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Differences Between 254SMO and 654SMO Stainless Steels

Date:2023-10-08View:169Tags:ss 304 sheet price,stainless steel hot rolled coil,"316L plate"
Introduction to 254SMO
254SMO, also known as DIN 1.4547, UNS S31254, and 00Cr20Ni18Mo6CuN, is a type of super austenitic stainless steel alloy with six molybdenum elements. It was developed to withstand the presence of halides and acids.

254SMO has an extremely low carbon content, minimizing the risk of carbide precipitation after heating.

Even after sensitizing heat treatment at temperatures ranging from 600 to 1000 degrees, 254SMO can pass the Strauss Test (ASTM A262 Practice E). However, due to its high alloy content, there may be precipitation of metallic mesophase on the grain boundaries within this temperature range. Nevertheless, these precipitates do not lead to intergranular corrosion in corrosive environments. As a result, 254SMO stainless steel can be welded without the risk of intergranular corrosion. The recommended welding rods are ENICRMO-3, and the welding wire is ERNICRMO-3. It's important to note that these precipitates could undergo intergranular corrosion in heat-affected zones if exposed to hot concentrated nitric acid.

Introduction to 654SMO
654SMO, also referred to as S32654 and DIN 1.4652, is a seven-molybdenum super austenitic stainless steel. It is derived from 254SMO but has increased chromium, nickel, and molybdenum content. Additionally, 654SMO contains 0.5% Nitrogen, 24% Chromium, and 3% low manganese. It has a high PRE value of 55, making it excellent in terms of resistance to various forms of corrosion. 654SMO can serve as a substitute for iron-nickel-based alloys and even some nickel-based alloys in numerous corrosive environments. It fills the gap between stainless steels and high-nickel corrosion-resistant alloys, offering high pitting and crevice corrosion resistance. 654SMO is the primary type of seven-molybdenum super stainless steels. The addition of 3% manganese enhances the nitrogen content in the steel, facilitating the formation of a high-chromium, nickel, and molybdenum combination, resulting in a PRE value of 55. This value surpasses that of all six-molybdenum stainless steels, as well as existing super ferritic and super duplex stainless steels. The solid solution of 654SMO exhibits a single austenitic structure, similar to six-molybdenum super austenitic stainless steels. However, inappropriate thermal processing, thermoforming, and heat treatment can lead to the precipitation of intermetallic phases and Cr2N on the substrate, which can negatively affect the performance of the stainless steel.

In terms of general corrosion resistance, 654SMO outperforms 304, 316, and 904L in H2SO4 medium. It also exhibits superior corrosion resistance compared to 254SMO in HC and HF acids.

Regarding pitting and crevice corrosion, 654SMO surpasses both six-molybdenum super austenitic stainless steels and super duplex stainless steel 2507. It even outperforms Inconel 625 and Hastelloy C-276 in certain environments.

When it comes to stress pressure, 654SMO is prone to stress corrosion fracture when the external pressure reaches σ0.2 of the steel. In comparison, 254SMO and super duplex stainless steel 2507 have lower fracture values and are thus less susceptible to stress corrosion fracture than 654SMO.

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