Sinter grate bars and pallet parts: what they do and why they fail
Published · Updated · Ni-Chrome Alloy Castings
A sinter strand is a loop of pallets that carries a bed of ore fines, flux and coke under an ignition hood and over suction wind boxes. A travelling grate in a pellet plant works the same way with green pellets. Every pallet is a set of castings: grate bars on the floor, side walls on the edges, a body underneath, and chain links, rods and bushes that join it to the next one. They run hot on top and cooler below, carry an abrasive load, and get tipped and knocked at the discharge end every lap. This guide takes the parts one at a time. For each we say what it does and how it fails, then which grade family is usually specified and what the RFQ should contain.
Which alloys are used for sinter grate bars and pallet parts?
Two families cover most of it. High-chromium cast irons are common for sinter grate bars, where oxidation resistance and hardness both matter (Zhiru et al., 2019; Shi et al., 2024). Heat-resistant Cr-Ni cast steels, of the kind listed in ASTM A297 (HH, HK and similar), are used for pellet-plant grate plates and links that run hotter or need more toughness (Bagheri et al., 2020; Kishore et al., 2023). Pallet bodies and cooler structural parts are often plain or low-alloy cast steel. The grades page lists what we pour, and the wear irons page covers the high-chromium irons.
What do sinter grate bars do?
Grate bars form the floor of each pallet. They sit side by side in rows with narrow gaps between them, so suction air can be drawn down through the burning bed while the fines stay on top. A strand carries a very large number of them, and they are replaced in bulk.
Why do grate bars fail?
The top of the bar sits under the flame front, so oxidation does the first damage. Zhiru et al. (2019) tested a high-chromium cast iron grate bar and found it protected by Fe3O4, Fe2O3 and Cr2O3 oxide films at temperatures below 800 °C. Where the bar runs hotter than that, or the film keeps spalling off, it loses section. Abrasion at high temperature follows, from the sinter cake and return fines sliding over the bar. Thermal fatigue adds to it, because the bar heats under the hood and cools on the return run every lap. Shi et al. (2024) characterised the structure and failure mechanisms of a failed high-chromium cast iron grate bar. Sticking and burning-in is the last problem: molten slag and fines fuse into the gaps, the bars stop moving freely, the gaps block and suction drops. A bar that thins or cracks then drops out of its row and lets material fall through.
Grades usually specified: high-chromium cast irons for most sinter strands, and heat-resistant Cr-Ni cast steels where the bar runs hotter or must take more knocking.
What to put in an RFQ: bar drawing (length, section, foot and hook profile), the gap the bar must keep, grade or chemistry, hardness if specified, the pallet or machine it fits, and quantity per lot.
We cast grate bars to drawing in high-chromium iron and heat-resistant steel. Enquire about grate bars →
What do upper and lower side walls do?
Side walls bolt along the two long edges of each pallet and hold the bed in. The lower wall sits on the pallet body. The upper wall raises the bed height. Together they set how deep a bed the strand can carry, and they seal the edge so air is drawn through the bed rather than around it.
Why do side walls fail?
The upper wall sees the flame front and the hottest part of the bed edge, so it oxidises and warps. Both walls take abrasion from the bed and from the cake breaking away at discharge, and they crack from thermal cycling at bolt holes and corners. Sinter sticks to the inner face and builds up, which narrows the bed and has to be knocked off. A warped wall leaves a gap at the edge, and air leaks past the bed.
Grades usually specified: heat-resistant Cr-Ni cast steel or high-chromium iron for upper walls, and the same or a leaner grade for lower walls, which run cooler.
What to put in an RFQ: separate drawings for upper and lower walls, bolt hole pattern, grade, which side of the pallet each fits, and quantity.
What do pallet bodies and travelling-grate plates do?
The pallet body is the cast frame of the car. It carries the grate bars and side walls, rides on wheels along the rails, and passes over the wind boxes. On a pellet-plant travelling grate the same job is done by grate plates or grate carriers, which carry the pellet bed through the drying, firing and cooling zones.
Why do pallet bodies and grate plates fail?
A pallet body runs cooler than its grate bars but heats and cools every lap, and it is large, so it cracks from thermal fatigue at changes of section and at the seats for the bars. It also distorts, and a distorted body no longer seals against the wind boxes. Grate plates on a travelling grate run hotter. Bagheri et al. (2020) examined used heat-resistant stainless steel grate plates from an iron-ore pelletizing plant and found intergranular cracking. They traced it to Mn, S and C rich precipitates between the dendrites, not to chromium depletion, and suggested lowering sulphur to narrow the solidification range. Oxidation and abrasion from the pellet bed add to the wall loss.
Grades usually specified: cast steel or heat-resistant Cr-Ni steel for pallet bodies, depending on how hot the body runs, and heat-resistant Cr-Ni stainless grades for travelling-grate plates.
What to put in an RFQ: pallet or plate drawing, grade and any limit on sulphur or other residuals, machining on wheel seats and mating faces, inspection and NDT required, and quantity.
We cast pallet bodies, grate plates and pallet car assemblies to drawing. Enquire about pallet bodies and grate plates →
What do chain links, side plates, through-rods and bushes do?
On a travelling grate the plates are carried by a chain of cast links and side plates, joined by through-rods that run across the width of the grate. Bushes sit where rods pass through links and take the wear as the chain turns over the sprockets. On a sinter strand, rods and bushes do similar work where pallets and their parts are pinned together.
Why do chain links and rods fail?
Links carry the pull of the whole grate while running hot, so they fail by embrittlement and cracking more than by wear. Kishore et al. (2023) studied failures of heat-resistant stainless steel travelling grate links and attributed them to sigma phase embrittlement, a brittle phase that forms in Cr-rich steels after long service at temperature. A link that has gone brittle can snap under a shock load it once took easily. Rods and bushes wear at the joint and oxidise, and once the joint opens up the chain loses its pitch and runs badly on the sprockets.
Grades usually specified: heat-resistant Cr-Ni cast steels for links and side plates, with the chromium and nickel balanced against the risk of sigma in long service. Rods and bushes in heat-resistant steel, or a harder grade for bushes where wear governs.
What to put in an RFQ: link, side plate, rod and bush drawings, the grade, the chain pitch and the mating parts, the zone of the grate where they run, any hardness or mechanical test needed, and quantity.
We cast chain links, side plates and bushes for travelling grates. Enquire about chain links and rods →
Why buy grate bars from a foundry that already supplies steel plants?
Grate bars look simple, but a strand takes them in bulk and every one has to hold its gap, sit flat in its row and last as long as the one beside it. A bad lot shows up as blocked gaps, dropped bars and an early shutdown. We have cast heat-resistant and wear-resistant parts for steel plants for decades. We pour both the high-chromium irons and the Cr-Ni heat-resistant steels, so the grade follows the duty. Every heat is checked in our own lab, and we work from your drawing or a sample bar. Send the drawing with your first enquiry and we will come back with questions before we quote.
Frequently asked questions
Should grate bars be high-chromium iron or heat-resistant steel?
High-chromium iron is harder and resists abrasion and oxidation well, which suits most sinter strands. Heat-resistant Cr-Ni steel is tougher and holds up better where the bar runs very hot or takes a lot of impact. If your bars crack, look at toughness. If they thin, look at oxidation and wear resistance.
Why do grate bars stick in the pallet?
Fines and slag melt into the gaps and around the bar feet, then freeze. Bars that have grown or warped from oxidation and heat make it worse. Stuck bars block suction and are hard to change, so they come out broken.
What causes chain links on a travelling grate to break?
Often embrittlement after long service at temperature rather than overload. Sigma phase forms in Cr-rich heat-resistant steels and makes them brittle (Kishore et al., 2023). A metallographic check on a broken link shows whether that is the cause.
Can you match the grate bars we already use?
Yes. Send the drawing, or a sample bar if there is no drawing, and the grade if you know it. If you do not, a chemical analysis of a used bar is a starting point.
Do upper and lower side walls need the same grade?
Not always. The upper wall runs hotter and sees more oxidation, so it often gets the richer grade. The lower wall can be leaner.
What do I need to send for a quote?
A drawing, the grade or the service conditions if the grade is open, quantity, and any inspection or test requirements. Our guide to getting a quote lists the rest.
Sources
- ASTM A297/A297M, heat-resistant iron-chromium and iron-chromium-nickel steel castings
- Zhiru et al. (2019), Casting process and high temperature oxidation resistance of high chromium cast iron grate bar, Engineering Review
- Shi et al. (2024), Structure characterization and failure mechanisms of high chromium cast iron grate bar, Engineering Failure Analysis
- Bagheri et al. (2020), Damage characterization of heat resistant stainless steel grate plates in an iron ore pelletizing plant, Engineering Failure Analysis
- Kishore et al. (2023), Sigma phase embrittlement-induced failures of heat-resistant stainless steel traveling grate links, Engineering Failure Analysis