A. Material definitions

Carbon steels, sometimes referred to as plain carbon steels, consist essentially of iron with with deliberate additions of carbon in the approximate range 0.05 to 1% by weight. ↩ However, chemical analyses will show the presence of silicon and manganese, which are added to all pressure vessel steels to combine with detrimental impurities such as oxygen and sulfur to reduce their effects. Thus, a typical analysis of a carbon steel will show silicon in the range 0.10 to 0.35% and manganese 0.30 to 1.0%.

Plain carbon steels are generally categorized into three broad groups according to the level of carbon present:

• Up to 0.30%-low-carbon (or mild) steels

• 0.30 to 0.60%-medium-carbon steels

• 60 to 1.0%-high-carbon steels

Carbon is a very powerful strengthening element, but as a rule the higher the carbon level the lower the ductility of the steel and the poorer its weldability. For heat exchangers, all the plain carbon steels used for the pressure parts will be low-carbon steels.