Study on Decarburization Mechanism and Law of GCr15 Bearing Steel during Heat Treatment

In order to understand the eects of heating temperature and holding time on decarburization of GCr15 bearing steel, an experimental device for heat treatment of bearing steel was designed and the sample was heated and held by a tubular heating furnace.emicrostructure and depth of decarburized layer of bearing steel under various working conditions were observed and measured by a metallographic microscope. e results show that the total decarburization depth increases with the increase of temperature; almost no decarburization occurred below 700°C; ferrite decarburization occurred at 750°C; partial decarburization began to occur at 820°C; after 900°C, the decarburization layer is mainly partial decarburization. When the heating temperature is below 1000°C, the depth of total decarburization layer increases slowly and increases rapidly after 1000°C. e total depth of decarburization layer is directly proportional to the square root of holding time. e relationship between the depth of the total decarburization layer of bearing steel and the heating temperature and holding time in the annealing process was obtained.


Introduction
GCr15 bearing steel is a widely used raw material for gears, bearings, and transmission shafts due to its excellent mechanical properties [1]. It mainly reduces friction between various parts in mechanical equipment, xing parts, and transmitting loads. ey are widely used in machinery manufacturing, shipbuilding, aerospace, and many other industries. Decarburization occurs during the annealing of steel in air, O 2 , CO 2 , H 2 O, and H 2 when carbon from the steel surface reacts with these gases and forms CO and CO 2 or CH 4 . e loss of carbon atoms in the steel matrix [2][3][4] results in a micron-scale ferrite decarburization layer or partial decarburization layer [5]. Decarburization is only bene cial for a few materials that require lower carbon content for better performance. Van Hall et al. [6] performed experiments to improve the aring capability of the rivets. e process of intentional decarburization has yielded up to a three-fold improvement in aring performance in both rivet geometries studied. However, decarburization is harmful to most steels, and the e ect of ferrite decarburization is the worst [7]. Decarburization will reduce steel's surface hardness and fatigue strength and reduce its service life [8][9][10][11][12]. Carroll et al. [9] investigated the e ect of decarburization on rolling contact fatigue and wear. It was found that as the depth of decarburization increased, the wear rate and the crack growth rate of the rail disc increased. Zhao et al. [11] found that the wear rate of a decarburized rail is over twice that of the same steel without decarburization. Also, the decarburized layer changes the damage mechanism of the rail steel. With an increase in the depth of decarburization, the damage of rail roller turns from major spalling to pitting and peeling. Because of this series of adverse e ects of decarburization, it is necessary to understand the factors that a ect decarburization in the heat treatment process of bearing steel.
ere are many factors a ecting steel decarburization, mainly alloy composition, heating atmosphere, heating temperature, and holding time. Generally speaking, alloying elements can be used to improve the mechanical properties of steel and can also be used to improve the decarburization of steel [13][14][15][16][17]. Zhang et al. [14] investigated the e ect of Sb elements on the decarburization of transformation-induced plasticity steels. It was shown that the addition of Sb suppresses the decarburization of TRIP steel under different water vapor contents. Liu et al. [17] performed experiments on the effects of Si and Cr on complete decarburization behavior.
e results showed that Si promotes while Cr prevents the complete decarburization of steels in the atmosphere of 2 vol. % O 2 and 98 vol. % N 2 . Previous studies have shown that heating temperature has the most significant impact on the decarburization rate. Temperature can affect both the diffusion rate of carbon and the phase transition of the decarburized layer, thereby affecting the morphology and depth of the decarburized layer [18][19][20]. Zhang et al. [18] studied the ferrite decarburization behavior of 60Si2MnA spring steel wires for automotive suspensions.
e results show that ferrite decarburization, which has a strong temperature dependence due to phase transformation, is produced between 675°C and 875°C. e maximum depth is found at 750°C. e heating atmosphere and holding time also significantly influence the decarburization of steel [21][22][23][24]. Liu et al. [21] investigated the effect of oxygen concentration on the characteristics of decarburization of 55SiCr spring steel. It was found that complete decarburization was eliminated by decreasing the oxygen concentration to 2%. Baud et al. [22] found that no decarburization was observed in the presence of 31% water vapor in the air, even for an oxidation time of 128 h. Li et al. [4] performed experiments on the influence of different thermal cycles on the decarburization process. e experiments showed that the decarburized depth was strongly increasing with increased temperature and longer holding time. Zhu et al. [24] established the equation by the second law of diffusion and theoretically deduced that the depth of the decarburization layer was proportional to the square root of holding time. Although researchers have studied the decarburization of some steel, less research has been performed on the annealing process of bearing steels. e influence of heating temperature and holding time on the decarburization of bearing steel was analyzed in this paper, and obtains the relationship between the total decarburization depth with heating temperature and holding time in the annealing process of bearing steel.

ermodynamic Analysis of Decarburization.
e essence of steel decarburization is that the carbon atoms inside the steel are heated and vibrated under the action of high temperatures. e work function rises and breaks away from the bondage of the metal lattice. ermal diffusion occurs and migrates to the surface to react with the atmosphere in the furnace, resulting in a certain distance from the surface of the steel-the phenomenon of the loss of internal carbon atoms. Generally speaking, decarburization includes two processes: (1) Physical diffusion: the diffusion of carbon atoms from the inside of the metal to the surface. (2) Chemical reaction: the carbon atoms on the metal surface react with the atmosphere in the furnace.
During the heating process of the steel, the atmosphere in the furnace greatly influences the decarburization of the steel. Different atmospheres cause different reactions on the surface of the steel. For the decarburization of bearing steel, the primary reactions are When the reaction goes from the left to right, decarburization occurs.

Diffusion
Mechanism. Since carbon atoms are smaller than iron atoms, the diffusion of carbon atoms in billet decarburization belongs to the diffusion in the interstitial solid solution. e carbon atoms jump to another adjacent gap in the lattice gap. e carbon concentration in the surface layer is low, and there is enough interstitial space. Fick's law is the basic macroscopic theory about diffusion in solid-state physics. Fick's second law is derived from the first law combined with the conservation of mass equation and applied to the unsteady diffusion of atoms to predict the change of concentration caused by diffusion over time. Surface decarburization is a process in which carbon atoms diffuse from the inside of the metal to the surface. erefore, the change of the decarburization depth with the heating temperature and time can be described according to Fick's second law: where D is the diffusion coefficient, m 2 /s; C is the carbon content, %; t is time, s; and x is the distance from the steel surface. When the diffusion coefficient is assumed constant and independent of the composition, equation (2) can be simplified to Generally, the diffusion coefficient, D, is known to be temperature dependent, it can be calculated by the Arrhenius formula: where D 0 is the preexponential, m 2 /s; Q is the activation energy, J /mol; R is the gas constant, J /(mol·K); and T is the absolute temperature, K.
In the process of billet decarburization, if the steel piece is large, there will be no decarburized region inside the steel piece, and the central carbon concentration remains unchanged. e differential equation can be given according to the boundary conditions and initial conditions.
(1) Initial conditions: when t � 0, C(x, 0) � C 0 , the initial carbon content at time 0 is the original carbon content of steel; (2) e outer boundary condition: when there is a constant carbon concentration on the surface of the steel, which is the same as the carbon potential of the atmosphere (3) Internal boundary conditions: when x � m, C(m, t) � C 0 , at a certain distance from the surface, the carbon content in the steel remains unchanged and is the same as the initial carbon content.
For such conditions, equation (3) is simplified to Equation (5) is the basic dynamic theoretical formula for decarburization of bearing steel surface. If x � m, C(m, t) � C 0 , then, the relationship between decarburization depth and diffusion time can be simplified as where K is a constant related to materials.

Experimental Method
e raw materials for the experiment were taken from GCr15-bearing steel hot-rolled bar, and the main chemical compositions are shown in Table 1.
It is cut into several small samples with a size of 10 mm * 10 mm * 15 mm, and a small hole with a diameter of 2 mm is made in the direction of height for hanging samples. Figure 1 shows the appearance of the samples prior to doing any experiment. Grind the sample's surface to 600 mesh with sandpaper, remove the oxide layer and decarburized layer on the initial surface of the sample. Furthermore, the surface stains were removed with anhydrous ethanol in the ultrasonic cleaning machine, and then, the sample was dried and preserved in isolation from the air for practical use.
An experimental device for heat treatment of bearing steel was designed to study the effect of heating temperature and holding time on surface decarburization of GCr15 bearing steel. e schematic diagram of the experimental device is shown in Figure 2. e heating program of the GSL-1700X tubular heating furnace was set by computer. e temperature of the sample was monitored in real-time through the K-type thermocouple to ensure the accuracy of the heating temperature of the sample. Set the dry nitrogen flow rate through the gas distribution system and adjust the flow rate of the double plunger pump to obtain the atmosphere required for the experiment in the steam generator. e sample is heated to the experimental temperature in a nitrogen atmosphere, and then the experimental gas is continuously introduced. e experimental gas is a nitrogen-containing 0.3% H 2 O (g). e experimental gas enters from the bottom of the tube furnace and flows out from the top. e sample is then incubated for some time in the experimental atmosphere. After the incubation, the sample was cooled to room temperature at a 5°C/min rate under a nitrogen atmosphere.
To better observe the metallographic structure of the edge of the sample, the sample after heating treatment was inlaid with epoxy resin, and then the sample was polished and polished. Figure 3 shows the appearance of the sample after mounting and polishing. A 4 vol % nital was used as an etchant to reveal the microstructures of the interfaces. e microstructures were observed by metallographic microscopy (LEICA DM4M). According to GB/T224-2008 "Measurement of Decarburization Layer Depth of Steel," 5 times of measurements were made in a field of view in the deepest uniform decarburized zone and the average value was taken as the depth of the decarburized layer.

Effect of Heating Temperature on Decarburization of Bearing Steel.
e samples were held at different heating temperatures from 700°C to 1150°C for 40 min. Figure 4 demonstrates microstructures of specimens in the experiment.
It is observed that temperature has an important role on decarburization. No decarburization phenomenon was observed at 700°C, indicating that decarburization reaction did not occur in samples below 700°C. When the temperature is 750°C, the sample has an obvious decarburization phenomenon, and the ferrite structure is columnar perpendicular to the decarburization surface. A partial decarburization layer appears in the sample at 850°C, and the thickness of the complete decarburization layer decreases. e decarburization layer comprises a complete decarburization layer and a partial decarburization layer. is phenomenon may be that at this temperature, the steel structure has been transformed, part of the ferrite into austenite. After 900°C, the sample is mainly a partial decarburization layer because, at this temperature, the steel structure is austenitized. e average depth of the total decarburization layer of each sample is arranged as shown in Figure 5. As shown in Figure 5, when the heating temperature is below 1000°C, the thickness of the total decarburization layer increases slowly. However, after 1000°C, the thickness increases rapidly, showing exponential growth.

Effect of Holding Time on Decarburization of Bearing Steel.
e annealing temperature of bearing steel is about 780-820°C. In order to study the effect of holding time on the decarburization of bearing steel, the samples are selected to be tested at different heating temperatures of 780-820°C for 20-70 min.
e experimental results are shown in Figures 6-10.
In order to understand the relationship between the depth of the decarburization layer and the holding time more intuitively, the average depth of total decarburization of samples under various working conditions was sorted out as shown in Figure 11.
It can be seen from Figure 11 that the total decarburization depth of GCr15 bearing steel increases with the holding time, but the growth rate gradually slows down. Also, the changing trend of the decarburization layer depth with the holding time at each temperature is the same. e depth of the total decarburization changes approximately parabolically with the holding time. After analysis, the main Advances in Materials Science and Engineering     Advances in Materials Science and Engineering gradually slows down. With the increase of holding time, the carbon concentration gradient on the surface decreased, which made the decarburization rate gradually slow down. e data is processed to obtain the variation curve of the total decarburization depth with the square root of holding time, as shown in Figure 12.
As can be seen from Figure 12 that the total decarburization depth is proportional to the square root of the holding time, which is the same as the theoretical result obtained by the equation established by this paper and Zhu et al. [24] through the second law of diffusion. e calculation and fitting formula of the total decarburization depth is as follows: where k 1 is the slope obtained by fitting and t is the holding time, min. e functional relationship between the depth of the total decarburization layer and the holding time at 780, 790, 800, 810, and 820°C can be obtained through calculation, as shown in Table 2.
It can be seen from Table 2 that at different temperatures, the total decarburization depth of bearing steel is proportional to the square root of the holding time. Fit k 1 with heating temperature, the fitting formula is  where T is the heating temperature,°C; and a is a constant. e fitting results are shown in Figure 13. When the heating temperature is between 780-820°C, the relationship between the depth of the total decarburization layer, the heating temperature, and the holding time is

Conclusions
By studying the decarburization of GCr15 bearing steel under different heating temperatures and holding times, the following conclusions are obtained: (1) e depth of the total decarburization layer of GCr15 bearing steel increases with the increase of temperature; basically, no decarburization occurs below 700°C; more obvious full decarburization can occur at 750°C; partial decarburization starts at 820°C; after 900°C, the decarburization layer is mainly partial decarburization layer; when the heating temperature is below 1000°C, the depth of total decarburization layer increases slowly, and increases rapidly after 1000°C. (2) e depth of the total decarburization is approximately proportional to the square root of the holding time.
(3) In the annealing process of GCr15 bearing steel, the approximate relationship between the depth of the total decarburization and the heating temperature and holding time is h � (0.0625T − 36.908) � t √ .

Data Availability
No data were used to support this study.

Conflicts of Interest
e authors declare no conflicts of interest.