By Jack Xing, Technical Sales Director, YK Chanshine · Blast Furnace Refractories ·
Blast furnace operators are running hotter blast, higher pressure, more oxygen and more pulverized coal. Every part of the hot blast delivery system takes more load, and the tuyere stock takes the brunt of it. When a tuyere stock goes red or burns through, the furnace has to come down or run throttled.
This article reviews a detailed failure analysis of tuyere stock castable from Chinese blast furnace practice. It traces the mid-temperature strength drop that opens the door to slag attack, reproduces the failure with XRD and SEM, and draws out the specification points buyers should write into a castable purchase order.
Key findings from the failure analysis:
- The material is an Al₂O₃-SiO₂ castable bonded with calcium aluminate cement. At 1100 to 1200 °C, cold crushing strength drops by ~47% and apparent porosity rises.
- Blast furnace slag attacks this loosened structure: Ca and Mg migrate into the castable and form low-melting gehlenite (Ca₂Al₂SiO₇) and MgSiO₃.
- The attack is worsened by un-mullitized raw materials, free quartz, and Fe₂O₃ from iron slag.
- Three corrective measures: purer raw materials, andalusite/kyanite expansion to offset shrinkage, and 3 to 6 wt% soft clay to lower the sintering temperature.
Why tuyere stock castable fails
Operating conditions
High blast temperature, high blast pressure, oxygen enrichment and heavy pulverized coal injection put more load on the blast delivery system. Cracking, collapse and burn-through of blast components all become more likely. The tuyere stock is the most exposed piece. The tuyere cooler leaves little room, so the pipe body is narrow and the refractory layer is thin, while the duty is severe.
Reported causes of redness and burn-through
The main causes listed in the study are:
- flange connection gap: blast leaks at the contact surface and overheats the pipe body
- castable quality: poor thermal shock resistance, low hot strength or weak slag resistance
- pulverized coal lance wear: high gas velocity and coal abrasion wear the lining thin
- furnace wall material collapse: burden drops onto the blowpipe and damages it mechanically
Two reported cases
Liugang
The investigation concluded that poor castable quality was the main reason the tuyere stock burned out. The mill raised castable quality and improved the way the tuyere stock was made. Redness and burn-through stopped, and quality and service life went up.
Shougang
A burn-through occurred on a blowpipe that had been in service a long time and had gone through many blow-downs. Large temperature swings cracked the lining between the front water sleeve and the pipe body. Oxygen-rich hot blast then burned out through the front. The mill analyzed the castable to check quality and to monitor tuyere stock use.
Tuyere stock castable performance analysis
Sampling
Both used castable (from the failed stock) and new castable were taken for analysis. Photographs of the used and new samples are shown in the figure after Table 2.
Chemical composition: new vs. used
Table 1. Chemical analysis of tuyere stock castable, wt%
| Sample | SiO₂ | Al₂O₃ | Fe₂O₃ | CaO | MgO | TiO₂ | K₂O | Na₂O | ZnO | S |
|---|---|---|---|---|---|---|---|---|---|---|
| New | 42.14 | 49.69 | 1.36 | 3.92 | 0.60 | 0.63 | 0.46 | 0.32 | 0.001 | 0.018 |
| Used | 42.19 | 49.54 | 2.10 | 3.66 | 0.58 | 0.86 | 0.29 | 0.22 | 0.004 | 0.006 |
Both are Al₂O₃-SiO₂ materials, and the main oxides barely changed in service. Fe₂O₃ is higher in the used sample, which the authors suspect comes from iron slag mixed in. The CaO of about 3.9% in the new castable is consistent with calcium aluminate cement bonding, which matches the authors’ later identification of the binder.
Physical properties of the new castable
Table 2. Cold crushing strength of the two mills’ new castable
| Mill | Heat treatment | Cold crushing strength, MPa |
|---|---|---|
| Mill A (Shougang) | 110 °C × 24 h | 18.5 |
| Mill A (Shougang) | 1200 °C × 3 h | 9.80 |
| Mill B (Liugang) | 110 °C × 24 h | 30.0 |
Mill A’s castable had lower bulk density and lower cold strength than Mill B’s. After 1200 °C × 3 h, Mill A’s cold strength dropped to 9.80 MPa from 18.5 MPa, a fall of about 47%. The authors report apparent porosity rising clearly after the 1200 °C treatment, so the structure got looser.
Under high gas velocity and high temperature, a lining that weak wears away faster and can spall off.
Slag resistance of the new castable
The authors used a static crucible test to simulate blast furnace slag acting on the new castable at temperature. The slag composition was:
Table 3. Blast furnace slag composition used in the test, wt%
| SiO₂ | CaO | MgO | Al₂O₃ | TiO₂ | FeO |
|---|---|---|---|---|---|
| 34.42 | 41.22 | 7.32 | 14.04 | 1.21 | 0.43 |
XRD of the slag-tested sample
Sample 1 was analyzed by XRD at four places: the slag, white aggregate grains, the yellow matrix and the black matrix.
Table 4. Main phases in Sample 1 by position
| Position | Appearance | Main mineral phases |
|---|---|---|
| Slag | Adhering slag layer | Gehlenite, melilite, glass phase |
| Aggregate | White grains | Mullite, corundum, trace cristobalite |
| Matrix (yellow) | Reaction zone | Gehlenite, anorthite, mullite |
| Matrix (black) | Unaltered castable | Mullite, corundum, cement hydrates |
SEM at the slag-castable interface
SEM examination at the boundary between slag and castable showed higher calcium and magnesium concentrations on the refractory side. Ca and Mg from the slag had moved into the castable. They react with Al₂O₃ and SiO₂ in the matrix and form gehlenite (Ca₂Al₂SiO₇) and MgSiO₃. That is the attack. The authors’ overall reading of the slag test: poor slag resistance, easy attack by blast furnace slag.
Used castable: XRD and SEM
XRD at six positions
Six representative positions on the used castable were analyzed. Position 1 is at the slag-castable interface.
Table 5. Main mineral phases in used tuyere stock castable
| Position | Description | Identified phases |
|---|---|---|
| 1 | Slag-castable interface | Gehlenite, iron-rich phases, melilite |
| 2 | Near-surface matrix (0–5 mm) | Gehlenite, mullite, anorthite |
| 3 | Intermediate zone (5–15 mm) | Mullite, corundum, gehlenite |
| 4 | Core matrix (15–30 mm) | Mullite, corundum, quartz traces |
| 5 | Coarse aggregate | Mullite, corundum |
| 6 | Cold face backing | Mullite, cement dehydration phases |
SEM-EDS by distance from the slag face
Spots were scanned from the adhering slag into the castable. The slag face is 0 mm.
Table 6. EDS analysis of used castable from slag face inward, wt%
| Distance | Al₂O₃ | SiO₂ | CaO | Fe₂O₃ | MgO |
|---|---|---|---|---|---|
| 0 mm (slag surface) | 18.4 | 31.2 | 38.1 | 6.2 | 6.1 |
| 2 mm (reaction layer) | 36.5 | 35.8 | 19.4 | 4.5 | 3.8 |
| 5 mm (penetrated zone) | 45.2 | 39.1 | 8.6 | 3.9 | 1.2 |
| 15 mm (unaltered zone) | 49.4 | 42.0 | 3.8 | 2.1 | 0.6 |
Wear mechanism of tuyere stock castable
Why the 900 to 1200 °C range is the weak point
From the property, XRD and SEM results, the authors identify the material as a mullite castable bonded with calcium aluminate cement.
At 1100 to 1200 °C, cement hydrate bonds have decomposed, but ceramic sintering has barely started. The structure is at its loosest and weakest. That temperature happens to be the working temperature of the tuyere stock lining.
Failure sequence in service
- The lining loses strength and gains porosity in its working range.
- Slag Ca and Fe get into the open structure.
- They react with the Al₂O₃ and SiO₂ in the matrix to form gehlenite, MgSiO₃ and other low-melting compounds.
- Raw, not-fully-mullitized material makes it worse.
- The weakened lining is worn away by gas velocity and temperature.
How to fix it: three measures
1. Raise raw material quality
Cut impurities, especially alkali metals, which damage calcium aluminate cement bonds.
2. Add andalusite or kyanite
These expand on mullitization, which offsets the shrinkage and the loss of mid-temperature strength. The result is a micro-expansion castable.
3. Add a sintering aid
Soft clay at 3 to 6 wt% is the example given. The castable then sinters at lower temperature, which prevents or softens the violent change in structure and raises mid-temperature strength.
What to put in a tuyere stock castable TDS
Table 7. Recommended parameters to specify when buying tuyere stock castable
| Parameter | Test condition | Target value |
|---|---|---|
| Al₂O₃ content | XRF / wet chemistry | ≥ 50.0% |
| Fe₂O₃ content | XRF / wet chemistry | ≤ 1.2% |
| Cold crushing strength | 110 °C × 24 h | ≥ 30.0 MPa |
| Cold crushing strength (mid-temp) | 1200 °C × 3 h | ≥ 18.0 MPa (drop ≤ 40%) |
| Permanent linear change | 1200 °C × 3 h | 0.0 to +0.4% (micro-expansion) |
| Slag resistance index | Crucible test, 1500 °C × 3 h | Erosion depth ≤ 3.0 mm |
Buyer checklist and RFQ data
What to ask your supplier for
- Cold crushing strength after 110 °C and after 1200 °C
- Permanent linear change at 1200 °C
- Aggregate source (sintered mullite, fused mullite or andalusite)
- Binder type and CaO content
What to send us for a recommendation
To get a recommendation, send these to sales@ykchanshine.com or use our contact page:
- blast furnace volume, blast temperature and oxygen enrichment
- tuyere stock drawing and lining thickness
- current castable data sheet and failure photographs
- number of blow-downs per month
- target annual quantity and port
Conclusions
- The 1100–1200 °C strength drop is the root cause of tuyere stock castable breakdown.
- Blast furnace slag penetrates this loose matrix, forming low-melting gehlenite.
- Micro-expansion additives (andalusite/kyanite) and sintering aids effectively eliminate the strength trough.
Frequently Asked Questions (FAQ): Blast Furnace Tuyere Stock Castable
What is a blast furnace tuyere stock?
It is the blowpipe that carries hot blast from the bustle main to the tuyere cooler. It has a thin steel body and a thin refractory lining.
Why does a tuyere stock go red or burn through?
The reported causes are leaking blast at the contact surface, poor castable quality, heavy coal lance wear and material falling off the furnace wall. Repeated blow-downs can also crack the lining near the front water sleeve.
Why is cement-bonded castable weak at 1100 to 1200 °C?
Hydrate bonds are gone by then and ceramic bonds have not formed. The structure loosens and shrinks, so strength drops. A tuyere stock operates in that range.
What does andalusite or kyanite do in a tuyere stock castable?
These minerals expand when they convert to mullite. The expansion offsets the shrinkage of the cement bond and gives a micro-expansion castable.
How much soft clay is added as a sintering aid?
The study suggests 3 to 6 wt%. It helps the castable sinter at a lower temperature and lifts mid-temperature strength.
Why does Fe₂O₃ go up in used castable?
The authors suspect iron slag mixed into the sample. One EDS spot at the slag face read 37.57% Fe, which supports that.
What should I ask for besides strength at 110 °C?
Mid-temperature strength, strength and porosity after 1200 °C, alkali content, the binder system and a slag test result.
Send Us Your Blast Furnace Operating Parameters & Tuyere Drawings
Experiencing hot blast redness, burn-through, or blowpipe lining cracking? Send us your blast temperature, oxygen enrichment rate, tuyere stock drawings, and current castable TDS. Our refractory engineering team will provide a tailored micro-expansion castable formulation and technical proposal within 24 to 48 hours.