1. Introduction
The vigorous development of the gear industry is greatly promoted by the rapid rise of today’s manufacturing industry, especially the automobile industry. Gear is a precision mechanical part, which usually needs to be forged, carburized, and quenched, after cutting into teeth. Gear is easily damaged, since it will bear a variety of stresses when it works, such as impact force of variable load, contact stress, friction force, and so on, for a long time. Therefore, gear steel must have high strength, toughness, fatigue strength, and wear resistance.
At present, gear steels are mainly medium-carbon and low-carbon alloy steels. After forming, the gear parts need to be carburized and, then, undergo thermal refining (quenching + high temperature tempering). This production method has a long process time and high energy consumption. Besides, the heat treatment parameters (heating temperature, time, carburization process, etc.) have a great impact on the performance of the gear [
1]. Furthermore, the deformation, which is difficult to control during heat treatment, seriously restricts the further improvement of gear quality. Therefore, it is urgent to develop environmentally friendly gear steel with better performance and lower cost.
So far, the steels used in high-speed and heavy-duty gears mainly include 20Cr2Ni4, 12Cr2Ni4, 17CrNiMo6, 18Cr2Ni4W, etc. The gear steel 20CrMnTi is widely used in China. The addition of Ti in the gear steel 20CrMnTi leads to the formation of TiN, which is of high hardness. The site of TiN is prone to a fatigue source and results in the reduced life of gear [
2]. A series of carburized gear steels with high hardenability, small quenching deformation, and good mechanical properties have been manufactured in America, Japan, Germany, and other countries. The carburized gear steels in Germany are mainly Cr series, Cr–Mn series, and Cr–Mo series. The carburized gear steels in Japan are mainly Cr series and Cr–Mo series, and the carburized steels SCM882 and SNCM415 are widely used for heavy-duty gear. The Cr–Ni–Mo series of carburized gear steels are the most common ones in America, and almost all of them contain the Mo element.
In recent years, superfine bainitic steel has become one of the most popular research directions in the field of advanced steel materials because of the high strength, good plasticity, toughness, excellent wear resistance, and fatigue resistance. The main characteristics of this kind of steel are high contents of C (0.6~1.3, wt.%) and Si (1.2~3.0, wt.%), and they also contain a certain amount of Cr, Ni, Mo, Mn, and other elements. This kind of steel can perform bainite transformation at low temperatures (125~325 °C), and the nano-bainite microstructure, consisting of bainitic ferrite lath with a thickness of 20~40 nm and thin-film carbon-rich retained austenite, can be obtained [
3]. The mechanical properties of nano-bainite steel are not only much better than those of the quenched and tempered steel but are, also, comparable to maraging steel. However, the cost of raw materials for nano-bainite steel is only 1/30 of that for maraging steel [
4]. The ultra-high strength of nano-bainite steel is due to the nanoscale bainitic ferrite with a carbon-supersaturation state, and its excellent plasticity and toughness are derived from the thin-film carbon-rich retained austenite with high stability. Nano-bainite steel not only has better mechanical properties than all the series of bainitic steels but also has a simple production process. Nowadays, nano-bainite steel has been gradually found in many applications, such as railway tracks [
5], springs [
6,
7], bearings [
8,
9], and gears [
10].
Compared with conventional bainite steel, the austempering temperature of nano-bainite steel is lower, which increases the number of bainitic ferrite nucleation sites, while slowing down the speed of bainitic ferrite growth. In the early studies of nano-bainite steel, it took 2–60 days to complete the bainite transformation within the isothermal temperature range of 125–325 °C [
3], which undoubtedly increases the manufacturing cost and limits its applications. In recent years, a series of significant achievements have been obtained in the exploration of accelerating bainite transformation, such as adding the alloying elements of cobalt and aluminum to the steel [
11,
12,
13,
14], making the undercooled austenite deform under different temperatures, straining before austempering [
15,
16,
17,
18,
19], and austempering at a temperature slightly lower than the Ms point [
20,
21,
22,
23,
24]. These methods accelerate the bainite transformation process, by shortening the incubation period or speeding up the growth rate of bainitic ferrite. However, the isothermal time required to obtain the whole nano-bainite microstructure is still the bottleneck of large-scale production for nano-bainite steel in the industry.
To shorten the production period of bainitic steel and make full use of the advantages of martensite and bainite, researchers have recently developed the bainite-martensite composite quenching process [
25,
26,
27,
28,
29]. Studies have shown that the mixed microstructure of martensite and bainite not only accelerates the entire phase transformation process and shortens the production period, but also gives the steel high strength and toughness.
In this paper, a new low-carbon alloy gear steel is designed via Si/Al alloying. The addition of aluminum can not only accelerate the bainite transformation but also play a similar role to Si in hindering the precipitation of carbide [
4,
10]. This paper considers not only taking advantage of superfine bainitic ferrite in improving the performance of surface of the gear steel, but also making use of the austempering process to enhance the toughness of martensite in core. Thus, after carburizing, the austempering temperatures were selected at slightly higher than the Ms of the surface and much lower than the Ms of the core. After carburizing and austempering, martensitic and bainitic composite microstructure gear steel with a short heat treatment period and excellent mechanical properties was prepared, which is expected to be utilized in heavy-duty gears.
2. Materials and Methods
The experimental steel was smelted in an electric furnace under vacuum condition and cast to ingot, whose chemical composition is Fe-0.20C-0.72Mn-0.74Si-1.26Ni-1.45Cr-0.34Mo-0.78Al (wt.%). Then, it was forged into steel rods with a diameter of 60 mm and annealed at 700 °C after forging. The phase-transition temperatures of the experimental steel were tested by Gleeble-3500. The test results showed that the phase transformation temperatures were Ac1 = 762 °C, Ac3 = 865 °C, and MS = 365 °C, respectively. A cylinder carburized specimen with an outer diameter of 10 mm, an inner diameter of 6 mm, and a length of 70 mm was used to test the MS temperature of the carburized surface. The specific test procedure was: with a temperature rate of 10 °C/s rising to 600 °C from room temperature, then, with a temperature rate of 1 °C/s rinsing to 900 °C, holding for 30 min, and, next, with a temperature rate of 30 °C/s dropping to room temperature. The test results showed that the MS of the carburized surface layer was 195 °C.
The carburizing and heat treatment processes is shown in
Figure 1. Firstly, the specimens were carburized in a drop-injection-controlled atmosphere full-automatic carburizing furnace, model RQ
2-25-9. The specimens were carburized at 930 °C for 10 h. The first 8 h were the strong carburizing period, with the carbon potential (Cp) of 1.0%, and the last 2 h were the diffusion period, with the Cp of 0.8%. After carburizing, the steel was sealed with anti-oxidation coating to avoid oxidation and decarburization during heat treatment. The process of secondary quenching after carburizing was austenitizing for 30 min at 900 °C. Then, the specimens were transferred quickly into salt baths with temperatures of 200 °C and 230 °C holding for different times, and, finally, tempered at 200 °C for 1 h.
After heat treatment, the anti-oxidation coating on the surface was removed. Then, the specimen surface was ground off 0.05 mm from the top surface using sandpaper and polished using polishing paste on a metallographic polishing machine. The microstructures of the experimental steel in different states were observed by OM (Axiovert200MAT, Carl Zeiss AG, Oberkochen, Germany), SEM (S-4800, Hitachi Limited, Tokyo, Japan), and TEM (JEM-2010, JEOL, Tokyo, Japan). The phase composition of the specimens was analyzed by XRD (D/MAX-2500/PC, Rigaku Corporation, Akishima, Japan).
The carbon content distribution of the carburizing layer was measured by a direct-reading spectrometer (PDA-5500II, Shimadzu, Kyoto, Japan). The hardness analysis required the dual analysis of surface hardness and cross-section hardness distribution. A digital Rockwell hardness tester (HRS-150, Laizhou Huayin Test Instrument Co., Ltd., Laizhou, China) was used to test the surface hardness of specimens. The hardness distributions of the carburized layer were tested on a Vickers hardness tester (FM-ARS9000, FUTURE-TECH, Tokyo, Japan), with an automatic micro-hardness testing system under the conditions of a load of 500 gf and a load retention time of 10 s. The tensile and impact tests were carried out on a MTS810 Landmark Servohydraulic Test System (MTS Systems, Eden Prairie, MN, USA) and NI300F impact test machine (NCS, Beijing, China), respectively. The friction and wear tests were carried out on a screen display end-face testing machine (MMU-5G, JiNan FangYuan testing machine, JiNan, China). The technical parameters selected for the wear test were as follows: the spindle speed was 200 r/min, and the load was 300 N. The total wear time of each specimen was 4 h. The quality change of the specimen was weighed by an electronic balance every 15 min during the wear process.
Author Contributions
Conceptualization, Y.W. and Q.H.; formal analysis, Q.Y., D.X. and F.Z.; investigation, Z.Y. and F.Z.; methodology, Q.H. and Q.Y.; project administration, Y.W., D.X. and Z.Y.; writing—original draft, Y.W., Q.H. and Z.Y.; writing—review & editing, F.Z. All authors have read and agreed to the published version of the manuscript.
Funding
The work in this paper is thanks to the support of the National Natural Science Foundations of China (No. 52001105, 52122410 and U20A20272), the Natural Science Foundation of Hebei Province (E2019402433 and E2020203058), the University Science and Technology Research Project of Hebei Province (BJ2021012), and the Key Project of the Handan Scientific Research Program (21122015004).
Data Availability Statement
The data presented in this study are available on request from the corresponding author.
Conflicts of Interest
The authors declare no conflict of interest.
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Figure 1.
Schematic diagrams of carburizing and heat treatment processes.
Figure 2.
The optical micrographs of carburized surface austempered at 200 °C: (a) 2 h, (b) 4 h, (c) 8 h, (d) 12 h, (e) 24 h, and (f) 48 h.
Figure 3.
The optical micrographs of carburized surface austempered at 230 °C: (a) 2 h, (b) 4 h, (c) 8 h, (d) 12 h, (e) 24 h, and (f) 48 h.
Figure 4.
The SEM images of the carburized surface austempered at 200 °C: (a) 2 h, (b) 8 h, and (c) 48 h. Notes: BF—bainitic ferrite, RA—retained austenite, and M—martensite.
Figure 5.
The SEM images of the carburized surface austempered at 230 °C: (a) 2 h and (b) 48 h. Notes: BF—bainitic ferrite, RA—retained austenite, and M—martensite.
Figure 6.
The TEM micrographs of the carburized surface after isothermal treatment: (a) 200 °C × 8 h, (b) 200 °C × 48 h, (c) 230 °C × 8 h, and (d) 230 °C × 48 h. Notes: BF—bainitic ferrite, RA—retained austenite, and M—martensite.
Figure 7.
XRD patterns of carburized surface austempered at 200 °C (a) and 230 °C (b).
Figure 8.
Carbon content distribution curve of the specimen after carburizing.
Figure 9.
Vickers hardness distributions of carburized layer austempered at 200 °C (left) and 230 °C (right) for different times.
Figure 10.
Relationship curves between weight loss and wear time of the carburized surface austempered at 200 °C.
Figure 11.
Wear morphology of carburized surface austempered at 200 °C: (a) 2 h, (b) 8 h, (c) 48 h.
Figure 12.
XRD patterns of the surface at 200 °C for 8 h before and after wear.
Figure 13.
Optical micrographs of the transition layer (a) and core (b) of the experimental steel, after carburizing and austempering at 200 °C for 8 h.
Table 1.
Retained austenite content (Vγ), and the carbon content in it (Cγ), on carburized surface.
Isothermal Time | 2 h | 4 h | 8 h | 24 h | 48 h |
---|
200 °C | Vγ (Vol.%) | 23.3 | 30.1 | 27.0 | 20.7 | 15.6 |
Cγ (wt.%) | 0.93 | 0.99 | 0.90 | 0.87 | 0.85 |
230 °C | Vγ (Vol.%) | 25.6 | --- | 17.4 | 15.3 | 14.6 |
Cγ (wt.%) | 0.94 | --- | 0.87 | 1.26 | 1.21 |
Table 2.
Rockwell hardness of carburized surface (HRC).
Isothermal Time | 2 h | 4 h | 8 h | 12 h | 24 h | 48 h |
---|
200 °C | 59.5 ± 0.3 | 59.0 ± 0.2 | 58.1 ± 0.3 | 57.8 ± 0.4 | 56.7 ± 0.1 | 56.9 ± 0.2 |
230 °C | 58.0 ± 0.4 | 57.8 ± 0.3 | 56.4 ± 0.1 | 55.7 ± 0.3 | 55.7 ± 0.2 | 56.3 ± 0.3 |
Table 3.
The retained austenite content (Vol.%) of surface austempered at 200 °C before and after wear.
Isothermal Time | 2 h | 8 h | 48 h |
---|
Before Wear | 23.3 | 27.0 | 15.6 |
After Wear | 12.8 | 8.0 | 4.9 |
Ratio of Transformation | 45.1% | 70.4% | 68.6% |
Table 4.
Mechanical properties in the core, after carburizing and austempering at 200 °C for 8 h.
Hardness | Impact Energy | Yield Strength | Tensile Strength | Elongation | Reduction of Area |
---|
34.4 HRC | 156 J | 731 MPa | 1217 MPa | 11.6% | 40.3% |
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