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Requirements and restrictions for the use of duplex stainless steel

Duplex stainless steel has been continuously developed and improved over the years, and some fields have replaced austenitic stainless steel. In recent years, the use of super duplex stainless steel has also created favorable conditions for expanding the use of duplex stainless steel and developing new steel grades. Duplex stainless steel not only has the commonality of ordinary stainless steel, but also has its own characteristics.
The performance requirements and limitations of using duplex stainless steel are detailed below.
First of all, to control the gold ratio, the most suitable ratio is that the ferrite phase and the austenite phase each account for about 50%, and the maximum number of one phase cannot be greater than 65%, so as to ensure the best overall performance. If the two ratios are out of tune, it is very easy to form a single-phase ferrite in the welded HAZ, which may be sensitive to stress corrosion cracking in the medium.
Secondly, it is necessary to master the metallographic transformation of duplex stainless steel and familiarize with the TTT and CCT transition curves of each steel grade, which is very important for correctly guiding the process of heat treatment and thermoforming of duplex stainless steel.
Third, the continuous use temperature range of duplex stainless steel is -50 ° C to 250 ° C, the lower limit is subject to the brittle transition temperature of stainless steel, the upper limit is limited by the brittleness of 475 ° C, and the upper limit temperature should not exceed 300 ° C.
Fourth, the duplex stainless steel should be directly cooled quickly after solution treatment. Slow cooling will lead to the precipitation of brittle phase, which will reduce the toughness and local corrosion resistance of stainless steel.
Fifth, the lower limit temperature of hot working and thermoforming of high chromium molybdenum duplex stainless steel shall not be less than 950 °C, super duplex stainless steel shall not be less than 980 °C, and low chromium molybdenum duplex stainless steel shall not be less than 900 °C to prevent precipitation due to brittle phase. Surface cracks are caused during processing.
Sixth, it is not possible to use 650-800 ° C commonly used in austenitic stainless steel to eliminate stress treatment, usually using solution annealing treatment. For the double-phase stainless steel after surfacing the surface of low-alloy steel, it is necessary to consider the toughness and corrosion resistance due to the precipitation of the brittle phase, especially the local corrosion resistance, when performing the overall treatment at 600-650 °C. Reduce the problem and minimize the heating time in this temperature range.
Seventh, the point is to master the welding rules of duplex stainless steel. The equipment of duplex stainless steel can be safely used and has a great relationship with the welding process. Some equipment failures are usually related to welding. The key is the control of line energy and interlayer temperature. It is also important to choose the right solder material.
Finally, when using duplex stainless steel in different corrosive environments, it is necessary to understand that the corrosion resistance of stainless steel is relative, and there are certain applicable conditions, which are affected by temperature, pressure, medium concentration and the like. Therefore, pay special attention to the selection of materials.
Source: China Duplex Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

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