Published: Jan 2005
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AN UNDETECTED, OR IGNORED, INGOT RELATED defect can be expected to show up as a forging defect, and may not be revealed until late in the production process for the component. Although some ingot internal problems, such as fully enclosed solidification voids, can be healed during forging, others such as surface cracks, pouring laps, and gross piping will give rise to defective forgings. This subject was discussed in Chapters 4 and 13, but it should be noted that it is not always apparent that problems encountered in a forging were the direct result of a condition in the ingot. As an example, during magnetic particle examination of cylindrical open die forgings in a modified SAE 4330 alloy steel, several transverse indications were detected. These were about 0.75 in. (18 mm) in length, and when probed, persisted to a depth of about 0.5 in. (12 mm). At the time they were detected about 1 in. (25 mm) of stock had been removed from the original forging surface, but later tests on other forgings showed that they actually began closer to the original forged surface. Etching of the indications in situ revealed that a decarburized envelope surrounded them, suggesting that they were present during heating for forging. Curiously, it was noted during the probing operations that the length of the indications essentially did not change until they were removed. All of this directed attention to the ingots that were from a big end down fluted forging ingot mold with a top diameter of 30 in. (750 mm). Since ingot porosity was suspected, cross sections from an ingot were hot acid etched. Those sections from the upper part of the ingot showed radially disposed, cylindrical near surface pores about the same length as the indications in the forgings, while those from lower in the ingot were pore free. The origin of the magnetic particle indications was now clear. Near surface, cylindrically shaped gas pores had been formed by reaction between carbon and oxygen in the steel during the initial solidification after the manner of a rimming steel. The ferrostatic head pressure in the mold had suppressed the pore formation lower in the ingot, and during heating for forging the proximity to the surface had permitted sufficient oxidation of the internal pore surface to impede the forge welding of the pores. The fix was to correct both deoxidation and vacuum degassing procedures during steel making. If surface NDE had not been a requirement, these defects would not have been discovered before delivery.