Behind the Technology

Metal-cored wires have a different structure and composition than solid wires, which results in distinctive operating characteristics, too. The wires consist of a hollow metal sheath into which filler metal manufacturers deposit metallic powders and/or alloys, including iron, that are designed to provide characteristics ranging from arc stabilization to higher tensile strengths and more. As a result of this tubular structure, metal-cored wires carry the welding current though the outside metal sheath to the work piece instead of through the entire cross section as with solid wires. At equivalent amperage settings, metal-cored wires also carry higher current densities. The result of both factors is a broad, cone-shaped arc and a wide penetration profile, as well as high burn-off rates that increase deposition rates and can provide faster travel speeds.

Metal-cored wires operate using the „spray transfer“ mode of droplet transfer with high argon shielding gas mixtures (a minimum of 75 percent argon is recommended). Using a constant voltage (CV) power source, the wires are capable of flat, horizontal, vertical-down and overhead welding and can also be used for vertical-up welding, but require a pulsing-capable power source to do so or must be adjusted to the short-circuit mode using a CV power source. Generally speaking, the wires operate at a lower deposition rate in the vertical-up position than flux-cored wires, and at about the same deposition rate as solid wire in the vertical-up position.

Typically, metal-cored wires are available in diameters ranging from 1.0 mm to 1.6 mm.

Identifying the Applications

Like any filler metal, metal-cored wires have applications for which they are better suited than others. They are appropriate for welding on mild, low-alloy and stainless steel, but are not recommended for welding sheet metal. Metal-cored wires work particularly well in the automotive industry for welding components such as chassis and steel wheels, primarily due to their ability to provide a wide bead profile and higher travel speeds. In the manufacturing and fabrication industries, metal-cored wires are well-suited for welding agricultural and heavy equipment, as well as rail cars, mostly due to their ability to weld through rust and mill scale (the fine oxide layer found on hot-rolled steels) — both common factors encountered in these applications. The wires can also aptly weld on 6 mm and thicker plate found in agricultural and heavy equipment manufacturing applications, and are usable for applications in the food and chemical industry, which, conversely, tend to be composed of thinner materials.

In addition, metal-cored wires can often be used as an alternative on certain applications currently using the submerged arc or gas-shielded flux-cored welding processes, as well as on many of the same applications that employ solid wire. These include solid wire applications requiring single-pass welding for welds that are 7 cm or longer, and those applications welded in the flat and horizontal position using the spray transfer mode. On such solid wire applications, companies who choose to convert to metal-cored wires can often increase the wire diameter by one size over a solid wire. Doing so often lets them standardize on a single wire diameter within their facility and allows for welding on a variety of joint sizes and material thicknesses.

Metal-cored wires are typically not recommended for applications requiring a lot of out-of-position welding, but they are good for single- and multi-pass welding using a robotic or automatic welding process. Other applications where metal-cored wires can be used include welding piping or other components where poor fit-up occurs, applications prone to burn-through and those requiring aesthetic bead appearances.

The Attributes and What They Mean

When used with the correct shielding gas mixtures (see previous section) and welding parameters (these are dependent on the application), metal-cored wires provide good weld penetration and high deposition rates, as well as fast travel speeds. Because of the way that these wires carry the welding current and burn off, they create very little spatter, as well as low amounts of slag, and offer very good gap bridging. Certain metal-cored wires offer chemical composition that helps minimize silicon deposits at the toes of the weld, while others are formulated to provide a specific chemistry and/or to increase tensile or impact strengths. Most are very capable of minimizing instances of porosity and providing good sidewall fusion to reduce instances of undercut in the final weldment and rework on defective parts.

On average, metal-cored wires cost more per kg than other filler metals, particularly solid wire. Many companies, however, still select this type of filler metal because of its potential for reducing costs in other areas of the welding operation, particularly in the pre- and post-weld areas. Often the pre-weld areas of a welding operation are used for activities like grinding, sandblasting or degreasing materials in preparation for moving them to the weld cell. In many cases, metal-cored wires can eliminate the need for these activities prior to welding because they can weld through mill scale and rust and they create little spatter. The lack of spatter means companies may be able to eliminate the need (and cost) of using anti-spatter and they can often take welded parts directly for painting without needing a separate cleaning operation beforehand. In certain cases, the elimination of such pre- or post-weld activities can allow companies to reallocate labor elsewhere in the welding operation.

In the welding cell, the fast travel speeds and high deposition rates that metal-cored wires provide can frequently increase productivity, as well. That improvement is most often seen in robotic and automatic welding applications.

What Else Is There to Know?

Metal-cored wire isn’t for every application. In certain ones, however, it can provide the welding performance a company wants and needs. For companies who are considering implementing this technology, it helps to assess the current welding operation and identify whether there are bottlenecks in productivity that exist due to pre- or post-weld activities like grinding or if there are quality issues, such as lack of fusion, spatter or undercut. In some cases, metal-cored wire can help eliminate these problems and improve the efficiency of the welding operation in the process.

Article based on ITW Welding global experience and knowledge.

When used with the right applications, metal-cored wire can help minimize costs, improve quality and increase productivity in the welding operation. Like any filler metal, metal-cored wire has unique characteristics, benefits, limitations and applications where the wire is best suited. Knowing when and how to use this wire can help companies achieve the best success with the product.

This article discusses the most appropriate applications for metal-cored wire, some of the characteristics to consider when choosing this filler metal and tips for welding successfully with metal-cored wire.

Understanding the basics

Metal-cored wire is a tubular wire filled with metallic powders, alloys and arc stabilizers, each of which offer distinct benefits such as lowering oxidation, providing higher impact strengths and reducing silicon deposits in the final weld.

The materials inside metal-cored wire can vary, depending on the desired properties and characteristics of the filler metal. Metal-cored wire is similar to flux-cored wire in that both have tubular construction and offer a higher deposition rate than solid wire. However, unlike flux-cored wire, metal-cored wire does not contain any slag-producing elements. This feature makes it more efficient, because more of the wire ends up deposited in the joint as weld metal. The lack of slag also contributes to the time- and cost-saving benefits of metal-cored wire, since it can be used to minimize pre- and post-weld activities such as grinding, chipping the slag or removing spatter.

While metal-cored wire offers a weld deposit similar to solid wire, its tubular structure causes the wire to operate differently than solid wire, which (as its name implies) is solid throughout the entire cross section. These different structures give metal-cored wire different arc and weld profile characteristics that can lead to significant benefits in the right application.

Where metal-cored wire excels

Metal-cored wire, because of the way that it is manufactured, is easily alloyed and available in many different chemistries, making it suitable for welding a wide variety of base metals. The wire can be used in many of the same applications that employ solid wire, but applications that benefit most are single-pass welds 8 cm or longer in flat or horizontal position using the spray transfer mode.

Using the spray transfer mode maximizes the benefits of metal-cored wire because this mode allows the welding operator to move faster. The spray transfer mode also generates little to no spatter, further enhancing the cleanliness of the metal-cored wire and minimizing the amount of post-weld cleanup.

Other applications that are well suited to metal-cored wire include those prone to burn-through; components presenting poor fit-up; and jobs where aesthetics are important.

The benefits of metal-cored wire

Because of the physical structure and the commonly used spray transfer mode, metal-cored wire produces a broad, cone-shaped arc, resulting in a wide penetration profile (compared to the more finger-like penetration of solid wire). This arc shape in turn creates a consistent bead profile that bridges gaps easily and accurately without burn-through. Another feature of metal-cored wire is the smaller droplets of liquid metal transferred across the arc, which result in less turbulence in the weld puddle.

Metal-cored wire offers faster travel speeds and higher deposition rates when compared to solid wire, resulting in increased productivity for welding operators. It’s also known to help minimize weld defects such as porosity, lack of fusion and undercut, which means using metal-cored wire may help reduce reject rates.

In addition, metal-cored wire has an increased ability to weld through mill scale and rust and still produce very little spatter, so it often helps eliminate the time and cost of activities before and after welding such as grinding, sand blasting or applying anti-spatter compound.

Consider the cost factors

While metal-cored wire offers advantages in many applications, it is not always the best product to use. There are some factors to consider when deciding if it’s the most cost-effective choice.

Metal-cored wire is more expensive than solid wire, so in applications where the advantages are not being utilized, that extra cost may not pay off in greater efficiency or productivity. Applications where the additional cost may not be justified include welding in the short circuit mode of transfer, welding out-of-position, and applications with a low operator factor (percentage of time in an operation actually spent welding).

For the higher cost of metal-cored wire, welding operators have the ability to get a higher deposition rate, but if the application can’t utilize that benefit the company isn’t really seeing any advantages for the extra cost.

Another consideration when weighing the pros and cons of metal-cored wire is shielding gas. High argon content gas (usually 90 percent argon/10 percent CO2, but mixtures range from 75 to 95 percent argon with the remainder CO2) must be used to achieve the beneficial spray transfer. However, argon is a more expensive gas, so this is another cost consideration when choosing metal-cored wire.

Companies should keep in mind that welding is only one step in the production process, and changes to that portion may require changes in other areas to avoid other issues such as product flow and inventory management. When productivity increases in the welding portion the rest of the process has to be able to handle that increase to realize a cost savings.

Tips and techniques for welding with metal-cored wire

Even though the physical characteristics of solid wire and metal-cored wire are different, the welding technique is basically the same. Here are a few differences:

Longer stickouts won’t cause erratic transfer. With metal-cored wire, the contact tip to work distance — the gap between the welding gun and the base material — can be slightly longer than it is with solid wire. The recommended gap for the best performance and arc stability with metal-cored wire is between12 mm and 25 mm, depending on wire diameter. Also, the general rule of thumb is the distance should increase as the diameter of the wire increases.
Using a larger wire diameter is OK. When switching to metal-cored wire from solid wire, welding operators typically can use one wire diameter larger. Since metal-cored wire has a broader metal transfer, the heat is not as concentrated and there is less chance of burn-through. The wire also is better at bridging gaps and providing consistent sidewall fusion.
Less need to manipulate the welding torch and puddle. The wider metal transfer/arc cone with metal-cored wire allows welding operators to make larger beads without the need to weave or manipulate the puddle.
Be careful with storage. Just as with any filler metal, proper storage is important. Metal-cored wire can pick up moisture in the chemical powders used to fill the wire and at the seams, which are left when the tubular wire is formed. Take care to store it in a dry place at room temperature. If filler metal is stored in cold temperatures, such as outside in a truck bed during winter, bring it inside and allow it to reach room temperature before welding with it to avoid condensation forming on the wire.

Other factors to note

The high argon shielding gases used with metal-cored wire impact the duty cycle of the gun. A welding gun is rated for a specific type of gas, so typically a 100 percent duty cycle rating at a specific amperage refers to using the gun with 100 percent CO2. Because CO2 does a better job of cooling the gun than argon does, the duty cycle can be reduced by 30 to 50 percent with high argon gas. Be sure to have a gun with a high enough amperage rating to account for a reduction in duty cycle when using metal-cored wire.

Also, the high argon gas spray transfer method used with metal-cored wire tends to result in lower visible smoke generation. These lower smoke levels can lead to noticeably more radiant light, since there is nothing in the area to minimize the light generated by the arc. The higher amperages often used with metal-cored wire also contribute to the increase in radiant light. Welding operators should take extra care to cover exposed skin, possibly increase welding lens shade and, when necessary, use screens in the area around the welding operation.

Making the right choice for the application

The selection of the right filler metal for the job is an important consideration. Metal-cored wire allows for greater travel speeds and higher deposition rates, but it also costs more than solid wire. Knowing when it’s most advantageous to use metal-cored wire can help increase productivity and save money by allowing welding operators to weld more efficiently, deposit more weld metal, reduce quality issues and spend less time cleaning welds.

Article based on ITW Welding global experience and knowledge.

Seamless wire basics

Seamless wires are available in flux-cored and metal-cored options for semi-automatic welding.

As the name suggests, these wires have no seam, making them more resistant to moisture absorption, even under extreme climate conditions such as tropical temperatures and/or high humidity. The lack of seam essentially eliminates a point of moisture entry.

The production of seamless wires begins with a strip of metal, typically carbon steel. The strips are folded round, closed using high-frequency welding, then are drawn to the required filling diameter. The tube is then densely filled with flux. In several subsequent steps the wire is drawn to its final diameter, copper-coated and spooled.

This manufacturing process differs compared to producing a standard flux-cored or metal-cored wire. The result is a completely sealed wire that offers extreme resistance to moisture absorption during storage and use.

Advantages of seamless wires

In addition to the ability to resist moisture, the copper coating found on seamless wire acts as another level of protection and provides optimal current transfer from contact tip to wire. This reduces contact tip wear and improves wire feedability. These properties also make seamless wires well-suited for robotic welding applications.

Seamless wires have a carefully controlled cast, helix and diameter, resulting in consistent wire feeding and straight delivery at the contact tip.

Seamless wire options

Choosing the right seamless wire depends on the needs of your specific application. Consider the various types available:

•         Seamless flux-cored wires: Standard flux-cored wires can require the addition of specific elements that scavenge out hydrogen from the weld; however, these scavengers may affect how smoothly the wires can weld. Seamless flux-cored wires do not require these additions and, as a result, provide excellent weldability. They are also available with sub-H4 hydrogen levels, or those with 4 ml of hydrogen or less per 100 grams of weldment, to further reduce the opportunity for hydrogen-induced cracking. Seamless flux-cored wires also tend to have a thin slag layer that removes easily, reducing time spent in weld cleanup. Seamless flux-cored wires are a good choice for multiple industries, but have proven especially beneficial within the oil and gas segment (e.g. offshore, pressure vessel, process pipe and spool fabrication applications) and infrastructure applications. This industry tends to have applications in more challenging climates and can require more challenging requirements for mechanical properties and low hydrogen.

•         Seamless metal-cored wires: Seamless metal-cored wires are especially well-suited for applications that require high strength and superior mechanical properties, as well as those requiring fast travel speeds and good gap bridging. These wires offer advantages for certain critical applications where moisture and wire feedability are concerns. Market conditions, climate conditions and the critical nature of the application are all drivers that can lend themselves to choosing seamless metal-cored wires.  Like seamless flux-cored wires, metal-cored options are also good for the oil and gas industry, large infrastructure, as well as heavy equipment applications, especially those requiring more stringent mechanical properties (e.g. higher tensile/yield strength and/or low temperature toughness).

In high-strength applications where ensuring low hydrogen levels and resistance of moisture absorption are critical, seamless wires are a good option to address these issues.

An option for high-strength applications

In high-strength applications where ensuring low hydrogen levels and resistance of moisture absorption are critical, seamless wires are a good option to address these issues. Choosing seamless wires can help welding operations meet the specific needs of demanding welding applications and reduce potentially costly rework.

Article based on ITW Welding global experience and knowledge.

Use low hydrogen filler metals when possible

Most filler metal manufacturers offer a variety of products, particularly flux-cored wires and stick electrodes that produce low levels of diffusible hydrogen. When welding ferritic (or iron-based) steels, the use of these filler metals can be a particularly good defense against weld failures caused by hydrogen-induced cracking, also referred to as cold cracking. This type of weld failure typically occurs within hours to days after the weld has cooled, and is the result of residual stress from the base material being restrained along the weld, along with the presence of hydrogen in the weld. Thicker materials are more prone to the failure, since they tend to create areas of high restraint and can serve as a heat sink that leads to fast cooling rates — the ideal condition for hydrogen to coalesce and add to the residual stresses in the weld. High-strength steels and applications with constrained joints are also prone to weld failures via cold cracking.

Filler metals with an H4 or H5 designator are a good choice to prevent weld failures associated with cold cracking, as they minimize the amount of hydrogen going into the weld in the first place, and with it, the opportunity to cause cracking upon the weld cooling. These filler metals contain less than 4 or 5 ml of hydrogen per 100 g weld metal, respectively.

In certain cases, using filler metals with a basic slag system can also help reduce the risk of weld failures from cold cracking. These filler metals typically contain high levels of hydrogen scavengers, including fluoride, sodium and calcium that can combine with hydrogen to remove it from a cooling weld.

Take care with fit-up and joint design

Proper part fit-up and good joint design are both key in preventing weld failures, particularly those associated with hot cracking. When presented with either of these conditions, it is not uncommon for a welding operator to try to compensate by creating a wider weld bead to fuse the metal together. The danger in doing so, however, is that the resulting weld may have too thin of a throat, causing it to be weak and create stress on the center of the weld. The result is quite often a condition called bead-shape cracking, which is a specific type of hot cracking, and it appears immediately upon the weld cooling.

A good rule of thumb, when possible, is to design the joint so that the welding operator has easy access the root. Doing so ensures a proper bead depth to width ratio. A good range for that ratio is to make the depth 5:1 to 2:1 the size of the width.

Pre- and post-weld materials correctly

Some materials are particularly susceptible to weld failures due to cracking, including high-strength steels, which have high carbon and/or high alloy levels. Because these materials are less ductile, they tend to generate residual stresses along the base metal and the finished weld during the cooling process.

It is important to always preheat such materials for the recommended time and temperature according to the welding procedure, and to ensure that adequate and uniform heat soak has occurred throughout. Preheating prevents rapid cooling and with it helps maintain a more ductile internal grain structure (pearlitic) in the heat-affect zone. It also limits shrinkage stresses  in the material and helps reduce instances of martensite formation in the grain structure — areas where hydrogen can dwell and ultimately cause cracking.

Similarly, when called for by a given welding procedure, post-weld heat treatment (PWHT) should be implemented as directed. PWHT relieves residual stresses and drives diffusible hydrogen from the weld to prevent weld failures by way of cold cracking.

Properly match filler metal and base material strengths

Selecting the appropriate filler metal strength can also help minimize the risk of weld failures. Most applications require matching the filler metal tensile or yield strength to that of the base material. The strengths should be as close as possible and selected as applicable to the design requirements of the application. If welding a lower strength material to a higher strength one, always match the filler metal to the lower strength one, as it will allow for greater ductility and help mitigate the risk of cracking. When making certain fillet welds or when welding on an application requiring only partial joint penetration (PJP), it may be desirable to undermatch the strength of the filler metal to the base material. Doing so can sometimes minimize the residual stresses in the finished weld.

Implement proper filler metal storage and handling procedures

To prevent filler metals from picking up moisture, dust, debris or oil that could lead to contamination — and ultimately weld failure — it is critical to follow proper storage procedures. Store filler metals in a dry area in their original packaging until ready for use. Ideally, keep the storage area the same temperature as the welding cell to avoid the condensation that occurs when moving from a cold area to a warm area, which could lead to moisture being absorbed by the filler metal. Allowing the filler metal to acclimate to the temperature of the welding prior to opening the package can also protect against hydrogen pickup that could lead to cracking and weld failure.

Welding operators should always wear gloves when handling filler metals to protect it from moisture from their hands, and they should cover any open spools with a plastic bag when not in use. Doing so protects that filler metal from accumulating contaminants from the air that may lead to poor weld quality and/or failure. Too, companies should never place grinding stations near an area where filler metal spools are present, as particles can settle on the wire, causing potential inclusions in the weld. If using stick electrodes, always follow proper storage and reconditioning procedures prior to welding.

Undergo the appropriate training

The importance of training as a first defense against weld failures cannot be emphasized enough. Proper education helps instill good welding techniques, as well as the ability to make sound decisions that positively affect the welding operation. Welding operators should be trained to always follow the prescribed welding procedure and to troubleshoot the common causes of weld defects, such as undercutting, slag inclusions or porosity that may lead to weld failures. They also need to be trained to attend to the special requirements of the alloys they may encounter. Check with a local welding distributor or welding (or filler metal) manufacturer for training opportunities. They can often assist with initial welding operator training and also assist with their continuing education. If the resources allow, companies may consider implementing their own training programs as well.

In the end, welding operators who know to follow procedure and also adjust properly to the various facets of the welding operation stand a good chance of achieving the desired weld quality and preventing weld failures.

Article based on ITW Welding global experience and knowledge.

Stainless steel is not a specific material, but a common term for a group of corrosion-resistant steel types. Stainless steels are steels which normally have a chromium content of at least 10.5%.

They can also contain nickel, molybdenum, nitrogen, copper, manganese, tungsten, titanium, niobium, cerium and other substances in varying degrees. Interest in nitrogen as an alloying element is increasing and many stainless steels, both austenitic and duplex with significant nitrogen levels have been in commercial use for the last ten years. It is well demonstrated that nitrogen in combination with molybdenum significantly increases resistance to pitting corrosion. The nitrogen also increases yield strength by solid solution strengthening of austenite.

Since the mechanical properties and the usefulness of each type of steel is dependent on its composition, it is important to take into account the different qualities of each type before choosing the steel and welding consumable for the application concerned.

Metallurgy
Alloying elements are usually divided into two groups – austenite stabilizers and ferrite stabilizers – as shown in table 1. Some alloying elements used in stainless steels are described in table 2.

Stainless steels are divided into four subgroups:

• Ferritic stainless steel
• Martensitic stainless steel
• Austenitic stainless steel
• Duplex stainless steel

Increasing the amount of austenite stabilizing elements or decreasing the amount of ferrite stabilizing elements will therefore promote a fully austenitic structure. In the same way, an increase in ferrite stabilizing elements or a decrease in austenite stabilizing elements would promote the ferritic phase.

Consider the phase diagram for Iron-Chromium-Nickel as shown in figure 2. Note that if we follow the composition 22% Cr-10% Ni (A) during cooling from liquid to room temperature, the first phase formed is delta. This is a ferritic phase also called delta-ferrite. At about 1400 °C, the melt is fully solidified and the phase present is delta-ferrite. Below this temperature, some of the delta-ferrite will be transformed to the gamma phase. Gamma is an austenitic phase which, compared to delta-ferrite, is non-magnetic.

If the composition of the melt had been 17% Cr-15% Ni (B) instead, the gamma phase would have formed initially. Below 1400 °C, the only existing phase is gamma and the steel is fully austenitic. By adding elements shown in table 2, it is possible to control which phase is formed and in which amount it is present.

For welding applications, it is often desirable to have a small amount of delta-ferrite in the weld metal. The reason for this is that ferrite has a higher solubility for sulphur and phosphorous than austenite. Primary austenite solidification causes Atmospheric corrosion resistance as a function of chromium content rejection of sulphur and phosphorus to the remaining liquid. A low melting point segregates results, which become trapped between growing dendrites (austenite grains) causing cracks along grain boundaries. Primary ferrite solidification does not cause sulphur and phosphorus rejection, thus preventing solidification cracking.

Sometimes, however, it is not desirable to have any delta ferrite at all. This is the case in high temperature applications, where delta ferrite during service will transform to the very brittle sigma-phase causing weld metal embrittlement. Type 310 fully austenitic steel is often used in such applications.

Corrosion resistance
A characteristic common to all stainless steels is that they contain chromium (min 10.5%), which inhibits corrosion. This excellent resistance results from the naturally occurring, chromium-rich oxide layer which always exists on the surface of stainless steel. This oxide layer has the unique property of self-healing, which cannot be achieved with layers applied by other means.

If the oxide layer is removed or damaged by abrasion, or if the raw metal surface is exposed when the steel is cut, a new layer is immediately formed by reaction between the steel and the atmosphere or other sources of oxygen. Because protection is re-established immediately, it is possible to choose steel that is not affected even in aggressive marine environments, or by many acids, alkalis and other chemicals.

Strength and formability
Stainless steel is sometimes cold stretched to increase strength, mainly for pressure vessels. Also in the embodiment, the tensile strength of the original stainless steel exceeds that of carbon steel. Similarly, hardness also varies from relatively soft annealed austenitic stainless steel to extremely hard martensitic materials, particularly for razor blades and ball bearings.

In general, ductility is inversely proportional to strength. Soft austenitic steels have outstanding ductility with an elongation exceeding 50%. Austenitic stainless steels can be cold worked to form a large number of semi-finished and finished products. Cold working can be optimized so that the final product achieves the best combination of strength and hardness. Ferritic stainless steels offer good strength and ductility, but without the outstanding formability of the austenitic variants. Martensitic steels can be formed in the annealed condition and subsequently heat treated to achieve the required strength and hardness.

Temperature spectrum
Stainless steel discolours if heated to very high temperatures, but this does not lead to scaling as in ordinary carbon steel and it retains much of its strength when heated. Consequently it is used in industry for many applications where durability at high temperatures is vital.

Strength decreases when steel is heated. The extent of the reduction is dependent on many factors, one of the most important being the actual alloy composition. Compared with carbon steel, stainless steel retains its strength when heated. Therefore, it is used in high-temperature environments in industry for its so-called creep strength and is chosen by many designers thanks to this characteristic.

High-temperature corrosion (scaling) must be avoided, although heat-resistant stainless steels are superior in this regard, because they are stable in contact with air and most of the products of combustion in temperatures up to +1100 ºC. A lot of industrial processes are performed at very low temperatures, down to -196 ºC (or even lower), and at such temperatures many materials lose their ductility and toughness and fail by brittle fracture. In such applications, specific austenitic stainless steels or nickel alloyed steel are ideally suited.

Several austenitic steels in table 3 contain about 18% Cr and 8% Ni and are often known by the popular term 18-8 stainless steels. Types 304 and 304L are very common grades and differ only in carbon content.

The ‘L’ grades are designed to avoid sensitization (Cr depletion close to grain boundaries due to chromium carbide formation) which can result from the heating cycle during welding. 316 and 316L contain additional molybdenum, which improves strength at high temperatures but most of all reduces the risk for intergranular corrosion.

The 347 type is known as stabilized stainless steel. This steel is used for elevated temperature applications, where a higher carbon content than that found in 304L and 308L is necessary to achieve good creep resistance. To avoid problems with chromium depletion as mentioned above, steel has to be stabilized. This is done by the addition of niobium (or columbium) and tantalum, which are strong carbide formers (stronger than chromium).

Article based on ITW Welding global experience and knowledge.

Properties of Stainless Steels

Ferritic
The main alloy is Cr (~10-30%) and ferritic steels are therefore called chrome steels. They do not normally contain any Ni. They are divided into two groups:

Semi-ferritic
Not ferritic all the way to melting point. (PHT 200-300 ºC and PWHT 700-800 ºC)

Fully ferritic
Ferritic all the way to melting point. (PHT 20 °C, PWHT not necessary). Use small electrodes and low amperage.

There are a few important differences compared to austenitic steels.
• Cheaper to manufacture
• Higher yield strength
• Lower elongation
• Better thermal conductivity
• Not sensitive to restraint cracks
• Not the same weldability as austenitic
• Good machining

Weldability of ferritic stainless steel varies depending upon the composition. Modern grades are reasonably weldable. However, all ferritic stainless steels suffer from grain growth in the HAZ resulting in loss of toughness. Consequently, interpass temperature and heat input must be limited.

Consumables for welding ferritic stainless steels can match parent metal composition or be austenitic.

Martensitic
Main alloys are Cr 11-17%, Ni up to 5% and C up to 0,4%. Martensitic steels are used for tools, but do not have the same hardness or durability as CMn/low-alloy steels used for tools or wear parts.

• May need both PHT and PWHT
• Mostly used as austenitic filler metal
• Hardens in air
• High mechanical properties

Martensitic stainless steels weldability is comparatively poor, and becomes worse with increasing carbon content. It normally requires preheating, well controlled interpass and cooling, as there is a significant risk of cold cracking in HAZ.

Matching-composition martensitic consumables are used when weld metal properties must match parent material. However, to decrease the risk of cracking, austenitic consumables may be used.

Austenitic
For welding applications, it is often desirable to have a small amount of delta-ferrite in the weld metal (310%). These steels do not normally need any post-weld-heat treatment (PWHT). They have about 50% higher thermal expansion compared to ferritic and duplex stainless steels.

Mechanical properties such as yield strength are lower than for ferritic steels.
• Expensive because of alloying element (Ni)
• Good resistance against corrosion (PRE)
• High thermal expansion
• Sensitive to hot cracks
• Good weldability
• Cold hardens when machined

Austenitic stainless steels are the most common and in most cases have really good weldability. Austenitic stainless steels are welded with consumables with a similar or over-alloyed chemical composition compared to the parent metal. Over-alloying is required in the more highly alloyed grades to optimize corrosion resistance.

In some cases there are requirements on fully austenitic weld structures e.g. for higher temperatures.

Duplex
Welding metallurgy has played a key role in the alloy development of duplex stainless steels. In terms of a common engineering material, modern duplex stainless steels are now well established as an alternative to other more general types of stainless steels and for certain applications, even nickel base alloys.

Duplex grades are readily weldable by all commonly used processes such as SMAW, FCAW, GTAW, BMAW, SAW and a large variety of joint designs. When planning welding operations, it is therefore of paramount importance to carefully consider the choice of welding processes and consumables, the establishment of comprehensive welding procedures and the need for proper control when storing consumables. During production, it is also important to understand the problems associated with storage, handling and fabrication of stainless steel plates and pipes.

Typical properties for DSS, grade 1.4462 compared to other stainless steel types: As can be seen in table 6, the duplex 1.4462 is 50% more expensive compared to the standard 304L grade in terms of price/kg, but less expensive if we compare Price/PRE and Price/Yield strength, valuable design and service life factors.

The greatest benefit of molybdenum and nitrogen in stainless steels is the improved resistance to pitting and crevice corrosion, especially in environments containing chloride.

One way of measuring this benefit is by determining the critical pitting temperature. Higher PRE means better corrosion resistance. This is normally established with the ASTM G48 test. The critical pitting temperature – CPT – is the point at which pitting corrosion starts in a test specimen immersed in a ferric chloride solution.

Article based on ITW Welding global experience and knowledge.