Durable duplex pipe has good welding performance, compared with ferritic stainless steel and austenitic stainless steel, it is neither like ferritic stainless steel welding heat-affected zone, due to severe grain coarsening and plastic toughness is greatly reduced, but also not like austenitic stainless steel, welding heat cracking is more sensitive.
Duplex stainless steel due to its special advantages, widely used in petrochemical equipment, seawater and wastewater treatment equipment, oil pipeline, paper machinery and other industrial fields, in recent years, also be studied for the bridge load-bearing structure field, has a very good prospect for development.
Duplex stainless steel is in its solid quenching organization in the ferrite phase and austenite phase each accounted for half, the general content of the least phase maybe to reach 30%.
- Need to control the phase ratio, the most appropriate ratio is about half of the ferrite phase and austenite phase, where the number of a particular phase should not exceed 65% at most, so as to ensure that there is an optimal combination of performance. If the ratio of the two phases is out of proportion, for example, the number of ferrite phase is too much, it is easy to form a single-phase ferrite in the welded HAZ, sensitive to stress corrosion rupture in some media.
- Need to master the organization of durable duplex pipe transformation law, familiar with each steel TTT and CCT transformation curve, which is a correct guide to the development of duplex stainless steel heat treatment, thermoforming and other process key, duplex stainless steel brittle phase precipitation than austenitic stainless steel sensitive.
- Duplex stainless steel tube continuous use temperature range of -50 ~ 250 ℃, the lower limit depends on the brittle transition temperature of steel, the upper limit by 475 ℃ brittle limit, the upper limit temperature can not exceed 300 ℃.
- Durable duplex pipe solution treatment needs to be fast cooling, slow cooling will cause precipitation of brittle phases, which leads to the toughness of the steel, especially the decline in local corrosion resistance.
- High chromium molybdenum duplex stainless steel pipe heat processing and hot forming of the lower temperature can not be lower than 950 ℃, super duplex stainless steel can not be lower than 980 ℃ low chromium molybdenum duplex stainless steel can not be lower than 900 ℃, to avoid precipitation of brittle phases in the processing process caused by the surface cracking.
- Can not use austenitic stainless steel commonly used 650-800 ℃ stress relief treatment, the general use of solution annealing treatment. For low alloy steel surface surfacing duplex stainless steel, the need for 600-650 ℃ overall stress relief treatment, must take into account the precipitation of brittle phases due to the toughness and corrosion resistance, especially resistance to local corrosion degradation, as far as possible to shorten the heating time in this temperature range. Low-alloy steel and duplex stainless steel composite plate heat treatment issues should also be considered with this.
- Need to be familiar with the understanding of durable duplex pipe welding law, can not be used in all sets of austenitic stainless steel welding, duplex stainless steel equipment can be used safely and correctly grasp the steel welding process has a great relationship between some of the equipment failure is often related to welding. The key lies in the line energy and interlayer temperature control, the correct choice of welding material is also very important. Welded joints (weld metal and welded HAZ) of the two-phase ratio, especially welded HAZ to maintain the necessary amount of austenite, which is important to ensure that the welded joints have the same properties as the base material.
- In different corrosive environments selected duplex durable duplex pipe, pay attention to the corrosion resistance of steel is always relative, although the duplex stainless steel has a better resistance to local corrosion, a duplex stainless steel, he also has a range of media conditions, including temperature, pressure, media concentration, pH, etc., need to be carefully selected. Data from the literature and manuals are many laboratory corrosion test results, often with the actual conditions of the project there is a gap, so in the selection of materials need to pay attention to the need for corrosion tests in the actual medium or field conditions, or even simulation of the device test.
Due to the characteristics of two-phase organization, through the correct control of chemical composition and heat treatment process, DSS has the advantages of both ferritic stainless steel and austenitic stainless steel.
Compared with austenitic stainless steel, the advantages of durable duplex pipe
- Yield strength than ordinary austenitic stainless steel is more than twice as high, and has enough plastic toughness needed for molding. The use of duplex stainless steel manufacturing tanks or pressure vessels to reduce the wall thickness than the commonly used austenitic 30-50%, is conducive to reducing costs.
- Durable duplex pipe has excellent resistance to stress corrosion rupture, even the lowest alloy-containing duplex stainless steel has a higher resistance to stress corrosion rupture than austenitic stainless steel, especially in the environment containing chloride ions. Stress corrosion is common austenitic stainless steel is difficult to solve the outstanding problems.
- In many media, the most common application of 2205 duplex stainless steel corrosion resistance is better than ordinary 316L austenitic stainless steel, and super duplex stainless steel has very high corrosion resistance, and then some media, such as acetic acid, formic acid, etc. and even replace the high-alloyed austenitic stainless steel, and even corrosion-resistant alloys.
- Durable duplex pipe has good resistance to localized corrosion, with a comparable alloy content of austenitic stainless steel, its resistance to wear corrosion and fatigue corrosion are better than austenitic stainless steel.
- Than the austenitic stainless steel has a low coefficient of linear expansion, and carbon steel is close to the carbon steel, suitable for connection with carbon steel, has important engineering significance, such as the production of composite panels or linings.
- Whether in dynamic or static load conditions, than the austenitic stainless steel has a higher energy absorption capacity, which is structural components to cope with sudden accidents such as impact, explosion, etc., duplex stainless steel advantage is obvious, there are practical applications.
Compared with austenitic stainless steel, durable duplex pipe disadvantages
- The universality and versatility of the application is not as good as austenitic stainless steel, for example, its use temperature must be controlled at 250 ℃ or less.
- Its plastic toughness is lower than austenitic stainless steel, cold, hot working process and molding performance is not as good as austenitic stainless steel.
- The existence of medium temperature brittle zone, need to strictly control the heat treatment and welding process system, in order to avoid the emergence of harmful phases, damage performance.