High strength low alloy (HSLA).

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1 7 Alloy Steels

2 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

3 TYPICAL CHEMICAL COMPOSITION OF HSLA

4 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.

5 Microstructure of HSLA A517 Grade 7,quenched from 925 C,tempered at 650C = tempered martensite. A533 grade b, quenched from 900C and tempered at 620C. This is a heavy section plate, the microstructure consists of ferrite and tempered bainite.

6 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.

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8 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.

9 Type of Maraging Steels Type of Maraging Steels 1. 18% Nickel 2. 23% Nickel 3. 12% Nickel Composition 18% Ni, 8% Co, 5% Mo, 0.9% Ti 23% Ni, 2.0% Al, 2.5% Ti 12% Ni, 12% Cr, Mo, less amount of Al and Ti Mechanical properties ~high quality to fracture toughness and ease of welding. ~the strenght is maintained with increase in section thickness and also up to 350 C ~high hardness results from the intermetallic compounds Ni3Mo and Ni3Ti. ~very high resistance to stress corrosion compared to carbon steels. ~these steels develop notch strenght higher than carbon steels.

10 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 maraging.

11 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.

12 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.

13 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.

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