UNS S34700 Chemical Composition

347 The composition of stainless steel is the core foundation of its performance. It not only contains the common chromium and nickel elements found in stainless steel—chromium provides the material with basic corrosion resistance, while nickel ensures the stability of the austenitic structure—but more importantly, it includes niobium (Nb) as a key alloying element. The addition of niobium acts like a “protective shield” for the material, effectively preventing intergranular corrosion during welding. During welding, ordinary stainless steel may form carbides due to the combination of carbon and chromium, leading to a reduction in chromium content at grain boundaries and thereby weakening corrosion resistance. However, the niobium in 347 stainless steel prioritizes binding with carbon, preventing chromium loss and ensuring that the material retains excellent resistance to intergranular corrosion after welding.

347 stainless steel Mechanical Properties

Common Standards

ASTM A240 – 347 stainless steel plate, sheet, and strip

ASTM A312 – 347 seamless and welded pipes

ASTM A182 – 347 forged flanges and fittings

ASTM A276 – 347 stainless steel bars

ASTM A403 – 347 stainless steel butt-weld pipe fittings

Characteristics

347 stainless steel (UNS S34700, also known as 1.4550) is a chromium-nickel austenitic stainless steel with a composition similar to that of common 304 stainless steel, but with the key difference being the addition of two stabilizing elements: niobium (Nb) and tantalum (Ta).

Outstanding resistance to intergranular corrosion: This is the most notable feature of 347 stainless steel. Within the temperature range of 425°C to 870°C, ordinary stainless steel is prone to intergranular corrosion due to the precipitation of chromium carbides at grain boundaries. However, the niobium and tantalum in 347 stainless steel preferentially react with carbon to form stable niobium carbides, thereby preventing the consumption of chromium and significantly enhancing resistance to intergranular corrosion.

Excellent high-temperature performance: It exhibits outstanding creep strength and sustained strength, with oxidation resistance up to 870°C, making it suitable for long-term high-temperature use.

Good weldability: The addition of stabilizing elements allows it to maintain corrosion resistance without post-weld heat treatment, simplifying the manufacturing process.

Non-magnetic: It is non-magnetic in the annealed state.

High-temperature resistance

347 stainless steel exhibits exceptional high-temperature resistance. While many metals soften or oxidize at high temperatures, 347 stainless steel maintains its structural integrity with minimal loss of strength. Its excellent oxidation resistance also means it is unlikely to degrade in performance even after prolonged exposure to high temperatures.

As a result, 347 stainless steel has become the material of choice for high-temperature equipment, such as boiler components, high-temperature sections of pressure vessels, and exhaust pipes for automobiles and industrial furnaces. In these demanding application environments, 347 stainless steel can withstand prolonged thermal stress, ensuring the safety and reliability of the equipment.

Corrosion Resistance

347 alloy stainless steel plate offers corrosion resistance comparable to that of SUS304. This material was specifically developed for the chromium carbide precipitation temperature range (800–1500°F (427–816°C)), whereas non-stabilized alloys like SUS304 are prone to intergranular corrosion. Within this temperature range, the overall corrosion resistance of 347 alloy stainless steel plate is superior to that of 321 alloy stainless steel plate. In strongly oxidizing environments up to 1500°F (816°C), the performance of 347 stainless steel is also slightly superior to that of 321 alloy.

This alloy can be used in nitric acid solutions and most diluted organic acids (at moderate temperatures), pure phosphoric acid (at low temperatures), and diluted solutions with concentrations up to 10% (at high temperatures). 347 stainless steel plates can resist stress corrosion cracking caused by polysulfuric acids in hydrocarbon media. It can also be used in alkaline solutions without chlorides or fluorides at moderate temperatures.

Finishes

Acid washing and passivation

First, the surface is acid washed to remove oxide scale and impurities. Then, passivation treatment is performed to form a dense oxide film, significantly improving corrosion resistance and preventing rust.

Mechanical polishing

Tools such as sandpaper or polishing wheels are used to grind and polish the surface to achieve a smooth and flat finish. This not only improves the appearance but also enhances its resistance to contamination.

Sandblasting

This process uses abrasive particles to blast the surface, creating a fine matte texture. It increases surface roughness, which is highly beneficial for improving coating adhesion and decorative effects.

Electrolytic Polishing

An electrochemical method used to eliminate microscopic surface irregularities. It produces a mirror-like finish, enhancing corrosion resistance and aesthetic appeal.

Coating

Applying a protective coating to the surface, such as fluorocarbon paint or polytetrafluoroethylene (PTFE). This adds an extra layer of protection, further improving corrosion resistance and wear resistance.

Hot Working

Hot working of 347H stainless steel, such as forging and upsetting, should be performed within the temperature range of 1149°C to 1232°C (2100°F to 2250°F). Within this temperature range, the material exhibits optimal plasticity, allowing for easy shaping while avoiding issues such as cracking due to low temperatures or excessive grain growth due to high temperatures.

Cold Working

347H stainless steel exhibits excellent cold working properties, primarily due to its superior toughness and ductility. It can be easily subjected to various cold forming operations, such as:

Stamping: Suitable for mass-producing complex thin-sheet parts.

Blanking: Used to cut specific shapes from large sheets of material.

Spinning: Suitable for producing symmetrical hollow components, such as pipes or containers.

Drawing: Suitable for manufacturing deep cup-shaped or cylindrical parts.

Advantages of Choosing 347 Stainless Steel

Reliable for long-term high-temperature service

Maintains corrosion resistance after welding

Works well in oxidizing and some reducing environments

Long service life with lower maintenance costs

Applications

Aerospace: Aircraft exhaust manifolds, piping, and engine components.

Petrochemical: Petroleum refining equipment, high-temperature piping, and vessels.

Power and Energy: High-temperature steam pipelines and boiler components.

Food Processing and Pharmaceutical: Equipment requiring welding where post-weld annealing is not feasible.

Heat Treatment Equipment: High-temperature furnace components.

Comparison: 347 vs. 304 vs. 321 Stainless Steel

Conclusion

347 stainless steel is a high-performance material combining excellent high-temperature strength with intergranular corrosion resistance, making it an ideal choice for petrochemical, aerospace, and boiler applications. Whether for corrosion resistance or high-temperature performance, 347 stainless steel provides stable and reliable service in demanding conditions.

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So, what is the difference between 2205 duplex stainless steel and 316 stainless steel? Which one is more suitable for your engineering application? Below we will conduct a comprehensive analysis from the aspects of chemical composition, corrosion resistance, cost, and application areas. Contact us

2205 Duplex Stainless Steel

2205 Duplex Stainless Steel refers to a super duplex stainless steel. It consists of a slightly higher content of chromium (22%) as well as nickel and molybdenum (5% and 3% respectively). It has a ferrite and martensitic duplex. Due to the ferrite, it is inherently magnetic. Alloy 2205 has very high corrosion resistance and is best suited for marine applications. Common uses include shipbuilding, offshore oil platforms and wind turbines. The high strength-to-weight ratio and low thermal expansion of duplex 2205 make it more demanding for low-temperature applications.

Advantages and Disadvantages of 2205 Duplex Stainless Steel

Advantages: The main advantage of alloy 2205 is its cost-effectiveness. It is less costly than other alloys with the same or lesser properties. It has high resistance to crevice corrosion, stress corrosion and acetic acid due to its high Ni and Mo content. Therefore, it is considered a marine stainless steel. Alloy 2205 has very high strength. This means that the thinner sections of the alloy have strength comparable to other steels with thicker sections.

Disadvantages: The limitation of Duplex Steel is that it has low toughness at low temperatures below 50°C. It has two phases which limits its operation at temperatures above 250°C. This is because it starts to form intermetallic phases which affect its weldability and other properties.

Advantages and limitations of 316 stainless steel

Advantages: The 316 grade has high corrosion resistance due to its high Cr and Mo content. This makes it highly resistant to acetic acid, crevice corrosion and stress corrosion. The non-magnetic properties of 316 steel make it suitable for the food processing industry. This is because it does not attract any dirt and is easy to clean. 316 also has higher formability and weldability than other austenitic steels used in piping systems and structural supports.

Disadvantages: 316 steel has high corrosion resistance due to its high Cr and Mo content, which limits its cost. In addition, due to its brittleness at low temperatures, it is not suitable for complex geometries and sharp bends.

316 Stainless Steel

316 stainless steel is an austenitic alloy. It contains up to 18% chromium, 14% nickel and 3% molybdenum. It is non-magnetic in nature. 316 alloy has high corrosion resistance and formability. It is mostly used in the food processing industry due to its corrosion resistance and high temperature resistance. Medical devices and tools are mostly made of 316 grade as they are less reactive with body fluids. High toughness makes it also highly demanded in the industrial sector.

2205 Duplex Stainless Steel vs 316 Stainless Steel

Chemical Composition

Core Differences:

2205 enhances strength and corrosion resistance through its duplex structure and higher Cr, Mo, and N content, particularly excelling in resistance to pitting and crevice corrosion.

316 relies on high Ni content to maintain austenitic stability, offering good corrosion resistance in non-chloride environments but with lower strength.

Welding and Processing Performance

 Corrosion Resistance

1. Resistance to Pitting and Crevice Corrosion

316 Stainless Steel: Contains molybdenum, making it more resistant to pitting and crevice corrosion than 304. It performs well in most mild chloride environments, but pitting and crevice corrosion can still occur in crevices with higher chloride concentrations, at high temperatures, or where there is an oxygen concentration cell. The critical pitting temperature (CPT) is relatively low.

2205 Duplex Stainless Steel: Due to its higher Cr and Mo content and the addition of nitrogen, 2205’s resistance to pitting and crevice corrosion far exceeds that of 316. Its critical pitting temperature (CPT) is significantly higher than that of 316, which means that it maintains excellent corrosion resistance in harsher chloride environments.

2. Stress Corrosion Cracking (SCC)

316 Stainless Steel: Austenitic stainless steels are generally at risk of stress corrosion cracking, particularly at high temperatures, in chloride-containing environments, and under tensile stress.

2205 Duplex Stainless Steel: Due to the presence of the ferrite phase in its duplex structure, 2205 has significantly better resistance to chloride stress corrosion cracking than 316 stainless steel. Because it can effectively avoid the sudden brittle fracture caused by the combined effect of stress and corrosion.

3. Resistance to Chloride Ion Stress Corrosion Cracking (SCC)

2205 Duplex Steel: Due to its high chromium and molybdenum content and duplex structure, 2205 exhibits extremely strong resistance to stress corrosion in chlorine-containing environments, which is far superior to 316 stainless steel.

316 stainless steel: although more than ordinary 304 stainless steel chloride corrosion resistance, but in the high temperature and high concentration of chloride ion environment may still occur stress corrosion cracking.

Cost

Due to its high alloying content (especially molybdenum and nitrogen) and the high manufacturing process requirements, 2205 typically has a higher raw material unit cost than 316. 2205 steel’s high strength makes it correspondingly more difficult to process (especially cold working, welding and machining). Its welding requires strict control of heat input to avoid the production of embrittled phases, cutting due to high hardness, high tool life and processing parameters. In contrast, 316 steel plasticity, easy to weld, easy to form, processing efficiency and tooling costs are relatively low.

Main Areas of Application

2205 Duplex Stainless Steel:

Marine and offshore platforms: Excellent seawater corrosion resistance for offshore oil and gas platforms and equipment.

Chemical, petrochemical and fertilizer industries: vessels, piping and heat exchangers handling highly chlorinated, acidic or alkaline, abrasive media.

Desalination plants: for critical equipment and piping.

Pulp and paper industry: resistance to chloride corrosion.

Structural components: Due to their high strength, they can be used in structures that are subjected to higher pressures.

316 Stainless Steel:

Food processing equipment: easy to clean and sterilize, resistant to acids in food.

Pharmaceutical and medical equipment: high hygiene requirements, resistance to disinfectant corrosion.

Ships and coastal buildings: can resist a certain degree of seawater corrosion, but not as good as 2205.

Chemical equipment: Suitable for a wide range of chemical handling, but still has limitations for high concentration chloride environments.

Architectural decoration, containers and piping.

Conclusion

The choice of 316 or 2205 depends critically on the specific requirements of the application environment:

– If your application environment has low chloride levels, moderate temperatures, and is more cost-sensitive, and you need good formability and machinability, then 316 stainless steel is a very economical and reliable choice.

– If your application environment involves high chlorides (e.g. seawater), high pressure, or risk of stress corrosion cracking, and you need higher strength and longer service life, then although 2205 duplex stainless steel is more costly and more complex to process, its superior performance will bring higher long-term value and reliability.

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