Brittle Fracture of a Crane Hook

From: Failure Analysis: The British Engine Technical Reports, F.R. Hutchings and P.M. Unterweiser, Ed., American Society for Metals, 1981

Abstract: During the lifting of a piece of machinery by means of an overhead travelling crane the hook fractured suddenly. The load was attached to the hook by means of fiber rope slings and rupture occurred in a plane which appeared to coincide with the sling loop nearest to the back of the hook. The rated capacity of the crane was 15 tons. At the time of the mishap it was being used to lift one end of a hydraulic cylinder with a total weight of about 27 tons. Fracture was of the cleavage type throughout. There was no evidence of any prior deformation of the material in the vicinity, nor was there any indication of a pre-existing crack or major discontinuity at the point of origin. A sulphur print suggested the hook had been forged from a billet cogged down from an ingot of semi-killed steel. Failure of this hook was attributed to strain-age embrittlement of the material at the surface of the intrados.

Keywords: Cranes

Material: Semi-killed steel (Nonresulfurized carbon steel)

Failure type: Brittle fracture


During the lifting of a piece of machinery by means of an overhead travelling crane the hook fractured suddenly, resulting in serious injury to one of the workmen. The load was attached to the hook by means of fibre rope slings and rupture occurred in a plane which appeared to coincide with the sling loop nearest to the back of the hook. The rated capacity of the crane was 15 tons; at the time of the mishap it was being used to lift one end of a hydraulic cylinder with a total weight of about 27 tons, the exact weight lifted being unknown.
The hook is illustrated in Figure 1 and the leading dimensions are given in Figure 2, which also indicates the positions from which the principal specimens were cut for examination. The hook was stamped “S.W.L. 15 tons.” Based on the data given in British Standard No. 482, the safe working load of this hook varied from a minimum of about $7\genfrac{}{}{0.1ex}{}{1}{2}$ tons at the shank thread to approximately 20 tons at the point of rupture. Measurements of the pitch of the shank thread did not reveal any significant variation throughout its length, an indication that the hook had not been subjected at any time to a load sufficient to raise the stress at this part above the yield point of the material. The intrados showed evidence of general wear over the lower half, extending towards the back of the hook beyond the plane of rupture; in places very shallow grooves had developed due to abrasion between the lifting ropes and the hook, as shown in Figures 3 and 4. Neither the nature nor extent of this wear was abnormal.
Figure 1

Fig. 1  

Figure 2

Fig. 2  

Figure 3

Fig. 3  

Figure 4

Fig. 4  

As can be seen from Figure 4, the fracture was of the cleavage type throughout, there being no evidence of any prior deformation of the material in the vicinity, nor was there any indication of a pre-existing crack or major discontinuity at the point of origin. The surface of the fracture is depicted in Figure 5, this having, unfortunately, rusted slightly in places prior to receipt. The unusual islands of elliptical form to be seen in places were found to be associated with local segregation, as shown by a sulphur print. This print, Figure 6, suggests that the hook had been forged from a billet cogged down from an ingot of semi-killed steel. There was no indication of any major segregation in the region of the origin of the rupture. While it was clear from the sulphur print that the material was of a quality, not suitable for the manufacture of a highly stressed component such as a hook, it seemed unlikely that this was the primary cause of the failure.
Figure 5

Fig. 5  

Figure 6

Fig. 6  

The conditions of service to which hooks and many other parts of lifting components are subjected give rise to severe pressure and abrasion of the wearing surfaces, which become work-hardened in consequence. It is well known that a work-hardened surface is liable to initiate a fracture of the brittle type, and the primary object of the periodical heat treatment of certain lifting components is to rectify this dangerous condition. As the mode of failure of this hook suggested that the state of the surface might have played a major part, this aspect was investigated.
The hook was sectioned transversely, in the positions indicated in Figure 2, and a series of Brinell hardness measurements were carried out, using a 2 mm. ball with a load of 120 kg. The results are indicated in Figure 7. It will be noted that there is a sharp increase in the surface hardness at the transition from the sides of the hook to the working surface the values obtained on the latter are uniform, whereas at a depth of 4 mm. below this surface there is a progressive increase from either side to a maximum at the crown. It is evident that the surface layer had been work-hardened to the limit of its capacity; the depth to which the hardening extended increased progressively towards the crown, as might be expected, this being the part subjected to the maximum surface pressure in service. The radial hardness gradient was also measured, using a diamond indenter with a 3 kg load, the measurements obtained being shown by the full-line curve in Figure 7. It will be observed that near the surface the hardness gradient is very steep, the whole of the severely hardened material being confined to a layer less than 2 mm. in depth. The specimen on which these measurements were made was given a sub-critical heat treatment, being maintained at a temperature of 650°C. for a period of half-an-hour. The hardness gradient was then measured in the same manner as previously and the results are indicated by the dotted line in Figure 7, from which it will be noted that the gradient has been virtually eliminated.
Figure 7

Fig. 7  

With a view to ascertaining the effect of the hardened surface on the behaviour of the material when subjected to a bending stress, as at the time of failure, two strips were cut from the position indicated in Figure 2 with their axes parallel and in the plane of the hook. These strips were tested by bending over a former having a diameter equal to three times the specimen thickness at the centre, the wearing surface being on the tension side. The load was applied by means of a hydraulic press. The first strip was tested in the “ as received ” condition and it snapped without any significant prior deformation as soon as an appreciable load was applied, the two halves flying across the Laboratory. The second specimen was given a sub-critical anneal, being maintained at a temperature of 650° C. for a period of half-an-hour; when tested it bent to an angle of 90°, and was subsequently closed to 180°, without developing any cracks. These two specimens are depicted in Figure 8. The fracture of the first specimen was of the cleavage type and showed the same characteristics as the fracture which had occurred in service. The test was repeated on two smaller specimens with identical results. A further specimen, cut from a position about 1 in. from the wearing surface and lying within the more heavily segregated zone, was subjected to a similar test in the “ as received ” condition, and this bent to an angle of 90° without developing any cracks. It was evident from these tests that the brittle service fracture was associated with the presence of the severely work-hardened surface at the intrados of the hook.
Figure 8

Fig. 8  

In order to obtain some data on the work-hardening capacity of this material, a specimen was cut from the side of the hook and severely indented by pressing into it a hard steel ball of $\genfrac{}{}{0.1ex}{}{3}{4}$ in. diameter. As work hardening by pressure alone did not simulate the conditions to which the surface of the hook had been subjected in service, where abrasion due to sliding of steel ropes, rings, etc., over the wearing surface of the hook plays an important part, the ball was oscillated while under load. The hardness of the material prior to indentation was 148; at the impression it rose to a value of 205, a figure, it will be noted, substantially below the maximum of 241 at the wearing surface. Another specimen, 25 in. square and 5 in. long, was compressed to a length of 25 in., but this severe deformation only served to raise the hardness from 153 to 179; this specimen was then skidded, under pressure, over a hard steel plate and this had the effect of increasing the hardness to 205. Sundry other specimens tested in the same manner gave similar results; the most severe deformation and abrasion failed to produce a hardness in excess of 205 and it was evident that some other factor was responsible for the higher hardness at the wearing surface of the hook. The small rectangular specimen referred to above was therefore placed in boiling water for 10 minutes in order to ascertain whether the hardness could be increased by accelerated ageing; as a result of this treatment the hardness rose to a value of 241 and it was apparent that the hardness of the wearing surface was the result of strain-ageing.
Having established that the material of which this hook was made was prone to strain-ageing, a number of further tests were carried out, the results of the more important being summarised as follows:
The chemical composition of the material in the region of the fracture, obtained from drillings taken to a depth of $\genfrac{}{}{0.1ex}{}{3}{8}$ in. from the wearing surfaces, was as follows:
 
Per cent.
Carbon
0.16
Silicon
0.05
Manganese
0.53
Sulphur
0.043
Phosphorus
0.062
Nitrogen
0.009
The composition suggests that the material was produced by the Bessemer process. The phosphorus content is a little in excess of the amount that might be regarded as permissible in a non-segregated steel of satisfactory quality. The nitrogen content, while low for a Bessemer steel, is nevertheless slightly above what is generally regarded as the safe upper limit of 0.0075% if some degree of embrittlement is to be avoided. These figures relate, of course, to the relatively pure outer zone of the material; towards the centre the content of impurities would be appreciably higher.
Microscopical examination of a specimen cut from a position adjacent to the origin of the fracture revealed the grain size to be large, while the carbides showed a marked tendency to lie along the grain boundaries, as depicted in Figure 9; this constituent also revealed some degree of spheroidisation. In places the structure was more of the Widmanstätten type, as depicted in Figure 10. The content of non-metallic inclusions was not excessive. The microstructure indicated that the hook had cooled from a rather high finishing temperature at the time of forging and had not been given a suitable heat treatment subsequently; the spheroidisation of the carbides suggests that on some subsequent occasion it may have been subjected to a sub-critical annealing treatment.
Figure 9

Fig. 9  Etched. (×100).

Figure 10

Fig. 10  Etched. (×100).

Near the origin of the service fracture several branching cracks were to be seen which tended to run parallel with the fracture. They exhibited, in general, a tendency to pass straight across the ferrite grains, changing direction at the boundaries. This straight, transgranular cracking in a normally ductile constituent is an indication of some degree of embrittlement. A specimen cut from a position adjacent to the fracture origin was normalised at 880°C., a treatment which resulted in considerable refinement of the grain size and a more normal distribution of the carbides; the structure of this specimen is depicted in Figure 11, which also shows cracks, of the type referred to above, that were present in the pre-existing ferrite crystals.
Figure 11

Fig. 11  Etched. (×250).

The failure of this hook is attributable to strain-age embrittlement of the material at the surface of the intrados. The cause of strainage embrittlement is controversial, but there are good reasons for believing that it is associated with the oxygen, and in particular the nitrogen, content of the material. Under service conditions the surface of the intrados of the hook would be subject to compression and abrasion, with resultant work-hardening. In consequence, the yield point and the ultimate tensile strength of the material comprising the affected layer would be raised and its ductility decreased. In the case of a material prone to strain-ageing, as in this instance, the effect of this work-hardening would intensify spontaneously with time. Ultimately, a stage would be reached when the stress required to cause plastic deformation of the strain-aged material exceeded that necessary to initiate fracture; rupture of the surface material would occur, and the crack thus initiated may be expected to result in a cleavage type fracture, as occurred in the case of this hook. It is clear from the tests carried out that in this case it is the condition of the surface layer of material which determines whether or not a cleavage fracture will occur; if the surface is in a severely work-hardened and aged condition, rupture occurs in a wholly brittle manner, but ductile behaviour can be restored either by a sub-critical annealing treatment which results in recrystallisation of the work-hardened layer and the re-solution of the precipitates responsible for the ageing, or by removing the surface layer mechanically; severe work-hardening of the surface alone, i.e., not followed by ageing, was not found sufficient to initiate a cleavage fracture.
It is obvious that a material of the class from which this hook was made is quite unsuited for the manufacture of lifting components. Steel of good quality is not susceptible to brittle failure as a result of work-hardening in service and in consequence lifting components made from this material are exempted from periodical annealing called for by statutory requirements.* It is clear from this investigation, however, that such exemption is not justified if the steel of inferior quality liable to strain-age embrittlement; had this particular hook been heat-treated under suitable conditions at regular intervals there is little doubt that the accident would not have occurred. Unfortunately, there is no non-destructive test that can be applied to lifting components in the course of a routine examination to ascertain if the material is susceptible to strain-age embrittlement.

Related Information

F.L. Jamieson, Failures of Lifting Equipment, Failure Analysis and Prevention, Vol 11, ASM Handbook, ASM International, 1986, p 514–528
W.T. Becker and D. McGarry, Mechanisms and Appearances of Ductile and Brittle Fracture in Metals, Failure Analysis and Prevention, Vol 11, ASM Handbook, ASM International, 2002, p 587–626
B.A. Miller, Overload Failures, Failure Analysis and Prevention, Vol 11, ASM Handbook, ASM International, 2002, p 671–699

Footnote

*
Factories Act, 1937. Docks Regulations, 1934. Building Regulations, 1948.