Chromium carbide precipitation

The austenitic stainless steels that are not stabilized or that are not of the extra-low-carbon types, when heated in the temperature range of 450 to 843°C (850 to 1,550°F), have chromium-rich compounds (chromium carbides) precipitated in the grain boundaries. This causes grain-boundary impoverishment of chromium and makes the affected metal susceptible to intergranular corrosion in many environments. Hot nitric acid is one environment which causes severe... 

Titanium added to prevent chromium carbide precipitation Cr,... 

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. 

In many cases, a small decrease in the corrosion resistance of weldments is tolerable. When the environment is particularly severe for the alloy being used, the weld may be attacked preferentially. This condition can be exaggerated by the area effects of the more noble base metal compared to the small weld zone. An alternate approach for field welding is fo selecf a higher alloy welding consumable so fhe weld deposif is more noble than the base metal. Preferential attack can also occur in the heat-affected zone of the base metal. This is typical of weldments made in standard type 304 where the carbon content will lead to chromium carbide precipitation. Of course, this condition cannot be avoided by using a different filler mefal and the only remedy is a postweld anneal. 

Austenitic steels are susceptible to grain boundary chromium carbide precipitation sensitization when heated in 540-820 °C range. Whenever sensitization is to be avoided, refineries use the stabilized grades of Type 347 (with Nb) or Type 321 (with Ti). 

The corrosion resistance is improved if compared with ferritic steels but the steels are sensitive to inter-crystalline corrosion as a result of chromium carbide precipitation at grain boundaries. Low carbon content or additional alloying metals like titanium, niobium, and tantalum, can suppress this sensitivity. 

Solution heat treatment is not performed on completed or partially-fabricated components. Rather, the extent of chromium carbide precipitation is controlled during all stages of fabrication as described below. 

Remove the Nitric Acid Test from ASTM A 262 and establish it as a separate ASTM Test At present, the boiling 65 % nitric acid test (Practice C) is specified for materials to be used in nitric acid service. Only this test is sensitive to sigma-phase in molybdenum-bearing austenitic stainless steels. Also, problems such as end-grain corrosion associated with hexavalent chromium derived from corrosion products are unique to this solution. While this test also detects susceptibility to intergranular attack associated with chromium carbide precipitates, there are other tests that perform this function in less time and with greater simplicity. 

As shown in Fig. 6, the weld decay zone which contains chromium carbide precipitate is not adjacent to the cast metal, but at some distance from it, in austenitic stainless steels. The reason for this is that the temperature of the metal in the zone adjacent to the molten zones has been... 

The most common microstructural effect on the corrosion resistance of nickel alloys is intergranular sensitization, as previously mentioned. This is the result of chromium carbide precipitation in many Ni-Fe-Cr alloys but can result from intermetallic Mu-phase precipitation in low-carbon highly alloyed materials such as alloy C-276 (UNS N10276). Several standard IGA tests (discussed in the Intergranular Corrosion section) are available for determining (1) if stabilized alloys have been properly annealed to prevent subsequent sensitization, and (2) if nonstabilized alloys are free from significant sensitization as produced. 

Chromium carbides in themselves do not suffer from poor corrosion resistance. The detrimental effect is in the fact that chromium is depleted from the surrounding matrix. In fact, the chromium depletion can be so severe as to lower the chromium content locally to below the 11% content considered to be the minimum for stainless steel. In actuality, any depletion can be significant if the environment is severe enough to cause the depleted zone to become anodic to the matrix. In high-temperature service, even where the component is used at a temperature that will cause chromium carbide precipitation, grain boundary chromium depletion is usually not a concern. Due to diffusion of chromium from within the grain toward the grain boundary, chromium depletion at elevated temperatures is short-lived. 

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. The austenitic family of stainless also prompted another approach to avoiding the effects of chromium carbide precipitation. 

Chromium carbide precipitate at the grain boundary. The precipitation of carbide is a time-temperature dependent phenomenon. The carbide precipitated is mainly Cr23C6, so that a single atom ties up almost four chromium atoms. 

Chromium carbide precipitation in nickel base alloys, such as alloy 600. 

Some of the atomic hydrogen reacts with chromium carbide precipitated at the grain boundaries. 

Sensitization, or carbide precipitation at grain boundaries, can occur when austenitic stainless steels are heated for a period of time in the range of about 425 to 870 °C (800 to 1600 °F). Time at temperature will determine the amount of carbide precipitation. When the chromium carbides precipitate in grain boundaries, the area immediately adjacent is depleted of chromium. 

---