Tom Bereznak, Area Manager - United States Steel Corp. Bill Heid, Project Engineer M7 Technologies

Transcription

1 TO: FROM: Tom Bereznak, Area Manager - United States Steel Corp. Dan Yemma, Engineer - M7 Technologies Bill Heid, Project Engineer M7 Technologies DATE: August 12, 2011 SUBJECT: Deburrer Gearbox Failure Analysis USS PO# INTRODUCTION US Steel issued the subject purchase order to M7 Technologies to perform an incoming inspection and failure analysis on a Deburrer Gearbox. The workscope is also to include recommended repairs and costs associated with these repairs. The gearbox experienced a field failure as the input shaft fractured on the coupling end. The input shaft failure is the focus of this report. OVERALL GEARBOX EVALUATION Incoming inspection of the gearbox did not produce evidence that could lead to identifying the failure of the input shaft. The output gear was in good condition, both input and output couplings were in good condition, as were the bearings and seals. All shafts properly turned and binding did not exist. Failed input shaft Figure 1: Photos showing the as received condition of the Deburrer Gearbox. Gears, bearings, and seals did not show sign of fatigue or wear. 1

2 Portable CMM measurements of the gearbox bearing bores did detect excess cylindricity, or out of round conditions up to M7 is unable to determine if the cylindricity resulted from the shaft failure or if it was a pre-existing condition. Figure 2: Portable CMM measurements of the Deburrer Gearbox bearing bores. The bottom number in each cylinder block reports measured cylindriicty. INPUT SHAFT EVALUATION Upon completion of the initial gearbox inspection M7 specifically focused on the failed input shaft. Visual inspection, review with a handheld digital microscope, hardness measurements, and a dye penetrant test of the fractured surfaces were tools utilized to complete the analysis. Failure point 2

3 Figure 3: Top identifying the failure point of the input shaft. Left and Right surfaces of each remaining piece of the failed input shaft. Surface hardness measurements were taken across the input shaft using a portable rebound tester. Units of measurement are Brinell Hardness. The area showing lower hardness is on the surface only due. The lower hardness is due to the weld repair that was specified for the subject gearbox. The weld has a maximum thickness of 1/16 per side, the shaft then returns to base material having hardness levels uniform with the remainder of the shaft. 243, 229, , 131, , 204, , 263, , 237, , 129, 122 Figure 4: Brinell hardness measurements of the input shaft. Appendix A shows photos obtained using a digital microscope. All photos are magnified at a scale of 60X. 3

4 FINDINGS Review of the fracture surfaces does not show sign of an impact failure. Also, signs of slow crack or indication propagation could not be detected. The fractured areas do not show sign of plastic deformation which would be a sign of failure due to excess material ductility. The photos included in Figure 3 and Appendix A provides evidence supporting possible torsional failure. The photos show an overlap of material on the edges of the shaft as well as a general swirl pattern on both fracture surfaces. The dye test proved open surfaces exist around the edge, or outer surface diameter of the shaft, while fractures, cracks, or indications did not exist throughout the body or core of the shaft diameter. Figure 5: Dye penetrant results showing open surfaces on the outer edge of the shaft. The last photo in Appendix A shows a step in the radius. This step has opportunity to create a stress riser at the fillet. Torsional load has opportunity to build up at this point making the combination of load and step in the radius a potential root cause of failure. This is also supported by the open surfaces found on this edge during the dye penetrant test. 4

5 CONCLUSION The subject failure analysis report was complete using the equipment available to M7 Technologies. The evidence reported shows signs that the shaft failure was torsional in nature. The exact cause of the torsional failure cannot be detected. It is likely the case the input shaft experienced excessively high levels of torque and/or the output shaft experienced a sudden stop while the input torque continued. RECOMMENDATIONS M7 Technologies has identified several continuous improvement opportunities we believe worth further review and discussion: - Weld Repair o When weld repairs are necessary to restore the diameter M7 recommends extending the weld through the radius onto the beginning of the diameter. This will ensure the complete radius is machined into the weld preventing a weld start/stop line or stress riser from being created in the radius. Weld up to this area, then machine to drawing specification - Radius o Review altering the geometry of the radius between the and diameters. A larger radius, compound radius, or undercut radius would allow for additional load carrying capability. - Material Grade 5

6 o M7 is certain review of material grade and strength took place during the design stage of the subject gearbox. However, review of the failed shaft from a third party would lead to suggesting the strength be reviewed. o Hardness measurements in the 240 Brinell range equate to an approximate tensile strength of 118ksi. Upgrading the material and hardness to 300 Brinell gives an approximate tensile strength of 150ksi, 340 Brinell a tensile strength of 170ksi. o M7 recognizes the challenge in altering material grade or strength due to the fact the gears on the input shaft have a carburize requirement. This can be a limiting factor when selected grade. Additional testing can be performed if desired per the remaining recommendations below. These tests will require an outside laboratory and will have additional costs associated with performing the tests. - Ductility Requirement o Destructively test a coupon from the current shaft material to determine steel ductility properties which include: elongation and reduction of area. Action to be reviewed based on results. - Perform a Hardness Trace o Obtain a slice from the shaft diameter and measure hardness every 1/8 from the surface to the core. Determine hardness decrease as measurements approach the core and if an increase in alloy material (increasing thru hardening capability) would be beneficial. - Fatigue Testing o Use a third party lab to perform torsional fatigue testing on a sample piece replicating the input shaft. Torsion inputs to be based off actual steel mill parameters and usages. 6

7 APPENDIX A Digital Microscope Photos Top view looking down on the fractured surface of the input shaft The dark edge at the top is the radius. Reference Figure 3 the photo on the left to compare orientation. Overlap of material is a sign of a torsional fracture. 7

8 Similar orientation to the photo above. 8

9 Photo is further towards the center of the shaft. A clean separation of surfaces and torsional shearing are evident. 9

10 Center, or core, of the input shaft where separation occurred. The twisting or swirl effect is believed to be from excess torsional load. 10

11 Top / separated surface of input shaft Radius Shaft diameter Side view of the input shaft. This photo shows a step where the radius joins the diameter with the diameter. 11