High-Temperature Performance of RTSi5 Heat-Resistant Cast Iron Grate Bars

RTSi5 is a national-standard heat-resistant cast iron grade with 4.5%-5.5% silicon and a mainly ferritic matrix. Its designed long-term safe service temperature is 750℃-850℃, with a short-term peak of up to 880℃. We supply grate bars in this grade and can check the material against the boiler type and operating conditions.

Material and high-temperature oxidation resistance

Under the normal boiler range of 750℃-850℃, the heated surface forms a dense, continuous film of SiO₂ silica that adheres to the casting surface, blocks oxygen and stops the matrix from further oxidising and burning away, so large-area scaling, spalling and thinning are not likely. In coal-fired and biomass furnaces running all year, grate wear and burn-off rates are low. At the same temperature, its oxidation life is 2.5-4 times that of grey cast iron.

Thermal shock resistance, strength and corrosion resistance

(1) Thermal shock: boiler start-up, banked firing and intermittent heating bring rapid temperature rises and falls, and RTSi5 resists thermal shock better than grey cast iron and high-chromium cast iron, though its toughness is below that of nodular cast iron and cast steel. Normal start-up and shutdown of an intermittent heating boiler will basically not crack or break the bars, but frequent severe quenching can still cause cracking. (2) High-temperature strength and deformation: at 750℃-850℃ creep is small, so the bars do not warp or bend easily, grate gaps do not widen and leak more coal, load bearing is stable, and strength is sufficient for the fuel bed of a chain grate.

(3) Flue gas corrosion: sulphur dioxide attack from ordinary low-sulphur coal is slight, and potassium-sodium alkaline ash from biomass fuels such as straw and wood chips can still be resisted. Resistance to high-sulphur corrosion, however, is far below that of chromium-bearing heat-resistant cast iron.

Over-temperature behaviour and limitations

When the temperature rises to 880℃-950℃, the silica film on the surface begins to soften, crack and spall, and the grate oxidises and thins quickly with edges scaling off. Creep then increases, the bars warp and bend, mating clearances get out of order, chains jam easily and coal leaks. With brittleness rising, slight vibration or thermal shock is enough to break a bar, and service life is cut by more than half.

Above 950℃ for long periods silicon 5 is no longer suitable; oxidation is fast and the bars may burn out within months. When burning high-sulphur or poor-quality coal, SO₂ penetrates the silica film and attacks the matrix, producing pitting and rust pits and a clear drop in life, and such duty is better served by RTCr1 and RTCr2 low-chromium cast iron. For the push-pull impact duty of a reciprocating grate, cast steel or nodular iron is also preferred.

Service life reference

The reference life under different duties is shown in the table below. Actual life is also affected by coal quality, furnace temperature and start-stop frequency.