A number of machined end frame steel forgings made of Cr-Si-Mn alloy showed tiny cracks during magnetic particle inspection after heat treatment. The cracks were mostly confined to base edges and fillet radius. No significant abnormality was observed in chemical composition and microstructure. SEM, optical microscopy, and gas analysis revealed that the subsurface discontinuous cracks at the bore edges and in the fillet radius of the heat-treated end frame component had occurred due to hydrogen embrittlement, and not because of faulty heat treatment. This conclusion was supported by the presence of cracklike indications in machined bore surface of the annealed part.

Two complete aircraft undercarriage-leg 2014 aluminum alloy forgings and a number of sectional ends that exhibited cracks during nondestructive testing were examined to determine the extent of damage and the type of cracking. Cracks were primarily confined to the diaphragm and adjoining wall between the steel sleeve and the steel diaphragm washer. Metallographic analysis and accelerated corrosion tests showed that the cracks had originated as stress-corrosion failures.

The repeated occurrence of random cracks in the fillet radius portion of low-alloy steel (38KhA) end frame forgings following heat treatment was investigated. Microstructural analyses were carried out on both the failed part and disks of the rolled bar from which the part was made. Subsurface cracks were found to be zigzag and discontinuous as well as intergranular in nature. A mixed mode of fracture involving ductile and brittle flat facets was observed. Micropores and rod-shaped manganese sulfide inclusions were also noted. The material had a hydrogen content of 22 ppm, and cracking was attributed to hydrogen embrittlement. Measurement of hydrogen content in the raw material prior to fabrication was recommended. Careful control of acid pickling procedures for descaling of the hot-rolled bars was also deemed necessary.

An investigation was conducted to determine the factors responsible for the occasional formation of cracks on the parting lines of medium plain carbon and low-alloy medium-carbon steel forgings. The cracks were present on as-forged parts and grew during heat treatment. Examination revealed that areas near the parting line exhibited a large grain structure not present in the forged stock. High-temperature scale was also found in the cracks. It was concluded that the cracks were caused by material being folded over the parting line. The folding occurred because of a mismatch in the forgings and from material flow during trimming and/or material flow during forging.

Two outboard main-wheel halves (aluminum alloy 2014-T6 forged) from a commercial aircraft were removed from service because of failure. One wheel half was in service for 54 days and had made 130 landings (about 1046 roll km, or 650 roll mi) when crack indications were discovered during eddy-current testing. The flange on the second wheel half failed after only 31 landings, when about 46 cm (18 in.) of the flange broke off as the aircraft was taxiing. Stains on the fracture surfaces were used to determine when cracking was initiated. The analysis (visual inspection, liquid penetrant inspection, and micrographs with deep etching in aqueous 20% sodium hydroxide) supported the conclusion that failure on both wheel halves was by fatigue caused by a forging defect resulting from abnormal transverse grain flow. The crack in the first wheel half occurred during service, and the surfaces became oxidized. Because the fracture surface of the second wheel half had chromic acid stains, it was obvious that the forging defect was open to the surface during anodizing. No recommendations were made except to notify the manufacturer.

An octagonal steel ingot weighing 13 tons made of manganese-molybdenum steel developed gaping cross-cracks on all eight sides in the forging press during initial pressure application. It was reported that the steel had been melted in a basic 12-ton arc furnace, oxygenated, furnished with 42 kg of 75% ferrosilicon and 12 kg aluminum additions, alloyed with 160 kg of 80% ferromanganese, and finally deoxidized in the ladle with 42 kg calcium silicon. For metallographic examination a plate approximately 100 mm thick was cut parallel to one of the eight planes. Platelet-like particles could be discerned on the conchoidal fracture planes with the SEM. The precipitates proved to be thin and partially transparent platelets of a hexagonal crystal lattice whose parameters resemble those of AIN. The precipitates were at least in part still undissolved in spite of the long holding period at high initial forging temperature. Another block melted under the same conditions and immediately after the defective one, was forged into a gear ring without any trouble. This ring was free of grain boundary precipitates, but it contained only 0.012 % AI and 0.0102 % N.

A cross-recessed die of D5 tool steel fractured in service. The die face was found to be subjected to shear and tensile stresses as a result of the forging pressures from the material being worked. The presence of numerous slag stringers was revealed by microscopic examination of an unetched longitudinal section taken through the die. The pattern was microscopically revealed after etching with 5 % nital to be due to severe chemical segregation or banding. Considerable variation in the hardness, corresponding to the banded and non-banded regions across the face of the specimen was observed. The fracture was found to have originated near the high-stress region of the die face examination of the fracture surface. Failure of the die was concluded to have originated in an area of abnormally high hardness which is prone to microcracking during heat treatment for this grade of tool steel

The primary purpose of this article is to describe general root causes of failure that are associated with wrought metals and metalworking. This includes a brief review of the discontinuities or imperfections that may be common sources of failure-inducing defects in the bulk working of wrought products. The article addresses the types of flaws or defects that can be introduced during the steel forging process itself, including defects originating in the ingot-casting process. Defects found in nonferrous forgings—titanium, aluminum, and copper and copper alloys—also are covered.

Field fatigue failures occurred in a hand-operated gear shift lever mechanism made of 1049 medium carbon steel hardened to 269 to 285 HB. The failures occurred in the 3.18 mm (0.127 in.) radius. Redesign increased the shift lever's diameter to 25 mm (1 in.) and the radius to 4.75 mm (0.187 in.). Also, instead of the as-forged surface, it was expedient to machine the radius. The as-forged surface at 360 MPa (52 ksi) maximum working stress would not ensure satisfactory life because the recalculated maximum stress was 390 MPa (57 ksi). However, the machined surface with a maximum working stress of 475 MPa (69 ksi) gives a safe margin above the 390 MPa (57 ksi) requirement for design stress. Interpreting these values, the forged surface should have a life expectancy of 1,000,000 cycles of stress. However, because the load cycle was somewhat uncertain, the machined radius was chosen to obtain a greater margin of safety. Redesigning eliminated the failures.

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.

During a routine shear-pin check, the end lug on the barrel of the forward canopy actuator on a naval aircraft was found to have fractured. The lug was forged from aluminum alloy 2014-T6. Investigation (visual inspection, 2x views, and 140X micrographs etched with Keller's reagent) supported the conclusion that the cause of failure was SCC resulting from exposure to a marine environment. The fracture occurred in normal operation at a point where damage from pitting and intergranular corrosion acted as a stress raiser, not because of overload. The pitting and intergranular attack on the lug were evidence that the surface protection of the part had been inadequate as manufactured or had been damaged in service and not properly repaired in routine maintenance. Recommendations included anodizing the lug and barrel in sulfuric acid and giving them a dichromate sealing treatment, followed by application of a coat of paint primer. During routine maintenance checks, a careful examination was suggested to look for damage to the protective coating, and any necessary repairs should be made by cleaning, priming, and painting. Severely corroded parts should be removed from service.

A motorboat engine connecting rod forged from carbon steel fractured in two places and cracked at the small end during service. The analysis (visual inspection, 50x micrographs of sections etched with 2% nital, magnetic-particle inspection, and metallographic examination) supported the conclusion that the connecting rods were rendered susceptible to fatigue-crack initiation and propagation by the notch effect of coarse folds formed during the forging operation. One fracture was caused by fatigue resulting from operating stresses, and the other was a secondary tensile fracture. No recommendations were made.

The cause of fracture of two piston rods of hammers of a drop forge was determined. The first rod of 180 mm diam consisted of an unalloyed steel with 0.37% C and 0.67% Mn and had a strength of 56 kp/sq mm at 26% elongation. Fatigue fractures propagated from several points which could be recognized as flaky cracks already in the fracture, and which later were united. No material defects could be detected in the cross section parallel to the fracture plane except for these very short cracks. These comparatively insignificant defects were sufficient to cause the fracture during high impact fatigue stresses in the drop forge. The second piston rod of 120 mm diam consisted of a steel with 0.25% C and 1.00% Mn. It allegedly had 57 kp/sq mm tensile strength and 26% elongation. The basic structure of the 120 mm piston rod was ferritic-pearlitic and hardness of 155 Brinell was accordingly low, corresponding to approximately 53 kp/sq mm tensile strength. The incipient fractures had no connection with the material defects in this shaft and therefore the fracture could not have been caused by them. Probably the low strength of the piston rod was insufficient for the high stresses.

The pointed ends of several stainless steel forceps split or completely fractured where split portions broke off. All the forceps were delivered in the same lot. The pointed ends of the forceps are used for probing and gripping very small objects and must be true, sound, and sharp. Analysis supported the conclusion that the failures to be the result of seams in the steel that were not joined during hot working. Recommendations included that closer inspection of the product take place at all stages of manufacturing. Inspection at the mill will minimize discrepancies at the source, and the inspection of the finished product will help detect obscure seams.

Alloy steel forgings used as structural members of a ski chair lift grip mechanism were identified to have contained forging laps (i.e., sharp-notched discontinuities) during an annual magnetic particle inspection of all chair lift grip structural members at a mountain resort. The material was confirmed to be 34Cr-Ni-Mo6. A heavy oxide on the dark area of one of the broken-open laps was revealed by scanning electron microscopy in conjunction with EDS. A bright area that contained ductile dimple rupture was observed adjacent to the dark area. The oxidized portion of the fracture was established to be the preexisting forging lap while the bright area was created during the breaking-open process. As a corrective action all forgings showing laps were recommended to be removed from service. Critical review and revision of the forging process and revisions to the nondestructive evaluation procedures at the forging supplier was recommended.

A ring clamp (8740 (AMS 6322), steel forged and cadmium plated) used for attaching ducts to an aircraft engine became loose after three hours of service. When the clamp was removed from the engine, the hinge tabs on one clamp half were found to be broken. Analysis (visual inspection and microscopic and metallographic examination) supported the conclusion that both hinge tabs on the clamp half fractured in a brittle manner as the result of gross overheating, or burning, during forging. The mechanical properties of the metal, especially toughness and ductility, were greatly reduced by burning. Evidence that burning was confined to the hinge end of the clamp indicated that the metal was overheated before or during the upset forging operation. Recommendations included notifying the supplier of the burned condition on the end of the clamp. The clamps should be macroetched before cadmium plating to detect overheating. The clamps in stock should be inspected to ensure that the metal had not been weakened by overheating during the upset forging operation.

Field failures, traced to internal cracks that were initiated from gross nonmetallics, were encountered in the upset portion of forged 4118 steel shafts. Ultrasonic inspection was thought to be the best method for detection from the location of these cracks, their orientation, and the size of the shaft. A longitudinal beam was sent in from the end of the shaft. The shaft was observed to have a radially drilled oil hole 9 mm in diam. Since there was a variation in flaw orientation, testing of the shaft was desired from both the long and short end. The rejection level was set at 20% of full screen and was based on the size of flaws observed when the shafts were cut up. The inclusions were considered to be rejectable if the size was larger than 20 mm diam. Similar flaws were observed in larger shafts, but no flaws were observed once the shafts were sectioned. It was interpreted that the flaw signals were false and had happened when a portion of the beam struck the oily surface of the longitudinal oil hole. The problem was solved by removing the oil film from the longitudinal oil hole.

Forged austenitic steel rings used on rotor shafts in two 100,000 kW generators burst from overstressing in a region of ventilation holes. A variety of causes contributed to the brittle fractures in the ductile austenitic alloy, including stress concentration by holes, work hardened metal in the bores, and a variable pattern of residual stress.

To forged AISI 4140 steel trailer kingpins fractured after 4 to 6 months of service. Fractographic and metallographic examination revealed that cracks were present in the spool-flange shoulder region of the defective kingpins prior to installation on the trailers. The cracks grew and coalesced during service. Consideration of the manufacturing process suggested that the cracks were the result of overheating of the kingpin blanks prior to forging, which was exacerbated during forging by deformation heating in the highly-strained region. This view was supported by results of two types of tensile tests conducted near the incipient melting temperature at the grain boundaries. All kingpins made by the supplier of the fractured ones were ultrasonically inspected and six more anticipated to fail were found. It was recommended that the heating of forging blanks be more carefully controlled, especially with respect to the accuracy of the optical pyrometer temperature readout. Also, procedures must be developed such that forging blanks that trigger the over-temperature alarm are reliably and permanently removed from the production line.