HT Consultants


1. Introduction
1.1: A request was received for an investigation to be carried out on the subject crankshafts to determine why they had fractured after approximately one year in service (It was reported that pumps of the same type are still running satisfactorily after approaching ten years in service).
1.2: Information provided with the fractured shafts stated that they had been manufactured from rolled low alloy steel bar to specification BS970 817M40 supplied in the annealed condition and subsequently heat treated by oil quenching from 850°C followed by tempering at 650°C. This heat treatment would be expected to give a tensile strength of approximately 1000 N/mm² (65 tonf/in²). In order to produce the crankshafts the bar would need to be greater than 225 mm in diameter.
2. Examination and Experimental Results: Visual and macroscopic examination of the crankshafts in the as received condition gave the following information:
(a): The crankshafts were three throw and both had fractured at the junction of the connecting rod big end journal with the related crank on one side at the centre throw position. The crankshaft bearing journal had been cut from the same end on both shafts. The fracture was at the journal to crank junction furthest from this end.
(b): The fracture faces on both shafts were typical of those produced by a high cycle, low stress fatigue mechanism (see Plates 01, 02, 11 and 12). Multiple fatigue initiation points were present in the journal to crank junctions in both cases.
(c): The side faces of the cranks had been machined by turning down to protruding lands at each end of each big end journal. These lands, a generous blend-in radius and the journals had been finished by grinding.
(d): The faces of the lands and the blend-in radii had been finished by flank grinding and there was considerable evidence of 'chatter' in the radius.
2.2: Vickers hardness tests were carried out on the turned side face of the crank and on the surface of the ground flank at the fracture position on Crankshaft 2 with the following results:

Ground land
305, 311, 402 HV(2.5)

Turned surface; 311, 322, 322 HV(2.5)

The scatter in hardness at the land position indicates that machining abuse (grinding burns) may have taken place.
2.3: The journal surface and the surface of the blend-in radius and adjacent ground land at the fracture position on Crankshaft 1 were etched in 5% aqueous nitric acid. This clearly showed that rehardening and temper burns were present in the surface of the land and that untempered martensite was present in the surface of the radius (see Plates 04 to 06 inclusive). It must be remembered that abusively machined surfaces contain residual stresses resulting from both the thermal disturbance and differences in volume between untempered and tempered martensite. In addition, a layer of very soft material is always present under the untempered martensite due to the temperature gradient between the surface and the core material when the grinding abuse takes place. Both these conditions considerably reduce fatigue resistance.
2.4: Examination of a microsection prepared from material cut from the position on Crankshaft 1 indicated on Plates 03 and 06 showed that the material was coarse grained and that it contained non-metallic inclusions which were mainly of manganese sulphide (see Plate 7). Such inclusions form stress raisers where they break out to the surface in sensitive regions, e.g. journal to crank blend-in radii and can also reduce fatigue resistance.
2.5: Following etching to confirm the presence of grinding 'burns', a magnetic crack test was carried out on one of the other big end journals on Crankshaft 1. A large crack was indicated at the edge of the blend-in radius (see Plates 08 to 10 inclusive).
3. Conclusions
3.1: It is considered that the fractures on both the subject crankshafts were the result of a high cycle, low stress fatigue mechanism in service. It is also considered that the resistance of the shafts to the normal fatigue loading experienced in service had been considerably reduced by the presence of machining abuse (in this case grinding 'burns') in the blend-in radii between the big end journals and the adjacent cranks where fracture initiated.