8 requirements for the production of dual-phase steel heat exchange tubes
1. It is necessary to control the phase proportion, and the most appropriate proportion is about half of the ferrite phase and the austenitic phase, of which the number of a phase cannot exceed 65% at most, so as to ensure the best comprehensive performance. If the two phases are out of proportion, such as the number of ferrite phases is too much, it is easy to form a single phase ferrite in the welding HAZ, which is sensitive to stress corrosion cracking in some media.

2. Need to master the structure change law of duplex stainless steel pipe, familiar with the TTT and CCT transformation curve of each steel grade, which is the key to correctly guide the development of duplex stainless steel heat treatment, thermoforming and other processes, the precipitation of brittle phase of duplex stainless steel is more sensitive than austenitic stainless steel.

3. The continuous use temperature range of dual-phase steel heat exchange tube is -50 ~ 250 ° C, the lower limit depends on the brittle transition temperature of steel, the upper limit is limited by the brittleness of 475 ° C, and the upper limit temperature cannot exceed 300 ° C.

4. Fast cooling is needed after solid solution treatment of dual-phase steel heat exchange tube, and slow cooling will cause the precipitation of brittle phase, which leads to the toughness of steel, especially the decline of local corrosion resistance.

5. The lower limit temperature of hot processing and hot forming of high chromium molybdenum duplex stainless steel pipe can not be lower than 950 ° C, super duplex stainless steel can not be lower than 980 ° C Low chromium molybdenum duplex stainless steel can not be lower than 900 ° C, to avoid surface cracks caused by the precipitation of brittle phase in the process

6 Can not use austenitic stainless steel commonly used 650-800℃ stress relief treatment, generally using solid solution annealing treatment. For the low alloy steel surface welding of duplex stainless steel, the need for 600-650℃ overall sterilization treatment, we must take into account the toughness and corrosion resistance caused by the precipitation of brittle phase, especially the decline in local corrosion resistance, as far as possible to shorten the heating time in this temperature range. The heat treatment of low alloy steel and duplex stainless steel composite plates should also be considered in this way. [1]

7 need to be familiar with the welding law of duplex stainless steel pipe, can not all apply the welding of austenitic stainless steel, duplex stainless steel equipment can be used safely and correctly grasp the welding process of steel has a lot to do with the failure of some equipment is often related to welding. The key lies in the control of line energy and interlayer temperature, and the correct selection of welding materials is also important. The two-phase ratio of the welded joint (weld metal and welded HAZ), especially the maintenance of the necessary amount of austenite in the welded HAZ, is important to ensure that the welded joint has the same properties as the base material.

8. When choosing duplex stainless steel in different corrosive environments, it should be noted that the corrosion resistance of steel is always relative, although duplex stainless steel has better local corrosion resistance, for a certain duplex stainless steel, he also has a range of applicable media conditions, including temperature, pressure, medium concentration, pH value, etc., which needs to be carefully selected. Most of the data obtained from literature and manuals are corrosion test results in the laboratory, which often have a gap with the actual conditions of the project, so it is necessary to pay attention to the selection of materials, and if necessary, it is necessary to carry out corrosion test in the actual medium or the hanging test under field conditions, or even the test of the analog device.

The above eight requirements come from the Internet.

HAOSTEEL can supply products: professional duplex stainless steel (S22053, S25073, S22253, SAF2205, SAF 2507, S32205, S32750, S31803, S32304, etc.); Special stainless steel (904L, 347H, 316Ti, 317L, 310S, 309S, 254SMO, 316L, 304, 321); Nickel-based alloy steel 800H, 600H series of steel pipes/pipe fittings/flanges/round steel/forgings.

First of all, let's understand the characteristics and advantages of duplex stainless steel: duplex stainless steel is a medium alloy stainless steel composed of austenite and ferrite. Ferrite and austenite make up about 50% each. Generally, the content of less phase should be 30% stainless steel. Due to the characteristics of both austenitic and ferritic stainless steel, duplex stainless steel has high plasticity and toughness compared with ferrite, no room temperature brittleness, and while maintaining 475°C characteristics, the grain boundary corrosion resistance and weldability become significantly higher.

Compared with austenitic stainless steel, duplex stainless steel has high strength and significantly increases resistance to intergranular corrosion and chloride stress corrosion. These properties make it widely used in the petrochemical industry in environments with highly corrosive media such as chlorides and sulfides. Compared with austenitic stainless steel, duplex stainless steel has strong stress resistance and corrosion cracking ability. In this paper, the frequent occurrence of local corrosion and leakage of the heat exchange tube during the use of the hydrogenation unit cooler is studied in detail, and the duplex stainless steel tube is selected to replace it. It can provide some help for the rational use of dual-phase stainless steel heat exchange tube in engineering.

Under normal operating conditions, the axial compression stress of the heat exchange tube is not tensile stress, and there are no stress corrosion conditions, which is consistent with the field inspection. The leak occurred at the welded joint of the heat exchange tube and tube plate. Under the influence of compression stress and gap corrosion, stress corrosion will occur in welded joints of tube plate and heat exchange tube. The shell side medium of the cooler contains H2O and H2S. In the case of water and sulfur, when the operating temperature is ≤120 ° C, if the sulfide is not fused, it will encounter H2S-H2O type corrosion, which is difficult to control, including general corrosion and corrosion cracks, and the carbon steel heat exchange tube can not reach the current working conditions.