Conventionally, as technology for joining layered steel sheets configured by overlapping two or more steel sheets, a penetration welding method by plasma-MIG welding has been known (e.g., refer to Patent Document 1). This penetration welding method includes the two processes of a plasma welding process of forming a through hole (so-called key-hole) in the layered steel sheets by way of a plasma arc generated by a plasma torch, and a MIG welding process of filling a filler into the above-mentioned through hole using a MIG torch, thereby backfilling this through hole. With such a penetration welding method, in order to realize sufficient strength, it has been important to form a through hole of a bore of sufficient size in the plasma welding process, which is the leading process, from a top surface of the layered steel sheets to the back surface.
FIG. 6 is a view showing the sequence of the plasma welding process described in Patent Document 1. With the technology of Patent Document 1, a through hole with a bore of sufficient size is formed in the layered steel sheets by performing the two processes of a hole digging process to cause a hole to penetrate from the top surface of the layered steel sheets to the back surface, and a melt diameter enlarging process of expanding the diameter of the through hole formed in the above process. Upon doing so, if controlling the arc current to a constant, since a proportional relationship is established between the length of the plasma arc column generated between the plasma torch and the layered steel sheets, and the arc voltage according to Ohm's law, the arc voltage will vary as shown at the bottom of FIG. 6. With the technology of Patent Document 1, it is determined that a hole penetrated from the top surface of the layered steel sheets to the back surface by detecting a sudden increase in arc voltage such as that shown by the dotted line in FIG. 6, and then the process advances from the hole digging process to the melt diameter enlarging process.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2013-180309