A jack cylinder split open during simulated service testing. The intended internal test pressurization was reportedly analogous to typical service. The material and mechanical properties of the cylinder pipe were unknown, although subsequent testing showed that the pipe satisfied the requirements for a grade 1045 medium-carbon, plain carbon steel. Investigation (visual inspection, chemical analysis, 2% nital etched 119x images, and tension testing) supported the conclusion that the cylinder pipe burst in a mixed brittle-ductile manner due to overpressurization. It is likely that the bearing strength of the pipe was slightly compromised by a low-strength layer of decarburization. Recommendations included evaluating the testing procedure for the possibility of inadvertent overpressurization and analyzing successfully tested cylinders to identify changes in material, and perhaps heat treatment, that may have contributed to this failure.

A drawing plant which processed steel wire of designation 105 Cr 2 for ball bearings had losses due to crack formation and wire breakage during drawing. To establish the reason for the breakage, seven fractures were submitted for investigation with contiguous wire segments on both sides of the fracture of 300 mm each. Missing in the lamellar surface structure, with the exception of the remnants of a coarse network, were the pre-eutectically precipitated carbides to be expected in this steel. Surrounding the ferritic region in the surface structure, a ring of lamellar pearlite is seen, which turns into the granular annealed structure towards the core. The described structural phenomena were noted in all of the seven fracture regions. Their intensity always decreased with increasing distance from the fracture. Surface decarburization caused the formation of lamellar pearlite during annealing. This investigation further revealed that the localized decarburization and pearlite formation was present already in the rolled wire in uneven distribution over the entire coil length.

One percent Cr-Mo low alloy constructional steel is widely used for high tensile applications, e.g., for manufacture of high tensile fasteners, heat treated shafts and axles, for automobile applications such as track pins for high duty tracked vehicles etc. The steel is fairly through hardening and heat treatment does not present any serious difficulty. Care is still required in processing to avoid decarburization. In an application of track pins for tracked vehicles, bars about 22 mm diam were required in heat treated and centerless-ground condition prior to induction hardening of the surface. Indifferent results were obtained in induction hardening; cracks were noticed, and patchy hardness figures were obtained on the final product in several batches. Metallographic examination of transverse sections through the defective areas showed decarburization to varying degrees, i.e., from partial to total decarburization. Observations suggested the defects originated at the stages of ingot making and rolling. This was apparently the reason for complete decarburization of the area with original surface defect which opened up further in the oxidizing atmosphere of the furnace with low melting clinkers from scale and furnace lining filling up the crevice of the original defect.

A plant had manufactured and heat treated their product in house for years. As time went on, the special steel that they had been using became more expensive, and a switch was made to a more common and less highly alloyed material. However, no change in hardness specifications were made, because calculations of ideal critical diameter and analysis of available hardenability data indicated that the original hardness specification could be met. There was, however, less room for process variation. The parts ended up containing temper carbides, developed heavy decarburization, and experienced excessive distortion because they were left in the furnace for extended and varying periods with the temperature “turned down a couple hundred degrees.”

A pushrod made by inertia welding two rough bored pieces of bar stock installed in a mud pump fractured after two weeks in service. The flange portion was made of 94B17 steel, and the shaft was made of 8620 steel. It was disclosed by visual examination that the fracture occurred in the shaft portion at the intersection of a 1.3 cm thick wall and a tapered surface at the bottom of the hole. The fatigue crack was influenced by one-way bending stresses initiated at the inner surface and progressed around the entire inner circumference. A heavily decarburized layer was detected on the inner surface of the flange portion and sharp corner was found at the intersection of the sidewall and bottom of the hole. It was concluded that the stress raiser due to the abrupt section change was accentuated by decarburized layer. As a corrective measure, the design of the pushrod was changed to a one-piece forging and circulation of atmosphere during heat treatment was permitted through a hole drilled in the flange end of the rod to avoid decarburization.

A bolt manufacturer observed that products made from certain shipments of steel 41 Cr4 wire were prone to the formation of quench cracks in their rolled threads. The affected wire was tested and found to be highly sensitive to overheating because of the metallurgical method by which it was produced. A stronger decarburization of the case was a contributing factor that could not be prevented by working because the thread was rolled. Hardening tests conducted by the bolt manufacturer showed that quench cracks did not occur in specimens that were turned down before hardening and when notches were machined instead of beaten with a chisel.

Some examples of equipment failures involving high temperature operation are presented. They include some steam generator superheater components and a pump shaft that should not have been at high temperature. Metallographic analysis is used to determine the causes of failure in each case.

An aircraft engine in which an in-flight fire had occurred was dismantled and examined. A bracket assembly fabricated from 2024 aluminum, one of several failed components, was of prime interest because of apparent heat damage. Scanning electron microscopy was used to compare laboratory-induced fractures made at room and elevated temperatures with the bracket failure. The service failure exhibited grain separation and loss of delineation of the grain boundaries due to melting. SEM revealed deep voids between grains and tendrils that connected grains, which resulted from surface tension during melting. Microscopic examination of polished, etched section through the fractured surface verified intergranular separation and breakdown of grain facets. The absence of any reduction of thickness on the bracket assembly at the point of fracture, along with evidence of intense heat at this point, indicated that little stress had been applied to the part. Comparisons of the service failure and laboratory-induced failures in conjunction with macroscopic and metallographic observations showed that the bracket assembly failed because an intense, localized flame had melted the material.

Welds in two CMo steel catalytic gas-oil desulfurizer reactors cracked under hydrogen pressure-temperature conditions that would not have been predicted by the June 1977 revision of the Nelson Curve for that material. Evidence of severe cracking was found in five weld-joint areas during examination of a naphtha desulfurizer by ultrasonic shear wave techniques. Defect indications were found in longitudinal and circumferential seam welds of the ASTM A204, grade A, steel sheet. The vessel was found to have a type 405 stainless steel liner for corrosion protection that was spot welded to the base metal and all vessel welds were found to be overlaid with type 309 stainless steel. Long longitudinal cracks in the weld metal, as well as transverse cracks were exposed after the weld overlay was ground off. A decarburized region on either side of the crack was revealed by metallurgical examination of a cross section of a longitudinal crack. It was concluded that the damage was caused by a form of hydrogen attack. Installation of a used Cr-Mo steel vessel with a type 347 stainless steel weld overlay was suggested as a corrective action.

Unalloyed steels and the pure nickel steels frequently used in the past can sustain significant damage from hydrogen attack in ammoniacal environments. The attack causes decarburization that leads to a loosening of the structure due to the precipitation of methane along grain boundaries. It occurs between 200 and 300 deg C, depending on hydrogen pressure. Parts of an apparatus that operate in these types of environments must be checked constantly if they are not made from hydrogen-resistant steel. The results of two such examinations serve to illustrate the challenges.

Decarburization of steel may occur as skin decarburization by gases either wet or containing oxygen, and as a deep ongoing destruction of the material by hydrogen under high pressure. Guidelines are given for recognizing decarburization and determining at what point cracks occurred. How decarburization changes workpiece properties and the case of hydrogen decarburization are addressed through examples.

The fractured end of a piston rod of a hydraulic press failed in line with the leading face of the piston retaining nut. Although the nut apparently had been seated uniformly, the face was polished, indicating that relative movement between it and the piston had taken place. Failure resulted from the culmination of two principal fatigue cracks which developed on approximately parallel planes from the roots of adjacent threads. A longitudinal section through the screw thread on the piston rod showed it had been carburized but not hardened, and that subsequent surface de-carburization to a depth of approximately 0.001 in. had occurred. It was concluded that insufficient tightening, as evidenced by the polish markings, was the main reason for failure, the portion of the rod therefore being subjected to a greater variation of cyclic stress during operation. The presence of the de-carburized layer lowered its resistance to the initiation of a fatigue crack to that of iron, considerably less than the resistance of the mild steel from which the rod was made and well below that shown by the carburized layer.

Evidence of destructive pitting on the gear teeth (AMS 6263 steel) in the area of the pitchline was exhibited by an idler gear for the generator drive of an aircraft engine following test-stand engine testing. The case hardness was investigated to be lower than specified and it was suggested that it had resulted from surface defects. A decarburized surface layer and subsurface oxidation in the vicinity of pitting were revealed by metallographic examination of the 2% nital etched gear tooth sample. It was concluded that pitting had resulted as a combination of both the defects. The causes for the defects were reported based on previous investigation of heat treatment facilities. Oxide layer was caused by inadequate purging of air before carburization while decarburization was attributed to defects in the copper plating applied to the gear for its protection during austenitizing in an exothermic atmosphere. It was recommended that steps be taken during heat treatment to ensure neither of the two occurred.

A connecting rod from a failed engine ruptured in fatigue without evidence of excessive stresses, detonation, overheating, or oil starvation. The origin of the fatigue failure was completely mutilated but decarburization was observed. Significant amounts of decarburization (0.010 to 0.015 in.) were found also in other forgings, such as exhaust rocker arms, main rotor drag brace clevises, bolts of carriage diagonal struts, and spring legs of main landing gears. The failure mode was low-stress, high-cycle fatigue involving tension and bending loads. The main cause was a manufacturing deficiency. The usual way to eliminate decarburization is to machine off the soft skin or employ better quality control when making them. Many aircraft manufacturers employ forged parts with machined surfaces or with shot-peened as-forged surfaces without excessive decarburization.

In a spring leg of a main landing gear, large brittle fracture zones indicated a predominately cleavage pattern with some ductile dimples, and a tiny fatigue segment disclosed fine striations. Factors influencing failure were surface decarburization, notch sensitivity of the modified SAE 6150 spring steel, Canada's cold weather which may have had an embrittling effect on the steel, and cumulative fatigue damage from severe landing loads during service life. Replacement with heavier-duty spring legs will probably not eliminate this type of failure, but their use has reduced the number of failures substantially. Precautionary measures recommended to preclude accidents include removal of decarburization, proper operation of main landing gears, and adequate magnetic particle inspection of the legs at the beginning and end of the ski season to detect any fatigue cracks that might develop in attachment holes.

Breech bolt assemblies from the Gatling guns used on fighter aircraft failed during firing tests. Metallography of the failed components revealed considerable decarburization which resulted in a loss of surface hardness. It was also determined that the maraging steel components were not in the nitrided condition as was required. This resulted in lower wear and fatigue resistance. These components also had a silicon content nearly double of that specified. The high silicon content lowered the notch tensile strength and toughness of the components.

A 75 mm OD x 7.4 mm wall thickness carbon steel boiler tube ruptured. A substantial degree of corrosion on the water-side surface leaving a rough area in the immediate vicinity of the rupture was revealed by visual examination. Decarburization and extensive discontinuous intergranular cracking was revealed by microscopic examination of a cross section through the tube wall at the fracture. It was concluded that the rupture occurred because of hydrogen damage involving the formation of methane by the reaction of dissolved hydrogen with carbon in the steel. Hydrogen was produced by the chemical reaction that corroded the internal tube surface. Steel embrittled by hydrogen can be restored only if grain boundary cracking or decarburization had not occurred but since the material embrittled in this manner, its replacement was recommended.