Welding dissimilar steels

Galvanic corrosion may occur at stainless steel welds if sensitization has taken place or if welding has produced unfavorable dissimilar phases (see Chap. 15, Weld Defects, particularly Case History 15.1). These forms of microstructural galvanic corrosion do not involve the joining of two different metals in the usual sense. 

In the selection of consumables for a dissimilar steel joint, it is only necessary to match the strength of the weaker steel. In fact, it may be an advantage to use the lowest strength weld metal possible to minimise the stresses on both HAZ and weld metal. 

Gooch, T G Stainless steel consumables for dissimilar metal welds . In Welding dissimilar metals , ed N Bailey, Ch 3, 13-15. Pub TWI, 1986. 

Stainless steel develops a passive protective layer (<5-nm thick) of chromium oxide [1118-57-3] which must be maintained or permitted to rebuild after it is removed by product flow or cleaning. The passive layer may be removed by electric current flow across the surface as a result of dissinulat metals being in contact. The creation of an electrolytic cell with subsequent current flow and corrosion has to be avoided in constmction. Corrosion may occur in welds, between dissimilar materials, at points under stress, and in places where the passive layer is removed it may be caused by food material, residues, cleaning solutions, and bmshes on material surfaces (see CORROSION AND CORROSION CONTROL). 

The electrochemical examination of fusion joints between nine pairs of dissimilar metal couples in seawater showed that in most cases the HAZ was anodic to the weld metals" . Prasad Rao and Prasanna Kumarundertook electrochemical studies of austenitic stainless steel claddings to find that heat input and 5Fe content significantly affected the anodic polarisation behaviour under active corrosion conditions whilst Herbsleb and Stoffelo found that two-phased weld claddings of the 24Cr-13Ni type were susceptible to inter-granular attack (IGA) as a result of sensitisation after heat treatment at 600°C /pa was unaffected by heat input. 

Transition joints are used to join dissimilar metals where flanged, screwed, or threaded connections are not practical. They are used when fusion welding of two dissimilar metals forms interfaces that are deficient in mechanical strength and the ability to keep the system leak-tight. Transition joints consist of a bimetallic composite, a stainless steel, and a particular kind of aluminum bonded together by some proprietary process. Some of the types in use throughout the cryogenic industry are friction- or inertia-welded bond, roll-bonded joint, explosion-bonded joint, and braze-bonded joint. 

Inertia (friction) welding in the solid state, which is useful for joining dissimilar metals, i.e., aluminum to steel and stainless steel. At least one of the pieces to be welded must be circular and capable of being rotated at high velocities before being brought into contact with pressure to the second member. 

In order to facilitate this shift toward lighter materials, designers have had to identify an appropriate method of joining new and dissimilar materials. The joining method must be fast and easy to achieve, and the resultant joints need to be strong and durable (at least on a par with the performance of welding and other traditional methods of joining steel components). This is where structural adhesives have helped to make a big difference. 

Almost all metals can be welded the most common of these are aluminium, copper, carbon steels, stainless steels, titanium and refractory metals. The process can also be used to weld dissimilar metal combinations. It is used in the aerospace and car industries. 

LEW is a versatile process and can be used to weld a variety of materials including carbon steels, stainless steel, titanium, aluminium, nickel alloys and plastics. Lasers are often used in high-volume production applications as they have high welding speeds and a level of automation which allows them to be used in numerically controlled machines and robots. They are also used to weld dissimilar metal combinations. 

Epoxy resin, as an adhesive, is nsed for many applications in building/construction, (i.e., it is possible to bond a new rebar to existing steel in concrete instead of welding it, by use of special epoxy adhesives). Epoxy can bond to almost any material (for strnctural or non-structural bonding) with high adhesive strength in various environments and temperatures. In civil engineering applications, epoxy adhesives are used to bond concrete in a number of different ways. Epoxy can be used to bond plastic concrete (or wet concrete) to cured concrete, it can be used to bond cured concrete to cured concrete, or cured concrete to cracked concrete, as well as to bond cured concrete to other materials with similar or dissimilar thermal expansion coefficients and elastic moduli. 

When ferritic-martensitic steels are used for structural components, usually dissimilar welded joints with austenitic stainless steels are used, too. When these are subject to elevated temperatures, creep property evaluation and creep-fatigue evaluation are needed. In the case of dissimilar welds, close attention should be paid to the location of failure. Under certain conditions, failure could occur at the interface between the two materials [27]. 

S. Takaya, Evaluation of fatigue strength of similar and dissimilar welded joints of modified 9Cr—IMo steel, J. Pressure Vessel Technol. 138 (1) (2015), 011402-011402-9. 

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