When Ni, Cr, Mo, and other alloy elements content consist of less than 10.5% are defined as low alloy steels. The low alloy steels consists alloys with small additions of chrome and nickel up to 11/13Cr steels with 4% nickel. It improves the high temperature performance and transmits some corrosion resistance. The addition of chrome and nickel improves the thermal stability of steel and makes it popular for applications which suffer wide temperature ranges without getting too hot.
Low alloy steel fabrication and welding
Low alloy steels present certain challenges with regards to fabrication and welding. The weldability complication is as diverse as the alloy composition and its application. The alloy combination accords its own weldability challenge and hence they must be treated accordingly. And there will not be one solution in these cases. What can be generic is the fact that each of these grades and even the modified versions needs full knowledge of their metallurgical structures and that must be matched with the utility demands on that material after the welding is done. Steel with 9% chromium and molybdenum is such steel that is regularly used in industrial and heat exchangers construction. In order to have a successful weldment, developed welding procedure with well-laid-out heat control plans to prevent formation of a detrimental phase in steel is required.
Low Alloy Steel Applications
Applications for low-alloy steels ranges from military vehicles, oil drilling platforms, pressure vessels and piping, earthmoving and construction equipment, and structural steel. Several common groupings of low-alloy steels, beginning with HY 80, HY 90, and HY 100 steels, are used for submarines, bridges, building ship hulls, and off-highway vehicles. It contains nickel, molybdenum, and chromium, which adds to the material's weldability, yield strength and notch toughness. For welding low-alloys steels, preheat and post heat treatments typically are not required.
Low Alloy steel (Cr – Mo Steel) grades and suitable welding consumables
|Steel category||ASTM / ASME standard|
|Low Alloy Steel Plate||Low Alloy Steel Pipe||SMAW Electrode/ Rod||Flux Cored Wires|
|0.5%Mo||A204 Grade A,B,C A336 Grade F1||A209 Grade T1 A335 Grade P1||GL76/78A1||GMX 811A1|
|0.5%Cr-0.5%Mo||A387 Grade 2 CI.1,2||A213 Grade T2 A335 Grade P2||GL86/88B1|
|1.25%Cr-0.5%Mo||A387 Grade 12 Cl.1,2 A387 Grade 11 CI.1,2 A336 Grade F11||A213 Grade T11, 12 A335 Grade P11, 12 A182 Grade F11||GL86/88B2||GMX 811B2|
|2.25%Cr-1.0%Mo||A387 Grade 22 CI.1,2 A336 Grade F22||A213 Grade T22 A335 Grade P22 A182 Grade F22||GL86/88B3||GMX 911B3|
|5.0%Cr-0.5%Mo||A387 Grade 5 CI.1,2||A335 Grade P5||GL86/88B6|
|9.0%Cr-1.0%Mo||A387 Grade 9 CI.1,2||A335 Grade P9||GL86/88B8|