Wow, today is very heat, the sun is like burning this place, me who working inside the container at the factory is really felt so hot because of the sun shining too bright. Lets forget about this heat and Piping & Fabrication will continue the post with Intergranular Attack and Sigmatization, and still on Piping Materials.
Intergranular Attack
When an unstabilized austenitic stainless steel is held at a temperature within the range of 850 to 1500°F (454 to 816°C), chromium carbides will quickly and preferentially form at the austenitic grain boundaries. The formation of these carbides deletes the surrounding grain matrix of chromium atoms, rendering the thin zone adjacent to grain boundary susceptible to corrosive attack in aqueous environments. This condition is called sensitization, and the resulting corrosion is termed intergranular attack (IGA).When also in the presence of local high-tension stresses, the result can be intergranular stress corrosion cracking (IGSCC). Avoidance of these failure mechanisms is best achieved by minimizing sensitization (fast cool from anneal; stabilized or L-grade steels), and eliminating local stresses.
The area of piping components most often attacked is weld regions. Sensitization can readily occur in a narrow band of base material in the heat-affected zone, caused by the heat of the weld pool. Corrosion of this area has been called knife line attack due to the characteristic appearance of a thin crack along a weld edge.
Sigmatization
A hard, brittle, nonmagnetic phase will form in some Fe-Cr and Fe-Ni-Cr alloys upon prolonged exposure to temperatures between about 1100 and 1475°F (593 and 800°C). Those austentic stainless steels containing higher alloy content, such as type 310 (25% Cr–20% Ni) are susceptible, as well as any grades that possess residual ferrite in their microstructure, a constituent which will transform to sigma,
preferentially at grain boundaries.
The most deterimental effect of sigma is reduction of toughness. Charpy Vnotch impact toughness can degrade to less than 10 ft • lb (14 joules) at room temperature if as much as 10 percent of the volume of material transforms. Toughness is usually not significantly degraded at higher temperatures, above about 1000°F (538°C).
Chemically, sigma is not as resistant to oxidizing media as the austenite, suchas acidic environments, thus, the materials will undergo intergranular attack. At normal metal operating temperatures in power plants, sigmatization of pressure piping made of these high-alloy materials takes very long times to form. Once formed, the phase can be redissolved by subjecting the material to an annealing heat treatment.
When an unstabilized austenitic stainless steel is held at a temperature within the range of 850 to 1500°F (454 to 816°C), chromium carbides will quickly and preferentially form at the austenitic grain boundaries. The formation of these carbides deletes the surrounding grain matrix of chromium atoms, rendering the thin zone adjacent to grain boundary susceptible to corrosive attack in aqueous environments. This condition is called sensitization, and the resulting corrosion is termed intergranular attack (IGA).When also in the presence of local high-tension stresses, the result can be intergranular stress corrosion cracking (IGSCC). Avoidance of these failure mechanisms is best achieved by minimizing sensitization (fast cool from anneal; stabilized or L-grade steels), and eliminating local stresses.
The area of piping components most often attacked is weld regions. Sensitization can readily occur in a narrow band of base material in the heat-affected zone, caused by the heat of the weld pool. Corrosion of this area has been called knife line attack due to the characteristic appearance of a thin crack along a weld edge.
Sigmatization
A hard, brittle, nonmagnetic phase will form in some Fe-Cr and Fe-Ni-Cr alloys upon prolonged exposure to temperatures between about 1100 and 1475°F (593 and 800°C). Those austentic stainless steels containing higher alloy content, such as type 310 (25% Cr–20% Ni) are susceptible, as well as any grades that possess residual ferrite in their microstructure, a constituent which will transform to sigma,
preferentially at grain boundaries.
The most deterimental effect of sigma is reduction of toughness. Charpy Vnotch impact toughness can degrade to less than 10 ft • lb (14 joules) at room temperature if as much as 10 percent of the volume of material transforms. Toughness is usually not significantly degraded at higher temperatures, above about 1000°F (538°C).
Chemically, sigma is not as resistant to oxidizing media as the austenite, suchas acidic environments, thus, the materials will undergo intergranular attack. At normal metal operating temperatures in power plants, sigmatization of pressure piping made of these high-alloy materials takes very long times to form. Once formed, the phase can be redissolved by subjecting the material to an annealing heat treatment.
No comments:
Post a Comment