Handbook-Steels Classified

An ASTM steel specification usually covers steel in a particular form (sheet, plate, pipe, tube, forging, casting, etc.).

(For example, ASTM Specification A 210 covers ”Seamless Medium-Carbon Steel Boiler and Superheater Tubes”.) It may or may not cover chemical composition. When it does not cover composition specifically, the purchaser then specifies composition by reference to SAE or AISI grade. An ASTM specification typically covers such things as test requirements, forming operations, marking, packaging, etc. It may be cross-referenced to several other ASTM specifications. The system appears complex, but is extremely useful to industry. All ASTM specifications applying to steel start with the letter ”A”, followed by three digits. The complete specification number also includes two additional digits at the end (as in ”A 210-73” ) which indicate the year in which the specification was issued or revised. In the 1974 ”Annual Book of ASTM Standards”, steel specifications alone (5 volumes out of a total of 47) ran to about 3000 pages! Rimmed vs. Killed Steel Some low-carbon steels are available in three grades: ”rimmed”, ”semi- killed”, and ”fully-killed”. The terms themselves are derived from the action of the steel when it is poured into an ingot mold after leaving the furnace. In terms of welding, they indicate whether or not there is oxygen in the steel which may cause weld porosity when certain welding processes are used. All carbon steels contain some oxygen. The very nature of the furnaces in which they are made makes this inevitable. It’s what happens to the oxygen when a weld is made that is significant. When a rimmed steel is welded, some of the oxygen will usually combine with some of the carbon to form bubbles of carbon monoxide (CO). These will cause weld porosity if they cannot escape from the molten weld metal before it solidifies. In oxy-acetylene welding, these minute bubbles of gas always have time to escape. In some other processes, such as tig welding (GTAW), however, they may be trapped in the solidified metal. To make a killed steel, aluminum (which has a stronger affinity for oxygen than carbon, manganese, or silicon) is added to the molten steel before it is poured. The aluminum locks up the oxygen, in the form of aluminum oxide, so that it cannot form gas bubbles during welding. In a semi-killed steel, silicon may have been used, with or without aluminum, as a deoxidizing addition, and there may be some bubbles of carbon monoxide gas formed during welding.