Back Shielding Welding of Stainless Steel

The rapid development of the petrochemical industry has a higher requirement for welding of stainless steel pipes and plate, the early stainless steel welding backing to be being washed out gradually and now more using argon arc welding backing welding, with more cleanliness and higher efficient. At the same time there are also appeared some problems, namely, the welding process with argon arc welding of stainless steel base back be oxidized easily and produce defects so on the back protection measures must be taken, so maintaining the weld mechanical properties and corrosion resistance, etc., today here we introduced several kinds of commonly used stainless steel welding back shielding methods:

Back shielding with Ar

The commonly used shielding gas can be pure argon and mixed gas. In fact, a specific proportion of argon and nitrogen mixed gas is more conducive to the welding of Austenitic stainless steel. Some inert gas is not used because of the high cost. The argon filling is the most commonly used back shielding method, which is characterized by good effect, easy operation, high cleaning and high qualified rate. It can be divided into protective cover filling argon shielding, local filling argon shielding, welded junction filling argon shielding, etc.

Protection cover filling argon

Used in stainless steel sheet and large diameter pipe welding. A metal shield connected pipe and argon hose, make the shield filled with argon gas, welder handheld metal pipe as handle make the shield on the back of molten pool slide and plate or pipe welding together, such making that the back got effective protection, greatly reduce the waste of argon.

Local filling argon

Used in locally small space or short size of the pipeline. The welding joint of the pipeline should be sealed with adhesive tape (to prevent air leakage), and both ends of the pipeline should be sealed with sponge, adhesive tape or paper, etc. One end of the argon hose should be filled with argon. It is better to make a small hole at the other end of the pipe (sponge is not required), which is conducive to the final backing welding joint and will not sag due to excessive internal pressure. Disadvantages are slow filling argon and costly.

Welded junction filling argon

For too long and large pipe diameter pipelines, the cost of local argon filling is high and the quality can not be guaranteed, so the welding junction filled argon methods can be directly used. The argon shielding can be judged according to the color of the inner weld joints, and welders can adjust Argon according to the color to achieve the best protection. White and gold are the best, while gray and black are the worst. But in the process of operation, there are some tips for stainless steel back shielding:

(1) Before argon arc welding, the welding parts can be protected by filling argon with a large flow on the back in advance, and the flow gradually decreases after the air is discharged. During the welding process, fill the pipe with argon continuously and stop after the welding is complete. In addition, the welding can only be carried out after the air is cleared, otherwise, the protection effect of argon filling will be affected.

(2) Argon gas flow should be appropriate. Too small flow is not good protection, the back of the weld is easy to oxidize; Excessive flow will cause concave defects at the root of the weld and affect the welding quality.

(3) The argon inlet should be placed as low as possible in the closed section, and the air outlet should be placed slightly higher. Because argon is heavier than air, charging it from a lower position ensures a higher concentration and provides better protection.

(4) In order to reduce the argon leakage from the joint gap, adhesive tape can be used along the welding gap before welding, leaving only the length of a continuous welding for the welder, and the adhesive tape can be removed while welding.

Self-shielded welding wire

The back self – shielding wire is a kind of welding wire with a flux-cored coating. During welding, the shielding coating will penetrate into the weld pool to form a dense protective layer, so that the back of the weld bead will not be oxidized. After cooling, the protective layer will fall off automatically and will be cleaned up with the purge pressure test.

The self-shielding welding stainless steel wire is not restricted by various welding conditions, and the operation is quick and simple. But because the flux cored coating may appear smoke and poison gas, also sag and other defects, so there are certain requirements for welders. Self – shielded wire is suitable for backing welding due to high cost. The method of this welding wire is basically the same as that of ordinary solid cored argon arc welding wire, and the weld metal can meet the use requirements in performance.

Can I weld ASTM A387 Gr22 and 304 steel plate together?

The welding of dissimilar steel has a wide application in the field such as aerospace, petrochemical industry, machinery industry. The dissimilar steel is really different in chemical composition, metallurgical compatibility and physical properties and etc., which will appear of alloy element migration, uneven chemical composition and metallographic organizations in the welding process, also can produce thermal stress and welding deformation or cracks, this will reduce the mechanical properties of welded joints. In this paper, the weldability of dissimilar steel welded joints of ASTM A387 GR22 Chromoly steel plate and S30408 stainless steel plate was analyzed, and the appropriate welding methods, welding materials and welding process parameters were selected,  as well as the post-welding heat treatment.

A387 GR220.110.350.462.
Chemical composition comparation

S30408 is a commonly used Austenitic stainless steel, ASTM A387 GR22 is a low alloy heat resistant steel with good high-temperature resistance and resistance to hydrogen, mainly used in hydrogenation plant reactor and heat exchanger and other equipment. Chromium and molybdenum can significantly improve the hardenability of steel, and , the weld metal and heat-affected zone may form microstructure sensitive to cold cracking at a specific cooling rate. Progressive embrittlement occurs when the total content of hazardous residual metals exceeds the allowable limit at 350-550℃ for long periods of operation. The main difficulties we have to face are:

  • Dilution of weld

The weld metal is diluted by the deposited metal during the welding process. A transition layer is formed in the weld metal close to the fusion zone on one side of the ASTM A387 GR22 steel plate. The composition of the transition layer is different from that of the weld metal. The higher the base metal alloy content is, the higher the fusion ratio is and the higher the dilution rate is. The transition layer on the ASTM A387 GR22 side may produce a brittle Martensite structure due to dilution.

  • Carbon migration

Chromium and carbon atoms under high temperature is easy to form compounds of chromium carbide, ASTM A387 Gr22 steel plate side forms carbon atoms from decarburization area due to poor chromium in the process of welding, in turn, softening, coarse grains, increase brittleness, corrosion resistance, and S30408 side for enriching chromium and carbon atoms to form the carburization layer migration, and hardening, grain size and performance better.

  • Welding stress

Due to the different thermal conductivity and linear expansion coefficient of the two materials, thermal stress will be generated in the high temperature zone during the welding process, which cannot be eliminated, resulting in additional stress near the weld and fusion zone, and welding residual stress generated in the cooling process due to inconsistent shrinkage, resulting in cracks on the side of ASTM A387GR22 steel plate.

After knowing the possible problems, the materials for this experiment are ASTM A387GR22 and S30408 stainless steel plates, with specifications of 400mm×150mm×10mm. The chemical composition of the two materials is shown in the table:

  • Welding method

In order to reduce the dilution of welding joints and prevent cold crack and reheat crack, nickel-based alloy welding material is first surfaced on the side of ASTM A387GR22 during welding. Welding methods with small fusion ratio and low dilution rate are selected, such as argon tungsten arc welding and electrode arc welding. In this experiment, the argon arc welding is used as the backing and the welding method of arc welding cover.

  • Welding Materials

Nickel-based electrodes and wires ERNiCr-3/ENiCr-3 are used to block the formation of carbide by the graphitization of nickel, reduce the transition layer and prevent the generation of brittle martensite structure, and further inhibit the carbon migration in ASTM A387GR22 steel plate.

  • Welding groove

The type of welding groove should consider the number of welding layers, the amount of filling metal and the fusion ratio and the welding residual stress. The type and size of the designed groove are shown below:

  • Preheating and interlayer temperature control

The microstructure of ASTM A387 GR22 is tempered bainite and that of S30408 is Austenite. The former has hardenability, reheat crack tendency and tempering brittleness, while the latter has good weldability. According to the chemical composition, joint form, welding method and welding material of the materials, we determined that the preheating temperature was about 200℃, and the temperature between the welding passes was within 100℃. After welding, the heat treatment was conducted at 350℃×2h immediately.

  • Welding process parameter
Welding  layerWelding MethodsWelding wiresWelding  electrodeWelding current I/AWelding pressure U/VWelding speed v/cm
Surfacing  SMAWERNiCr-3, 4.0mmDCEP140-16023-2616-20
Spot welding/1GTAWERNiCr-3, 2.4mmDCSP120-15013-158-10
2-endSMAWERNiCr-3, 4.0mmDCEP140-16023-2616-20

Before welding, clean up the oxide layer, oil, moisture, rust, etc. within 200mm of the groove and both sides of the steel plate. The specific welding process parameters are shown in the table.

  • Post-weld stress relief heat treatment

Post-welding stress relief heat treatment is an important process to prevent welding cracks. Large welding residual stress will be generated during welding, so 690±10℃×2h heat treatment is required after welding to eliminate the welding residual stress and avoid the generation of cracks.

  • Results and analysis

We conducted an appearance inspection on the steel plate according to the welding evaluation standard for pressure bearing equipment, and found that there were no defects such as pores, slag inclusion and cracks on the surface. Then, we conducted 100% radiographic inspection and mechanical properties tests such as tensile, bending and impact. The test results are shown in the table.

ItemWidth/mmThickness/mmCSA/mm²Maximum loadTensile strength
I120.3039.72806.3507.12625 Mpa
I220.2839.78806.7482.83600 Mpa
Tensile Test

Sample No.Bend typeThickness/mmBend  diameterBend angleResults
C1Lateral bending10D=40 mm180°Qualified
C2Lateral bending10D=40 mm180°Qualified
C3Lateral bending10D=40 mm180°Qualified
Bend Test

Sample No.Sample size mmGap positionTest temperatureImpact absorbing energy/Akv
R110*10*55A387 GR22 side0℃152
R210*10*55A387 GR22 side0℃176
R310*10*55A387 GR22 side0℃122
Impuse Test

From the data above, it can be seen that the tensile, bending and impact tests are all qualified, indicating that our welding process plan is qualified, the dissimilar material steel plate welding between ASTM A387 Grade 22 and 304 are perfectly feasible.

What’s K-TIG welding?

Keyhole TIG welding, K-TIG for Short, is a new high current TIG welding technology developed by Australia CSIRO company in 2000. K-TIG welding technology is a high-speed one-side full penetration weld process, which requires no wire, no groove or no professional operators, but can weld metal up to 16mm thick and 100 times faster than conventional TIG welding.

K-TIG welding is based on the traditional TIG welding through the large current (> 300A) to form a large arc pressure and weld pool liquid metal surface tension to achieve a relative balance, forming a hole to achieve deep welding. Stable welding process, beautiful welding forming, weld microstructure and mechanical properties are better than TIG welding, is a high efficiency, high speed, low-cost welding method. The tungsten electrode diameter is greater than 6mm (commonly used diameter is 6.3-6.5mm, the Angle of the end is 60o), the welding current is up to 600~650A, the arc voltage is 16~20V, under the action of such a high specification parameters, the electromagnetic shrinkage of the arc is greatly improved, showing the arc straightness, arc force and penetration ability are significantly enhanced.

In welding, the arc “sinks” deeply into the base metal, forcing the molten metal around the molten pool to form keyholes. If the arc pressure, the reaction force of metal vapor on the sidewall of the keyhole, the surface tension of the liquid metal and the pressure inside the liquid metal reach a dynamic balance, the keyhole will exist stably. As the arc progresses, the pool metal is fused behind the arc and cooled to form a weld, similar to the plasma “keyhole” welding method.

304 Stainless steel K-TIG
1One Side Welding Both Sides FormationNoYesYesYes
2Maximum depth of fusion20mm3mm10mm16mm
3Medium and thick plate grooveNoYesNoNo
4Medium and thick plate once formingNoNoYesYes
5Apperance qualityGoodGoodNot goodGood
6Welding alignment clearance≤2≤2≤0.5≤2
7Welding ConsumablesNeed welding wire and fluxNeed more wiresNeed plasma gasNeed less wires
8Maximum welding speed400mm/min200mm/min500mm/min1000mm/min
9Arc initiation/closure defectsYesNoYesNo
10Rated load ratio60%60%60%100%
Comparation of SAW, TIG, K-TIG

The features of K-TIG welding

  • Automatic welding, simple operation, no welding wire filling, the whole process is one-way full penetration welding.
  • High efficiency, low energy consumption, fast welding speed, the speed is more than 5-10 times of ordinary TIG/GTAW. It allows weld defects to form highly stable, self-correcting full penetration weld holes in a variety of special metals. The clearance of welding parts can be up to 2 mm.
  • The medium thickness metal plate can realize single-sided welding and double-sided forming without groove opening, with beautiful welding seam, small deformation and no root cleaning on the back. 
  • It can weld carbon alloy steel, stainless steel, titanium alloy, nickel base alloy, cobalt alloy and zirconium materials, especially for large diameter Austenitic stainless steel. K-TIG deep penetration argon arc welding in 16mm titanium, 14mm zirconium, 13mm Austenitic stainless steel, Hastelloy, nickel-chromium and various nickel-cobalt alloys and 9mm conductive materials (such as Ferritic steel and carbon steel) to achieve a single side welding double side forming. When the welding speed is 250-300mm/min, it can weld through the stainless steel below 14mm at a time.