Heat affected zone

Fig. 5. (a) Temperature isotherms in the region of a moving welding arc. (b) Heat-affected-zone thermal cycles at various locations in the base plate (see... 

Thus, the metallurgy of welds, comprising the weld metal and surrounding heat-affected zone, is influenced not only by the composition of the materials involved, but also by the welding process, the specific procedures for applying the process, and the heat-transfer characteristics (deterrnined by material, mass, and geometry) of the welded joint (9—12). 

Excellent high strength welds have been produced by inertia-welding, or friction-welding, which develops essentially no heat-affected zone. The actual interface is wrought molybdenum because the molten metal and the adjacent soHd metal that has been raised to very high temperatures have been expelled from the joint. 

Many instances of intergranular stress corrosion cracking (IGSCC) of stainless steel and nickel-based alloys have occurred in the reactor water systems of BWRs. IGSCC, first observed in the recirculation piping systems (21) and later in reactor vessel internal components, has been observed primarily in the weld heat-affected zone of Type 304 stainless steel. 

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). 

Zirconium is completely resistant to sulfuric acid up to Foiling temperatures, at concentrations up to 70 wt %, except that the heat-affected zones at welds have lower resistance in >55 wt % concentration acid (Fig. 1). Fluoride ions must be excluded from the sulfuric acid. Cupric, ferric, or nitrate ions significantly increase the corrosion rate of zirconium in 65—75 wt % sulfuric acid. 

AU other carbon steel, low-intermediate, and high-aUoy steels, ferritic steels Base metal Deposited weld metal and heat-affected zone (See Note 1) 2. Except when conchtions conform to Note 2, the material shall be heat-treated to control its microstructure by a method appropriate to the material as outlined in the specification applicable to the product form and then impact-tested. (See Note 1.) Deposited weld metal and heat-affected zone shall be impact-tested. 

Any tests and associated acceptance criteria which are part of the welding-procedure qualification for filler materials and heat-affected zone need not he repeated. 

Corrosive chemicals and external exposure can cause tank failure. Materials of construction should be chosen so that they are compatible with the chemicals and exposure involved. Welding reduces the corrosion resistance of many alloys, leading to localized attack at the heat-affected zones. This may be prevented by the use of the proper alloys and weld materials, in some cases combined with annealing heat treatment. 

Hastelloy C-4 is almost totally immune to selective intergranular corrosion in weld-heat-affected zones with high temperature stabihty in the 650-I040 C (I200-I900 F) range Hastelloy C-22 has better overall corrosion resistance and versatihty than either C-4 or C-276 (in most environments). 

The weld was riddled with mildly undercut, gaping pits. Attack was confined to fused and heat-affected zones, with a pronounced lateral or circumferential propagation (as in Fig. 6.10). The resulting perforation at the external surface was quite small. Pits were filled with deposits, friable oxides, and other corrosion products. Black plugs embedded in material filling the gaping pit contained high concentrations of iron sulfide. Bulk deposits contained about 90% iron oxide. Carbonaceous material was not detected. 

General description. In weld-root cracking, cracks originate at the root of the weld (Fig. 15.17). Such cracks may propagate into the weld, through the weld, into adjacent components, or through a relatively brittle heat-affected-zone base metal. 

There is one major pitfall which must be avoided in using stainless-steel components joined by welding it is known as weld decay. It is sometimes found that the heat-affected zone - the metal next to the weld which got hot but did not melt - corrodes badly. 

Large differences in microstructure between parent metal, heat-affected zone and weld bead. Sharp changes in mechanical properties give local stress concentrations. 

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. 

Stressed, such as heat-affected zones near welds, in areas of high acid-gas concentration, or at a hot gas-liquid interface. Therefore, stress-relieving all equipment after manufacturing is necessary to reduce corrosion, and special metallurgy in specific areas such as the still overhead or the reboiler tubes may be required. 

HEAT AFFECTED ZONE (NOT VISMLE ON DRILL STEM)... 

Welding It is possible to weld molybdenum using a TIG-shielded arcwelding process. A heat-affected zone is unavoidable and grain growth must be anticipated. 

Selective corrosion in the heat-affected zone of a weld occurs most commonly when unstabilised stainless steels are used in certain environments. The obvious answer is to use an extra-low-carbon grade of stainless steel, e.g. types 304L, 316L or a stabilised grade of steel, e.g. types 321 and 347. Knifeline attack at the edge of a weld is not commonly encountered and is seldom predictable, and it must be hoped that it is revealed during preliminary corrosion testing. 

For both heat-affected zone corrosion (intergranular attack) and knifeline attack the heat flux during welding and the time at temperature can critically affect the severity of the attack. Both these factors may vary from one welder to another, and when preparing pieces for corrosion testing not only should fabrication welding conditions be accurately reproduced, but the work of more than one welder should be evaluated. 

---