Thermal path dependence of secondary carbide precipitation kinetics and morphology in high-Cr cast irons

• Heating rate is the dominant factor controlling secondary carbide (SC) size and fraction, outweighing holding time. • Slow heating (1 °C/min) yields SC 3–5 × larger than fast heating (10 °C/min) due to precipitation during the ramp. • Furnace cooling increases carbide volume fraction by ∼70% only in fast-heated samples; slow-heated samples are insensitive to cooling rate. • Hardness is driven primarily by destabilization temperature and holding time, with heating rate marginal and cooling non-significant as a main effect. High-chromium cast irons (HCCIs) are optimized for wear resistance through controlled heat-treatments. A 26 wt% Cr HCCI was investigated by varying the heating rate (1 or 10 °C/min), cooling mode (water-quenching or furnace-cooling), and hold time (0 or 6 h at 800–980 °C). Microstructures were characterized by SEM and quantitative image analysis, from which secondary-carbide size, carbide volume fraction (CVF), and phase constituents were obtained. The overall precipitation trajectory was governed by the heating rate: under slow heating, CVF and particle size were approximately three to five times higher than under fast heating, and the final microstructure was dominated by this difference. The cooling rate had a comparatively minor effect, except after fast heating, when furnace cooling produced additional fine carbides that increased CVF by about 70% without changing the mean size. A 6 h hold promoted coarsening; at 900 °C, CVF was maximized, whereas at 980 °C, a decrease in CVF with an increase in size indicated partial dissolution of small carbides. Slow-heated samples evolved gradually because substantial precipitation occurred during heating, whereas fast-heated samples changed mainly during the hold and cooling stages. Thus, heating rate, together with hold duration and cooling path, must be considered in process design