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550M is the cold-drawn version of 520M, the latter being an M-Steel®, so-named because, via treatment of the melt with calcium, improved machinability is achieved without detriment to strength, toughness or weldability. The steel is micro-alloyed via a small addition of vanadium. Machinability is improved even further in 550M as a result of cold drawing.
In relation to chemical analysis, 550M is similar to S355J2C +C which is standardised in SS-EN 10077 - 2 and for which the (discontinued) SS-designations are 2172 and 2174. However, 550M has a higher yield strength thanks to the vanadium addition. Furthermore, and at variance with S355J2C +C, the Charpy-V impact toughness is guaranteed to 27 Joule or better at +20°C.
The diameter tolerance for 550M as cold-drawn rounds is good, h9 as defined in ISO 286 - 2 or better. Furthermore, straightness is considerably better than for 520M hot-rolled round bars and the maximum height of arc is 0.001 times bar length.
520M has good weldability with a carbon equivalent between 0.45 and 0.47% depending on diameter. Welding without preheat is normally quite feasible. Machinability is excellent especially in comparison with that of S355J2C +C.
If the application requires that the part has a wear-resistant surface, 550M responds well to both case hardening and nitriding. Furthermore, its strength can be increased via quenching and tempering provided cooling during hardening is fast enough (preferably in water or polymer quenchant).