Austenitic Stainless Steel

  The superior ductility and corrosion resistance of austenitic stainless steels are attributed to their face-centered cubic (FCC) crystal structure. These steels are mostly alloyed with nickel and chromium, which stabilize the austenitic phase at room temperature. They are also very weldable, which makes them appropriate for a variety of applications requiring sturdy joints. Austenitic stainless steels of kinds 304, 316, 321, and 347 are frequently utilized. These steels are very well-liked for applications involving welding. The most used type, 304, has good formability and resistance to corrosion. While kinds 321 and 347 are made to withstand high temperatures and are resistant to carburization and oxidation, type 316 offers excellent corrosion resistance, especially in marine situations. Phenomena like Knife Line Attack (KLA) and Intergranular Attack (IGA) can happen while welding these steels, and it’s crucial to take them into account to preserve material integrity. [1]
  Austenitic stainless steel is stainless due to its high chromium content, which forms a passive oxide layer on the surface that resists corrosion. Additionally, the presence of nickel stabilizes the austenitic structure, enhancing its resistance to oxidation and rust. Resistance to corrosion is the result of adding enough Cr to Fe as a substitutional solute, with greater resistance generally afforded as the concentration of Cr increases above around 14 wt%. Chromium preferentially reacts with oxygen from the atmosphere compared to iron, forming a translucent layer of Cr2O3 just a few molecules thick, but with tremendous adhesion to the underlying iron and imperviousness to oxygen atoms from the atmosphere diffusing in and to iron atoms from the substrate diffusing out. As such, it provides a passivating layer on the surface of the stainless steel alloy that renders the underlying iron unreactive, thereby protecting it from corrosive attack. As it forms naturally, this passivating layer is self-healing if scratched. All stainless steels contain a relatively low concentration of C (typically, less than 0.08 wt%) to provide interstitial strengthening without extending the so-called gamma-loop/γ-loop on the Cr–Fe equilibrium phase diagram.[2]

Intergranular Attack

  One kind of corrosion that happens along a metal’s grain boundaries is called intergranular attack (IGA). This is caused by the depletion of alloying elements, like chromium in stainless steel, near the grain boundaries, which increases the susceptibility of these regions to corrosion. This might happen as a result of poor welding or heat treatment, which forms chromium carbides at the grain boundaries and depletes the chromium in the surrounding areas, decreasing their resistance to corrosion.[3]

Knife-Line Attack

  Typically, the weld metal of a single-pass weld does not get sensitized, because the material exceeds the temperature for sensitization, and then cools through the sensitization temperature range too rapidly for chromium carbides to form. Sensitization rarely occurs in small single-pass welds or heat-affected zones on thin stainless steel, because these materials do not remain in the sensitization temperature range long enough.[7]

Intergranular Corrosion of Austenitic Stainless Steel

  Austenitic stainless steels containing more than about 0.06–0.065 wt% C are often rendered susceptible to intergranular corrosion in the HAZ of any welds.
The heat affected zone (HAZ): The portion of metal that has not melted but has changed in composition due to being heated to relatively high temperatures during welding is known as the heat affected zone, or HAZ. The unaffected base metal and the weld are separated by the HAZ.
  The general phenomenon that can be the result of any prolonged exposure in the range 500–800 /∼925–1475 is known as sensitization for the way the material is rendered sensitive to corrosive attack and as weld decay for the way the sensitized region heated above about 800 /1472 in the HAZ of a weld is eaten away by corrosion. In fact, sensitization of the microstructure leads to weld decay around the welds.

Figure 1 Intergranular attack in the HAZ of austenitic stainless steel [3]

  Fig. 1 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 parallels the fusion boundary, these are sometimes called “wagon tracks” because they are symmetrical and parallel on either side of the weld. In cross section, several 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 leads to sensitization occurs in the temperature range from about 600 to 850°C (1110 to 1560°F). above this temperature range, carbides go back into solution and this the region adjunct to the fusion boundary is relatively free of carbides (assuming that cooling rates are rapid enough to suppress carbide precipitation during cooling). [3]

  Intergranular corrosion is the pronounced localized attack that occurs in narrow regions at or immediately adjacent to grain boundaries of an alloy. Type 304 stainless steel (which contains 18% Cr and 8% Ni as well as small amounts of carbon) is subject to intergranular corrosion if the stainless steel is heated to the temperature range of 425–790◦C (and then cooled). The stainless steel is said to be sensitized and is susceptible to intergranular corrosion.

Figure 2 Schematic representation of sensitized stainless steel. Chromium-depleted zones adjacent to grain boundaries are susceptible to intergranular corrosion

  During sensitization, carbon diffuses to the grain boundaries where it combines with chromium to form chromium carbide precipitates (such as Cr23C6). This process depletes chromium from the areas in and adjacent to the grain boundaries so that these regions locally contain less than the 12% Cr required for a stainless steel. Thus, localized corrosion occurs in certain aqueous environments in the form of intergranular corrosion, as depicted in Fig. 2. [4]

Figure 3 Ulcerative and intergranular corrosion in the metal of the welded joint of chromium-nickel steel.[5]

  When Cr-rich carbides form as Cr and C in a solid solution in the austenite diffuse to grain boundaries, Cr content is depleted or “denuded” in regions surrounding the newly formed grain boundary carbides . Once the Cr content in the solution drops below around 12 wt%, the corrosion resistance of the material is drastically reduced. In fact, the Cr-depleted region forms a galvanic couple with the nondepleted, higher Cr region, becoming relatively anodic and susceptible to corrosive attack.[2]
Sensitization in austenitic stainless steels is more severe under the following conditions:
⦁ the higher the carbon content above about 0.06 wt%
⦁ the greater the net linear heat input to a weld
⦁ with any residual cold work done before welding
⦁ in the absence of any alloying additions (i.e. substitutional solutes) with greater affinity for C than Cr.

Preventing Intergranular Attack of Austenitic Stainless Steel
Preventing sensitivity 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
⦁ Use base metals that are “stabilized” by additions of niobium (Nb) and titanium
⦁ Use annealed base material or anneal prior to welding to remove any prior cold work effects.These elements are more potent carbide formers than chromium and this tie up the carbon, minimizing the formation of Cr-rich grain boundary carbides. Cold work accelerates carbide precipitation.
⦁ Use low weld heat inputs and low interpass temperatures to increase weld cooling rates, therefore 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 sensitivity of the id resulting from subquent passes.
⦁ Solution heat treatment after welding. Heating the structure into the temperature range 900 to 1100°C (1650 to 2010°F) dissolves any carbides that may have formed along grain boundaries in the HAZ. 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 later approach. Distortion during quenching is a serious problem for plate structures. The inability to quench complex pipe weldings is also a limiting factor.[3]

Knife-Line Attack of Austenitic Stainless Steel
Knifeline attack intergranular attack can also occur in certain situations in the stabilized grades, such as types 347 and 321. This attack, normally occurs in a very narrow region just adjacent to the fusion boundary. It is sometimes called knifeline attack because the weld appears as if it were crying out with a knife. This type of attack occurs when the stabilized carbides (NbC or TiC) dissolve at elevated temperatures in the region just adjacent to the fusion zone. upon cooling, Cr-rich carbides will form faster than the NbC or TiC, resulting in a narrow sensitized region. further from the fusion boundary, NbC and TiC do not dissolve and sensitization does not occur. [3]