Weak terms and characteristics of duplex stainless steel

The performance of duplex stainless steel is generally strong, but it also has weaknesses. Especially, the weakness of high alloy duplex stainless steel containing high amounts of chromium, molybdenum and nitrogen is obvious.

One is thermal processing performance, the deformation temperature in thermal processing, because the two microstructure of duplex stainless steel in strength, plasticity and the deformation behavior of different has the difference, it will reduce the thermoplastic properties, and thermoplastic decline will lead to thermal processing performance of duplex stainless steel worse.

The other is the cold forming properties. The cold forming properties of the duplex stainless steel are less than those of the chromium nickel austenitic stainless steel, and are lower than some chromium containing ferritic stainless steels. This is related to the low yield strength, large deformation resistance, low elongation, large anisotropy, R value (plastic strain ratio) and n value (cold working hardening coefficient) of duplex stainless steel. Therefore, the duplex stainless steel is usually not suitable for the use of strict cold forming (such as deep drawing). For pipe and tubesheet expansion technology, we must use the larger external force to cause the steel tube yield, so that we can successfully complete the expansion joint operation, in addition, it can also make cold bending, but the resilience is higher.

The following features of the lower double phase stainless steel are introduced.

The first point is that the yield strength of duplex stainless steel is nearly twice as high as that of austenitic stainless steel, so the amount of deformation needed to be used is lower than that of the latter. When the cold rolling is done, the proper speed and delivery must be selected, so as to reduce the uneven longitudinal wall thickness and rolling. The rate of reducing the surface of the steel tube should not be too large when it is cold drawn.

The second point is the middle of the pipe softening using continuous roller hearth type heating furnace and heat treatment furnace duplex stainless steel (water cooling device with exit), do not use electric contact heating, which is not easy to control the temperature, and the heating speed is very fast, easily lead to overheating, the ferrite microstructure content exceed the standard, especially in the rolling of thin walled tube, if the control is not good, it is easy to form bamboo like uneven wall thickness, even if the wall thickness is not too mean. In electric contact heating, thin wall parts rapid warming, cause local overheating, will greatly increase the amount of ferrite, and even the formation of single-phase coarse ferrite, reduced corrosion resistance, acid pickling process after when the site is prone to local pickling, resulting in damage to scrap of duplex stainless steel tube.

The third point is the solid solution treatment of finished duplex stainless steel pipes, which is the key link. It not only eliminates the residual stresses in the finished products, but also ensures the proper two phase ratio and the surface quality of the finished products. During heat treatment, we should pay attention to the uniformity of material temperature and enough holding time, so as to avoid stress corrosion caused by insufficient solid solution and high residual stress on the surface of steel pipe.

Finally, for the steel pipe, which is mainly used for stress corrosion resistance, the delivery of the solid solution in the pickling state is needed to avoid the abrasive delivery. When the pipe surface is distributed with small cracks and folding defects, when the whole tube is needed to abrade, the steel pipes after grinding must be treated again after solid solution and acid washing. The main reason is that the tangential stress in the residual stress produced by grinding is the main stress to produce SCC damage.

Source: China Duplex Stainless Steel Pipe Manufacturer - Yaang Pipe Industry Co., Limited (www.yaang.com)