Stainless steel 316L VS 2205 duplex in biomedicine fields

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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.

Back Shielding Welding of Stainless Steel

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    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?

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      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.

      GradesCSiMnCrMoCuNiNPS
      A387 GR220.110.350.462.211.060.120.22/0.010.006
      3040.050.621.8319.16//8.970.060.0270.015
      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.