Turret trucks operating in deep-reach warehouses are suffering catastrophic structural failures. The mast I-beam channels are snapping laterally under load. The root cause is a severe lack of metallurgical understanding regarding the Heat Affected Zone (HAZ) in induction-hardened steels.
To resist the 800 MPa Hertzian contact stress of the load rollers, the front face of the mast channel is induction hardened to 55-60 HRC (Rockwell C) to a depth of 2.5mm. However, to attach mounting brackets or reinforcement gussets, manufacturers are welding directly onto the web of the channel.
The arc welding process introduces extreme heat, pushing the base metal immediately adjacent to the weld (the HAZ) past the Ac3 critical temperature (approx. 850°C). When this zone cools rapidly against the mass of the cold channel, it forms untempered martensite-a microstructure that is incredibly hard but catastrophically brittle, with a fracture toughness (K1c) dropping below 30 MPa√m. Under a 4,000 lb load at a 30-foot elevation, the cyclic bending stress of 350 MPa initiates microscopic intergranular cracking in the HAZ. The crack propagates at a rate of 0.2mm per cycle. After just 4,000 lift cycles, the channel snaps cleanly in half. Welding on induction-hardened mast channels without an immediate post-weld stress relief normalization (heating to 650°C and slow cooling) is engineering malpractice.
