Austenitic weld metal

Austenitic stainless steels are often susceptible to sigma-phase embrittlement after extended exposure at 1050°F to 1700°F (565°C to 925°C). Occurrence depends primarily on the temperature and the presence of ferrite. While Type 304 SS can develop sigma-phase embrittlement, it is more common in austenitic products that contain small amounts of ferrite. Examples include austenitic weld metal and castings. 

However, both types require some preheat for the more difficult steels, they cannot be effectively stress relieved thermally, and welds are difficult to inspect non-destructively for cracking. Austenitic weld metals can be selected which are of higher strength than the common suitable nickel alloys. 

Superalloyed filler metals with high ferrite numbers (15 to 40%) are often used in mixed weld connections between low-alloy filler metals and stainless steel. Weldability is very good. By using such filler metals, mixed weld metals of the austenitic type can be obtained. The use of filler metals of the ordinary austenitic type for welding low-alloy filler metals to stainless steel can, owing to dilution, result in a brittle martensitic-austenitic weld metal. 

Fully austenitic weld metals. Sometimes ferrite-free metals are required because there is usually a risk of selective corrosion of the ferrite. Fully austenitic weld metals are naturally more susceptible to hot cracking than weld metals with a small percentage of ferrite. To reduce the risk, they are often alloyed with manganese, and the level of trace elements is minimized. Large weld pools also increase the risk of hot cracks. 

Micro-fissures are caused by thermal contraction stresses during weld solidification and are a problem that plagues austenitic stainless steel fabrications. These weld metal cracks are more likely to form when phosphorus and sulfur levels are higher (that is, more than 0.015% P and 0.015% S), with high heat input welding, and in austenitic weld metal in which the a-ferrite content is low (< 3%). 

Austenitic Steel weld has a well defined transcrystalline (oriented) macrostructure with continuously changing orientation of the crystal axis - from the periphery towards the centre the angle between the axis of the crystal and the axis of the weld is changed from 90 to 0 degrees. Weld metal eould be possible to approximate in the form of a discrete combination of crystals with parallel axes of the crystallites. 

Experiments were carried out on samples, made of austenitic steel with thickness from 10 to 50 mm using equipment, described above. The samples were a) multipass austenitic weld and b) base metal. 

Figure 2 a) multipass austenitic weld and b) base metal. 

Austenitic stainless steel 3(3. If (1) the carbon content by analysis is greater than 0.10 percent or (2) the material is not in the solution-heat-treated conchtion, then impact testing is required for design temperatures below-29 C (-20 F). See Note 2. ib. When materials are fabricated or assembled by wel(hng, the deposited weld metal shall be impact-tested for design temperature below —29 C (—20 F) unless cou(htious conform to Note 2. 3. The material shall be impact-tested. See Note 2. 

Austenitic types. These are susceptible to hot cracking which may be overcome by balancing the weld metal composition to allow the formation of a small amount of 5-Fe (ferrite) in the deposit, optimum crack resistance being achieved with a 5-Fe content of 5-10%. More than... 

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. 

J. O Donnell, H. Huthmann, and A. A. Tavassoli, The Fracture Toughness Behavior of Austenitic Steels and Weld Metal Including the Effects of Thermal Aging and Irradiation , Int. J. Pres. Ves. Piping, 65 (1996), 209-220. 

Delong W. T., Ferrite in austenitic stainless steel weld metal. Weld. Res. Suppl. 53 (1974), 273s —286s... 

In rare cases, a relatively small area near the weld will be an anode to the relatively large cathodic surface area of the parent metal. In moderately corrosive media, this zone may corrode much faster than either the weld metal or the parent metal. Postweld heat treatment is usually helpful. In some instances, normalizing (or even solution annealing in the case of an austenitic stainless steel) the weldment is necessary, a measure that can cause significant distortion problems. In most cases, the weld metal, HAZ, and parent metal do not have significant galvanic differences. 

Aluminum, copper, and other face-centered cubic metals and alloys (such as the austenitic stainless steels and nickel-base alloys) do not become brittle at low temperatures, except if heavily cold worked. Most such alloys are exempt from impact testing for design temperatures down to -320°F (-195°C). Some types, such as Type 304, are exempt down to 25°F (-255°C). The exemption temperatures for weld metals and HAZs are usually higher than those for the parent metal. 

Often the very engineers who insist on post-weld heat-treatment with carbon steel or low-alloy weld metals would be willing to omit the post-weld heat treatment if an austenitic stainless steel electrode, particularly type 309 (25-Cr, 12-Ni) were used. 

Ironically, their reasoning that the hardened, heat-affected zone of the 5-Cr steel would have ductile, austenitic stainless steel weld metal on one side of it and ductile, 5-Cr steel parent metal on the other, is equally valid in the rejected case where the weld met is carbon steel. 

Similar comments are applicable to weld metal hydrogen cracks, with the further observations that cracking in weld metals is frequently transverse to the weld and that in weld metals of the acicular ferrite type, cracking is most common in the primary ferrite (if it is present) at the prior austenite grain boundaries, as in Fig. 1.3(a). In relatively hard, alloyed weld metals (e.g. in the Cr-Mo steels) any transverse weld metal cracking is usually perpendicular to the weld surface, but in C-Mn weld metals the chevron type of cracking (Fig. 1.3(c)) at 45° to the surface is common. 

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