That's like asking why a carpenter needs to be able to tell the difference between pine, maple, oak, or black walnut.

I differentiate between a rod burner that deposit consistent looking weld beads day in and day out and a welder that understands the difference between the different types of steels and other metals that he or she works with and how to weld them properly.

BCC metals are typically harder and stronger than FCC metals at the expense of ductility. FCC metals are softer, not as strong, but have better ductility than BCC metals. HCP metals generally have improved corrosion resistance in certain environments, but they tend to be brittle when loaded in compression.

There are several good books on basic metallurgy. A good book for introductory metallurgy (Materials Science and Engineering by Carter & Paul)) is available from ASM, Metals Park, Ohio. AWS also has a couple of good books, perhaps a little more advanced, one is Welding Metallurgy by George Linnert.

A production welder that welds the same thing day after day probably can get by without a working knowledge of metals. On the other hand, a welder involved with maintenance or working in a job shop will have an advantage if he has a good working knowledge of metallurgy.

Years ago, when I had my welding shop, many of the jobs that came in the door were because I had a reputation of being able to weld materials the other shops struggled with. The difference wasn't my ability to lay a good looking weld, the difference was I understood basic metallurgy and how the metals responded to heating and cooling. I knew when to use high preheat, when to limit interpass temperature, how to select a compatible filler metal.

Al

Al