1. Overview
The structures of metals and alloys change due to phenomena such as diffusion, recovery, recrystallization and others at high temperatures.
Furthermore, prolonged exposure to high temperatures can impair the performance of metallic materials.
In equipment such as high-pressure steam boilers, steam turbines, diesel engines, aeroengines, chemical equipment and high-temperature and high-pressure piping, many parts are in service at high temperatures for long periods.
It is not enough to only consider the mechanical properties of such materials under normal temperature and short-term static load. For example, high-temperature and high-pressure pipes in chemical equipment, although the stress they withstand is lower than the flow resistance of the materials at their working temperature, they will undergo continuous plastic deformation over time, which gradually increases the diameter of the pipe and can even lead to pipe rupture.
The “high” or “low” temperature rating is relative to the melting point of the metal. The relationship between temperature and melting point (T/Tm) is often used as a reference, where Tm refers to the melting point of the material. If T/Tm is greater than 0.4 to 0.5, it is considered high temperature.
The temperature of a civil aircraft is approximately 1,500°C, while that of a military aircraft is approximately 2,000°C. The spacecraft's local working temperature can even reach 2,500°C.
2. Influencing factors
Temperature has a significant impact on the mechanical properties of materials. The duration of charging at high temperatures also has a major influence on these properties. It is important to note that high temperature mechanical properties are not the same as room temperature mechanical properties.
As a general trend, as the temperature increases, the resistance of metallic materials decreases while their plasticity increases. The duration of the load also affects the mechanical properties. Under short-term loads, tensile strength decreases and plasticity increases, but under long-term loads, plasticity decreases significantly, notch sensitivity increases, and brittle fracture often occurs.
The combined effect of temperature and time also influences the fracture path of the material. For example, creep may occur during long-term use, eventually leading to fracture. The tensile strength of steel at high temperatures decreases with prolongation of the duration of the load.
As the temperature increases, both the grain strength and grain boundary strength decrease. However, the grain boundary resistance decreases more quickly due to the irregular arrangement of atoms at the grain boundary, facilitating the occurrence of diffusion.
The temperature at which the grain strength and grain boundary strength are equal is known as the “equal strength temperature” (TE). When the material operates above the TE, the fracture mode of the material changes from typical transgranular fracture to intergranular fracture.
It is important to note that the TE is not fixed and is influenced by the strain rate. Because grain boundary strength is more sensitive to strain rate than grain strength, TE increases with increasing strain rate.
In conclusion, to study the mechanical properties of materials at high temperatures, both temperature and time must be considered as factors.
3. Creep phenomenon
Creep is the gradual plastic deformation of metal that occurs under constant temperature and load, even if the stress is below the yield point at that temperature, over a long period of time. This type of material fracture caused by creep deformation is called creep fracture.
Although creep can occur at low temperatures, it is only significant when the temperature is greater than approximately 0.3. If the temperature of carbon steel exceeds 300°C or that of alloy steel exceeds 400°C, the effect of creep must be taken into account.
It is important to note that the creep curve of the same material varies with stress and temperature.
Typical creep curve
The first stage, labeled “ab,” is known as the Creep Deceleration Stage or Creep Transition Stage. The creep rate at the beginning of this stage is very high and gradually decreases over time until it reaches its minimum at point “b”.
The second stage, labeled “bc”, is known as the Constant Velocity Creep Stage or Steady State Creep Stage. This stage is characterized by a relatively constant creep rate. The creep rate of a metal is generally expressed by the creep rate ε during this stage.
The third stage is the Accelerated Creep Stage. As time progresses, the creep rate gradually increases until creep fracture occurs at point “d”.
Change the creep curve diagram with different stresses and temperatures
As illustrated in the figure, when the stress is low or the temperature is low, the second stage of creep lasts a considerable time and, in some cases, the third stage may not occur at all. On the other hand, when the stress is high or the temperature is high, the second stage of creep is very brief or may not occur at all, resulting in the specimen breaking in a very short time.
4. Creep fracture surface characteristics
Macro characteristics of the fracture surface
Plastic deformation occurs close to the fracture surface and there are numerous cracks in the vicinity of the deformed area (these cracks can be seen on the surface of the fractured part). In cases of high-temperature oxidation, the fracture surface is coated with a layer of oxide film.
Microfeatures of the fracture surface
Intergranular fracture morphology of crystal sugar-like patterns
5. Index and performance measurement
Creep limit, breaking strength, relaxation stability and other mechanical properties are commonly used to evaluate the creep behavior of materials.
5.1 Creep limit
Creep limit is a measure of a metallic material's resistance to plastic deformation under long-term loading at high temperatures and is a crucial factor in the selection and design of high-temperature service components.
There are two ways to express the creep limit in MPa: one is to determine the maximum stress that the sample can withstand at a specified constant creep rate within a specified time and temperature; the other is to determine the maximum stress that causes the sample to undergo a specified creep elongation within a specified time and temperature.
Example 1 shows that the creep limit of the material is 80MPa when the temperature is 500 ℃ and the constant creep rate is 1×10 -5 %/h;
Example 2 shows that the creep limit of the material is 100 MPa when the temperature is 500 ℃, 100,000 hours, and the creep elongation is 1%.
Creep Test Equipment and Schematic Diagram
Creep testing must be performed under consistent temperature conditions and a range of stress levels, with a minimum of 4 creep curves recorded.
Creep curves should be created based on the recorded results, with the slope of the straight line on the curve representing the creep rate.
The relationship curve is plotted in logarithmic coordinates using the obtained stress creep rate data.
By applying relatively high stress levels, multiple creep curves can be generated with relatively short test times. The stress value for a specified creep rate can be determined through interpolation or extrapolation of the measured creep rate, allowing determination of the creep limit.
At a constant temperature, there is a linear empirical relationship between the second stage creep stress (σ) and the steady-state creep rate (ε) in double logarithmic coordinates.
σ- ε curve of S-590 alloy
(20.0%Cr, 19.4%Ni, 19.3%Co, 4.0%W, 4.0%Nb, 3.8%Mo, 1.35%Mn, 0.43%C)
5.2 Resistance force
Durable strength refers to a material's ability to resist fracture for a long period of time under high temperature loads. It is the maximum stress that a material can withstand without suffering creep fracture under specific conditions of temperature and time. Durable strength is a measure of a material's resistance to fracture, while creep limit refers to its resistance to deformation.
For some materials and components, creep deformation is minimal and their only requirement is not to break during their useful life (like the superheated steam tube in a boiler). In these cases, mechanical strength is the main criterion used to evaluate the suitability of the material or component for use.
Stress rupture strength curve of S-590 alloy
The strength of metallic materials is determined by the high temperature tensile strength test.
During the testing process, it is not necessary to measure the elongation of the sample as long as the time it takes to fracture under a specified temperature and stress level is recorded.
For machine components with long lifespans (tens of thousands to hundreds of thousands of hours or more), it is challenging to perform long-term testing, so data is typically generated using high stress levels and short fracture times. The strength of the materials is then calculated by extrapolation.
Extrapolate the empirical formula:
(fracture time t, stress σ, constants A, B related to temperature and test material)
Take the logarithm of the above formula to get:
Take log t-log σ Fig., the linear relationship can be extrapolated from the data with short fracture time to the lasting strength with long time.
5.3 Residual voltage
When subjected to constant deformation, the elastic stress of materials gradually decreases over time, known as stress relaxation.
The resistance of metallic materials to stress relaxation is referred to as relaxation stability, which can be determined through stress relaxation tests by measuring the stress relaxation curve.
Residual stress is a metric used to evaluate the relaxation stability of metallic materials. The higher the residual stress, the better the relaxation temperature.
Stress relaxation curve
Stage 1: Stress drops quickly at first;
Stage 2: stage in which the drop in stress gradually decreases;
Relaxation Limit: Under certain initial stresses and temperatures, the residual stress will not continue to relax.
5.4 Factors influencing high temperature mechanical properties
To increase the creep limit, it is important to control the rate of rise of the dislocation based on the creep deformation and fracture mechanism.
To improve rupture strength, it is necessary to control grain boundary sliding and vacancy diffusion.
Several factors can impact high-temperature mechanical properties, including chemical composition, casting process, heat treatment process, and grain size.
Influence of the chemical composition of the alloy
The base materials for heat-resistant steels and alloys typically consist of metals and alloys with high melting points, high self-diffusion activation energy, or low stacking fault energy.
Metals with higher melting points, such as chromium (Cr), tungsten (W), molybdenum (Mo), and niobium (Nb), have slower self-diffusion rates.
The low stacking fault energy makes it easier for extended dislocations to form and harder for dislocations to slip and rise.
The dispersed phase can effectively block the sliding and rising of the dislocation.
The addition of elements such as boron and rare earths, which increase the activation energy of grain boundary diffusion, not only makes grain boundary sliding difficult, but also increases the surface energy of grain boundary cracks.
Heat-resistant materials with face-centered cubic structures have greater resistance to high temperatures compared to those with body-centered cubic structures.
Influence of the casting process
Reviewed:
It is important to reduce the content of inclusions and metallurgical defects.
When using directional solidification, the number of transverse grain boundaries is reduced, leading to an improvement in rupture strength as cracks are more likely to form at transverse grain boundaries.
Influence of the heat treatment process
Heat-resistant pearlitic steel typically goes through a normalizing process followed by high-temperature tempering.
The tempering temperature should be 100 to 150 degrees Celsius higher than the service temperature to increase structural stability under operating conditions.
Heat-resistant steel or austenitic alloys are typically treated through dissolution and aging to achieve the appropriate grain size and improve the distribution of reinforcing phases.
Thermomechanical treatment can further increase the strength of the alloy by changing the shape of the grain boundaries (forming serrations) and creating polygonal subgrain boundaries within the grain.
Effect of grain size
Grain size: When the operating temperature is below the constant strength temperature, fine-grain steel has greater strength, while when the operating temperature exceeds the constant strength temperature, coarse-grain steel has greater creep resistance and toughness.
Irregular grain size: When stress is concentrated at the junction between large and small grains, cracks are more likely to form and result in premature fractures.
