1. Refractory materials in the Global market:
How to assess Quality and Defects Generation Potential
Ing. S. Tiozzo, Dr. S. Sanchetti, Stazione Sperimentale del Vetro Scpa – Italy
2017 ICG Annual Meeting & 32nd Sisecam Glass Symposium, October 22-25th 2017, Istanbul

2. The context – why so serious?
Furnace service lifetime is getting increasingly longer, reaching up to 12+ years for “typical” container glass furnaces, and approaching 20 years for “float” tanks.
Service conditions for refractories are becoming more and more severe, due to more frequent pull and color changes, increasing insulation (to reduce heat loss), more reduced combustion conditions (for primary reduction of NOx), etc.
Localized critical failures from even a single refractory block can spell disaster over whole sections of the furnace, can severely compromise the surrounding equipment and pose a great threat to worker’s safety.
Early failures may lead to major production problems and significant losses.
New “Far-East” refractory suppliers have accessed the refractories market, and “traditional” Western players have started up production facilities in non- Western countries, all proposing appealing prices and boasting top-notch quality performances.
It is necessary to assess both:
The QUALITY of the PRODUCT
&
The RELIABILITY of the PRODUCER

3. Performance assessment approach
The SSV’s integrated experimental approach to the assessment of refractories (and suppliers) performances can be divided into the following steps:
Step 4: Regular Follow-up by endoscopic and thermografic inspections, and post mortem evaluation at the end of the furnace lifetime
Step 3: Audit on the technical and technological proficiency of suppliers
Step 2: Sampling & Characterization campaign on products of various suppliers
Step 1A: Definition of a post-mortem benchmark for end-of life final state of refractories
Step 1: Definition of a benchmark for «virgin» materials based on products of known quality and that demonstrated good reliability in the past campaigns

4. BENCHMARK analytical plan
integrated approach: step 1 – benchmark
Create a benchmark as reference, based on characterisations of well known materials with good performance in the past in similar operating conditions
BENCHMARK analytical plan
Furnace zone
Material type
Producer
XRF
XRD Rietveld
BD/AP
Exudation
Static corrosion
Dynamic corrosion
Blistering
Vapor test
CCS
CTE
Creep
Thermal conductivity
Sidewalls
AZS ? 3 2 1
Paving
Superstructure
Various
Throat
Crown
Silica
Feeder
Regenerator

5. step 1A – post mortem benchmark
The definition of a post-mortem benchmark is not immediately useful in the choice of supplier, but helps in defining what should be the final expected state of “good” refractories at the end of their service life, for future comparison with the post mortem of new campaigns.
Microchemical profile from metal line to the core of refractory

6. step 2 – sampling and analysis campaign
Furnace zone
Material type
XRF
XRD Rietveld
BD/AP
SEM
microstructure
Exudation
Static corrosion
Dynamic corrosion
Blistering
Vapour test
CCS
CTE
Creep
Sidewalls
AZS X
Paving
Superstructure
Various
Throat
Crown
Silica
Feeder
Regenerator
AZS materials are by far the most critical for glass industry, due to their peculiar fused cast “nature” and to their huge impact on furnace survivability and defect generation potential.

7. AZS sampling: an extremely critical step
The AZS microstructure is the result of a direct solidification process, and exhibits an intrinsic level of heterogeneity. In order to compare the analytical results, it is crucial to standardize the sampling procedure.
glass side
High
sample
AZS soldier block
Casting direction
Courtesy of P. Carlo Ratto
Small «test blocks» cast «on demand» should be avoided: AZS samples should be cut from standard-size blocks;
Test samples must always be cut from the same standardized section of the cast block;
Specimens for analysis must always be cut from this sub- sample following a standardized pattern;
This standard pattern must consider for each specimen the correct cutting position (skin, core, transversal, etc) as a function of the specific property to be tested.
Courtesy of P. Carlo Ratto

8. AZS inhomogeneity: microstructure (SEM)
Back-scattered electrons SEM images of two different zones of a sample piece, cut from an AZS block
Surface
Core
Baddeleyite crystals (ZrO2) are split into two different families:
Large white globular grains, slightly elongated, with different degrees of clusterization (cross section of dendritic crystal growth).
Smaller crystals, closely packed with Corundum crystals, forming a mixed ZrO2-Al2O3 eutectic microstructure.
Corundum (Al2O3) crystals are fine grained and are visible as dark grey areas in the pictures, mainly in association with small sized ZrO2 grains (eutectic microstructure).
The amorphous phase looks light gray and “smoother” than the rest of the sample, especially if compared with Al-rich and eutectic regions.

9. AZS for glass contact: static corrosion
Specimen: bar h = 125 mm l = 25 mm
s = 15 mm
Pt crucible: h = 120 mm
f = 100 mm
Thermal cycle: °C for hours
Measure: reduction in thickness

10. AZS for glass contact: static corrosion
Measurement of the residual thickness
with optical microscope at 10x magnif.

11. AZS for glass contact: dynamic corrosion
Specimen: cylinder h = 250 mm f = 30 mm
Refractory crucible coated with Pt: h = 300 mm
f = 150 mm
Thermal cycle: °C for hours
Rotational speed: 15 rpm
Measure: reduction in thickness
Sample
Test performed with a SINGLE sample turning along its cylindrical (radial) symmetry axis, at the center of the molten glass mass.

12. Dynamic corrosion at 1475°C for 2 days
AZS for glass contact: dynamic corrosion
Sample 3: bad corrosion resistance
Thickness reduction = 13.5%
Sample 2: Mid corrosion resistance
Thickness reduction = 5.7%
Sample 3: Good corrosion resistance
Thickness reduction = 1.3%
Dynamic corrosion at 1475°C for 2 days
Axial rotation at 15 rpm

13. AZS for glass contact: dynamic corrosion
Dimensions : cylinder h = 150 mm f = 15 mm
Refractory crucible coated with Pt: h = 300 mm
f = 150 mm
Thermal cycle: °C for hours
Rotational speed: 6 rpm
Measure: reduction in thickness and calculation
of wear in mm/h
Test performed with four (up to 6) pencil specimens inserted in a carousel-like sample holder, all revolving along circular paths inside the molten glass.

14. AZS for glass contact: defect generation
Passivation
Changes in composition, temperature and pull rate can drag small parts of the passivation layer into the glass, forming cords of different composition (enriched in Al2O3 and ZrO2), or releasing stones (typically baddeleyite) into the glass.
AZS
AZS alteration/passivation layer, where corundum and glassy phase have been partly dissolved, and baddeleyite crystals form a coral like skeleton.
Hight viscosity glass layer, rich in dissolved Al2O3 and ZrO2, with possible presence of ZrO2 stones & bubbles

15. azs defect generation: bubbles
REDOX
REBOIL
FINING
ELECTROCHEMICAL
REDUCTION
REFRACTORY
AIR N2 X
CO2
SO2 O2 H2 Ar
Guide to the interpretation of bubble analysis, courtesy of Dr. Stefano Ceola, SSV
Physical chemical equilibrium at the passivation interlayer
bubbles nucleation
into the pores
traces of polyvalent metals
gradient of alkali concentration 15

16. TOTAL NUMBER OF BUBBLES TRAPPED WITHIN ONE “CRUCIBLE”
Bubbles generated by azs: blistering test
AZS Product 32% ZrO2
TOTAL NUMBER OF BUBBLES TRAPPED WITHIN ONE “CRUCIBLE”
AZS Product 32% ZrO2
AZS Product 34% ZrO2
AZS Product 40% ZrO2
Specimen A B
AVG
Total bubbles 12 18 15 2 4 3 48 39 44
AZS Product 34% ZrO2
Performance index:
<10 bubbles/g of glass
bubbles/g
bubbles/g
>1500 bubbles/g
AZS Product 40% ZrO2

17. Superstructure azs: glassy phase exudation
The exudation is the expulsion of the glassy phase of fused cast AZS during operation, which is especially strong at the beginning of the furnace life (1-2 months). The result of this phenomenon is the release into the melt of a glassy phase rich in Al2O3 e ZrO2, with an high defect potential for the final product.
Possible Causes
Expansive Reboil of gases inside the AZS glassy phase at high temperatures.
Isteretic transformation of ZrO2 from tetragonal to monoclinic and vice-versa close to 1100°C.
New formation of gases due to oxidation of C e S inclusions, with formation of CO, CO2, SO2, with a consequent “pump effect”.
High Na2O initial content, with consequent reduction of viscosity of the glassy phase of AZS.
Large domains of glassy phase, or poor interlocking of crystals in the skin of the AZS.
Al2O3-ZrO2 enriched cord in soda lime glass
Al2O3-ZrO2 stone in soda lime glass

18. SSV repeats 2 times the thermal cycle ASTM C1233 standard procedure
Superstructure azs: exudation tests
Specimen: cylinder or bar h = 100 mm, dbase = 25 mm
Thermal cycle: 20 → 1510°C in 12 h;
palier 4 h; natural cooling.
SSV repeats 2 times the thermal cycle
Volume measurements (ASTM C20) before and after the thermal cycle. The exudation is expressed as % of volume increase after test.
PRO’S: fast, easy, practical
CON’S: no compensation for PLC, it might not exhaust the specimen whole exudation potential
ASTM C1233 standard procedure
TC11 has developed a more accurate test method, consisting of cycles of heating / cooling, including PLC compensation, that gives a better overview on exudation evolution; the drawback is the high expensiveness of the procedure.
Exudation: 2,53%
Exudation: 5,82%

19. Superstructure: Alkali vapor attack tests
Microchemical profile after attack with 1/3 K2CO3 and 2/3 Na2CO3 on AZS 32% ZrO2
Specimen: prism 55x55x20 mm
Reactant similar to the vapor encountered in service (often mix K2CO3 + Na2CO3)
Thermal cycle: 1370°C/24 h.
Measures: depth of corrosion, mineralogic changes, thickness reduction, cracks, chemical composition depth profiling, etc
ASTM C 987
AZS 32% ZrO2
Special Silica

20. Al2O3-ZrO2 castable fired at 3 different T
Crown: creep in compression (CIC) tests
Creep in compression (CIC, according to ISO 3187) refers to the deformation (shrinkage) of a refractory test piece exposed to a constant high temperature over a long period of time under a constant mechanical load. 
Example of critical failure of a sample of poor quality silica tested at 1450° with a load of 0.4 Mpa
Al2O3-ZrO2 castable fired at 3 different T
Specimen: cylinder 50 mm in diameter and 50 mm in height, with a central 12.5 mm hole
Thermal cycle and mechanical load are functions of type of material and service T
Measure: shrinkage of the specimen as a function of time.

21. Step 3: supplier’s tech proficiency audit
Since it is impossible to prooftest 100% of the blocks, samples and analyses alone cannot completely guarantee good quality performances, so the refractory supplier’s RELIABILITY must also be evaluated, in parallel with their products.
The rationale behind this is that if the tested samples are good, the installed technologies are adequate, the process control and quality management are satisfactory, and good manufacturing practices are followed, then all the blocks should be “good enough”.
Assessing supplier’s reliability requires a thorough audit to the production facilities, taking into consideration technologies, production capabilities, technical proficiency, process control and quality management systems, chemical lab, logistics, etc, and can be only performed by highly trained experts of fused cast AZS science and technology.
Tier 1a: e.g. Western manufacturers mastering top world level technology and products processed in their “traditional” western sites.
Tier 1b: e.g. Western manufacturers processing products in delocalized “low-cost” production hubs
Tier 2: e.g. Low-cost independent manufacturers operating a “Tier 2” level of technology, processing products in low-cost manufacturing sites.
Tier 3: Eastern Low-cost independent manufacturers operating a “Tier 3” level of technology, processing products in low-cost manufacturing sites, generally without direct export capability.
Courtesy of P. Carlo Ratto

22. Step 3: supplier’s tech proficiency audit
Typical parameters taken into account when auditing a refractory supplier are:
Raw material procurement and quality control protocol
Process control and feedback systems (e.g. chemical analysis / batch formula)
Process traceability and logistics management
Moulds materials, workshop and management
Melting, Oxidizing (e.g. type of O2 lance) and Casting (e.g. hot stripping or not) technologies available on site, and production capabilities (shapes and sizes)
Choice and management of annealing media
Quality controls in the process and quality assurance protocols
Grinding shop equipment and finishing capability
Furnace design and After – Sales support services

23. STEP 4: periodic inspection and post mortem
Incremental learning !
Air phase

24. conclusions
The choice of refractory suppliers is a critical step for the future of a furnace, and always involves a delicate balance between risks and cost savings, especially in the case of “complex” materials such as fused cast AZS blocks.
To minimize (or be aware of) the risks and make a well grounded choice, it is necessary to apply an integrated assessment approach, combining together the verification of the performances of the products (QUALITY) through a dedicated sampling and testing campaign, and the evaluation of the proficiency of the producer (RELIABILITY) by means of focused suppliers audits.
Sampling, Testing and Auditing “Protocols” have to be carefully conceived through close cooperation between technological and laboratory experts, providing refractory manufacturing know-how and glass production knowledge within the same team, especially in the case of critical materials such as AZS.
A follow up with periodic inspections is necessary to monitor refractory “aging”, and to plan ahead of time the eventual hot (or cold) repairs that might be necessary to extend the service life of the furnace.

25. Thank you for your attention !
Stefano Sanchetti Simone Tiozzo
Stazione Sperimentale del Vetro scpa
via Briati 10, Murano (VE), Italy