Mainstream Grate Bar Material Classification, Performance, and Application Scenarios

Grate bars work for long periods in hot flue gas, ash abrasion and repeated heating and cooling, so the core requirements are high-temperature resistance, thermal shock resistance, wear resistance, oxidation resistance and resistance to ash corrosion. Judged by how often they are used on industrial boilers, mainstream materials fall into three groups: plain cast iron, alloy heat-resistant cast iron and cast steel.

Plain cast iron

Small coal-fired boilers at low temperature and atmospheric pressure usually take plain cast iron, which keeps purchase cost low and is convenient to cast and supply. Grey iron HT150 and HT200 is simple to cast and conducts heat well; its graphite provides a self-lubricating effect, wear resistance is acceptable and the price sits at the low end among the grades. Its drawbacks are brittleness and poor heat resistance: long-term tolerance is ≤650-700℃, above 700℃ it oxidises and scales readily, and cold water shock makes it crack, so service life is short. It therefore suits domestic heating stoves, small 1-4t chain grates, low-temperature biomass boilers and passive grate bars that carry no tensile load.

Malleable iron (KTH350) is clearly tougher than grey iron, does not snap easily and absorbs light impact. Driving grate bars of a chain grate carry the drive load and rub at the pins, so this position suits malleable iron, and old small boilers often used the same material. Nodular iron QT450 and QT500 has spheroidal graphite, with better strength, toughness and thermal fatigue resistance than grey iron and a slight gain in heat resistance. It suits medium-load reciprocating grates and biomass combustion furnaces, sitting between plain iron and heat-resistant iron.

Alloy heat-resistant cast iron

Alloy heat-resistant cast iron is used widely on industrial coal-fired and biomass boilers, and high-silicon iron RTSi5 (known as silicon 5) takes a large share of that use. Its silicon content is 4.5%-5.5%; at high temperature the surface forms a dense silica protective film, so oxidation resistance is strong and the price is moderate. Long-term service temperature is 750-850℃, and it is a common material for straight, four-claw and scale-type grate bars. Chain grates, reciprocating grates, biomass boilers and most coal-fired industrial boilers can use it according to duty.

Among the chromium irons, RTCr1 and RTCr2 are low-chromium grades with slightly better heat resistance than silicon 5 and better sulphur corrosion resistance, suiting boilers that burn high-sulphur coal. RTCr16 is a high-chromium heat-resistant iron with 15%-18% chromium that tolerates above 900℃, with strong oxidation and corrosion resistance. It is mostly used in high-temperature combustion zones, waste incinerators, circulating fluidised bed auxiliary grates and duties burning poor high-sulphur fuel.

Aluminium heat-resistant iron RTAl has strong high-temperature performance but is brittle and hard to cast, so it is mostly used in small high-temperature furnaces. Because casting is difficult, the range of sizes available for general grate service is fairly limited.

Cast steel and special materials

Duties with heavy load, high temperature and frequent reciprocating motion generally call for cast steel. Plain carbon cast steel ZG230-450 is tough, impact resistant and does not break easily, with average wear resistance, and it oxidises and deforms readily at high temperature. It suits moving bars of reciprocating grates, heavy cross-beam grates and tumbling grates of waste incinerators, positions that take repeated push-pull load.

Heat-resistant alloy cast steel covers high-chromium cast steel and 1Cr18Ni9 stainless cast steel, tolerating 950-1100℃. These steels resist acid, flue gas corrosion and slag abrasion well, and the price is higher. Grate bars for municipal solid waste incinerators, hazardous waste incinerators and large high-temperature power station boilers mostly use this group.

Among niche special materials, high-chromium white cast iron leans towards wear resistance with average high-temperature performance, and is mostly used for anti-wear plates on the side of the grate. Compacted graphite iron balances heat conduction and strength, and is rarely used for grate bodies.