Forming Process Anomalies in Diesel Fuel Injection Control Sleeve
The cold start advance solenoid sleeve was found leaking through the wall during troubleshooting complain of a diesel engine that failed to start in cold weather. The component was revealed to be a tubular product with a “bulb” section at one end and threads on the other. The manufacturing method used to create the bulb shape was hydroforming, using a 300 series stainless steel tube in the full-hard condition. The leak was attributed to a crack in the sleeve in the radius between the bulb area and the cylindrical portion of the sleeve. Fatigue cracks initiated at multiple sites near the OD of the sleeve were revealed by scanning electron microscopy of the broken-open crack. It was revealed by analysis that during the hydroforming process, heavy biaxial strains were imparted to the sleeve wall. It was interpreted that when combined with the heavy strains inherently present in the full-hard 300 series stainless steel, the hydroforming strains in the radius caused the microcracking. The root cause for this failure was identified to be omission of an intermediate stress relief or annealing treatment prior to hydroforming to the final shape.