Sensitization welding effect

Thermal imaging is sensitive to iafrared radiation that detects temperature changes over the surface of a part when heat has been appHed. Thermal diffusion ia a soHd is affected by variatioa ia composition or by the preseace of cracks, voids, delamiaatioas, etc the effects are detected by surface temperature changes. Defects cannot be detected if their depth below the surface is more than two to three times their diameter. Nondestmctive testing has been primarily used for composites and analysis of adhesive bonds or welds. Several studies are documented ia the Hterature (322—327). 

Austenitic alloys also make use of the concept of stabilization. Stainless types 321 and 347 are versions of type 304 stabilized with titanium and niobium, respectively. These elements will preferentially combine with carbon that comes out of solid solution during weld solidification. Rather than a loss of corrosion resistance associated with formation of harmful chromium carbides, the carbides of titanium and niobium are not detrimental to corrosion resistance. The austenitic family of stainless also prompted another approach to avoiding the effects of chromium carbide precipitation. Because the amount of chromium that precipitated was proportional to the carbon content, lowering the carbon could prevent sensitization. Maintaining the carbon content to below about 0.035% vs. 

Beggs, D.V., and R.W. Howe. 1993. Effects of Welding and Thermal Stabilization on the Sensitization and Polythionic Acid Stress Corrosion Cracking of Heat and Corrosion-Resistant Alloys. CORRO-SION/93, Paper No. 541. Houston, TX NACE International. 

Fig. 7.68 Effect of welding on sensitization as a function of distance from the weld fusion line of type 304 stainless steel as determined by the EPR test. Redrawn from Ref 104...

This work shows that absorption is very sensitive to the level of carbon black in the polymer formulation and provides some data to quantify this effect. This sensitivity occurs at very low levels of carbon black. Thus, when a polymer is selected for infrared welding, it will be important to know the concentration of carbon black in the formulation. If it falls below 0.07 percent, there will be increasing depth of heating and less surface heating. 

These steels can also be liable to intergranular corrosion. In addition to Cr carbides, Cr nitrides may be precipitated and thus contribute to the depletion of Cr at the grain boundaries. The mechanism is in principle the same as for austenitic steels, but in the ferritic steels the attacks can occur for a wider spectrum of environments and lower contents of C + N. The critical range of temperatures that makes the metal sensitive is higher, namely above 925°C, and the attack occurs therefore close to the weld or in the weld metal itself. The material becomes resistant again after annealing for 10-60 minutes at 650-815°C. It is noted that the effects of the different temperature ranges are opposite to what is the case for the austenitic steels. 

The materials behavior (austenitic, austeno-ferritic, and ferritic steels - base metal, welds, heat affected zones) in terms of thermal aging and its effect on mechanical properties, embrittlement, sensitiveness to corrosion (in normal and incidental environment), and radiation effect on the potentially exposed structures. 

Throughout the world, many arc welding methods are used to fabricate large-capacity structures from these steels. The weldability of both steels was evaluated using various methods of welding. Thin samples of both steels were satisfactorily welded. Nevertheless, weld metal of the same composition was susceptible to pore and hot crack formation and resulted in decreased ductility and low impact strength at low temperatures. Both steels were sensitive to the welding heat effect this was... 

Extensive testing on stainless steel mockups, fabricated using production techniques, has been conducted to determine the effect of various welding procedures on the susceptibility of unstabilized 300 series stainless steels to sensitization-induced intergranular corrosion. Only those procedures and/or practices demonstrated not to produce a sensitized structure are used in the fabrication of RCPB components. The ASTM standard A 708 (Strauss Test) is the criterion used to determine susceptibility to intergranular corrosion. This test has shown excellent correlation with a form of localized corrosion peculiar to sensitized stainless steels. As such, ASTM A 708 is utilized as a go/no-go standard for acceptability. 

The obvious effect of specimen geometry on strength indicates that data obtained from tests of notched specimens should be interpreted with caution. Considerably different ratios were obtained with the various notches and no one notch is wholly suitable for the evaluation of notch sensitivity under conditions other than those in this test. However, the data are sufficient to indicate that plain and welded aluminum-magnesium alloy plate is relatively insensitive to notches, at least to temperatures as low as -320 F. 

This leaves new environments for which there are no actual or even comparable data on their effects on sensitized alloys. Even when such a new environment is thought to be harmless, evaluation tests should, nevertheless, be applied to assure optimum metallurgical conditions. There have been a number of costly corrosion failures on material sensitized by welding when exposed... 

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