Austenitic Stainless Steel.docx

Austenitic Stainless Steel Austenitic stainless steel represent the largest of the general group of stainless steel and are produced in higher tonnages than any group. They have good corrosion ressistance in most environmentns. The austenitic stainless steel have sternghts equivalent to those of mild steels, approximately 210 Mpa (30Ksi) minimum yield stergth at room temperature, and are not ttansformation hardenable. Low-temperature impact properties are good for these alloys, making them useful in cryogenic applications. Service temperatures can be up to 760 C (1400F) or even higher, but tehe strenght and oxidation resistance of most of these steels are limited at such high temperatures. Austenitic satinless steel can be strengthened significantly by cold working. They often used in applications requiring god atmospheric or elevated temperature corrosion resistance. They are generally considered te be weldable, if proper precautions are followed. Element that promute teh formation of austenite, most notably nickel, are added to these steels in large quanntities (generally over 8 wt%). Other austenirtic-promoting elements are C, N and Cu. Carbon and nitrogen are strong austenite promoters, as can be seen from the various values in nickel equivalency formulas (see Chapter 3). Carbon is added to improve strength (creep resistance) at high temperatures. Nitrogen is added to some alloys to improve strength, mainly at ambient and cryogenic temperatures, sometimes more than doubling it. Nitrogen-strengthened alloys are usually designated with a sufix N added to their AISI 300 series designation . Austenitic stainless steels generally have good ductility and toughness and exhibit significant elongation during tensile loading. They are more expensive than the martensitic and low medium Cr ferritic grades, due to the higher alloy content of these isulaalloys. Despite the steel cost, they offer distinct engineering advantages, particularly with respect to formability and weldablity, that often reduce the overall cost compared to other groups of stainless steels. Although there are a wide variety of austenitic stainless steels, the 300 series alloys are the oldest and most commonly used. Most of these alloys are based on the 18Cr – 8Ni system, with additional alloying elements or modifications to provide unique or enhanced properties. Type 304 is the foundation of this alloy series, and along with 304L, represent the most commonly selscted austenitic grade. Type 316 substitutes approximately 2%Mo for a nearly equal amount og Cr to improve pitting corrosion resistance. The stabilized grades, 321 and 34, contain small additions of Ti and Nb, respectively, to combine with carbon and reduce the tendency for intergranular corrosion due to Crcarbide precipitation. The L grades became popular in the 1960s and 1970s with the advent of AOD (argon-oxygen decaburization) melting practice that reduced the cost differential between standard (not low carbon) and L grades. These low-carbon grades (304L,316L) have been widely used in aapplications where intergranular stress corrosion

cracking are a concern. These forms of corrosion attack are discussed later in the chapter. Austenitic stainless steel are used in awide range of applications, including structural support and containment, architectural uses, kitchen equipment, and medical products. They are widely used not only because of their corrosion resistance but because they are readily formable, fabricable, and durable. Some highly alloyed grade are used for very high temperature servis ( above 1000 c ) for aplications such as heat-treating baskets. In addition to higher chromium levels, these alloys normally contain higher levels of silicon (and sometimes aluminium) and carbon, to maintain oxidation and /or carburization resistance and strength, respectively. Chloride-containing media, or in highly caustic environments. This is due to their susceptibility to strees corrosion cracking. A phenomenon that afflicts the base metal, HAZ, and weld metal in these alloys. Care should be taken when selecting stainless steel that will be under signifivicant strees in these environments. The aspect and consequences of stress corrosin cracking will be described later.

6.6.1

Intergranular Corrosion

Represents the appearance of a weld that has undergone intergranular attack in the HAZ. On the surface of the weld exposed to the corrosive environment, there often appears a linear area of attack that parallel on either side of the weld. In cross section, severe attack (or weld “decay”) can be observed along a sensitized band in the HAZ. Note that this band is at some distance from the fusion boundary. This is due to the fact that the carbide precipitation that lead to sensitization occurs in the temperature range from about 600 to 850 C (1110 to 1560 F). Above this temperature range, carbide go back into solution and this the region adjacent to the fusion boundary is relatively free

of carbided (assuming that cooling rates are rapid enough to suppress carbide precipitation during cooling). In the HAZ of most austenitic stainless steels, CR-rich M23C6 carbides from prefentially along grain boundaries, as shown in figure 6.48. this result in a chromiumdepleted zone along the grain boundary that is “sensitive” to corrosive attack. Hence, the term sensitization is often used to describe the metallurgical condition leading to intergranular attack. The exception to this the stabilized grades of stainless steel containing Nb and/or Ti (such as types 347 and 321). In these steels the Nb and Ti ties up carbon in the form of stable MC-type carbides and minimizes the formation of M23C6 carbides at the grain boundaries. Intergranular corrosion result from the localized precipitation requires shortrange diffusion of Cr from the adjacent matrix and produces a Cr-depleted region surrounding the precipitate, as shown in figure 6.49 this reduces the loca; corrosion resistance of the microstructure and promotes rapid attack of the grain bundary region. In certain corrosive environments the effect is a local “ditching” at the grain boundary, as shown in the metallographic section in figure 6.49. In extreme cases, the grains will actually drop out of the structure because of complete grain boundary attack and dissolution. Diffusion of Cr from the adjacent matrix and produces a Cr-depleted region surrounding the precipitate, as shown in figure 6.49. This reduces the local corrosion resistance of the microstructure and promotes rapid attack of the grain boundary region. In certain corrosive environment the effect is a local “ditching” at the grain boundary, as shown in the metalographic section in figure 6.49. In extreme cases, the grains will actually drop out of the structure because of complete grain boundary attack and dissolution. Carbon content has the most profound influence on susceptibility to IGC in austenitic stainles steels. The use of low-carbo (L grade) alloys minimizes the risk of sensitization by slowing down the carbide precipitation reaction. The time-temperatureprecipitations curves shown in figure 6.50 demonstrate the effect of carbon content on the time to precipitation. Note that at low carbon content (C<0.04 wt%), the nose of the curve is beyonfd 1 hour, while for carbon levels from 0.06 to 0.08 wt% the time for precipitation may be less than a minute. This diffrence demonstrates the benefit of the low-carbon alloys (L grades) for reducing or eliminating HAZ grain boundary sensitization during welding. The presence of residual stresses in the HAZ may also serve to accelerate the precipitation reaction. In most cases, sensitization occurs in the HAZ as direct result of the weld thermal cycle. It should be noted, however, that the stress relief temperature range for most austenitic stainless steels overlaps the carbide precipitation range. Care must be taken not to sensitize the entire structure during PWHT. This is a particular concern with alloys containing more than 0.04 wt%C. In general, weld metals such as 308 and 316 are less likely to be sensitized than correponding 304 and 316 base metals. The ferritw that is normally found in the weld

metal is richer in Cr than the austenite, and Cr depletion. M23C6 carbides tend to precipitate at the tortous ferrite-austenite boundaries instead of usually much straighter austenite-austenite boundaries. All of these factors greatly limit the tendency for sensitization in austenitic stainless steel weld metals, sensitization in largely a HAZ problem, not a weld metal problem.

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Preventing sensitization it is possible to minimize or eliminate intergranular corrosion in austenitic stainless steel welds by the following methods. Select base and filler metals with as low a carbon content as possible (L grades such as 306L and 316L). Use base metals that are “stabilized” by additions of niobium (Nb) and titanium (Ti). These elements are more potent carbide formers than chromium and thus tie up the carbon, minimizing the formation of CR-rich grain boundary carbides. Use annealed base material or anneal prior to welding to remove any prior cold work (cold work accelerates carbide precipitation). Use low weld heat inputs and low interpass temperatures to increase weld cooling rates, thereby minimizing the time in the sensitization temperature range. In pipe welding, water cool the inside of the pipe after the root pass. This will help to eliminate sensitization of the ID resulting from subsequent passes. Solution heat treat after welding. Heating the structure is then quenched from this temperature to prevent carbide precipitation during cooling. Note, however, that there are a number of practical considerations that tend to limit the usefulness of the latter approach. Distortion during qunching is a serious problem for plate structures, Inability to quench complex pipe weldments is also a limiting factor.