In general, simple chromium nickel, and chromium manganese nitrogen austenitic stainless steel pipe not only used for rust and oxidation resistance of medium (such as nitric acid, etc.) conditions, the use of molybdenum austenitic stainless steel as the important alloy elements added to the further expansion of the use of steel, the effect of molybdenum is mainly in the reducing medium steel (than – H2SO4, H3PO4, and some organic acid and urea environment) corrosion resistance, and improve the pitting corrosion and crevice corrosion resistance and other properties of the steel.
1, molybdenum impact to the organization
Chromium and molybdenum are the formation and stability of ferrite and expand in ferrite phase element, molybdenum and the ability of ferritic chromium. Also promotes molybdenum austenitic stainless steel pipe, intermetallic phase in the sigma phase, for example, kappa, predominate and Laves equal precipitation, corrosion resistance and mechanical properties of steel will produce adverse effect, plasticity, toughness, farewell is for keep the austenitic stainless steel pipe single austenitic organization, along with the increase of the content of molybdenum in steel austenite formed elements (such as nickel, nitrogen and manganese) levels will increase, in order to keep the steel ferrite and austenite balance between form elements.
2, molybdenum impact on performance
Oxidation of the molybdenum in the austenitic stainless steel pipe was not significant, so when cr-ni austenitic stainless steel pipe in single austenitic organization and no intermetallic precipitation, the effect of molybdenum on the mechanical properties at room temperature is not big, but along with the increase of the content of molybdenum steel high temperature strength increase, such as persistent, creep properties are improved, thus containing molybdenum stainless steel tubes are often in high temperature applications, however, the addition of molybdenum make high temperature deformation resistance of steel increases, and often there is a small amount of delta ferrite in steel and molybdenum stainless steel of workability than does not contain molybdenum steel for the poor, and molybdenum content is higher, the hot working performance is bad, in addition, the molybdenum austenitic stainless steel easy one million kappa (sigma) precipitation, predominate in this will significantly worse plasticity and toughness of steel, therefore in molybdenum austenitic stainless steel production, equipment manufacturing and application in the process, must pay attention to prevent the formation of intermetallic phase in steel.
Molybdenum in the austenitic stainless steel pipe, the main role is to improve the steel tube of the reducing medium corrosion resistance and resistance to corrosion, crevice corrosion resistance and other performance (304 stainless steel round) respectively on chrome molybdenum nickel austenitic stainless steel in nitric acid, sulfuric acid, acetic acid, phosphoric acid and urea, etc. The effect of medium corrosion resistance, it can be seen that, in addition to oxidizing medium HNO3 in place, the role of molybdenum are beneficial and therefore molybdenum austenitic stainless steel pipe generally don’t day nitric acid corrosion resistance, unless in the nitric acid containing F -, Cl – plasma, while the effect of alloying elements molybdenum reducing medium resistance of austenitic stainless steel tubes, the reason of surface corrosion and crevice corrosion is not entirely clear, but a lot of experimental has pointed out that corrosion resistant effect of molybdenum is only contains high amount of chromium in steel is effective, molybdenum mainly strengthening of chromium in steel corrosion effect, at the same time, the corrosion inhibition of molybdenum formed after acid salt have been experimentally.
In resistance to stress corrosion of high concentration chloride solution, although molybdenum alloy elements on the reducing medium resistance of austenitic stainless steel of the resistance to pitting corrosion and crevice corrosion is not entirely clear, but a lot of experimental has pointed out that the effect of molybdenum only when contains high amount of chromium in steel is valid, molybdenum mainly strengthening of chromium in steel corrosion effect, at the same time, molybdenum salt molybdate formed after the cushion has been experimentally.
Due to the more common cr-ni austenitic stainless steel pipe in water medium containing trace chlorine and oxygen saturation, the stress corrosion and pitting corrosion as origin, therefore molybdenum nickel chromium molybdenum austenitic stainless steel pipe due to pitting corrosion resistance is higher, so in practice often than does not contain molybdenum steel tube has better resistance to chloride stress corrosion performance.
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)