How to eliminate work hardening in 316 stainless pipes?
After cold working of 316 stainless steel pipes, due to the flattening and elongation of the grains, the distortion of the crystal lattice and the deformation of the grains, the plasticity of the metal is reduced and the strength and hardness are increased. This phenomenon is called work hardening.
Work hardening is one of the methods to improve the strength of 316 stainless steel pipes, which is especially important for metals that cannot be strengthened by heat treatment. The high strength of cold drawn tubes and finished rolled tubes is the result of work hardening. In addition, work hardening also creates conditions for cold working. Because the pipe hardens to a certain extent during the cold working process and can no longer be processed, it will break due to deformation under certain conditions.
Work hardening improves deformation resistance, but also brings difficulties to the continued processing of stainless steel product pipes, because its hardness and strength are increasing, while plasticity and toughness are decreasing. Among stainless steels, the work hardening phenomenon of austenitic and ferritic stainless steels is the most prominent.
For example, the tensile strength of austenitic stainless steel pipe after hardening reaches 1470 ~ 1960 MPa, and as the tensile strength increases, the yield limit increases; the austenite yield limit in the annealed state does not exceed the tensile strength of 30% ~ 45% , and after work hardening it reaches 85% to 95%. The depth of the work-hardened layer can reach 1/3 or more of the cutting depth; the hardness of the hardened layer is 1.4 to 2.2 times higher than the original.
Because the 316 stainless steel pipe has high plasticity, the character is distorted during plastic deformation, and the strengthening coefficient is very large; and the austenite is not stable enough. Under the action of cutting stress, part of the austenite will transform into martensite; coupled with compound impurities Under the action of cutting heat, it is easy to decompose and become dispersedly distributed, causing a hardened layer to be produced during cutting. The work hardening phenomenon caused by the previous process seriously affects the smooth progress of subsequent processes.
Therefore, in order to eliminate work hardening of stainless steel welded pipes, special heat treatment must be carried out during the processing, and subsequent processing can only be continued after recrystallization and softening. After recrystallization, only the shape of the crystal grains changes, but the lattice type does not change and remains the same as the original grains.
Recrystallized nuclei are generally formed at the grain boundaries or slip zones of deformed grains and in places with severe lattice distortion. After the nuclei are formed, they rely on the diffusion and movement of atoms to grow nearby until each crystal nucleus grows. Until they contact each other and form new equiaxed grains. Through recrystallization, the microstructure of the 316 stainless steel pipe has been completely changed, so its strength and hardness are significantly reduced, while the plasticity and toughness are greatly improved, the work hardening phenomenon is eliminated, and all the mechanical and physical properties of the deformed metal are restored to the cold state. The state before transformation.
Why is 316 stainless steel pipe more resistant to high temperatures?
The high temperature range of 316 stainless steel is roughly 1200-1300 degrees. The reason why 316 stainless steel can withstand such high temperatures is that the addition of Mo element to 316 stainless steel greatly increases its corrosion resistance and high temperature strength.
Comparing 316 stainless steel with other steels:
Excellent work hardening (non-magnetic); excellent high-temperature strength; non-magnetic in solid solution state; cold-rolled products have good gloss and beautiful appearance; compared with 304 stainless steel, the price is higher. When the usage scenario has high requirements for chloride ion corrosion resistance and heat resistance, 316 stainless steel is better. Because 316L stainless steel contains molybdenum (Mo) element, the carbides formed are extremely stable, which can prevent the grain growth of austenite when heated and reduce the overheating sensitivity of steel. In addition, the molybdenum element can make the passivation film denser and stronger, thereby effectively improving the corrosion resistance of stainless steel to chloride ions (Cl-). 316 stainless steel uses: seawater utilization equipment, chemical, dye, papermaking, oxalic acid, fertilizer and other production equipment; photography, food industry, coastal area facilities, ropes, CD rods, bolts, nuts.
How to detect the welds of 316 stainless steel pipes?
X-ray examination.
It can pass through substances that cannot be penetrated by light, and check whether there are pores in the welding head, unstable welding, cracks, slag inclusions, shrinkage holes and other phenomena; it can detect the plane position and size of the defect, and can detect the defects at the defect. Plane position and size, etc. However, it is usually difficult to find defects in the pipeline. There are some small cracks on the welds, and factors such as the depth of the defects cannot be detected.
Ultrasonography.
Using the principle of ultrasonic detection, it will be reflected back when encountering a certain interface. This system can detect raw materials and 316 stainless steel pipes, and can also find problems during the welding process. It can determine the location of the problem and the corresponding size parameters. It has extremely strong penetrability and can penetrate up to 10cm thick. However, problems existing in welds of coarse-grained materials cannot be detected. In addition, the location and shape of defects have a great impact on the detection results.
Magnetic particle inspection.
The basic principle is that after the stainless steel pipe is magnetized, if there are some internal problems, the distribution of the magnetic force will change to a certain extent. This method mainly detects problems with surface openings or near surfaces in magnetic materials. But this method often has difficulty detecting relatively complex stainless steel pipes.
Penetration testing.
It uses the capillary phenomenon to allow the penetrant to enter the defects, and then cleans the 316 stainless steel pipe to remove excess penetrant. Use the developer to absorb water and adsorb the penetrant in the defects, thereby discovering some minor problems in time.
For 316 stainless steel pipes, X-ray, ultrasonic, magnetic particle, permeability and other inspection methods all use non-destructive testing methods. The quality inspection of this link is to detect flaws in the stainless steel pipes to ensure their normal and safe operation. The accuracy of the final inspection results completely depends on whether the various operations during the inspection process are reasonable and complete. When purchasing 316 stainless steel pipes, you must pay special attention to the merchant's quality inspection management system, which directly determines the quality of stainless steel welded pipes.