Investigation of crack-like defect in 15CDV6 (ESR) steel forging for aerospace applications

•A crack-like defect was discovered during the final machining of a 15CDV6 steel dome.•The defect surface displayed the presence of oxide scale and a decarburization layer.•A pre-existing oxide scale, formed during the soaking process, became encapsulated within the steel during forging.•It is recommended to eliminate any deep depressions, laps, or folds before initiating the forging operation. During final machining of High-altitude Escape Solid Motor (HEM) head end dome forging made of 15CDV6 high strength low alloy (HSLA) steel, a crack-like defect was observed. The defect was through thickness and clearly visible on the dome profile from both inner diameter (ID) side and outer diameter (OD) side. The length of the defect was 38 mm at the ID side and 34 mm at the OD side. The forging was realized from Electroslag remelted (ESR) 15CDV6 steel and was in heat treated condition. Detailed metallurgical failure analysis was carried out to identify the nature and root cause of the crack-like defect, assessing the possibility of defect generation right from the melting stage to fabrication stage on the basis of evidences/results. Metallography analysis on the opened defect revealed continuous oxide scale present throughout the defect surface. Extensive decarburization was also observed on the surface next to the oxide scale. The observations revealed the presence of an oxide scale with a thickness of approximately 18 µm, an oxygen-enriched layer measuring 35 µm, and extensive decarburization extending up to 1360 µm from the edge of the defect. Based on the metallurgical analysis and evidences, the root cause of the defect is found to be a pre-existing surface defect, which got oxidised during preheating (soaking), then embedded inside during forging and opened out during machining operation as part of HEM dome realization. The evaluation of the penetration of surface defects within the forging has also been carried out through simulation.