An electric transport vehicle, similar to an electric trolley or subway rail car, experienced frequent breakdowns due to in-service fractures of torsion springs that support the weight of an overhead electric pickup assembly. Scanning electron microscopy and metallographic examinations determined that the fractures stemmed from electric arc damage. Intergranular quench cracks in the transformed untempered martensite on the surface of the spring provided crack initiations that propagated during operation causing fatigue fracture.

A steel spring used in an automotive application suddenly began to fail in the field, although “nothing had changed” in the fabrication process. Fatigue tests using springs fabricated prior to field failures lasted 500,000 cycles to failure, whereas fatigue tests performed on springs fabricated after field failures lasted only 50,000 cycles to failure. It was discovered that the percent coverage of shot peening prior and subsequent to the increase in failure incidence was much less than 100%, with a shot peening time of 12 min. The residual-stress state of “as fabricated” springs in three conditions were evaluated using XRD: springs manufactured prior to failure incidence increase, 12 min peen; springs manufactured following failure incidence increase, 12 min peen; and 60 min peen. The conclusion was that the failure occurred because low peening time significantly decreased the compressive residual-stress levels in the springs. Recommendation was made to increase the time the spring was shot peened from 12 to 60 min.

Decarburization of steel may occur as skin decarburization by gases either wet or containing oxygen, and as a deep ongoing destruction of the material by hydrogen under high pressure. Guidelines are given for recognizing decarburization and determining at what point cracks occurred. How decarburization changes workpiece properties and the case of hydrogen decarburization are addressed through examples.

A cadmium-plated music-wire return spring that operated in a pneumatic cylinder designed for infinite life at a maximum stress level of 620 MPa failed after 240,000 cycles. An extremely hard and small kernel, which looked like a weld deposit, was observed at the center of the fractured surface. The kernel was assumed to have resulted from extreme localized overheating. These springs were reported to have been barrel electroplated after fabrication. The intermittent contact with the dangler (suspended cathode contact) as the barrel rotated allowed high local currents when the last contact was broken was revealed to have resulted in an arc that caused local melting of the metal being plated. The molten metal was interpreted to have been quenched instantly by the plating solution and by the mass of the cold metal of the spring. The hard spot caused by arcing during plating was concluded to be the reason of the fatigue failure. Rack plating or barrels with fixed button contacts at many points instead of dangler-type contacts were recommended to avoid hard spots.

Presence of transverse marks which were remnant of grinding was indicated in a failed valve spring made from ground rod. The shot-peening pattern was light at this location. A transverse crack was found to grow from one such mark under the influence of local stress fields until it was reoriented to the plane normal to the major tensile axis by sufficient loading. The shot-peening procedure was altered to create adequate surface compression at all stressed points on the springs.

The springs formed from 3.8 mm diam cold-drawn carbon steel wire failed to comply with load-test requirements. A split wire in the spring was revealed by investigation. A smooth heat-tinted longitudinal zone was observed in the fracture. It was concluded that the spring failed in the load test due to the split wire. The reason for the condition was interpreted to be overdrawing which resulted in intense internal strains, high circumferential surface tension, and decreased ductility.

Type 302 stainless steel springs used in a printing operation failed by breaking into several pieces after two months in service. The springs were operating over a very small deflection and were regulating the flow of ink, in which they were constantly immersed. Fatigue fractures on every piece of the spring were revealed by visual examination. Each of the fractures was found to be oriented at 45 deg to the wire axis. Clear evidence of pitting corrosion at the fatigue fracture origin was also observed. Free chloride ions were revealed to be present in the ink in which the spring was operating. An alternative ink that contained no free chloride ions was recommended.

Majority of the water feeders in a new chicken house had stopped working. The water feeders were found to be operated on the principle that when the chickens pecked a plastic bowl, a compressed spring released a squirt of water. The small compression springs were made from type 302 stainless steel, and the operating stresses were safely within the design limits given by the Goodman diagram. The springs were revealed by scanning electron microscopy to contain numerous cracks on their inside surface, and these cracks were all at 45 deg to the wire axis. The solution was recommended as to select a grade of spring steel that would be more corrosion resistant than 302 stainless.

A helical compression spring with ten turns made of 1.8 mm thick wire which was under high pressure during tension applied to a rocker arm broke on the test stand in the third turn. The fracture was a torsion fracture that initiated in the highly loaded inner fiber and showed in its origin the characteristics of a fatigue fracture. A longitudinal fold was located at the fracture crack breakthrough which could still be observed at the fourth and fifth turns, where a further incipient crack originated. A metallographic section was made directly next to the fracture path and the fold was cut. It showed decarburized edges in the outer slanted part and this most likely occurred during rolling. The inner radially proceeding part, however, was probably a fatigue fracture originating in the fold. The fracture of this highly stressed spring was therefore accelerated by a rolling defect. In order to decrease the stress, the construction has meantime been modified.

A fractograph of the failed spring was found to indicate light streaks are parallel to the wire axis. A darker depressed area was visible between the streaks and below the center of the fractograph in which distinct outlines that represent sharp corners in the depressions were revealed by careful examination. A hard material (mill scale) was assumed to have been impressed during drawing of the wire and was broken out during peening, leaving the depressions with sharp-bottomed corners. Spring was concluded to have failed due to a surface defect.

A valve spring made of 4.1 mm diam wire, designed to withstand 10,000,000 stress cycles, fractured after only 2,000,000 cycles. The surface displayed impressions which indicated it had been treated by shot blasting. The spring has broken in two places. Fracture 1 was a torsional fatigue fracture which has started from a lobe-like surface defect and not, as is usual, from a point on the most highly stressed inner surface. Fracture 2, on the other hand, was a bending fatigue fracture with a starting point on both the inner and the outer surface of the spiral. The objective of the shot blasting, to put the surface into a state of even compressive internal stress, which must first be overcome during subsequent bending and torsional loading before the boundary zone comes under tensile stress, was therefore not realized in this case. On the contrary, the shot blasting led to a state of internal stress which favored fracture of the spring.

Spring failures were investigated in this study. A seam that extended more than 0.05 mm below the wire surface was revealed and the fatigue-fracture front progressed downward from several origins. A crack that is triangular in outline was produced by each of the fronts. This was reported to have occurred when the fracture plane changed to an angle with the wire axis in response to the torsional strain. The spring failure was concluded to have originated at the seam.

A steel valve spring meeting Steel-Iron-Test 1570 fractured during the high-stress condition of the operation of its valve. Metallographic examination of a transverse section adjacent to the fracture and a longitudinal section through the crack showed the steel was free of major defects and was of high purity, although a number of minor surface defects such as rolling laps were found. The spring was heat treated and its surface strengthened by shot-peening, but the surface was also decarburized to a depth of approximately 0.03 mm which resulted in a lowering of the surface hardness. The fracture of this valve spring is therefore primarily due to surface defects, and secondly perhaps also to weak surface decarburization. No recommendation resulted from the investigation except to note that comparatively minor effects suffice to cause fractures in highly stressed springs.

To samples of helical compression springs were returned to the manufacturer after failing in service well short of the component design life. Spring design specifications required conformance to SAE J157, “Oil Tempered Chromium Silicon Alloy Steel Wire and Springs.” Each spring was installed in a separate heavy truck engine in an application in which spring failure can cause total engine destruction. The springs were composed of chromium-silicon steel, with a hardness ranging from 50 to 54 HRC. Chemical composition and hardness were substantially within specification. Failure initiated from the spring inside coil surface. Examination of the fracture surface using scanning electron microscopy showed no evidence of fatigue. Final fracture occurred in torsion. X-ray diffraction analysis revealed high inner-diameter residual stresses, indicating inadequate stress relief from spring winding. It was concluded that failure initiation was caused by residual stress-driven stress-corrosion cracking, and it was recommended that the vendor provide more effective stress relief.