Study on Mechanism and Influencing Factors of Wheel Strengthening and Toughening of High-Speed and Heavy-Load Train

Abstract

When maximum speed of 160 km/h is reached and the axle load reaches 25–30 tons, the train wheels need to have high strength and toughness. The main chemical elements affecting the strength and toughness of the wheel were determined by the mechanical features of the samples with different chemical compositions. Through analysis of the impact fracture of typical specimens, the difference of wheel toughness was mainly reflected in the dimple band, crack source, and cleavage pattern. By SEM analysis of fracture cracks, the critical size difference was found to exist between the grains during brittle fracture, where the intergranular fracture between grains of different sizes is mainly due to the different interfacial stresses between grains of different sizes.

1. Introduction

The heavy-load train has the great advantage of high efficiency and economy, which is the important direction of railway technology development. For all of countries in the world, axle load increasing is the main way to develop heavy haul transport. Along with the execution of railway heavy haul traffic and high speed, the wheels working environment become more and more harsh [1]. Most of the world’s heavy-haul railway technology is found in relatively developed countries, such as the United States, Canada, Australia and so on. The axle load of Canadian railway freight cars is 32.5~37.5 tons, while the United States is the most important country in terms of heavy railway loads, with an axle load of approximately 29.8~35.7 tons. The country with the most developed heavy-haul railway is Australia, whose heavy-haul freight technology is at the forefront of the world. The axle weight of freight cars is 40 tons, and in Australia it is developing to exceed 42 tons. However, the speed of a heavy load train is generally below 80 km/h.

High-speed and heavy-load train have an axle load of 25–30 t and a maximum speed of 160 km/h. In this case, the wheel-rail relation deteriorates, so the wheels need to see continuous breakthroughs in terms of materials and technology. On the one hand, the strength and hardness of the wheels need to be improved to carry more weight, increase the wear life, thermal fatigue, and mechanical fatigue damage [2,3,4]. On the other hand, the wheels need to maintain enough toughness, impact resistance, and fatigue crack resistance to improve the safety performance of the train in fast operation [5].

High-speed and heavy-load train wheels need high strength, and increasing carbon content is the main method to improve the strength of wheels. However, with the increase of carbon content, the brittleness trend of wheels also increases, and the possibility of abnormal microstructures, such as bainite and martensite, on wheel tread also increases [6]. With the increase of wheel strength, the crack initiation resistance increases, while the crack propagation resistance decreases. Research shows that pearlite steel with high strength has good fatigue strength and high fatigue threshold [7]. Therefore, in the case of higher carbon content, to maintain the high strength of the wheel while improving the toughness of the wheel is one of the important topics in the research on the material and technology of high-speed and heavy-load train wheels.

The wheels material of railway wagon in China is CL60, while the wheels of railway heavy-load train in other countries and regions mostly comply with AAR standards of the Association of American Railways [8]. The comparison of chemical components between the two is shown in

Table 1. Comparison of chemical composition in different standards.

Material	C/wt.%	Si/wt.%	Mn/wt.%	P/wt.%	S/wt.%	Cr/wt.%	Ni/wt.%	Cu/wt.%	Mo/wt.%	V/wt.%
CL60	0.55~0.65	0.17~0.37	0.50~0.80	≤0.035	≤0.040	≤0.25	≤0.25	≤0.25	-	-
AAR C	0.67~0.77	0.15~1.00	0.60~0.90	≤0.030	0.005~0.040	≤0.30	≤0.25	≤0.35	≤0.10	≤0.04

.

The AAR C wheel has high strength and can meet the requirements of heavy load, but the AAR standard does not require the toughness and surface microstructure of the wheel. Therefore, to develop high-speed and heavy-load technology, it is necessary to improve the toughness of the wheel, and the microstructure of the wheel rim should be pearlite, which does not allow the existence of brittle structures, such as bainite and martensite [9]. Therefore, it is necessary to design a reasonable chemical composition and appropriate technology in order to improve the strength, toughness, and microstructure of the wheel.

2. Materials and Methods

2.1. Mechanical Tests

Carbon plays a decisive role in terms of the mechanical properties of wheel, so to achieve a reasonable strength and toughness matching, it is necessary to establish a reasonable range of C content. In this experiment, a large number of forged and normalize samples with different carbon contents were made using a small smelting electric furnace. Two different carbon contents of 0.61–0.64% and 0.70–0.73% (Table 1 two material), in which there are 54 test rods with carbon contents of 0.61–0.64% and there are 29 test rods with carbon contents of 0.70–0.73%, were designed. The actual carbon content of test rods was tested, and different amounts of test rods were obtained between 0.58% and 0.75% (actual carbon content of the rods). The sample is φ40 × 200 mm (see Figure 1). The distribution of different carbon rods is shown in Figure 2. Then, the mechanical analysis of these test rods was tested, and the influence rule of carbon on mechanical properties was obtained.

In order to analyze the influence of chemical elements on wheel strength and toughness, a 50 kg vacuum induction furnace was used to add different contents of Si, Mn, Cr, Mo, V, and other elements. Using the recipe of different elements, the influence law of wheel plasticity toughness was studied. The test process was as follows: scrap steel and alloy vacuum melting → ingot → ingot slow cooling → rolling → steel plate slow cooling → heat treatment → test.

When the ingot is rolled, the forging ratio is 4 and the size is 250 × 250 × 65 mm³, which is similar to the forging ratio of the wheel hot forming. Because the steel plate is smaller than the wheel size, the heat treatment does not adopt the water quenching method like the wheel, but adopts the normalizing process to ensure the fine pearlite structure of the steel plate. Figure 3 presents the normalizing cooling curve, and samples from steel plate after heat treatment are shown in Figure 4.

2.2. Fracture Analysis

In the toughness test of the heavy-load train wheel, it is shown that the impact of web is unstable, which affects the toughness level of wheels, so the test analyzes the samples with abnormal impact.

3. Results and Discussion

3.1. Mechanical Property

3.1.1. Influence of Carbon on Wheel Strength and Toughness

The content of C varied from 0.58% to 0.75% in the samples. The mechanical results are shown in Figure 5 and Figure 6.

It can be seen from Figure 5 that increasing the content of C can significantly improve the strength of wheel, but the yield strength increases little when C content is 0.65–0.68%. This shows that when the C content is between 0.69% and 0.74%, the impact declines slowly, the fluctuation is small, the hardness high, and the strength and toughness match well with Figure 6.

3.1.2. Influence of Other Elements on Wheel Strength and Toughness

Two samples with different Cr/Si/Mn were designed for testing. The contents and test results are shown in

Table 2. The test results of different Cr/Si/Mn content.

Pos.	Cr = 0.22 wt.%		Cr = 0.16 wt.%		Si = 0.77 wt.%		Si = 0.31 wt.%		Mn = 1.02 wt.%		Mn = 0.79 wt.%
	Rm	Impact	Rm	Impact	Rm	Impact	Rm	Impact	Rm	Impact	Rm	Impact
	/Mpa	KU2/J	/Mpa	KU2/J	/Mpa	KU2/J	/Mpa	KU2/J	/Mpa	KU2/J	/Mpa	KU2/J
T1/1-3	1001	16/14/14	1012	25/26/19	1229	13/10/17	1179	17/14/21	1187	16/12/23	1169	16/24/25
T2/4-6	980	23/21/12	959	37/18/21	1201	23/11/18	1127	22/16/25	1150	21/29/20	1138	23/26/27

.

As the result of Cr test shows that the strength of the steel plate is low, the normalizing process of the steel plate was adjusted in this test, and water spraying cooling was adopted for heat treatment.

As the previous tests have verified the strength and impact at different positions, only the T1 strength and the impact 4&5&6 were tested in this test. The other residual elements, such as V, Ni, and Mo, were added in the test [10,11,12], and the test results are shown in

Table 3. The test results of other element contents.

Sample	V/wt.%	Ni/wt.%	Mo/wt.%	Rm/Mpa	Rp0.2/Mpa	A5/%	Z/%	KU2 (Notch Depth 2 mm)/J
1#	0.002	0.011	0.001	1153	649	15.5	32.0	26/29/23
2#	0.060	0.009	0.001	1205	689	14.5	31.5	27/23/22
3#	0.006	0.100	0.001	1197	674	15.5	28.5	18/24/22
4#	0.003	0.010	0.011	1184	689	16.5	32.0	23/26/27

.

Studies show that Cr can refine pearlite lamellar and improve wheel strength and toughness [13,14]. The test shows that tensile T1 has a high strength and the results of the two different Cr are similar. This is because tensile T1 is close to the edge of the steel plate where the normalizing cooling is sufficient. The strength of tensile T2 is obviously higher with the increase of Cr content. It can be concluded that the increase of Cr content has no obvious effect on strength under quenching condition. However, the increase of Cr content can effectively improve strength under normalizing condition. Because the wheel rim is quenched by water spray, increasing Cr content has little effect on improving wheel rim strength when Cr exceeds 0.15%. However, an increase of Cr content can significantly improve wheel web strength in the absence of quenching treatment. For the impact, when Cr content is high, the overall impact level decreases and the impact fluctuation increases, so increasing Cr content is unfavorable to wheel toughness.

Adding a certain amount of Si/Mn is an important means to improve the wheel strength, but the test shows that Si/Mn and C have similar effects, where a higher content decreases the toughness of the wheel and increases instability. When Si content reaches above 0.70%, the strength increases by 6%, while the impact minimum value decreases by 30%. When Mn content over 1% will increase the instability of toughness, the difference between the maximum and minimum of impact double. Adding a small amount of other residual elements can effectively improve the strength of the wheel, especially the yield strength, but it has no obvious effect on improving the toughness. The elements Cr, V, Ni, and Mo increase the strength of wheel, but reduce the mean impact of wheel by about 20%, then present great difficulties in the process control of microstructures such as bainite on the surface of the wheel. Therefore, the microalloyed composition design of wheel has limited effect on high-carbon heavy-load train wheels, and the combined effect of residual elements and Cr, Mn, and Si greatly promotes the hardenability of the wheel, then presents great difficulties in the process control of microstructures such as bainite on the surface of the wheel.

3.2. Toughness Mechanism Analysis

The mechanical test results are shown in

Table 4. Mechanical properties of wheel.

Sample	ReH/MPa	Rm/MPa	A5/%	Z/%	KU2(+20 °C, Notch Depth 2 mm)/J			Under Tread 30 mm/HB
					1#	2#	3#
Rim	703	1109	15.0	41	29.2	20.0	27.5	314
Web	/	888	14.5	/	9.1	24.5	18.5

. According to the test results of web impact, after USC (ultrasonic cleaning) of the web impact 1# and 2#, SEM analysis was conducted on the fracture of the sample.

3.2.1. SEM Analysis of Fracture

The typical fracture morphology is shown in Figure 7 and Figure 8.

The SEM results of the specimen fracture show that the impact 1# has obvious transverse and longitudinal lines at the gap. There is no dimple at the fracture edge, which indicates a complete brittle fracture. There is an obvious concentrated crack source on the secondary surface. The fracture morphology is cleavage plane, and there are many intergranular secondary cracks, large cleavage patterns corresponding to coarse grains. The impact 2# also has transverse and longitudinal lines at the gap, dimples with a certain width at the edge of the fracture. There is no obvious concentrated crack source. There are many secondary cracks and large cleavage patterns corresponding to coarse grains.

3.2.2. OM Analysis of Fracture

Impact 1# and impact 2# were assessed by OM. The test results are shown in Figure 9 and Figure 10 and

Table 5. OM test results.

Sample	Inclusion					Microstructure	Grain Size (Grade)
	A Sulfur	B Aluminate	C Silicate	D Globular Oxide	Ds Single Globular Oxide
1#	0	0.5	0	0.5	0	Pearlite + fine pearlite + a little ferrite	6.5 and a little 3.5
2#	0	0	0.5	0.5	0	Pearlite + fine pearlite + a little ferrite	6.5 and a little 4.0, some field 3.0

.

The microstructure of the web is lamellar pearlite and pro-eutectoid ferrite along the grain boundary, and there are coarse grains and obvious mixed grains field in the sample, but no large inclusions.

The SEM fracture analysis results of typical samples show that there are many longitudinal and transverse lines in notch, which is a common phenomenon and impact toughness is not affected, so the notch morphology has no direct correlation with impact toughness. The fracture pattern of typical samples is cleavage fracture, and there are many secondary cracks. The shape and number of secondary cracks are found not to be related to impact toughness, nor is any inclusion found.

The difference in wheel toughness is mainly reflected in dimple band, crack source, and cleavage pattern. The sample with high impact toughness has dimples of a certain width at the fracture edge, and the specimen with abnormal or very low impact toughness has complete brittle fracture. It can be concluded that the dimple band is directly related to the impact toughness, and the wider dimple band means the higher impact toughness. The secondary surface of the sample with low impact value has obvious concentrated crack source. The size of cleavage pattern is consistent with the size of grains, which represents the brittleness trend of materials [15]. According to the statistics of many impact data regarding web, the impact toughness of most samples is normal, i.e., most of the samples have a dimple band of a certain width on the impact fracture. The complete brittle fracture is occasional.

From the other characteristics of the fracture, there are more secondary cracks at all impact fracture and coarse grains on the cleavage plane, but the samples with low toughness have concentrated crack sources on the cleavage plane. Therefore, it can be inferred that during the impact fracture, more cracks are generated at the grain boundary between coarse grains and fine grains. In the uniform fine grain plan, it has a dimple region on the impact fracture, which increases the fracture energy and has a higher toughness.

This shows that there are no inclusions in the sample [16], the grain size at web is 6–6.5 grade, and there are mixed crystals by OM. Because the web is normalized, the cooling rate is low during heat treatment, the coarse grain after forging cannot be completely eliminated. Since the impact toughness fluctuation of the web is only an occasional phenomenon [17,18], it can be seen that the mixed crystals in the web do not cause the decrease of the toughness level.

The results of SEM and OM analysis show that there is a little coarse grain in the wheel, but not all of the coarse grains can lead to the reduction of wheel toughness. Different grain sizes have different tendencies to form concentrated crack sources. Due to the different interfacial stress of grain sizes, the crack generation trend during impact fracture is different. Wheel impact fracture, whether brittle or ductile, is carried out along the grain. When the size difference of grain reaches a critical value, there will be a large stress in the interface, and then brittle fracture generates during impact. Critical grain size or interfacial stress still needs further study [19,20,21].

For the same wheel, due to the different heat treatment states of the rim and web, the impact toughness values differ greatly (Table 2). Although the microstructure of the two parts is the same, the rim cooling rate during quenching is fast, the grain size can reach 7–8 grade, and the pearlite lamellar is finer, so it has higher strength and toughness [22,23]. However, the toughness fluctuation phenomenon of the rim and web is the same. It can be inferred that the fracture mechanism of the rim and web is the same, except the frequency is different. The average grain size has a proportional relationship with the impact toughness value, and the impact toughness has random fluctuations. One of the mechanisms is that there is a critical difference in the grain size. This leads to a concentrated crack source during impact fracture, resulting in brittle fracture and abnormal reduction of toughness.

4. Conclusions

• Carbon plays a decisive role in the performance of high-speed and heavy-load train wheels. When the content of carbon is between 0.69% and 0.74%, the impact decline is the slowest, the fluctuation is the smallest, the strength is higher, and it has a good matching of strength and toughness.
• Si/Mn is an important strengthening element of wheels and has little influence on their toughness. However, when Si content reaches above 0.70%, the strength increases by 6%, while the impact minimum value decreases by 30%. Mn content exceeding 1% will increase the instability of toughness, doubling the difference between the maximum and minimum of impact.
• The elements Cr, V, Ni, and Mo increase the strength of the wheel, but reduce the mean impact of the wheel by approximately 20%, then present great difficulties in the process control of microstructures such as bainite on the surface of the wheel.
• The different toughness of the wheel is reflected in the dimple band of the fracture, the source of the crack, and the pattern of cleavage.
• The mechanism of random fluctuation of impact toughness is that there is a critical difference in grain size, which leads to a concentrated crack source during impact fracture, resulting in brittle fracture and abnormal reduction of toughness.
• The abnormal fluctuation of toughness on wheels is mainly due to the different interfacial stress of the grain size. It remains necessary to improve the toughness level of wheels, in addition to refining the grain, but also to control the grain grade difference.