[**Carbon steel pipe**] is steel in which the main interstitial alloying constituent is carbon in the range of 0.122.0%. The American Iron and Steel Institute (AISI) definition says:Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, molybdenum, nickel, niobium, titanium, tungsten,vanadium or zirconium, or any other element to be added to obtain a desired alloying effect;when the specified minimum for copper does not exceed 0.40 percent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.6, copper 0.6.
Effect of Carbon:
Generally, carbon is the most important commercial steel alloy. Increasing carbon content increases hardness and strength and improves hardenability. But carbon also increases brittleness and reduces weldability because of its tendency to form martensite. This means carbon content can be both a blessing and a curse when it comes to commercial steel.