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V155- Indurenti per precipitazione

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Valbruna Grade

V155

Steel type

Precipitation Hardening

Description of material

It is a martensitic precipitation hardening stainless steel with strength and good toughness comparable to standard V174 and V174LC. The mechanical properties are obtained by a solution treatment (Cond. A) that brings the Cu in solution in the Austenitic matrix followed by a rapid cooling, obtaining a super-saturated Cu martensitic structure. A re-heating (ageing) at t° = 480°C gives a maximum Hardness and resistance Rm with a low Kv impact due to a precipitation of a Cu –rich phase while at t°= 620°C this results in a higher Kv impact with a reduction of Rp0, 2 and Rm due to a progressive softening of Martensite and the formation of both Cu-globules with a loss of coherence within the matrix, and stable Austenite. It is important to know that the transformation of Austenite to Martensite is completed below 30°C and the formation of stable Austenite during aging can start to appear at the lowest aging temperature. This strongly depends on the specific chemical balance, to avoid the formation of Ferrite, thereby providing better transversal toughness.

Applications

All parts or devices where better toughness, and resistance to certain kinds of corrosion, compared to standard V174, are required. Typical applications are aircraft components, valves, large forging, shafts, chemical industry and nuclear devices.

Melting practices

AOD, AOD + ESR, AOD + VAR

Corrosion resistance

This grade has the same general resistance corrosion as 304, but better than the group of standard martensitic 400 series. However, solution treatment (cond. A) without aging should be avoided. For maximum resistance to Chloride stress corrosion cracking, it should be aged at a higher temperature, not less than 550-580°C. For Sulfide aggressive environments, aging at 620° C or overaging, is better. The same choice should be made in the case of situations or environments prone to cause H-embrittlement. It should also be noted that for this grade, as for every kind of stainless steel, surfaces should be free of contaminants and scale and passivated for optimum resistance corrosion.

Cold working

This grade has limited cold deforming capacity in the annealed condition (cond.A) due to untempered Martensite. More severe cold working requires aging at highest temperature or overaging. For restoring or increasing mechanical properties, such as Tensile Rm, a new solution treatment (cond.A) followed by a suitable aging temperature, should be done.

Machinability

Machinability is good in both solution-treated (cond.A) and precipitation hardened conditions, considering that this property improves when hardness decreases. A certain amount of dimensional changes, in terms of contraction, happens after the aging of parts: these dimensional variations should be evaluated.

Weldability

This grade has a good weldability and doesn’t normally need preheating, but welding design should be well evaluated in order to avoid situations prone to generate stress. In short, small sections could be welded in the solution treatment condition followed by an aging; large or heavy sections require a high temperature aging or overaging obviously followed by a new solution treatment (cond. A) and an aging. Autogenous high energy welding, with their typical fast cooling rates, together with high welding speeds, may cause weld solidification cracking in fused zones.

Hot working

Ingots or large forgings require a suitable preheating, in order to avoid thermal cracking. Avoid overheating (high temperatures could cause the formation of delta ferrite, jeopardizing transverse toughness) and also avoid improper cooling. Large forging bars should be equalized at 1030 -1040°C in the heating furnace prior to cooling. Both small or large forgings, rolled rings or bars must be cooled under 30°C after solution treatment (cond. A) in order to complete the transformation of martensite obtaining both a good structure and mechanical properties after aging.

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