Tips of welding Austenitic stainless steel

Austenitic stainless steel is the most widely used type of stainless steel, mainly Cr18-Ni8, Cr25-Ni20, Cr25-Ni35 type. The welding of austenitic stainless steel has obvious characteristics:

  • Welding hot crack.

Austenitic stainless steel is easy to form a bulky columnar grain structure when the welding joint parts of high temperature and retention time is longer because of small thermal conductivity and large linear expansion coefficient. In the process of solidification, if the content sulphur, phosphorus, tin, antimony, niobium and other impurity element are higher, This leads to the formation of low melting point eutectic between grains. When the welded joint is subjected to high tensile stress, solidification cracks are easy to form in the weld seam and liquefaction cracks are easy to form in the heat-affected zone, which are welding thermal cracks. The most effective method to prevent hot crack is to reduce the impurity elements which are easy to produce low melting point eutectic in steel and welding materials and to make the Cr – Ni austenitic stainless steel contain 4% ~ 12% ferrite structure.

  • Intergranular corrosion.

According to the theory of chromium depletion, the precipitation of chromium carbide on the intergranular surface, resulting in chromium depletion at the grain boundary, is the main reason for the intergranular corrosion. Therefore, choosing ultra-low carbon grades or welding materials containing stabilized elements such as niobium and titanium are the main measures to prevent intergranular corrosion.

  • Stress corrosion cracking.

Stress corrosion cracking (SCC) is usually presented as a brittle failure, and the processing time of failure is short and the damage is serious. Welding residual stress is the main cause of stress corrosion cracking in austenitic stainless steel. The microstructure change of the welded joint or the stress concentration of local corrosive media are also the reasons.

  • σ phase embrittlement of welded joints

σ phase is a kind of brittle intermetallic compound which mainly concentrated in the grain boundary of columnar grains. For Cr-Ni austenitic stainless steel, especially for Ni-Cr-Mo stainless steel, it is prone to the δ-σ phase transition and the change will be more obvious when the δ ferrite content in weld joints by more than 12%, making obvious embrittlement in the weld metal, that is why the delta ferrite quantity of hot wall hydrogenation reactor wall surfacing layer will be controlled in 3%~10%.

What welding material is suitable for 304 stainless steel welding?

Type 308 welding material is recommended when welding 304 stainless steel because the additional elements in 308 stainless steel can better stabilize the weld zone. 308L wires are also an acceptable option.

Low carbon stainless steel carbon content is less than 0.03%, while standard stainless steel can contain up to 0.08% carbon content. Manufacturers should give special consideration to the use of L-carbon welding materials because their low carbon content reduces the tendency for intergranular corrosion. Manufacturers of GMAW welding also use 3XXSi welds such as 308LSi or 316LSi because Si improves the wetting of welds. In cases where the weldment has a high hump or where the pool connection is poor at the toe of the fillet or lap weld, using an air-shielded wire containing Si can moisten the weld and increase the deposit rate. Type 347 welding materials with a small amount of Nb can be selected if carbide precipitation is considered.

How to weld stainless steel and carbon steel?

Some structural parts are welded to the surface of carbon steel with a corrosion-resistant layer to reduce costs. When welding carbon steel to alloy base metal, the use of higher alloy content welding material can balance the dilution rate in the weld. For example, when welding carbon steel and 304 or 316 stainless steel, as well as other dissimilar stainless steels, 309L wire or electrode is a suitable choice.

If you want to get a higher Cr content, use 312 welding material. It should be pointed out that the thermal expansion rate of austenitic stainless steel is 50% higher than that of carbon steel. When welding, the difference in thermal expansion rate will produce internal stress, which will lead to the crack. In this case, it is necessary to select the appropriate welding material or specify the appropriate welding process (Fig. 1). It shows when welding carbon steel and stainless steel, the warping deformation caused by different thermal expansion rates needs more compensation.

What is the proper pre-weld preparation?

Before welding, use chlorine-free solvent to remove grease, marks and dust to avoid the corrosion resistance of stainless steel base material from carbon steel. Some companies use separate storage of stainless steel and carbon steel to avoid cross-contamination. When special grinding wheels and brushes with stainless steel are used to clean the area around the bevels, it is sometimes necessary to perform a secondary cleaning of the joints. Because the electrode compensation operation of stainless steel welding is more difficult than that of carbon steel, the joint cleaning is important.

What is the correct post-weld treatment?

First of all, let’s recall that the reason why stainless steel does not rust is that Cr and O react on the surface of the material to generate a layer of the dense oxide layer, and play a protective role. Stainless steel rust is caused by the precipitation of carbide and heating during the welding process resulting in the formation of iron oxide on the welding surface. Perfected weldments in the welding state may also produce undercut in the rusted area at the boundary of the welding heat-affected zone within 24 hours. Therefore, in order to regenerate the new chromium oxide, stainless steel needs to be polished, pickled, sanded, or washed after welding.

How to control carbide precipitation in Austenitic stainless steel?

When the carbon content exceeds 0.02% at 800-1600℉, C diffuses to Austenitic grain boundaries and reacts with Cr at grain boundaries to form chromium carbides. If a large amount of Cr is cured by element C, the corrosion resistance of stainless steel will decrease, and intergranular corrosion will occur when exposed to a corrosive environment. The experimental results show that intergranular corrosion occurs in the heat-affected zone of welding in the water tank with corrosive media. Using low carbon or special alloy welding materials can reduce the tendency of carbide precipitation and enhance corrosion resistance. Nb and Ti can also be added to solidify C. Compared with Cr, elements Nb and Ti have a greater affinity with C. The grade347 welding material is designed for this purpose.

Why are stainless steel wires magnetic?

Stainless steels with full Austenitic structure are non-magnetic. However, the higher welding temperature makes the grains in the microstructure grow larger and the susceptibility to crack increases after welding. To reduce thermal crack sensitivity, the welding consumable manufacturer adds ferrite forming elements to the welding material (Fig. 2). The ferrite phase reduces the austenite grain size and increases the crack resistance. The following picture shows the ferrite phase (gray part) distributed on the austenite matrix in 309L welding material.

The magnet does not adhere firmly to the Austenitic weld metal, but a slight suction can be felt when thrown. This also leads some users to believe that the product is mislabeled or that the wrong solder material is used (especially when the label is removed from the package). The amount of ferrite in the welding material depends on the service temperature of the application. Excess ferrite, for example, reduces toughness at low temperatures. As a result, the ferrite quantity for grade 308 welding materials used in LNG pipelines is between 3 and 6, while the ferrite count for standard Type 308 welding materials is 8. In short, the welding materials may look similar, but even small differences in composition can sometimes make a big difference.

How to avoid the porosity of self-shielded flux-cored welding wires

In the last article we introduced what is copper-free welding wire and its advantages. As we know, there are mainly two kinds of welding wire according to its protection: One is the welding wire that relying on flux or gas protection, the welding wire plays as filling metal and conducting electricity, such as submerged arc welding, solid cored welding wire and part of flux-cored welding wire used in CO2 gas shielded welding; The other kind is the flux-cored welding wire without external gas protection, it relies on the alloy elements of the wire itself and high temperature to prevent the invasion of oxygen, nitrogen and other gases in the air and adjust the composition of the weld metal, which is called self-shielding flux-cored wire, is a kind of a little expensive but potential welding wire.

At present, self-shielding flux-cored wire is widely used in pipeline construction, ocean engineering, outdoor large steel structure manufacturing, high-rise steel structure building, surface surfacing, especially the welding of light structures such as thin carbon steel and galvanized steel plate. The self-shielding flux-cored wire protects the droplet and molten pool by the gas and slag produced by the slag-forming and gas-forming agent in the arc core under the action of high temperature, and the welding porosity or welding pores are a common problem in the semi-automatic welding of self-shielding flux-cored wire, so we analyze and make some control measures to avoid them.

The cause of welding pores for the self-shielded flux-cored welding wires

Welding cooling rate

Due to the gravity of the liquid metal itself in the vertical welding section, the welding speed is faster and the weld pass melting depth is shallow, which speeds up the cooling rate of the liquid metal in the weld, reduces the gas escape, and causes more pores in the weld pass.

Welding spatter

When the metal oxide spatter adhered to the front end of the conductive nozzle reaches a certain amount, it enters the molten pool with the moving welding wire. This becomes more serious with the increase of the amount of metal in the weld pass, resulting in the occurrence of porosity in the weld pass.

Weld joint

The weld joint of the hot welding layer, filling layer and cover layer is easy to superposition, which increases the chance of dense pores in the weld bead.

External environment

When the welding wire is placed in an open-air environment with high humidity, which is easy to cause the welding wire to be damp. In addition, if no wind protection measures are taken when the wind speed is greater than 8m/s, it is also an important reason for the occurrence of pores in the weld pass.

Welding process parameters

If there is a narrow adjusting range of welding process parameters of the semi-automatic welding of self-shielding flux-cored. Generally, the arc voltage is between 18 and 22V, and the wire feeding speed is between 2000 and 2300mm/min. Otherwise, the high voltage is easy to cause the slag protection effect on the weld pass surface is not good, easy to produce pores.


How to avoid the welding pores?

  • Adjust the arc voltage and welding parameters before welding.

The welding power supply adopts DC and inverter power supply, DC direct connection (DC-): the welding parts are connected to the positive pole of the power supply, and the welding gun is connected to the negative pole of the power supply. The welding ground wire is close to the welding area as far as possible, and it should be confirmed that the conduction is good (whether the ground wire is oxidized, whether the connection is firm, and there can be no rust in the contact place between the ground wire and the base metal). If the conduction is not good, it will cause arc instability.

The welding parameters directly affect welding quality. Too small current is easy to cause the incomplete fusion, slag and other defects, while too large current is easy to cause the burn through, splash increase, down to the welding caused by slag and molten iron drip, can not be applied to welding, also easy to appear pores. Voltage is too low, it’s easy to cause arc instability, top wire, incomplete molten pool and slag inclusion. Voltage is too high, the arc is too far from the molten pool, air involved in the molten pool, and holes occurs.

Specifications Size PackagingPolarity
AWS A5.20 E71T-11
AWS A5.20 E71T-GS
0.8mm
0.9mm
1.0mm
1kg
5kg
DC- connection, positive grounding wire, negative welding gun
  • Angle of welding torch

Before welding the cover layer, if the filling layer in the vertical welding section is too low or too high, it shall be trimmed until the welding height of the filling layer is about 0.5~1.0mm lower than the base metal, before the welding of the next procedure can be carried out.

  • Control the extension length and Angle of the welding wire

Generally should be controlled in 6 ~ 10 times the diameter of the welding wire, generally 15~20mm, such as dry elongation is too long, will make the welding wire melting too fast, reduce the arc blowing force. Too short will cause the metal oxide spatter at the front of the conductive nozzle to accumulate too fast; Too long will reduce the arc voltage and affect the quality of welding. In addition, you need to check and clean the conductive nozzle before welding. The Angle of the welding wire is generally required to maintain 800 ~ 900 between the welding wire and the workpiece to avoid the downward flow of molten slag and molten iron near the vertical position, which affects the smooth welding operation and is prone to defects such as slag inclusion and porosity.

  • Necessary preparation before welding.

The surface of the welded parts should be uniform and smooth, and there should be no rust, slag, grease and other harmful substances that affect the welding quality.

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.