Structure of centrifugally cast austenitic stainless steels: Part 1 HK 40 as cast and after creep between 750 and 1000°C

Centrifugally cast 0·4C–25Cr–20Ni (wt-%) steel (HK 40) has been subjected to detailed microscopic examination both in the as-cast condition and after long-term creep. The as–cast alloy contained a eutectic of M7C3 and austenite, which was subjected to energy-dispersive X-ray analysis. Observations by transmission electron microscopy revealed high dislocation densities near cell boundaries. After creep in the range 750–1000°C, the eutectic carbides coarsened and transformed from M7C3 to M23C6; moreover, the M23C6 carbide precipitated predominantly on dislocations as a finer dispersion within the grains. During coarsening, the M23C6 particles generated further dislocations in the austenite. The higher creep strength of the cast HK 40 compared to wrought alloys is discussed in terms of the detailed microstructural observations, in particular the M23C6 dispersion.MST/135