Steel consists primarily of iron, and it contains carbon to make the steel harder. Steel is available in a variety of grades, such that the grade provides information on the amount of alloying agents in the steel. The welding process itself is generally the same for different types of steel, but the preparation is specific to each grade. Tungsten inert gas (TIG) welding is a common method of welding 4140 steel.

Analyze 4140 steel. The grading system for steel consists of four digits. The first digit indicates the primary alloying agent, the second digit indicates the secondary alloying agent and the third and fourth digits indicate the carbon content in hundredths of a percent. Grade 4140 steel contains 0.12 to 0.30 percent molybdenum and 0.50 to 0.95 percent chromium. The carbon content of 4140 steel is 0.40 percent, making it a medium carbon steel.

Choose your electrode. The electrodes in TIG can be pure tungsten for softer steels, but they may require alloying agents for harder steels. Grade 4140 steel is relatively hard and typically uses tungsten electrodes with 2 percent cerium oxide.

Select your shielding gas. TIG welding generally uses argon as the shielding gas. Some alloys may require small amounts of other gases, but you can weld 4140 steel with at least 15 pounds per square inch of pure argon.

Adjust the setting for electrical current on your welder. A TIG welding machine has settings indicating the type of current and charge on the electrode. You typically weld 4140 steel with the direct current electrode negative, or DCEN, setting.

Preheat 4140 steel. You generally need to preheat steel with a carbon content greater than 0.30 percent. Heat 4140 steel to 700 degrees Fahrenheit before you weld it.