Welds heat affected zone, HAZ

Recrystallization and grain growth in the weld heat-affected zone (HAZ)... 

As in the case of many metal—ahoy systems, weld ductihty is not as good as that of the base metal. Satisfactory welds can be made in vanadium ahoys provided the fusion zone and the heat-affected zone (HAZ) are protected from contamination during welding. Satisfactory welds can be made by a variety of weld methods, including electron-beam and tungsten-inert-gas (TIG) methods. It is also likely that satisfactory welds can be made by advanced methods, eg, laser and plasma techniques (see Lasers Plasma technology). 

A listed material may be used at any temperature not lower than the minimum shown in the tables in Appendix IX, provided that the base metal, weld deposits, and heat affected zone (HAZ) are qualified as required by the applicable entry in Column A of Table GR-2.1.2(b)(2). 

Weld metal across the weld, with notch in the weld metal notch axis shall be normal to material surface, with one face of specimen S 1.5 mm (Vj6 in.) from the material surface Heat affected zone (HAZ) across the weld and long enough to locate notch in the HAZ after etching notch axis shall be approximately normal to material surface and shall include as much as possible of the HAZ in the fracture... 

Welded microstructures can be extremely complex and often change drastically over a very short distance. The fusion zone or weld metal is a dendritic structure that has solidified from a molten state. Bordering the fusion zone are transition, unmixed and partially melted zones, and the heat-affected zone (HAZ). These zones can be reheated and altered by subsequent weld passes, in multipass welding. For alloys with structures that depend strongly on thermal history, such as steels, the final microstructure can be extremely complex. Since welded structures are often quite susceptible to corrosion, overalloyed filler metals are often used to enhance the weld corrosion resistance. For stainless steels with sufficiently high carbon content, sensitization in the HAZ is another major problem. (Frankel)5... 

Carbon steels. The corrosion behavior of carbon steel weldments produced by fusion welding can be due to metallurgical effects, such as preferential corrosion of the heat-affected zone (HAZ) or weld metal, or it can be associated with geometric aspects, such as stress concentration at the weld toe, or creation of crevices due to joint design. 

Select low-temperature steels for fracture-critical structural members designed for tensile stress levels greater than a ksl (40 MPa) and specify a minimum Charpy V notch Impact energy absorption of 20 ft-lb (27 J) for base metal, heat-affected zones (HAZs), and welds when the structures are exposed to low-ambient temperatures. Fracture-critical members are those tension members whose failure would have a significant economic impact. 

The manufacturer shall perform all fabrication and welding in accordance with an established written procedure. The first production pipe shall be sectioned and tested. Included in the testing shall be the normal physical property and nondestructive testing as well as a microhardness traverse across the weld and heat-affected zone (HAZ). The hardness shall not exceed 280 HV1Q at any location. Test results from previous production runs of these grades may be considered to fulfill this requirement if the chemical composition and wetding procedure used are substantially the same as proposed for this order. 

Results (fig.2) show that the parent material is almost stress free. In the weld pool tensile bulk (around 60 MPa) stresses were found, whereas the opposite occurs in the heat-affected zones (HAZ). Curve shapes are consistent with theoretical and experimental behaviours reported in literature [6,7], and suggest the presence of structure change stresses , according to Macherauch s definition [7], This means that a superposition of the shrinkage stress due to different cooling rates inside and at the borders of the weld, and structural change stresses due to the presence of different phases can be outlined. 

As the hquid metal in a weld solidihes, it becomes at least partially constrained by the surrounding parent metal. When the weld metal cools to ambient temperature, the resulting residual stress in the weld is approximately equal to the ambient temperature yield strength of the parent metal. Stress relief heat treatment for welds is called postweld heat treatment. An additional benefit of such treatment is a reduction of the hardnesses of the weld metal and heat-affected zone (HAZ), thns reducing the risk of stress corrosion cracking. 

Heat-affected zone (HAZ) is the volume of parent metal in which the mechanical properties and/or the microstructure have been changed by the heat of welding or thermal cutting. For most welds in carbon and low-alloy steels, the HAZ is a band, usually about 1/8 in. (3 mm) wide, adjacent to the fusion line of the weld. In austenitic stainless steels, a narrow, secondary HAZ may be generated some distance from the fusion line as illustrated in Figure 21.4. 

Fig. 4.14 (a) Nugget grain size vs. rotation speed and (b) nugget center and heat-affected zone (HAZ) minimum hardness variation as a function of rotation speed with constant welding speed and z-axis force, Fz... 

Fig. 5.26 Crack tip opening displacement in friction stir welded 5051-T5 Al in both the nugget and heat-affected zone (HAZ) compared to the base material. RW repair weld PW, production weld. Source Ref 72...

Fig. 7.11 Microhardness data for a traverse across the entire weld region for friction stir welding on Ti-6Al-4V. Note the increase in hardness of the heat-affected zone (HAZ)...

The presence of tensile stress in the component may cause individual ligaments of HIC to form a stack of through-thickness array. This is a necessary precursor to stress-oriented hydrogen-induced cracking (SOHIC). SOHIC is common in heat affected zones (HAZs) of welds in low-strength steels. 

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. 

The weldability of the Fe-12Ni-0.5Al alloy is illustrated in Fig. 3, where the ratios of toughness for either weld metal or the heat-affected zone (HAZ) to that of... 

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