Failure Investigation of Cast Diesel Electric Loco Wheel +

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1 Failure Investigation of Cast Diesel Electric Loco Wheel + M. K. Mukhopadhyay Former Dy. Chief Chemist & Metallurgist in Indian Railways monoj48@gmail.com Introduction To meet the demand for Indian Railways, loco wheels both for diesel and electric are imported. In addition to BoxN wheels, RWF started production of diesel wheels in pressure die-casting route to meet partially the demand for Indian Railways. It is made from plain carbon-manganese steel micro alloyed by Mo-V. One failure of such microalloyed cast wheel is discussed in this paper. In this context it is relevant to mention that one such wheel and axle plant has been set up recently at Chhapra, Bihar to reduce the quantum of import. Failure Details One cast Diesel Electric Loco Wheel supplied by RWF to Central Railway was reported fracture from hub radially towards rim fillet during press-fitting of axle at Parel Workshop, Central Railway. After Preliminary investigation, it has been revealed that the wheel got cracked while press fitting on axle and at that time pressure was 120 MT, which is well within the specified limits of MT. The crack length was measured about 270mm emanating from bore and extending upto rim fillet. Refer to Figs.1& 2. Technical data of pressing of the wheel are as follows: Interference used: 0.43 mm General limits of Interference: 0.40 to 0.44mm Wheel bore diameter: mm. Wheel seat diameter before pressing: mm Final load reached before cracking: 122 tonne. Rate of loading: Continuous Extent of pressing length in terms of bore length before wheel cracking: 140mm. Further, the wheel was taken for detailed metallurgical investigation. Physical Observation The wheel found fractured from hub extended to plate about 14 inches and open crack visible on both front Safe reaching of bonafide passengers is the prime importance of Indian Railways and hence all components of rolling stocks are periodically checked to ensure the reliability and if required, the doubtful components are replaced immediately. One of the important components is rail wheel, which has been taken as case study for its failure. During periodic checking, a major defect has been observed on a new wheel and immediately been replaced. In addition to replacement, root cause of the defect was analysed to prevent such reoccurrence in future. This paper deals with the investigation of cause of failure and suggests preventive measures to avoid such defect in future. Wheel set comprises two wheels press-fitted on one axle. During periodic overhauling, wheels are changed depending on the wear and other defects, if any. New wheels are press-fitted at the workshop, where maintenance is undertaken. One such new wheel, during pressing, revealed crack extending upto rim. This was a micro-alloyed wheel manufactured in cast route and immediately the same was rejected and sent for investigation. During investigation, it was revealed that hub was cut in cold condition when a small crack was developed due to thermal shock. But due to compressive force in the wheel generated during rim quenching, the growth of minor crack was arrested in as-cast condition. The presence of scales might have shaded the crack from detection during Magnetic Particle Test. But the crack propagated during pressing stresses at maintenance workshop. Metallurgical investigation followed by analysis of manufacturing history reveals the cause of the defect. Remedial measures have been suggested to prevent such defect. and back sides of the wheel. No other defects were noticed on any other portion of the wheel. Since crack is open and prominent, no other physical checks carried out on the wheel other than visual inspection. The location and its orientation of the crack can be seen on Figs. 1 & 2. The defect wheel is further subjected to other destructive and metallurgical tests for further investigation. + The paper was originally presented at 62nd Indian Foundry Congress held during February 7-9, 2014 in Gandhinagar, Gujarat. 40

2 Fig-1(a): Complete view of the wheel from back plate. Fig.2a: Crack on back plate. Fig-1(b): Sectional view of the crack from back plate. Fig.2b: Crack inside bore. Fig. 2c: Crack on back hub. Verification of Residual Stress Wheel has been sliced close to the fractured area for closure test as per the IRS specification procedure and measured for closure value of 3.5 mm, which is well within the limits of specification. Visual Examination of Fractured Faces After closure test, the fractured face of the hub is opened carefully without damaging any evidence after cutting portion of wheel containing crack. Photographs of fractured slices are placed as Fig.2a, 2b and 2c. Based on the above crack appearance in Figs. 3a, 3b and 3c, it is clear that the crack is opened fully after application of mounting pressure. It was noticed while separating the slices, crack was further getting closed and the slices were separated by hammering. This reveals that some compressive stresses still exist on the hub. No plastic deformation observed indicating the brittle nature of the fracture, typical of cast steel. The black band i.e., the oxidised layer in Fig.4c suggests preexistence of crack prior to fracture during pressing. Arrow Fig 3a: Sectional view of the cracked portion after opening from back plate mark at Fig. 3c is the crack initiation point. Destructive Tests Slices are cut from the wheel under investigation to carry out destructive tests. Accordingly, three slices were cut from 41

3 Fig.3b: Sectional view of the cracked portion after opening from front plate. the wheel and subjected to mechanical and metallurgical testing, found satisfactory. Chemical Analysis Elements Carbon Manganese Silicon (min.) 0.15 Phosphorus (max.) 0.03 Sulphur (max.) 0.03 Chromium (max.) 0.25 Nickel (max.) 0.25 Copper (max.) 0.28 Molybdenum (max.) 0.06 Vanadium (max.) Permissible Percentage Aluminium (max.) 0.02* (preferable) *Aluminium upto 0.03 per cent (max.) is permitted provided that the manufacturer ensures Al inclusions (non-metallic) to not worse than 1.5 for both thick and thin series. Hydrogen in liquid steel should not exceed 2.5ppm Mechanical Properties Mechanical properties of the failed wheel indicate no deviation from the specification and fully comply with the requirements of the relevant IRS specification. Found satisfactory. } combined 0.50% (max) Fig.3c: Complete sectional view of the cracked slices separated. Hardness Survey Hardness survey has been carried out on the wheel slice taken out from the defective wheel. Hardness distribution pattern exhibits smooth transition from rim to hub. Hardness survey is found normal and typical of quenched and tempered cast wheel. Macrostructure A second slice underwent macro test. The slice etched in 1:1 HCl at 80 to 90 0 C. The central segregation found better than C1 level of ASTM E-381 standard, which is within the acceptable limits. No other macro defects were observed on the etched slice. Inclusion Rating Inclusion rating test conducted on the sample taken out from the tensile test-piece end. The sample revealed that the inclusion content is very low and no voids or sulphides found on the micro sample. Microstructure Fig.4: Inclusion content at 100X One sample was cut and taken out from the rim tensile test-piece end used for the study of microstructure. The microstructure is found uniform fine pearlitic structure of

4 ASTM grain size 7. Two more micro samples containing fractured surface, one close to inside bore and the other close to the back hub face were cut from the fractured area to study the nature and origin of the fracture. A thin decarburised layer/broken grains was observed on the edges of fractured surface at 100X but microstructure has been pearlitic which is very clear from the Figs.5a and 5b. Whereas microstructures opposite to the cracked face in the same sample Figs.5c and 5d do not show any decarburisation/distortion of the grains. Fig.5d: Microstructure opposite to fracture surface on the back hub face. Results and Discussion Fig.5a: Microstructure close to fracture on the back hub face. Fig.5b: Microstructure close to fracture surface. Fig.5c: Microstructure opposite to fracture surface inside the hub. The chemical composition conforms to the specification. The closure value of 3.5mm as against 1mm (min) specified indicates that wheel was having sufficient compressive residual stresses even after fracture. Therefore, cracks were not allowed to grow further towards radial direction upto the rim, indicating the effectiveness of rim quenching during heat treatment. Hardness pattern on slice shows the smooth transition of hardness from rim to hub. Mechanical properties of tensile strength, yield strength, elongation, impact charpy results are conforming to specification. The mechanical property of the hub, especially elongation %, indicates adequately ductile hub of cast wheel. The metallographic analysis results show conformance to the specification with respect to structure, non-metallic inclusion and grain size. The wheel is satisfactory with all the mechanical and metallographic tests and does not reveal any abnormalities in the test results which indicates that heat treatment of wheel is in order. Figure 5c shows oxidised band inside the fractured area, it gives room to pre-existence of a hot crack further supported by micro-examination and the revelation of a decarburised layer (Figs. 5a and 5b). This layer should have been of 5 considerable thickness before boring the wheel. This suggests that the generation of the crack prior to the heat treatment points to the previous operation of hub cutting as the generic location. From the process data, it is seen that the wheel is hub cut after a long time of casting and there are sufficient reasons for cold-cutting resulting in bore crack, especially on micro-alloyed wheels, which are thermally sensitive. The crack should have been narrow hairline in nature, which is left undetected during Magnaglo inspection. Further, the compressive stress available on the hub hold the crack tighter. The scales present inside the bore makes it still difficult to detect. 43

5 The chevron pattern originated from the centre of the hub and propagated towards both back hub and front hub during progressive loading of axle into the bore. It is reported that the pressing tonnage is 122T and the interference 0.43mm is within the limit. The interference appears to be on the higher side of the specified range. The microstructure examination reveals a very minor layer of decarburisation indicating HAZ due to the thermal input observed. Conclusion There was no considerable carbon depletion on the surface of fractured faces. This shows that the strength of the wheel on hub is considerably satisfactory. Mechanical and Metallurgical test results are conforming to the specification and as such, the defect is not attributable to heat treatment during manufacturing but obviously it was inflicted by the process. The wheel has undergone cold hub cutting resulting in thermal cracks during hub-cut process, which is undetected during magnetic particle inspection due to scales present inside the bore. The presence of residual stress arrested the growth of thermal cracks radially in which case the wheel would probably have broken immediately. While mounting the wheel, this minor pre-existing crack with eccentric oil groove and interference on higher side in the range may have aggravated the fracture leading to failure during pressing. Suggestions/Remedial Measures All the wheels in a specific batch undergoing cold process should be strictly and periodically tested for MPI on hub area. Henceforth, no wheel should be cold processed during manufacture in hot line area. Any abnormalities found on heat treatment of wheels during normalising and quenching, such wheels should be monitored and removed from the line without going to tempering furnace for reprocess. Ensure scales inside the bore are cleaned properly by adjusting velocity of steel shots in cleaning machine before loading to magnetic particle inspection. All Micro-alloyed wheels should be re-inspected for bore cracks after final boring operation since these wheels are thermally sensitive to develop cracks. All the operators involved on hot line and heat treatment area should be trained and instructed with immediate effect for corrective and preventive action. It is suggested that the mean interference may be set while pressing cast wheels to avoid sudden impact loading. 44