Product introduction
US321 stainless steel (UNS S32100) is a very stable stainless steel.
Chinese name: SUS321 stainless steel
Foreign name: 321 stainless steel
Introduction: a kind of stainless steel with excellent stability
Chemical composition: ASTM A240 and ASME SA-240
Uniform corrosion: alloy 321 and 347
Under the conditions of 800-1500 ° F (427-816 ° C) temperature and chromium carbide precipitation, it can still maintain good resistance to intergranular corrosion. Due to the addition of titanium in the composition, 321 alloy can still remain stable when chromium carbide is formed.
321 alloy stainless steel has advantages in high temperature environment due to its excellent mechanical properties. Compared with 304 alloy, 321 alloy stainless steel has better ductility and stress fracture resistance. In addition, 304L can also be used for anti sensitization and intergranular corrosion.

General attributes
Alloy 321 (UNS S32100) is a very stable stainless steel. Under the conditions of 800-1500 ° F (427-816 ° C) temperature and chromium carbide precipitation, it can still maintain good resistance to intergranular corrosion. Due to the addition of titanium in the composition, 321 alloy can still remain stable when chromium carbide is formed. The stability of 347 alloy is maintained by adding Ke and Ta.
321 and 347 alloys are commonly used for long-term operations at 800-1500 ° F (427-816 ° C) at high temperatures. If the application only involves welding or short time heating, 304L can be used instead.
The advantages of 321 and 347 alloys in high-temperature operation also depend on their good mechanical properties. Compared with 304304L, 321 and 347 have creep stress resistance and stress rupture resistance. This makes the pressure of these stable alloys still meet the specification at a higher temperature. Therefore, the high service temperature of 321 and 347 alloy can reach 1500 ° F (816 ° C), while 304304L is limited to 800 ° F (426 ° C)
321 and alloy 347 also have varieties with high carbon content. Their UNS numbers are S32109 and S34709 respectively.

Chemical composition
ASTM A240 and ASME SA-240
  碳C* 錳Mn 磷P 硫S 硅Si 鉻Cr 鎳Ni 鈳Co+鉭Ta** 鉭Ta 鈦Ti** 鈷Co 氮Ni 鐵Fe
321 0.08 2.00 0.045 0.030 0.75 17.00-19.00 9.00-12.00 -- -- 5×(C+N)小0.70大 -- 0.10 Rest
347 0.08 2.00 0.045 0.030 0.75 17.00-19.00 9.00-12.00 ten × C small 1.00 large -- -- -- -- Rest
*Grade H carbon content is 0.04? 0.10%
**Stabilizers of low grade are of different formulations

Corrosion resistance
Uniform corrosion
Alloys 321 and 347 have the same resistance to general corrosion as the unstable nickel chromium alloy 304. Long time heating in the temperature range of chromium carbide degree may affect the corrosion resistance of alloys 321 and 347 in harsh corrosive medium.
In most environments, the corrosion resistance of the two alloys is similar; However, the corrosion resistance of annealed alloy 321 in strong oxidizing environment is slightly inferior to that of annealed alloy 347. Therefore, alloy 347 is better in water environment and other low temperature environments. When exposed to the temperature range of 800 ° F - 1500 ° F (427 ° C - 816 ° C), the overall corrosion resistance of alloy 321 is much worse than that of alloy 347. Alloy 347 is mainly used for high temperature applications. High temperature applications require strong anti sensitization of materials to prevent intergranular corrosion at lower temperatures.
Intergranular corrosion
Unstable nickel such as alloy 304? Steel is sensitive to intergranular corrosion, and alloy 321 and alloy 347 are developed and applied in this field.
When unstable chromium nickel steel is placed in an environment with a temperature of 800 ° F - 1500 ° F (427 ° C - 816 ° C) or is slowly cooled within this temperature range, chromium carbide precipitates at the grain boundary. When placed in some highly corrosive media, these grain boundaries are eroded first, which may weaken the effectiveness of the metal and may completely collapse.
In organic medium or corrosive water, milk or other dairy products or under atmospheric conditions, even if there is a large amount of carbide precipitation, intergranular corrosion will rarely occur. When welding thin plates, because the time to stay in the temperature range of 800 ° F - 1500 ° F (427 ° C - 816 ° C) is very short, intergranular corrosion is not easy to occur, so unstable grades can be qualified. The extent to which carbide precipitates is harmful depends on the length of time that the alloy is exposed to 800 ° F - 1500 ° F (427 ° C - 816 ° C) and the corrosive medium. Although the heating time for welding thicker plates is longer, due to the unstable level L, the carbon content is 0.03% or lower, and the carbide precipitation is not enough to harm this level.
The strong sensitization resistance and intergranular corrosion resistance of stable 321 and alloy 347 stainless steels are shown in the following table. (Copper copper sulfate - 16% sulfuric acid test (ASTM A262, Practice E)). Before the test, the samples annealed in the steel mill shall be subject to soaking photosensitive heat treatment at 1050 ° F (566 ° C) for 48 hours.
Stress corrosion cracking
Alloy 321 and 347 austenitic stainless steels are sensitive to stress corrosion cracking in halides, similar to alloy 304 stainless steels. This result is due to their similar nickel content. The conditions causing stress corrosion cracking are: (1) exposure to halide ions (usually chloride), (2) residual tensile stress, and (3) ambient temperature exceeding 120 ° F (49 ° C). Cold deformation in forming operations or thermal cycles encountered in welding operations may produce stresses. Stress relief heat treatment after annealing or cold deformation may reduce the stress level. The stable alloys 321 and 347 are suitable for the operation environment that eliminates stress and may cause intergranular corrosion to unstable alloys.
321 and 347 are particularly useful in environments where polythionic acid stress corrosion occurs on unstable austenitic stainless steels, such as alloy 304. Unstable austenitic stainless steel will produce chromium carbide precipitation at grain boundary if exposed to temperature where sensitization will occur. When cooled to room temperature in a sulfur containing environment, sulfide (usually hydrogen sulfide) will react with water vapor and oxygen to form hydropolysulfuric acid that attacks the sensitized grain boundary. Under the condition of stress and intergranular corrosion, the stress corrosion cracking of polythionic acid occurs in the refining environment where sulfide is ubiquitous. Stable alloys 321 and 347 solve the problem of polythionic acid stress corrosion cracking due to their anti sensitization in the heating operation environment. If the operating environment conditions will cause sensitization, these alloys should be used under thermal stable conditions to achieve good anti sensitization.
Pitting corrosion
The pitting corrosion resistance and pitting corrosion resistance of stable alloys 321 and 347 in the environment containing chloride ions are similar to those of alloy 304 or 304L stainless steel, because their chromium content is similar. In general, for unstable and stable alloys, the upper limit of chloride content in the water environment is 100 ppm, especially when crevice corrosion exists. High chloride ion content will lead to crevice corrosion and pitting corrosion. In case of severe conditions such as high chloride content, low PH value and/or high temperature, molybdenum containing alloy, such as alloy 316, shall be considered. Stable alloys 321 and 347 passed the 100 hour 5% salt spray test (ASTM B117), and the tested samples did not produce rust and fade. However, if these alloys are exposed to salt fog from the sea, pitting corrosion, interstitial corrosion and severe discoloration may occur. Exposure of alloys 321 and 347 to the marine environment is not recommended.

High temperature oxidation resistance
The oxidation resistance of sus321 stainless steel is comparable to other 18-8 austenitic stainless steels. Expose the sample to high temperature laboratory atmosphere. Regularly take the sample out of the high temperature environment for weighing, and the degree of rust formation can be calculated. The test result is expressed by the weight change (mg/cm2), and the average of the small values of two different tested samples is taken.
The main difference between 321 and 347 is the subtle alloy additive, but it does not affect the oxidation resistance. Therefore, these test results are representative for both levels. However, the oxidation rate is affected by inherent factors such as the exposure environment and product form, so these results should only be considered as the usual values of these levels of oxidation resistance.

Physical property
The physical properties of alloy 321 and 347 are quite similar, in fact, they can be regarded as the same. The values listed in the table are applicable to both alloys.
After proper annealing treatment, alloy 321 and 347 stainless steels mainly contain austenite and titanium carbide or niobium carbide. A small amount of ferrite may or may not occur in the microstructure. If exposed to temperatures between 1000 ° F and 1500 ° F (593 ° C and 816 ° C) for a long time, a small amount of sigma phase may be formed.
Heat treatment does not harden stable alloy 321 and 347 stainless steels.
The total heat transfer coefficient of a metal depends on other factors besides its thermal conductivity. In most cases, heat dissipation coefficient of the film, rust scale and surface condition of the metal. Stainless steel can keep the surface clean, so its heat transfer is better than other metals with high thermal conductivity.
Magnetic conductivity
Stable alloys 321 and 347 are generally non-magnetic. In the annealed state, its magnetic conductivity is lower than 1.02. Permeability will change due to composition and increase due to cold work. The magnetic permeability of welds containing ferrite will be higher.

Mechanical properties
Ductility at room temperature
The mechanical properties of stable alloy 321 and 347 chromium nickel grades in the annealed state (2000 ° F [1093 ° C], air cooling) are shown in the following table.
Ductility at high temperatures
The typical mechanical properties of alloys 321 and 347 at high temperatures are shown in the table below. In 1000 ° F (538 ° C) and high temperature environments, the strength of these stabilized alloys is significantly higher than that of unstable 304 alloys.
Alloys 321H and 347H (UNS32109 and S34700) with high carbon content have high strength in environments above 1000 ° F (537 ° C). The ASME allowable design stress data of alloy 347H show that the strength of this grade is higher than that of alloy 347 with lower carbon content. Alloy 321H is not allowed to be used in Section VIII applications, and is limited to 800 ° F (427 ° C) or below for Section III applications.
Creep and stress rupture properties
Typical creep and stress rupture data of alloy 321 and 347 stainless steels are shown in the following table. The creep and stress rupture strength of stable alloy at high temperature is higher than that of unstable alloy 304 and 304L. The superior properties of alloy 321 and 347 make it suitable for pressure parts in high-temperature operation, such as our common boilers and pressure vessels.

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