Welding stainless steel

Welding stainless steel is a process of obtaining permanent joints by establishing interatomic bonds between the welded parts of stainless metal during their local or general heating, plastic deformation or the combined action of both.

In general, the process of welding stainless steel is divided into several stages:

1. Preparatory (treatment of stainless steel before welding).
2. Welding.
3. Post-weld metal treatment (heat treatment in the weld zone and seam for some grades of stainless steel to relieve stress; treatment of the weld seam of stainless steel: cleaning, grinding, polishing; passivation;).

The high popularity of stainless steel in the world is undeniable for most manufacturing and service applications. When examining the mechanisms of metal destruction, the devastating effects of corrosion can be easily mentioned. Therefore, one of the most preferred methods to prevent corrosion can be considered the advantage of stainless steels for target products. The importance of stainless steel is well known to most technology enthusiasts. However, the methods of joining stainless steel are not as popular as stainless steel. One of the most difficult parts of the production stage of the desired products is considered to be the assembly process. Stainless steel has high strength properties, so the deformation processes of stainless steel can be quite complex. However, stainless steel welding is just the solution for most assembly processes that cannot achieve the desired shape of the products using plastic forming methods. Stainless steel welding allows manufacturers to create a variety of complex shapes of products. In addition, stainless steel welding can be used for specific areas of the target products. For example, if a supplier requires only some parts of stainless steel, stainless steel welding can be used to combine two or more different parts into one product.

The excellent properties of stainless steels have created a wide range of applications for them, so stainless steel welding has become a necessity for most engineering applications. Stainless steel welding has some excellent properties, but the methods used to weld stainless steel are very similar to those used to weld regular carbon steel.

Pre-weld treatment of stainless steel

Regardless of the method or method of welding stainless steel (stainless sheet, stainless pipe, stainless flange and other products), the welding technology requires preliminary preparation of the surface of the welded metal. The strength, reliability and durability of the future connection depend on how responsibly and efficiently it is performed. Surface treatment occurs in several stages:

1. Mechanical processing. Such mechanical processing is carried out in the form of cleaning, removing dirt, eliminating irregularities. For this, abrasive materials, a metal hard brush, etc. are used.
2. Degreasing. To degrease the future connection areas, wipe the surface with acetone, white spirit, alcohol or another special degreasing agent. The absence of grease and similar substances will allow better conduction of electric current and guarantee arc stability during the welding process.
3. Treatment with a special agent that prevents metal spatter from sticking. Molten metal particles inevitably spatter during the stainless steel welding process. Therefore, after stainless steel welding is complete, they are much easier to remove if they are not firmly attached to part of the surface of the product.
4. Arrangement of welded parts. The correct arrangement of welded metal parts is a very important stage of the welding process, which is very useful for ensuring unimpeded shrinkage of the material. To do this, a small gap is left between the edges of the parts to be joined.

Stainless steel welding methods

Let's look at the stainless steel welding processes for further understanding. The list of welding methods is quite numerous, but only a few of them are used in practice. So, let's look at only the most popular stainless steel welding methods that you can use when working with stainless metal:

1. Arc welding with a non-consumable electrode in a protective atmosphere of inert gas (GTAW | WIG | TIG)
2. Plasma arc welding (PAW)
3. Arc welding with a consumable electrode in a protective atmosphere of inert / active gas (GMAW | MIG | MAG)
4. Shielded (Stick - rod) arc welding of metal (SMAW)
5. Contact spot welding
6. Electron beam welding
7. Laser welding

Arc welding with a non-consumable electrode in a protective atmosphere of inert gas (GTAW | WIG | TIG)

TIG welding is a common process for welding stainless steel. This process is also known as TIG: T - Tungsten or WIG: W - Wolfram. The energy required to melt the workpiece is generated by creating an arc between a tungsten electrode and the base metal. When creating the arc, an inert or reducing atmosphere is selected. This is due to the desire to prevent unwanted joints during the welding process. Welding stainless steel has its own requirements, even if the gas tungsten arc welding method is the usual process for most alloy steels.

The polarity of the electrodes and the type of current (DC or AC) can have a fundamental effect on the width and depth of the weld. Therefore, stainless steel is welded using DC electrodes with negative or positive DC polarity. Under these conditions, the electrons strike the metal, where deeper penetration is ensured. The tungsten electrode loses a small amount of material during arc operation. Arc stability during welding is the most important parameter for the correct course of the process. Here, creating an inert atmosphere can be useful for improving the quality of welding. By providing an inert gas atmosphere, the stability of the generated arc increases. The type of shielding gases may depend on the base metal. Usually, mixtures of argon, helium and hydrogen are preferred. However, the advantage of a shielding gas mixture is also important for welding stainless steel. When welding stainless steel, mixtures of argon with hydrogen, argon with nitrogen and argon with helium and hydrogen are used in certain quantities. The wrong choice of shielding gas type can lead to the loss of alloying elements. Moreover, the loss of alloying elements can deteriorate the corrosion-resistant properties of stainless steel. So, the choice of the exact atmosphere affects the welding quality of stainless steel.

Plasma arc welding (PAW)

Plasma arc welding (Plasma Arc Welding - PAW) is very similar in its operating method to gas inert gas arc welding (GTAW | WIG | TIG). However, the application of arc plasma is somewhat different from GTAW.

In plasma arc welding, the arc plasma is supplied from a nozzle that restrains the arc propagation. Thus, the process can produce an arc with excess energy. The process arc is narrower than conventional arc welding operations, so a wider flow of shielding atmosphere can be useful for stainless steel welding operations. Shielding gas mixtures are similar to those of GTAW for stainless steel welding processes. The plasma arc welding process has some advantages over the TIG welding process. Especially when welding stainless steel, the controlled plasma arc allows better control of the input energy. Since the amount of alloying elements is high for stainless steel, the heat affected zone can be a problem for stainless steel welding operations. Narrowing the arc of plasma arc welding reduces the size of the possible heat affected zone.

Arc welding with a consumable electrode in a protective atmosphere of inert / active gas (GMAW | MIG | MAG)

The gas metal arc welding process, also known as metal inert / active gas welding, is very similar in principle to the GMAW and PAW processes. Here, an arc is created between the electrode and the base metal. However, this method differs from GMAW and PAW in that it uses a consumable electrode. A high current density is maintained on the consumable wire electrode. The mode for welding stainless steel can be selected as DC electrode positive or DC reverse polarity.

GMAW welding (Gas Metal Arc Welding) is welding with a metal electrode in a gas environment. It is divided into welding in an inert gas (Metal Inert Gas, MIG) and in an active gas (Metal Active Gas, MAG).

This type of welding is used for manual, semi-automatic and automatic welding in various spatial positions of stainless metals and alloys with thicknesses from tenths of particles to tens of millimetres.

Shielded (Stick - Rod) Metal Arc Welding (SMAW)

Although the SMAW method is a very old welding method, it is still common in most welding applications due to its simplicity. Therefore, the SMAW method is preferred for stainless steel welding operations. The electrode of the method contains a metal core coated with a flux material. The flux prevents the formation of unwanted compounds that can be harmful to stainless steel welding operations. In the SMAW process, the slags formed can be easily removed. Rutile or lime electrodes are used for welding.

Contact spot welding

Resistance spot welding is mainly used to join stainless steel sheets or plates. The melting of the base metals is ensured by passing an electric current from the workpiece. The opposite currents create excess heat between the base metals, which causes the boundaries to melt. Due to its simplicity and speed, resistance spot welding is one of the best methods for welding stainless steel.

Electron beam welding

The electron beam welding process passes electrons through the base metal where high energy is generated. Thus, the melting of the workpiece is ensured by electron collisions. Deep and thin welds can be created with electron beam welding. Thus, susceptibility to the heat affected zone is minimized, which is an advantage for welding stainless steel.

Laser welding

Laser welding of stainless steel is a fusion joining process that fuses metal using the heat generated by a laser beam. Laser welding is an efficient solution for industrial processes that helps to produce strong and aesthetic welds using fewer resources.

Is it difficult to weld stainless steel ?

Stainless steel is considered worldwide to have good welding properties. It is also very suitable for working and welding with various welding processes, including spot, support, electron beam, arc, MIG or friction welding. For any of these methods, you must know the type of stainless steel you are working with and prepare the surface by cleaning it thoroughly.

Stainless steel has about a 50% higher coefficient of thermal expansion than regular carbon steel. Better heat retention means less heat dissipation when welding, meaning you'll need to generate less heat to weld. It also has better electrical conductivity, so you can use less current during resistance welding processes.

Some types or grades of stainless steel require special care when welding to obtain the best results.

• Martensitic stainless steel. Martensitic stainless steel grades require preheating and post weld heat treatment for best welding results.
• Ferritic stainless steel. Most ferritic stainless steels perform better with minimal preheating (150 ℃ to 230 ℃).
• Austenitic stainless steel. When welding metal parts made of austenitic stainless steel grades, be sure to use the appropriate filler metal. This will help to avoid possible thermal cracking.
• Duplex stainless steel. The welding process of material made of two-phase (duplex) stainless steel is not particularly complicated. However, the temperature during welding must be strictly controlled. After all, leaving this feature without due attention, you can lose all the benefits of using this wonderful material.

What is the best method for welding stainless steel ?

Depending on the type, brand and grade, thickness and processing of the metal, the method of welding stainless steel varies. Although there are quite a few welding methods, the above are most often used.

The answer to this question is not so simple. It depends on the result you want to achieve. Each of these processes will produce a slightly different result. To choose the best welding process for your project, you should consider the following factors: the skill level of the welder, the aesthetics of the final part, including the appearance of the weld, the thickness of the metal, and the factors of cost and time. If skill is of the utmost importance, then the elegance of TIG welding may be suitable, but if speed and efficiency are a priority, then MIG welding may be the best process.

Let's take a closer look at the weldability of different types of stainless steel.

Welding of austenitic stainless steel

Like other types of stainless steel, austenitic stainless steels, especially chromium-nickel steels, are resistant to corrosion and oxidation due to the presence of chromium, which forms a self-healing protective film on the surface of the steel. They also have very good strength at extremely low temperatures, so they are widely used in cryogenic structures and components. They can be hardened and their strength increased by cold working, but not by heat treatment. They are the easiest of the stainless steel family to weld and can be welded by all welding processes. The main challenges are to prevent hot cracking and maintain corrosion resistance in the weld zone.

The alloying elements in austenitic stainless steel can be divided into two groups: those that promote the formation of austenite, and those that promote the formation of ferrite. The main creators of austenite are nickel, carbon, manganese and nitrogen. Important ferrite formers are chromium, silicon, molybdenum and niobium. By varying the amounts of these elements, the steel can be made completely austenitic or it can be designed to contain a small amount of ferrite.

In 1949, Anton Schaeffler published a phase diagram illustrating the effect of composition on microstructure. In the diagram, Schaeffler assigned a coefficient to the various elements, reflecting the force of action on the formation of ferrite or austenite. The elements are then combined into two groups, forming equivalents of chromium and nickel. Knowing the composition of austenitic stainless steel, the proportions of the phases can be determined.

Although all austenitic stainless steels are susceptible to hot cracking, fully austenitic steels such as type AISI 310S are particularly sensitive.

The main culprits are sulfur and phosphorus. To this end, the amount of these reject elements has been gradually reduced, so that steels with sulfur contents of less than 0.010% and phosphorus contents of less than 0.020% have become available. Ideally, an alloy such as AISI 310 or AISI 317 should have sulfur and phosphorus levels below approximately 0.003%. Cleanliness is also very important, and thorough degreasing should be carried out immediately before welding.

Stainless steels such as AISI 304, AISI 316, AISI 347 contain a small amount of delta ferrite and, although they are not protected from hot cracks, have increased resistance to the formation of sulfur-containing liquid films. The reasons for this are as follows:

1. ferrite can dissolve more sulfur and phosphorus than austenite, so they are contained in solution rather than being able to form liquid films along grain boundaries;
2. the presence of a small enough amount of ferrite enlarges the grain boundary region such that any liquid films must spread over a large area and can no longer form a continuous liquid film;

100% austenitic steels do not have this advantage.

One of the problems encountered with very low sulfur stainless steels is a phenomenon known as "cast to cast variation" or "variable penetration." The weld pool of low sulfur steels (<0.005%) tends to be wide and shallow. Steels with sulfur content greater than 0.010% have a narrower, more deeply penetrating weld pool.

This is usually only a problem when using a fully automated TIG process, as the manual welder can cope with variations in penetration due to differences in sulfur content in different steel castings. However, automated TIG procedures designed for high sulfur steels may result in no penetration defects when used to weld low sulfur steels. The reverse situation may result in excessive penetration.

Changes in procedure that have mitigated but not eliminated the problem have included slow travel speeds, pulsed current, and the use of Ar/H₂ shielding gas mixtures. Other methods include setting a minimum sulphur content, say 0.010%, or separating the steels into batches of known penetration characteristics and developing appropriate welding procedures. The A-TIG activated flux process has also been found to be beneficial.

Some grades of austenitic stainless steels also become brittle, but this occurs as a result of the formation of hard brittle phases. The embrittlement occurs in the temperature range from approximately 500 °C to 900 °C. This is a slow process and is not a problem when welding austenitic stainless steels, but can occur when working at elevated temperatures or if the component being welded is stress relieved.

The formation of these phases is favoured by high chromium and molybdenum contents (ferrite-forming elements), so steels such as AISI 310 and AISI 316 are particularly sensitive and can show significant loss of ductility after stress relief. Delta ferrite also transforms faster than austenite, so alloys containing large amounts of this phase degrade faster than austenitic steel with a small percentage of ferrite, hence the problems with duplex and super duplex stainless steels.

When stress is to be relieved from a component, the loss of ductility must be taken into account. In steels containing delta ferrite, this phase should be kept to a minimum, which corresponds to minimizing the risk of hot cracking by controlling the ferrite-forming elements, typically requiring a delta ferrite content of 2% to 5%.

Welding of ferritic stainless steel

There are a number of problems with welding ferritic stainless steels. Although they are not considered hardened steels, they can form a small amount of martensite, which causes a loss of ductility. Also, if the steel is heated to a high temperature, very rapid grain growth can occur, which also causes a loss of ductility and toughness.

Although ferritic steels generally contain only small amounts of carbon, when rapidly cooled, precipitation of carbides at grain boundaries can "sensitize" the steel, making it susceptible to intergranular corrosion. When this occurs in a weld, it is often referred to as weld failure. However, the development of ultra-low carbon, titanium or niobium grades in recent years has improved this situation.

Ferritic stainless steels are usually welded in thin sections. Most are less than 6mm thick, so any loss of strength is less. Most common arc welding processes are used, although it is considered good practice to limit the heat input for these steels to minimise grain growth (heat input of 1 kJ/mm and maximum interpass temperature of 100 °C - 120 °C are recommended), bearing in mind that high deposition rate processes are undesirable. Preheating is not required, although it may be useful when welding sections of, say, 10mm thickness, where grain growth and weld restriction may lead to joint cracking.

Welding consumables for ferritic steels are generally of the austenitic type. AISI 309L (low carbon) is the most commonly used filler metal. This is to ensure that any dilution does not result in the formation of an austenitic/ferritic/martensitic weld metal microstructure with low ductility. However, with dilution control, AISI 308 and AISI 316 can be used. Nickel-based consumables can also be used, which will result in improved performance during thermal cycling of the component. Suitable filler metal is available for welding AISI 409, a steel often used in automotive exhaust systems.

Post weld heat treatment at temperatures around 620 °C is rarely performed, although the reduction of residual stresses will lead to improved fatigue properties: in this context, nickel-based fillers are a better choice than austenitic Cr/Ni consumables.

Welding of martensitic stainless steel

Martensitic stainless steels are used in more demanding environments and, as the name suggests, present many more challenges than ferritic steels. Both the higher carbon (>0.1%) and low carbon (<0.1%) versions, with a few exceptions, require preheating and post weld heat treatment to avoid problems with weld cracking and to ensure a sufficiently strong and ductile joint.

Suitable welding consumables are available in most grades so that corrosion resistance and mechanical properties can be matched to those of the base metal. To reduce the risk of hydrogen cracking, low hydrogen welding processes are required and preheating temperatures of 200 °C to 300 °C are recommended. A weld that has completely transformed into untempered martensite as the joint cools to room temperature may be extremely brittle and must be handled with extreme caution to prevent brittle fracture. Such joints are also susceptible to stress corrosion cracking even under normal shop conditions. It is therefore strongly recommended that post weld heat treatment be carried out as soon as possible.

A typical heat treatment cycle involves cooling the joint to below 100 °C to ensure complete conversion of the weld and weld zone to martensite, carefully controlled heating to minimize stresses caused by temperature variations, post weld heat treatment at approximately 700 °C for one to four hours, and cooling to ambient temperature.

Hydrogen evolution treatment at a preheat temperature of, say, 350 °C for four hours is unlikely to reduce the risk of cold cracking. Unless the metal is allowed to cool low enough to allow complete transformation to martensite, austenite will be present during hydrogen evolution treatment.

This austenite will retain hydrogen and may crack as it transforms to martensite as the joint cools to ambient temperature. If cold cracking is a real problem even with good hydrogen control, it may be necessary to perform a post weld heat treatment directly from the preheat temperature, cool to ambient temperature, and repeat the post weld heat treatment to temper any martensite that formed after the first post weld heat treatment cycle.

For most martensitic stainless steels, filler metals are available that match the composition of the base metal, often with small additions of nickel to ensure that ferrite does not form in the weld. Nickel lowers the temperature at which martensite transforms to austenite, so for these filler metals it is important that the post weld heat treatment temperature does not exceed approximately 750 °C, otherwise untempered martensite will form in the weld when the workpiece cools to ambient temperature.

Traditionally, when welding dissimilar metals, the filler metal is selected to match the composition of the low alloy steel. Experience has shown that this can lead to problems with cold cracking, so filler metals that match the martensitic steel should be used. An alternative is to weld using austenitic stainless steel filler metals, such as AISI 309, but in this case the weld may not match the tensile strength of the ferritic steel, and this must be taken into account when calculating the weld. Nickel-based alloys can also be used, for example alloy 625 (Inconel 625) has a 0.2% tensile strength of about 450 MPa and will give a better match to the coefficient of thermal expansion.

The metallurgy of these types of steel is complex and they are often used in difficult and hazardous conditions.

Welding duplex stainless steel

It should be noted that welding of duplex stainless steel is not particularly difficult. But it is different from other steels. In fact, the weldability and welding characteristics of duplex stainless steels are better than those of ferritic steels, although in general they are not as good as those of austenitic steels. Modern duplex steels with a high nitrogen content are easy to weld. However, the properties of the duplex welded structure are greatly affected by such welding parameters as heat input. Therefore, it is necessary to follow the correct welding procedures to obtain an acceptable structure and properties of the welded product.

Duplex stainless steels typically harden with a fully ferritic structure, with austenite nucleating and growing during cooling. Rapid cooling from high temperatures can result in increased ferrite levels in the weld metal and adjacent base metal. Therefore, filler metals are specifically formulated with higher nickel contents to provide a phase balance similar to that of the base material. For this reason, autogenous welding (without filler) is generally not recommended for duplex steels. Even though duplex steels are not completely resistant to hardening and hydrogen cracking, this is less of a problem than with other stainless steels. The duplex microstructure is more sensitive to the effects of subsequent passes than standard austenitic grades.