Technical points of the stainless steel welding process
The stainless steel welded tube is formed on the welded tube forming machine by rolling and welding the stainless steel plate through the Ruo main road mold.
Because stainless steel has high strength and its structure is a face-centered cubic lattice, it is easy to form work hardening, therefore, when forming a welded tube:
On the one hand, the mold must withstand great friction, so that it is easy to use;
On the other hand, the stainless steel sheet is easy to form adhesion (bite) with the die surface, resulting in deformation of the welded tube and die surface.
Therefore, a good stainless steel forming die must have high wear resistance and anti-sticking (bite) performance.
Our analysis of imported welded pipe dies shows that the surface treatment of these dies is super-hard carbide or nitride coating.
Compared with traditional fusion welding, laser welding and high frequency welding have the characteristics of fast welding speed, high energy density and small heat input.
Therefore, the heat-affected zone is narrow, the degree of grain growth is small, the welding deformation is small, and the cold working formability is good.
It is easy to perform automatic welding and single-pass penetration on thick plates. The most important feature is that I-slot butt welding does not need filler materials.
Welding technology is mainly applied to metal-based materials.
Common welding technologies include electric arc welding, argon arc welding, CO2 shield welding, oxygen acetylene welding, laser welding, electroslag pressure welding and so on.
Plastics and other non-metallic materials can also be welded.
There are more than 40 metal welding methods, which are mainly divided into three categories: fusion welding, pressure welding and brazing.
Fusion welding is a method of heating the part interface to the molten state during the welding process and completing the welding without pressure.
During fusion welding, the heat source will quickly heat and melt the interface between the two parts to be welded to form a molten pool.
The molten pool moves forward with the heat source, forms a continuous weld after cooling, and connects the two parts into one.
In the fusion welding process, if the atmosphere is in direct contact with the high-temperature molten pool, the oxygen in the atmosphere will oxidize metals and various alloy elements.
When nitrogen and water vapor from the atmosphere enter the weld pool, defects such as pores, slag inclusions and cracks will be formed in the weld during subsequent cooling, which will deteriorate the quality and performance of the weld.
Pressure welding is the combining of atoms between two parts in the solid state under the condition of pressure, also known as solid state welding.
The commonly used pressure welding process is resistance butt welding. When current passes through the connecting end of two workpieces, the temperature increases due to the large resistance. When heated to a plastic state, it fuses into one under the action of axial pressure.
The common feature of various pressure welding methods is to apply pressure without filler material in the welding process.
Most pressure welding methods such as diffusion welding, high frequency welding and cold pressure welding do not have a melting process, so there are no problems such as burning of beneficial alloy elements and intrusion of harmful elements into the weld, which simplifies the welding process. and improves welding safety and health conditions.
At the same time, because the heating temperature is lower than that of fusion welding and the heating time is short, the heat-affected zone is small.
Many materials that are difficult to weld by fusion welding can often be pressure welded into high-quality joints with the same strength as the base metal.
Brazing is a method of using metallic materials with a melting point lower than that of the part as solder, heating the part and solder to a temperature higher than the melting point of the solder and lower than the melting point of the part, wetting the part with liquid solder , filling the interface gap and realizing mutual diffusion between the atoms with the workpiece, so as to realize welding.
The joint between two connected bodies formed during welding is called a weld.
Both sides of the weld will be affected by welding heat during welding, resulting in changes in microstructure and properties. This area is called the heat-affected zone.
During welding, due to different workpiece materials, welding materials and welding current, overheating, embrittlement, hardening or softening may occur in the weld and in the heat-affected zone after welding, which also reduces the welding performance and worsens weldability.
This requires adjusting the welding conditions. Preheating at the welding interface before welding, heat preservation during welding and post-welding heat treatment can improve the welding quality of welding.
Types of stainless steel pipe welding technology
Welding process adopted:
Small specifications can prevent intergranular corrosion, thermal cracking and deformation, and the welding current is 20% lower than that of low-carbon steel;
To ensure stable arc combustion, DC reverse connection is adopted;
The short arc welding arc must be stopped slowly, the arc well must be filled, and the surface in contact with the medium must be finally welded;
During multilayer welding, the temperature between layers must be controlled and forced cooling can be adopted after welding;
Do not start the arc outside the groove and the ground wire must be well connected;
Deformation after welding can only be corrected by cold working.
1) Argon arc welding
When argon arc welding is used for stainless steel, due to the good protection, difficult burning of alloy elements and high transition coefficient, the weld is well formed, there is no slag shell, and the surface is smooth.
Therefore, the welded joint has high heat resistance and good mechanical properties.
At present, manual argon tungsten arc welding is widely used in argon arc welding.
It is used to weld 0.5~3mm thin stainless steel plates.
The composition of the welding wire is generally the same as that of welding. Industrial pure argon is generally used as a shielding gas.
The welding speed should be appropriately faster and lateral oscillation should be avoided as much as possible.
For stainless steel with a thickness greater than 3mm, MIG welding can be used.
The advantages of MIG welding are high productivity, small heat-affected weld zone, small deformation and good welding corrosion resistance, and easy automatic operation.
2) Gas welding
Because gas welding is convenient and flexible, it can weld welds in various spatial positions.
For some stainless steel parts, such as thin plate structures and thin-walled tubes, gas welding can sometimes be used without corrosion resistance requirements.
To avoid overheating, the welding nozzle is generally smaller than when welding low carbon steel of the same thickness.
Neutral flame must be used for gas welding.
The welding wire should be selected according to the composition and welding performance.
Gas flux 101 should be used for gas welding powder.
It is better to use the left welding method.
During welding, the inclination angle between the welding torch nozzle and the welding should be 40~50°, the distance between the flame core and the molten pool should not be less than 2mm, and the end of the wire welding rod must be in contact with the molten pool.
And move along the weld with the flame. The welding torch does not oscillate sideways.
The welding speed should be fast and interruption should be avoided as much as possible.
3) Submerged arc welding
Submerged arc welding is suitable for welding stainless steel plates (6~50mm) with medium and thick thickness.
Submerged arc welding has high productivity and good weld quality, but it is easy to cause segregation of alloy elements and impurities.
4) Manual welding
Manual welding is a very common and easy-to-use welding method.
The arc length is adjusted by human hands, which depends on the size of the space between the welding electrode and the workpiece.
At the same time, when used as an arc carrier, the electrode is also the weld filler material.
This welding method is very simple and can be used to weld almost all materials.
For external use, it has good adaptability, even when used underwater.
Most electric welding machines can be TIG welded.
In stick welding, the arc length depends on the human hand: when you change the distance between the electrode and the workpiece, you also change the arc length.
In most cases, direct current is used for welding, and the electrode is used not only as an arc carrier, but also as a weld filler material.
The electrode is composed of alloy or non-alloy metal core wire and electrode coating.
This coating protects the weld from air and stabilizes the arc.
It also causes the formation of slag layer and protects the weld to form it.
The electrode can be titanium or sealed, which depends on the thickness and composition of the coating.
The titanium electrode is easy to weld, and the weld is flat and beautiful. Furthermore, welding slag is easy to remove.
If the electrode is stored for a long time, it must be baked again.
Because moisture in the air will soon accumulate on the electrode.
5) MIG/MAG welding
This is an automatic gas shielded arc welding method. In this method, the arc is burned between the current-carrying wire and the workpiece under shielding gas.
The metal wire fed by the machine is used as a welding rod and melted under its own arc.
Related Reading: How to Choose the Right Welding Rod?
Due to the advantages of universality and particularity of the MIG/MAG welding method, it is still the most used welding method in the world.
It is used on steel, non-alloy steel, low-alloy steel and high-alloy based materials.
This makes it an ideal welding method for production and repair.
When welding steel, MAG can meet the requirements of thin steel sheet with a thickness of only 0.6mm.
The shielding gas used here is an active gas such as carbon dioxide or mixed gas.
The only limitation is that when external welding is performed, the part must be protected from moisture to maintain the effect of the gas.
6) TIG Welding
The arc is generated between the refractory tungsten welding wire and the workpiece.
The shielding gas used here is pure argon, and the welding wire coming in is not charged.
The welding wire can be fed manually or mechanically.
There are also some specific uses that do not require feeding welding wires.
The material to be welded determines whether DC or AC will be used. When direct current is adopted, tungsten electric welding wire is defined as negative electrode.
Due to its deep penetration ability, it is very suitable for different types of steel, but it does not have a “cleaning effect” on the weld pool.
Related Reading: MIG vs TIG Welding
Inspection method for stainless steel welding process
Welding inspection includes inspection of materials, tools, equipment, processes and finished product quality used throughout the production process, from design to product production. It is divided into three stages: pre-welding inspection, welding process inspection and finished product inspection after welding.
Inspection methods can be divided into destructive inspection and non-destructive testing depending on whether they cause damage to the product.
1) Pre-welding inspection
Pre-welding inspection includes inspection of raw materials (such as base metal, welding rod, flux, etc.) and inspection of welding structure design.
2) Inspection during welding
Including inspection of welding process specifications, weld size, fixture condition and structural assembly quality.
3) Inspection of finished products after welding
There are many methods for inspecting finished products after welding, including the following:
Appearance inspection
Appearance inspection of welded joints is a simple and widely used inspection method.
It is an important content of finished product inspection.
It is mainly to find weld surface defects and size deviations.
Generally, inspection is carried out through visual observation with the aid of a standard template, meter, magnifying glass and other tools.
If there are defects on the surface of the weld, there may be defects inside the weld.
Compactness Test
For welding vessels that store liquid or gas, non-dense weld defects such as penetrating cracks, pores, slag inclusion, incomplete penetration and loose structure can be found through tightness testing.
Compactness test methods include: kerosene test, water transport test, water impact test, etc.
Pressure vessel resistance inspection
For pressure vessels, in addition to the tightness test, a resistance test must also be carried out.
There are two common types: hydrostatic testing and pneumatic testing.
They can test the tightness of the welds of vessels and pipes that work under pressure.
Pneumatic testing is more sensitive and faster than hydraulic testing.
At the same time, the products after testing do not require drainage treatment, which is especially suitable for products with difficult drainage.
But the test is more dangerous than the hydrostatic test.
During testing, corresponding technical safety measures must be observed to avoid accidents during testing.
Verification of physical methods
The physical inspection method consists of using some physical phenomena for measurement or inspection.
Inspection of internal defects of materials or parts generally adopts the non-destructive testing method.
At present, there are ultrasonic flaw detection, radiographic flaw detection, penetrant flaw detection, magnetic flaw detection and so on.
① Radiographic inspection
X-ray flaw detection is a flaw detection method that uses the characteristics of the ray-penetrable material and the attenuation of the material to find defects.
According to the different rays used in flaw detection, it can be divided into X-ray flaw detection, Y-ray flaw detection and high-energy ray flaw detection.
Due to the different defect display methods, each radiographic inspection is divided into ionization method, fluorescent screen observation method, photographic method and industrial television method.
Radiographic inspection is mainly used to inspect cracks, incomplete penetration, pores, slag inclusion and other defects in the weld.
② Ultrasonic flaw detection
When the ultrasonic wave propagates in metal and other uniform media, it can be used to inspect internal defects because it will reflect at the interface of different media.
Ultrasound can inspect the defects of any welding material and any part, and can find the location of the defect more sensitively, but it is difficult to determine the nature, shape and size of the defect.
Therefore, ultrasonic flaw detection is often used in conjunction with radiographic inspection.
③ Magnetic inspection
Magnetic inspection consists of finding defects using magnetic leakage generated by the magnetic field that magnetizes ferromagnetic metal parts.
According to different magnetic flux leakage measurement methods, it can be divided into magnetic particle method, magnetic induction method and magnetic recording method, among which the magnetic particle method is the most widely used.
Magnetic flaw detection can only find defects on the surface and near the surface of the magnetic metal, and can only do quantitative analysis of defects, and the nature and depth of defects can only be estimated according to experience.
④ Penetrating inspection
Penetrating inspection uses the permeability and other physical properties of some liquids to find and display defects, including dye inspection and fluorescent flaw detection.
It can be used to check surface defects of ferromagnetic and non-ferromagnetic materials.
Key Points and Precautions for Welding Stainless Steel
1. The power supply with vertical external characteristics is adopted, and the positive polarity is adopted in DC (the welding wire is connected to the negative pole)
2. It is generally suitable for welding thin plates below 6mm, with the characteristics of beautiful weld formation and small welding deformation.
3. The shielding gas is argon with a purity of 99.99%. When the welding current is 50 ~ 150A, the argon flow is 8 ~ 10L/min, and when the current is 150 ~ 250A, the argon flow is 12 ~ 15L/min.
4. The length of the tungsten electrode protruding from the gas nozzle is preferably 4 to 5 mm.
In places with little shielding, such as fillet welding, it is 2 to 3 mm, and in places with deep grooves, it is 5 to 6 mm.
The distance from nozzle to work is generally no more than 15 mm.
5. To prevent the occurrence of welding pores, welding parts should be cleaned if there is rust, oil stains, etc.
6. For welding arc length, when welding common steel, 2~4mm is preferred, while when welding stainless steel, 1~3mm is preferred.
If it is too long, the protection effect is not good.
7. During butt reinforcement, to prevent the back of the lower weld bead from being oxidized, the back also needs to be protected with gas.
8. In order to protect the argon welding puddle well and facilitate the welding operation, the angle between the center line of the tungsten electrode and the workpiece in the welding position should generally be maintained at 80 ~ 85 °, and the angle included between the filler the surface of the wire and the part should be as small as possible, generally about 10°.
9. Windproof and ventilation. Where there is wind, take measures to block the net and take adequate ventilation measures indoors.
Requirements for stainless steel welding operation
Combined with the construction practice of various large-scale projects.
Prepared construction organization design or construction scheme including pipeline, storage tank, tower container, steel structure, etc.
Through implementing these construction organization projects on the project, I accumulated some experience and gained a greater knowledge of stainless steel welding. Now combined with the construction of gas pipelines.
With a view to the technical management and quality control of stainless steel in welding, this article discusses some experiences.
1. Determine the welding process
There are many types of stainless steel.
According to the alloy composition, it can be divided into chromium stainless steel and chromium nickel stainless steel.
According to the metal structure of stainless steel, it can be divided into austenite type, ferrite type, martensite type, etc.
The most commonly used in construction is austenitic type, such as 0Crl9Ni9, 1Cr18Ni9Ti, etc. Austenitic stainless steel has good weldability and is relatively easy to weld.
The welded joint has high toughness even in the welded state.
However, compared with ordinary carbon steel, its thermal conductivity is about 1/3 of that of carbon steel, but its expansion coefficient is 1.5 times that of carbon steel.
As austenitic stainless steel has low thermal conductivity and high coefficient of expansion, it will produce large deformation and stress in the welding process.
Therefore, the welding quality mainly depends on whether the welding process is suitable for the base metal.
Therefore, when determining the welding process, the following aspects must be considered.
Selection of welding methods Common stainless steel welding methods include manual arc welding, gas shielded welding and automatic submerged arc welding.
It is mainly determined according to the designed average parameters, construction conditions and operating environment, as well as construction cost.
In the pipeline construction process, due to the different diameters of the pipes and the greater number of valves and pipe fittings in the pipeline, changing the position of the welded joint is more complex.
Therefore, manual arc welding is generally used.
Argon arc welding is generally used to prepare pipelines that transport flammable, explosive or media with certain cleanliness requirements.
Manual arc welding overlay welding to improve internal weld quality. Selection of welding materials Stainless steel electrode is divided into chromium stainless steel electrode (the brand has the prefix “g”) and the chromium-nickel stainless steel electrode (the brand has the prefix “a”).
Chromium stainless steel electrode is mainly used for welding martensitic stainless steel.
The selection of welding rod mainly considers the chemical composition of the base metal, temperature and pressure of the pipeline medium, welding current (AC or DC), welding method and ambient temperature during welding.
Generally speaking, through selection and confirmation, there will be several brands of welding rods that can meet the welding requirements.
At this time, the welding rod can be selected according to its cost performance.
The selection of the weld groove shape is generally based on the stress condition of the weld.
The shape of the groove must be indicated in the construction drawing, and the corresponding specifications or standards must be adopted.
However, commonly used specifications or standards do not subdivide the groove size according to the difference between the base metal and the welding material. It is determined only according to the thickness of the base metal and the welding method.
But in fact, different base metals and welding materials have different requirements for the size of the groove during welding.
This is because the chemical composition and physical properties of materials are different, and the penetration (penetration) during welding is also different.
Therefore, during construction, the butt gap, blunt edge and groove angle of the groove must be adjusted according to the specific material.
If the size of the groove is too large, it will not only increase the construction cost, but also make the welding stress too large, easy to deform and crack;
If the groove size is too small, it will be subject to quality defects such as incomplete penetration and slag inclusion.
When manual arc welding is used, the penetration of the stainless steel electrode is smaller than that of the carbon steel electrode.
Therefore, the groove angle and end clearance must be increased accordingly.
It can be controlled according to the positive deviation value given in the specification or determined through experimental welding.
When selecting welding current, the specific strength of austenitic stainless steel is almost 5 times that of carbon steel.
Therefore, the electrode is easy to overheat and burn red during welding.
Using high current will cause overheating of the welding rod and burning loss of effective components in the coating, so poor weld protection can easily cause defects.
At the same time, the expected weld metal composition cannot be obtained, so the welding current should not be too large.
Generally, a lower welding current is preferred.
2. Make all preparations before welding
Before welding.
Targeted preparations must be made.
This preparation is an important part of ensuring welding quality.
Its content is mainly considered from the following three aspects:
Confirmation of welding operator skills Welders involved in welding must work with certificates and operate in strict accordance with the permitted welding items indicated in the operating certificate.
The welder must have more than two years of experience welding stainless steel or chrome molybdenum steel.
Welding materials management before use, the welding rod should be baked in accordance with the provisions of the operation manual (if there is no provision, it is generally treated in accordance with the drying temperature of 150 ~ 200 ℃ and the drying time 1H drying).
To bake, a special oven with a controllable temperature must be used. Bake as much as you want and take it however you want.
The dry electrode must be placed into the isolation cylinder for use. If exposed for more than 2 hours, it must be baked again. Repeat a maximum of 3 times.
The stainless steel pipe groove can be machined or plasma cut before welding.
The oxide layer and burrs in the groove must be removed first.
To facilitate the removal of spatter after welding, chalk paste can be painted within 50 mm on both sides of the weld and removed after welding.
Due to the “carburization” phenomenon caused by contact between stainless steel and carbon steel, a special grinding wheel and stainless steel brush must be used to clean the weld bead and spatter.
3. Prevent deformation and cracking
Deformation prevention Due to the large expansion coefficient and small thermal conductivity of austenitic stainless steel, it is easy for stainless steel to have large deformation during welding.
Therefore, during assembly, different types of anti-deformation accessories should be used according to the welds in different positions.
The position of spot welding and fixed welding should be smaller than that of general carbon steel.
The welding sequence must be reasonably determined during personnel welding.
For example, for large diameter pipes, two people can weld symmetrically in the same direction at the same time.
When the base metal is more than 8 mm thick, the weld bead must be welded in multiple layers and with low linear energy.
The welding adopts the “reverse connection” method of connecting the welding with the negative electrode to reduce the welding temperature.
Avoid cracks.
After baking the electrode, use a thermos to hold it.
The welding ambient temperature should be above 0℃ and should not fluctuate greatly during welding.
When the temperature is lower than 0 ℃, the welding must be preheated, and the preheating temperature is 80 ~ 100 ℃.
The reverse method is adopted to start the arc at slope 13, and the arc cannot be started on the base metal.
The straight-line strip transportation method of pulling forward without swinging is adopted for strip transportation.
If horizontal oscillation is required during vertical welding, the oscillation range must be minimized. Excessive horizontal rocking can easily cause thermal cracking and insufficient protection.
The arc length should be as short as possible.
The long electric arc will not only cause the burning loss of alloy components, but also lead to the reduction of ferrite and heating cracks due to the invasion of nitrogen in the air.
The arc crater must be filled during arc termination.
In particular, it is easier to ignore crater filling in spot welding and it is difficult to avoid thermal cracks in concave craters.
