revealed"woody"fracture features(Fig. 12b), as a result of decohesion between a high volume fraction of manganese sulfide stringers and the matrix(Fig. 13). The matrix fracture features showed ductile dimple rupture Fig. 12 Crack (a) and broken-open fracture surface(b)of failed wedge-shaped middle tapered ring.6× Fig. 13 Higher-magnification view of fracture surface shown in Fig. 12 at origin of cracking arrows indicate large manganese sulfide inclusion at origin Chemical analysis of the material revealed a resulfurized grade of carbon steel (Sae type 1144, UNS G11440),as required by the manufacturer. This type of steel is marketed as having a rather unusual combination of high strength and high machinability. The source of the high strength is in the carbon content and the cold-drawing process used to produce the bar material, giving rise to enhanced longitudinal tensile properties. The high volume fraction of manganese sulfide inclusions(Fig. 14)impart the high machinability properties, due to the well-documented enhancement to chipmaking
revealed “woody” fracture features (Fig. 12b), as a result of decohesion between a high volume fraction of manganese sulfide stringers and the matrix (Fig. 13). The matrix fracture features showed ductile dimple rupture. Fig. 12 Crack (a) and broken-open fracture surface (b) of failed wedge-shaped middle tapered ring. 6× Fig. 13 Higher-magnification view of fracture surface shown in Fig. 12 at origin of cracking. Arrows indicate large manganese sulfide inclusion at origin. Chemical analysis of the material revealed a resulfurized grade of carbon steel (SAE type 1144, UNS G11440), as required by the manufacturer. This type of steel is marketed as having a rather unusual combination of high strength and high machinability. The source of the high strength is in the carbon content and the cold-drawing process used to produce the bar material, giving rise to enhanced longitudinal tensile properties. The high volume fraction of manganese sulfide inclusions (Fig. 14) impart the high machinability properties, due to the well-documented enhancement to chipmaking
luring machining. The trade-off to this combination of properties, however, is the loss of transverse properties, including th, ductility, and tough ess Fig. 14 Significant volume fraction of manganese sulfide inclusions in wedge-shaped tapered ring microstructure. 73x Analysis of the forces present in the tapered-ring locking device revealed that when the fastening screws were torqued, a significant hoop stress was placed on the middle rings due to the wedging action between the inner and outer rings as well as the relatively small cross section of the middle rings at the fastener holes(see Fig. 11). Since the large inclusion was present at the minimum section thickness zone of the middle ring, the stresses applied to the middle rings during norma torquing caused failure at the inclusion. Since the material contained a high volume fraction of these inclusions, this material choice was not appropriate for this application. The material was weak in an orientation of relatively high stress ailure prevention recommendations involved specification of a nonresulfurized grade of a low-alloy steel Example 2 illustrates some of the complexity and subtlety of RCA. The material was no doubt chosen for its ease of machining. The designer may not have been heavily involved in the material specification or may not have realized the sensitivity of this particular design to material anisotropy. The material itself was not defective or bad, and the part design was reasonable too, except for the material selection, which turned out to be the critical factor in this case Material Defects Unacceptable imperfections or discontinuities in materials are defects, and some types of imperfections may be generally detrimental to the performance or appearance of a product or system. Some of the classical types of material discontinuities that have been identified as causal factor(s)in failures include Metal product form Types of discontinuities Forgin Bursts Flakes Thefileisdownloadedfromwww.bzfxw.com
during machining. The trade-off to this combination of properties, however, is the loss of transverse properties, including strength, ductility, and toughness. Fig. 14 Significant volume fraction of manganese sulfide inclusions in wedge-shaped tapered ring microstructure. 73× Analysis of the forces present in the tapered-ring locking device revealed that when the fastening screws were torqued, a significant hoop stress was placed on the middle rings due to the wedging action between the inner and outer rings as well as the relatively small cross section of the middle rings at the fastener holes (see Fig. 11). Since the large inclusion was present at the minimum section thickness zone of the middle ring, the stresses applied to the middle rings during normal torquing caused failure at the inclusion. Since the material contained a high volume fraction of these inclusions, this material choice was not appropriate for this application. The material was weak in an orientation of relatively high stress. Failure prevention recommendations involved specification of a nonresulfurized grade of a low-alloy steel. Example 2 illustrates some of the complexity and subtlety of RCA. The material was no doubt chosen for its ease of machining. The designer may not have been heavily involved in the material specification or may not have realized the sensitivity of this particular design to material anisotropy. The material itself was not defective or bad, and the part design was reasonable too, except for the material selection, which turned out to be the critical factor in this case. Material Defects Unacceptable imperfections or discontinuities in materials are defects, and some types of imperfections may be generally detrimental to the performance or appearance of a product or system. Some of the classical types of material discontinuities that have been identified as causal factor(s) in failures include: Metal product form Types of discontinuities Forgings Laps Bursts Flakes Segregation The file is downloaded from www.bzfxw.com
Metal product form Types of discontinuities Castings Porosity, gas, and microshrinkage Segrega Cold shuts Plate and sheet Edge cracking Lamination Extrusions and drawn products Edge cracking More detailed descriptions, with physical characteristics and mechanisms for the creation of these defects, are contained in subsequent sections of this Volume. Problems that may develop during subsequent processing, such as heat treating and welding are discussed in the section "Manufacturing/Installation Defects" in this article These material defects can be generally described as discontinuities that degrade the performance of a product in some way. Despite measures taken to control, document, measure, analyze, and improve the processes involved in manufacturing the metal product(such as in TQM and Six Sigma systems), material defects occur. Many defective products are prevented from leaving the mill, foundry, or forge through diligence in adhering to internal procedures and uality-assurance systems. Yet defective materials are sometimes delivered Depending on the criticality, periodic field inspection may be required and may reveal defects not previously identified. A case study of one such occurrence lustrates the effectiveness of a maintenance plan that includes periodic inspection chair lift grip mechs Laps in Ski Chair Lift Grip Components. Alloy steel forgings used as structural members of a ski Example 3: Forg hanism identified to have contained forging laps during an annual magnetic particle inspection of all chair lift grip structural members at a mountain resort. A lap in one of the lift grip components(Fig. 15)measured 4.8 mm( in )long on the surface. An example of the metallurgical cross section through a similar la ap is provided in Fi Ig. 16. In accordance with the AsTM standard for magnetic particle inspection, the paint on the forgings was stripped prior to performing the magnetic particle inspection, since the thickness of the paint slightly exceeded the maximum allowable 0.05 mm(0.002 in )thick paint layer. It should be noted that prior annual inspections, performed at a contracted magnetic particle inspection facility, revealed no significant indications on these forgings. However, the paint was not stripped prior to the magnetic particle inspection at that time
Metal product form Types of discontinuities Cavity shrinkage Centerline pipe Parting line grain flow Inclusions Castings Porosity, gas, and microshrinkage Cavity shrinkage Segregation Cold shuts Inclusions Plate and sheet Edge cracking Laminations Flakes Extrusions and drawn products Edge cracking Seams Steps Central bursts More detailed descriptions, with physical characteristics and mechanisms for the creation of these defects, are contained in subsequent sections of this Volume. Problems that may develop during subsequent processing, such as heat treating and welding, are discussed in the section “Manufacturing/Installation Defects” in this article. These material defects can be generally described as discontinuities that degrade the performance of a product in some way. Despite measures taken to control, document, measure, analyze, and improve the processes involved in manufacturing the metal product (such as in TQM and Six Sigma systems), material defects occur. Many defective products are prevented from leaving the mill, foundry, or forge through diligence in adhering to internal procedures and quality-assurance systems. Yet defective materials are sometimes delivered. Depending on the criticality, periodic field inspection may be required and may reveal defects not previously identified. A case study of one such occurrence illustrates the effectiveness of a maintenance plan that includes periodic inspection. Example 3: Forging Laps in Ski Chair Lift Grip Components. Alloy steel forgings used as structural members of a ski chair lift grip mechanism were identified to have contained forging laps during an annual magnetic particle inspection of all chair lift grip structural members at a mountain resort. A lap in one of the lift grip components (Fig. 15) measured 4.8 mm ( 3 16 in.) long on the surface. An example of the metallurgical cross section through a similar lap is provided in Fig. 16. In accordance with the ASTM standard for magnetic particle inspection, the paint on the forgings was stripped prior to performing the magnetic particle inspection, since the thickness of the paint slightly exceeded the maximum allowable 0.05 mm (0.002 in.) thick paint layer. It should be noted that prior annual inspections, performed at a contracted magnetic particle inspection facility, revealed no significant indications on these forgings. However, the paint was not stripped prior to the magnetic particle inspection at that time
Fig. 15 Forging lap on ski lift fixed jaw Fig. 16 Microstructure of forging lap in another ski lift grip component. As-polished 111 The presence of the laps, which are rejectable according to the manufacturer's drawings, indicates the forgings were delivered from the manufacturer in this condition. Aside from the obvious procedural roots related to the quality system of the manufacturer, the present issue was whether or not the laps (i.e., sharp-notched discontinuities) had"grown"in progressive manner, such as by fatigue or stress-corrosion cracking, during the five years that the components had been The material was confirmed to be 34CrNiMo6(a European Cr-Ni-Mo alloy steel containing 0. 34%C), as required. The broken-open lap(Fig. 17)revealed a darkened area on the fracture surface that was consistent with the dimensions of the lap. The darkened area extended 0.89 mm(0.035 in )deep. Adjacent to the darkened area, a small area of bright, fibrous fracture features was observed, as well as a transition to a bright, faceted fracture appearance. Scanning electron microscope examination in conjunction with energy-dispersive x-ray spectroscopy(EDS)revealed a heavy oxide on the dark area of the fracture surface(Fig. 18). The bright area adjacent to the dark area contained ductile dimple rupture, which changed to cleavage fracture beyond this area. It was determined through stereomicroscopy, fractography, and metallography that the oxidized portion of the fracture was the preexisting forging lap and that both bright fracture areas were created in the laboratory during the breaking-open process. a cross-sectional view of the broken-open lap is shown Fig. 19, depicting the field of oxides in the material beneath the lap surface Thefileisdownloadedfromwww.bzfxw.com
Fig. 15 Forging lap on ski lift fixed jaw Fig. 16 Microstructure of forging lap in another ski lift grip component. As-polished. 111× The presence of the laps, which are rejectable according to the manufacturer's drawings, indicates the forgings were delivered from the manufacturer in this condition. Aside from the obvious procedural roots related to the quality system of the manufacturer, the present issue was whether or not the laps (i.e., sharp-notched discontinuities) had “grown” in a progressive manner, such as by fatigue or stress-corrosion cracking, during the five years that the components had been in service. The material was confirmed to be 34CrNiMo6 (a European Cr-Ni-Mo alloy steel containing 0.34% C), as required. The broken-open lap (Fig. 17) revealed a darkened area on the fracture surface that was consistent with the dimensions of the lap. The darkened area extended 0.89 mm (0.035 in.) deep. Adjacent to the darkened area, a small area of bright, fibrous fracture features was observed, as well as a transition to a bright, faceted fracture appearance. Scanning electron microscope examination in conjunction with energy-dispersive x-ray spectroscopy (EDS) revealed a heavy oxide on the dark area of the fracture surface (Fig. 18). The bright area adjacent to the dark area contained ductile dimple rupture, which changed to cleavage fracture beyond this area. It was determined through stereomicroscopy, fractography, and metallography that the oxidized portion of the fracture was the preexisting forging lap and that both bright fracture areas were created in the laboratory during the breaking-open process. A cross-sectional view of the broken-open lap is shown in Fig. 19, depicting the field of oxides in the material beneath the lap surface. The file is downloaded from www.bzfxw.com
Fig. 17 Broken-open lap 6x um Fig 18 Scanning electron micrograph of surface features in dark area. Fig. 19 Micrograph of lap. As polished 58x This case is particularly significant in that it is a successful example of failure prevention through periodic field inspections. The previously unknown defects were discovered only after magnetic particle inspection procedures adhering
Fig. 17 Broken-open lap. 6× Fig. 18 Scanning electron micrograph of surface features in dark area. Fig. 19 Micrograph of lap. As polished. 58× This case is particularly significant in that it is a successful example of failure prevention through periodic field inspections. The previously unknown defects were discovered only after magnetic particle inspection procedures adhering