Heat-resistant 347 stainless steel (S34700) is a very stable type of stainless steel. It retains good resistance to intergranular corrosion even under chromic carbide precipitation conditions at temperatures of 800-1500°F (427-816°C).
Due to the addition of titanium in its composition, heat-resistant 347 stainless steel maintains stability even when chromic carbide is formed.
Due to its excellent mechanical properties, heat-resistant 347 stainless steel has significant advantages when working in high temperature environments.
Compared with alloy 304, heat-resistant 347 stainless steel has superior ductility and resistance to stress rupture.
Additionally, 304L can also be used to resist sensitization and intergranular corrosion.
I. General features
Alloy 321 (UNS S32100) is a highly stable stainless steel. It maintains excellent resistance to intergranular corrosion under chromium carbide precipitation conditions at temperatures of 800-1500°F (427-816°C).
Thanks to the addition of titanium in its composition, alloy 321 maintains stability even in the presence of chromium carbide formation. The stability of heat-resistant 347 stainless steel, on the other hand, is maintained by the addition of collubium and tantalum.
Heat resistant stainless steels 321 and 347 are commonly used for long term operations in high temperature environments ranging from 800-1500°F (427-816°C). However, if applications only involve welding or short-term heating, 304L can be used as a substitute.
The advantages of using heat-resistant stainless steels 321 and 347 in high-temperature operations also lie in their impressive mechanical properties.
Compared to 304 and 304L, 321 and 347 exhibit superior creep resistance and stress rupture properties. This allows these stable alloys to withstand pressures at slightly higher temperatures that still meet the regulations for boilers and pressure vessels established by the American Society of Mechanical Engineers.
Therefore, the maximum use temperature for heat-resistant stainless steels 321 and 347 can reach up to 1500°F (816°C), while 304 and 304L are limited to 800°F (426°C).
There are also high-carbon versions of alloys 321 and 347, designated as UNS S32109 and S34709, respectively.
II. Chemical composition
ASTM A240 and ASME SA-240:
| Composition | Unless specifically stated otherwise, the values listed in the table represent the maximum percentage by weight. | |
| 321 | 347 | |
| Carbon | 0.08 | 0.08 |
| Manganese | 2:00 | 2:00 |
| Phosphor | 0.045 | 0.045 |
| Sulfur | 0.030 | 0.03 |
| Silicon | 0.75 | 0.75 |
| Chrome | 5pm-7pm | 5pm-7pm |
| Nickel | 9:00-12:00 | 9am-1pm |
| Strontium + Tantalum | – | 10x C – Minimum 1.00 Maximum |
| Tantalum | – | – |
| Titanium | 5x(C+N) minimum 0.70 maximum |
– |
| Cobalt | – | – |
| Nitrogen | 0.10 | – |
| Iron | Remaining part | Remaining part |
| Observation | * The carbon content of H grade is between 0.04 and 0.10%. *Minimum stabilizer for grade H varies depending on the specific formula. |
|
III. Corrosion resistance
1. Uniform Corrosion
Alloys 321 and 347 have a similar ability to resist general corrosion as the unstable nickel-chromium alloy 304. Prolonged heating in the chromium carbide grade temperature range can affect the corrosion resistance of alloys 321 and 347 in media aggressive corrosives.
In most environments, the corrosion resistance of both alloys is quite comparable; however, the resistance of annealed alloy 321 in strong oxidizing environments is slightly lower than that of annealed alloy 347.
Thus, alloy 347 is superior in aquatic environments and other low temperature conditions. Exposure to temperatures ranging from 800°F to 1500°F (427°C to 816°C) significantly reduces the overall corrosion resistance of alloy 321 compared to alloy 347.
Alloy 347 is mainly used for high temperature applications where strong sensitization resistance is required to prevent intergranular corrosion at lower temperatures.
2. Intergranular Corrosion
Unstable nickel-chromium steel such as alloy 304 is susceptible to intergranular corrosion, while alloys 321 and 347 were developed to solve this problem.
When unstable chromium-nickel steel is placed in an environment with temperatures ranging from 427°C to 816°C (800°F – 1500°F) or slowly cooled within this temperature range, chromium carbide precipitates at the grain boundaries .
When exposed to aggressive corrosive media, these grain boundaries can be the first to corrode, potentially weakening the metal's performance and leading to total disintegration.
In organic media or weakly corrosive aqueous solutions, milk or other dairy products, or in atmospheric conditions, intergranular corrosion is rarely observed, even in the presence of substantial precipitation of carbides.
When welding thinner plates, short exposure to temperatures between 800°F – 1500°F (427°C – 816°C) reduces the likelihood of intergranular corrosion, making unstable grades suitable for the task.
The extent of harmful carbide precipitation depends on the duration of exposure to temperatures between 800°F – 1500°F (427°C – 816°C) and corrosive media.
For welding thicker sheets, despite longer heating times, unstable L class, with a carbon content of 0.03% or less, results in insufficient carbide precipitation to pose a threat to this class.
The strong resistance to sensitization and intergranular corrosion of stabilized 321 stainless steel and 347 alloy is demonstrated in the table below (Copper-Copper Sulphate-16% Sulfuric Acid Test (ASTM A262, Practice E)).
Before testing, samples annealed in steel plants are subjected to a sensitization heat treatment at 1050°F (566°C) for 48 hours.
| Results of grain boundary corrosion tests under long-term sensitization effects. ASTM A262 Practice E |
|||
| turns on | Rate (ipm) | To bend | Rate (mpy) |
| 304 | 0.81 | dissolved | 9720.0 |
| 304L | 0.0013 | IGA | 15.6 |
After a 240-hour annealing process at 1100°F, the Alloy 347 samples showed no signs of intergranular corrosion, indicating that they did not sensitize when exposed to such heat conditions. The low corrosion rate of the Alloy 321 samples suggests that although they underwent some intergranular corrosion, their corrosion resistance was superior to that of Alloy 304L under these conditions.
In the environment of this test, all of these alloys performed significantly better than standard Alloy 304 stainless steel.
Generally speaking, alloys 321 and 347 are used for the manufacture of heavy-duty welding equipment that cannot undergo annealing treatment, as well as equipment operating or slowly cooling in the range of 800°F to 1500°F ( 427°C to 816°C). .
Experience gained under a variety of operating conditions provides ample data to predict the likelihood of intergranular corrosion in most applications. Please also review some of our opinions published in the heat treatment section.
3. Stress corrosion cracking
Austenitic stainless steels from alloys 321 and 347 are sensitive to halide stress corrosion cracking, similar to stainless steel from alloy 304. This is due to their similar nickel content. Conditions that lead to stress corrosion cracking include:
(1) exposure to halide ions (usually chlorides)
(2) residual tensile stress
(3) ambient temperatures above 120°F (49°C).
Cold deformation in forming operations or thermal cycling encountered in welding operations can generate stress. Annealing treatment or stress relieving heat treatment after cold deformation can reduce stress levels.
Stabilized alloys 321 and 347 are suitable for stress relief operations that can cause intergranular corrosion in unstable alloys.
Alloys 321 and 347 are particularly useful in environments that cause polythionic acid stress corrosion cracking in unstable austenitic stainless steels such as alloy 304. Unstable austenitic stainless steel, when exposed to temperatures that cause sensitization, will precipitate chromium carbides in the grain boundaries.
After cooling to room temperature in a sulfur-containing environment, sulfides (usually hydrogen sulfide) react with steam and oxygen to form polythionic acids that erode sensitized grain boundaries.
Polythionic acid stress corrosion cracking occurs in refinery environments where sulfides are predominant, under conditions of stress and intergranular corrosion.
Stabilized alloys 321 and 347 solve the problem of stress corrosion cracking of polythionic acid due to their resistance to sensitization during heating operations. If operating conditions may cause sensitization, these alloys should be used under thermally stabilized conditions to obtain optimal resistance to sensitization.
4. Pitting/crevice corrosion
The pitting and crevice corrosion resistance of stable alloys 321 and 347 in environments containing chloride ions is approximately the same as that of stainless steel alloys 304 or 304L due to their similar chromium content.
Generally, for unstable and stable alloys, the maximum chloride content in an aquatic environment is one hundred parts per million, especially when there is crevice corrosion. Higher chloride ion content can cause pitting and pitting corrosion.
In adverse conditions with higher chloride content, lower pH and/or higher temperatures, the use of molybdenum-containing alloys, such as alloy 316, should be considered. Stable alloys 321 and 347 passed the 100-hour 5% salt spray test (ASTM B117) with no rust or discoloration in the tested samples.
However, if these alloys are exposed to marine salt spray, crevice corrosion and severe discoloration can occur. Exposure of alloys 321 and 347 to marine environments is not recommended.
4. High temperature oxidation resistance
The oxidation resistance of 321 and 347 can be compared to other 18-8 austenitic stainless steels. Samples are exposed to high-temperature laboratory atmospheres.
Regular weighing of samples removed from the high temperature environment can predict the degree of scale formation. Test results are represented by weight changes (milligrams/square centimeter), averaging the minimum values of two different tested samples.
| Weight variation (mg/cm 2 ) | |||||
| Exposure period | 1300°F | 1350°F | 1,400°F | 1,450°F | 1,500°F |
| 168 hours | 0.032 | 0.046 | 0.054 | 0.067 | 0.118 |
| 500 hours | 0.045 | 0.065 | 0.108 | 0.108 | 0.221 |
| 1,000 hours | 0.067 | – | 0.166 | – | 0.338 |
| 5,000 hours | – | – | 0.443 | – | – |
The main difference between 321 and 347 lies in their subtle alloying additives, but this does not affect their antioxidant properties.
Therefore, these test results are representative for both series. However, oxidation rates are affected by inherent factors such as the exposure environment and the shape of the product.
Consequently, these results should merely be considered as typical antioxidation values for these grades.
V. Physical properties
The physical properties of Alloys 321 and 347 are quite similar, in fact, they can be considered identical. The values listed in the table apply to both leagues.
With appropriate annealing treatment, alloy 321 and 347 stainless steels mainly contain austenite and titanium carbides or niobium carbides. A small amount of ferrite may or may not appear in the microstructure. If exposed to temperatures between 1000°F and 1500°F (593°C to 816°C) for an extended period, a small amount of sigma phase may form.
Heat treatment cannot harden stabilized alloy 321 and 347 stainless steels.
The overall heat transfer coefficient of the metal depends not only on the thermal conductivity of the metal, but also on other factors.
In most cases, these include the film cooling coefficient, scale, and metal surface condition. Stainless steel maintains a clean surface, making its heat transfer better than metals with higher thermal conductivity.
Magnetism
Stabilized alloys 321 and 347 are generally non-magnetic. In the annealed condition, its magnetic permeability is less than 1.02. Magnetic permeability changes with composition and increases with cold working. The magnetic permeability of welds containing ferrite is slightly higher.
| Physical properties | ||
| Density | ||
| Level | g/ cm3 | lb/in. 3 |
| 321 | 7.92 | 0.286 |
| 347 | 7.96 | 0.288 |
| Elastic Traction Module | |||
| 28×10 6 psi | |||
| 193 GPa |
| Linear Coefficient of Thermal Expansion | |||
| Temperature range | |||
| °C | °F | cm/cm°C | in/in °F |
| 20-100 | 68 – 212 | 16.6 x 10 -6 | 9.2 x 10 -6 |
| 20 – 600 | 68 – 1112 | 18.9 x 10 -6 | 10.5×10 -6 |
| 20 – 1000 | 68 – 1832 | 20.5×10 -6 | 11.4 x 10 -6 |
| Thermal conductivity | |||
| Temperature range | |||
| °C | °F | W/m•K | Btu•in/h•ft 2 •°F |
| 20-100 | 68 – 212 | 16.3 | 112.5 |
| 20 – 500 | 68 – 932 | 21.4 | 14.7 |
| Specific heat | |||
| Temperature range | |||
| °C | °F | J/kg K | Btu/lb•°F |
| 0-100 | 32 – 212 | 500 | 0.12 |
| Resistivity | ||
| Temperature range | ||
| °C | °F | microhm•cm |
| 20 | 68 | 72 |
| 100 | 213 | 78 |
| 200 | 392 | 86 |
| 400 | 752 | 100 |
| 600 | 1112 | 111 |
| 800 | 1472 | 121 |
| 900 | 1652 | 126 |
| Fusion range | |
| °C | °F |
| 1398 – 1446 | 2550 – 2635 |
SAW. Mechanical properties
1. Ductility at room temperature
The minimum mechanical properties of nickel-chromium stable alloys 321 and 347 in the annealed state (2000°F (1093°C), air-cooled) are shown in the table below.
2. High temperature ductility
Typical mechanical properties of alloys 321 and 347 at high temperatures are shown in the table below. In environments of 538°C (1000°F) and higher temperatures, the strength of these stable alloys is significantly greater than that of unstable alloy 304.
High carbon alloys 321H and 347H (UNS32109 and S34700) exhibit greater resistance in environments above 1000°F (537°C). ASME maximum allowable design stress data for alloy 347H shows that the strength of this grade is greater than that of low-carbon alloy 347.
Alloy 321H is not permitted for use in Section VIII applications and, for Section III applications, is limited to temperatures of 427°C (800°F) or below.
3. Creep and stress rupture properties
Typical creep and stress rupture data for stainless steel alloys 321 and 347 are shown in the table below. The creep and tension rupture strength of stable alloys at high temperatures is greater than that of unstable alloys 304 and 304L.
The superior performance of alloys 321 and 347 makes them suitable for pressure parts that operate at high temperatures, such as commonly seen boilers and pressure vessels.
| Impact resistance of 321 and 347 | |||
| Test Temperature | Energy absorption from impact loading | ||
| °F | °C | Foot-lb | Joules |
| 75 | 24 | 90 | 122 |
| -25 | -32 | 66 | 89 |
| -80 | -62 | 57 | 78 |
|
ASTM A 240 and ASME SA-240
Minimum required mechanical performance at room temperature |
|||
| Type | Yield strength 0.2% compensation psi (MPa) |
Tensile strength psi (MPa) |
Stretching (%) |
| 321 | 30,000 (205) |
75,000 (515) |
40.0 |
| 347 | 30,000 (205) |
75,000 (515) |
40.0 |
|
ASTM A 240 and ASME SA-240 Minimum required mechanical performance at room temperature |
|||
| Type | Hardness, maximum value. | ||
| Sheet | Plate | Range | |
| 321 | 217 Brinell |
95Rb | 95Rb |
| 347 | 201 Brinell |
92Rb | 92Rb |
|
Tensile strength under high temperature conditions Alloy 321 (0.036 inch thick / 0.9 mm thick) |
||||
| Test Temperature | Yield Strength 0.2% compensation psi (MPa) |
Tensile strength psi (MPa) |
Elongation Rate (%) |
|
| °F | °C | |||
| 68 | 20 | 31,400 (215) |
85,000 (590) |
55.0 |
| 400 | 204 | 23,500 (160) |
66,600 (455) |
38.0 |
| 800 | 427 | 19,380 (130) |
66,300 (455) |
32.0 |
| 1000 | 538 | 19,010 (130) |
64,400 (440) |
32.0 |
| 1200 | 649 | 19,000 (130) |
55,800 (380) |
28.0 |
| 1350 | 732 | 18,890 (130) |
41,500 (285) |
26.0 |
| 1500 | 816 | 17,200 (115) |
26,000 (180) |
45.0 |
|
Tensile strength under high temperature conditions Alloy 347 (0.060 inch thick / 1.54 mm thick ) |
||||
| Test Temperature | Yield strength 0.2% compensation psi (MPa) |
Tensile strength psi (MPa) |
Elongation Rate (%) |
|
| °F | °C | |||
| 68 | 20 | 36,500 (250) |
93,250 (640) |
45.0 |
| 400 | 204 | 36,600 (250) |
73,570 (505) |
36.0 |
| 800 | 427 | 29,680 (205) |
69,500 (475) |
30.0 |
| 1000 | 538 | 27,400 (190) |
63,510 (435) |
27.0 |
| 1200 | 649 | 24,475 (165) |
52,300 (360) |
26.0 |
| 1350 | 732 | 22,800 (155) |
39,280 (270) |
40.0 |
| 1500 | 816 | 18,600 (125) |
26,400 (180) |
50.0 |
4. Impact force
Both alloys 321 and 347 exhibit excellent impact resistance, whether indoors or in subzero environments.
The Charpy V impact test of alloy 347 after annealing, which was left at a specified test temperature for one hour, is shown in the following graph. The situation in league 321 is similar to 347.
5. Fatigue strength
In fact, the fatigue strength of each metal is affected by factors such as corrosion environment, surface finish, product shape and average stress.
For this reason, it is impossible to represent the fatigue strength value under all operating conditions with an accurate number. The fatigue limit of alloys 321 and 347 is approximately 35% of their tensile strength.
VII. Processing
Welding
Austenitic stainless steel is considered the easiest alloy steel to weld and can be welded with all melting substances as well as resistance welding.
When welding austenitic stainless steel, two factors must be considered: 1) maintaining its corrosion resistance and 2) preventing cracking.
During welding, it is crucial to preserve the stabilizing elements in alloys 321 and 347. The titanium in alloy 321 is more prone to depletion, while the niobium in alloy 347 is often easily lost. It is necessary to avoid carbon elements from petroleum and other sources of contamination, as well as nitrogen elements from the air.
Therefore, whether welding stable or unstable alloys, cleanliness and protection against inert gases must be maintained.
When welding metals with austenitic structure, cracks are easily caused during operation. For this reason, alloys 321 and 347 require a small amount of ferric salt to be added during resolidification to minimize crack sensitivity. Stainless steel containing niobium is more prone to hot cracking than that containing titanium.
Matching filler metals can be used to weld stable steels such as alloys 321 and 347. The matching filler metal of alloy 347 can sometimes also be used to weld 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 serve this purpose.
VIII. Heat treatment
The annealing temperature range for alloys 321 and 347 is 1800 to 2000°F (928 to 1093°C). Although the primary purpose of annealing is to improve the softness and ductility of the alloy, stress can also be eliminated within the carbide precipitation range of 800 to 1500°F (427 to 816°C) without causing intergranular corrosion.
Although prolonged heating within this temperature range may somewhat reduce the overall corrosion resistance of the alloy, alloys 321 and 347 may relieve stress after annealing for a few hours within the temperature range of 800 – 1500°F (427 to 816°C), and its overall corrosion resistance will not be significantly reduced.
As emphasized, low temperature annealing in the range of 800 to 1500°F (427 to 816°C) will not lead to intergranular corrosion.
To achieve optimal ductility, it is recommended to use a higher annealing temperature of 928 to 1093°C (1800 to 2000°F).
When processing these nickel-based stainless steels in equipment that needs to avoid chromium carbide precipitation as much as possible, it must be recognized that the stability of columbium is not the same as that of titanium. For these reasons, when using alloy 321, the stability and protection results are not so obvious.
When maximum corrosion resistance is required, alloy 321 must undergo stabilization annealing treatment. Heat in the temperature range of 843 to 899°C (1,550 to 1,650°F) for up to 5 hours, with heating time depending on thickness.
This temperature range exceeds the temperature range for the formation of chromium carbide and is also sufficient to decompose and dissolve previously formed chromium carbide.
Furthermore, at this temperature, titanium can combine with carbon to form harmless titanium carbide. The result is that the chromium is reduced to a solid solution and the carbon is forced to combine with the titanium to form harmless carbides.
Stabilized alloy 347 containing collubium does not always require this additional treatment.
After completion of heat treatment in an oxidizing environment, the oxides formed on the annealed surface are removed in a pickling solution such as a mixture of nitric acid and hydrofluoric acid. After pickling, the stainless steel surface must be rinsed well to remove residual acid solution.
These alloys cannot be hardened by heat treatment.
IX. Cleaning
Regardless of its corrosion resistance, stainless steel requires surface cleaning throughout its use and manufacturing process, even under normal working conditions.
During welding, an inert gas process is used and the oxides and slag formed are removed with a stainless steel brush. Ordinary carbon steel brushes leave carbon steel particles on the surface of stainless steel, which can lead to surface rust. In severe circumstances, the welding area must be treated with a rust remover solution (such as a mixture of nitric acid and hydrofluoric acid) to eliminate oxides and slag.
After removing the rust, the stainless steel surface must be rinsed well to remove any residual acid solution.
In landlocked areas, materials used in light industries require less maintenance. Only protected areas occasionally require pressurized water cleaning. Heavy industries, however, are recommended to clean frequently to remove accumulated dust, which can lead to corrosion and damage the appearance of the stainless steel surface.
Appropriate design aids in cleaning. Equipment with round fillets, internal radii and no gaps facilitate cleaning and surface polishing.
Reference data is simply a typical analysis and cannot be used as a specification or maximum or minimum value of the final product. Data for a specific material may not align with the reference data above.
