ASTM A213 TP321 Stainless Steel Seamless Tube For Heat Exchange Application

ASTM A213 TP321  Stainless Steel Seamless Tube for Heat Exchange Application

Stainless Steel 321 is a basic austenitic 18/8 steel (Grade 304) stabilised by Titanium (321) addition. SS 321 is used because they are not sensitive to intergranular corrosion after heating within the carbide precipitation range of 425-850°C. SS 321 is the grade of choice for applications in the temperature range of up to about 900°C, combining high strength, resistance to scaling and phase stability with resistance to subsequent aqueous corrosion. SS 321H is a modification of SS 321 with a higher carbon content, to provide improved high temperature strength.

Specifications

Mechanical & Physical Properties

Chemical Composition

Alloys 321 (S32100) and 347 (S34700) are stabilized stainless steels which offer as their main advantage an excellent resistance to intergranular corrosion following exposure to temperatures in the chromium carbide precipitation range from 800 to 1500⁰F (427 to 816⁰C). Alloy 321 is stabilized against chromium carbide formation by the addition of titanium. Alloy 347 is stabilized by the addition of columbium and tantalum.

While Alloys 321 and 347 continue to be employed for prolonged service in the 800 to 1500⁰F (427 to 816⁰C) temperature range, Alloy 304L has supplanted these stabilized grades for applications involving only welding or short time heating.

Alloys 321 and 347 stainless steels are also advantageous for high temperature service because of their good mechanical properties. Alloys 321 and 347 stainless steels offer higher creep and stress rupture properties than Alloy 304 and, particularly, Alloy 304L, which might also be considered for exposures where sensitization and intergranular corrosion are concerns. This results in higher elevated temperature allowable stresses for these stabilized alloys for ASME Boiler and Pressure Vessel Code applications. The 321 and 347 alloys have maximum use temperatures of 1500⁰F (816⁰C) for code applications like Alloy 304, whereas Alloy 304L is limited to 800⁰F (426⁰C).

High carbon versions of both alloys are available. These grades have UNS designations S32109 and S34709.
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Resistance to Corrosion of 321 Stainless Steel Pipes and Tubes

General Corrosion
Alloys 321 and 347 offer similar resistance to general, overall corrosion as the unstabilized chromium nickels Alloy 304. Heating for long periods of time in the chromium carbide precipitation range may affect the general resistance of Alloys 321 and 347 in severe corrosive media.

In most environments, both alloys will show similar corrosion resistance; however, Alloy 321 in the annealed condition is somewhat less resistant to general corrosion in strongly oxidizing environments than annealed Alloy 347. For this reason, Alloy 347 is preferable for aqueous and other low temperature environments. Exposure in the 800⁰F to 1500⁰F (427⁰C to 816⁰C) temperature range lowers the overall corrosion resistance of Alloy 321 to a much greater extent than Alloy 347. Alloy 347 is used primarily in high temperature applications where high resistance to sensitization is essential, thereby preventing intergranular corrosion at lower temperatures.

Physical Properties of 321 Stainless Steel Pipes and Tubes

The physical properties of Types 321 and 347 are quite similar and, for all practical purposes, may be considered to be the same. The values given in the table may be used to apply to both steels.
When properly annealed, the Alloys 321 and 347 stainless steels consist principally of austenite and carbides of titanium or columbium. Small amounts of ferrite may or may not be present in the microstructure. Small amounts of sigma phase may form during long time exposure in the 1000⁰F to 1500⁰F (593⁰C to 816⁰C) temperature range.
The stabilized Alloys 321 and 347 stainless steels are not hardenable by heat treatment.
The overall heat transfer coefficient of metals is determined by factors in addition to thermal conductivity of the metal. In most cases, film coefficients, scaling, and surface conditions are such that not more than 10 to 15% more surface area is required for stainless steels than for other metals having higher thermal conductivity. The ability of stainless steels to maintain clean surfaces often allows better heat transfer than other metals having higher thermal conductivity.