The Case of the Badly Welded High Pressure Hydraulic Cylinders
The weld between the rod and end-cap of a high-pressure hydraulic cylinder installed on a concrete boom pumping assembly failed in-service. The weld was a Gas Metal Arc Weld (GMAW) installed at an overseas manufacturing facility. The steel for the rod and cap was a fine grained alloy steel Type StE690 (Yield Strength of 100 ksi). The rod and cap were attached to a piston which moved inside the cylinder housing.
The fracture surfaces of the weld exhibited incomplete weld penetration and weld non-fusion on the rod’s surface and at the root of the groove-joint. There were metallurgical notches with areas of incomplete penetration at the root of the weld with micro-fissures that propagated in the weld. A contributor to the weld size variations was off-centered welding electrode placement into the groove-joint. Observations revealed fissures with brittle microhardness features. The cracks propagated as “fretting” over an extended period of time. The failure mode was a defective weld subjected to cyclic fatigue stresses.
The failed weld in the hydraulic cylinder while in-service resulted in numerous personnel injuries at the jobsite, material damages inside the building elevator shaft, and several weeks of project delays. Areas of incomplete penetration in the root and weld non-fusion were causes of the failure.
The Lessons Learned:
The high-strength steel welding procedure, heat input, electrode placement and welding techniques were unreliable and inconsistent. The incomplete weld penetration and off-center electrode at the root of the groove-weld-joint resulted in non-fused regions and uneven weld beads. Manufacturers that rely on welding high-strength steel rods and cylinders should establish, maintain, monitor and nondestructively test GMAW in high-strength steel applications.