347/347H Heat-Resistant Stainless Steel: Essential Guide

347 heat-resistant stainless steel (S34700) is a very stable type of stainless steel. It retains good resistance to intergranular corrosion even under conditions of chromic carbide precipitation at temperatures of 800-1500°F (427-816°C).

Due to the addition of titanium in its composition, 347 heat-resistant stainless steel maintains stability even when chromic carbide forms.

Due to its excellent mechanical properties, 347 heat-resistant stainless steel has significant advantages when working in high-temperature environments.

Compared to the 304 alloy, 347 heat-resistant stainless steel exhibits superior ductility and stress rupture resistance.

Additionally, 304L can also be used to resist sensitization and intergranular corrosion.

I. General Characteristics

Alloy 321 (UNS S32100) is a highly stable stainless steel. It maintains excellent resistance to intergranular corrosion under conditions of chromium carbide precipitation at temperatures of 800-1500°F (427-816°C).

Thanks to the addition of titanium in its composition, 321 alloy retains stability even in the presence of chromium carbide formation. The stability of 347 heat-resistant stainless steel, on the other hand, is maintained by the addition of columbium and tantalum.

Both 321 and 347 heat-resistant stainless steels 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 321 and 347 heat-resistant stainless steels in high-temperature operations also lie in their impressive mechanical properties.

Compared to 304 and 304L, 321 and 347 have superior creep resistance and stress rupture properties. This allows these stable alloys to withstand pressures at slightly higher temperatures that still comply with the regulations for boilers and pressure vessels set by the American Society of Mechanical Engineers.

Therefore, the maximum usage temperature for 321 and 347 heat-resistant stainless steels 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:

III. Corrosion Resistance

1. Uniform Corrosion

Alloys 321 and 347 possess a similar ability to resist general corrosion as the unstable nickel-chromium alloy 304. Prolonged heating at the temperature range of chromium carbide degree may affect the corrosion resistance of alloys 321 and 347 in harsh corrosive media.

In most environments, the corrosion resistance of both alloys is pretty comparable; however, the resistance of annealed alloy 321 in strong oxidizing environments is slightly inferior to that of annealed alloy 347.

Thus, alloy 347 is superior in water environments and other low-temperature conditions. Exposure to temperatures ranging from 800°F — 1500°F (427°C — 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 resistance to sensitization is required to prevent intergranular corrosion at lower temperatures.

2. Intergranular Corrosion

The unstable nickel-chromium steel like alloy 304 is susceptible to intergranular corrosion, while alloys 321 and 347 were developed to address this issue.

When unstable chromium-nickel steel is placed in an environment at temperatures ranging from 800°F — 1500°F (427°C — 816°C) or slowly cooled within this temperature range, chromium carbide precipitates at the grain boundaries.

When exposed to aggressive corrosive media, these grain boundaries may be the first to corrode, potentially weakening the metal’s performance and leading to total disintegration.

In organic media or weakly corrosive water solutions, milk or other dairy products, or atmospheric conditions, intergranular corrosion is rarely observed even in the presence of substantial carbide precipitation.

When welding thinner plates, the 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 the corrosive media.

For welding thicker plates, despite longer heating times, the unstable L grade, with a carbon content of 0.03% or less, results in insufficient carbide precipitation to pose a threat to this grade.

The strong resistance to sensitization and intergranular corrosion of stabilized 321 and alloy 347 stainless steel is demonstrated in the table below (Copper-Copper Sulfate-16% Sulfuric Acid Test (ASTM A262, Practice E)).

Before the test, the steel mill annealed samples are subjected to a sensitizing heat treatment at 1050°F (566°C) for 48 hours.

After a 240-hour annealing process at 1100°F, Alloy 347 samples showed no signs of intergranular corrosion, indicating that they didn’t sensitize when exposed to such heat conditions. The low corrosion rate of the Alloy 321 samples suggests that although they endured some intergranular corrosion, their corrosion resistance was superior to that of Alloy 304L in these conditions.

In the environment of this test, all these alloys performed significantly better than the standard Alloy 304 stainless steel.

Generally speaking, Alloys 321 and 347 are used for manufacturing heavy-duty welding equipment that cannot undergo annealing treatment, as well as equipment operating or cooling slowly within the range of 800°F to 1500°F (427°C to 816°C).

The experience gained under various operating conditions provides ample data to predict the likelihood of intergranular corrosion in most applications. Please also review some of our views published in the heat treatment section.

3. Stress Corrosion Cracking

Alloys 321 and 347 austenitic stainless steels are sensitive to stress corrosion cracking in halides, similar to Alloy 304 stainless steel. 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) environmental temperatures above 120°F (49°C).

Cold deformation in forming operations or thermal cycles encountered in welding operations can generate stress. Annealing treatment or stress relief heat treatment after cold deformation can reduce stress levels.

Stabilized Alloys 321 and 347 are suitable for stress-relieved operations that could cause intergranular corrosion in unstable alloys.

Alloys 321 and 347 are particularly useful in environments that cause polythionic acid stress corrosion in unstable austenitic stainless steels, such as Alloy 304. Unstable austenitic stainless steel, when exposed to temperatures that cause sensitization, will precipitate chromium carbides at grain boundaries.

Upon cooling to room temperature in a sulfur-containing environment, sulfides (usually hydrogen sulfide) react with steam and oxygen to form polythionic acids which corrode sensitized grain boundaries.

Polythionic acid stress corrosion cracking occurs in refinery environments where sulfides are prevalent, under conditions of stress and intergranular corrosion.

Stabilized Alloys 321 and 347 solve the issue of polythionic acid stress corrosion cracking due to their resistance to sensitization during heating operations. If operating conditions can cause sensitization, these alloys should be used under thermally stabilized conditions for 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 a water environment is one hundred parts per million, especially when crevice corrosion is present. Higher chloride ion content can lead to crevice and pitting corrosion.

In harsh 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 have passed the 100-hour 5% salt spray test (ASTM B117) without rust or discoloration on the tested samples.

However, if these alloys are exposed to marine salt spray, pitting, crevice corrosion, and severe discoloration may occur. The exposure of alloys 321 and 347 to marine environments is not recommended.

IV. 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.

Regularly weighing the 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.

The primary difference between 321 and 347 lies in their subtle alloy additives, yet this doesn’t affect their antioxidation properties.

Therefore, these test results are representative for both grades. However, oxidation rates are impacted by inherent factors such as exposure environment and product form.

Consequently, these results should merely be regarded as typical values of antioxidation 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 alloys.

With appropriate annealing treatment, Alloys 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 a prolonged period, a small amount of sigma phase may form.

Heat treatment cannot harden stabilized Alloys 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 surface condition of the metal. 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, their magnetic permeability is less than 1.02. Magnetic permeability changes with composition and increases with cold working. The magnetic permeability of ferrite-containing welds is slightly higher.

VI. Mechanical Properties

1. Ductility at Room Temperature

The minimum mechanical properties of stable alloys 321 and 347 at the chromium-nickel level in the annealed state (2000°F [1093°C], air cooled) are as shown in the table below.

2. Ductility at High Temperature

The typical mechanical properties of alloys 321 and 347 at high temperatures are as shown in the table below. In environments of 1000°F (538°C) and higher temperatures, the strength of these stable alloys is significantly higher than that of the unstable 304 alloy.

The high carbon alloys 321H and 347H (UNS32109 and S34700) have higher strength in environments above 1000°F (537°C). The ASME maximum allowable design stress data of alloy 347H shows that the strength of this grade is higher than that of the lower carbon alloy 347.

Alloy 321H is not permitted for use in Section VIII applications, and for Section III applications, it is limited to temperatures of 800°F (427°C) or below.

3. Creep and Stress Rupture Properties

The typical creep and stress rupture data of stainless steel alloys 321 and 347 are as shown in the table below. The creep and stress rupture strength of stable alloys at high temperatures is higher than that of unstable alloys 304 and 304L.

The superior performance of alloys 321 and 347 makes them suitable for pressure parts operating at high temperatures, such as commonly seen boilers and pressure vessels.

4. Impact Strength

Both 321 and 347 alloys exhibit excellent impact toughness, whether in indoor conditions or in environments below freezing.

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 for alloy 321 is similar to 347.

5. Fatigue Strength

In fact, the fatigue strength of each metal is affected by factors such as the 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 a precise 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 fusion 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’s crucial to preserve the stabilizing elements in alloys 321 and 347. Titanium in alloy 321 is more prone to depletion, while 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 inert gas protection must be maintained.

When welding metals with an austenitic structure, splitting is easily caused during the operation. For this reason, alloys 321 and 347 need 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 for welding stable steels such as alloys 321 and 347. The matching filler metal of alloy 347 can sometimes also be used for welding 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 — 2000°F (928 to 1093°C). While the primary objective of annealing is to enhance the alloy’s softness and ductility, stress can also be eliminated within the carbide precipitation range of 800 — 1500°F (427 to 816°C) without causing intergranular corrosion.

Although prolonged heating within this temperature range can somewhat reduce the alloy’s general corrosion resistance, alloys 321 and 347 can relieve stress after annealing for a few hours within the temperature range of 800 — 1500°F (427 to 816°C), and their general corrosion resistance will not be significantly reduced.

As emphasized, low-temperature annealing within the range of 800 — 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 1800 to 2000°F (928 to 1093°C).

When processing these nickel-based stainless steels into equipment that needs to prevent chromium carbide precipitation to the maximum extent, 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 results of stability and protection are not as obvious.

When maximum corrosion resistance is required, alloy 321 must undergo stabilization annealing treatment. Heat within the temperature range of 1550 to 1650°F (843 to 899°C) for up to 5 hours, with the heating time dependent on thickness.

This temperature range exceeds the temperature range for chromium carbide formation and is also sufficient to decompose and dissolve previously formed chromium carbide.

Moreover, at this temperature, titanium can combine with carbon to form harmless titanium carbide. The result is that chromium is reduced back to a solid solution, and carbon is forced to combine with titanium to form harmless carbides.

Columbium-containing stabilized alloy 347 does not often require this additional treatment.

After heat treatment is completed 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 should be thoroughly rinsed to wash off residual acidic solution.

These alloys cannot be hardened through heat treatment.

IX. Cleanliness

Regardless of its corrosion resistance, stainless steel requires surface cleanliness throughout its use and manufacturing process, even under normal working conditions.

During welding, an inert gas process is employed, and the oxides and slag formed are removed using a stainless steel brush. Ordinary carbon steel brushes leave carbon steel particles on the stainless steel surface, which could ultimately lead to surface rusting. In stringent circumstances, the welding area has to be treated with a rust-removal solution (such as a mixture of nitric acid and hydrofluoric acid) to eliminate oxides and slag.

After rust removal, the stainless steel surface should be thoroughly rinsed to wash away any residual acidic solution.

In landlocked areas, materials used in light industries require less maintenance. Only shielded areas occasionally require pressurized water cleaning. Heavy industries, however, are recommended to frequently clean to remove accumulated dust, which could potentially lead to corrosion and damage to the stainless steel surface appearance.

Appropriate design aids in cleaning. Equipment with round fillets, internal radii, and no gaps facilitates cleaning and surface polishing.

The reference data is simply a typical analysis and cannot be used as the specification or maximum or minimum value of the final product. The data for a specific piece of material may not align with the above reference data.