32

"Argon or argon + max. 3% hydrogen.

bFor MAG Welding use of the shielding gas Cronigon He30S is recommended. (Cronigon He30S = argon + 30helium + 2hydrogen + 0.05 carbon dioxide). In all gas-shielded welding operations, ensure adequate back shielding. These figures are only a guide and are intended to facilitate setting of the welding machines and should not be taken as absolute.

TABLE 7.20 Guidelines for Energy Input per Unit Length During Welding

Energy Input"/Unit Length Welding Process During Welding (kJ/cm)

GTAW (manual, automatic) 10 max

Hot wire GTAW 6 max

Plasma arc 10 max

GMAW (MIG/MAG)—manual, automatic 11 max

SMAW (manual metal arc) 7 max

"Heat input = ———^qqq kJ/cm> where U = arc voltage, volts; I = welding current, amps; V = welding speed cm/ min.

processes, where as Table 7.20 give the basic guidelines for heat input during these processes. It must be noted that these are just basic guidelines and will have to be optimized for individual processes, positions, machines, and operators during any weld procedure qualifications.

Due to the need to manipulate the weld metal in the weld joint, nickel alloys require more openness (wider root gaps and larger included angles) to permit the use of a slight weaving technique, which should not exceed three times the diameter of the weld wire. These parameters are necessary in comparison to welding of mild steel because of lower thermal conductivity and higher thermal expansion characteristics of nickel alloys. Generally, welding can be done by all the conventional processes such as GTAW (TIG), GMAW (MIG), and SMAW (coated electrode). Submerged arc welding can be used, but, depending on the flux used, the danger of carbon and silicon pick up or chrome depletion in the weldment exists that could lower the corrosion resistance. Plasma arc welding has also been successfully used for welding nickel alloys. Another new technique in recent use is GTAW hot-wire welding, which uses a 2% hydrogen addition to the argon shielding gas. This has resulted in a significant increase of welding speed, thereby reducing the cost and increasing production efficiency. This process has been very successfully used in welding roll-clad steel with alloy 59 for FGD systems of large coal-fired power plants. Due to the extremely low iron content of alloy 59 filler metal, the iron dilution is kept at very low levels thus fully maintaining the corrosion resistance of the weld joint.

Another welding process, a variation of GMAW process known as the MAG process (metal active gas) is gaining popularity. In this the shielding gas contains an active gas component such as carbon dioxide (0.05%) in argon plus 30% helium and 2% hydrogen. This gas mixture is known as CronigonHe30S and is very popular in Germany.

Welding products suitable for welding Ni-Cr-Mo-type alloys are matching filler metal or overalloyed filler metal such as alloy 59, which is designated under AWS/ANSI A5.14 as ERNiCrMo-13 and ENiCrMo-13 in AWS A5.11. For welding the superaustenitic SS (6% Mo SS), and other low alloys such as alloy 825 or 904L, it is recommended that a minimum 9% Mo containing alloy such as alloy 625 (ERNiCrMo-3) be used because autogenous welding will significantly

FIGURE 7.7 Hot-cracking sensitivity of various alloys as measured in a modified vare-straint test.

lower the corrosion resistance through microsegregation of molybdenum during solidification. Since alloy 625 is a niobium-containing Ni-Cr-Mo alloy (9% Mo), its high hot cracking sensitivity has been known to cause some problems during welding (Fig. 7.7). Also, there has been an embrittling tendency when using this alloy as a filler metal for welding duplex and superduplex stainless steels, due to formation of niobium nitrides. Also, weldments of this alloy in the power industry and other high-temperature applications have been known to embrittle, due to formation of gamma double prime Ni3Nb at temperatures above 1000 F. A new improved niobium-free version alloy of this Ni-Cr-Mo family, with even higher Mo content, has been developed by Krupp VDM to overcome the limitations of alloy 625 mentioned above. This alloy is known as alloy 50 with a UNS N06650. Its coverage in AWS and ASTM specifications is pending. Due to its higher Mo content with additions of some tungsten, alloy 50 (Ni bal, Mo 12%, W 2%, Fe 13.5%) would be an excellent filler metal, not only for welding 6% Mo alloys, duplex, and superduplex stainless steels, but also for low nickel alloys such as alloys 825, 904L, 20, and even stainless steels. Details on this alloy are published elsewhere [46, 47].

The alloy in the initial testing has shown promising results in weld overlay of superheater tubes in the power boilers of coal-fired power plants and refuse to energy power plants. Alloys C-276 and 59 have also been successfully used for welding these superaustenitic alloys. For welding of Ni-Mo alloys only the matching filler metals of the Ni-Mo family are recommended.

Neither preheating nor postweld heat treatment is required for these alloys, which are designed to be used in the as-welded condition due to their very low carbon contents. No major problems have been reported from the field if the welding was done properly. For dissimilar welding of these alloys to carbon steel, stainless steel, and other nickel alloys, the welding practices are not significantly different except for the selection of a proper filler metal, which is a very important parameter. Also, when joining these alloys to carbon or low-alloy steels, the arc may have a tendency to migrate on to the steel side of the weld joint. Hence, proper grounding procedures and techniques, short arc length, and torch/electrode manipulation capabilities are essential to counteract and compensate for this problem. When welding carbon steel/low-alloy steel to any stainless or superstainless steel, or Ni-Cr-Mo alloy, it is advisable to use either alloy C-276 or alloy 59, which fall in the category of fully austenitic overalloyed filler metals. Some fabrication shops have also used alloy 625 (ERNiCrMo-3) filler metal depending on the intended service. Under above conditions, when one of the base metals is of the Ni-Mo family, such as alloy B-2, then the recommended filler metals are alloys B-2 or B-4. As a general rule the choice of the filler metal will be greatly influenced by the intended corrosive service. It has been shown that coated electrode welding of Ni-Mo alloys is difficult and is not recommended for Ni-Mo alloys.

Figure 7.7 shows the sensitivity to hot cracking for various alloys as measured in a modified varestraint test. As is obvious, the sensitivity to hot cracking of tungsten and columbium free alloys, such as alloy 59 and C-4, is low compared to the tungsten and columbium containing alloys in the Ni-Cr-Mo family such as alloys C-276, 22, G-3, and 625. Another often-raised question with these alloys is whether to remove the heat tint, produced during welding. On the higher alloys as those discussed in the Ni-Cr-Mo and Ni-Mo family, it may not be necessary to remove the heat tint if proper welding procedures have been followed. Extensive field case histories in FGD systems of many coal-fired power plants in United States have shown that removal of heat tint (discoloration) is not necessary for the high alloys like C-276 but is necessary for lower alloys like 904L and stainless steels. Lab tests in simulated FGD environments have also confirmed this conclusion [48]. If excessive heat input or inadequate shielding has resulted in an inferior weld and excessive heat tint, then other remedial measures need to be taken, which will depend upon the specific condition in question.

Information on these remedial measures and answers to specific questions on welding are also available from the producers of the various Ni-Cr-Mo, Ni-Mo, and superaustenitic stainless alloys.

One question has been frequently raised on the manufacturing process of tubes and or pipes of Ni-Cr-Mo, Ni-Mo alloys, and lower grades of Ni-Cr-Mo alloys. The question relates to the need for a "full draw of the as-welded tube" prior to final anneal or the adequacy of the "welded or welded and bead-worked annealed tubes." Discussions with many people in this field and tests run have shown that although this requirement of welded and fully drawn and annealed tubes holds true for standard 18-8 variety stainless steel tubes for critical chemical services, this is not so for superaustenitics or high alloys of the Ni-Cr-Mo family. A very prominent and respected engineer, Dillon [49] states, "in the CPI the requirement to fully draw a longitudinally welded tube prior to annealing has been applied only to the standard 18-8 stainless steels. There have been no reports of selective weld corrosion in either the super-austenitic grades or high Ni-Cr-Mo alloys like C-276. Likewise, no cold-work is needed. In fact many of the field welds in this Ni-Cr-Mo alloy C-276 are left in the as-welded condition without the benefit of any solution annealing, and these too in the industry have performed well. I know of no instances in which welds in tubing, pipe or vessels have been selectively attacked, except in the case of defective welds or if the environment has been to severe for alloy C-276 base metal itself. Hence the bead worked and annealed tubes of these alloys, as opposed to fully drawn and annealed tubes, are totally adequate for the CPI corrosive environments." Not only are these tubes adequate for the service but they are significantly cheaper than the fully drawn and annealed tubes.

0 0

Post a comment