How to Use Converter Hearth Self-Flow Repair Mix: Erosion Criteria, Hot Charging and Sintering Control

How to Use Converter Hearth Self-Flow Repair Mix: Erosion Criteria, Hot Charging and Sintering Control

The converter hearth wall - the lower cone and the charging side - wears faster than any other zone in the vessel: scrap impact, tapping-line wash and local overheating from slag foaming all land here. A full relining costs seven to ten days of vessel time, while one hot repair takes only a tapping gap. Whether that repair survives or drops into the slag pot within three heats depends on a few numbers: self-flow mix versus gunning material, the charging temperature window, and how thick each layer goes.

First Judge the Wear: When Is Charging Worth It

Inspect the hearth in two phases: visual checks every 50 heats before 200 heats, then a thickness measurement every 30 heats after - laser gauging combined with visual inspection when the vessel is tilted. Three triggers call for repair: a pit deeper than 50 mm; working-layer magnesia-carbon brick below 120-150 mm residual (from an original 500-600 mm, less than a third left); or continuous spalling on the slag line exposing a whitened carbon skeleton. Charging before the trigger line only buries the measurement under fresh mix and costs more later in the campaign.

Why Self-Flow Mix Beats Gunning Here

Gunning material bonds at 70-85% adhesion, and the rebound goes into the slag pot along with half the money; a gunned layer relies on chemical binders and resists slag and wash worse than the magnesia-carbon system it patches. A hot-charged self-flow magnesia-carbon mix, typically 8-14% carbon, flows and self-levels into the pit, packs the edges, and sinters into one body on residual lining heat - almost no rebound, and chemically identical to the lining brick, so the interface fuses well. Its limit: it suits flat areas visible and reachable from the mouth. Awkward corners are easier to gun. The two divide the work; neither replaces the other.

The Charging Window: 800-1100 degrees C Is a Hard Line

Hot charging lives or dies on lining surface temperature. Below 800 degrees C the layer cannot sinter onto the old brick - it hangs loose and peels off whole at the first blow. Above 1100 degrees C the mix slumps down the slope, too thin where it must build and lumpy where it should not, and never shapes the profile you need. Practical routing: after tapping and tilting, read the hearth surface with an infrared or optical pyrometer; too hot means hold on slag and let it cool, too cold means a short oxygen trim, then charge within 800-1100 degrees C. Spread the mix along the charging side from the mouth, low spots first, never dumping a whole bag on one point.

Layering and Sintering Time

Charge 100-150 mm per layer and let each layer sinter 15-25 minutes on residual heat before the next; deep pits take two or three layers to the contour line, with material budgeted at 1.2 times pit volume. Do not rock the vessel while a layer sinters - a finished layer shows a uniform dull-red face with no loose dust. Keep clear of the tap-hole and oxygen-lance footprints; mix that falls into either does more harm than skipping the repair. A charging frequency of every 30-80 heats is normal, densifying late in the campaign. If it climbs to every 20 heats, wear has outrun repair - time to schedule a reline.

Keep the Log, Do the Arithmetic

Record four items per repair: date, heat count, charged mass, and residual thickness before charging; log thickness again at the next inspection, and the difference is the real consumption rate of the repair layer. That rate forecasts hearth life three to four weeks out, so both repairs and reline land inside planned windows. The hard lines for relining: local shell temperature above 400 degrees C or visible red heat, repair frequency at every 20 heats, or spalling signs appearing on the trunnion side at the same time - any one of the three starts the reline process. Hot charging pays not in single repairs but in days of vessel time won back from the unplanned column; keep the log for three months and the arithmetic makes the case by itself.