120 for the precipitation of carbides during the carburizing process in a short carburizing time, so it is judged that carbides were deposited on the surface. In particular, during the quenching process with a fast cooling rate, not only does the carbon solubility of the matrix phase decrease, but there is very little time for carbon to diffuse within the carbides, so the carbides in the carburized structure after quenching are preserved in a precipitated state during the carburization process.

As shown in Figure 5, the microstructure of the outermost surface of the specimen carburized for 5 min was observed in the transverse direction using SEM equipment. On the surface of the AISI 1020 specimen, a carbon film with a size of about 6 µm and a flake shape was observed. Most of the plate martensite was observed on the surface of the AISI 8620 specimen, but on the surface of the AISI 4120 specimen, carbide with a size of about 3 µm was additionally precipitated on the matrix of plate martensite. Abnormal layers such as carbon deposited and carbide observed on the surface of AISI 1020 and AISI 4120 specimens play a role in inhibiting carbon penetration during the carburizing process. The abnormal layer formed on the surface will be identified in a later chapter through quantitative phase analysis.

Figure 6 shows the X-ray diffraction results of vacuum carburized specimens according to steel type at a carburizing temperature of 950 °C and carburizing time of 5 min. Martensite and retained austenite peaks were detected in all steel types regardless of alloy composition. Martensite peaks were detected as (110), (200), and (211) planes at 2θ = 44°, 65.1°, and 82°, respectively. This is due to the transformation of the austenite matrix into a martensite structure by carbon dissolved during the carburizing process, and since the carburized surface is mostly a martensitic structure, the peak was detected higher than that of other phases. In particular, the (100) plane peak of AISI 1020 steel has a peak pattern that is somewhat different from that of other steel types. This is because ferrite is transformed and interferes with the martensite peak at surrounding degrees even though quenching was performed at a high rate after carburization [23,24]. Retained austenite peaks were detected in the (200) and (220) planes at 2θ = 50.7° and 74.6°. The largest amount of retained austenite is detected in AISI 1020 steel. This is because the lower the Cr content, the higher the concentration of carbon that can be dissolved in the austenite phase (C_γ/θ). Therefore, there are many peaks of retained austenite that are proportional to the carbon solubility, which is an austenite stabilizing element (Figure 2). In AISI 4120, unlike other steel types, carbide peaks were detected at 2θ = 22°, 40.3°, and 41°.

Figure 7 shows the average carbon flux according to the boosting time, and the mass gain measured in Figure 3 was calculated by substituting Equation (2). The average carbon flux follows the reciprocal law in all steel types and decreases with longer boosting times. This is because during the carburizing process, the surface carbon concentration reaches the saturation point, and the concentration gradient is almost eliminated and is dominated by the diffusion reaction to the interior. The AISI 1020 specimen had the lowest average carbon flux due to the low equilibrium carbon concentration and the formation of a carbon layer that lowers carbon penetration on the surface, even though the carbon diffusivity is similar to that of other steel types. The average carbon flux of the AISI 4120 specimen had a comparatively higher value than the AISI 8620 specimen until the beginning of carburization (2 min). Still, later, the carburization time was conversely lower. It is judged that the carbon flux increases by obtaining additional mass gain from the carbide precipitated on the surface at the beginning of carburization, and after the critical time, the carbon flux decreases due to the role of carbide inhibiting carbon diffusion [25,26].

Figure 8 shows the Cr, Mo, and Ni component content analyzed using EPMA equipment in the carburizing layer (within 20 μm) of the test specimen carburized for 5 min for each steel type. The surface Cr content of the AISI 1020 specimen was detected to be about 0.12 wt.%, and Mo and Ni components were not detected. The alloying element content of the carburizing layer of the AISI 8620 specimen was detected to be 0.52 wt.%Cr, 0.13 wt.%Mo, and 0.46 wt.%Ni on average. On the other hand, in the AISI 4120 specimen, Ni was not detected in the carburizing layer, and Mo was detected at about 0.19 wt.%. The Cr content can be divided into a Cr-rich zone and a Cr-deletion zone according to the location and has a concentration of 3.3 wt.% and 0.7 wt.%, respectively. It is believed that Cr around the grain boundary diffused into the intergranular carbide and precipitated (Fe,Cr)₃C containing some Cr content [27,28].

Figure 9 shows the microstructure observed through SEM analysis of the cross-section of the carburized AISI 4120 specimen and the EDS analysis results for the designated positions. In addition to microstructure differences, four points (carbide interior (position 1), carbide interior (position 2), carbide boundary (position 3), and carburized zone (position 4)) were analyzed to identify differences in chemical composition and the phases. Position 1 and position 2 are judged to be (Fe,Cr)₃C, a cementite-type carbide, and are distributed from the surface to a depth of about 12 μm, as shown in Figure 4f. In position 3, relatively little Cr was analyzed, unlike the inside of the carbide, which is believed to be due to Cr moving from the carbide boundary into the inside of the carbide to form a depletion area. This is due to the grain-boundary diffusion of Cr, which is generally much faster than diffusion inside the grain, and because the carbon in the grain boundary carbide plays a role in attracting Cr, a depletion area is formed in a short period of time [29,30]. Position 4, which consists of a martensite structure, showed almost similar results to the Cr concentration of the base material.

Figure 10 shows the carbon concentration curve in the depth profile for a 5 min carburized test specimen for each steel type. The carbon concentration at 25 μm from the surface is 0.92 wt.%C (AISI 1020), 1.18 wt.%C (AISI 8620), 1.29 wt.%C (AISI 4120), and as the Cr concentration contained in the steel increases, the carbon concentration near the surface increases. The reason why the carbon concentration of the AISI 4120 specimen is higher than the carbon solubility limit (C_γ/θ) specified in Table 2 is that the carbon concentration, including the intergranular carbide, precipitated on the surface, was analyzed. In addition, because the intergranular carbide hinders carbon diffusion, the carburizing depth of the AISI 4120 specimen is 0.178 mm, which is the thinnest compared to other steel types. On the other hand, the carbon diffusion depth of the AISI 8620 specimen is 0.201 mm, which is the thickest because there is no abnormal layer on the surface that inhibits carburization, and the carbon diffusivity is the highest. The carburizing depth of the AISI 1020 specimen is 0.194 mm, which is similar to that of AISI 8620, but the carbon penetration rate is lowered due to the reduction of the carburizing catalyst area by the carbon layer in Figure 4d, so the integral value of the carbon concentration at the total depth is the lowest.

Figure 11 is a Vickers hardness profile in the cross-sectional depth direction in a specimen carburized for 5 min according to the steel types. The hardness gradient tends to be proportional to the carbon content according to the depth from the surface in Figure 10, regardless of the steel type. The surface hardness of the AISI 1020 specimen was measured at 768 Hv, and the effective case depth was 0.15 mm, which is a thinner hardening layer than other steel grades. The surface hardness of the AISI 4120 specimen was measured at 850 Hv, and the effective hardening depth was measured at 0.16 mm. Although it has a relatively high surface hardness due to high carbon concentration and carbide precipitation hardening on the surface and contains hardenability-improving elements such as Cr and Mo, it has a steep hardness gradient because the carbon diffusion depth is thin. The surface hardness of the AISI 8620 piece was measured to be about 796 Hv, and the effective hardening depth was 0.2 mm. This has a relatively thick hardening depth compared to other steel types because it contains no abnormal layer that inhibits carbon diffusion on the surface of the carburized piece and contains hardenability-improving elements.

Figure 12 shows the carbon crystallinity analyzed using Raman spectroscopy to identify the carbon structure formed on the surface of the specimen carburized for 5 min according to the steel type. Since most of the surfaces of the AISI 8620 and AISI 4120 specimens are martensitic structures to which no carbon is applied, no specific carbon peak is detected. The surface of the AISI 1020 specimen was coated with a carbon layer, and various carbon peaks were detected. The peaks located at 1585 cm⁻¹ and 2680 cm⁻¹ mean the G band with sp² bonding characteristics and the 2D band with multiple scattering characteristics, respectively [31,32]. In particular, a D band was detected around 1350 cm⁻¹, which means a peak caused by a defect in the crystal, and because it cannot be observed by Raman scattering in a perfect lattice structure due to the symmetry of vibration, it usually appears in graphite and carbon black [33,34]. The carbon layer applied on the surface of the AISI 1020 specimen is judged to be a crystalline graphite with many defects, considering the Fe-Cr-C phase diagram and Raman peak.

To more closely observe the atomic composition and bonding state of the outermost surface of the carburized specimen, the XPS intensity according to binding energy is shown in Figure 13. In the case of the AISI 1020 carburized specimen, it can be found that the binding energy of C_1s at 284.4 eV corresponds to a graphitic structure that has sp² bonding, which explains that the surface of the carburized layer presents graphite above 5 min or more. After carburizing from 5 min to 10 min, the peak of C_1s increases noticeably (Figure 13a). As for the XPS results of the AISI 8620 carburized specimen, the carbide peak was not confirmed as in the XRD diffraction pattern in Figure 6 until the carburization time was 5 min, but the carbide peak (282.7 eV) was analyzed when the carburization time was 10 min (Figure 13b). Although it is a steel type that inhibits carbide formation as it contains a lot of austenite forming elements, carbides were precipitated due to excessive carburization time without diffusion. In the case of the AISI 4120 carburized specimen, the carbide peak (282.7 eV) was analyzed from the beginning of carburization, and the peak clearly tends to increase as the carburization time increases (Figure 13c). Considering that the carbide peak was detected from the beginning of carburization, it is believed that a carbide film was formed on the outermost surface. Since XPS analysis is generally performed in nanometer units, it compensates for the difficulty of observing carbide under a microscope.

As described above, the phenomenon that the carburizing behavior varies from the surface of the treated specimen according to the Cr content of the steel during vacuum carburization is more effectively explained by using a schematic diagram in which carbon penetrates and diffuses into the steel as shown in Figure 14. In the case of AISI 1020, the carburizing rate decreases because the graphite film is formed on the outermost surface as the saturated carbon concentration is reached in an instant at the beginning of carburization, and the area where acetylene can be additionally catalyzed and decomposed is reduced. AISI 8620 not only has a low Cr content but also contains Ni, an austenite stabilizing element, so it is difficult to form an abnormal layer such as carbide on the surface. Therefore, it exhibits excellent carburizing behavior due to the high carbon diffusivity, as well as the excellent degree of carbon penetration from the surface. In AISI 4120, as shown in Figure 14c, (Fe,Cr)₃C is precipitated on the outermost surface and grain boundaries, and as the catalyst area decreases, the degree of carbon penetration decreases, and carbon diffusion into the interior is greatly hindered [35]. In order to obtain efficient carburizing properties, it is necessary to set carburizing conditions with maximum carbon flux without forming an abnormal structure on the surface in consideration of the alloy elements contained in the steel type. For example, when adjusting the carburizing schedule under carburizing conditions of 950 °C and 30 sccm, the AISI 1020 specimen should set the boosting time to 60 s or less to minimize the formation of a graphite layer, which is a factor interfering with carburizing behavior. In the case of the AISI 4120 specimen, the boosting time should be set to 120 s or less to inhibit the precipitation of carbide at the grain boundary.

4. Conclusions

In order to observe the influence of the alloying elements on carburizing behavior during vacuum carburizing, we investigated the relationship between the chemical composition of steel, the distribution of alloying elements, and the phase of the surface. Additionally, the carbon profile in the matrix was also analyzed. These investigations yielded the following.

(1)

As the Cr content of the steel increases, the equilibrium carbon concentration (C_γ/G) increases, so the carburization rate increases based on the carbon flux equation, while the area in which the cementite phase can be stable expands.

(2)

As the carburizing time increases, the surface carbon concentration of the steel increases, so the mass gain increases from a linear step to a parabolic step (the more Cr it contains, the shorter the linear time is)

(3)

The graphite layer formed on the surface of the AISI 1020 carburized specimen at the initial stage of carburization (60 s) inhibits the carburization rate by about 14%, and the amount of carbon penetrated is the lowest among the steel types used in the experiment.

(4)

In the AISI 4120 carburized specimen, grain boundary carbide was observed on the surface at a carburization time of 120 s, and it was identified as (Fe,Cr)₃C with a Cr depletion region around it. Precipitated carbide plays a role in inhibiting the carbon penetration rate and diffusion rate.

(5)

The carbon diffusivity of the AISI 8620 carburized specimen is high and has the best hardness profile because an abnormal layer that inhibits the carburization rate is not formed.