What is residual stress
Let’s start with an official definition of residual stress:
Residual stress refers to the effect and influence of various process factors in the manufacturing process;
When these factors disappear, if the above effects and influences on the component can not disappear completely, and some of them remain in the component, then the residual effect and influence is residual stress.
A little dizzy?
Let’s talk about it in a popular way.
For example, a person who was very thin bought a pair of L-size jeans.
However, after a year, he becomes fat. When he wears these jeans again, he will feel that the trousers are too tight because he has gained weight and the trousers have not changed.
At this time, there is a strong force between the body and pants, if the force is too strong, it is easy to tear the pants.
This destructive force is the effect of residual stress.
From the perspective of energy work, when the external force causes the plastic deformation of the object, it will cause the internal deformation of the object, thus accumulating part of the energy;
When the external force is eliminated, the energy with uneven internal stress distribution will be released.
If the brittleness of the object is low, it will deform slowly, and if the brittleness is high, it will form cracks.
Fig. 1 Effect of residual stress
Residual stress is very common in mechanical manufacturing, and it often occurs in every process.
However, in essence, the causes of residual stress can be divided into three categories.
- Non-uniform plastic deformation;
- Uneven temperature change;
- Inhomogeneous phase transition.
Harm of residual stress
From the classification of residual stress, it can be seen that residual stress can cause slow deformation and change the size of the object.
As a result, the size of machined workpiece is unqualified, the precision of the whole instrument is lost, and the casting and forging workpiece is cracked or even broken.
At the same time, it also has a very important impact on its fatigue strength, stress corrosion resistance, dimensional stability and service life.
During the cooling process, the residual thermal stress is produced due to the uneven cooling caused by the unreasonable process, which leads to the fracture of the casting.
Fig. 2 fracture of casting during cooling
During the quenching process of heat treatment, the martensitic transformation of undercooled austenite is easy to cause material fracture.
Fig. 3 metal fracture during quenching
Measurement of residual stress
The measurement of residual stress can be divided into mechanical method, chemical method and X-ray method.
The most common mechanical method is drilling method (also known as blind hole method).
Fig. 4 Schematic diagram of drilling method
Figure 5 drilling method
During the operation, a section of bar (or pipe) whose length is three times of its diameter is cut from the object, a through hole is drilled in the center, and then a thin layer of metal is removed from the inside by the bore rod or drill bit, each time about 5% of the cross-sectional area is removed.
After removal, the elongation of the length and diameter of the sample were measured.
The relationship curve between these values and borehole section area is drawn, and the derivative of any point on the curve is obtained by drawing method to characterize the change rate of elongation and borehole section, and then the residual stress value can be obtained by substituting the corresponding stress formula.
There are two ideas of chemical methods.
One idea is to intrude the sample into the appropriate solution, measure the time from the beginning of corrosion to the discovery of cracks, and judge the residual stress according to the time.
The solution used can be mercury and mercury containing salts for tin bronze and weak base and nitrate for steel;
Another idea is to immerse the sample in a suitable solution and weigh it at intervals.
In this way, a curve of weight reduction versus time can be obtained and compared with the standard curve to determine the residual stress.
The higher the position of the curve is, the greater the residual stress is.
Fig. 7 weighing curve of chemical method
X-ray method can use X-ray to penetrate metal parts, and Laue method can qualitatively determine the residual stress by interfering with the change of spot shape.
Figure 8 principle of X-ray method
When there is no residual stress, the interference spots are distributed as dots. When there is residual stress, the interference spots elongate and show “Star” shape.
(a) No residual stress (b) residual stress exists
Fig. 9 measurement results of Laue method
Debye method can measure the residual stress quantitatively, which can be determined according to the position, width and intensity of the diffraction line on Debye diagram.
(a) transmission method (b) back reflection method
Fig. 10 schematic diagram of Debye method
Wrap it up
Mechanical method and chemical method are destructive testing methods, which need local sampling of the object to be tested, and the damage is irreversible after testing;
X-ray method is a non-destructive testing method, which can maintain the integrity of the object.
Mechanical method can accurately determine the size and distribution of residual stress, which is generally suitable for bar or tube shaped objects;
Chemical method is suitable for objects of wire and sheet type, but chemical method can only make qualitative judgment, it is difficult to achieve quantitative description;
Although X-ray method is a “non-destructive” method, it is only suitable for some materials that can give clear and sharp diffraction lines, and because of the small projection ability of X-ray, it can only detect the part of the object close to the surface.
Elimination of residual stress
Since there are so many hazards of residual stress, the effective elimination method is very necessary.
There are four elimination methods: heat treatment, static load pressurization, vibration stress relief and mechanical treatment.
Heat treatment is to use the thermal relaxation effect of residual stress to eliminate or reduce residual stress. Generally, annealing and tempering are used.
Fig. 11 annealing treatment of heat treatment
static load pressurization
Static load pressurization is to adjust the residual stress of workpiece by plastic deformation of whole or part or even micro area.
For example, the large pressure vessels are pressurized inside after welding, which is called “bulging”, so that the welding joint has a small amount of plastic deformation so as to reduce the welding residual stress.
Fig. 12 large oil tank after bulging
Vibration stress relief
Vibration stress relief (VSR) is a common method to eliminate the internal residual stress of engineering materials.
When the vector sum of the residual internal stress and the additional vibration stress in the workpiece exceeds the yield strength of the material by vibration, a small amount of plastic deformation occurs in the material, so that the internal stress in the material can be relaxed and reduced.
Fig. 13 strain quantifiable VSR system
Mechanical treatment is to reduce the residual stress by using the method of small plastic deformation on the surface of the object, including parts colliding with each other, surface rolling, surface drawing, surface sizing and fine pressing in the mold.
For example, one of the advantages of ironing is the elimination of residual stress.
Fig. 14 forging with iron