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.

Equipment for the transportation and storage of LNG must have good properties at temperatures down to -196°C.

The most important material property is good toughness at low temperatures. Weld metal properties are often the limiting factor.

Weld metal toughness can vary depending on factors such as welding methods, welding procedure and the choice of filler material.

The most common welding methods are TIG, Submerged Arc and basic coated electrodes.

Article based on ITW Welding global experience and knowledge.

Welding spatter is not only visually unappealing, but it also impacts the efficiency of a welding operation. In most cases, the spatter must be removed to pass a company’s quality checks. Companies also have to factor in the cost for purchasing grinding equipment and abrasives to remove the spatter, as well as maintenance and the associated safety risks of using grinders.

Anti-spatter compound can prevent spatter from accumulating on a part; however, it should be a last resort. Anti-spatter compound adds to an operation’s expenses and has the potential to introduce weld defects like porosity. It is also notoriously messy and can adhere to equipment, tools and the floor, which poses a slipping hazard.

There are several ways to reduce spatter that result in better-looking welds and greater efficiencies without the use of an anti-spatter compound.

No. 1: Adjust wire and welding parameters.

The diameter of wire used, along with the power source settings —particularly voltage — impact spatter generation.

For example, larger diameter wires operating at lower or colder welding parameters (less voltage), are prone to creating higher levels of spatter. In this situation, the combination of wire type and size, along with certain corresponding welding parameters, will operate in a short circuit transfer. In this mode, the welding wire makes electrical contact as it touches the base material repeatedly per second. Or the combination may move toward a globular transfer mode, causing large droplets of weld metal (larger than the wire diameter) to transfer across the arc. Both can cause spatter.

When welding with less than ideal settings with a larger wire, it may be beneficial to go down to a smaller size — for instance, from an 1.2mm to an 1.0mm wire. A smaller wire with more optimal settings allows for spray transfer mode that sprays tiny droplets of weld metal across the arc. The result is a smoother arc that reduces spatter.

Shielding gas selection also factors into the ability to achieve a smooth spray transfer mode. When welding with solid wires, it’s necessary to use a minimum of 80% argon in the shielding gas mixture. Tubular wires, like metal-cored wire, require a minimum of 75% argon with a CO2 balance. There is a tradeoff with higher argon levels; they produce deep, narrow joint penetration that may be less forgiving than a wider joint penetration. Welding operations will need to determine if that is more of an issue and a cost factor than dealing with
spatter.

No. 2: Avoid mill scale when possible.

The presence of mill scale or scale is a common problem in welding operations. This flaky surface found on hot rolled steel is made up of mixed iron oxides and melts off at a higher temperature than the actual base metal, essentially blocking electrical current to the arc during welding. The result is a colder weld deposit that tends to „ball up,“ as opposed to wetting out smoothly, and it causes welding spatter.

When possible, weld on base material that is free of scale. This can be achieved by purchasing plate that has already been cleaned or by grinding the mill scale off with a grinder or flap disc. Both add cost to the welding operation but can help avoid downtime for spatter removal.

If welding on scale-free material isn’t possible, be sure to ground the power source securely on a clean surface. Grounding over scale can cause interruptions to arc starting that leads to spatter. Using certain filler metals, like metal-cored wire, can also help minimize issues with mill scale and spatter.

No. 3: Consider metal-cored wires.

When possible and appropriate for the welding application, converting from solid wire to metal-cored wire is a good way to control spatter levels. As opposed to a solid wire that has a solid cross section, metal-cored wires are tubular and filled with metallic powders, alloys and arc stabilizers. These wires carry the current through the outside metal sheath, which creates a broader, cone-shaped arc for a wider penetration profile with little to no spatter.

Metal-cored wires also operate in the spray-transfer mode described earlier and can weld well through mill scale without pre-cleaning. Again, the combination reduces the opportunity for spatter.

No. 4: Follow proper welder training and best practices.

Less skilled welders can often produce welds with more spatter. As with any part of the welding operation, welder training and following some best practices are key.

Using the appropriate work and travel angles based on the application, wire type and joint configuration, as well as maintaining a proper contact-tip can also reduce spatter and should be instilled into training programs for new welders.

Also, using the proper consumables and replacing them when needed can help reduce spatter.

Taking steps to reduce welding spatter can help companies streamline their welding processes and be more efficient.

No. 5: Use pulsed MIG welding.

If a welding operation has a power source capable of pulsed MIG welding or is in a position to purchase one, the waveforms it provides can help reduce spatter. Pulsed MIG welding operates by switching between a high peak and low background current approximately 30 to 400 times per second. As the switch occurs, a droplet of wire is pinched off during the peak current and propelled to the weld pool. The background arc is responsible for maintaining the arc during this process, but at a low heat input that prevents metal transfer from occurring.

Pulsed MIG welding pairs well with solid wire and metal-cored wire to reduce spatter and helps with out-of-position welding. Because metal-cored wire already produces little to no spatter, the benefit of spatter reduction is more noticeable when using pulsed MIG welding with solid wire.

Pulsed MIG welding is also relatively easy for new welders to learn, which is an added benefit to creating consistent welds with low spatter, and the process can often weld through mill scale.

Supporting productivity, quality and cost savings.

Taking steps to reduce welding spatter can help companies streamline their welding processes and be more efficient. This is particularly true for applications that require parts to be painted. By lowering or eliminating spatter, the part can be moved more quickly into that portion of the welding operation. Spatter reduction can also support weld quality, increase throughput and minimize unnecessary costs.

Article based on ITW Welding global experience and knowledge.

When it comes to choosing filler metals for critical applications, it’s essential to find ones with the right mechanical and chemical properties. Having the right properties can help provide the proper impact toughness for the application, which is especially important when welding high-strength, low alloy materials. It is also important for applications subject to:

– rapid loading

– cyclic loading

– low service temperatures

– seismic activity

So, what is impact toughness and why is it important?

Impact toughness defined

By definition, impact toughness is the ability of a weld to permanently deform and absorb energy before fracturing, after applying rapid stress to it. Simply stated, it’s how much rapid impact energy a weld can take before it cracks. The application of the stress is typically under one second. In the real world, impacts can result from any number of events, such as:

– high winds

– earthquakes

– intentional and unintentional collisions

– explosions

The value of proper impact toughness

Filler metals with proper impact toughness provide several benefits in critical applications found in structural steel, oil and gas industries, shipbuilding and more. These include:

1. Reducing the risk of brittle fractures in steel that are associated with the combination of impact and cyclic loading and loss of toughness at low service temperatures
2. Helping to arrest the propagation of a crack so emergency repairs can be made

Ideally, however, using a filler metal with good impact toughness and following proper welding procedures will help prevent cracking altogether.

Selecting a filler metal

Filler metal manufacturers follow strict standards for formulating their products, including those set forth by the European Committee for Standardization or American Welding Society (AWS) „A5“ filler metal specifications. These specifications provide testing criteria for the filler metal, along with minimum impact toughness requirements for each filler metal classification. For general purpose applications, select a filler metal that provides minimum impact toughness properties that meet or are better than the impact toughness requirements of the application.

Article based on ITW Welding global experience and knowledge.