Failure Analysis of a Hammer Drill Shaft Under Complex Loading Paths and Severe Environmental Conditions. P. A. Manohar

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1 Materials Science Forum Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Failure Analysis of a Hammer Drill Shaft Under Complex Loading Paths and Severe Environmental Conditions P. A. Manohar Robert Morris University, Engineering Department, JJ 130, 6001 University Boulevard, Moon Township, PA , USA Manohar@rmu.edu Keywords: Hammer Drill, Torsional Fatigue, Rotating Bending Fatigue, Machining Marks Abstract. This paper describes the failure investigation of a tubular shaft that is part of a hammer drill assembly. The failure investigation was particularly challenging as the fracture surfaces were completely damaged during and subsequent to the failure process. However, careful examination of the component and its assembly revealed many clues that pointed to the root causes of failure. It was determined that the shaft was subjected to impact, fatigue, bending and torsional loads simultaneously at elevated temperatures. The basic failure mode was identified as a combination of torsional fatigue and rotating bending fatigue failure that originated on the inside diameter of the shaft. The root causes were determined to be operational overload in combination with rough machining marks on the bore surface and higher than necessary operating torque required to overcome the dry adhesive friction in the system. The preventative measures recommended were many-fold including improving surface finish on the bore diameter, reducing dry sliding friction, decreasing the overall level of dynamic loads by appropriate design changes and adding a surface strengthening heat treatment Introduction The hammer drill shaft is a tubular shaft and a key component of an assembly called Hydraulic Rotation Percussion Hammer. The function of the percussion hammer is to drill tap holes in the blast furnace. The failure of the shaft was somewhat baffling because it occurred on a section that is supposed to have been rotating freely with no major torsional or bending loads acting upon it. The fracture plane was almost perpendicular to the longitudinal axis, which meant that the shaft failed either in a single event of torsional overload or in fatigue. The fracture surfaces were completely damaged during and subsequent to the failure. The failure mode identification was thus the most challenging task, akin to catching a thief who left no fingerprints! In general, it is difficult resolve torsional ductile failure from torsional fatigue failure because of heavy localized shear deformation involved in both the fracture processes. Nevertheless, it is crucial to be able do so because the determination of root causes dictates the choice of preventative measures. This paper focuses on finding pieces of secondary evidence so that the two failure modes are resolved with a greater certainty and appropriate preventive measures are recommended. The blast furnace contains molten iron and slag operating at high internal temperature (~2500 o F). The percussion hammer receives radiative heat from the blast furnace and therefore operates at somewhat elevated temperature (estimated to be o F). The shaft is made from AISI 4340 material, hardened and tempered in the range HRC. The schematic arrangement of the percussion hammer assembly is shown in Figure 1. The basic operation of the percussion hammer involves a pneumatic system that impacts upon a shaft at a frequency of over two thousand beats per minute. A hydraulic system rotates the shaft simultaneously at a speed greater than 200 rpm. A drill body is mounted on the worm threads on the shaft via a coupling. It was informed that the design of the shaft was changed recently where the length of the shaft from the splined flange to one end (at All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (# , Pennsylvania State University, University Park, USA-19/09/16,09:40:24)

2 3890 THERMEC 2009 drill side) was increased by 50%. Metallurgical design parameters were not changed after this modification. The failed shaft was of the modified design that failed within four weeks of service. The shafts were expected to give a service life of the order one year or greater based on past experience. Therefore, the failure of the subject shaft was considered premature. Impact Load Sleeve Torsional Load Impact Bushing Hammer Drill Shaft Drill Body Coupling Rotating Bending Load (a) Failure Location (b) Figure 1: Schematic diagram showing failure location on the shaft with respect to the hammer drill assembly. The shaft is impacted upon by a pneumatic piston on smaller diameter (~ 45 mm), rotated by a hydraulic system via the splines on the shaft. Fig. 1(a) and 1(b) show the location of the shaft before and after one stroke. The bending load is due to the inertia of the shaft and the drill body mounted on the larger diameter (~ 57 mm,). The shaft has a central hole through which a metallic tube is inserted until the location where bore diameters change. The tube carries a mixture of air and water that is delivered at the cutting face. Visual Examination The failure location on the small diameter as shown in Figure 1b. This failure location is at the entry surface during the return stroke of the shaft when the section remains unsupported. The overall view of the failed shaft is shown in Figure 2a. It is evident from Fig. 2a that the shaft does not exhibit any gross plastic deformation. A detailed view of the fracture surface is shown in Figure 2b. It is clear from Fig. 2b that fractographic featyures have been completely damaged due to rubbing that occurred during the failure process.

3 Materials Science Forum Vols (a) (b) Figure 2: (a) Photograph showing the failed shaft. No gross plastic deformation of the shaft is evident. The fracture plane is inclined ~ 1-2o to the transverse plane. Some post-failure rusting of the splines is evident, and (b) Photograph showing the fracture surface on the small diameter of the shaft. The fracture surface is completely damaged by metal smearing caused by rubbing during and after the failure. The fracture plane is flat and nearly perpendicular to the longitudinal axis of the shaft. Shear lips are visible on the outside periphery of the fracture surface. Note the jagged edge and longitudinal cracks on the ID of the shaft. The shaft exhibited dry adhesive wear at several locations. Scoring and spalling due to adhesive wear on the shaft is shown in Figure 3. (a) (b) (c) Figure 3: (a) Scoring and spalling due to dry adhesive wear on the small diameter of the shaft, left edge of the photograph represents the fracture plane, (b) stereomicroscopic view of the area in (a) indicating circumferential and longitudinal contact pattern; and (c) adhesive spalling on the large diameter of the shaft. The surface roughness of the bore of the small diameter indicates the presence of deep machining marks and cracks as shown in Figure 4. (a) (b) Figure 4: (a), Scanning Electron Microscope (SEM) image of the bore surface exhibiting the presence of deep machining marks on the bore surface. and (b) SEM image of the cracks at bore surface. The cracks are longitudinally oriented and perpendicular to the fracture plane.

4 3892 THERMEC 2009 Metallurgical Examination The shaft was sectioned for examination of microstructures under light optical microscope. Metallographic sections were taken parallel and perpendicular to the fracture surface. The microstructure of the shaft just beneath the fracture surface is shown in Figure 5. Figure 5: Light optical micrograph showing fine tempered martensite microstructure beneath the fracture surface. Note the twisting of the grains close to fracture surface. No decarburization is evident. 200X, 2.5% Nital. Full size tensile and impact samples were machined longitudinally from the shaft and it was determined that the UTS was 1496 MPa, 0.2% proof strength to be 1365 MPa, % elongation (%E) of 14% and % reduction in area (%RA) of 48%. These parameters are acceptable for the material grade and heat treatment specified. The impact testing was performed using Charpy V notch geometry using longitudinal specimen orientation at 100 o C. The results of the impact test showed that the average energy absorbed was 20.3 J and 40% of the fracture surface with shear appearance. Typical value of absorbed energy at 100 o C is reported [1] to be around 22 J at a UTS of 1517 MPa. Therefore, it is clear that the impact energy absorbed by the shaft material is quite reasonable for the material grade and heat treatment specified. The hardness of the shaft was found to be uniform in the range HRC as required. Chemical analysis confirmed the composition of the shaft to be according to AISI 4340 specifications. Discussion Identification of the Failure Mode. The nearly flat and transverse nature of the fracture plane along with the fact that there is no gross plastic deformation of the shaft suggests that the basic failure mode is one of the following three alternatives or a combination of some of them: torsional ductile (shear) failure, torsional fatigue or rotating bending fatigue.while fractographic examination was not possible due to damage, some pointers presented here identify the nature of the failure: Shear lip exists on the outside diameter of the shaft (Fig. 2b) this means that the fracture process terminated at the OD. The shear lips form in ductile materials under the conditions of gross shear overload. Metal crystals exhibit significant twisting just beneath the fracture plane (Fig. 5) indicating torsional nature of the stresses that caused the failure. The fracture plane is inclined about 1-2 o to the transverse plane, which means that bending loads have also contributed to the torsional failure it is not a purely torsional failure. Deep machining marks (Fig. 8) exist just below the fracture on bore surface such machining marks act as stress raisers and help initiate fatigue failures. Presence of secondary cracking on the fracture surface at the inside bore (Fig. 4) suggests that the failure initiated at the bore surface.

5 Materials Science Forum Vols Thus it appears that the failure initiated at the ID in fatigue, grew outwards and terminated at the OD. The stresses that caused fatigue failure are of mainly torsional nature with an additional bending component. The torsional stresses are generated most likely due to the dry friction in the system while the bending stresses are a result of the inertial mass of the system acting upon the unsupported section of the shaft during the return stroke. Single-overload torsional shear failure mode is ruled out because such failures initiate at the outer fiber and grow towards the center. Choice of Material and Heat Treatment. AISI/SAE 4340 combines deep hardenability with high ductility, toughness, and strength. It has high fatigue, creep resistance and hish temperature strength. It is immune to temper embrittlement. It is used where severe service conditions exist and where high strength in heavy sections is required. Grade 4340 can be heat-treated to various levels of tensile strengths in the range MPa [2]. The specified strength range for the shaft MPa appears to be within the typical levels at which this material is usually employed. In this regard, the current choice of material and the heat treatment condition seems to be reasonable. Design Aspects. The location of failure on the shaft with respect to the assembly is an important factor that needs to be discussed. According to the weakest link theory, the failure usually occrs at a location where stress first exceeds the strength [3]. In the present case, the shaft failed at a location that is not supported during the return stroke of the shaft. This situation contributes to the more bending stresses on this section. In such a case, the operating stresses may become just high enough to make the machining marks on the bore surface more critical. The stress intensification caused by the machining marks may exceed the fatigue limit of the metal on the diameter leading to fatigue failure. It is also noteworthy that the bore diameter in the small section is twice the size compared to that in the larger diameter (See Fig. 1). Reduced wall thickness in the small diameter also means a steep stress gradient in this section. Fatigue limit of metals depends on their tensile strength, type of loading, part size and surface roughness. As an example consider the following equation [4] to determine the design fatigue limit of a metal: Design Fatigue Limit (ksi) = K 1 * K d * K s * i (1) Where, K 1 is the correction factor for type of loading (= 0.58 for torsional loading), K d is the correction factor for specimen size (= 0.9 for torsional loading of shaft 10.0 mm < Shaft Diameter 50.0 mm), K s is the correction factor for surface roughness (= 0.55 for rough machined part, 1.0 for mirror finish) and N i is the fatigue limit determined from laboratory tests. The fatigue limit, i, of 4340 at MPa tensile strength is about MPa [4]. It is clear from Eq. (1) that a high surface roughness under torsional loading of shafts can decrease the fatigue limit of the 4340 steel shaft to as low as 1/3 rd of its value determined in laboratory testing. Appropriate design changes such as decreasing the length of the shaft from splined flange to drill body, decreasing the bore size in the small diameter to increase the wall thickness and improve lubrication on the shaft to relieve dry adhesive wear conditions could thus be made to decrease the overall level of operating stresses on the shaft. Surface finish on the bore surface needs to be improved. Fatigue strength of the shaft can be improved further by an additional nitriding heat treatment

6 3894 THERMEC 2009 Conclusions 1. The basic failure mode is determined to be a combination of torsional fatigue and rotating bending fatigue failure modes, and not a single-event torsional overload failure. This conclusion is supported by a number of other observations including the existence of shear lips on the outer periphery of the fracture plane, the significant twisting of metal crystals just beneath the fracture surface, the presence of deep machining marks on bore surface, failure location being situated on the unsupported section during the return stroke, and the presence of ratchets on the fracture surface close to the bore 2. The primary cause of the failure of the shaft is identified to be the high operating stresses on the shaft that exceed fatigue limit of the metal at failure location. The stresses consist of a range of dynamic loads generated during the operation of the shaft including torsional, rotating bending, fluctuating compressive, frictional and vibrational stresses. In addition, the overall stress level is amplified further by the following factors: Stress concentration due to deep machining marks in the bore Higher than necessary torque required to overcome dry friction in the system due to a complete lack of lubrication on the shaft surface Stress concentration at fillet radius regions at the root of the worm threads. Significant circumferential cracking of the worm threads also indicates a condition of operational overload Recommendations The recommendation consists of several measures to decrease the overall stress on the shaft and to relieve the stress concentration such as: Reduce stress concentration by improving surface finish on the bore Decrease frictional stresses by applying a suitable high performance lubricant on the shaft surface Reduce the overall stress level on the shaft by decreasing the length of the shaft from splined flange to drill head, and Relieve the steep stress gradient in the shaft wall thickness by decreasing the bore size which increases wall thickness Monitoring of operational parameters is also important to make sure that the drilling speed and feed are within the recommended limits and that sufficient care is taken to handle the sudden overload situation caused by the jamming of the cutter body. Finally, the fatigue resistance of the shaft could be improved further by a suitable surface heat treatment such as nitriding rather than shot peening or induction hardening treatments that would not strengthen the bore surface. References [1] G. J. Roe and B. L. Bramfitt: Notch Toughness of Steels, 10 th Ed., Vol. 1: Irons, Steels and High-Performance Alloys, ASM International, pp , (1990). [2] T. V. Philip and T. J. McCaffrey: Ultrahigh-Strength Steels, ASM Handbook, 10 th Ed., Vol. 1: Irons, Steels and High-Performance Alloys, ASM International, pp , (1990). [3] D. J. Wulpi: Understanding How Components Fail, 2 nd ed., ASM International, (2000). [4] B. Boardman: Fatigue Resistance of Steels, ASM Handbook, 10 th Ed., Vol. 1: Irons, Steels and High-Performance Alloys, ASM International, pp , (1990).