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.

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.

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 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.

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 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.