Look at the specification sheet for a modern 20-ton excavator, and you'll see yield strengths for the boom and stick listed at 700 MPa or higher. OEMs are using High-Strength Steel (HSS)-like Domex or Weldox-to build lighter, stronger booms that can lift more without adding structural weight. It works brilliantly in the factory, but it is creating a catastrophic failure epidemic in the field when contractors try to modify or repair these machines on site.
For decades, welders on job sites have grabbed a stick welder, thrown on an E7018 rod, and welded lifting lugs onto excavator booms or patched cracks without a second thought. If you do that on an HSS boom, you will kill someone.
HSS achieves its strength through precise chemistry and controlled cooling at the mill. When a welder lays a 600-degree bead on HSS without pre-heating the massive steel plate, the metal cools incredibly fast. This rapid quench creates a microscopic crystalline structure in the Heat Affected Zone (HAZ) called martensite. Martensite is glass-hard and extremely brittle. The very next time the operator takes a heavy dig, the brittle HAZ cracks instantly under the tension, and the boom snaps in half like a dry stick. To weld HSS, the steel must be preheated to 200-250 degrees Fahrenheit using rosebud torches, welded with specific low-hydrogen rods, and then slow-wrapped in thermal blankets to cool over several hours. Mechanics used to welding mild steel are completely unaware of this metallurgy, and brittle boom failures are on the rise.
