I understand that the diamond symbol on M1 bolts and receivers indicates "8620" steel. What's the composition of 8620 steel?
8620 steel is used even today in camshafts, gears, piston pins, shafts, splines and other machinery parts where a tough, ductile interior needs to be complemented by a hard outer surface. Its chemistry is:
8620 steel welds very well, in the annealed form, with appropriate rod, welding technique, etc.
The problem with welded M1 and M14 receivers is that heat-treated 8620 does not weld well due to the case hardening; typically 60 Rockwell "C" on the case to a depth of 12-18 thousandths, then dropping quickly to a residual core of around 28-35.
Micro fractures, voids, stress & other dangerous anomolies will occur, usually resulting in cracking, that can lead to catastrophic failure.
Hope this answers your question!