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Description of material
V174LC is a martensitic precipitation hardening stainless steel with high strength and hardness. This grade is designed to obtain the highest Rp0, 2/Rm ratio at a low aging temperature (i.e. as 480°C) with a structure free of stable austenite. 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 550°. This strongly depends on the (Cr/Ni) equivalent balance that, besides, also influences the amount of Ferrite in the matrix.
V74LC is used for stainless steel load cells and applications where the highest values of Rp0, 2/Rm are required.
Argon Oxygen Decarburization
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. When V174LC is used for load cells requiring the highest Rp0, 2/Rm, a low aging (as 480° C) should be done and, in this condition, there is no stable austenite in the structure and a small amount of ferrite. For maximum resistance to Chloride stress corrosion cracking, it should be aged at a higher temperature, not less than 550-580°C, but in this case the performances of load cells could be jeopardized. It’s important point out that in the 480°C aging condition, V174LC does not have a good resistance to stress corrosion, sulfide stress cracking and 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.
This grade has a limited cold deforming capacity in the annealed condition (cond.A) due to untempered Martensite and a certain amount of ferrite. More severe cold working requires aging at the highest temperature or overaging. For restoring or increasing mechanical properties, such as high Rp0, 2/Rm ratio, a new solution treatment (cond.A) followed by a low temperature aging should be carried out.
Machinability is good in both the solution-treated (cond.A) and precipitation hardening conditions, considering that this property improves when the hardness decreases. A certain amount of dimensional changes, in terms of contraction, happens after the aging of parts: these dimensional variations should be evaluated.
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. V174LC is less prone to cause hot cracks in the fused zone, in case of autogenous high energy welding.
Ingots or large forgings require a suitable preheating, in order to avoid thermal cracking. Avoid overheating and improper cooling. Large forging bars and shapes 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. It is useful to point out that a certain amount of Ferrite could be in the V174LC structure.