Search Results for nonmetallic inclusions

The master connecting rod of a reciprocating aircraft engine revealed cracks during routine inspection. The rods were forged from 4337 (AMS 6412) steel and heat treated to a specified hardness of 36 to 40 HRC. H-shaped cracks in the wall between the knuckle-pin flanges were revealed by visual examination. The cracks were originated as circumferential cracks and then propagated transversely into the bearing-bore wall. No inclusions in the master rod were detected by magnetic-particle and x-ray inspection. Three large inclusions lying approximately parallel to the grain direction and fatigue beach marks around two of the inclusions were revealed by macroscopic examination of the fracture surface. Large nonmetallic inclusions that consisted of heavy concentrations of aluminum oxide (Al2O3) were revealed by microscopic examination of a section through the fracture origin. The forging vendors were notified about the excess size of the nonmetallic inclusions in the master connecting rods and a nondestructive-testing procedure for detection of large nonmetallic inclusions was established.

The 1040 steel crankshaft in a reciprocating engine cracked within one year of operation. The journals of the main and crankpin bearings were inspected by the magnetic-particle method. Three to six indications of 1.5 to 9.5 mm long discontinuities were observed in at least four of the main-bearing journals. A crack along the fillet, almost entirely through the web, was observed in one of the main-bearing journals. Numerous coarse segregates, identified as sulfide inclusions, were identified by macroetching the surface during metallographic examination of a section taken through the main-bearing journal at the primary crack. Fatigue cracking with low-stress high-cycle characteristics was disclosed during macroscopic examination of the crack surface. Sulfide inclusions, which acted as stress raisers, were found to be present in the region where cracking originated. As a corrective measure, ultrasonic inspection was used in addition to magnetic-particle inspection to detect discontinuities.

An aircraft was grounded when illumination of the transmission oil-pressure light and an accompanying drop in pressure on the oil-pressure gage was reported by the pilot. No discrepancy in the bearing assemblies and related components was revealed by teardown analysis of the transmission. The center bearing of the transmission input-shaft ball-bearing stack had a broken cage and one ball was found to have been split into several pieces. Several scored balls and flaking damage in the raceways of the inner and outer rings was observed. The origin (area in rectangle) was oriented axially in the raceway and was flanked by areas of markedly different-textured flaking damage. Stringers of nonmetallic inclusions were revealed at the origin during metallographic examination of a section parallel to the axially oriented origin. Thus it was concluded that the failure was caused by contact fatigue mechanism (flaking) activated by the subsurface nonmetallic inclusions.

The failure of a high-speed pinion shaft from a marine diesel engine was investigated. The shaft, which had been in service for more than 30 years, failed shortly after the bearings were replaced. Examination of the shaft revealed cyclic fatigue, with a substantial distribution of nonmetallic inclusions near the fracture initiation site. Fracture mechanics analysis indicated that, if stresses acting on the shaft were induced only by normal service loads, there was little likelihood that the inclusions served as failure initiation sites. Further examination of the bearing elements revealed an abnormal wear pattern, consistent with the application of elevated bending loads. The root cause of failure was determined to be an increase in service stresses after bearing replacement along with the presence of nonmetallic inclusions in the shaft.

The failure of a high speed steel twist drill which caused injury to the user was investigated thoroughly to settle a legal suit. The drill was being used to remove a stud that broke in the vertical wall of a metalworking machine (upsetter) after drilling a pilot hole. The drill had shattered suddenly with a bang which caused a chip to be dislodged and cause the injury. A large nonmetallic inclusion parallel to the axis near the center of the drill was revealed in an unetched longitudinal section. Carbide bands in a martensitic matrix were indicated in an etched sample. It was concluded by the plaintiff's metallurgist that the failed drill was defective as the steel contained nonmetallic inclusions and carbide segregation which made it brittle. It was revealed by the defendant that the twist drill met all specifications of M1 high-speed steel and investigated several other drills without failure to prove that the failure was caused by use in excessive conditions. It was revealed by examination that the point of the broken drill was not the original point put on at manufacture but came from regrinding. Both technical and legal details have been discussed.

A pilot-valve bushing fractured after only a few hours of service. In operation, the bushing was subjected to torsional stresses with possible slight bending stresses. A slight misalignment occurred in the assembly before fracture. The bushing was made of 8617 steel and was case hardened to a depth of 0.13 to 0.4 mm (0.005 to 0.015 in.) by carbonitriding. Specifications required that the part be carbonitrided, cooled, rehardened by quenching from 790 deg C (1450 deg F), then tempered at about 175 deg C (350 deg F). Visual examination, hardness testing, and metallographic and microstructural investigation supported the conclusion that the bushing fractured in fatigue because of a highly stressed case-hardened surface of unsatisfactory microstructure and subsurface nonmetallic inclusions. Cracks initiated at the highly stressed surface and propagated across the section as a result of cyclic loading. The precise cause of the unsatisfactory microstructure of the carbonitrided case could not be determined, but it was apparent that heat-treating specifications had not been closely followed. Recommendations included that inspection procedures be modified to avoid the use of steel containing nonmetallic stringer inclusions and that specifications for carbonitriding, hardening, and tempering be rigorously observed.

The cause of the fatigue failure in the retaining ring of the compressor region of an aero-engine turbine was found to be the presence of a high concentration of nonmetallic inclusions. The results of chemical analysis were used to estimate the phases present. The most frequently observed inclusions were spinel solid solutions of the type MO middot; N2O3, where M = Fe, Mn, or Mg and N = Cr or Al. The detrimental inclusions were corundum, calcium aluminates, cristobalite, and silicates. The most detrimental phases were traced on the surfaces of the specimens fractured using impact loading; the comparison is being made with the polished surfaces and the tensile specimen fracture surfaces. The inclusions in the failed retaining ring were compared with the ones in a similar component obtained from a used engine. In the case of the latter, a large number of fine and elongated (Mn, Cr, Fe)S inclusions were present along with spinels. The nondeformable, rigid oxide particles are considered more undesirable than the sulfides as far as fatigue life of the component is concerned. It has been reported that the presence of sulfides may eliminate the stresses due to oxides.

Pressure testing of a batch of AISI type 316L stainless steel thermowells intended for use in a nuclear power-plant resulted in the identification of leakage at the tips in 20% of the parts. Radiography at the tip region of representative thermowells showed linear indications along the axes. SEM examination revealed the presence of longitudinally oriented nonmetallic inclusions that were partly retained and partly dislodged. Electron-dispersive x-ray analysis indicated that the inclusions were composed of CaO. Based on the overall chemistry of the inclusion sites, the source of the CaO was determined to be slag entrapment during the steel making process. It was recommended that the thermowell blanks be ultrasonically tested prior to machining and that the design be modified to make internal pressurization possible.

Wire ropes, pulleys, counterweights, and connecting systems are used for auto tensioning of contact wires of electric railways. A wire rope in one such auto tensioning system suffered premature failure. Failure investigation revealed fatigue cracks initiating at nonmetallic inclusions near the surface of individual wire strands in the rope. The inclusions were identified as Al-Ca-Ti silicates in a large number of stringers, and some oxide and nitride inclusions were also found. The wire used in the rope did not conform to the composition specified for AISI 316 grade steel, nor did it satisfy the minimum tensile strength requirements. Failure of the wire rope was found to be due to fatigue; however, the ultimate fracture of the rope was the result of overload that occurred after fatigue failure had reduced the number of wire strands supporting the load.

A steel lifting eye, manufactured from grade 1144 steel, failed during service. The eye ring fractured in two places, adjacent to the threaded shank and diametrically opposite to this region. Woody overload features, typical for resulfurized steels were revealed by SEM. The directionality of the features was found to be suggestive of shear overload. It was observed that fracture preferentially followed the nonmetallic inclusions. The fracture was revealed to be parallel to the direction of the manganese sulfide stringer inclusions. The presence of significant banding of the ferrite and pearlite microstructure was revealed by etching. It was also observed that the fracture is primarily along the inclusions and through bands of ferrite. It was concluded that the lifting eye failed as a result of overload. Fracture occurred parallel to the rolling direction, through manganese-sulfide stringers and ferrite bands in the base metal matrix. The material used for this application was very anisotropic, exhibiting substantially poorer long and short transverse mechanical properties than longitudinal properties.

Two A6 tool steel (free machining grade) shafts, parts of a clamping device used for bending 5.7 cm OD tubing on an 8.6 cm radius, failed simultaneously under a maximum clamping force of 54,430 kg. The shaft was imposed with cyclic tensile stresses due to the clamping force and unidirectional bending stresses resulting from the nature of operation. Nonmetallic oxide-sulfide segregation was indicated by microscopic examination of the edge of the fracture surface. Both smooth and granular areas were revealed on visual examination of the fracture. The shaft was subjected to a low overstress as the smooth-textured fatigue zone was relatively large compared with the crystalline textured coarse final-fracture zone. The fatigue crack was nucleated by the nonmetallic inclusion that intersected the surface and initiated in the 0.25 mm radius fillet at a change in section due to stress concentration. To minimize this stress concentration, a larger radius fillet shaft at the critical change in section was suggested as corrective measure.

Carbon steel axle forgings were rejected due to internal cracks observed during final machining. To determine the cause of the cracks, the preforms of the forging were analyzed in detail at each stage of the forging. The analysis revealed a large central burst in the intermediate stage of the forging preform, which subsequently increased in the final stage. A high upset strain during forging, especially in the final stage, accentuated the center burst by high lateral flow of the metal. It was concluded that the center burst of the axle forging resulted from a high concentration of nonmetallic inclusions in the central portion of the raw bar stock rather than the usual problem of improper forging temperature. Strict control over the inclusion content in the raw material by changing the vendor eliminated the problem.

Two outer valve springs made from air-melted 6150 pretempered steel wire broke during production engine testing. The springs were 50 mm in OD and 64 mm in free length, had five coils and squared-and-ground ends, and were made of 5.5 mm diam wire. It was revealed that fracture was nucleated by an apparent longitudinal subsurface defect. The defect was revealed by microscopic examination to be a large pocket of nonmetallic inclusions (alumina and silicate particles) at the origin of the fracture. Partial decarburization of the steel was observed at the periphery of the pocket of inclusions. Torsional fracture was indicated by the presence of beach marks at a 45 deg angle to the wire axis. It was established that the spring fractured by fatigue nucleated at the subsurface defect.