High-temperature creep is the slow, time-dependent deformation of a refractory material under a sustained load at a specified temperature. Unlike sudden cracking or crushing, creep develops gradually. A brick may retain its apparent integrity while its dimensions and position slowly change during long-term furnace operation.
Creep is particularly important in refractory structures that continuously carry their own weight or support other lining components. Typical examples include glass furnace regenerators, checker structures, arches, furnace roofs, support bricks and hot-blast stoves. Excessive deformation in these areas may cause settlement, misalignment, opening or compression of joints, changes in load distribution, or narrowing of gas-flow passages.
The engineering significance of creep can be summarized as follows:
Creep = f(temperature, stress, time, material structure, atmosphere)
A refractory brick with high cold crushing strength or high refractoriness may still deform under long-term high-temperature loading. These properties describe different aspects of material performance. Creep resistance specifically indicates the ability of a refractory to maintain controlled dimensional stability when temperature, load and time act together.
For this reason, creep data are especially valuable when selecting materials for long-term, high-temperature, load-bearing structures.
A typical strain–time curve may contain three stages.
During primary creep, deformation initially develops relatively quickly and then slows. This early dimensional change may include elastic deformation, closure of microcracks, compression of contact surfaces, continued sintering and structural adjustment within the material.
During secondary creep, deformation continues at a more stable rate. This stage is particularly useful for evaluating and comparing the long-term dimensional stability of refractory materials.
During tertiary creep, the deformation rate accelerates as the internal load-bearing structure becomes progressively unstable. However, many refractory creep tests conducted for a specified duration do not continue until a complete tertiary stage is reached.
The creep behaviour of a refractory is influenced by several interacting factors:
Laboratory creep data normally represent the refractory in its tested condition. They may not fully predict its behaviour after prolonged chemical attack or structural changes inside an operating furnace.
For refractory bricks, creep is commonly evaluated through a creep in compression, or CIC, test. A prepared specimen is subjected to a specified axial compressive stress, heated according to a controlled schedule and held at a constant temperature and load. Its axial dimensional change is then measured continuously or at defined intervals.
A creep result can be expressed as cumulative dimensional change over a specified period:
εc = [(Lt₂ − Lt₁) / Lt₁] × 100%
Average creep rate = (εt₂ − εt₁) / (t₂ − t₁)
A technically complete result should therefore be written in a form such as:
Creep in compression at 1500°C under 0.2 MPa, measured from 20 h to 50 h: maximum compressive deformation 0.30%.
Writing only “Creep: 0.30%” is insufficient because the value cannot be interpreted without its test conditions.
Before comparing data from different suppliers, engineers should confirm that the following conditions are equivalent:
The sign convention requires particular attention. Compressive deformation may be reported as a negative value in some data sheets, while other suppliers report its absolute magnitude as a positive percentage. A result of −0.30% and “maximum compression: 0.30%” may describe the same dimensional change.
Final values should also be considered together with the strain–time curve. One material may experience greater initial deformation before becoming stable, while another may show a smaller initial change followed by a continuously increasing creep rate. The second material is not necessarily safer for long-term service.
The objective is controlled and predictable dimensional stability—not simply the value closest to zero without considering the test conditions and deformation trend.
Creep is sometimes confused with other refractory properties, but each test answers a different engineering question.
| Property | Main test conditions | What it evaluates |
|---|---|---|
| CCS | Room-temperature compression until failure | Short-term compressive strength at room temperature |
| HMOR | High-temperature bending until failure | Short-term flexural strength at a specified temperature |
| RUL | Increasing temperature under a specified load | Temperature at which specified deformation occurs |
| PLC | Specified temperature and time, normally without a working load | Permanent shrinkage or expansion after heating |
| Creep | Constant temperature and load for a specified time | Time-dependent deformation under high-temperature loading |
Refractoriness under load, or RUL, measures deformation while temperature progressively increases under a specified load. It indicates the temperature range in which the material begins to lose load-bearing stability according to the defined deformation criterion.
Creep testing uses a different approach. The temperature and load remain constant while dimensional change is measured over time. RUL therefore asks, “At what temperature does specified deformation occur during heating?” Creep asks, “How does deformation develop over time at a particular temperature and load?”
A high RUL value is beneficial, but it does not automatically guarantee equally strong long-term creep resistance.
Permanent linear change, or PLC, measures irreversible shrinkage or expansion after a material is exposed to a specified temperature for a specified time, normally without an external working load.
Creep focuses on time-dependent deformation under sustained stress. However, continued sintering, phase transformation or residual expansion may also occur during a creep test. PLC data can therefore help determine whether measured dimensional change is caused mainly by the load or partly by ongoing thermal and mineralogical changes.
Cold crushing strength, or CCS, measures the short-term compressive force required to break a specimen at room temperature. It is useful for evaluating handling strength, manufacturing consistency and resistance to mechanical damage.
It does not represent prolonged high-temperature loading. A refractory may have high CCS but still show substantial creep if its grain-boundary phases soften or its load-bearing structure becomes unstable at the operating temperature.
Hot modulus of rupture, or HMOR, measures short-term resistance to bending at a specified high temperature. It is relevant where refractory components experience flexural stress.
Creep mainly evaluates long-term deformation under compression. For roofs, suspended structures, anchor bricks and other components exposed to bending, tension or shear, CIC results alone are insufficient. HMOR, thermal-shock resistance and structural design must also be considered.
In glass furnace regenerators, excessive creep may deform checker openings, misalign bricks and restrict gas flow. Settlement can also redistribute loads to adjacent bricks and support structures.
In arches, roofs and other supporting structures, gradual deformation may cause roof settlement, changes in arch thrust, movement of joints and local stress concentration. In hot-blast stoves and similar high-temperature structures, creep may contribute to checker settlement, wall deformation or displacement of supporting layers.
Improving creep resistance usually involves developing stable high-temperature phases and an effective load-bearing skeleton, controlling fluxing impurities and low-melting grain-boundary phases, optimizing particle grading and aggregate–matrix bonding, and using sufficient firing to reduce continued shrinkage during service.
However, the lowest creep value should not be pursued in isolation:
Material selection should therefore balance creep with RUL, PLC, HMOR, thermal-shock resistance, corrosion resistance, thermal conductivity and the actual structural requirements of the application.
A refractory inquiry or technical specification should include:
A clear specification would be:
Creep in compression at 1500°C under 0.2 MPa, measured from 20 h to 50 h: maximum compressive deformation 0.30%.
Terms such as “low creep” or an isolated value such as “Creep ≤0.30%” do not provide sufficient information for reliable technical comparison.
High-temperature creep describes the ability of a refractory material to maintain dimensional stability under the combined effects of temperature, sustained load and time. It cannot be replaced by CCS, RUL, PLC or HMOR because each property represents a different loading condition and failure mechanism.
Creep values are directly comparable only when the test temperature, stress, duration, measurement interval, standard and calculation method are consistent. Even then, laboratory results should be evaluated together with the furnace atmosphere, structural load, thermal cycling and possible chemical attack.
Firebird evaluates refractory materials according to their operating temperature, application position, structural load and long-term dimensional-stability requirements. If you are selecting refractory bricks for a high-temperature load-bearing structure, please share the application, service temperature and required test conditions for a technical review.
