A method for failure analysis of rail fracture caused by external damage

Abstract

The application discloses a failure analysis method for cracking caused by external damage of a steel rail, and belongs to the technical field of failure analysis of steel rails. The method is used for quickly judging the failure analysis of cracking of the steel rail caused by surface external damage (such as contact fatigue, straightening process defects, punching operation damage and the like) through macroscopic external damage observation combined with micro fracture feature identification. The purpose of the application is to provide a failure analysis method for cracking caused by external damage of a steel rail. The method directly and quickly displays the white layer or hardened layer of a crack source area caused by external damage through a specific sample preparation and observation mode, so that the cracking reason of the steel rail can be accurately judged, the analysis process is simplified, and the accuracy of the judgment is improved.

Description

Technical Field

[0001] This invention belongs to the field of rail failure analysis technology, and relates to a failure analysis method for rail fracture caused by external damage. Specifically, it relates to a failure analysis method that rapidly determines rail cracking caused by surface damage (such as contact fatigue, straightening process defects, drilling damage, etc.) by combining macroscopic external damage observation with microscopic fracture feature identification. Background Technology

[0002] As a key component of railway transportation, rails are subjected to wheel-rail contact stress, bending stress, and residual stress over a long period of time. Their surfaces are prone to various external injuries, which often become the source of crack initiation, eventually leading to rail breakage and seriously threatening traffic safety.

[0003] Current rail failure analysis typically employs the following method: macroscopic observation of fracture morphology to locate crack initiation points; to find evidence at the crack initiation points, a sample is usually cut perpendicular to the fracture surface near the crack initiation point, mounted, and then ground to form a cross-section perpendicular to the fracture surface; corrosion is performed; finally, the microstructure is observed under a microscope. However, in practice, this method has been found to have the following significant drawbacks:

[0004] Key microscopic features easily overlooked: External damage to the rail surface (such as abrasions and peeling) often forms an extremely thin modified layer (such as a cryptocrystalline martensite layer, commonly known as the "bright white layer") on the surface. This layer is extremely hard and brittle, and is a key source of crack initiation. Because this layer is extremely thin and extremely hard, it is very easy to peel off and break during the cutting and polishing process in traditional vertical cutting sample preparation, causing analysts to overlook this crucial evidence.

[0005] The analysis cycle is long and the efficiency is low: the metallographic sample preparation process is complicated, and if the altered layer is missed, the analyst often needs to conduct a re-inspection or carry out other supplementary experiments, which makes the analysis process lengthy and cannot meet the needs of rapid on-site troubleshooting.

[0006] This can easily lead to misjudgment: If the bright white layer or hardened layer in the source area is not observed, the analyst may misjudge the cause of the fracture as a problem with the quality of the material matrix or excessive inclusions, thus misleading the direction of improvement.

[0007] Therefore, there is an urgent need for a failure analysis method that can quickly and accurately locate crack initiation and intuitively reveal the characteristics of the surface metamorphic layer leading to fracture, in order to solve the problems existing in the current technology. Summary of the Invention

[0008] The purpose of this invention is to provide a failure analysis method for rail fracture caused by external damage. This method uses a specific sample preparation and observation method to intuitively and quickly display the white or hardened layer in the crack initiation zone caused by external damage, thereby accurately determining the cause of rail fracture, simplifying the analysis process, and improving the accuracy of the determination.

[0009] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0010] This invention discloses a failure analysis method for rail fracture caused by external damage, comprising the following steps:

[0011] Step a, Macroscopic observation and location: Clean the fracture surface of the failed rail, observe the morphological characteristics of the fracture surface (such as radial stripes) macroscopically, trace back to locate the crack source, and confirm whether the crack source is located at the external damage site on the rail surface.

[0012] Step b, specimen cutting: Cut a block-shaped specimen including the fracture surface and the nearby matrix from the confirmed crack initiation area. The specimen size should be easy to handle or grind by hand or clamp. Avoid touching or damaging the original surface of the crack initiation area during the cutting process.

[0013] Step c, Fractal surface grinding: Instead of using the traditional method of cutting perpendicular to the fracture surface, the fracture surface corresponding to the crack source is gently ground directly;

[0014] Step d, corrosion imaging: chemically corrode the ground fracture surface using a 4% nitric acid alcohol solution for 5-10 seconds.

[0015] Step e, Microscopic observation and judgment: Place the corroded sample under a metallographic microscope for observation; if obvious white bright layer or hardened layer features are observed at the crack initiation site, and the microstructure of the layer is cryptocrystalline martensite or fine martensite, then it is determined that the rail fracture is a crack induced by surface damage; if no white bright layer is observed, then external damage-induced cracking factors are ruled out, and other causes are analyzed.

[0016] Step f, Cause Analysis: Based on the morphology and distribution of the white layer, combined with the type of external injury (contact fatigue, straightening defects, drilling damage, etc.), further confirm the specific failure mode (e.g., martensitic phase transformation caused by local frictional heat, which in turn induces cracking).

[0017] Furthermore, step c specifically involves: using metallographic sandpaper to grind the fracture surface of the crack initiation area from coarse to fine, with the grinding direction parallel to the fracture surface. It is recommended that the grinding direction be parallel to the fracture surface or at a very small angle, in order to remove the oxide scale, contaminants, and loose layer from the fracture surface, while retaining the extremely thin characteristic metamorphic layer of the crack initiation area.

[0018] Furthermore, the external damage to the rail surface includes, but is not limited to: wheel-rail contact fatigue damage, straightening process defects during rail production, drilling damage during on-site construction, scratches, abrasions, etc.

[0019] Furthermore, the sample cutting method differs from the traditional method of cutting perpendicular to the fracture surface near the crack source. Instead, the crack source area is placed in the middle of the sample, with the crack source 8-12mm away from the cutting positions on both sides, to avoid the white gloss layer peeling off due to cutting.

[0020] Furthermore, the crack source is 10mm away from the cutting positions on both sides.

[0021] Furthermore, the term "light grinding" refers to controlling the grinding force and depth to avoid excessive grinding that could cause the extremely brittle white gloss layer to peel off or be completely removed.

[0022] Furthermore, the bright white layer is mainly a cryptocrystalline martensite layer, which appears as a bright white area that is not easily corroded by conventional reagents under a microscope, and its hardness is much higher than that of the substrate.

[0023] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0024] Highly intuitive and less prone to missed detection: This invention directly grinds and observes the fracture surface of the crack initiation zone, rather than just observing the vertical section. This method can preserve and visually display the extremely thin, bright white layer to the greatest extent, avoiding the missed detection problems caused by cutting angles or sample breakage in traditional methods.

[0025] Accurate and intuitive judgment: By grinding and imaging the fracture surface, the distribution range and morphology of the white layer covering the crack initiation area can be directly observed, directly confirming the causal relationship between external damage (such as martensitic phase transformation caused by frictional heat) and crack initiation. A direct evidence chain of "external damage - altered layer (white layer) - crack initiation" is established, which can quickly distinguish whether the crack is caused by surface process defects or by internal material quality problems, thus avoiding misjudgment.

[0026] Wide range of applications: Applicable to various rail failure analyses caused by surface damage, especially with extremely high sensitivity for typical failure modes such as contact fatigue, straightening defects and drilling damage. Attached Figure Description

[0027] The present invention will be further described below with reference to the accompanying drawings.

[0028] Figure 1 : Macroscopic morphology of rail fracture, arrows indicate the location of crack initiation and surface damage, and the box indicates the location where the sample was taken.

[0029] Figure 2 Image of the bottom of the rail; the area within the box indicates external damage.

[0030] Figure 3 Image showing the surface morphology of the fracture surface; the area within the box indicates the fracture origin.

[0031] Figure 4The photograph of the ground fracture surface under a microscope clearly shows the morphology of the "bright white layer" on the surface.

[0032] Figure 5 : Macroscopic morphology of rail fracture. Arrow 1 indicates the location of crack initiation and surface damage, and arrow 2 indicates the location of the fracture initiation zone.

[0033] Figure 6 The photograph of the ground fracture surface under a microscope clearly shows a "bright white layer" at the crack initiation point. Detailed Implementation

[0034] The present invention will be further described in detail below with reference to specific embodiments.

[0035] Example 1: Fracture analysis caused by defects in the straightening process of U75V rails (e.g.) Figures 1-4 (As shown)

[0036] Macroscopic observation: The U75V rail fractured after straightening. Macroscopic observation of the fracture surface revealed a flat fracture, exhibiting brittle fracture characteristics. Based on the convergence direction of the radial striations, the crack origin was determined to be located at a minor indentation (external injury) on the bottom corner surface of the rail.

[0037] Sample cutting: Use a wire EDM machine to cut a block sample of about 20mm × 20mm from the crack initiation point.

[0038] Grinding the crack source area: Use fine sandpaper to gently grind the fracture surface of the crack source area. Control the pressure during grinding to remove only the surface iron oxide scale and avoid damaging the subsurface structure.

[0039] Corrosion imaging: The ground fracture surface is chemically etched using a 4% nitric acid alcohol solution for 5 seconds.

[0040] Microscopic observation and determination: When the ground surface was observed under a metallographic microscope, a continuous and distinct bright white layer was found in the crack initiation area. This confirmed that the layer was a martensitic hardened layer caused by straightening friction.

[0041] Conclusion: The rail fracture was determined to be caused by improper straightening process, which resulted in the formation of a friction hardening layer (white bright layer) on the surface. The brittle hardening layer fractured under tensile stress, inducing crack propagation.

[0042] Example 2: Crack analysis of U78CrV rails after one year of service on the line (e.g.) Figures 5-6 (As shown) Macroscopic observation: Macroscopic observation of the fracture surface revealed obvious external damage at the rail head arc position corresponding to the crack source on the fracture surface.

[0043] Sample cutting: Use a wire EDM machine to cut a block sample at the crack initiation point.

[0044] Grinding the crack source area: Use fine sandpaper to gently grind the fracture surface of the crack source area. Control the pressure during grinding to remove only the surface iron oxide scale and avoid damaging the subsurface structure.

[0045] Corrosion imaging: The ground fracture surface is chemically etched using a 4% nitric acid alcohol solution for 7 seconds.

[0046] Microscopic observation and judgment: When the polished surface was observed under a metallographic microscope, a bright white layer with a depth of about 0.035 mm was found at the edge of the fracture source, and cracks were found on the bright white layer.

[0047] Conclusion: It was determined that the rail suffered surface damage during service due to external impact or abnormal contact at the rail head arc. Friction and plastic deformation in this area led to the formation of a brittle, bright white layer (martensite). During subsequent operation, the brittle bright white layer fractured, forming microcracks that propagated deeper under contact fatigue stress, ultimately causing the rail to break.

[0048] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A failure analysis method for rail fracture caused by external damage, characterized in that: Includes the following steps: Step a, Macroscopic observation and location: Clean the fracture surface of the failed rail, observe the morphological characteristics of the fracture surface macroscopically, trace back to locate the crack source, and confirm whether the crack source is located at the external damage site on the rail surface. Step b, specimen cutting: Cut a block-shaped specimen including the fracture surface and the nearby matrix from the confirmed crack initiation area. The specimen size should be easy to handle or grind by hand or clamp. Avoid touching or damaging the original surface of the crack initiation area during the cutting process. Step c, Fractal surface grinding: Instead of using the traditional method of cutting perpendicular to the fracture surface, the fracture surface corresponding to the crack source is gently ground directly; Step d, corrosion imaging: chemically corrode the ground fracture surface using a 4% nitric acid alcohol solution for 5-10 seconds. Step e, Microscopic observation and judgment: Place the corroded sample under a metallographic microscope for observation; if obvious white bright layer or hardened layer features are observed at the crack initiation site, and the microstructure of the layer is cryptocrystalline martensite or fine martensite, then it is determined that the rail fracture is a crack induced by surface damage; if no white bright layer is observed, then external damage-induced cracking factors are ruled out, and other causes are analyzed. Step f, Cause Analysis: Based on the morphology and distribution of the white layer, combined with the type of external injury, further confirm the specific failure mode.

2. The failure analysis method for rail fracture caused by external damage according to claim 1, characterized in that: Step c specifically involves using metallographic sandpaper to grind the fracture surface of the crack initiation zone from coarse to fine. The grinding direction should be parallel to the fracture surface or at a very small angle. The aim is to remove the oxide scale, contaminants, and loose layer from the fracture surface, while retaining the extremely thin characteristic metamorphic layer of the crack initiation zone.

3. The failure analysis method for rail fracture caused by external damage according to claim 1, characterized in that: The surface damage to the rails includes, but is not limited to: wheel-rail contact fatigue damage, straightening process defects during rail production, damage from drilling operations during on-site construction, scratches, abrasions, etc.

4. The failure analysis method for rail fracture caused by external damage according to claim 1, characterized in that: The sample cutting method differs from the traditional method of cutting perpendicular to the fracture surface near the crack source. Instead, the crack source area is placed in the middle of the sample, with the crack source 8-12mm away from the cutting positions on both sides, to avoid peeling off the white gloss layer due to cutting.

5. The failure analysis method for rail fracture caused by external damage according to claim 4, characterized in that: The crack source is 10mm away from the cutting positions on both sides.

6. The failure analysis method for rail fracture caused by external damage according to claim 1, characterized in that: The term "light grinding" refers to controlling the grinding force and depth to avoid excessive grinding that could cause the extremely brittle white gloss layer to peel off or be completely removed.

7. The failure analysis method for rail fracture caused by external damage according to claim 1, characterized in that: The bright white layer is mainly a cryptocrystalline martensite layer, which appears as a bright white area that is not easily corroded by conventional reagents under a microscope, and its hardness is much higher than that of the substrate.

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