METL Metallographic Analysis is in, and here's the short version. I'll let y'all know when/if the long version gets published
I believe the burst was caused by an obstruction, likely the shot wad from the previous shell, lodged in and just past the forcing cone, caused by a light powder drop in reloading and incomplete combustion from the very low temperature. There may have been a pre-existing bulge also.
1. Did the barrel fail related to low cycle fatigue? NO
The fracture surface exhibited a mixture of ductile overload (plastic deformation with both tensile overload and shear) and transgranular cleavage indicating a ductal failure mode with rapid failure. The cleavage failure appeared to form preferentially in the steel component. No evidence of fatigue failure was observed; there were no striations on the fracture surface.
2. Did the barrel fracture at a ribband edge weld, between iron and steel rod welds, or within a rod? NO
The fracture did not appear to trace along the ribband (spiral) welds. Some cracking was seen along the individual bands within the crolle pattern, but this was not always the case.
3. Did the barrel burst related to interlaminar rust, inclusions, voids, or embrittlement? NO
No evidence of embrittlement was observed. There were a large number of inclusions but there was no apparent fracture jumping from one inclusion to another. The composition of the inclusions was predominantly silicon, phosphorus, and sulfur ie. slag.
No voids or interlaminar rust were observed.
While the microstructure was banded, and the bands had different grain size, inclusion content, and inclusion form (globular in the iron and linear in the steel)... “the overall material appeared to be a single piece of metal...(without)...microstructural defects.”
Chemical composition was similar to AISI 1005 low alloy steel. The low range of tensile strength is 40,000 psi, but may be heat treated to much higher numbers.
