12.2.4.1 Carbon steel
Carbon steel, a carbon and iron alloy, is the cheapest choice of metal for piping and valves. This material is classified as a noncorrosion-resistant alloy that can be corroded by corrosive compounds such as carbon dioxide, hydrogen sulfite, etc. Other elements, such as silicon, copper, and manganese, can be added to carbon steel. Carbon steel can be divided into different categories, such as low carbon with a carbon content less than 0.25%, medium carbon with a carbon content between 0.25% and 0.5%, and high carbon with a carbon content between 0.5% and 1.25%. It should be noted that the classification of carbon steel based on carbon content can differ from what is mentioned here. If the carbon content of an iron-carbon alloy is more than 2%, the material is called cast iron. Cast iron was a popular choice for piping swage and water pipe before the advent of plastic piping. Carbon steel can be corroded in the form of metal loss due to carbon dioxide CO2. Therefore 3 mm corrosion allowance as an average can be added to carbon steel pipe and valve bodies according to Norsok M-001, “material selection” and Norsok L-001, “piping and valves.” In addition, hydrogen-induced cracking (HIC) is a big risk for carbon steel; hydrogen atoms can accumulate in the void spaces in the microstructure of carbon steel; after collision they get bigger and cause cracking or other types of failures in the metal (see Fig. 12.20). One approach to mitigating HIC corrosion is to used killed carbon steel. Killing carbon steel process involves deoxidating the steel with aluminum or silicon, which makes the steel homogenous and free of porosity.
Fig. 12.21. V Notch on metal for the Charpy impact test.
The other important parameter for carbon steel is carbon equivalent (CE), which affects the weldability of carbon steel. Increasing the percentage of some of the alloys, such as carbon and manganese, in the carbon steel improves the mechanical strength of the carbon steel, but also increases its hardness and brittleness. Some piping and valve specifications put limitations, such as 0.39 or 0.40 or 0.42, on CE to prevent cracking the carbon steel due to welding or sour corrosion such as HIC. CE can be calculated through Eq. (12.8):
Carbon equivalent calculation
(12.8)CE=C+Mn6+Cr+Mo+V5+Ni+Cu/15
where:
C: carbon; Mn: magnesium; Cr: chromium; Mo: molybdenum; V: vanadium; Ni: nickel; Cu: copper.
Carbon steel, according to ASME B31.3, “process piping code,” can be used for design temperatures between − 29°C and approximately 400°C. Alternatively, low-temperature carbon steel (LTCS) can be used if the design temperature of the piping and valve will be between − 46°C and − 29°C. LTCS is probably more suitable for lower temperature than carbon steel due to the addition of some alloys such as nickel and magnesium. A Charpy impact test is applied on LTCS at − 46°C to evaluate the resistance of the materials against breakage. A V-notch is created on a sample of the material (see Fig. 12.21) and the required energy to break the material through the V-notch is measured. The mechanism of low temperature failure in the material is cracking, which is evaluated by the impact test. Adding 3 mm of corrosion allowance is also applicable for piping and valve bodies in LTCS material.