550MW+ is the cold-drawn version of 520MW+. The latter is an M-Steel® in which treatment of the melt with calcium is supplemented with an addition of sulphur to further promote machinability. The steel is micro-alloyed via a small addition of vanadium. Moreover, the machinability of 550MW+ is improved further as a consequence of the cold drawing. 550MW+ is a realistic alternative to leaded steels if it is desired to mitigate contamination of the workplace by lead.
Apart from the sulphur addition, the chemical analysis of 550MW+ is similar to that for S355J2C +C which is standardised in SS-EN 10077 - 2 and for which the (discontinued) SS-designations are 2172 and 2174. However, 550MW+ 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 550MW+ as cold-drawn rounds is good, h9 as defined in ISO 286 - 2 or better. Furthermore, straightness is considerably better than for 520MW+ hot-rolled round bars and the maximum height of arc is 0.001 times bar length.
The weldability of 550MW+ is limited as a consequence of the high sulphur content. However, machinability is excellent especially in comparison with that of S355J2C +C.
If the application requires that the part has a wear-resistant surface, 550MW+ 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).