High-Chromium Wear-Resistant Cast Iron and Alloy Steel Grate Bars

Grate bar materials fall into two groups by duty. High-chromium wear-resistant cast iron suits heavy-load positions with strong abrasive erosion and temperatures not above 850℃, while heat-resistant austenitic cast steel suits high-temperature and thermal-shock duty.

High-chromium wear-resistant cast iron

The representative grade is KMTBCr26 (Cr26 high-chromium cast iron); its long-term service temperature is ≤850℃, its hardness is HRC58-62, and its wear resistance is high among cast iron materials. The structure is austenite plus hard chromium carbides, so resistance to abrasive erosion and crushing wear is strong, while oxidation resistance is moderate. The advantages are a wear life 2-3 times that of ordinary heat-resistant cast iron, resistance to ash erosion and little creep deformation at high temperature; the limitations are moderate resistance to rapid heating and cooling, chloride corrosion resistance lower than heat-resistant steel, and no long service above 900℃.

This material suits large-capacity chain grates, reciprocating pusher grates and biomass pellet boilers burning high-ash fuel such as straw and wood chips. It is also used for the auxiliary grate of circulating fluidised bed boilers in heating power plants and for the heavy-load carrying positions of stoker-fired boilers. In coal-fired duty with furnace temperature not above 900℃ where wear life is the aim, its application share in domestic stoker-fired furnaces is about 50%.

Heat-resistant austenitic cast steel

Chromium-nickel austenitic heat-resistant cast steel is better than all cast irons in thermal-shock resistance, high-temperature strength and oxidation resistance. There are 3 common grades.

1. ZG3Cr18Mn12Si2N is a chromium-manganese-nitrogen steel with a temperature rating of 850-950℃; manganese and nitrogen replace costly nickel, so cost is moderate. Its resistance to alternating thermal shock is good, carburisation resistance and high-temperature toughness are good, and its oxidation resistance is lower than that of high-nickel steel. It suits industrial boilers with frequent starts and stops, reciprocating grate bars, medium-sized biomass boilers and moving grate bars of chain grates.
2. ZG35Cr24Ni7SiN (247 alloy) is rated 1050-1100℃, with a balanced chromium-nickel ratio and fairly balanced high-temperature strength, oxidation resistance and sulphide corrosion resistance. Its thermal-shock performance is good, and it suits large industrial coal-fired boilers, high-temperature biomass incinerators, heat-recovery boiler grates and the normal positions of the drying and combustion zones of waste incinerators.
3. ZG40Cr25Ni20Si2 corresponds to rolled 310S heat-resistant steel, has a temperature ceiling of 1200℃, and is the higher-rated conventional material for commercial grate bars. Its high-chromium high-nickel austenite gives high-temperature oxidation resistance, creep resistance and sulphide corrosion resistance, and it withstands severe temperature swings; the limitation is a high price. It suits municipal solid waste incinerators, hazardous waste incinerators, the high-temperature section of power station boiler grates and severe duty with furnace temperature above 1000℃.