Mechanical Properties of Materials Under High Temperature

1. Overview

The diffusion, recovery, recrystallization and other phenomena in metals and alloys at high temperatures will change their structures.

The performance of metal materials will also be damaged if they are exposed to high temperature for a long time.

In high-pressure steam boilers, steam turbines, diesel engines, aeroengines, chemical equipment, high temperature and high pressure pipelines and other equipment, many parts have been in service under high temperature for a long time.

For such materials, it is not enough to only consider the mechanical properties under normal temperature and short-time static load.

For example, high temperature and high pressure pipes in chemical equipment, although the stress they bear is less than the yield strength of materials at this working temperature, will produce continuous plastic deformation during long-term use, which will gradually increase the pipe diameter, and even lead to pipe rupture.

The “high” or “low” temperature is relative to the melting point of the metal.

Generally, the approximate temperature T/Tm (Tm refers to the melting point of the material) is used.

If T/Tm > 0.4~0.5, it is considered as high temperature.

The temperature of civil aircraft is close to 1500 ℃, that of military aircraft is about 2000 ℃, and the local working temperature of spacecraft can reach 2500 ℃.

2. Influencing factors

The temperature has a great influence on the mechanical properties of materials.

The duration of loading at high temperature also has a great influence on the mechanical properties.

High-temperature mechanical properties ≠ room temperature mechanical properties.

Generally, with the increase of temperature, the strength of metal materials decreases and the plasticity increases.

Effect of load duration: σ<σ s. During long-term use, creep may occur, which may eventually lead to fracture;

With the extension of load duration, the tensile strength of steel at high temperature decreases;

The plasticity of the material increases when it is stretched at high temperature for a short time;

However, under long-term load, the plasticity of metal materials is significantly reduced, the notch sensitivity is increased, and brittle fracture often occurs;

The combined effect of temperature and time also affects the fracture path of the material.

When the temperature increases, the grain strength and grain boundary strength will decrease.

However, because the atoms on the grain boundary are arranged irregularly, diffusion is easy to occur through the grain boundary, so the grain boundary strength decreases rapidly.

The temperature at which the strength of both grains and grain boundaries is equal is called “equal strength temperature” TE.

When the material works above TE, the fracture mode of the material transits from the common transgranular fracture to intergranular fracture.

The TE of the material is not fixed, and the deformation rate has a great influence on it.

Because grain boundary strength is much more sensitive to deformation rate than grains, TE increases with the increase of deformation rate.

To sum up, two factors, temperature and time, must be added to study the mechanical properties of materials at high temperatures.

3. Creep phenomenon

Creep is the phenomenon that metal slowly produces plastic deformation under the action of constant temperature and load (even if the stress is less than the yield strength at this temperature) for a long time.

The material fracture caused by creep deformation is called creep fracture.

Creep also occurs at low temperatures, but it is only significant when the approximate temperature is greater than 0.3.

If carbon steel exceeds 300 ℃ and alloy steel exceeds 400 ℃, the influence of creep must be considered.

The creep curve of the same material varies with the stress and temperature.

Typical creep curve

The first stage ab is the deceleration creep stage, also known as the transition creep stage.

The creep rate at the beginning of this stage is very high, and gradually decreases with time, and reaches the minimum at point b;

The second stage, bc, is a constant speed creep stage, also known as a steady state creep stage. The characteristic of this stage is that the creep rate is almost unchanged.

Generally speaking, the creep rate of metal is expressed by the creep rate ε at this stage;

The third stage is accelerated creep stage. With the extension of time, the creep rate increases gradually, and creep fracture occurs at point d.

Change diagram of creep curve with different stress and temperature

It can be seen from the figure that when the stress is small or the temperature is low, the second stage of creep lasts for a long time, and even the third stage may not occur;

On the contrary, when the stress is high or the temperature is high, the second creep stage is very short, or even completely disappears, and the specimen breaks in a very short time.

4. Characteristics of the creep fracture surface

Macro characteristics of the fracture surface

Plastic deformation occurs near the fracture surface, and there are many cracks near the deformation area (cracks appear on the surface of the fracture part);

High-temperature oxidation, the fracture surface is covered by a layer of the oxide film.

Micro characteristics of the fracture surface

Intergranular fracture morphology of crystal sugar like patterns

5. Performance index and measurement

Creep limit, rupture strength, relaxation stability and other mechanical properties are often used for creep performance of materials.

5.1 Creep limit

Creep limit is the plastic deformation resistance index of metal materials under long-term load at high temperature, and is one of the main bases for high temperature materials and design of high temperature service parts.

There are two ways to express the creep limit (MPa), one is to make the specimen produce the maximum stress of the specified steady creep rate within the specified time under the specified temperature;

One is the maximum stress that causes the specimen to produce the specified creep elongation within the specified time at the specified temperature and time.

Example 1 shows that the creep limit of the material is 80MPa when the temperature is 500 ℃ and the steady creep rate is 1×10-5%/h;

Example 2 shows that the creep limit of the material is 100MPa when the temperature is 500 ℃, 100000 hours, and the creep elongation is 1%.

Creep test equipment and schematic diagram

Creep test shall be conducted under the same temperature and different stress conditions, and at least 4 creep curves shall be measured.

Creep curves shall be made according to the measured results. The slope of the straight line on the curve is the creep rate;

According to the obtained stress creep rate data, the relationship curve is drawn on the logarithmic coordinate;

Several creep curves can be drawn with relatively short test time by using relatively large stress. According to the measured creep rate, the stress value of the specified creep rate can be obtained by interpolation or extrapolation, and the creep limit can be obtained.

At the same temperature, there is a linear empirical relationship between the second stage creep stress σ and the steady creep rate ε in the double logarithmic coordinates.

S-590 alloy σ- ε curve

(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 Endurance strength

Durable strength refers to the ability of materials to resist fracture under long-term load at high temperature, that is, the maximum stress of materials without creep fracture under certain temperature and time conditions (creep limit refers to the deformation resistance of materials, and durable strength refers to the fracture resistance of materials).

For some materials and parts, the creep deformation is very small, and they are only required not to break during the service life (such as the superheated steam pipe of the boiler).

At this time, it is necessary to use the endurance strength as the main basis for evaluating the use of materials and parts.

Stress rupture strength curve of S-590 alloy

The endurance strength of metal materials is determined by high temperature tensile endurance test.

During the test, it is not necessary to measure the elongation of the sample, as long as the time from the specified temperature and certain stress to fracture is measured.

For the machine parts with long design life (tens of thousands to hundreds of thousands of hours or more), it is very difficult to test for a long time, so generally, the test data with large stress and short fracture time are made, and the endurance strength of materials is calculated by extrapolation.

Extrapolate empirical formula:

(t-fracture time, σ-stress, A, B-constants related to test temperature and material)

Take the logarithm of the above formula to get:

Make 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 stress

Under the condition of constant deformation, the elastic stress of materials decreases gradually with time, which is called stress relaxation.

The resistance of metal materials to stress relaxation is called relaxation stability, which can be evaluated by the stress relaxation curve measured by stress relaxation test.

Residual stress is an index to evaluate the stress relaxation stability of metal materials.

The higher the residual stress, the better the relaxation temperature.

Stress relaxation curve

Stage 1: the stress drops rapidly at the beginning;

Sstage 2: the stage where the stress drop gradually slows down;

Relaxation limit: under certain initial stress and temperature, the residual stress will not continue to relax.

5.4 Influencing factors of high temperature mechanical properties

According to the creep deformation and fracture mechanism, the rate of dislocation climb must be controlled to increase the creep limit;

To improve the rupture strength, it is necessary to control the grain boundary sliding and vacancy diffusion.

Influence factors of high temperature mechanical properties: chemical composition, smelting process, heat treatment process, grain size.

Influence of chemical composition of the alloy

The base materials of heat-resistant steels and alloys are generally metals and alloys with high melting point, high self diffusion activation energy or low stacking fault energy.

The higher the melting point of metals (Cr, W, Mo, Nb), the slower the self diffusion;

The stacking fault energy is low, easy to form extended dislocations, and the dislocations are difficult to cross slip and climb;

The dispersed phase can strongly block the slip and climb of dislocations;

The added elements (such as boron and rare earth) that can increase the activation energy of grain boundary diffusion can not only hinder grain boundary sliding, but also increase the surface energy of grain boundary cracks;

The high temperature strength of face centered cubic structure is higher than that of body centered cubic structure.

Influence of smelting process

Reduce the content of inclusions and metallurgical defects;

Through the directional solidification process, the transverse grain boundary is reduced and the rupture strength is improved, because cracks are easily generated on the transverse grain boundary.

Influence of heat treatment process

Pearlitic heat-resistant steel generally adopts normalizing+high temperature tempering process.

The tempering temperature shall be 100~150 ℃ higher than the service temperature to improve the structural stability under the service temperature;

Austenitic heat-resistant steel or alloy is generally treated by solution and aging to obtain proper grain size and improve the distribution of strengthening phase;

The alloy can be further strengthened by using thermomechanical treatment to change the shape of the grain boundary (forming serrations) and form polygonal subgrain boundaries in the grain.

Effect of grain size

Grain size: when the service temperature is lower than the constant strength temperature, the fine grain steel has higher strength, whereas when the service temperature is higher than the constant strength temperature, the coarse grain steel has higher creep resistance and endurance strength;

Uneven grain size: when stress concentration occurs at the junction of large and small grains, cracks are easy to occur here and cause premature fracture.

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