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Maraging steels 05/21/22 1

Maraging Steels

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maraging steel

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  • Maraging steels

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  • Introduction MARAGING STEELS are low-carbon, iron-nickel steels that possess an excellent combination of strength and toughness superior to carbon-hardened steelsvery high nickel, cobalt, and molybdenum contents and very low carbon contentsStrength due to the formation of iron-nickel martensite and then formation of iron nickel intermetallic compounds as precipitate phase during agingHence the name maraging steel - martensite+aging

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  • Advantages of maraging steelsHigh yield strength(2000-3000MPa)High weldabilityHigh fracture toughnessIntricate shapes can be easily machined**

  • History of maraging steel development20 % and 25% Ni added to steel - with 0.3% Al, 1.4% Ti, and 0.4% Nb, 0.03%C - resulted in precipitation hardening of the low-carbon martensitic structure when aged at 425 to 510 CThese alloys exhibited good combinations of strength and ductility at hardness levels of 53 to 56 HRC but were abandoned because of their brittleness at extremely high strength levelsWith 18% Ni, martensite of required hardness and toughness was obtained through the addition of cobalt and molybdenumCurrent Maraging steels based on Fe-18%Ni-Co-Mo quaternary alloy system eg: 18 Ni Marage 200, 250, 300 and 350 alloysCarbon content restricted to less than 0.03%**

  • Compositions of commercial maraging steelsUltrahigh-strength maraging steel18 Ni Marage 350 alloy is essentially amodified version of the 300 grade that contains higher cobalt and titanium levels and a slightly reduced molybdenumContentYield strength > 2100MPaLow cobalt maraging steels also developed**

  • Strength of maraging steelsFormation of lath martensitePrecipitation/age hardening of lath martensite**

  • 1. Formation of iron nickel lath martensiteIron nickel martensite martensitic structure formed when iron nickel alloy is heated above 800C(austenite formation temperature) and then cooled to temp below 300C Iron nickel martensite is called lath martensite with BCC structure formed by diffusionless transformation of FCC austenite(no diffusion to phase during cooling,but crystal structure changes from fcc to bcc )LATH martensite is not as hard as ordinary martensite but more tough and ductile

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  • Phase diagram of iron nickel alloysnickel levels > 18% results in the retention of austenite and thus prevent complete transformation into martensite18 Ni chosen as the standard maraging steel composition as it promotes complete transformation into martensite during quenching**

  • Martensite formation & morphologyFor iron containing 5 to 10% Ni, martensite is formed with rapid cooling Excess of 10% Ni lowers the cooling rate required for martensite formationlath martensite is formed in iron containing up to 23% NiIf the nickel content is increased above 23%, the lath martensitic structure is replaced by a twinned martensitelath martensitic structure is preferred in maraging steels because, following aging, this structure is tougher than a twinned martensitic structure**

  • Lath martensiteThe lath martensitic structure of maraging steels consists of several martensitic packets and numerous blocks within each packetpackets and blocks are planar, lie along one direction, and are parallel to each otherPackets are the predominant structure of lath martensite followed by the block structures that appear as discrete areas within each packet**

  • 2. Precipitation Aging of lath martensiteLath martensite is again reheated in the temperature range 450-500C and aging is done for a certain time period( 1 6 hr)Aging is done to minimize or eliminate the reversion of martensite into austenite() and ferrite() During aging, ordered precipitate phase of nickel-rich intermetallic compounds is formed in the lath martensitic structureprecipitation of the Ni3Mo and Ni3Ti intermetallic compounds

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  • Effect of Ti, Mo and CobaltTi strong hardener and Mo - moderate hardenerCo helps the precipitation of Ni3Mo and a finer, more uniform distribution of Ni3Mo precipitates is formedthe good structural fit between (Ni3Mo and Ni3Ti ) precipitates and the bcc martensitic matrix gives coherence and ordered precipitate phase necessary for required hardness and toughness**

  • Effect of aging time on hardness Plots of aged hardness versus aging time at 455 C (850 F) for Fe-18Ni-5Mo and Fe-18Ni-5Mo-8Co maraging steels**

  • Effect of aging time on hardness Hardness of 18Ni(250) maraging steel versus aging time for various aging temperatures**

  • Effect of alloying elements Ni, Mo, Ti, Co, AlNi, Mo, Ti lowers Ms (martensite formation start 200-300C) temperature, low Ms favors formation of twinned martensite which is not desirableNi, Mo, Ti helps in precipitation hardening(desirable)Co increases Ms temp and favors formation of lath martensite (upto 6-8% composition)Hence Co added to compensate for excess use of Ni, Mo, Ti and maintains Ms temperature required for lath martensite formationIn the absence of cobalt, other elements such as nickel, molybdenum, and titanium must be maintained or reduced to levels to ensure an adequately high Ms temperatureNickel lowers Ms temperature heavily if in excess of 18%, hence Ni level kept at 18%Upto 0.1% Al slightly increases Ms temperature**

  • Variation of strength and hardness in maraging steels**

  • Strength/toughness combination of 18 Ni maraging steels compared to conventional high-strength carbon steels**

  • Weldability of maraging steelsWeldability of maraging steels is due to tough, ductile lath martensite Heat-affected zone(HAZ) in maraging steels can be divided into three regionsFirst region closest to the fusion line contains coarse martensite Second region is a narrow region containing reverted austenite produced by temperatures 595 to 805 CThird region contains martensite that has been age hardened by temperatures from ambient up to 595 C**

  • Strength of welded zoneHeat-affected zone in an as-welded structure is relatively softBecause the metal in the area immediately surrounding the weld is soft and ductile, residual stresses are low, and weld cracking is considerably less than in steels hardened by quenchingSubsequent aging brings the hardness of the weld zone up to that of the base metal**

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