Take You To A Comprehensive Understanding Of Duplex Stainless Steel

Take you to a comprehensive understanding of duplex stainless steel

In order to help users better understand, select, process and apply this kind of stainless steel, this paper will introduce the various characteristics of duplex stainless steel, and around the application of duplex stainless steel processing and welding one of the difficulties, give the processing and welding of duplex stainless steel basic principles and practical information.

Mechanical property
High strength:
Duplex stainless steel has excellent mechanical properties, and the mechanical properties of standard duplex stainless steel grades are shown in Table A. Their yield strength at room temperature in the solution annealing state is more than twice that of standard austenitic stainless steels without added nitrogen, allowing designers to reduce wall thickness in some applications. Figure B compares the yield strength of several duplex stainless steels with that of 316L austenitic stainless steels from room temperature to 300 ° C (570 ° F). Due to the risk of brittleness of the ferritic phase at 475 ° C (885 ° F), duplex stainless steel should not be used for long periods of time at temperatures higher than those specified in the pressure vessel design specifications.

The mechanical properties of forged rolled duplex stainless steel are highly anisotropic, that is, the properties change with the direction of the test sample. This anisotropy is formed by elongated grains and the crystal texture produced by hot or cold rolling. Although the solidification structure of duplex stainless steel is usually isotropic, there are two phases in the structure after rolling or forging and subsequent annealing. The shape of the two phases of the final product reveals the directivity of the processing, and the strength of the duplex stainless steel perpendicular to the rolling direction is higher than that along the rolling direction. When the notch of the impact sample is perpendicular to the rolling direction, the impact toughness is higher than that along the rolling direction. The toughness of the "longitudinal" (L-T) Charpy impact sample is higher than that of the other direction. The impact energy of a transverse specimen of a duplex stainless steel plate is generally equivalent to 1/2 to 2/3 of a longitudinal specimen.

Good toughness and ductility:
Despite their high strength, duplex stainless steels exhibit good plasticity and toughness. The toughness and ductility of duplex stainless steel are significantly better than that of ferritic stainless steel and carbon steel, and maintain good toughness even at very low temperatures such as -40 ° C /F. But not up to the excellent degree of austenitic stainless steel.

Corrosion resistance
In the vast majority of standard austenitic stainless steel applications in the environment, duplex stainless steel has shown high corrosion resistance, it is worth noting that they have a very obvious advantage in some cases, this is due to their high chromium content, very favorable in oxidizing acids, and contain sufficient amounts of molybdenum and nickel, resistant to corrosion of medium reducing acid media.

The relatively high chromium, molybdenum and nitrogen content of duplex stainless steels also gives them good resistance to chloride pitting and crevice corrosion, and their duplex structure is an advantage in environments where chloride stress corrosion cracking can occur.

If the microstructure of duplex stainless steel contains at least 30% ferrite, its resistance to chloride stress corrosion fracture is far better than that of austenitic stainless steel 304 or 316. However, ferrite is sensitive to hydrogen embrittlement, so in environments or applications where hydrogen may enter the metal and cause hydrogen embrittlement, duplex stainless steel does not have high corrosion resistance.