Ball joints made from carburized En 353 (BS970:815A16) steel failed after several hours of being fitted into vehicles. The parts were forged, machined, and thread rolled. The threads were copper plated to prevent carburization. The heat treatment consisted of carburizing in a cyanide bath for 12 hours at 930 deg C. After tempering for 2 h at 170 to 175 deg C, the copper plate was removed by immersing in an acid bath for 45 min. The investigations found the microstructure, hardness, and chemistry all met the specification. The case depth was approximately 0.75 mm to 1.0 mm. The SEM studies showed that it was a brittle fracture and completely intergranular to a depth of about 2.5 mm. It was concluded that the failure was due to hydrogen embrittlement for the following reasons: (i) failure did not occur immediately after loading, (ii) the fracture was intergranular to a depth of two to three times the case depth, (iii) secondary cracks were observed at the surface. The hydrogen was introduced during copper plate removal by acid dipping. If the tempering operation was performed after the acid dip operation, the hydrogen would have been driven out.

The piston rod of a steering damper on a single decker bus fractured after 100,000 miles of service in the fully-extended left full-lock position. The steering damper, which is similar in shape and operation to a telescopic shock absorber, was secured by ball joints locked with slotted nuts. The steel piston rod fractured at the axle end leaving approximately 5 mm of rod welded to a securing ferrule. The failure was caused by a fatigue mechanism. Small surface cracks formed during welding in the heat-affected zone close to an unradiused shoulder in the piston. Under alternating stresses in normal service these cracks propagated through the piston rod made less tough by the extended weld heat-affected zone.

To forged AISI 4140 steel trailer kingpins fractured after 4 to 6 months of service. Fractographic and metallographic examination revealed that cracks were present in the spool-flange shoulder region of the defective kingpins prior to installation on the trailers. The cracks grew and coalesced during service. Consideration of the manufacturing process suggested that the cracks were the result of overheating of the kingpin blanks prior to forging, which was exacerbated during forging by deformation heating in the highly-strained region. This view was supported by results of two types of tensile tests conducted near the incipient melting temperature at the grain boundaries. All kingpins made by the supplier of the fractured ones were ultrasonically inspected and six more anticipated to fail were found. It was recommended that the heating of forging blanks be more carefully controlled, especially with respect to the accuracy of the optical pyrometer temperature readout. Also, procedures must be developed such that forging blanks that trigger the over-temperature alarm are reliably and permanently removed from the production line.

Several heavy truck Cr-Mo steel steering arms in service less than three years fractured during stationary or low-speed turning maneuvers that required power-assisted steering. Metallographic examination of the cracked AISI 4135 arms, heat treated to a hardness of 285 to 341 HB, revealed that fatigue crack initiation occurred from the tip of oxide scale inclusions forged into the U-shaped arm at the inside radius. Corrective action involved redesigning the steering arm to increase the minimum forging radius and reduce the stress level at the inner-bend radius, and reducing the level of power assistance to the wheels to encourage the driver to put the vehicle in motion prior to turning.