HASTELLOY C-4 alloy (UNS N06455) is the most (microstructurally) stable of the widely used nickel-chromium-molybdenum materials,which are well known for their resistance to many aggressive chemicals, in particular hydrochloric acid, sulfuric acid, and chlorides. This stability means that the alloy can be welded without fear of sensitization, i.e. the nucleation and growth of deleterious, second phase precipitates in the grain boundaries of the weld heat-affected zone (HAZ).Like other nickel alloys, it is ductile, easy to form and weld, and possesses exceptional resistance to stress corrosion cracking in chloride-bearing solutions (a form of degradation to which the austenitic stainless steels are prone). With its high chromium and molybdenum contents, it is able to withstand both oxidizing and non-oxidizing acids, and is resistant to pitting and crevice attack in the presence of chlorides and other halides. The butt welding cap is a component of the container and is connected to the cylinder by welding. (As shown below) According to different geometric shapes, it can be divided into spherical, elliptical, dish-shaped, spherical crown, cone shell and flat cover. Among them, spherical, elliptical, dish-shaped and spherical crown heads are collectively referred to as convex heads. In terms of welding, it is divided into butt welding heads and socket welding heads. It is used for various container equipment, such as storage tanks, heat exchangers, towers, reactors, boilers and separation equipment. The materials include carbon steel (A3, 20#, Q235, Q345B, 16Mn, etc.), stainless steel (304, 321, 304L, 316, 316L, etc.), alloy steel (15Mo3 15CrMoV 35CrMoV45CrMo), aluminum, titanium, copper, nickel and nickel alloys, etc.
