Weldability, Structural Alloys

Poor Weldability a. Underbead cracking, high hardness in heat-affected zone. b. Sensitization of nonstabilized austenitic stainless steels. a. Any welded structure. b. Same a. Steel with high carbon equivalents (3), sufficiently high alloy contents. b. Nonstabilized austenitic steels are subject to sensitization. a. High carbon equivalents (3), alloy contents, segregations of carbon and alloys. b. Precipitation of chromium carbides in grain boundaries and depletion of Cr in adjacent areas. a. Use steels with acceptable carbon equivalents (3) preheat and postheat when necessary stress relieve the unit b. Use stabilized austenitic or ELC stainless steels. 

There is hardly a metal that cannot, or has not, been joined by some welding process. From a practical standpoint, however, the range of alloy systems that may be welded is more restricted. The term weldability specifies the capacity of a metal, or combination of metals, to be welded under fabrication conditions into a suitable structure that provides satisfactory service. It is not a precisely defined concept, but encompasses a range of conditions, eg, base- and filler-metal combinations, type of process, procedures, surface conditions, and joint geometries of the base metals (12). A number of tests have been developed to measure weldability. These tests generally are intended to determine the susceptibility of welds to cracking. 

Once the corrosion resistance plateau in ferritic alloys of 18%i chromium is reached, the addition of about 8%i nickel is required to cause a transition from ferritic to austenitic. The primary benefit of this alloy addition is to achieve the austenitic structure, which, relative to the ferritics, is very tough, formable, and weldable. The added benefit, of course, is the improved corrosion resistance to mild corrodents. This includes adequate resistance to most foods, a wide range of organic chemicals, mild inorganic chemicals, and most natural environmental corrosion. 

Positive influence of scandium has a complex character. Thus, the additions of Sc not only increase the yield stress and the ultimate tensile strength, but improve also the workability of alloy and its production ductility, decrease the grain size in as-cast condition, eliminate the dendrite structure and lead to the formation of a fine-grained equiaxial structure, sharply increase the recrystallization temperature (Tab.2), improve metal weldability and raise its corrosion resistance. 

In this context NijNb with a DO22 structure should be mentioned, which is of practical importance for some Ni-base superalloys. These alloys are strengthened by precipitated metastable NijNb particles and show excellent weldability (Oblak etal., 1974). 

In addition to requirements for the steels and alloys used in cryogenic structures, good weldability requires (1) resistance to metallurgical defects in the weld and heat-affected zone (HAZ), (2) stability of structural and physical-mechanical properties during welding, (3) the development of a special weld alloying system, and (4) the possibility of application of welding joints without post-heat treatment. 

In the USSR, the E. O. Paton Institute of Electrowelding carries out systematic studies of the weldability of structural steels and alloys for cryogenic use and the development of new materials and technological processes for welding. In recent years, data have been obtained and summarized on the main types of steels and alloys, distinguished by their specific strength, structural composition, and alloying system. 

The Al-6Cu-Mn alloy differs from other aluminum alloys in that it has relatively good mechanical properties at low temperatures and, therefore, can be successfully used for low-temperature structures. Unlike the duralumin-type alloys, the Al-6Cu-Mn heat-treatable alloy contains more copper (5.8 to 6.8%) and no magnesium [ ]. It is similar to the USA alloy 2219. It has satisfactory weldability and mechanical properties over a wide range of temperatures from 20 K up to 523 K, making it suitable for welded vessels for liquefied gas transportation and storage. 

Timetal 15-3 Ti-15V-3Cr-3Sn- 3A1) Cold formable and weldable, this strip alloy is primarily used for aircraft ducting, pressure vessels, and other fabricated sheet metal structures up to 300°C. 

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