A TIG+MAG welding design of 304 Stainless steel pipe

Compared with all argon welding and argon-electric welding, the production efficiency and welding quality of stainless steel pipe TIG+MAG welding are greatly improved, and it has been widely used in power plant pipeline welding. The horizontal fixed all-position joint of 304 stainless steel large-diameter pipe is mainly used in power plant lubricating oil pipeline. It is difficult to weld and requires higher welding quality and inner surface forming. PT and RT inspection is required after welding.

TIG welding or manual arc welding has low efficiency and poor welding quality can not be guaranteed. We use TIG inner and outer filling wire welding bottom layer, MAG welding filling and cover surface layer to get good welding joints. Compared with carbon steel and low alloy steel, the thermal expansion rate and conductivity of TP304 stainless steel are larger, and the pool flow and forming are poor especially in the all-position welding. In the process of MAG welding, the extension length of the welding wire must be less than 10mm, and the appropriate welding torch swing amplitude, frequency, speed and edge retention time should be maintained. The Angle of the welding torch should be adjusted at any time to make the weld surface edge fuse neatly, good forming to ensure the quality of the filling and cover layer.

The sample TP304 steel pipe with size 530mm *11mm, manual argon tungsten arc welding backing was used, mixed gas (CO2+Ar) welding filling and cover welding, horizontal fixed all-position welding. Before welding , we should do some preparation projects:

1. Clean up dirt such as oil and rust, and polish the groove and the surrounding 10mm range;

2. Assembly according to the size, the positioning welding using the floor fixed (2, 7, 11 points for the positioning block fixed), can also use groove point solid welding;

3. The tube is protected by argon gas.

TIG welding process

Welding parameters

2.5mm WCE-20 tungsten electrode is used. The tungsten electrode extends 4~6mm without preheating, and the nozzle diameter is 12mm

Welding wireO.DWelding current I/AArc voltage U/VGas flow L/minAr purity, %Polarity
TIG-ER3082.580-9012-14Positive9-12Backing 9-399.99DCSP

Operation process

  • The horizontal fixed all-position welding of the pipe is difficult. In order to prevent the internal sag of the welding seam, the overhead position welding part (60°on both sides of six points) is used to fill the wire, and the vertical and horizontal welding parts are used to fill the wire as the backing welding.
  • Before starting the arc, the tube should be filled with argon to clean the air. In the welding process, the welding wire should not contact with the tungsten electrode or go directly into the arc column area of the arc, otherwise, tungsten inclusion will be caught in the weld seam and the arc stability will be damaged.
  • Start welding from close to 6 points to make the tungsten electrode always perpendicular to the axis of the steel pipe, which can better control the size of the molten pool, and make the nozzle evenly protect the molten pool from oxidation.
  • The extreme part of the tungsten is about 2mm away from the welding piece, and the welding wire should be sent to the front end of the welding pool along the groove. The arc is preheated at one end of the groove after ignition, and the first drop of welding wire is immediately sent to melt the metal after the metal is melted, and then the second drop of welding wire is sent to melt the metal at the other end of the groove, and then the arc swings laterally and stays on both sides for a while so that the welding wire is evenly and intermittently sent to the molten pool. At 12 points, the end is polished into a slope, and the wire is suspended when welding to the slope, it is melted into a hole closure with an arc. Attention should be paid to reduce the internal protective gas flow to 3L/min at the end of welding to prevent the weld from concave due to excessive air pressure.

MAG welding process

Welding parameters

The diameter of the nozzle is 20mm, the distance between the nozzle and the specimen is 6~8mm, the temperature between layers is less than 150℃, and the thickness of the welding seam is 11mm.

Mixing protective gas with Ar80%+CO2 20% ratio (volume) makes AR arc stable, small splash, easy to obtain axial jet transition. The oxidation of arc overcomes the defects of argon welding, such as high surface tension, thick liquid metal and easy drift of cathode spots, and improves the weld penetration depth.

Welding wireO.DWelding current I/AArc voltage U/VShielding gasGas flow L/minPolarity
E-308L1.0100-11017-19Positive 80%Ar+20%CO2,Backing Ar9-12,3DCEP

The operation process

  • Inspection before welding: Inspect the nozzle, conductive nozzle cleaning, gas flow, hit the bottom surface, temperature between layers.
  • When gas welding in the filling, cover surface layer, the length of welding wire extended will affect the stability of the welding process. Too long extension length will increase wire resistance value and wire overheating, causing splashing and poor weld forming; a too short extension length will increase the current, the distance between the nozzle and the workpiece is shortened to cause overheating, which may cause splashes to block the nozzle, thus affecting the gas flow and weld bead forming.
  • During welding, the welding gun Angle is perpendicular to the pipe axis to avoid pores and slag inclusion in the welding seam. Small amplitude swing, both sides stay slightly faster in the middle speed, which can avoid the welding seam convexly, uneven; In the welding process, uniform and appropriate swing amplitude and frequency of the welding torch should be used to ensure that the welding surface size and the edge of the cover layer are fused properly.

Stainless steel 316L VS 2205 duplex in biomedicine fields

The pharmaceutical and biotech industry has relatively high requirements about the steel materials used in processing vessel and pipeline system, that must have excellent corrosion resistance and cleanness to ensure the purity and quality of the drug product, they must also be able to tolerate the production environment and disinfection and cleaning processes of temperature, pressure and corrosion, also have good weldability and can satisfy the requirements of the industry of surface finish.

316L (UNS S31603, EN 1.4404) Austenitic stainless steel is the main material for equipment in the manufacturing of pharmaceutical and biotechnology industries. 316L stainless steel has excellent corrosion resistance, weldability, and electrolytic polishing properties, making it an ideal material for most pharmaceutical applications. Although 316L stainless steel performs well in many process environments, customers continue to improve the performance of 316L stainless steel through careful selection of specific 316L stainless steel chemical composition and the use of improved production processes such as electroslag remelting (ESR).

For highly corrosive media, customers who can accept increased maintenance costs can continue to use 316L stainless steel, or choose to use 6% molybdenum super austenitic stainless steel with higher alloy composition, such as AL-6XN® (UNS N08367) or 254 SMO® (UNS S31254, EN 1.4547). Currently, 2205 (UNS S32205, EN 1.4462) dual-phase stainless steel is also used in the manufacture of process equipment in this industry.

The microstructure of 316L stainless steel includes the Austenite phase and a very small amount of Ferrite phase, which is formed mainly by adding a sufficient amount of nickel to the alloy to stabilize the Austenite phase. The nickel content of 316L stainless steel is generally 10-11%. 2205 duplex stainless steel is formed by reducing the content of nickel to about 5% and adjusting the manganese and nitrogen added to form about 40-50% Ferrite and contains roughly the same amount of ferrite phase and austenite phase microstructure, with large to considerable corrosion resistance. The increase of nitrogen content and the fine grain microstructure of 2205 duplex stainless steel make it have higher strength than common austenitic stainless steels such as 304L and 316L. Under annealing conditions, the yield strength of 2205 duplex stainless steel is about twice that of 316L stainless steel. Due to this higher strength, the allowable stress of 2205 duple stainless steel can be much higher, depending on the design specifications for manufacturing process equipment. It can reduce wall thickness and cost in many applications. Let’s see the chemical composition and mechanical property comparison between 316L and 2205(specified in ASTM A240)

GradesUNSCMnPSSiCrNiMoN
316LS316030.032.00.0450.030.7516.0-18.010.0-14.02.0-3.00.1
2205S322050.032.00.030.021.022.0-23.04.5-6.53.0-3.50.14-0.2
GradesTensile strength, Mpa(ksi)Yield strength Mpa(ksi)ElongationHardness,HRB(HRC)
316/316L515(75)205(30)40%217(95)
2205655(95)450(65)25%29331()

Corrosivity performance

Pitting corrosion resistance

In pharmaceutical and biotechnology applications, the most common corrosion of stainless steel is pitting in chloride media. 2205 duplex stainless steel has higher chromium, molybdenum and nitrogen content, which is significantly better than 316L stainless steel in pitting and crevice corrosion resistance. The relative corrosion resistance of stainless steel can be determined by measuring the temperature (critical corrosion temperature) required for pitting in a standard test solution of 6% ferric chloride. The critical corrosion temperature (CPT) of 2205 duplex stainless steel is between 316L stainless steel and 6% molybdenum super Austenitic stainless steel. It should be noted that the CPT data measured in ferric chloride solution is a reliable ranking of the resistance to chloride ion pitting and should not be used to predict the critical corrosion temperature of the material in other chloride environments.

Stress corrosion cracking

When temperatures are higher than 150°F (60°C), 316L stainless steel is prone to crack under the combined action of tensile stress and chloride ions, and this catastrophic corrosion is known as chloride stress corrosion cracking (SCC). When selecting materials in hot fluid conditions, 316 stainless steel should be avoided in the presence of chloride ions and temperatures of 150°F (60°C) or above. As shown in the figure below, 2205 duplex stainless steel can withstand SCC at least 250°F (120°C) in a simple salt solution.

Processing properties

The machining of 2205 duplex stainless steel is similar to that of 316L in many ways, but there are still some differences. Cold forming processing must take into account the higher strength and work hardening characteristics of dual-phase stainless steel, equipment may be required to have a higher load capacity, and in operation, stainless steel 2205 will show higher resilience than standard austenitic stainless steel grades. The higher strength of 2205 duplex stainless steel makes it more difficult to cut than 316L.

2205 duplex stainless steel can be welded in the same way as 316L stainless steel. However, the heat input and interlaminar temperature must be strictly controlled to maintain the expected austenite-ferrite phase ratio and to avoid the precipitation of harmful intermetallic phases. The welding gas contains a small amount of nitrogen to avoid these problems. In the welding qualification of duplex stainless steel, the commonly used method is to evaluate the Austenite-ferrite ratio by ferrite tester or metallographic examination. The ASTM A 923 test method is typically used to verify the presence of harmful intermetallic phases. The recommended filler metal for the weld is ER2209 (UNSS39209, EN 1600). Self-fusion welding is recommended only if the weld solution annealing treatment can be performed after welding to restore corrosion resistance. It does not use filler metal. To perform solution annealing, the components are heated to a temperature of at least 1900°F (1040°C) and then rapidly cooled.

The penetration and fluidity of Duplex stainless steel 2205 are poor than 316L stainless steel, so the welding speed is slower and the shape of the joint needs to be modified. 2205 duplex stainless steel requires a wider groove Angle, a larger root clearance and a smaller blunt edge than 316L stainless steel in order to obtain a fully fused weld. If the welding equipment allows the use of filler wire, the 2209 filler wire is used to handle the track welding of 2205 stainless steel pipe, or the filler wire can be used instead of the appropriate alloying consumable insert.

Electrolytic polishing

Many pharmaceutical and biotech applications require the surface in contact with the product to be electrolytically polished, so high-quality electrolytically polished surfaces are an important material property. 2205 Duplex stainless steel can be electrolytically polished to a finish of 15 microinches (0.38 microns) or higher, which exceeds the ASME BPE standard for surface finish of electrolytically polished surfaces, but the electrolytically polished 2205 stainless steel surface is not as bright as 316L stainless steel surface. This difference is due to the slightly higher metal solubility of ferrite compared to austenite during the electropolishing process.