Dear José:
The Type 321 alloy offers similar resistance to general, overall corrosion as the unstabilized chromium nickel Type 304. Heating for long periods of time in the chromium carbide precipitation range may affect the general resistance of Type 321 in corrosive media.
Type 321 has been developed for applications where the unstabilized chromium-nickel steels, such as Type 304 would be susceptible to intergranular corrosion.
When the unstabilized chromium-nickel steels are held in or slowly cooled through the range of 800 to 1500° F (427 to 816° C), chromium carbide is precipitated at the grain boundaries. In the presence of certain strongly corrosive media, these grain boundaries are preferentially attached, a general weakening of the metal results, and a complete disintegration may occur.
When thin gauge material is welded the time in the temperature range of 800 to 1500° F (427 to 816° C) is so short that with most corroding media the unstabilized type material is generally satisfactory. The extent to which carbide precipitation may be harmful depends upon the length of time the alloy was exposed to 800 to 1500° F (427 to 816° C) and upon the corrosive environment. Even the longer heating times involved in welding heavy gauges are not harmful to the unstabilized "L" grade alloys where the carbon content is kept to low amounts of 0.03% or less.
In general, Type 321 is used for heavy welded equipment which is operated between 800 to 1500° F (427 to 816° C) or slowly cooled through this range. Experience gained in a wide range of service conditions has provided sufficient data to generally predict the possibility of intergranular attach in most applications. Please review the comments under the HEAT TREATMENT section.
Type 321 is particularly useful under conditions which cause polythionic acid stress corrosion of non-stabilized austenitic stainless steels such as Type 304. Exposure of non-stabilized austenitic stainless steel to temperatures in the sensitizing range will cause the precipitation of chromium carbides at grain boundaries. On cooling to room temperature in a sulfide-containing environment, the sulfide (often hydrogen sulfide) reacts with moisture and oxygen to form polythionic acids which attach the sensitized grain boundaries. Under conditions of stress, intergranular cracks form. Polythionic acid SCC has occurred n oil refinery environments where sulfides are common. The stabilized Type 321 alloy offers a solution to polythionic acids SCC by resisting sensitization during elevated temperature service. For optimum resistance, these alloys should be used in the thermally stabilized condition if service related conditions may result in sensitization.
The annealing temperature range for Type 321 is 1800 to 2000° F (928 to 1093° C). While the primary purpose of annealing is to obtain softness and high ductility, this steel may also be stress relief annealed within the carbide precipitation range 800 to 1500° F (427 to 816° C), without any danger of subsequent intergranular corrosion. Relieving strains be annealing for only a few hours in the 800 to 1500°F (427 to 816° C) range will not cause any noticeable lowering in the general corrosion resistance, although prolonged heating within this range does tend to lower the general corrosion resistance to some extent. As emphasized, however, annealing in the 800 to 1500° F (427 to 816° C) temperature range does not result in a susceptibility to intergranular attack.
For maximum ductility, the higher annealing range of 1800 to 2000° F (928 to 1093° C) is recommended.
When fabrication chromium-nickel stainless steel into equipment requiring the maximum protection against carbide precipitation obtainable through use of a stabilized grade, it is essential to recognize that there is a difference between the stabilizing ability of columbium and titanium. For these reasons the degree of stabilization and of resulting protection may be less pronounced when Type 321 is employed.
When maximum corrosion resistance is called for, it may be necessary with Type 321 to employ a corrective remedy which is known as a stabilizing anneal. It consists of heating to 1550 to 1650° F (843 to 899° C) for up to 5 hours depending on thickness. This range is above that within which chromium carbides are formed and is sufficiently high to cause dissociation and solution of any that may have been previously developed. Furthermore, it is the temperature at which titanium combines with carbon to form harmless titanium carbides. The result is that the chromium is restored to solid solution and carbon is forced into combination with titanium as harmless carbides.
After heat treatment, you may need to remove oxides with a mixture of nitric and hydrofluoric acid to restore corrosion resistance.
-- The Type 321 alloy offers similar resistance to general, overall corrosion as the unstabilized chromium nickel Type 304. Heating for long periods of time in the chromium carbide precipitation range may affect the general resistance of Type 321 in corrosive media.
Type 321 has been developed for applications where the unstabilized chromium-nickel steels, such as Type 304 would be susceptible to intergranular corrosion.
When the unstabilized chromium-nickel steels are held in or slowly cooled through the range of 800 to 1500° F (427 to 816° C), chromium carbide is precipitated at the grain boundaries. In the presence of certain strongly corrosive media, these grain boundaries are preferentially attached, a general weakening of the metal results, and a complete disintegration may occur.
When thin gauge material is welded the time in the temperature range of 800 to 1500° F (427 to 816° C) is so short that with most corroding media the unstabilized type material is generally satisfactory. The extent to which carbide precipitation may be harmful depends upon the length of time the alloy was exposed to 800 to 1500° F (427 to 816° C) and upon the corrosive environment. Even the longer heating times involved in welding heavy gauges are not harmful to the unstabilized "L" grade alloys where the carbon content is kept to low amounts of 0.03% or less.
In general, Type 321 is used for heavy welded equipment which is operated between 800 to 1500° F (427 to 816° C) or slowly cooled through this range. Experience gained in a wide range of service conditions has provided sufficient data to generally predict the possibility of intergranular attach in most applications. Please review the comments under the HEAT TREATMENT section.
Type 321 is particularly useful under conditions which cause polythionic acid stress corrosion of non-stabilized austenitic stainless steels such as Type 304. Exposure of non-stabilized austenitic stainless steel to temperatures in the sensitizing range will cause the precipitation of chromium carbides at grain boundaries. On cooling to room temperature in a sulfide-containing environment, the sulfide (often hydrogen sulfide) reacts with moisture and oxygen to form polythionic acids which attach the sensitized grain boundaries. Under conditions of stress, intergranular cracks form. Polythionic acid SCC has occurred n oil refinery environments where sulfides are common. The stabilized Type 321 alloy offers a solution to polythionic acids SCC by resisting sensitization during elevated temperature service. For optimum resistance, these alloys should be used in the thermally stabilized condition if service related conditions may result in sensitization.
The annealing temperature range for Type 321 is 1800 to 2000° F (928 to 1093° C). While the primary purpose of annealing is to obtain softness and high ductility, this steel may also be stress relief annealed within the carbide precipitation range 800 to 1500° F (427 to 816° C), without any danger of subsequent intergranular corrosion. Relieving strains be annealing for only a few hours in the 800 to 1500°F (427 to 816° C) range will not cause any noticeable lowering in the general corrosion resistance, although prolonged heating within this range does tend to lower the general corrosion resistance to some extent. As emphasized, however, annealing in the 800 to 1500° F (427 to 816° C) temperature range does not result in a susceptibility to intergranular attack.
For maximum ductility, the higher annealing range of 1800 to 2000° F (928 to 1093° C) is recommended.
When fabrication chromium-nickel stainless steel into equipment requiring the maximum protection against carbide precipitation obtainable through use of a stabilized grade, it is essential to recognize that there is a difference between the stabilizing ability of columbium and titanium. For these reasons the degree of stabilization and of resulting protection may be less pronounced when Type 321 is employed.
When maximum corrosion resistance is called for, it may be necessary with Type 321 to employ a corrective remedy which is known as a stabilizing anneal. It consists of heating to 1550 to 1650° F (843 to 899° C) for up to 5 hours depending on thickness. This range is above that within which chromium carbides are formed and is sufficiently high to cause dissociation and solution of any that may have been previously developed. Furthermore, it is the temperature at which titanium combines with carbon to form harmless titanium carbides. The result is that the chromium is restored to solid solution and carbon is forced into combination with titanium as harmless carbides.
After heat treatment, you may need to remove oxides with a mixture of nitric and hydrofluoric acid to restore corrosion resistance.
On Thu, Jul 18, 2013 at 4:28 PM, José Luís Ferreira <jluis.nf@gmail.com> wrote:
Dear all,
The specification of my client requires to be carried stabilization heat treatment in WPS qualification for material TP 321, but does not specify parameters for this treatment, says it should be agreed in advance. Usually the parameters which are adopted?
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