Hastelloy alloy special material properties and introduction
Hastelloy alloy
I. Introduction
Hastelloy is a type of nickel-based alloy. It is currently divided into three series: B, C, and G. It is mainly used for strong corrosion that cannot be used in iron-based Cr-Ni or Cr-Ni-Mo stainless steel, non-metallic materials, etc. It has been widely used in petroleum, chemical industry, environmental protection and many other fields abroad. Its grades and typical usage situations are shown in the table below.
In order to improve the corrosion resistance and cold and hot working properties of Hastelloy, Hastelloy has made three major improvements. The development process is as follows:
B series: B → B-2(00Ni70Mo28) → B-3
C series: C → C-276(00Cr16Mo16W4) → C-4(00Cr16Mo16) → C-22 (00Cr22Mo13W3) → C-2000(00Cr20Mo16)
G series: G → G-3 (00Cr22Ni48Mo7Cu) → G-30 (00Cr30Ni48Mo7Cu)
The most widely used materials at present are N10665 (B-2), N10276 (C-276), N06022 (C-22), N06455 (C-4) and N06985 (G-3). The third generation materials N10675 (B-3), N10629 (B-4), and N06059 (C-59) are in the promotion stage. Due to the advancement of metallurgical technology, multiple brands of so-called "super stainless steel" containing ~6% Mo have appeared in recent years, replacing G series alloys, causing a rapid decline in the production and use of G series alloys.
2. Mechanical properties
The mechanical properties of Hastelloy are very outstanding. It has the characteristics of high strength and high toughness, so it is difficult to machine. Moreover, its strain hardening tendency is extremely strong. When the deformation rate reaches 15%, it is about 18-8 Twice that of stainless steel. Hastelloy also has a medium-temperature sensitization zone, and its sensitization tendency increases with the increase in deformation rate. When the temperature is high, Hastelloy easily absorbs harmful elements, causing its mechanical properties and corrosion resistance to decrease.
3. Commonly used Hastelloy alloys
Hastelloy B-2 alloy(Hastelloy B-2 alloy)
Corrosion resistance
Hastelloy B-2 alloy is a Ni-Mo alloy with extremely low carbon and silicon content. It reduces the precipitation of carbides and other phases in the weld and heat-affected zone, thereby ensuring that even under welding conditions Also has good corrosion resistance. As we all know, Hastelloy B-2 alloy has excellent corrosion resistance in various reducing media and can withstand corrosion at any temperature and concentration of hydrochloric acid under normal pressure. It has excellent corrosion resistance in non-aerated medium-concentration non-oxidizing sulfuric acid, various concentrations of phosphoric acid, high-temperature acetic acid, formic acid and other organic acids, bromic acid and hydrogen chloride gases. At the same time, it is also resistant to corrosion by halogen catalysts. Therefore, Hastelloy B-2 alloy is usually used in a variety of harsh petroleum and chemical processes, such as the distillation and concentration of hydrochloric acid; the alkylation of ethylbenzene and low-pressure oxo synthesis of acetic acid and other production processes. However, it has been found in the industrial application of Hastelloy B-2 alloy for many years:
(1) Hastelloy B-2 alloy has two sensitization zones that have a considerable impact on intergranular corrosion resistance: a high temperature zone of 1200~1300°C and a medium temperature zone of 550~900°C;
(2) Due to dendrite segregation in the weld metal and heat-affected zone of Hastelloy B-2 alloy, intermetallic phases and carbides precipitate along the grain boundaries, making them more sensitive to intergranular corrosion;
(3) Hastelloy B-2 alloy has poor medium-temperature thermal stability. When the iron content in Hastelloy B-2 alloy drops below 2%, the alloy is sensitive to the transformation of the beta phase (Ni4Mo phase, an ordered intermetallic compound). When the alloy stays in the temperature range of 650~750℃ for a slightly longer time, the β phase is generated instantly. The existence of β phase reduces the toughness of Hastelloy B-2 alloy, making it sensitive to stress corrosion, and even causes Hastelloy B-2 alloy to be damaged during raw material production (such as hot rolling process) and equipment manufacturing process (such as Hastelloy B-2 alloy equipment post-weld overall heat treatment) and Hastelloy B-2 alloy equipment cracks in the service environment. Nowadays, the standard test method designated by my country and other countries around the world for the intergranular corrosion resistance of Hastelloy B-2 alloy is the normal pressure boiling hydrochloric acid method, and the evaluation method is the weight loss method. Since Hastelloy B-2 alloy is an alloy resistant to hydrochloric acid corrosion, the normal pressure boiling hydrochloric acid method is quite insensitive to test the intergranular corrosion tendency of Hastelloy B-2 alloy. Domestic scientific research institutions used high-temperature hydrochloric acid method to study Hastelloy B-2 alloy and found that the corrosion resistance of Hastelloy B-2 alloy not only depends on its chemical composition, but also depends on its thermal processing control process. When the thermal processing process is improperly controlled, the grains of Hastelloy B-2 alloy not only grow, but also the high Mo σ phase will precipitate between the grains. At this time, the intergranular corrosion resistance of Hastelloy B-2 alloy is significantly reduced. , in the high-temperature hydrochloric acid test, the grain boundary etching depth of the coarse-grained plate and the normal plate differed by about twice.
