Abstract: The surface cracks of railway passenger car bearing rollers were analyzed using methods such as magnetic particle inspection, cold acid washing, and metallographic inspection. The results showed that secondary quenching burns generated during the roller manufacturing process were the main cause of roller rolling surface cracks. By taking measures such as reducing the grinding allowance of the roller end face, increasing the grinding allowance of the outer diameter, and strengthening the management of the sleeve, the burn caused by secondary quenching has been eliminated.
Keywords: Railway passenger car bearings; Roller; Grinding; Burns; crack
During normal maintenance of NJ (P) 3226X1 railway passenger car roller bearings after running for a maintenance period, magnetic particle inspection revealed one or several cracks roughly distributed along the axial direction on individual roller rolling surfaces. The roller material is GCr15, and the longest installation and use time of the roller is 18 months. The maximum driving distance of the passenger car has reached 700000 kilometers. The following text provides a detailed analysis of four cracked roller samples.
1. Inspection and analysis
1.1 Macroscopic morphology observation
Macroscopic observation and magnetic particle inspection were conducted on the crack morphology of 4 samples (numbered 1 # to 4 #), and no other forms of damage were found on their rolling surfaces and end faces except for the cracks. The crack morphology of rollers 1 # to 4 # is shown in Figure 1, where a total of 10 cracks with a length of 4.7-9 mm were observed on roller 1 #; Four cracks with a length of 4-9.5mm were observed on the 2 # roller; A total of 11 cracks with a length of 3-20 mm were observed on the 3 # roller; Two cracks with lengths of 40 and 45 mm were observed on the 4 # roller.
1.2 Routine project inspection
According to the quality control standards of TB/T 3010-2001 "Technical Conditions for Ordering High Carbon Chromium Bearing Steel for Rolling Bearings of Railway Vehicles", JB/T 1255-2001 "Technical Conditions for Heat Treatment of High Carbon Chromium Bearing Steel Rolling Bearing Parts", and TB/T 2235-2010 "Rolling Bearings of Railway Vehicles", the chemical composition, hardness, inclusions, quenching and tempering structures of the four rollers were inspected, The results indicate that the raw materials and heat treatment quality of the rollers are within the qualified range and meet the relevant standard requirements.
1.3 Pickling inspection
Four cracked rollers were subjected to cold acid washing according to JB/T 1255-2001 standard. The results showed that there were multiple light gray white or bright white marks along the circumferential direction on the rolling surfaces of the four rollers. The edges of the bright white area were surrounded by dark bands, and the crack positions were all in the light gray white or bright white areas, which were typical burn morphology. The burn morphology of the 3 # and 4 # rollers after acid washing is shown in Figure 2 The area indicated by the arrow in Figure 3.
Figure 1 Crack morphology of rollers
Figure 2 Burn morphology of 3 # roller after cold acid washing
Figure 3 Burn morphology of 4 # roller after cold acid washing
1.4 Fracture analysis
Using a wire cutting device, the 4 # roller was horizontally cut open. While protecting the original crack from being damaged, the other end of the crack on the 4 # roller was artificially cut open. The fracture morphology after opening is shown in Figure 4. Observing the fracture surface, it can be observed that there are radial prisms on both sides of the elliptical box (corresponding to the surface gray white area marked by the elliptical box in Figure 3), as indicated by the arrow in the figure. Based on their direction, it can be determined that the burn area on the roller surface is the crack source area.
Figure 4 Fracture morphology of 4 # roller crack
1.5 Metallographic structure inspection on both sides of the crack
Select the bright white area along the circumference of roller 4 # and cut it horizontally, grind it, and use 4% nitric acid alcohol corrosion to place it under a microscope for observation. It is found that there is no decarburization on both sides of the crack, and the extremely thin white bright layer on the surface is a secondary quenched martensitic structure, with a depth of about 10-30 μ m. Underneath the surface is a high-temperature tempered structure, with a depth of approximately 70-140 μ m. The core of the matrix is a normal metallographic structure. The crack starts from the secondary quenching of the martensitic white bright layer on the surface and extends towards the matrix. The metallographic structure on both sides of the 4 # roller is shown in Figure 5. From the above inspection results, it can be seen that one or several cracks on the rolling surface of individual rollers in railway passenger car bearings are related to the secondary quenching burn on the surface of the rollers.
Figure 5 Metallographic Structure on Both Sides of Roller 4 #
2. Analysis of burn causes
2.1 Investigation and analysis of the use of cracked rollers
Railway passenger car bearings are key components of vehicle bogies, installed in pairs in axle boxes and lubricated with railway grade IV grease. They must withstand certain radial and axial loads. Common faults include peeling of the working surface, fatigue cracks, pitting, and electrical corrosion marks. In response to the occurrence of roller cracks, on-site investigation found that except for a few rollers with burns and cracks, the appearance, lubricating grease color, and lubrication status of other parts of the bearing were normal. After magnetic particle testing, it was determined that no cracks were found on the inner and outer ring surfaces; According to the temperature data recorded by the vehicle axle temperature alarm, the temperature rise of the axle box bearings is also normal. The rolling surface of the cracked roller was corroded with a 3% nitric acid alcohol solution, and there were no signs of electrical corrosion on the rolling surface after observation under a 400x microscope. Due to the fact that the crack morphology of the faulty roller is a longitudinal straight crack with varying lengths, and the location where it occurs is also on the rolling surface, the distribution pattern of the burn layer is also different from that caused by electrical corrosion (the distribution pattern of the electrical corrosion layer is mostly circular arc). Therefore, it can be ruled out that the possibility of roller rolling surface burn and crack is caused by poor lubrication or electrical corrosion during bearing operation. Therefore, it can be determined that the burn on the rolling surface of the roller is generated during the manufacturing process, and the cracking of the rolling surface of the roller may be due to the initiation and propagation of fatigue cracks at the burned area under alternating contact stress.
2.2 Mechanism and characteristics of grinding burn formation
When bearing parts are ground after heat treatment, the grinding heat causes high temperatures on the surface of the parts, resulting in local changes in the surface structure and properties. This type of defect is usually referred to as grinding burn. Grinding burns are generally divided into two types: one is high-temperature tempering burns; Another type is secondary quenching burn.
The temperature range generated by high-temperature tempering type burns is from above the tempering temperature of the parts to below the critical point temperature of steel phase transformation Ac1, approximately between 200 and 745 ℃. At this temperature, the shallow martensite and residual austenite structure will decompose and transform into martensite or martensite structure. This type of structure has poor acid corrosion resistance and appears dark black after cold acid washing. Therefore, high-temperature tempering type burns are also known as "black burns".
The local instantaneous high temperature generated by the secondary quenching type burn is above the critical point temperature of steel phase transformation Ac1, about 800 ℃. The shallow martensitic structure undergoes phase transformation, transforming into austenite, and then being cooled and re quenched by cutting fluid to form a secondary quenching martensitic layer. This structure is not easily corroded by acid, so after cold acid washing, the burn surface appears gray white or bright white, and the surrounding area appears dark black. Therefore, Secondary quenching burn, also known as "white burn". The secondary quenching burn tissue will generate significant tensile stress on the surface of the workpiece, which can induce cracks under certain conditions. During the grinding process, the burn area caused by secondary quenching is prone to generating grinding cracks under the action of grinding stress. Usually, grinding cracks are very small and cannot be detected by naked eye observation. Magnetic particle testing must be used to identify them.
2.3 Investigation of Burns on Roller Processing Surface
The surface of roller grinding includes an outer diameter surface and two end faces. The outer diameter surface grinding process uses a through type centerless grinder for continuous grinding, which is divided into four processes: coarse grinding, fine grinding, fine grinding, and ultra precision, with a total of 10 rounds of grinding and ultra machining completed.
The outer diameter surface of the roller is processed using a through type centerless grinder, with a small feed rate and good cooling conditions, and generally does not produce secondary quenching burns. The precision grinding outer diameter process adopts a fully automatic CNC centerless grinder, which has automatic diagnosis, alarm, and protection functions. In case of any abnormalities during the processing, the equipment will issue a stop command, and it is also impossible to produce circumferential grinding burns. Investigating the rough grinding outer diameter process, a total of 5009 roller acid washing inspection records were reviewed, and no grinding burns were found.
The roller end face processing adopts a horizontal double end face grinder to simultaneously grind both end faces, divided into coarse grinding and fine grinding processes. After investigation, it was found that there were traces of steel sleeve pulling on the outer diameter surface of the rollers ground by the M775B1 horizontal axis double end face grinder. After fine grinding of the outer diameter and acid washing inspection, it was found that there were burns on the outer diameter surface of the rollers.
2.4 Reproduction test
A simulation grinding test plan has been developed for the characteristics of centerless through grinding, which involves artificially increasing the grinding amount and speed, not timely repairing the grinding wheel, shutting down or reducing the cutting fluid flow rate, or sudden power outage during grinding, resulting in a rapid increase in temperature in the grinding area. After multiple tests, only the outer diameter surface of the roller has been found to be bitten and the surface has spiral black burns. However, a simulation experiment was conducted using a more severe grinding condition on a double end face grinder (using experimental conditions such as increasing the amount of end face grinding, using worn old sleeves, reducing cutting fluid flow, and grinding wheel passivation), and a certain width of burn marks were generated on the outer diameter surface of the roller (Figure 6), indicating that grinding the double end face on a horizontal axis double end face grinder will cause secondary quenching burns on the outer diameter surface of the roller.
Figure 6 Burn marks generated by grinding the double end face
Through in-depth analysis of the grinding principle and characteristics of the double end face grinding process, it was found that during the grinding of rollers, the rollers enter 25 cylindrical sleeves on the circular grinding disc of the grinding machine. The grinding disc rotates and sends the rollers into the grinding area of the grinding wheel. The grinding disc and the grinding wheel rotate relative to each other, and the left and right grinding wheels simultaneously grind the two end faces. During grinding, the outer diameter surface of the roller generates significant friction with the inner wall of the steel sleeve, resulting in instantaneous high temperature, Due to the small gap between the sleeve and the roller, it is difficult for the cutting fluid to cool in a timely manner. When the grinding condition deteriorates (such as excessive machining allowance on the end face), the temperature will quickly rise above the critical point temperature of the steel phase transformation. When the roller is quickly cooled by the reflux of the cutting fluid in the lower part of the grinding disc, it will produce secondary quenching type burns. If the surface allowance is too small in the subsequent outer diameter machining process, The burn layer tissue was not removed and remained on the surface of the roller.
3. Preventive measures
When using a horizontal axis double end face grinder to grind the roller end face, the outer diameter surface of the roller is prone to burns. The method to prevent burns is to reduce the frictional heat generated by the inner wall of the sleeve and the outer diameter surface of the roller. The recommended preventive measures are:
(1) Adjust the process flow and increase the grinding allowance for the outer diameter of the roller after grinding the double end face. Increase the coarse grinding outer diameter process and adjust the original coarse grinding double end face process and fine grinding double end face process to the coarse grinding outer diameter process; Before the rough grinding outer diameter process, adjust the outer diameter grinding allowance after grinding the double end face from 0.06-0.08 mm to 0.18-0.20 mm to remove the burn tissue layer on the outer diameter surface of the roller;
(2) Reduce the grinding allowance of the double end face of the roller. The double end face allowance has been adjusted from 0.45-0.60 mm to 0.20-0.40 mm, and a reasonable turning length dimension has been formulated. The size change rate after heat treatment is strictly controlled to ensure that the roller length dimension entering the double end face grinding process is controlled within the standard tolerance range, reducing cutting force and friction on the inner wall of the sleeve;
(3) Improve the sleeve structure. Make a cutting fluid pouring port on the outer surface of the sleeve, which can allow the cutting fluid to flow smoothly into the inner wall of the sleeve during processing, reduce frictional heat, and avoid surface burns;
(4) Strengthen the management of important tools such as sleeves and strictly implement the regulations for replacing sleeves. When the number of rollers processed in the double end face grinding process reaches 200000 to 250000 or the end face runout of the rollers exceeds the tolerance, all sleeves must be replaced; When the outer diameter surface of the roller after double end grinding is still burned after acid washing inspection (until the rough grinding outer diameter process), all sleeves should be replaced;
(5) Improve process documents and standardize pickling inspection standards. Specify the timing, frequency, and quantity of acid washing inspections, clarify the requirements for handling burns found during acid washing, and promptly and accurately identify burn quality issues.
2023 December 2nd Week VAFEM Product Recommendation:
Agricultural machinery requires better seals for contaminated conditions, provisions for misalignment, and economical mountings for shafts and housings. VAFEM offers bearings for disc harrows, planters, mowers, rakes, all types of balers, tillage, seeding and forage harvester equipment, combines, cotton pickers/stripers, and other agricultural equipment like silo unloaders and grain elevators.
We also offer a complete line of insert ball bearings, shaft mounted with either tapered adapters or locking collar or set screws. Our proven seal designs include low torque shields to heavy-duty triple lip seals. Disc harrow bearings are also available with either round, square, or hexagonal bores. All bearings are available with or without re-lubrication holes.
https://www.vafem.com/products/Agricultural-Bearings/775.html
2023-12-20