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347 stainless Steel VS 321 stainless Steel

Comparison between 347 stainless steel and 321 stainless steel

Alloy 321 (UNS S32100) is a very stable stainless steel. When the temperature reaches 800-1500 ° F (427-816 ° C) and chromium carbide precipitates, it still has good intergranular corrosion resistance. Because of the addition of titanium in the composition, 321 stainless steel can still maintain stability in the case of chromium carbide formation. However, the addition of coltan and tantalum to maintain the stability of alloy 347.

Because of its excellent mechanical properties, 347 stainless steel has advantages in high temperature environment. Compared with 304 alloy, 347 alloy stainless steel has better ductility and stress fracture resistance. In addition, 304L can also be used to resist sensitization and intergranular corrosion.

321 and 347 alloys are commonly used for long-term operation at 800-1500 ° F (427-816 ° C) at high temperature. If the application only involves welding or short-time heating, use 304L instead.  
The advantages of 321 and 347 alloys in high temperature operation also depend on their good mechanical properties. Compared with 304 and 304L, 321 and 347 have better creep stress resistance and stress rupture resistance. This allows these stable alloys to withstand pressures that still comply with the ASME Boiler code and pressure vessel code at higher temperatures. Therefore, the maximum service temperature of 321 and 347 alloys can reach 1500 ° F (816 ° C), while 304 and 304L is limited to 800 ° F (426 ° C). 321 and 347 also have high carbon content varieties, and their UNS numbers are S32109, the same as S34709, 304 and 430.

Chemical composition

ASTM A240 and ASME SA-240:

Component Unless otherwise specified, the weight percentage is the maximum value listed in the table
321 347
Carbon* 0.08 0.08
Manganese 2 2
Phosphorus 0.045 0.045
Sulfur 0.03 0.03
Silicon 0.75 0.75
Chromium 17.00-19.00 17.00-19.00
Nickel 9.00-12.00 9.00-13.00
Coltan + Tantalum** 10xc min 1.00 Max
Titanium** 5x (c + n) min 0.70 Max
Nitrogen zero point one zero
Iron rest rest

* The carbon content of grade H is 0.04/0.10%.

** Grade H minimum stabilizers are different formulations.

Uniform corrosion

Alloys 321 and 347 have similar resistance to general corrosion as unstable NiCr 304. The corrosion resistance of alloys 321 and 347 may be affected by prolonged heating in the temperature range of chromium carbide.In most environments, the corrosion resistance of the two alloys is similar; However, the corrosion resistance of alloy 321 in strong oxidizing environment is slightly lower than that of alloy 347 after annealing. Therefore, alloy 347 is superior in water environment and other low temperature environment. 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 in high temperature applications, which require strong sensitization resistance to prevent intergranular corrosion at lower temperatures.

20220402034659 38418 - 347 stainless Steel VS 321 stainless Steel

Intergranular corrosion

Unstable nickel steels such as alloy 304 are sensitive to intergranular corrosion, and alloy 321 and alloy 347 are developed and applied in this field.Chromium carbide precipitates at grain boundaries when unstable chromium nickel steels are exposed to temperatures ranging from 800 ° F to 1500 ° F (427 ° C to 816 ° C) or slowly cooled in this temperature range. When placed in some corrosive medium, these grain boundaries are eroded first, which may weaken the efficiency of the metal and may disintegrate completely.In organic media or corrosive water, milk or other dairy products or atmospheric conditions, even if there is a large amount of carbide precipitation, it will rarely produce intergranular corrosion. When welding thin plates, because the time staying in the temperature range of 800 ° F – 1500 ° F (427 ° c-816 ° C) is very short, it is not easy to produce intergranular corrosion, so unstable grades can be competent. The extent to which carbides precipitate is harmful depends on the length of time the alloy is exposed to the temperature range of 800 ° F to 1500 ° F (427 ° C to 816 ° C) and the corrosive medium. Welding thicker plate is that although the heating time is longer, due to the unstable l grade, the carbon content is 0.03% or less, and the precipitation of carbide is not enough to harm this grade.The strong sensitization resistance and intergranular corrosion resistance of the stable 321 and alloy 347 stainless steels are shown in the table below. (copper copper sulfate-16% sulfuric acid test (ASTM a262, practice E)). Before the test, the annealed steel samples were treated by soaking and photosensitizing at 1050 ° F (566 ° C) for 48 hours.There is no intergranular corrosion in alloy 347, which indicates that they are not sensitized when exposed to this thermal environment. The low corrosion rate of alloy 321 shows that although it has suffered some intergranular corrosion, its corrosion resistance is better than that of alloy 304L in these environments. In this test environment, all of these alloys are much better than the ordinary alloy 304 stainless steel.In general, alloys 321 and 347 are used for making heavy welding equipment that cannot be annealed and for equipment operating in or slowly cooling from 800 ° F to 1500 ° F (427 ° C to 816 ° C).

Stress corrosion cracking

Alloy 321 and 347 austenitic stainless steels are sensitive to stress corrosion cracking in halides, similar to that of alloy 304 stainless steel. This result is due to their similar nickel content. The conditions leading to stress corrosion cracking are: (1) exposure to halide ions (generally chlorides), (2) residual tensile stress, (3) ambient temperatures above 120 ° F (49 ° C). Cold deformation in forming operations or thermal cycles encountered in welding operations can produce stresses. Stress relief heat treatment after annealing or cold deformation may reduce the stress level. Stable alloys 321 and 347 are suitable for stress-free and intergranular corrosion of unstable alloys.321 and 347 are particularly useful in environments where the stress corrosion of unstable austenitic stainless steels (e.g., alloy 304) occurs in the presence of polysulfate. If the unstable austenitic stainless steel is exposed to the temperature where sensitization will occur, chromium carbide precipitation will be produced at the grain boundary. When cooled to room temperature in a sulfur-containing environment, sulfide (usually hydrogen sulfide) reacts with water vapor and oxygen to form polysulphuric acid, which erodes and sensitizes grain boundaries. Under the condition of stress and intergranular corrosion, the stress corrosion cracking of polysulfate occurs in the refinery environment where sulfides are common. The stable alloys 321 and 347 have the anti sensitization ability in the temperature rising operation environment, which solves the stress corrosion cracking problem of polysulfoxylic acid. If the operating conditions can cause sensitization, these alloys should be used under thermal stability conditions in order to achieve the best sensitization resistance.

Pitting corrosion / crevice corrosion

The pitting and pitting resistance of stable alloys 321 and 347 in chloride containing environments is similar to that of alloy 304 or 304L stainless steel because of their similar chromium content. Generally speaking, for unstable and stable alloys, the upper limit of chloride content in the water environment is 100 parts per million, especially in the presence of interstitial corrosion. The higher content of chloride ion will lead to crevice corrosion and pitting corrosion. In severe conditions with higher chloride content, lower pH and / or higher temperature, molybdenum containing alloys such as alloy 316 should be considered. The stable alloys 321 and 347 passed the 5% salt spray test (ASTM B117) for 100 hours, and the tested samples did not produce rust and discoloration. However, if these alloys are exposed to salt spray from the sea, pitting corrosion, crevice 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 321 and 347 is comparable to other 18-8 austenitic stainless steels. Expose the sample to high temperature laboratory atmosphere. If the samples are taken out from the high temperature environment and weighed regularly, the degree of rust formation can be calculated. The test results are expressed by weight change (mg / cm2), taking the average of the minimum values of two different samples tested. The main difference between 321 and 347 is the fine alloy additive, but 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 the form of the product, so these results should only be considered as the normal values for these grades of oxidation resistance.

Physical properties

The physical properties of alloys 321 and 347 are quite similar, but in fact, they can be regarded as the same. The values listed in the table are applicable to both alloys.If properly annealed, alloy 321 and 347 stainless steels mainly contain austenite and titanium carbide or niobium carbide. A small amount of ferrite may or may not appear in the microstructure. A small amount of sigma phase may be formed if exposed to temperatures between 1000 ° F and 1500 ° F (593 ° c-816 ° C) for a long time.Heat treatment does not harden the stable alloy 321 and 347 stainless steels.The total heat transfer coefficient of metal depends not only on the thermal conductivity of metal, but also on other factors. In most cases, the heat dissipation coefficient of the film, rust scale and surface condition of the metal. Stainless steel keeps the surface clean, so it has better heat transfer than other metals with higher thermal conductivity.

Magnetic conductivity

Stable alloys 321 and 347 are generally not magnetic. In the annealed state, its magnetic conductivity is less than 1.02. The permeability changes with composition and increases with cold working. The permeability of weld with ferrite will be higher.

Mechanical properties

Ductility at high temperature

Typical mechanical properties of alloys 321 and 347 at high temperatures are shown in the table below. At 1000 ° F (538 ° C) and above, the strength of these stabilized alloys is significantly higher than that of the unstable 304 alloy.

Alloys 321h and 347h (uns32109 and s34700) with high carbon content have higher strength at temperatures above 1000 ° F (537 ° C). The ASME maximum allowable design stress data of alloy 347h show that the strength of this grade is higher than that of alloy 347 with low carbon content. Alloy 321h is not permitted for Section VIII applications and is limited to temperatures of 800 ° F (427 ° C) or less for section III applications.

Creep and stress rupture properties

Typical creep and stress rupture data for alloy 321 and 347 stainless steels are shown in the table below. The creep and stress rupture strength of the stabilized alloy is higher than that of the unstable alloy 304 and 304L at high temperature. These superior properties of alloy 321 and 347 make it suitable for high temperature service pressure parts, such as our common boilers and pressure vessels.

Impact strength

The impact toughness of 321 and 347 is very good both indoors and in the environment below zero.

Fatigue strength

In fact, the fatigue strength of each metal is affected by corrosion environment, surface finish, product morphology and average stress. For this reason, it is not possible to use an exact number to represent the fatigue strength values under all operating conditions. The fatigue limit of alloy 321 and 347 is about 35% of its tensile strength.


Austenitic stainless steel is considered to be the most easily welded alloy steel and can be welded with all fusion materials, as well as resistance welding.

Two factors should be considered in the production of austenitic stainless steel welding joints: 1) to maintain its corrosion resistance, 2) to avoid cracking.

Attention must be paid to maintaining the stabilizing elements in alloys 321 and 347 during welding. Alloy 321 is more likely to lose titanium, while alloy 347 is more likely to lose coltan. Carbon in oil and other sources of pollution and nitrogen in the air need to be avoided. Therefore, it is necessary to keep clean and protect inert gas when welding stable alloy or unstable alloy.

When welding the metal with austenite structure, it is easy to split during operation. Due to this reason, a small amount of ferrite is needed to reduce the crack sensitivity of alloy 321 and 347 to the minimum. The stainless steel containing coltan is more prone to hot cracking than the stainless steel containing titanium.

The matching filler metal can be used for welding of alloy 321 and 347. The matching filler metal of alloy 347 can also be used for welding of alloy 321.

These stable alloys can be added to other stainless steels or carbon steels. Alloy 309 (23% cr-13.5% Ni) or nickel based filler metals can be used for this purpose.

Source: Network Arrangement – China Stainless Steel Flange Manufactuer – Yaang Pipe Industry (

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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