77 AlloyAlloy SteelsSteels High strength low alloy (HSLA). a type of steel alloy that provides many benefits over regular steel alloys contain a very small percentage of carbon (less than one-tenth of a percent) and only small amounts of very specific alloying elements referred to as 'microalloyed', as they are indeed alloyed in extremely small amounts by comparison to other main commercial alloy steels group of low carbon steels that utilize small amounts of alloying elements to attain yield strengths in excess of 275 MPa in the as-rolled or normalized conditions SME 1613 : Materials Science 2007 TYPICAL CHEMICAL COMPOSITION OF HSLA SME 1613 : Materials Science 2007 MECHANICAL PROPERTIES: HSLA • Much stronger and tougher than ordinary plain carbon steels. • alloying elements : alter the microstructure of plain carbon steels, which is usually a ferrite-pearlite aggregate, to produce a very fine dispersion of alloy carbides in an almost pure ferrite. • Increases the material's strength by precipitation strengthening and by refining the grain size, which in the case of ferrite increases yield strength by 50% for every halving of the mean grain diameter. SME 1613 : Materials Science 2007 Microstructure of HSLA A517 Grade A533 grade b, quenched 7,quenched from from 900C and tempered 925°C,tempered at at 620C . This is a heavy 650C = tempered section plate, the martensite. microstructure consists of ferrite and tempered bainite. SME 1613 : Materials Science 2007 Application of HSLA • Applications: include oil and gas pipelines, automotive beams, offshore structures and shipbuilding. • Also used in cars, trucks, cranes, bridges and other structures that are designed to handle a lot of stress, often at very low temperatures. SME 1613 : Materials Science 2007 SME 1613 : Materials Science 2007 Maraging Steels • Fe-Ni martensite alloys (which belong to the family of iron-base alloys)additionally alloyed with cobalt, molybdenum, titanium and some other elements. • difference between carbon steels and maraging steels is that the martensite that forms in carbon steels,when quenched, transforms to a softer more ductile material when tempered. • maraging steels are not included in the AISI numbered listings since they are not machine steels. SME 1613 : Materials Science 2007 Type of Maraging Steels Type of Maraging Steels Composition Mechanical properties ~high quality to fracture toughness and 1. 18% Nickel 18% Ni, 8% Co, 5% Mo, ease of welding. 0.9% Ti ~the strenght is maintained with increase in section thickness and also up to 350°C ~high hardness results from the intermetallic 2. 23% Nickel 23% Ni, 2.0% Al, 2.5% Ti compounds Ni3Mo and Ni3Ti. ~very high resistance to stress corrosion compared to carbon steels. 3. 12% Nickel 12% Ni, 12% Cr, Mo, ~these steels develop notch strenght higher less amount of Al and Ti than carbon steels. SME 1613 : Materials Science 2007 Heat treatment: Maraging Steel • The maraging steels are strengthened by a process of martensitic transformation, followed by age or precipitation hardening . Precipitation hardenable stainless steels are also in this group. (1) Annealing Fe-Ni alloys to 820 °C, cooling from the austenitic condition, the alloy transforms to a fine lath type martensite. (2)(3)(4)The iron-nickel martensite starts out ductile and become hard and tough with aging. Cooling in air for 1 hours and tempered at 480 °C for 3 hours • The tempering results in strong precipitation hardening owing to the precipitation of intermetallics from the martensite, which is supersaturated with the alloying elements. By analogy with the precipitation hardening in aluminum, copper and other non-ferrous alloys, this process has been termed ageing, and since the initial structure is martensite, the steels have been called mar aging . SME 1613 : Materials Science 2007 Microstructure of Maraging Steels Fe-Ni Martensite Tempered Fe-Ni Martensite • The maraging steels have high fracture toughness due to a combination of the grain size of the martensite and the dislocation density,leading to a fine precipitation.(eg,Ni3TiAl,orthorhombic Ni,Mo) • The steels can be nitrided. The corrosion resistance is only slightly improved but the 12% Cr variety has been developed for corrosion resistance. SME 1613 : Materials Science 2007 Mechanical Properties: Maraging Steel Ultra-high strength at room temperature.(yield strength=1100MPa) Simple heat treatment, which results in minimum distortion. Superior fracture toughness compared to quenched and tempered steel of similar strength level. Low carbon content, which precludes decarburisation problems. Section size is an important factor in the hardening process. Easily fabricated. Good weldability. SME 1613 : Materials Science 2007 Application: Maraging Steel • Applications : shafts, and substitute for long, thin, carburized or nitrided parts, and components subject to impact fatigue, such as print hammers or clutches. • Also commonly used: air frame and engine components,injection moulds and dies due to its high resistance of decarburisation,distortion and cracking. • Maraging steels work well in electro-mechanical components where ultra-high strength is required, along with good dimensional stability during heat treatment. SME 1613 : Materials Science 2007 SME 1613 : Materials Science 2007.