Strong And Tough – It’s the HLSA alloy!

Posted in The World Of Metal on 16th November 2011

High strength low alloy steel (HLSA) is an alloy steel that provides mechanical properties and resistance to corrosion better than that of carbon steel. Their carbon content range is between 0.05 - 0.25% which retains formability and weld ability.

Other alloying elements include up to 2.0% manganese and small quantities of copper, nickel, niobium, nitrogen, vanadium, chromium, molybdenum, titanium, calcium, rare earth elements, or zirconium. Elements such as copper, titanium, vanadium and niobium are added to strengthen alloys, and alter the microstructure of carbon steels. This is usually a ferrite-pearlite aggregate that produces a very fine dispersion of alloy carbides as an almost pure ferrite matrix, and this eliminates the toughness-reducing effect of a pearlitic volume fraction, yet maintains and increases the material's strength by refining the grain size. This increases the yield strength of the ferrite by 50% for every half of the mean grain diameter. HSLA steels usually require 25 to 30% more power to form in comparison to carbon steels due to their higher strength and toughness.

Copper, silicon, nickel, chromium, and phosphorus are added to increase corrosion resistance. Zirconium, calcium, and rare earth elements are added for sulfide-inclusion shape control that increases formability. These are needed because most HSLA steels have directionally sensitive properties. Formability and impact strength can vary significantly when tested longitudinally and transversely to the grain. Bends that are parallel to the longitudinal grain are more likely to crack around the outer edge because it experiences tensile loads. This directional characteristic is substantially reduced in HSLA steels that have been treated for sulfide shape control.

HLSA's are used in cars, trucks, cranes, bridges, roller coasters and other structures that are designed to handle large amounts of stress or need a good strength-to-weight ratio. These steels are usually 20 to 30% lighter than carbon steel with the same strength.

HSLA steels are also more resistant to rust than most carbon steels, due to their lack of pearlite - the fine layers of ferrite (almost pure iron) and cementite in pearlite The Angel of the North at Gateshead, England is a well-known example of an unpainted HSLA structure (the actual alloy used is called corten steel and includes a small amount of copper).

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