Section 1. General Principles for Selection of Welding Materials
To obtain high-quality welded joints, the selection of welding materials must be reasonable. Due to the great differences in the operating conditions of welded components, the material properties and composition of the base material vary greatly, and the manufacturing process of components is complex and diverse.
Therefore, it is necessary to comprehensively consider various aspects to determine the corresponding welding materials.
The selection of welding materials must follow the following principles:
(1) Meet the performance requirements of welding joints, including short-term strength at room temperature and high temperature, flexural performance, impact resistance, hardness, chemical composition and special performance requirements for joints in technical standards and design drawings design, such as long-time resistance, creep limit, high temperature oxidation resistance, corrosion resistance, etc.
(2) Meet the requirements of manufacturing process performance and welding process performance of welded joints.
The components that make up the welded joint inevitably require various forming and cutting processes during the manufacturing process, such as stamping, rolling, bending, turning, planing, etc., requiring the welded joint to have a certain plastic deformation capacity, cutting, high-temperature comprehensive performance, etc.
The welding process requires good process performance of the welding material and the ability to resist defects such as cracking according to differences in the welding properties of the base material.
(3) Reasonable economy.
Although they meet the minimum requirements for various performance and manufacturing performance mentioned above, cheap welding materials should be chosen to reduce manufacturing costs and increase economic benefits.
For example, when welding low carbon steel for important components using manual arc welding, alkaline coated electrodes should be preferred because they are fully deoxidized, desulfurized and have low hydrogen content, with good crack resistance and impact toughness of the metal. solder.
For some non-critical components, acidic electrodes can be used because they can still meet the performance requirements of non-critical components, have good processability and are cheap, which can reduce manufacturing costs.
Section 2. Selection of welding materials for carbon steel and low alloy steel
When selecting welding materials for carbon steel and low-alloy steel (including heat-resistant low-alloy steel and high-strength low-alloy steel), the following factors should be considered:
(1) Principles of equal strength and equal toughness
For pressure-bearing components, strength calculations are generally based on the allowable tensile stress of the material.
The allowable tensile stress is related to the lower limit of the standard tensile strength of the material, that is, the allowable stress (σ) = σ b /n b (the values of n b vary according to different standards), where (σ ) is the allowable tensile stress of the material, σ b is the lower limit of the standard tensile strength of the material, and nb is the safety factor (values of nb vary according to different standards).
Therefore, as part of the component, the tensile strength of the weld should not be less than the lower limit of the standard tensile strength of the base material.
At the same time, attention should be paid to the fact that the tensile strength of the deposited metal of the welding material should not be much higher than the tensile strength of the base material, which may lead to a reduction in the plasticity of the weld and an increase in hardness, which is not conducive to subsequent manufacturing processes.
Although strength calculations only consider the tensile strength of the material and various process evaluation standards do not require the yield strength of the weld, when selecting welding materials, the yield strength of the deposited metal of the welding material must also be considered as not lower than the yield strength of the base material, and attention should be paid to ensure a certain relationship between yield and tensile strength.
When the joint operates at high temperatures, the calculation of allowable stress is generally based on the lower limit of the high-temperature short-term tensile strength specified by the material at the working temperature (or design temperature), that is, (σ t ). = σ b t /n b where (σ t ) is the allowable stress calculated based on the lower limit of high-temperature short-term tensile strength at temperature t, σ b t is the lower limit of short-term tensile strength at a high temperature specified by the material at temperature t, or the allowable stress is calculated based on the long-term strength and creep limit of the material at the working temperature, that is, ( σ D t ) = σ D t /n D where (σ D t ) is the allowable stress calculated based on the long-term resistance at temperature t, σ D t is the long-term resistance of the material at temperature ten D is the safety factor (values of n D vary from according to different standards).
Therefore, when selecting welding materials for welded joints operating at high temperature, their short-term high-temperature tensile strength or long-term strength should not be lower than the corresponding values of the base material.
For carbon steel and common low-alloy steel, the selection of welding materials mainly considers the tensile strength of the welding material, and the chemical composition correspondence between the deposited metal and the base metal may not be considered.
However, for Cr-Mo heat-resistant steel, the selection of welding materials should not only consider their equal strength, but also consider the combination of alloying elements to ensure that the comprehensive performance of the welded joint is consistent with the metal base.
In special cases where components are designed based on the yield strength of the material, the principle of equal yield strength should be an important factor to consider.
Due to the different operating conditions of components, brittle fracture often occurs during operation due to insufficient toughness, especially for components working at low temperatures or high-strength thick-walled components.
Therefore, relevant standards have clear requirements for the impact resistance of welded joints. When selecting welding materials, it is necessary to ensure that the impact resistance of the weld meets the requirements of relevant standards.
However, different standards have different requirements for the impact resistance of the joint. The Steam Boiler Safety Supervision Regulations stipulate that the impact resistance of the welded joint must not be less than the lower limit of the impact resistance specified by the base material.
If the base material does not have an impact resistance index, it must not be less than 27J. The GB150 standard for steel pressure vessels specifies that the impact resistance value of the joint is determined according to the lowest tensile strength of the steel. For carbon steel and low alloy steel, the minimum joint impact toughness is:
- When the lowest tensile strength of steel ≤450MPa, the minimum impact strength of the joint is 18J;
- When the lowest tensile strength of steel is >450-515MPa, the minimum impact strength of the joint is 20J;
- When the lowest tensile strength of steel is >515-655 MPa, the minimum impact strength of the joint is 27J.
For low-temperature vessels, the impact resistance value should not be less than the lower limit of the specified value of the base material.
However, ASME VIII-1 regulation determines whether the joint needs to guarantee impact resistance performance based on the strength level, thickness, working temperature, and the relationship between the design stress and the allowable stress of the material.
If the joint has impact resistance requirements, the minimum guaranteed impact resistance value is specified based on the resistance level and material thickness.
In summary, when selecting welding materials, we must determine the requirements for the impact resistance of the joint in accordance with product design, manufacturing and inspection standards, and select suitable welding materials to meet the standard requirements, that is, the requirements for usage performance.
When considering impact resistance requirements, attention should be paid to the design temperature and operating temperature of the structure.
If the operating temperature is equal to or greater than the ambient temperature, only the impact resistance of the joint at ambient temperature needs to be maintained; if it is below the ambient temperature, the impact resistance value specified in the standard or drawing at the corresponding temperature must be guaranteed.
Of course, the performance of the welded joint is not only related to the welding materials, but also to the specific welding process.
Therefore, the selection of welding materials for the joint is a complicated matter.
(2) Consider the requirements and impacts of manufacturing processes
After components are welded, they generally need to go through several forming processes such as rolling, pressing, bending and calibration.
Therefore, welded joints and base materials must have a certain deformation capacity, especially the cold deformation capacity, which is measured by the joint bending test. Many standards have established clear requirements for the flexural testing of welded joints of various materials.
The “Steam Boiler Safety Technical Supervision Regulation” stipulates that the bending shaft diameter D = 3a (a is the thickness of the sample) during the bending test, and the carbon steel is qualified for a bending angle of 180°, while the low – alloy steel is qualified to 100°.
GB150-99 steel pressure vessels and ASME section IX stipulate that when any material is subjected to bending test, the bending axis diameter D=4a and the bending angle of 180° are qualified.
Therefore, when selecting welding materials, the bending performance of the weld metal must meet the requirements of the above standards.
In addition, the selection of welding materials should also consider the effects of post-welding heat treatment processes (such as post-welding annealing, normalizing, quenching and tempering, etc.) on the properties of the weld metal.
It should be noted that post-weld annealing heat treatment, especially post-weld normalizing, can cause significant changes in the properties of the weld metal. When the welding component is relatively thin, stress relief heat treatment after welding is not necessary.
As long as the performance of the weld metal in the welded condition meets the relevant requirements. For thick-walled welding components, according to relevant manufacturing standards, stress relief annealing should be carried out after welding if the wall thickness exceeds a certain limit.
Different heating temperatures and holding times during heat treatment will lead to different changes in the properties of the weld metal.
In engineering, the Larson-Miller parameter, also known as the tempering parameter, is used to discuss joint properties affected by heating temperature and stress relief annealing retention time. The formula for the tempering parameter is:
(P)=T(20+logt)×10 -3
Where T is the absolute temperature in Kelvin and t is the time in hours.
Quenching parameters〔P〕=T（20+Logt）×10 -3
Generally, as the value (P) increases, the tensile strength and yield strength of the weld metal decrease, the elongation increases, and the impact strength fluctuates.
Figures 1 and 2 show the relationship between the tempering parameters of the deposited metal and the mechanical properties of the CMA96 and CMA106 welding rods, respectively.
Therefore, when selecting post-welding heat treatment for welding materials, it is necessary to consider whether the mechanical properties of the deposited metal at the corresponding (P) value meet relevant standards.
It should be noted that when the welded joint needs to undergo hot stamping, hot calibration, hot rolling or other hot forming processes after welding, if the heating temperature reaches above the AC3 temperature of the material and is maintained for a period of time before cooling in still air, the cooling rate during the normalization process is much slower than that during the welding process.
The standardized process will cause the weld metal to remain at 800-500°C longer than during the welding process.
Allowing the steel to be heated above AC3 during the normalizing process will cause complete austenitization, followed by recrystallization during cooling, which destroys the originally supercooled structure of the weld metal and greatly reduces the strength of the weld.
The most severe reduction can exceed 100 MPa. Therefore, for welded joints that need to undergo hot forming processes, the selected welding material must have a resistance level 50-100 MPa higher than that of the welded material in the welded condition or with stress relief treatment.
For example, for 19Mn6, the submerged arc welding wire in the as-welded condition is H08MnMO, while for normalized and tempered conditions, H08Mn2Mo should be used.
For SA675, a 300,000 kW steam drum lifting rod material with a minimum tensile strength of 485 MPa, J507 welding rod is typically used for manual arc welding.
However, in the case of welded joints in bent sections that undergo hot bending and normalizing treatment, J607 is recommended based on experimental results.
When selecting welding materials for welded joints subjected to normalizing and tempering treatments, not only must it be considered that the strength increases by 50-100MPa above usual conditions, but also the chemical composition of the weld metal must be equivalent to that of the material base. This is because the alloy composition and content determine the AC3 temperature of the material.
If the chemical composition of the weld metal and the base material differs greatly, the AC3 temperature will also be very different. When the base material and weld metal are normalized together, it is impossible to determine the appropriate normalization temperature.
Furthermore, if the welded joint requires quenching and tempering treatment, the impact of such treatment on the performance of the joint must also be considered. The strength of the welding material for quenched and tempered joints may be lower than that for standard and tempered joints.
For example, for BHW35, H10Mn2NiMo is used after arc welding and normalizing, while for quenching and tempering treatment, H10Mn2Mo can be used instead.
Consider the weldability of materials and the metallurgical characteristics of welding methods. Different materials have different weldability and there are different requirements for certain key element contents. When selecting welding materials, the weldability of the material must be considered.
For example, the weld metal of 2.25Cr-1Mo heat-resistant steel may experience the so-called quenching embrittlement phenomenon when maintaining or slowly cooling in the temperature range of 332-432°C, which causes a significant increase in temperature brittle transition of the weld metal.
Studies have shown that the sensitivity to tempering embrittlement of this type of weld metal is caused by impurities of P, As, Sb and Sn that deviate at grain boundaries. It is generally believed that the low-temperature quenching embrittlement of weld metal is related to the P and Si content. The P and Si content in the weld metal should be reduced to P≤0.015% and Si ≤0.15% .
Therefore, for Cr-Mo heat-resistant steel submerged arc welding, HJ350 welding flux with medium manganese and medium silicon should be selected instead of HJ431 combined with H08Cr3MnMoA wire. The sensitivity to quench embrittlement of the weld metal depends on the alloy series of the weld metal. Likewise, weld metals in the C-Mo, Mn-Mo and Mn-Ni-Mo series also present quenching embrittlement problems.
Welding materials with corresponding HJ350 welding flux should be used for submerged arc welding wire of the above-mentioned series to reduce the Si content in the weld metal. For example, H08Mn2Mo submerged arc welding wire should be combined with HJ350 welding flux for BHW35 welding. If higher impact resistance of the weld metal is required, the welding flux should also be HJ250 or HJ250+HJ350 mixed flux.
However, for welding wires with low silicon content, such as H08MnA and H10Mn2, there is no quenching embrittlement phenomenon in the weld metal. These two types of welding wires should be used with HJ431 high silicon high manganese welding flux when welding 20# or 16Mn steel.
When using high manganese and high silicon welding flux, the welding puddle will be siliconized, and a certain amount of silicon content in the weld metal is beneficial to the deoxidation process of the weld metal, preventing the occurrence of pores . When selecting welding materials, the metallurgical characteristics of different welding methods must also be considered.
For example, for gas metal arc welding with CO2 or CO2+Ar as shielding gas, there is no metallurgical reaction between the flux or welding wire and the metal during the welding process. However, there can be a reaction between CO2 and metallic elements to form iron oxide FeO.
Therefore, the welding wire must contain adequate amounts of silicon and manganese to reduce the reduction reaction and ensure the formation of a dense weld structure. In tungsten inert gas welding, there is no oxidation-reduction reaction and the filler wire and base material are actually melted again.
Therefore, the argon arc welding wire must be fully deoxidized, and boiling steel materials must not be used. Otherwise, pores will occur in the weld. Instead, calm steel material should be used, and it is not necessary to have a certain content of Si and Mn in the welding wire.
For example, when using 15CrMo heat-resistant steel for argon arc welding, H08CrMo welding wire should be selected; while for gas shielded fusion electrode welding, H08CrMnSiMo welding wire should be chosen.
Section 3. Selection of welding materials for austenitic stainless steel
The principle of the same strength of welding materials and original materials is not entirely applicable to austenitic stainless steel. When used in corrosive environments without specific strength requirements, the main concern is the anticorrosive properties of the welded joint.
If used in high temperature and high pressure conditions with short-term work, certain high-temperature and short-term resistance is required, while long-term work requires sufficient durable strength and creep limit of the weld metal.
For example, when SA213-TP304H pipes are used in high pressure and high temperature conditions, E308H welding materials should be selected.
When welding austenitic stainless steel, the selection of welding materials mainly considers that the chemical composition of the deposited metal must be equivalent to that of the base material.
As long as the chemical composition of the deposited metal of the welding material is the same as that of the base material, the performance of the weld metal can be equivalent to that of the base material, including mechanical properties, corrosion resistance, etc.
Special attention should be paid to special corrosion resistance requirements in manufacturing process conditions or designs.
To avoid intergranular cracks during welding, it is best to use stainless steel welding materials that are low in carbon (ultra-low carbon) and contain Ti and Nb.
If the SO2 content in the welding rod coating or flux is too high, it is not suitable for welding high nickel content austenitic steels.
To avoid hot cracking of the weld (solidification cracks), the content of impurities such as P, S, Sb and Sn must be controlled, and it is preferable to avoid the formation of a single-phase austenite structure in the weld metal as much as possible.
Although many materials suggest that the ferrite content in the weld metal of austenitic stainless steel is beneficial to reducing the cracking tendency of the weld metal, a large amount of pure austenitic stainless steel weld metal has been used for many years and the together they have performed well.
Adequate ferrite content is advantageous for corrosion resistance in certain media, but detrimental to weld metal impact under low temperature conditions.
Taking comprehensive factors into consideration, it is generally desirable that the ferrite content in austenitic stainless steel is between 4% and 12%, because a ferrite content of 5% can achieve satisfactory resistance to intergranular corrosion.
The ferrite content in the weld can be estimated through the chemical composition of the weld metal, converted into Cr equivalent and Ni equivalent, using a microstructure graph.
Commonly used charts include WRC-1988, Esptein and DeLong.
The WRC-1988 table is suitable for 300 series stainless steel and duplex stainless steel, but not applicable to materials with N>0.2% and Mn>10%. The Epstein chart is suitable for 200 series nitrogen reinforced austenitic stainless steel with Mn<1.5% and N<0.25%.
When selecting austenitic stainless steel welding materials, attention should be paid to the influence of welding methods on the chemical composition of the deposited metal. Tungsten inert gas welding has the least effect on changing the chemical composition of the weld metal, and the other changes except C and N are small in the undiluted weld metal.
In particular, the loss of C is the largest. For example, when the C content of the electrode is 0.06%, the content in the undiluted weld metal of argon arc welding is 0.04%, and the N content in the weld metal increases by about 0. .02%.
The content of Mn, Si, Cr, Ni and Mo in the deposited metal may undergo small changes during gas shielded arc welding with melting electrode, while the loss of C is only 1/4 of that of gas shielded arc welding. argon, and the increase in N content is much greater. The amount of increase differs according to different welding processes, up to a maximum of 0.15%.
During manual arc welding and automatic submerged arc welding, the alloying elements in the weld metal are jointly affected by the coating, flux, welding wire and electrode.
Especially for welding materials with transition of alloying elements through coating or flux, it is impossible to estimate the chemical composition of the weld metal from the chemical composition of the welding wire or electrode.
Of course, the ferrite content in the weld can be estimated from the alloy content in the weld metal, but this estimated value has a certain deviation from the actual value because the cooling rate during the welding process also affects the ferrite content. .
It is generally accepted that if the alloying element content in the weld metal is exactly the same, the ferrite content will be different depending on the welding method.
The ferrite content is highest in strip coating and lowest in argon arc welding. Even with the same strip coating, it was found that the ferrite content at the beginning and end of the weld was about 2-3% lower than that in the intermediate segment.
With the standardization of stainless steel materials and welding materials, the selection of austenitic stainless steel welding materials has become simple. Corresponding welding material grades can be selected based on stainless steel material types, such as selecting E316 electrodes for SA240-316 stainless steel.
Section 4. Selection of welding materials for martensitic stainless steel and ferritic stainless steel.
For martensitic stainless steel, it is best to use welding materials that are the same as the base material. For example, 1Cr13 steel must use E410 series welding materials, and the welding electrode number for manual arc welding is G217.
However, the weld metal structure of common welding materials corresponding to 1Cr13 has coarse martensite and ferrite, which are hard, brittle and prone to cracking. Furthermore, welding must be preheated to 250-350°C.
To improve performance, the S and P content in welding materials should be limited, the Si content should be controlled (≤0.30%), and the C content should be reduced. A small amount of Ti, Al and Ni can be added to refine the grain and reduce hardenability.
Some data shows that adding Nb content (up to about 0.8%) to welding materials can obtain a single-phase ferrite structure. In CO2 welding wire, Ti and Mn elements must be added to achieve the purpose of deoxidation.
Martensitic stainless steel can also use austenitic stainless steel welding materials. At this point, the influence of the dilution of the base metal on the composition of the weld metal must be considered. By the appropriate content of Cr and Ni, the formation of martensite structure in the weld metal can be avoided. For example, A312 (E309Mo) welding materials can be used to weld 1Cr13 martensitic steel.
For ferritic stainless steel, it is generally welded with welding materials that are the same as the base material. However, the ferrite structure of the solder is coarse and has low toughness. The microstructure of quenched ferrite can be improved by increasing the Nb content in the welding materials.
Meanwhile, heat treatment can be used to improve the toughness of the weld metal. For ferritic stainless steel that cannot be heat treated after welding, pure austenitic welding materials can also be used to obtain welded joints with comprehensive properties.
Section 5. Selection of welding materials for different steel of the same material, low carbon steel and low alloy steel
Welding between low carbon steels and low alloy steels, both belonging to common ferritic steel, as well as welding between different low alloy steels, belongs to the welding of different steels of the same material.
To weld these steels, welding materials are chosen based on the lowest quality material, referring to the lowest level of strength or the lowest content of alloying elements, in order to ensure that the metallurgical properties of the weld can meet the requirements of lower quality materials.
Selecting lower quality materials also provides better welding performance at a relatively cheaper price, which is beneficial to reduce manufacturing costs.
For example, when welding different steels of the same material to 20# steel, SA106 carbon steel, 16Mn, 19Mn6, 15MnMoV, BHW35 and other low alloy steels, the welding materials used are completely identical to those used to weld the low carbon steel itself.
The corresponding welding materials for manual arc welding, submerged arc welding and gas shielded welding are J507, H08MnA+HJ431 and H08Mn2Si, respectively.
Welding heat-resistant steel from low-alloy steel and heat-resistant steel from medium-alloy steel
Due to the discontinuity of the chemical composition of the weld seam in the same different steel material, there will be a corresponding discontinuity in performance. If this discontinuity significantly affects the usage performance, then welding materials cannot be selected based on low-grade principles.
For example, when welding SA213-T91 and SA213-T22 materials, choosing 2.25Cr-1Mo welding materials for welding according to the usual lower grade principle would result in severe carbon enrichment and decarburization near the base metal T91 of the fusion line on the T91 side.
This is because T91 contains about 9% chromium, while 2.25Cr-1Mo welding wire contains about 2.25% carbon.
After post-weld annealing treatment, the chromium content in the heat-affected zone on the T91 side is much higher than that on the weld bead side, causing a large amount of carbon to migrate toward the base metal and resulting in layers of carbon enrichment, which increase hardness and cause an even harder microstructure.
On the other hand, the weld bead side suffers severe decarburization, with lower hardness and softer microstructure, leading to degradation of joint performance.
If 9Cr-1Mo welding material is chosen, the weld seam on the T22 side will undergo carbon enrichment and decarburization of the base material. It should be noted that when components with such chemical composition discontinuities operate at high temperatures, carbon migration continues for a long time, severely deteriorating joint performance and causing operational failures.
Studies have shown that to avoid or reduce the above phenomena, welding materials with intermediate chemical compositions of 5Cr-1Mo can be used for welding, or carbide stabilizing elements such as Nb and V can be added to the welding materials to solidify the carbon element and reduce the occurrence of carbon diversion.
In preliminary experiments conducted by a domestic company, using T91 welding materials containing Nb and V, such as CM-9cb, TGS-9cb and MGS-9cb, to weld the same material as above, different steel, produced good results.
Section 6. Selection of Welding Materials for Welding Steels Other than Carbon Steel, Low Alloy Steel, and Austenitic Stainless Steel
When welding steel joints other than carbon steel, low alloy steel and austenitic stainless steel, the selection of welding materials should be based on the joint working temperature and stress conditions.
For dissimilar steel joints that withstand pressure and operate at temperatures below 315°C, welding materials with high content of Cr and Ni alloys in austenitic stainless steel can be used. Based on the chemical composition of carbon steel (alloy steel) and austenitic steel, as well as the size of the melting rate, suitable austenitic stainless steel welding materials with appropriate Cr and Ni contents are selected according to a certain equivalent of nickel and chromium equivalent structure diagram to avoid the formation of martensite in large quantities in the weld.
Of course, near the melting line of carbon steel or low alloy steel, small martensitic zones may occur. By reducing the carbon content of the welding material, the martensitic structure can become low-carbon martensite with better plasticity, which can ensure good joint performance.
For dissimilar steel joints that bear pressure and operate at temperatures above 315°C, nickel-based welding materials must be used. For example, ECrNiFe-2, ERCrNiFe-3, etc. The main reason is that the use of common austenitic stainless steel welding materials will cause the following problems:
a) Due to the significant difference in the coefficient of thermal expansion between ferrite and austenite, thermal stress and thermal fatigue damage may occur during high-temperature operation.
b) Due to the large difference in alloying element content, severe decarburization layers and carbon enrichment may occur in the welded joint under high temperature operation, leading to the deterioration of high temperature performance.
c) Due to the structure of the martensitic zone close to the fusion line, the local microstructure of the weld becomes tempered and hardened.
The use of nickel-based welding materials can avoid the above phenomena. This is because:
a) The thermal expansion coefficient of nickel-based materials is between that of ferrite and austenite.
b) Nickel-based materials will not cause decarburization or carbon enrichment in the welded joint.
c) Nickel-based materials will not produce martensite structure during welding.
This greatly improves the performance of the joint at high temperatures.
However, for pressureless welded joints operating at high temperatures, although the use of nickel-based electrodes can meet the performance requirements, the manufacturing cost is expensive and there is no need for their use.
Other cheaper welding materials can also achieve the same purpose. Through a large number of experimental studies, foreign countries have found that for pressureless fillet welds in boiler manufacturing, when the tube is made of carbon steel or low alloy steel and the fitting is made of austenitic stainless steel, the materials of Welding should be selected according to lower grade principles.
For example, when welding SA210C pipes and SA240-304 fittings, AWS E7018-A1 (GB E5018-A1) can be used for manual arc welding, and MGS-M or TGS-M (KOBE welding materials) can be used for gas protection. welding instead of using austenitic stainless steel welding materials.
The main reason is that the use of austenitic stainless steel welding material will produce a martensite zone close to the fusion line on the pipe side, and if cracks occur on the pipe side during operation, it will cause pipe leakage. However, the use of common low-quality welding materials will produce martensite zones close to the fusion line on the fastening side. Even if cracks occur, they will not damage the pipe on the clamping side.
On the other hand, when the pipe is made of austenitic stainless steel and the fixture is made of low carbon steel or low alloy steel, the welding material E309Mo(L) should be used to make the martensite zone occur close to the fusion line on the fastening side.
These principles were applied in the production of 300,000 kW and 600,000 kW surface heating tubes and were officially applied in the production of 200,000 kW surface heating tubes.
