Low alloy steels are defined as steels with a lower than 10% content of carbon, manganese, chronium, nickel, molybdenum, barium, vandium, and silicon combined. Low alloy steel testing for elemental composition mostly revolves around quantifying these.
The alloyants referenced above improve the strength and hardness of low alloy steels compared to the carbon steels and even to stainless steels (as long as their temperature remains well under 400° C). Low alloy steels serve as construction materials in chemical, petrochemical, and electrical energy plants and have a range of applications in process pressure vessels, load-bearing structures, in car parts, low-cost aerospace parts, tubing, and low-cost cutting tools. A special category of low alloy steels is Chrome-moly or Chromoly (Cr/Mo Steels). These steels, labeled 41** in the AISI / SAE classification, contain 1-9% Cr and 0.5-1% Mo and they are used in the petrochemical and electrical energy sectors for their good creep and high temperature resistance.
When it comes to steel scrap, low alloy scrap is usually not a very hot commodity because the concentration of valuable metals in it is low: the value distinctions between different alloy steel grades are small, while steel mills have exigent requirements concerning metal scrap composition as the quality of the output depends on that of the raw material. Base steel grades are more compatible with the presence of “tramp” elements, but making special steel grades purely out of alloy steel scrap requires careful sorting and preparation of the metal.
Copper and tin are usually found only in old scrap, where they can be present either in a pure form or in combination with steel parts. Chromium, nickel and molybdenum cannot be verified visually, thus requiring an accurate composition test.
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The presence of undesirable but persistent “tramp elements” tends to strengthen and harden steel, with a corresponding loss in malleability. For instance, molybdenum and chronium in an extra-low-carbon steel will increase its resistance to hot deformation, necessitating greater rolling loads to work with it.
The presence of tin and arsenic negatively influences the recrystallization kinetics during annealing of some cold-rolled steel grades meaning an increase in annealing temperature is required. Copper, often present at 0.2% or above, tends to cause surface defects as a result of scaling and cracking. nickel, when present in an equal quantity, can reduce copper's effect, however tin and arsenic will increase it. For example, when as little as 0.05% of Sn is added to a steel containing 0.22% copper, the tendency towards cracking is increased.
In addition, tramp inclusions also affect downstream steel processing and the resulting steel's properties. This can cause grain-boundary embrittlement even in low alloy structural steels, which may express itself as cracking caused by stress, stress-relief, fatigue or creep rupture. The list of problems caused by impurities does not end here, and they can be aggravated by heat treatment in steel processing.
Metallurgy has developed various complex ways to deal with the issues caused by tramp impurities. Those methods depend on knowing exactly which of the relevant elements are present in a steel, which returns us to our subject of effective low alloy steel testing with a Bruker S1 TITAN analyzer gun.