Good synopsis. One must consider China's steel making facilities are state of the art, being relatively new. But how they manage their quality control is, as mentioned above, unknown. Sulfur and phosphorous content tend to make steel brittle at temperature extremes,ie below zero F, can't quite remember which is which, I think the sulfur makes if cold short. I also think a huge factor here is joint design, the metallurgical changes in the heat affected zone from welding ie grain growth which makes the fracture look like cast iron, and fatigue stresses, which can be hard to estimate. Using a chrome-moly alloy usually requires post weld heat treatment, difficult and expensive for a small manufacturer. Plain old mild steel is pretty ductile and can take a lot, IF the joint is properly designed to keep the stresses well under the fatigue limits. I still think a broke frame can be welded IF properly reinforced to lower the stresses, including minimizing residual stresses from the welding.
The fracture is not actually at a weld but plumb in the middle between two welds and about 10cm away from each. There looks to be significant corrosion at the fracture site but this may have occurred post fracture. I wonder if anyone ever did finite element analysis on the chassis, as to me it looks like it is due to stress concentration at that point, leading to a fatigue failure but of an inadequate material. It has the classic jagged appearance of a fatigue failure. Hopefully this link to an image of the fracture on Google Images will work
https://photos.google.com/photo/AF1QipMb7iXACGtRN8eE6LePPSzGnwytVKQyB3sut88_Wilson
