Why Grate Bars Jam After Months: Cast Iron Grows at High Temperature

Jamming, arching, a grate that will not push: the first reaction on site is usually to check the gearbox, the chain and the tensioner. There is a class of causes where the bar itself has changed size. Cast iron grows irreversibly at high temperature and does not shrink back when it cools. That is not thermal expansion, and the mechanism is documented in the literature.

1. Growth and thermal expansion are two different things

Thermal expansion goes up with heat and back down with cooling; it is fully reversible. Growth is not: after repeated high-temperature service the same piece of cast iron is larger at room temperature than it was when it left the foundry, and the increase is net.

There are two sources, both established experimentally. The first is graphitisation of cementite, together with repeated graphitising and austenitising during cycling. The second is oxidation: the oxide that forms occupies more volume than the metal it consumed, and that oxide is packed into graphite channels and cracks, prising the matrix apart. The deeper the oxidation, the further it opens.

So on a used grate bar, growth has two measurable signatures: the dimensions increase, and there is usually layered scaling parallel to the working face together with a dark internally oxidised band along the graphite channels. Thickness does not necessarily drop; some bars look fatter than new ones.

2. Temperature and atmosphere set the rate: published data

Above 400℃ the graphite in cast iron is no longer stable in air and oxidises first. Once the graphite has burnt out it leaves pores and cracks, oxygen follows those channels inwards and subsurface oxidation begins (Lin et al., 2011)
With similar composition and a ferritic matrix in both cases, the parabolic oxidation rate constant of flake graphite cast iron is about 5× that of spheroidal graphite cast iron; the spheroidal iron behaves much like low carbon steel. The reason is that flake graphite is almost interconnected, which amounts to a road built for oxygen (same source)
Volume increase under repeated thermal cycling: when heated to 700℃ the increase comes mainly from oxidation; above 800℃ it is joined by porosity generated by repeated graphitising and austenitising, and that porosity makes further oxidation easier (Metal Science and Heat Treatment, 1971)
At 850℃ the volume grows by about 0.7% per cycle in air and about 0.05% per cycle in vacuum, a difference of 14×. In other words, oxygen matters as much as temperature in this chain (same source)

3. The same temperature ceiling does not mean the same life

GB/T 9437-2009 assigns service temperatures to heat-resistant cast iron grades, with the top grade reaching 1100℃. But a temperature ceiling is only an entry ticket. Close to that ceiling, the choice between grey iron and ductile iron is not decided on the strength table but on the oxidation and growth resistance that graphite morphology governs.

RTSi5 and RQTSi5 in our material comparison table are exactly that pair: nearly the same silicon target, one a grey iron with flake graphite and one a ductile iron with nodular graphite. Left in an oxidising atmosphere above 800℃ for long periods, the oxidation rates of the two can differ by a factor of 5 on the data above.

That is also why a common practice on site works: replace the rows in the main combustion zone with ductile iron or a high-chromium casting, and keep ordinary heat-resistant cast iron in the drying and burnout zones. What changes is not “a more expensive material” but graphite morphology and chromium content.

4. Four steps on site to confirm growth rather than wear

Measure: compare new and used bars at the same position, three readings each on bar width, bar length and head thickness. Growth means larger dimensions with thickness not necessarily reduced; abrasive wear means reduced thickness with dimensions largely unchanged
Look: check the section for layered cracks and scaling parallel to the working face, and for a dark internally oxidised band along the graphite channels
Sort by position: worst along the main combustion zone and mild at both ends points to temperature-driven growth; uniform thinning across the whole grate points more to atmosphere or an under-rated grade
Rule out: drive-side current, abnormal noise, chain tension and rail wear all cause a grate that will not push; clear those before blaming the material

The first two steps are the method described in how to check a casting certificate: measure the dimensions, then look at the section and the microstructure. For why bars break outright, see the four failure modes.

5. Four things you can change, and three to put in the purchase agreement

What can be changed: first, bring the bar surface temperature down by holding the main combustion zone in place with air distribution, bed thickness and grate speed, because growth accelerates markedly above 800℃; second, cut the number and amplitude of thermal cycles, since start-stop, cold-air shock during shutdown and long low-load running each add an irreversible increment; third, reduce the oxidising atmosphere by holding excess air at the lowest level combustion allows, which is where the 14× gap between air and vacuum comes from; fourth, select by graphite morphology and chromium content, zone by zone. On assembly, fit new bars at the design clearance rather than hoping a tighter fit seals better, and do not straighten and reuse bars that have already grown, because straightening only turns distortion into residual stress.

Put three items into the technical agreement instead of a grade alone: grade plus graphite morphology (grey or ductile), service temperature ceiling and atmosphere, and the acceptance basis (chemical composition, graphite morphology by metallography, hardness).