Welding Stainless Steel: MIG, TIG, ERW, PAW, SAW, Laser Welding

What is Stainless Steel?

Stainless steel is a group of iron-based alloys containing at least nearly 10.5 percent chromium, iron-containing chromium amount of 10.5% or more, in contact with oxidizing media, due to electrochemical effects, the surface quickly forms a layer of chromium-rich passivation film, to protect the metal from internal corrosion; but in non-oxidizing corrosive media, it is still not easy to form a strong passivation film. To improve the corrosion resistance of steel, usually increase the proportion of chromium or add alloying elements that can promote passivation, plus Ni, Mo, Mn, Cu, Nb, Ti, W, Co, etc., these elements not only improve the corrosion resistance of steel but also change the internal organization of the steel as well as physical and mechanical properties. The content of these alloying elements in the steel is different, the performance of stainless steel has various effects, some are magnetic, some non-magnetic, some can be heat-treated, and some can not be heat-treated.

Stainless Steel Welding Methods

Welding methods can be divided into two types: fusion methods and pressure methods.

Fusion methods

Fusion methods refer to the two edges or surfaces being joined are heated and joined with or without a filler material. These comprise:

• Manual Metal Arc (MMA)
• Metal Inert Gas (MIG)
• Metal Active Gas (MAG)
• Flux Cored Arc Welding (FCAW)
• Tungsten Inert Gas (TIG)
• Plasma Arc Welding (PAW)
• Submerged Arc Welding (SAW)
• Laser Beam Welding (LBW)
• Laser Hybrid Welding (LHW)

Pressure methods

Pressure methods refer to two clean surfaces that are brought into close contact to form a metallic bond between the two surfaces. These comprise:

• Resistance Spot Welding
• Seam Welding
• High-Frequency Welding

Tungsten Inert Gas (TIG)

TIG is the most widely used process because of its versatility, high quality, and the finished weld’s aesthetic appearance. The ability to weld at low currents and therefore low heat input, as well as the ability to add filler wire when required, makes it ideal for root runs on thin sheet material and single-sided welding of thick plates and pipes. The process is easy to mechanize and the ability to weld with or without the addition of filler wire (autogenous welding) makes it a process for pipe orbital welding. Pure argon is the most commonly used shielding gas, but argon-rich mixtures with the addition of hydrogen, helium, or nitrogen can also be used for specific purposes. Inert backing gases are used to protect the weld from oxidation and corrosion resistance by welding on one side.

Plasma Arc Welding (PAW)

Plasma arc welding refers to the use of plasma arc high energy density beam as a welding heat source welding method. Plasma arc welding has concentrated energy, high productivity, welding speed, and stress deformation is small, stable and suitable for welding thin plate and box material features, especially suitable for a variety of refractory, easy to oxidise and heat-sensitive metal materials welding.
The arc heats the gas to produce dissociation, compressed at high speed through the water-cooled nozzle, increasing the energy density and dissociation degree, forming a plasma arc. Its stability, heat generation and temperature are higher than the general arc, and therefore has a greater penetration and welding speed. Formation of plasma arc gas and its surrounding protective gas is generally used pure argon. According to the nature of the material of various workpieces, helium, nitrogen, argon, or a mixture of the two mixed gases is also used.

Manual Metal Arc (MMA)

MMA electrodes are manually operated in operation and are the oldest arc process because they are flexible to adapt to the variety of materials to be welded. Electrode coating types are produced to provide performance characteristics that make them suitable for different welding applications. The most widely used acid rutile-coated electrodes produce jet arc-type metal transfer, self-releasing slag and fine corrugated aesthetic weld profiles. Minimal post-weld dressings are required. They are primarily used for underhand positions when producing fillet and butt welds. Electrodes with this coating type can be used in appropriate positions but are limited in application and size, i.e. up to 3.2mm.

Basic coated electrodes produce a higher integrity weld metal with slag micro inclusions and porosity and are useful for fixing pipe weldments. Slag removal and weld profile are not as attractive as acid rutile types. Specially coated electrodes are produced for specific applications; e.g. vertical down and high recovery manual welding. Electrodes are available in sizes ranging from 2.5 to 5.0 mm (Types 308L, 347 and 316L are also available in 1.6 and 2 mm diameters).

Metal Inert Gas (MIG)

The arc welding method using a molten electrode with an applied gas as the arc medium and protection of the molten metal droplets, the weld pool and the high-temperature metal in the weld zone are known as fusion electrode gas-shielded arc welding. The inert gas (Ar or He) shielded arc welding method with solid cored wire is known as melting electrode inert gas shielded welding, or MIG welding for short.

Submerged Arc Welding (SAW)

A fully mechanized wire and flux powder-shielded arc process with high deposition rates, fast travel speeds and weld quality. Applications include continuous underhand fillet and butt welds in thicker section plates, pipes and vessels and stainless steel cladding of carbon steel components, particularly where long seams or long runs are involved. The electroslag process using a strip electrode can also be used for cladding, with a number of properties that are superior to SAW.

Resistance Welding (ERW)

Resistance welding refers to the use of electric current through the weldment and the resistance heat generated at the contact as a heat source will be localized heating of the weldment, and at the same time pressurized welding method. Welding, no filler metal, high productivity, small deformation of the weldment, easy to automate.
Resistance welding using current flow through the workpiece contact surface and the adjacent area of the resistance of the thermal effect will be heated to the melting or plastic state so that the formation of a metal bonding method. There are four main types of resistance welding methods, namely spot welding, seam welding, projection welding, and butt welding.

Laser welding

Laser welding is a method of welding with heat generated by bombarding the weldment with a focused laser beam as an energy source. Because the laser has refractive, focusing and other optical properties, laser welding is very suitable for micro-parts and poor accessibility of the parts of the welding. Laser welding is also characterized by low heat input, small welding distortion and independence from electromagnetic fields.

High-Frequency Welding

High-frequency welding is a welding method that uses high-frequency current, the resistance heat generated by flowing through the contact surface of the workpiece, and applying pressure (or no pressure) to form a connection between the workpiece metals. High-frequency welding is different from resistance welding. High-frequency welding, welding current only in the workpiece surface parallel contact; resistance welding current is perpendicular to the welding interface flow. Generally, the frequency of high-frequency welding is selected at 300 to 450 kHz: resistance welding uses 50 kHz industrial frequency electricity.

Stainless Steel Types

To meet the wide range of applications using stainless steel grades, there are more than 100 stainless steel grades. These grades and types are made by inserting nickel, silicon, nitrogen, manganese and carbon, as well as chromium, into alloys to impart properties such as strength, ductility, heat resistance and flexibility.

There are five main families, classified primarily by their crystal structure: austenitic, ferritic, martensitic, precipitation-hardening and duplex.

Stainless Steel Weldability and Notes

Due to the characteristics of stainless steel itself, compared with plain carbon steel stainless steel welding and cutting have their special characteristics, more likely to produce a variety of defects in its welded joints and heat-affected zone (HAZ). Welding should pay special attention to the physical properties of stainless steel.

Martensitic stainless steel is generally represented by 13% Cr steel. When it is welded, due to the heat-affected zone being heated to the region above the phase transition point γ-α (M) phase transition occurs, so there is low-temperature brittleness, low-temperature toughness deterioration, accompanied by hardening of the ductility of the decline and other issues. Thus, in general, martensitic stainless steel welding needs to be preheated, but low carbon and nitrogen content and the use of butyl welding materials may not need preheating. The welding heat-affected zone of the organization is usually hard and brittle. For this problem, the toughness and ductility can be restored by post-weld heat treatment. In addition, the lowest carbon and nitrogen content of the grade, in the welded state also has a certain toughness.

Ferritic stainless steel is represented by 18% Cr steel. In the case of low carbon content has good welding performance, welding cracks within the sensitivity are also low. However, due to be heated to 900 ℃ above the weld heat affected zone grain coarsening significantly, making the lack of elongation and toughness at room temperature, prone to low-temperature cracking. That is to say, generally speaking, ferritic stainless steel 475 ℃ embrittlement, 700-800 ℃ long time heating “phase embrittlement, inclusions and grain coarsening caused by embrittlement, low-temperature embrittlement, carbide precipitation caused by a decline in corrosion resistance and high alloy steel prone to delayed cracking and other issues. Usually should be pre-welding preheating and post-welding heat treatment, and welding in the temperature range with good toughness.

Austenitic stainless steels are represented by 18% Cr-8% Ni steels. In principle, preheating before welding and post-welding heat treatment are not necessary. Generally have good welding performance. However, the nickel, and molybdenum content of high alloy stainless steel for welding is prone to high-temperature cracking. Also prone to σ-phase embrittlement, ferrite generated under the action of ferrite-generating elements cause low-temperature embrittlement, as well as reduced corrosion resistance stress corrosion cracking, and other defects. After welding, the mechanical properties of welded joints are generally good, but when the grain boundaries in the heat-affected zone of chromium carbides will be easy to generate chromium-poor layer, and the emergence of chromium-poor layer will be easy to produce intergranular corrosion in the use of the process. To avoid the occurrence of problems, should use low carbon (C ≤ 0.03%) grade or add titanium, or niobium grade. To prevent high-temperature cracking, it is generally accepted that controlling δ-ferrite in austenite is certainly effective. Containing 5% or more δ ferrite at room temperature is generally advocated. For steels whose main purpose is corrosion resistance, low carbon and stable grades should be used with appropriate post-weld heat treatment; while steels whose main purpose is structural strength should not be post-weld heat treated to prevent distortion and σ-phase embrittlement due to precipitation of carbides and occurrence of σ-phase embrittlement.

Duplex stainless steel welding crack sensitivity is low. However, the increase in the ferrite content in the heat-affected zone will increase the intergranular corrosion susceptibility and therefore can cause a reduction in corrosion resistance and low-temperature toughness deterioration and other problems.

For precipitation hardening type stainless steel welding heat affected zone softening and other issues.

Choose the suitable filler metal

Depending on the type of stainless steel you must weld, choose a filler metal accordingly. If the metals you join are different, you must pick them based on less cracking chance and the most compatible with the base metal.

In the following tables, we help you choose the filling material for your stainless steel:

For martensitic stainless steel

AISI steel	Recommended input metal	Alternative input metal
403	410	308, 347, 309
410, 410S	410	308, 347, 309
414	410	410 NM, 309
420	420	309
431	410	309, 310
440A	312	309

For ferritic stainless steel

AISI steel	Recommended input metal	Alternative input metal
405	410	308L, 309, 410NM
409	409CB	430, 309LSi
430	430	308L, 309L
442	308L	309L
446	308L	309L

For austenitic stainless steel

AISI steel	Recommended input metal	Alternative input metal
201, 202, 205	240	308, 347, 309
301, 302, 302B	308	347, 309
304, 304L	308L	347, 309
304H	308H	347, 309
303, 303SE	312	309MO
316, 309S	309	309CB, 310
310, 310S, 314	310	310CB, 310MO
316, 316L	316, 316L	309MO, 317
316H	316H	309MO, 317
317	317	317L, 309MO, 318
321	347	309CB, 310CB, 321
347, 348, 347H	347	309CB, 310CB
320	320LR	320
330	330	–
904L	385	–

If you are unsure of the type of stainless steel, the 309 works well in most situations.

Inspection of finished product after welding

Appearance Inspection

Appearance inspection of welded joints is a simple and widely used inspection method, and is an important part of the finished product inspection, mainly to find the defects on the surface of the weld and size deviation. Generally through visual observation, with the help of standard samples, gauges and magnifying glasses and other tools for inspection. If there are defects on the surface of the weld, there is a possibility of defects inside the weld.

Tightness test

Storage of liquids or gases in the welded container, the weld is not dense defects, such as penetrating cracks, pores, slag, not welded through and loose tissue, etc., can be used to find the tightness test. Tightness test methods include paraffin test, water test, water flushing test and so on.

Strength test of pressure vessel

Pressure vessel, in addition to the sealing test, but also for strength test. Commonly, there are two kinds of water pressure test and air pressure test. They can test the tightness of the weld of the container and pipeline working under pressure. The pneumatic test is more sensitive and rapid than the hydraulic test, while the product after the test does not need to be drained, especially for products with drainage difficulties. However, the danger of the test is greater than that of the hydraulic test. When carrying out the test, must comply with the appropriate safety measures to prevent accidents during the test.

Physical methods of testing

Physical inspection method is to use some physical phenomena for measurement or inspection methods. Material or workpiece internal defects inspection, generally using non-destructive flaw detection methods. The current non-destructive flaw detection is ultrasonic flaw detection, ray flaw detection, penetration detection, and magnetic flaw detection.

Ray Detection

Ray flaw detection is the use of radiation that can penetrate the material and the material has the characteristic of attenuation to find defects in a flaw detection method. The different rays used in flaw detection can be divided into X-ray flaw detection, γ-ray flaw detection, and high-energy ray flaw detection. As it shows the defects of different methods, each ray detection is divided into ionization method, fluorescent screen observation method, photographic method and industrial television method. Ray inspection is mainly used to test the weld internal cracks, unwelded, porosity, slag and other defects.

Ultrasonic flaw detection

Ultrasound in the metal and other uniform media propagation, due to the interface in different media will produce reflections, so it can be used for internal defects inspection. Ultrasonic inspection of any weldment material, any part of the defects, and can be more sensitive to find the location of defects, but the nature of the defects, shape and size is more difficult to determine. So ultrasonic flaw detection is often used in conjunction with ray inspection.

Magnetic inspection

Magnetic inspection is the use of magnetic field magnetization of ferromagnetic metal parts produced by magnetic leakage to find defects. The different methods of measuring magnetic leakage can be divided into the magnetic powder method, magnetic induction method and magnetic recording method, in which the magnetic powder method is most widely used.

Magnetic flaw detection can only find defects on the surface and near the surface of magnetic metal, and can only do quantitative analysis of the defects, and the nature and depth of the defects can only be estimated based on experience.

Penetration test

Penetration test is the use of certain liquid permeability and other physical properties to find and show the defects, including coloring test and fluorescence flaw detection of two kinds, which can be used to check the ferromagnetic and non-ferromagnetic material surface defects.