We produce BA tube bright annealing stainless steel both welded tube and seamless tube, the typical specification is ASTM A269 for stainless steel tubing, stainless steel grades include 304/304L, 316/316L, 321, 904L, and duplex steels, the BA tube can be designed in a large range of sizes, standard diameters have 1/4″, 3/8, 1/2″, 3/4″ and 1″. After the bright annealed finish the surface from an oxidation film outside of the tube to prevent the tube from corrosion.
BA is an abbreviation of bright annealing, BA tube is annealed in a high-temperature heat treatment in a hydrogenated or vacuum condition that forms a passivation film on the surface of the tubes, the process improves the morphology and structure, removes internal stresses, and reduces the energy level, but not to improve the surface roughness. The surface of the BA tube is smoother and brighter.
BA Tube Bright Annealing Stainless Steel Specifications
| Welded tube | ASTM A249, A269, A789, EN10217-7 |
|---|---|
| Seamless tube | ASTM A213, A269, A789 |
| Grade | 304, 304L, 316, 316L, 321, 2205 etc. |
| Finish | Bright Annealing (BA) |
| OD | 3 Mm – 50.8 Mm; |
| Thickness | 0.3 Mm – 3 Mm |
| Forms | Round, U Bend, Coil tubing |
| Application | Heat Exchanger, Boiler, Condenser, Cooler, Heater, instrumentation tubing |
Bright Annealed Definition
Bright annealing (BA) refers to stainless steel material is heated in a closed furnace to reduce the atmosphere of inert gases, common Hydrogen gas, after fast annealing, and rapid cooling, stainless steel has a protective layer on the outer surface, no reflection in open air environment, this layer can resist corrosion attack. In general, the material surface is more smooth and brighter.
Bright Annealing Process
Before Bright Annealing
The surface of the tube must be clean, and no other foreign matter or any matter left on the tubing surface can cause damage during the process.
Inserted Gas
Annealing atmosphere should be free from Oxygen, isolating material, and creating a vacuum condition. Inserted gas, common dry hydrogen or argon, can obtain bright results.
Annealing Temperature
Annealing temperature should be noticed depending on different stainless grades, commonly Austenitic steel’s annealing temperature is at least 1040 degrees, soak time is not important. high temperature must be required to have brighter appearance. Heating is done as fast as possible, slow heat results in oxidation.
Some Ferritic stainless steels require lower annealing temperatures, such as TP439, which cannot be effectively bright annealed, Water quenching will cause scale formation.
Bright Annealing Welded Tube
Welded Tube is annealed in a vacuum-controlled atmosphere to keep the surface free of oxidation and bright, in this bright annealing process, protective gas is inserted in the closed furnace, usually, the gas is purified hydrogen, or a mixture of hydrogen and nitrogen atmosphere from dissociated ammonia, when the tube is heated up to certain specified temperature and rapid cooling, a bright and smooth protective layer is formed on the surface of the tube.
Bright Annealing Seamless Tube
Seamless tube is manufactured from stainless steel billet by coil colled and coil drawn process, annealing is a critical stage for material quality to eliminate residue stress and uniform structure, for some applications, bright annealed seamless tube is required, compared to BA welded tube, BA seamless tube is annealed using offline annealing furnace, ASTM A269 usually specified bright annealing seamless tubing for instrumentation tubing.
After bright annealing, it comes to the final steps of sizing and straightening, The stainless steel tube surface performs a bright appearance, it is unnecessary to pickle for the bright annealed tube. If the bright annealing process is not properly done, the tube surface can crack, and stain and may lead to easy corrosion.
Purpose and Advantages of Bright Annealed
- Eliminate work hardening and obtain a satisfactory metallographic structure
- Obtain a bright, non-oxidizing surface with good corrosion resistance
- The bright treatment maintains the smoothness of the rolled surface, and the bright surface can be obtained without post-processing
- No pollution problems caused by common pickling methods
Bright Annealed VS Solution Annealing
Bright annealed is processed in a closed furnace, this controlled atmosphere reduces the surface oxidation to a minimum which results in a more colorful surface and a much thinner oxide layer. Pickling is not needed after bright annealing since the oxidation is minimal. Since there is no pickling, the surface is much smoother, resulting in better resistance to pitting corrosion. Using protective gas inserted inside.
Bright Annealed products have a slightly higher hardness and tensile strength compared to solution-annealed products.
Solution Annealing is a standard step for stainless steel material, it is open-air annealing, commonly used to soften materials, minimize residual stresses, improve machinability and increase ductility. Some annealing requires only minutes at temperature whilst others can be prolonged soak times with controlled cooling rates from high temperatures, typically between 1010 & 1150ºC.
Factors Affecting the BA Tube
- Annealing temperature: whether the annealing temperature reaches the specified temperature is one of the key factors affecting the bright annealing quality of stainless steel tube. Stainless steel heat treatment is generally used solution heat treatment, that is, the annealing process, the temperature range is usually 1040 ℃ to 1120 ℃. Insufficient annealing temperature will lead to incomplete treatment, while too high a temperature may damage the surface of the fittings.
- Annealing atmosphere: the selection and control of the annealing atmosphere has a direct impact on the BA tube. Generally use pure hydrogen as the annealing atmosphere, the purity of the atmosphere is best to reach 99.99% or more. The atmosphere should not contain too much oxygen or water vapour, otherwise it will have a negative impact on the brightness.
- Furnace sealing: the sealing of the furnace is essential to maintain the annealing atmosphere, the annealing furnace should be isolated from the outside air. Poor sealing of the furnace body will lead to atmosphere leakage, affecting the annealing effect. The sealing of the furnace body joints should be checked regularly to ensure that there is no air leakage.
- Protective gas pressure: the furnace to maintain a certain positive pressure is the key to prevent micro-leakage. For hydrogen protective gas, the pressure is generally required to be above 20 kBar to ensure the stability and uniformity of the atmosphere.
- Furnace water vapour: the control of water vapour in the furnace is also an important factor affecting the quality of bright annealing. Furnace materials should be dried before the first use, and at the same time into the furnace stainless steel belt should not be left too much water, in order to avoid destroying the furnace atmosphere, affecting the annealing effect.