Early fatigue damage detection method for steel rail containing rare earth

Abstract

The invention discloses an early fatigue damage detection method for a rare earth-containing steel rail. The method comprises the following steps: 1) ultrasonic multi-angle probe positioning; 2) preparing and breaking a sample; 3) performing scanning electron microscope and energy spectrum analysis; 4) observing a metallographic structure: analyzing a microstructure around the fracture by adopting a metallographic microscope; and 5) microhardness testing: carrying out high-density hardness testing on a crack source and an adjacent region, and analyzing the correlation between the rare earth induced hardness distribution abnormity and fatigue crack initiation. The invention aims to provide the early fatigue damage detection and identification method for the rare earth element-containing steel rail material, a multi-step microscopic analysis means is adopted to identify a nuclear damage source, and a correlation mechanism between rare earth inclusions and fatigue crack formation is disclosed.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metallurgy and rail transit material detection, and particularly relates to an early fatigue damage detection method for a rare earth-containing rail. BACKGROUND

[0002] With the continuous improvement of the use conditions of rails by rail transit development, the problem of early fatigue damage of rails is increasingly prominent. The current detection technology for nuclear damage is mostly based on online nondestructive detection means such as ultrasonic waves, which can achieve preliminary positioning, but is still insufficient for the analysis of damage formation mechanisms and material intrinsic factors. Traditional rails mostly use high-carbon steel, and internal inclusions and abnormal structures are common fatigue crack initiation sources. In recent years, rare earth elements (such as La and Ce) have been gradually introduced into rail alloys to purify inclusions and refine grains in order to improve strength and toughness.

[0003] However, the introduction of rare earth elements also brings non-uniformity in microstructure and mechanical properties, which may become a new inducement for crack initiation and propagation. There is a lack of systematic detection and analysis methods for the early damage formation mechanism of rare earth-containing rails in existing literature. Therefore, there is an urgent need for a scientific and systematic method to qualitatively and quantitatively identify fatigue damage in such rails, providing a basis for optimizing formulations and service safety evaluation. SUMMARY

[0004] The purpose of the present application is to provide an early fatigue damage detection and identification method for rare earth-containing rail materials, which uses multi-step microanalysis methods to identify nuclear damage sources and reveals the correlation mechanism between rare earth inclusions and fatigue crack formation.

[0005] To solve the above technical problems, the present application adopts the following technical solutions:

[0006] The early fatigue damage detection method for a rare earth-containing rail of the present application comprises the following steps:

[0007] 1) Ultrasonic multi-angle probe positioning: use a high-sensitivity ultrasonic probe to perform three-dimensional positioning on the suspected fatigue damage area of the rail;

[0008] 2) Sample preparation and breaking: cut a sample from the suspected area, open a 2-4mm groove at the rail bottom, and then use a press machine to break it, so that the crack source area is fully exposed;

[0009] 3) Scanning electron microscope and energy spectrum analysis: observe the sample fracture under a high-power scanning electron microscope, and analyze the inclusion types by combining energy spectrum technology to distinguish the existence characteristics of rare earth oxides, sulfides or composite inclusions;

[0010] 4) Metallographic structure observation: analyze the microstructure of the fracture periphery using a metallographic microscope to evaluate grain distortion, precipitates, deformation bands and other structural abnormalities;

[0011] 5) Microhardness test: High density hardness test is carried out at the crack source and adjacent area, and the correlation between the rare earth induced hardness distribution anomaly and the fatigue crack initiation is analyzed.

[0012] Further, a 3mm slot is opened on the rail bottom.

[0013] Further, a single crystal multi-angle probe is used for flaw detection.

[0014] Further, after the alarm is found, the surface of the alarm is checked by using a mirror surface, and external influencing factors are excluded, and if the alarm occurs, the position of the nuclear injury is judged.

[0015] Further, through fracture and energy spectrum analysis, it can be known that the external microcracks develop inwardly, the fracture source is generated at the inclusion in the rail, and then the nuclear injury is formed.

[0016] Further, the method can distinguish the influence mechanism of different types of rare earth inclusions on the formation of fatigue damage.

[0017] Further, the method is suitable for transverse comparison analysis of rare earth rail under different steel plants or formula processes, and supports composition optimization and failure prediction.

[0018] Compared with the prior art, the beneficial technical effects of the present application are:

[0019] 1) The early crack source of rare earth rail can be accurately positioned and analyzed;

[0020] 2) The influence mechanism of different types of rare earth inclusions on the formation of fatigue damage can be distinguished;

[0021] 3) Through the combination of hardness gradient and structure analysis, the scientificity and repeatability of diagnosis are improved;

[0022] 4) The method can be used for transverse comparison analysis of rare earth rail under different steel plants or formula processes, and supports composition optimization and failure prediction. BRIEF DESCRIPTION OF DRAWINGS

[0023] The present application will be further described below in combination with the description of the drawings.

[0024] Figure 1 It is a typical nuclear injury morphology-white nucleus;

[0025] Figure 2 It is a typical nuclear injury morphology-black nucleus;

[0026] Figure 3 It is the rail head morphology of U75V (A steel plant);

[0027] Figure 4 It is the fracture source electron microscope scanning of U75V (A steel plant);

[0028] Figure 5 Rockwell hardness test results for damaged U75V and U76CrRE samples from Steel Plant A. Detailed Implementation

[0029] A method for detecting early fatigue damage in rare earth-containing rails, comprising:

[0030] 1. Flawed rails were located using ultrasonic testing at the metrology department, rail beam factory, and rail welding factory. Single-crystal multi-angle probes were used for detection. Upon detecting an alarm, a mirror was used to inspect the reflective surface of the alarm to eliminate external influencing factors. If an alarm still occurred, the location of the fault was determined.

[0031] 2. After determining the location, cut off the rail head, cut a 3mm groove at the bottom of the rail at the location of the core damage, and use a press to break the rail to expose the core damage morphology.

[0032] 3. Electron microscopy and energy dispersive spectroscopy examination of white nuclei

[0033] Figure 1 The U76CrRE rail shown is a typical white core with a metallic color, indicating no oxidation due to lack of external contact. Scanning electron microscopy (SEM) was used to observe the fracture morphology. Two inclusions were found in the fracture origin region. Energy dispersive spectroscopy (EDS) analysis revealed the main components of the inclusions to be C (5.64%), O (41.51%), Na (7.20%), Mg (7.20%), S (7.93%), Ca (13.18%), Fe (12.35%), and C (6.97%), O (14.04%), Al (1.27%), Si (5.18%), Ca (5.42%), La (13.3%), Ce (25.72%), Fe (26.7%). Analysis indicates that stress concentration caused by the inclusions during rail service led to crack initiation and the formation of the core injury.

[0034] 4. Electron microscopy and energy dispersive spectroscopy examination of black nuclei

[0035] Observation of the external appearance of the rail revealed that the rail was an outer strand of a curve, subjected to the combined effects of the wheel's gravity and centrifugal force. The inner rounded corners received a large cyclic load. Under these conditions, the rail was highly susceptible to developing oblique cracks and changes in the rail head morphology. Figure 3 It was observed that the morphology of the core damage consisted of two horizontal planes, with the rounded corners connected to the core damage. Analysis suggests that the lateral horizontal force exerted by the wheel on the curved track caused the rail to deform, and the external oblique cracks and deformations propagated into the interior of the rail, resulting in core damage in the presence of inclusions.

[0036] The fracture morphology was observed using a scanning electron microscope, such as Figure 4The fracture origin contains inclusions, the main components of which are C (18.81%), O (40.26%), Ca (37.86%), and Fe (3.07%). Analysis of the fracture surface and energy dispersive spectroscopy shows that the fracture originates from the external microcracks and develops inward at the inclusions inside the rail, eventually forming a core injury.

[0037] 5. Inclusion Analysis

[0038] For steel plant A's U75V rails with identified defects, a sample of the fir surface was taken for inclusion inspection. For steel plant B's U75V rails, since no defects were found, inclusion analysis was performed 15mm downwards from the rail head fillet, as shown in Table 2.

[0039] Table 2. Inclusion Inspection of U75V Steel from Steel Mill A and Steel Mill B

[0040]

[0041] Inclusion analysis showed that the inclusion level of U75V rails from steel plant A was relatively high, with more large inclusions and mixed inclusions of level 3 or higher. In contrast, the inclusion level of U75V rails from steel plant B was lower.

[0042] 6. Nuclear damage hardness gradient

[0043] Rockwell hardness tests were performed on the vicinity of the nuclear damage samples U75V and U76CrRE from Steel Plant A. The test points were located as follows: Figure 5 As shown:

[0044] The hardness near the nuclear damage points of the two types of rails was found to be relatively uniform, with no soft spots or areas of excessively high hardness observed. Specific data is shown in Table 3.

[0045] Table 3 Rockwell Hardness of U75V and U76CrRE Steel Samples from Steel Mill A (with Injury Samples)

[0046]

[0047] III. Conclusion of Rail Fault Analysis for Small Radius Curves by a Certain Railway Bureau

[0048] 1. Based on the flaw detection results, not all U75V and U76CrRE rails from Steel Plant A and U75V rails from Steel Plant B showed signs of inclusion after being pressed apart. Analysis suggests that because track flaw detection uses a flaw detection trolley, maintenance personnel adjust the probe sensitivity level to a relatively high level, typically twice the production technical requirements, to ensure rail operation safety. Therefore, even if the level of inclusions in the rail meets production inspection standards, the track flaw detection will still trigger an alarm.

[0049] 2、From the electron microscope and energy spectrum analysis results, the main reasons for the production of nuclear injury are two kinds, one is simply with inclusion as the fracture source caused by nuclear injury, the other is by rail head appearance change, micro crack and so on, the fracture source is produced in the position of inclusion in the rail, and the nuclear injury is formed. This nuclear injury caused by external injury is related to the road condition and maintenance of the line.

[0050] 3、From the comparison results of the inclusion inspection, the inclusion level of U75V of B steel plant is lower and the quantity is less compared with U75V rail of A steel plant.

[0051] The above-described embodiments are only to describe the preferred modes of the present application, and not to limit the scope of the present application. Without departing from the design spirit of the present application, various modifications and improvements of the technical solutions of the present application made by those skilled in the art shall fall within the protection scope determined by the claims of the present application.

Claims

1. A method of detecting early fatigue damage in a rare earth containing rail, characterised in that: The method comprises the following steps: 1) Ultrasonic multi-angle probe positioning: use a high-sensitivity ultrasonic probe to position the suspected fatigue damage area of the steel rail in three dimensions; 2) Sample preparation and breaking: cut a sample from the suspected area, open a 2-4 mm groove at the rail bottom, and then use a press to break it, so that the crack source area is fully exposed; 3) Scanning electron microscopy and energy spectrum analysis: observe the sample fracture under high magnification scanning electron microscopy, and analyze the inclusion types by combining energy spectrum technology to distinguish the existence characteristics of rare earth oxides, sulfides or composite inclusions; 4) Metallographic structure observation: analyze the microstructure of the fracture periphery using a metallographic microscope to evaluate the grain distortion, precipitated phase, deformation band and other abnormal structures; 5) Microhardness test: perform high-density hardness testing on the crack source and adjacent area to analyze the correlation between the hardness distribution anomaly induced by rare earth and the initiation of fatigue cracks.

2. The method for detecting early fatigue damage of rare earth-containing rails according to claim 1, characterized in that: Open a 3mm groove at the rail bottom.

3. The method for detecting early fatigue damage of rare earth-containing rails according to claim 1, characterized in that: Use a single crystal multi-angle probe to detect.

4. The method for detecting early fatigue damage of rare earth-containing rails according to claim 3, characterized in that: After finding the alarm, use the mirror to check the surface of the alarm reflection surface, and exclude external influencing factors. If there is an alarm, determine the position of the core damage.

5. The method of claim 1, wherein: Through fracture and energy spectrum analysis, it can be known that the external micro-cracks develop inward, and the fracture source is generated at the inclusion in the steel rail, and then the core damage is formed.

6. The method for detecting early fatigue damage of rare earth-containing rails according to claim 1, characterized in that: This method can distinguish the influence mechanism of different types of rare earth inclusions on the formation of fatigue damage.

7. The method for detecting early fatigue damage of rare earth-containing rails according to claim 1, characterized in that: This method is suitable for comparative analysis of rare earth steel rails under different steel plants or formula processes, and supports composition optimization and failure prediction.

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