Austenitic stainless steel should also pay attention to: cold work hardening, high temperature aging embrittlement

 The intergranular corrosion tendency test of stainless steel is a common content in design documents, and the relevant content in standards such as HG/T 20581 is relatively clear. The water pressure test or the chloride ion content in the operating medium is also the basic content of the austenitic stainless steel equipment design. In addition to chloride ions, wet hydrogen sulfide, polythionine and other environments that may generate sulfides can also cause stress corrosion cracking of austenitic stainless steel.

It is worth mentioning that although austenitic stainless steel is not mentioned in the chapter of HG/T 20581 wet hydrogen sulfide corrosion, the reference points out that although austenitic stainless steel has a much greater ability to dissolve atomic hydrogen than ferritic stainless steel, But hydrogen-induced wet hydrogen sulfide stress corrosion cracking will still occur, especially after cold work hardening appears deformed martensite structure transformation.

Cold work hardening increases stress corrosion cracking sensitivity

Austenitic stainless steel has excellent cold working properties, but its work hardening is very obvious. The greater the degree of cold working deformation, the higher the hardness rise. The increase in hardness caused by work hardening is also an important reason for the stress corrosion cracking of stainless steel, especially those where the base material is not welded.

There are some cases:

The first type of case is the cold spinning of austenitic stainless steel oval or dish-shaped head, the cold deformation of the transition zone is the largest, and the hardness also reaches the highest. After commissioning, chloride ion stress corrosion cracking occurred in the transition zone, resulting in equipment leakage.

The second type of case is a U-shaped corrugated expansion joint made by hydroforming after the stainless steel sheet is rolled. The cold deformation is the largest at the wave crest, and the hardness is the highest. The stress corrosion cracking occurs along the wave crest the most, and even cracking along the wave crest occurs. Explosion accident with low stress and brittle fracture.

The third case is the stress corrosion cracking of the corrugated heat exchange tube. The corrugated heat exchange tube is cold-extruded from a stainless steel seamless tube. The wave crests and troughs are subjected to different degrees of cold deformation and thinning. The crests and troughs may cause several stress corrosion cracks.

The essence of cold work hardening of austenitic stainless steel is to produce deformed martensite. The greater the cold work deformation, the more deformed martensite and the higher its hardness. At the same time, the greater the internal stress within the material. In fact, if solution heat treatment is carried out after its processing and forming, the effect of reducing the hardness and greatly reducing the residual stress can be achieved, and the martensite structure can also be eliminated, thereby avoiding stress corrosion cracking.

The embrittlement problem of long-term service under high temperature

At present, the container and pipe materials at 400～500℃ are mainly Cr-Mo steel with higher high temperature strength, and at 500～600℃ or even 700℃, various austenitic stainless steels are mainly used. In the design, people often pay more attention to the high temperature strength of austenitic stainless steel, and require its carbon content not to be too low. The allowable stress at high temperature is basically obtained by the extrapolated high temperature endurance strength test, which can ensure that no creep rupture occurs under the design stress of 100,000 hours of service.

However, the ageing embrittlement problem of austenitic stainless steel at high temperature cannot be ignored. After long-term service at high temperature, austenitic stainless steel will have a series of changes in the structure, which will seriously affect a series of mechanical properties of steel, especially the brittleness Significantly rise, resilience drops significantly.

The embrittlement problem after long-term service at high temperature is generally caused by two factors, one is the formation of carbides, and the other is the formation of σ phase. The carbide phase and σ phase continue to precipitate along the crystal after long-term service of the material, and even form a continuous brittle phase on the grain boundary, which is very easy to form intergranular fracture.

The formation temperature range of σ phase (Cr-Fe intermetallic compound) is about 600-980 ℃, but the specific temperature range is related to the alloy composition. As a result of the precipitation of σ phase, the strength of austenitic steel is greatly increased (the strength may be doubled), and it becomes hard and brittle. High chromium is the main reason for the formation of high-temperature σ phase, and Mo, V, Ti, Nb, etc. are alloy elements that strongly promote the formation of σ phase.

The formation temperature of carbide (Cr23C6) is in the sensitization temperature range of austenitic stainless steel, which is 400～850 ℃. Cr23C6 will dissolve above the upper limit of the sensitization temperature, but the dissolved Cr will promote the further formation of σ phase.

Therefore, when austenitic steel is used as a heat-resistant steel, the understanding and prevention of high-temperature aging embrittlement should be strengthened. Like the metal monitoring of thermal power plants, the metallographic structure and hardness changes can be checked regularly. If necessary, samples can be taken out for metallographic and hardness inspections, and even comprehensive mechanical properties and endurance strength tests can be performed.

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the Processing way of stainless steel elbow

 A stainless steel elbow is a pipe fitting that changes the direction of the pipe. According to the angle, there are three common types, such as 45°, 90°, and 180°. The processing methods of stainless steel elbows are generally mechanical processing such as: stamping method, forging method, roller processing method, rolling method, bulging method, stretching method, bending method, and combined processing method;

However, in the current production and processing, there are not many common methods for processing stainless steel elbows.

There are three commonly used forming methods for stainless steel elbows: stretching, stamping, and roller.

Forging method: The end portion or a part of the stainless steel pipe is punched by a swaging machine to reduce the outer diameter. The common type forging machine has a rotary type, a link type, and a roller type.

Stamping method: The end of the tube is expanded to the required size and shape with a tapered core in the stainless steel elbow.

The stamping forming of stainless steel elbows is the earliest forming process for mass production of elbows. At present, it has been replaced by hot push or other forming processes in the production of seamless elbows of common specifications, but in some specifications of elbows It is still used because of the small amount of production, excessive wall thickness or special requirements of the product. The stamping elbow is formed by using a tube blank equal to the outer diameter of the elbow, and is directly press-formed in the mold using a press.

Before the stamping, the tube blank is placed on the lower mold, the inner core and the end mold are loaded into the tube blank, the upper mold is moved downward to start pressing, and the punching elbow is formed by the constraint of the outer mold and the support of the inner mold.

Compared with the hot push process, the appearance quality of stamping is not as good as the former; when the punch elbow is in the stretched state when forming, there is no excess metal in other parts to compensate, so the wall thickness at the outer arc is about 10% thinner. . However, due to the characteristics of single-piece production and low cost, the stamping elbow process is mostly used for the manufacture of small batches and thick-walled elbows.

Stamping elbows are divided into cold stamping and hot stamping. Cold stamping or hot stamping is usually selected according to material properties and equipment capabilities.

The forming process of the cold extrusion elbow is to use a special elbow forming machine to put the tube blank into the outer mold. After the upper and lower molds are closed, the blank is reserved along the inner and outer molds under the push of the push rod. The gap moves to complete the forming process.

The elbow manufactured by the inner and outer mold cold extrusion process has beautiful appearance, uniform wall thickness and small dimensional deviation, so the stainless steel elbow, especially the thin-walled stainless steel elbow forming, is mostly manufactured by this process. The precision of the inner and outer molds used in the stamping elbow process is high; the wall thickness deviation of the billet is also demanding.

Roller method: The core is placed in the tube of the stainless steel elbow, and the outer circumference is pressed by the roller for the round edge processing.

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