Backwall loss in a forged shaft
A 42CrMo4 forged shaft, 300 mm diameter and 500 mm long, is examined from the cylindrical surface with a 2 MHz, 20 mm diameter straight-beam probe. In a clean area the backwall echo is 80% FSH. Over a 100 mm long zone the backwall echo falls to 30% FSH, and a cluster of small echoes at 10-30% FSH appears between 80 and 120 mm depth. There is no single large indication. The shaft has a 5 mm machining allowance on the radius and the client wants to accept it without further machining. The forging specification requires freedom from internal defects greater than 3 mm equivalent flat-bottom hole. You must decide whether the backwall loss indicates a rejectable defect and what additional testing is justified.
1. How do you interpret the 50% backwall echo loss and the cluster? What causes other than defects could produce it?
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A fall from 80% to 30% FSH is about 8.5 dB and is too large to dismiss. It can be caused by a defect that scatters or blocks sound, but also by curvature, poor coupling, a rough or tapered surface, coarse grain or banding in the forging. The cluster at 10-30% FSH is not by itself a 3 mm FBH-sized defect; dispersed inclusions or forging bursts often look like this. Check reproducibility: repeat at the same point, rotate the probe, move around the circumference and scan from the opposite end. Compare the backwall at the same depth in adjacent clean zones. If the loss is a geometry or coupling artefact it should change with probe orientation or position; a real internal reflector should be consistent. A 300 mm diameter with a 20 mm crystal also gives some curvature loss, so the reference backwall should be taken on the same surface.
2. What additional ultrasonic tests and acceptance logic would you use before deciding?
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Before acceptance, map the zone with at least two frequencies, 1 MHz and 4 MHz, and a focused probe, from both directions. Use angle-beam scanning to detect radial or circumferential planar defects that a straight beam may miss. Size the cluster by DGS or 6 dB drop against the 3 mm FBH reference; if any coherent reflector exceeds 3 mm FBH equivalent, reject. If the cluster stays well below that and the backwall loss is shown to be due to attenuation or curvature, the forging may be acceptable, but the loss itself can mask defects, so the 5 mm machining allowance should be used: machine and re-test, or accept only with a documented engineering assessment. Do not accept solely because no single large echo is seen.