310S Other Forms of Degradation

Species other than oxygen present in the high temperature environment can cause accelerated degradation of stainless steel. The presence of sulfur can lead to sulfidation attack. Sulfidation of the stainless steel is a complex process and depends strongly on the relative levels of sulfur and oxygen, along with the form of sulfur present (e.g., elemental vapor, sulfur oxides, hydrogen sulfide).

Chromium forms stable oxides and sulfides. In the presence of both oxygen and sulfur compounds, a stable external chromium oxide layer often forms which can act as a barrier to sulfur ingress. However, sulfidation attack can still occur at regions where the scale has become damaged or detached, and under certain circumstances sulfur can transport across a chromia scale and form internal chromium sulfide phases. Sulfidation is enhanced in alloy containing a significant (about 25% or more) amount of nickel. Nickel and nickel sulfide form a low melting point eutectic phase which can cause catastrophic damage to the underlying alloy at elevated temperatures.

High levels of carbon-bearing species in the environment can result in undesired carbon ingress and the subsequent formation of internal carbide. Carburization generally takes place at temperature above 1470°F (800°C) and at a carbon activity less than unity. The formation of a zone of internally carburized metal can cause undesired changes in mechanical properties and physical properties. Generally the presence of oxygen will prevent carbon ingress by formation of a protective external scale.

Higher levels of nickel and silicon are somewhat effective in reducing the susceptibility of carburization. Metal dusting is a specific form of carburization attack which generally occurs at lower temperature (660-1650°F or 350-900°C) and at a carbon activity greater than unity. It can result in catastrophic local attack via the formation of deep craters through a complex mechanism which converts solid metal to a mixture of graphite and metal particles.

Nitridation can occur in the presence of nitrogen gas. Oxides are generally more stable than nitrides so in an atmosphere which contains oxygen, an oxide scale typically forms. Oxide layers are good barriers to nitrogen ingress so nitridation is rarely a concern in air or in gases typical of combustion products. Nitridation can be a problem in purified nitrogen and is of special concern in dried, cracked ammonia atmospheres where the oxygen potential is very low. At relatively low temperatures a surface nitride film will generally form. At high temperatures (above about 1832°F or 1000°C) the diffusivity of nitrogen is fast enough that nitrogen penetrates deep into the metal and causes the formation of internal nitrides on grain boundaries and within grains. This can lead to compromised mechanical properties.

Metallurgical instability, or the formation of new phases during high temperature exposures, can adversely affect mechanical properties and reduce corrosion resistance. Carbide particles tend to precipitate at grain boundaries (sensitization) when austenitic stainless steels are held in or slowly cooled through the temperature range 800-1650°F (427-899°C). The higher levels of chromium and nickel contained in these alloys results in lower carbon solubility, which tends to increase the susceptibility for sensitization. Forced quenchant (gas or liquid) cooling is recommended through the critical temperature range, particularly for thicker sections. The time at temperature required to form chromium carbides increases with decreasing carbon content. Therefore, the low carbon versions of these alloys are more resistant but not immune to sensitization. When heated at temperatures between 1200-1850°F (649-1010°C) for long periods of time, Alloys 309/309S and 310/310S can exhibit decreased ductility at room temperature due to the precipitation of brittle second phase particles (sigma phase and carbides). Sigma phase often forms at grain boundaries and can reduce ductility. This effect is reversible, and full ductility can be restored by reannealing at the suggested temperatures.

Elevated temperature degradation is greatly affected by the atmosphere present and other operating conditions. General oxidation data can often be used only in estimating the relative oxidation resistance of different alloys. Sandmeyer Steel Company can supply data and prior experience pertaining to specific applications on request.

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