A “Toolbox” for Forensic Engineers

272 Forensic Materials Engineering: Case Studies
Figure 8.12 Polished and etched section of spring adjacent to fracture. The
wire diameter is 0.9 mm.
some appear as white areas, having a high hardness, surrounded by a shaded
heat-affected zone, while immediately adjacent to the fracture there is a knob
of light-etching metal with a crack in it and the underlying metal is soft.
(The black diamond shapes are Vickers hardness indentations; the larger they
are, the softer the metal.)
Obviously, this end of the spring had been affected by inadvertent heat
treatments, highly localized and related to the welding. Weld spatter droplets
had fallen onto the spring and transferred enough heat to raise the surface
locally to transformation temperatures. Adjacent to the fracture can be seen
where a molten droplet has bonded to the spring and transferred enough
heat to soften the center section. The weld droplet itself has a hot tear at the
outside caused by contraction stress as it was solidifying. Farther along,
smaller spatter particles have heated the metal to a hardening temperature
but only carried sufficient heat to affect the microstructure in a shallow zone
immediately below the surface.
As a consequence of these microstructural changes resulting from the
weld spatter, the spring was unable to withstand the bouncing up and down
of the trays as the frame was manhandled up the stairs. The unfortunate
young man’s eye must have been directly in line with the spring when it
suddenly broke under what was, in effect, a mechanical overstress of the
softened region while the spring was under tension. The firm that did the
welding accepted liability; they were well aware that spatter always occurs
when arc welding with flux-coated electrodes and should have taken precautions
to cover the springs and/or remove the top few trays.