Si₃N₄-Bonded Silicon Carbide Brick for Blast Furnace Bosh, Belly & Tuyere

Si₃N₄-bonded silicon carbide brick is the material that made long blast furnace campaigns in the bosh and belly practical. It works for a reason that is counter-intuitive if you are used to specifying refractory as a barrier: its job is not to keep heat in, it is to move heat out.

The Lining Survives by Conducting, Not Resisting

In the bosh and belly, the lining is being attacked from the inside by heat, alkali and abrasion. A low-conductivity material traps heat; the hot face temperature climbs, the critical isotherm moves outward, and wear accelerates. Adding thickness to a low-conductivity lining can actually make things worse, because you are moving the hot face further from the cooling staves.

SiC conducts. At 35 W/(m·K) at 300 °C and still 15 W/(m·K) at 1300 °C, XZK-NSiC grades pull heat into the cooling system, keeping the hot face below the temperature at which alkali attack and softening accelerate. This is why thermal conductivity is the first number to check on any bosh or belly material — before density, before strength.

Why the Bond Matters as Much as the SiC

Silicon carbide grain gives conductivity and abrasion resistance. The bond holds it together — and in the lower furnace, the bond is what fails first. Circulating alkali attacks oxide and clay bonds, and the brick loses cohesion long before the SiC itself is consumed. Si₃N₄ (and SiALON) bonding is far less reactive, which is the practical reason nitride-bonded grades outlast cheaper alternatives by a wide margin in this zone.

Verify the bond by asking for the Si₃N₄ content: XZK-NSiC grades are specified at ≥20% Si₃N₄, and the figure is on every batch Certificate of Analysis.

Grade Selection

• XZK-NSiC72 — 72% SiC, density 2.67 g/cm³, CCS 160 MPa, HMOR 45 MPa at 1400 °C. Inset bricks, lining bricks, tuyere combination bricks.
• XZK-NSiC70 — 70% SiC, density 2.55 g/cm³, CCS 120 MPa. Tuyere sleeves and less severe duties.
• XZK-SASiC — SiALON-bonded, density 2.69 g/cm³, porosity 15.0%, CCS 160 MPa. Where a SiALON bond is preferred.

All three are rated at 30 thermal shock cycles at 1100 °C water quench and RUL above 1700 °C.

Manufacturing and Documentation

XZK nitride-bonded SiC bricks are pressed, nitrided under controlled atmosphere, and machined where required. Thermal conductivity is verified on every production batch rather than only at type approval, because it is the property that governs performance. Bulk density and porosity per ASTM C830, crushing strength per ASTM C133, hot modulus of rupture per ASTM C583, RUL per ISO 1893 and chemistry per GB/T 16555 are tested per batch, with a Certificate of Analysis on every shipment.

Where This Grade Sits in the Lining System

Silicon carbide and carbon grades are selected for what they do with heat rather than for what they resist. Thermal conductivity decides whether the cooling system ever sees the load, and the bonding system decides how long the material survives alkali attack and oxidation once it does.

XZK supplies si₃n₄ bonded sic brick as one zone of a zoned package rather than as a standalone item: the grades normally zoned alongside it — among them sialon bonded sic brick — are documented on the silicon carbide and carbon refractories page, and the vessel-level architecture — where each zone starts and ends, and which mechanism actually limits the campaign — is set out on the blast furnace page.

Campaign data measured on this class of lining — including the wear profile that drove grade selection — is published in the MMK Magnitogorsk No.10 BF and 375 t ladle programme project reference.