347 stainless steel heat treatment
Alloy 347 (UNS S34700) is an austenitic stainless steel that contains columbium, which provides stabilization. It offers good overall corrosion resistance and better resistance in strong oxidizing conditions compared to Alloy 321 (UNS S32100).
When exposed to temperatures within the range of 800 – 1500°F (427 – 816°C), it exhibits excellent resistance to intergranular corrosion caused by chromium carbide precipitation. The alloy demonstrates favorable oxidation resistance, creep strength up to 1500°F (816°C), and good toughness at low temperatures.
Alloy 347H (UNS S3409) is a variation of the alloy with a higher carbon content (0.04 – 0.10). It was specifically developed to enhance creep resistance and provide higher strength at temperatures exceeding 1000°F (537°C). In most cases, the carbon content of the plate allows for dual certification.
Heat treatment does not harden Alloy 347; only cold working can achieve hardening. It is readily weldable and can be processed using standard shop fabrication practices.
347 Stainless Steel Properties
347 stainless steel is an austenitic stainless steel alloy that contains a stabilizing addition of niobium/columbium. It possesses a number of desirable properties:
- Good overall corrosion resistance: 347 stainless steel demonstrates resistance to corrosion in various environments, including both acidic and alkaline conditions.
- Excellent resistance to intergranular corrosion when welded: This grade of stainless steel exhibits exceptional resistance to intergranular corrosion, which can occur along the grain boundaries after welding or exposure to high temperatures.
- Easy to weld and shape: 347 stainless steel is known for its ease of welding and formability, making it well-suited for a wide range of fabrication processes.
- Strong toughness at low temperatures: It maintains its toughness even in low-temperature environments, making it suitable for applications in cryogenic conditions.
In its solution annealed state, typically achieved by heating the material to temperatures of 1000 – 1100°C (1832 – 2012°F) followed by rapid cooling in air or water, 347 stainless steel is primarily composed of austenite. It may also contain a small amount of ferrite and niobium/columbium carbonitrides, which contribute to its overall properties.
Chemical Compositions of 347 Stainless Steel
The chemical composition of 347 stainless steel is as follows:
- Carbon (C): 0.08% maximum
- Silicon (Si): 0.75% maximum
- Manganese (Mn): 2.00% maximum
- Phosphorus (P): 0.045% maximum
- Sulfur (S): 0.030% maximum
- Chromium (Cr): 17.00% – 19.00%
- Nickel (Ni): 9.00% – 13.00%
- Columbium (Niobium) (Cb): 10 x (C%) – 1.00% maximum
- Iron (Fe): Balance (remainder of the composition)
The addition of columbium (niobium) in 347 stainless steel promotes stabilization by preventing the formation of chromium carbide during heating or welding processes, which helps to maintain the material’s corrosion resistance. This makes 347 stainless steel suitable for applications where intergranular corrosion resistance is required.
347 Stainless steel annealing
The recommended temperature range for annealing alloys 321 and 347 is 1800 — 2000°F (928 to 1093°C). The main purpose of annealing is to enhance the softness and ductility of the alloys. It is worth noting that stress can be eliminated within the temperature range of 800 — 1500°F (427 to 816°C) without causing intergranular corrosion.
While prolonged heating within this temperature range may slightly reduce the general corrosion resistance of alloys 321 and 347, a few hours of annealing within the range of 800 — 1500°F (427 to 816°C) can still relieve stress without significantly compromising their general corrosion resistance.
It is important to emphasize that annealing at low temperatures within the range of 800 — 1500°F (427 to 816°C) will not result in intergranular corrosion.
For achieving optimal ductility, it is recommended to use a higher annealing temperature of 1800 to 2000°F (928 to 1093°C).
347 Stainless Steel Stabilization Heat Treatment
When it comes to nickel-based stainless steels that need to prevent chromium carbide precipitation to a high degree, it’s important to note that the stability of columbium is different from that of titanium. Consequently, when using alloy 321, the effectiveness of stability and protection is not as pronounced.
To achieve the highest level of corrosion resistance, alloy 321 requires stabilization annealing treatment. This involves heating the material within the temperature range of 1550 to 1650°F (843 to 899°C) for up to 5 hours, with the duration of heating dependent on the thickness of the material.
Within this temperature range, chromium carbide formation is prevented, and any previously formed chromium carbide is decomposed and dissolved.
Additionally, at this temperature, carbon can combine with titanium to form harmless titanium carbide. As a result, chromium returns to a solid solution, and carbon combines with titanium to form harmless carbides.
On the other hand, columbium-containing stabilized alloy 347 typically does not require this additional treatment.
347 Stainless Steel Stress Relieving Heat Treatment
Stress relieving heat treatment for 347 stainless steel is commonly performed to reduce internal stresses that may have been induced during fabrication or welding processes. The objective is to enhance the material’s dimensional stability and minimize the risk of distortion or cracking.
The recommended temperature range for stress relieving 347 stainless steel is typically 1100 to 1350°F (593 to 732°C). The specific temperature within this range should be selected based on considerations such as the level of stress, material thickness, and desired properties.
The steel is heated to the chosen temperature and held at that temperature for an appropriate duration to allow stress relaxation. The duration is generally determined based on the thickness of the material, with a typical guideline of one hour per inch (25 mm) of thickness.
After the designated holding time, the material is slowly cooled in still air or by using a controlled cooling method to prevent the reintroduction of stress.
It’s important to note that stress relieving heat treatment does not significantly alter the material’s mechanical properties, including its strength or hardness. Instead, it aims to reduce residual stresses and enhance the material’s stability.
Hot forming of 347 stainless steel
Hot forming of 347 stainless steel should be conducted within a temperature range of 1150 – 750°C (2102 – 1652°F). It is important to note that 347 stainless steel has a low thermal conductivity, so the material may require a longer holding time at the designated temperature compared to carbon-manganese steels (approximately 50% longer).
After hot forming, it is crucial to rapidly quench the material to achieve proper hardening. The surface cleanliness of the stainless steel is of utmost importance, and precautions should be taken to avoid any oil contamination.
During the hot forming process, it is recommended to maintain a neutral or slightly oxidizing atmosphere to ensure the desired results and prevent any adverse effects on the material. This helps in preserving the integrity and properties of the 347 stainless steel during the hot forming operation.
Pickling treatment for 347 stainless steel
To perform the pickling treatment for 347 stainless steel, a bath containing nitric and hydrofluoric acid is commonly used. The bath composition typically consists of 10-20% nitric acid (HNO3) and 1.5-5% hydrofluoric acid (HF). The pickling process is carried out at a temperature range of 20-60°C (68-142°F).
Alternatively, a pickling bath containing 10-20% sulfuric acid (H2SO4) and 1.5-5% hydrofluoric acid (HF) can also be employed.
For decontamination treatment, a nitric acid solution with a concentration of 10-20% by weight is commonly used. This solution aids in removing contaminants from the surface of the stainless steel.
It is important to rinse the material thoroughly after the pickling process to remove any residual acids or contaminants.