From there, Gene continued into a discussion of the properties of metals and particularly steels. We learned that the strength of steel can be affected by the rate at which it is allowed to cool. We learned that an excess of acetylene in the flame will introduce too much carbon into the weld, seriously weakening it. In a discussion of the various types of steel, Gene described the different characteristics of low carbon versus high carbon steel, and the effects of adding trace amounts of other materials to steel such as chromium and molybdenum. The numerical system for identifying the alloy components of steels was de-mystified.
Steel is described using a four-digit numerical system.
The first two numbers describe the trace metals added to the steel and the last two give the concentration of carbon. The concentration of carbon is one determining factor in its relative hardness. But, it is also brittle, which explains why if you use your knife blade to pry something apart, it may break rather than bend. Kearns demonstrates the proper flame for welding. Next, Gene progressed to the particulars of oxy-acetylene welding.