Effect of Ultrasonic Treatment on the Solidification Microstructure of GCr15 Bearing Steel

Experimental materials

Material used in this research is GCr15 bearing steel and its composition is as follows (wt%): C: 0.96–1.0, Mn: 0.35–0.4, Si: 0.2–0.3, Cr: 1.4–1.5, P < 0.012%, S < 0.005%.

Experimental equipment and analysis instruments

Figure 1 shows the schematic of the ultrasonic treatment apparatus, which consists of some assembled equipment. In this schematic, the JTS-3000 intelligent NC ultrasonic generator V6.0 (no. 1) and the transducer (no. 3) are manufactured by Cheng Gong Ultrasonic Wave Co. Limited, Hangzhou. The wave guide rod (no. 4) is homemade, and the shape of “C,” whose radius of curvature is the same as that of the interior radius of the mold, is designed at the point of contact between the wave guide rod and the mold.

Figure 1:

Sketch of ultrasonic experimental equipment.

The melting unit is the RKJ-12-16 high-temperature heating furnace, and the heating material is MoSi₂, which includes two parts: the gas protection, which is composed of flow meter, control valve, and nitrogen, and the heating furnace, as shown in Figure 2.

Figure 2:

Experimental apparatus of heating equipment.

The ZEISS-200MAT metallographic microscope is used to observe the solidification structure of steel by ultrasonic treatment with powers of 0 W, 300 W, 500 W, 700 W, and 1,000 W. The mechanical properties of steel are tested using the ZWICK-Z150 electronic stretcher from Germany.

The analytical instrument for element segregation is the original position analysis (OPA) apparatus (NCS Testing Technology Co., Ltd.). OPA can serve various functions, such as segregation, quantity and distribution of inclusion, as well as porosity and chemical composition of steel ingot and is widely used in the metallurgical industry because of its simple sample preparation, accurate quantization, and high-speed analysis compared with conventional analytical methods. The following technical parameters are used for OPA: (1) discharge parameters (continuous excitation system): frequency, 500 Hz; inductance, 120 H; capacitance, 5 F; voltage, 400 V; (2) spectrophotometric system: focal length, 750 mm; spectrum range, 120 nm to 800 nm; distinguishability surpass, 0.01 nm; (3) sample size: length 500 mm × width 245 mm; and (4) load bearing: 20 kg.

Experimental schemes

The experimental schemes employed in this study are shown in Table 1.

Experimental procedure

The experimental procedure is as follows.

Sample analysis

1. The polished samples were placed in a 50% hydrochloric acid solution and heated in a water bath of 100°C for 2 h. After the erosion, the sample was washed with anhydrous ethanol to observe its macrostructure.

2. OPA was adopted to quantitatively analyze the segregation of C, Si, Cr, and Mn in the steel ingot. The in situ scanogram spot of the ingot is shown in Figure 3. The line-by-line scan mode was adopted during the sample scanning. The X-axis was used as the continuous scan direction, and the scan speed was 80 mm min⁻¹. The Y-axis was used as the stepping scan, and the stepping spacing was 2 mm.

3. The sample, which was treated with different powers, was cut into small samples with dimensions of 20 mm×30 mm, and the distribution of carbide was observed through a metallographic microscope. Tensile samples were machined after forging them at an initial forging temperature of 1,150 °C and a final forging temperature of 850°C. The mechanical properties of the bearing steel were tested. The tensile samples were φ10 mm×70 mm, and the screw threads were M16 mm×20 mm.

Figure 3:

In situ scanogram spot of the steel ingot.

Influence of ultrasonic on the macrostructure of GCr15 bearing steel

Figure 4 shows the macrostructure of GCr15 steel treated with different powers.

Figure 4:

Macrostructure of GCr15 steel treated with different ultrasonic power.

Figure 4 shows that, during the solidification process, the columnar grains of GCr15 steel without ultrasonic are developed and the structure is not uniform. When the power increases to 300 W, the coarse columnar grains are broken, and a large number of equiaxed grains appear, which means that grain refinement is evident and that the structure is uniform. The ratio of equiaxed grains further increases when the power increases to 500 W. When the power increases beyond 500 W, the ratio of equiaxed grains no longer increases.

Cavitation and acoustic streaming effects are produced during the solidification process of liquid steel when the ultrasonic is introduced. When the wave spreads through the media, resonance occurs between the wave and tiny bubbles in the media. When the ultrasonic reaches a certain value, the pressure inside the bubbles becomes lower than the original pressure. The bubble expands quickly at the sparse part of the wave trough, but compresses abruptly, explodes, and then disappears at the dense part of the wave peak. The “formation–development–explosion” process of small bubbles constitutes the ultrasonic cavitation effect. The ultrasonic forms cavitation bubbles in the liquid steel, thereby reducing the temperature on the surface of the cavitation bubbles and facilitating the formation of the nucleus around the bubbles in the molten steel. Some solid particles in the liquid steel have surface defects because of the ultrasonic cavitation effect, and the weak particles may break into smaller ones and be distributed evenly in the liquid steel under the actions of ultrasonic acoustic streaming, heat flow, and cavitation effect. Similarly, the dendrite in the mushy zone is broken into pieces. Hence, the ratio of the columnar grains of the steel decreases, whereas the ratio of equiaxed grains increases [8–11].

The experimental results show that, if the ultrasonic power is too high or too low, the effect on the refinement of the solidification structure is not obvious. The power has a threshold value; when the power is lower than this threshold, the ultrasonic treatment can noticeably improve the solidification structure with an increase of power. However, when the power is beyond this threshold, the influence is not so obvious. In this experimental condition, the best effect occurs when the power is between 300 W and 500 W.

Influence of ultrasonic on composition segregation of GCr15 steel

Figure 5 shows the variations in carbon, silicon, manganese, and chromium segregations as a function of the power, which is assessed through the statistical and maximum segregations of the elements.

The statistical segregation is calculated according to the formula

where C₀ represents the average element content, C represents the element contents at different positions, N represents the collected spot number; σ represents confidence expansion range of C0 at 95 % confidence level and 95 % confidence interval is between (C0−σ) and (C0+σ). The larger is the statistical segregation, more serious is the segregation, but rather the contrary. The statistical segregation is zero if there is no segregation.

The maximum segregation is calculated according to the formula

where C₀ represents the average element content and C_max represents the maximum element content on the longitudinal section of steel ingot.

Figure 5:

Relation between the segregation and ultrasonic power.

Figure 5 shows that, when no ultrasonic is introduced during the solidification process of GCr15 steel, the statistical segregations of carbon and silicon are 0.0964 and 0.1152 and the maximum segregations are 2.541 and 2.861, respectively. When the ultrasonic power increases to 300 W, the statistical segregations and maximum segregation of carbon and silicon decrease to 0.0706, 0.1101 and 1.163, 1.445, respectively. By contrast, the statistical and maximum segregations of manganese and chromium increase slightly. When the power increases to 500 W, the statistical and maximum segregations of all the elements decrease, which shows that the ultrasonic effectively restrains the segregation of the elements in the solidification process. This occurrence may be attributed to the refinement of the solidification microstructure by the ultrasonic, which reduces the formation of columnar grains and leads to the uniform distribution of the alloy elements in the liquid steel as a result of the ultrasonic cavitation and acoustic streaming effects.

However, when the power further increases to 700 W and 1,000 W, the segregation increases instead. Excessive ultrasonic power initiates the violent motion of the molten steel, thereby removing the enriched steel surrounding the dendrites in the mushy zone and reducing the solute content in the region. As a result, severe element segregation occurs.

Influence of ultrasonic treatment on carbide and mechanical properties of GCr15 steel

Given that a bearing has to endure high impact and alternating load, the size and distribution of carbide must be controlled for good macro- and micro-structures, and the fatigue life and wear-resisting property must be guaranteed. With a sharp or rough surface for large carbide particles, stress concentration tends to form at the point of contact between the carbide and matrix, thereby shortening the service life of the ball bearing steel. Stress concentration does not easily form when the carbide particles are small and spherical in shape [13]. Figure 6 shows the average diameter and area percentage of carbide in GCr15 steel subjected to ultrasonic treatment with different powers. To facilitate the statistics of carbide in steel, the carbide is assumed to be (FeMe)₃C [12, 14, 15].

Figure 6:

Relation between area percentage, average diameter of the carbide, and ultrasonic power.

Prior to ultrasonic treatment, the area of the carbide accounts for 10.62% of the field of view, and its average size is 14.63 μm. When the powers are 300 W, 500 W, 700 W, and 1,000 W, the area of the carbide accounts for 3.31%, 3.71%, 5.80%, and 9.30%, and its average sizes are 2.96 μm, 3.05 μm, 3.72 μm, and 7.83 μm, respectively. Small carbide particles do not easily form a stress concentration, but their concentrated distribution may lead to stress concentration [14]. Therefore, the distribution and size of carbide have a significant influence on the properties of bearing steel, particularly fatigue life and toughness. The size of carbide must consequently be refined and the uniformity of its distribution be increased to improve the property of the ball bearing steel.

The metallograph for the distribution of carbide is shown in Figure 7(A). The metallographs are processed, as shown in Figures 7(B) and 8, to clearly observe the carbides in the steel. The white particles in Figure 7(B) are the carbides.

Figure 7:

Carbide distribution before (A) and after (B) the image processing (the ultrasonic power of 500 W).

Figure 8(A) shows the distribution of carbide before the introduction of the ultrasonic to the liquid steel. Figure 8(A) shows that the carbides have a mesh distribution and that the largest size in the statistical field of view is 14.63 μm. After introducing the ultrasonic, both the distribution and size of carbide improve noticeably. Many nonlinear effects, such as acoustic steaming and cavitation effects, produced by the ultrasonic in the liquid steel can disintegrate the dendritic crystals and carbide in the solidification process. Thus, the carbide particles are refined, and the uniformity of their distribution is increased. Figure 8(B) shows that, after treatment with 300 W ultrasonic power, the distribution of carbide becomes uniform and the mesh disappears, while large amounts of small and round carbide particles appear, thereby improving the property of the bearing steel. Figure 7(B) shows the distribution of carbide treated with 500 W ultrasonic power. The carbide distribution is uniform, and its size is similar to the carbides in Figure 8(B). After introducing 700 W and 1,000 W ultrasonic powers, the distribution of carbide remains almost unchanged, and its size increases (Figure 8(C) and (D)). The above analysis reveals that, when the power increases from 0 W to 500 W, the size of carbide decreases. However, the size increases as the power increases further.

Figure 8:

Carbide distribution of GCr15 steel with different ultrasonic power.

To further confirm the favorable influence of ultrasonic treatment on the solidification microstructure, the mechanical properties of GCr15 steel are tested. First, the steel ingot treated with the ultrasonic is forged and annealed. The changes in its tensile strength, yield strength, ductility, and reduction of its area are tested. After the ultrasonic treatment, its tensile strength, yield strength, ductility, and reduction of its area improve, as shown in Figure 9. When the power is 300 W, its tensile strength, yield strength, ductility, and reduction of its area improve to a maximum of 653 MPa, 370 MPa, 58%, and 27%, respectively. The mechanical property indexes worsen as the ultrasonic power increases further, which is in accordance with that shown in Figure 8. The smaller the size of the carbide, the more uniform its distribution, and the better its mechanical properties at room temperature.

Figure 9:

Mechanical properties of GCr15 steel with different ultrasonic power. (A) Tensile strength and reduction of area after ultrasonic treatment. (B) Yield strength and percentage of elongation after ultrasonic treatment.