Nowadays, duplex stainless steel is widely applied in various industrial fields, such as pulp and paper industry, oil and gas industry, chemical industry, dangerous goods transportation, pharmaceutical and food processing, and construction industry. In these industries, the duplex stainless steel is mainly used for the preparation of reaction containers, industrial equipment and various kinds of conveying pipes. Compared with many other materials, duplex stainless steel is generally recognized as the best cost effective material. It effectively fills the blank of intermediate austenitic stainless steel such as 316 and high alloy austenitic stainless steel.
It is generally believed that the application of duplex stainless steel is due to the material can resist chemical corrosion, corrosion medium such as urea fertilizer and organic acid production, production and processing of natural gas containing hydrogen sulfide will encounter, but more critical applications for those who do not have enough resistance to pitting corrosion of austenitic stainless steel and chlorine ion hot water solution of stress corrosion cracking the properties, such as heat exchanger, condenser and various types of containers and other equipment.
For duplex stainless steel, there are limitations and requirements for the following safe use.
The first is to control the phase ratio. The most suitable proportion is the ferrite phase and the austenite phase, each half of which must be occupied. The number of a phase can not be greater than 65%.
The second is to master the transformation rule of the duplex stainless steel, and be familiar with the constant temperature transformation and the continuous transformation of the ferrite in each steel. This is also the key to correctly guide the process of heat treatment and hot forming for duplex stainless steel. The sensitivity of duplex stainless steel to brittle phase is far more sensitive than austenitic stainless steel.
In addition, the continuous temperature range of the duplex stainless steel is from -50 to 250.
The duplex stainless steel should be cooled quickly after the solid solution treatment. The slow cooling will lead to the precipitation of the brittle phase and reduce the toughness and corrosion resistance of the steel.
It is important to pay attention to the elimination of stress treatment at 650-800 C, usually used for austenitic stainless steel, usually with a solution annealing treatment over 950 centigrade. If in the surfacing of low alloy steel duplex stainless steel after 600-650 DEG C to the overall stress relieving, we must consider the toughness and corrosion resistance decrease, shorten the heating time as far as possible in this temperature range.
It is also necessary to be familiar with the welding rules of duplex stainless steel and not to use all kinds of austenitic stainless steel welding technology. The key of welding is the control of line energy and interlayer temperature. The correct selection of welding materials is also very important. The heat affected zone of welding fusion line should maintain a certain amount of austenite so that the performance of the welded joint is equivalent to or close to the parent material.
The last point is the selection of duplex stainless steel in various corrosion environments, the need to pay attention to the corrosion resistance of stainless steel is limited, even if the double phase stainless steel has better resistance to local corrosion, but for any type of double phase stainless steel, the model is a suitable medium range of conditions, including temperature medium, pressure, concentration and pH value, so need to be carefully selected according to actual use. In order to meet the requirements of actual conditions, it is necessary to do some corrosion tests in actual media or hang up test under field conditions when necessary, and even more, we can do the test of analog devices.
Source: China Duplex Stainless Steel Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)