Because of the cracking sensitivity of nickel alloys, heat input needs to be limited and interpass temperatures should not exceed 150 oC. In order to decrease dilution, welding straight rows is advised to avoid electrode oscillation.
Until now, we’ve discussed dissimilar metal welds between unalloyed or low-alloyed materials and high-alloyed iron based materials. However in some cases, nickel based materials need to be alloyed with corrosion resistant steel, other nickel alloys or even copper alloys. Table 3 introduces common examples of these material pairings.
Table 3. Filler metals for dissimilar metal welds made of nickel based metal and special alloyed metal
Various strongly alloyed corrosion resistant steels frequently need to be welded together when a structure is being assembled. As a general rule, the filler metal for the more strongly alloyed metal is used to prepare the dissimilar metal weld. In this case, consulting the Schaeffler diagram to confirm that the weld composition doesn’t fall into the martensite area of the diagram is still advised.
In such situations, 309L or 309MoL filler metals are often the right choice. Of course other factors must also be taken into consideration, for example the fact that a completely austenitic weld is prone to hot cracks. This propensity towards cracking decreases slightly if the weld’s manganese content is above 4%. In the case of niobium stabilized corrosion resistant steels, stabilized welding materials or so-called ELC welding materials with less than 0.03% carbon content is advised.
In contemporary fossil fueled power plants, everything from flues to superheaters use multiple kinds of ferrite, bainite, martensite and austenite steels. The welds of these steels work at high temperatures, thus it’s very important to note the qualities of these welds at high temperatures, including hydrogen induced brittleness, aging behavior, oxidation, and creep strength.
Welding power plants boilers is regulated under the PED (European Pressure Equipment Directive), thus only pre-approved filler metals can be used. Welds between two different creep resistant steels, such as 10CrMo9-10, (0.1% C, 2.25% Cr and 1% Mo-content) and X20CrmOv11-1 (0.2% C, 12% Cr and 1% Mo and V- content) could be prepared with the correct highly-alloyed filler metals. However, high industrial temperatures induce carbon enrichment from the diffusion of carbon and chromium atoms on the one side and decarbonization on the other side, which considerably downgrades the weld’s mechanical qualities [1].
Here, nickel based materials can also be used, although the thermal expansion coefficient of austenitic nickel welds differs from that of ferrite steel, which results in excessive tension. It also makes testing the weld for flaws much more difficult.
As a rule in these cases, the filler metal for the lesser alloyed metal is used. If the chrome content between the two welded materials is high, carbon diffusion is inevitable. Carbon enrichment can be decreased by choosing a vanadium or niobium-containing filler metal, and if the post-weld heat treatment is done at the lowest possible temperature in the least possible amount of time.
Post-weld heat treatment parameters always require a compromise. The necessary heat treatment temperature and length of time should be between the prescribed heat treatment temperature and time length requirements of the two materials. If the difference between the prescribed heat treatment temperature and time length requirements of the two materials is too big, then a buttering layer must be welded on to eliminate the problem. Nickel based welding materials are the best for welds made of austenitic and ferritic steels working at high temperatures.
[1] J.Hilkes, V.Gross: Welding of CrMo Steels for Power Generation and Petrochemical Applications, internetes publikáció
The filler metals in the table are either purely austenitic or approximately 5-40% ferrite containing structures. General unalloyed steel, eg. S235 and 19/9 dissimilar metal welds welded with 309L filler metal consist of 92-95% austenite and 5-8% ferrite [1]. The weld’s high ductility can withstand heat contraction, and the low ferrite content ensures a high degree of resistance against hot cracks.
Because carbon steel’s welding melt pool has a lower viscosity, modifying the usual weld preparation with a somewhat larger beveling angle and improving dampening with a slightly greater gap opening is advised. Preheating usually isn’t necessary, except if the given steel is inclined towards quenching. This is the case for example, if the carbon content exceeds 0.2%, or the plate thickness is greater than 30 mm. In these cases, preheating is only necessary with unalloyed steel.
The necessary degree of pre-heating can be calculated as a function of the material’s chemical composition and plate thickness. The EN 1011-2 standard lists the prescribed preheating temperatures. It also defines its effect on the weldability of the compound using an equivalent based on the carbon related effects of individual alloys and provides limits for each compound.
In the case of those steels whose compositions (or the carbon equivalent thereof) do not match the prerequisites in the standard, the necessary preheating is usually based on expert advice. In general, those steels whose International Welding Institute’s equivalent value is between 0.4-0.6 are recommended to be preheated at a temperature between 150-200 oC, and anything higher at a temperature of 300 oC.
Table 1. General welding materials for black and white dissimilar metal welds [2].
In the case of those steels where post-welding heat treatment is required — these include creep-resistant steels in power plants — or where the application temperature is higher than 300 oC, the aforementioned filler metals cannot be used. Dissimilar metal welds used with power plant steels will be discussed later.
Welds prepared with 309L or similar iron based filler metals at 300 oC must account for brittleness from sigma-phase formation as well as intergranular corrosion. At room temperature, diffusion processes are very slow, with no considerable effects. As temperature increases, as a rule the speed of diffusion grows exponentially.
At a higher temperature, the diffusion of carbon atoms moves from unalloyed steel towards the lower carbon containing corrosion resistant steels, where it interacts with chrome to form carbide nt he grain boundaries. The decreasing chrome content results in corrosion.
In this case, a nickel based filler metal needs to be used. The structure of nickel alloys is completely austenitic, it won’t appear even after welding a martensite structure, and there is also no sigma-phase formation. Carbon diffusion in nickel welds is not considerable, thus neither the weld nor the surrounding area are sensitive to intergranular corrosion. Further advantages of nickel filler metals is high elongation and heat expansion qualities, which help bear the tension of heat contraction. Nickel’s heat contraction coefficient is between unalloyed steels and austenitic chrome steels. Table 2 illustrates the qualities of nickel alloyed filler metals.
Table 2. Most commonly used nickel based filler metals in dissimilar metal welds [2].
[1] R.E.Avery: Guidelines for welding dissimilar metals. Chemical Engineering Progress, May 1991.
First, the metallurgical properties of the two metals to be welded should be taken into account, as well as their differing chemical compositions which will determine the microstructure formed after the weld and thus the resulting joint’s physical properties. In addition to this, the welding parameters are also important, including heat input, the two metals’ physical and chemical qualities, and temperature. Finally, the newly joined components workability and other economic factors need to be considered.
There is no universal filler metal for every possible combination of welded nt heor operating condition in industry. In reality, choosing a good filler metal always requires a compromise. The fundamental requirement of the selected material is that the resulting weld and its heat affected zone not crack in operating conditions. The structure’s mechanical qualities should meet user requirement standards, and in order to guarantee its structural integrity, the weld’s strength should be equal to or greater than that of the weaker metal’s.
Selection depends largely nt he base materials and how the joint will be used. In addition to this, the characteristics of the weld also depend a lot on what process is used. This is especially true in the case of dissimilar metal welding, where care must be taken to keep weld dilution to a minimum.
The dilution value is defined by what percentage of the weld is made up of the melted material. This value is a function of the welding process and welding parameters. Table 1 depicts the average dilution of various welding processes.
The least amount of dilution is achieved with the least amount of heat input. The highest dilution levels are usually the result of Gas Tungsten arc welding (GTAW), Submerged arc welding (SAW), while the least amount of dilution is a result of specialized variations of Gas metal arc welding (GMAW).
If the dilution value is 20%, then each welded metal makes up 10% each of the weld’s chemical composition and the filler metal determines the remaining 80%. With certain processes, special properties also influence the dilution value. For example, processes that use shielding gasses are influenced by the gas composition.
When using the same parameters, shielding gases with higher energy conducting active gas content create a bigger melt through, which increases dilution. Gas mixtures that contain carbon-dioxide, oxygen or hydrogen are examples of these.
Naturally, shielding gases have additional side effects. For example, using austenite forming Nitrogen content in shielding gasses can be advantageous when welding corrosion resistant steels, however it can increase the risk of cracking.
In addition to those factors listed above, welding parameters also affect the dilution value. In general, combining lower welding power and/or tension and faster welding speed decreases heat input, which results in a smaller melt through and less dilution. When determining the process for welding dissimilar metals, the second factor to consider after the composition of the material is heat input, as the lowest heat input ensures the least amount of dilution.
In this article, we describe the filler metals and technological factors for frequently used dissimilar welds created with arc welding processes. The most frequent dissimilar metal weld is between unalloyed steel and some form of strongly alloyed corrosion resistant or heat resistant steel (ferrite, austenite, duplex, martensite or nickel based).
In spite of the fact that the two types of steel differ considerably in multiple physical qualities, including heat conductivity, heat expansion, magnetism, structure and corrosion resistance, this dissimilar metal weld remains a definitive weld in industrial structures.
These welds can be successfully welded using most arc welding processes. GMAW (FCAW), SMAW and GTAW are the most often used processes. If we weld carbon steel directly to corrosion resistant austenitic material, then a martensite structure with a low brittle-ductile capacity will form in the weld, resulting in cracking from thermal contraction.
Thus in practice, over-alloyed filler metal is usually used for dissimilar metal welds, which forms an austenite-ferrite structure after dilution and helps avoid cracking from thermal contraction.
The Schaeffler diagram depicts the typical structure of steels after rapid cooling and the risk of potential defects that may occur when they are implemented (including grain growth, sigma phase formation, cold and hot cracks) as a function of their composition. The effects of austenite forming elements are expressed with nickel equivalents, and the effects of ferrite forming elements are expressed with chrome equivalents.
In this way we’re able to predict the diluted structure of the weld as a function of the specified filler metal and dissimilar metals that are welded together. Diagram 1 presents a classic example of how the diagram should be used. In the diagram, the weld’s structure is indicated with a red dot, and is considered advantageous for both welding and implementing corrosion resistant steels.
Diagram 1. The weld’s location on the Schaeffler diagram indicates a 20% material dilution when S235 unalloyed structural steel and 304L corrosion resistant steel is welded together using 309L filler metal.
At the same time, Diagram 1 also indicates that if the weld is prepared with an electrode made for carbon steel, the red dot would fall directly into the martensitic zone. In this case, the weld would most likely crack due to its brittle structure.
Nickel-based welding fillers can help compensate the stresses of brittle material arising from shrinkage during welding and their large deformation capacity. As a result, the brittle and difficult to weld material can be welded with nickel base consumable. Different iron and steel casting with high carbon content can be welded with nickel or ferronickel electrodes at a low heat input. Due to the brittleness of naturally lemezgrafitos steel castings at room temperature they can require a certain degree of preheating, but at a much lower temperature than if it was welded with with iron-based fillers. Gray, or malleable cast iron is welded with pure nickel-based electrodes with about 1% carbon content. In this case, the hardness of the weld seam is fairly low. Greater strength and hardness can be achieved by using ferro-nickel electrodes developed for the welding of cast irons and their various joints. Application of this electrode type results in small heat expansion, similar to that of invar alloy.
The heat expansion of unalloyed steel is 12×10-6/K, while that of 304/309 type austenitic stainless steel is 19×10-6/K. The large heat expansion difference at the border of welding seams in black and white joints can led to the development of great stresses as a function of changing temperature. The heat expansion of nickel is between that of carbon and stainless steel (15×10-6/K), which can result in smaller stresses at the border of the weld and the base material. When choosing a filler, all paired materials should be considered. Working conditions are also a very important factor when choosing filters. If a nickel base alloy is welded to a steel, then corresponding or similar Nickel fillers should be used.
The weld pool of nickel alloys is less liquid than unalloyed or even stainless steel. Otherwise, the viscosity of the weld pool and its dilution depends on the type of the alloy, that is, how much nickel is in the weld. In general, depending on what technology is used, a wider bevel and bigger root gap should be used when welding nickel alloys.
Due to its protective oxide surface layer pure nickel is highly resistant to corrosion attacks even in highly chloridic environments. A typical base alloy is the Alloy 200 series, used for manufacturing chemical-, food- and power industry equipment. To weld the equipment, UTP 80 Ni and similar fillers have been developed (see Table 2.).
Pure nickel has a lower yield strength than general steels. Therefore pure nickel is not suitable for the manufacturing of structures working under relatively high stresses. Most alloying elements of nickel results in primarily solid solutions which has a strengthening effect. One of the alloying elements is copper, which can be completely dissolved in nickel. One example of this alloying system with a 30% copper addition to the nickel is MONEL. A representative example of this system is called the Alloy 400 series. Alloy 400 is typically used in so-called “offshore” applications, in systems working in sea water or similarly salt heavy environments. The welding fillers developed for this purpose, and their typical chemical compositions, are shown in Table 2. Nickel-copper alloyed welding demonstrates outstanding corrosion resistance, not only in high concentrated NaOH alkali medium but also in slightly acidic environments (see figure 2.).
Figure 2. Speed of corrosion in nickel-copper alloy.
The R405 alloy is part of the Alloy 400 series, and contains a small amount of sulphur (0.025-0.060%) in order to ease machinability. Welding of R405 is difficult. The dilution of base material should be kept as low as possible during welding, in order to avoid the segregation of sulphur. If Alloy 400 is to operate in a hydrofluoric or calcium fluoride environment, a stress relieving heat treatment between 550-650 C should be applied after repair welding to prevent stress corrosion cracking.
Different types of nickel-molybdenum alloys are similar to each other. Because of the molybdenum content, these materials have excellent corrosion resistance and especially high pitting corrosion resistance in chloridic, reductive environments. Previously, Alloy B types had to undergo a solid solution treatment at 1175 C after welding, due to the strong carbide precipitation in the heat affected zone, but the enhanced Alloy B-2 can be used in its welded condition. Because these alloys do not contain chromium, they have little resistant against oxidizing atmospheres. Alloy B-2 can be used in high temperatures in any concentration of hydrochloric acid. Depending on the welding process, either the UTP 703Kb basic electrode or UTP A 703 MIG wire or TIG rods are the most optimal.
Nickel-chromium-(cobalt-iron) alloyed fillers
Over the past 50 years, developments in the power plant industry, have resulted in built-in boilers, heat exchangers and turbines having to undergo three times more pressure and tension at up to 200-300 oC higher temperatures. This change led to the development of new materials and applications within nuclear power plants, conventional power plants and waste energy power plants as well. The main advantage of nickel-chromium alloys is their maintained strength at higher temperatures and consistent resistance against the effects of oxidation due to its automatically generated oxide surface layer. The most well-known such alloy type is Alloy 600, which contains at least 15% chromium. Furnace walls and reactors built from these kinds of materials can be welded with UTP 068 HH or similar fillers, which are excellent in both oxidizing or carburizing environments. The further developed Alloy 690 contains with 29% chromium content. This material is typically applied in joint welding or pipe cladding in nuclear power plants. Alloy 690 exhibits outstanding resistance against intergranular stress corrosion cracking. While the base material tends to crack from heat, it can be welded securely with the correct technologies, like the UTP 6229Mn or the Thermanit 690 type filler.
Nickel-chromium, nickel-iron-chromium and nickel-chromium-molybdenum alloys are susceptible to carbide precipitation, but this has no considerable effect on its corrosion resistance, so it does not require a heat treatment solution after welding. In order to decrease carbide precipitation susceptibility, low carbon and stabilizing elements containing filler metals are used. After welding Alloy 600, if the equipment is working in a high temperature, alkali environment, a stress-relief annealing is carried out at 900 oC in order to avoid stress corrosion cracking.
There are a lot of carbide forming companion elements in nickel alloys. The carbides form chemical compounds in nickel-based alloys. The presence and distribution of carbides have an effect on the mechanical properties of the material and heighten its resistance against creep at higher temperatures. Precipitation hardenable nickel alloys are used to manufacture devices working under high mechanical stresses. These alloys must be exposed to heat treatment after welding. Depending on the alloy type, the heat treatment temperature is between 600-760 oC. If high strength is not required than another filler type can also be used.
Alloy 617 is used as a base material for the pipes in conventional fossil-fueled boiler heat exchangers. As we can unequivocally establish from Figure 3, the creep limit of ferritic chromium steel is about 100 Mpa after 100,000 working hours at 600 oC, while Alloy 617 maintains a consistent creep strength, even at an elevated temperature of 700 oC.
Figure 3. Creep fracture of different types of materials as a function of temperature after 100,000 working hours.
The occurrence of nickel is quite rare in the world. The raw ores are mined from sulphides-, oxides- or in silicate form. The metallic ore is enriched through further processing and nickel is subsequently produced by electrolysis or direct reduction. Pure nickel wire, sheets or piping processed using this method is widely used in many industries. Nevertheless, nickel is used in its pure state for manufacturing nickel alloys and surface coating, with steelmakers using the largest quantity of it in order to alloy steels. Within this field the application of nickel-based welding fillers is important. These are used with materials operating under heavily corrosive and/or hot working conditions, like cryogenic liquid gas tanks, the cladding of water turbines and . Nickel is also used for manufacturing equipment with dissimilar joints. Use of nickel-based welding materials is based on their physical and metallurgical properties. Table 1. shows the physical properties of different alloys.
Table 1. Some basic physical properties of nickel alloys. (alloy type, heat conduction, heat expansion, density, melting point)
The main properties of different nickel alloys are: an austenitic structure, surface oxide layer, lack of allotropic transformation, and often also a precipitation-hardenable alloy. These properties specify the application fields of different alloys. Classification of nickel alloys according to alloying elements are shown in Figure 1.
Figure 1. Classification of nickel alloys according to their alloying elements
Among these alloying elements, copper is completely soluble in nickel, iron and chromium have a relatively good solubility, while molybdenum, cobalt, wolfram, vanadium, niobium and titanium are soluble to some extent in nickel as well. Welding filler manufacturers, including Böhler, have developed various kinds of welding materials for the different types of nickel alloy. Here we have introduced the fillers and examples of typical applications of them, in accordance with the classification of nickel alloys above. The data of the base materials and welding fillers mentioned in this article are demonstrated in Table 2.
Table 2. Data of typical nickel alloys and their welding fillers (number, nickel-based alloys/welding material, chemical composition, application, material number, EN name, ASTM name)
Dubai’s metro and rail network is also continuously growing, reaching close to 50 stations the underground system connects various points and artificial islands in the desert city, raising myriad challenges for architects and construction crews alike.
While a number of the financial district’s metro stations are underground, much of the Red Line’s stations can be found looming over the 50km long, city-spanning Sheikh Zayed Road that in itself is two time 8 lanes wide at certain points. The steel structures of these stations is similarly impressive, connecting rails for over 80 self-driving trains capable of transporting up to 600 people each.
Curved roofs will be the signature of the metro stations, with an elevated shell-shape roofs intended to evoke the country’s pearl diving heritage. Generally, the stations are designed to be elliptical in shape, with the principle design philosophy being to wrap the station around the tracks.
Air-conditioned, closed pedestrian bridges made up of modular elements lead up to the metro stations.
Before they are installed, each pedestrian bridge is fully assembled off-site, including the glazing, external cladding, internal fixtures and finishes, and electrical and mechanical infrastructure. The largest pedestrian bridge weighs well in excess of 200 tonnes.
Special self-propelled modular transporters transport the pedestrian bridges to the metro site and lift them onto piers. The piers have flared pier heads similar to those seen on the main viaduct structures of the metro.
Construction of the stations’ steel frameworks began after the Rail Agency erected the steel structures of every metro depot at the end of each line, where the trains will be parked and serviced. When construction is complete, nearly 5,000 tonnes of steelwork will have been used in the construction of the depots.
According to sources, construction of the Dubai metro project has exceeded 30B USD, a large part of which has Gond towards building, installing and welding the steel structures.
Heat input intake must be kept within a relatively narrow range when welding duplex steels, it needs have both an upper and lower limit. Upper limitations are due to the chemical composition of duplex steel, which is inclined to unfavourable precipitation formation when subjected to very high temperatures across a long period of time. It’s also important to note the embrittlement at 475 °C, as well as sigma phase formation at 600 °C. The risk of these phenomena grows as Cr content increases (this is also the reason for duplex steel’s max. 250 °C and super-duplex steel’s max. 220 °C limited working temperature). The impact of the welding temperature can diminish corrosion resistance and mechanical properties, especially if the interpass temperature is too high, or the heat can’t effectively dissipate from the weld due to the welded piece’s specific shape. For these reasons, the lowest welding heat intake possible is a fundamental requirement, though higher temperatures and lower cooling speeds may be favorable, so prescribing a heat intake minimum is also recommended.
Earlier, we mentioned that solidification is ferritic and that ferrite-austenite transformation happens in a solid state, so cooling speeds that are too high (with too little heat intake) limit austenite formation. For this reason, the time during which the weld pool cools from 1200 °C to 800 °C is crucial. Pre-warming is usually unnecessary, used at most in order to remove humidity from the steel’s surface (in this case, make sure that the reductive gas flame doesn’t increase the surface carbon content via diffusion processes). As the ferrite austenite transformation takes place between 1200-800 °C, the max. 250-300 °C preheating temperature can’t considerably decrease the speed of cooling from a higher temperature or thereby increase the amount of austenite that is formed. However, the cooling time period below 800 °C will increase (the cooling speed will decrease) and the risk of precipitation will grow, so preheating will probably have a negative effect. For this same reason, during multi-layer welding the interpass temperature must not exceed 150 °C.
When developing the technologies for welding duplex steels, the physical properties of the steel must also be taken into account. The heat conducting, heat expanding factors differ considerably from regularly weldable carbon steels, but also from classic austenitic corrosion resistant steel. As a result of all of this, we can expect above average deformation, and should pay special attention when putting together the structures (tackweld and welding order).
Duplex steels can be welded with the simplest and oldest shielded metal arc welding process (SMAW/MMA). The technique can be used flexibly with plate thicknesses that are at least 2mm thick. It’s recommended for onsite and repair welding, which is exceptionally useful for welding the pipes of “offshore” or chemical industry structures. In order to increase corrosion resistance, applying cleaning, pickling and passivation procedures is recommended. The acid used during the pickling process is more aggressive than those used with 304 and 316 types, and its activation time is longer. Of course, pickling time varies according to the material and the temperature of the environment. Covered electrode processes are often used combined with TIG/AVI welding, where the root pass is welded with a tungsten electrode technique, and the multi-pass welds are done with covered electrodes. To make thicker welds or when you’re welding equipment used in low temperatures, a basic coated electrode is recommended. In this case, the manufacturer can guarantee the industry standard level of power in as low as -60 °C environments.
Table 2. The chemical composition of welding materials used with various duplex steels
The composition of (TIG/GTAW, or AVI) and MIG/MAG (filler metal electrode with shielding gas) welding materials is basically the same. Table 2 [5] contains welding material types used with various duplex materials in each procedure. In addition to the composition of the welding material, the choice of shielding gas technology for the work gas and the gas protecting the root can influence the ratio of austenite-ferrite in the structure.
Though consumable electrode and shielding gas welding is impossible without welding materials, in certain circumstances TIG welding is possible without them if special protective gases are used. These circumstances may arise when welding the butt weld of thin plates. With this welding technique, the use of N2 containing shielding gases is an advantage. As the increased Ni-content in the welding material ensures a balanced ratio of ferrite-austenite, so too does the nitrogen from the shielding gas achieve this when no welding material is used. Nitrogen is a strong austenite forming element, and as it enters the base metal during the welding process it helps form austenite there.
Furthermore, nitrogen has the biggest impact on pittingcorrosion resistance and it also enhances mechanical properties in small doses. Nitrogen’s solubility at 1600 oC is 0,045% and it quickly grows as chrome content increases [6]. For example, austenite steel’s N solubility is around 0,4%. As mentioned before, too much N2 content in addition to the material’s N content can cause gas porosity from nitrogen enrichment during cooling and ferrite’s low nitrogen dissolving capacity. It should be noted, that tungsten electrodes “wear” faster due to the nitrogen content in shielding gases. The electrode needs to be ground and changed more often than when the welding is done in a pure argon atmosphere.
Pure argon is the commonly used shielding gas used when AVI welding duplex steels. With this gas, the majority of welding work can be done securely and cost-effectively. Argon-hydrogen mixes — which are often used to weld austenitic steels in order to increase the speed of welding — are not recommended, because cracks from hydrogen may form from the increased ferrite content of the weld. Argon-helium mixes offer the potential to increase heat intake, with a favorable impact on the viscosity of the material, and they broaden the setting options for the welding parameters. The weld arc and the heat input also increases together with the helium content. In summary, the AVI technique guarantees the “purest” good quality weld from the perspective of both corrosion resistance and mechanical properties, but it’s also inefficient. Its application in welding thin plates or preparing root welds is very advantageous.
The heat intake value needs to have an upper and lower limit when gas metal arc welding (MIG/GMAW) duplex steels. For example, if the welds are welded too thinly the heat intake will be too small, resulting in an undesired weld structure where too much ferrite has formed. In contrast, too much heat input will cause austenite formation, and heat input exceeding 2 kJ/mm increases the risk of the formation of intermetallic phases (sigma, chi), which considerably detracts from both mechanical properties and corrosion resistance. A maximum 1.5 kJ/mm heat input and max. 100-150 oC interpass temperature should be maintained when welding super-duplex steels [7]. In general, the best welding quality can be achieved with an industrial pulse mode welding machine. The correct welding parameters need to be individually defined and checked for each job.
Usually the same protective gases are used with gas metal arc welding as with austenitic steels. Pure argon is generally not used for gas metal arc welding, as the arc will be unstable and the penetration low. Usually argon rich gas mixed with oxygen or carbon dioxide is used. With argon-oxygen mixed gases (with about a 1-3% oxygen content) the arc is even, and the material transfer is spatter-free. Compared to argon and carbon dioxide mixes, the penetration shape is worse and the weld’s surface is more strongly oxidized. The penetration can be increased by increasing oxygen content, but this will further oxidize the welded joint, thus argon and carbon dioxide mixtures containing 2-3% CO2 are more broadly used as they ensure a deeper penetration and less oxidation [6]. Compared to regular austenitic steels the penetration is worse and the weld pool viscosity is low, which heightens the risk of weld defects.
If we add helium to the mix — usually 30% — the melt pool will be more fluid, which ensures a better surface between the weld and the base material. Compared to pure argon or argon carbon-dioxide mixtures the greater arc voltage means that in addition to the other parameters, the heat input is significantly greater. This is advantageous, especially when a balanced ferrite-austenite ratio is a strict requirement. In the event that the material’s nitrogen content is high, ferrite’s nitrogen solving capacity may trigger nitrogen enrichment during cooling, which increases the risk for porosity.
We can achieve good results when metal arc welding duplex steels with a consumable flux cored wire electrode (FCAW process). The productivity of welding with flux cored wire electrodes is greater than with solid wire, and the classic 15-25% carbon-dioxide argon mix or even an economical carbon-dioxide welding gas to shield. The liquid pool’s protection is guaranteed in part by the flux of wire, thus a higher active gas content is allowed. Other advantages of the process are a wide and even arc that lower the risk of weld defects, and a lower tendency towards porosity or spatter. Discoloration or oxidation on the surface of the weld is shallow, decreasing the need to clean the weld after welding. Disadvantages of the process are that the slag needs to be carefully removed. Any remaining slag will create an inclusion which will result in welding failure. With the right quality of wire and good parameters the slag will separate, leaving it easily removable from the weld’s surface.
This can also lead to joint flaws or perhaps slag inclusion formation. For these reasons, an above average (wider) root face, and more open beveling (bigger bevel angle) is recommended. Classic rutile flux cored wire can only be used in a PA- position. The preferred fine grained spray arc material transition can be effectively achieved with a regular welding machine. Using a welding machine on 4 rolling feeders and applying ceramic root protection achieves the best result. There are so-called fast cooling slag creating consumable cored wire electrodes for position welding. The quickly solidifying slag helps support the melt until it completely solidifies. The wire, suitable for position welding the slag, usually separates harder and the applicable amperage is also lower than that of classic flux cored wire. The impact value of the weld is lower by flux cored wire than those welded with a similar type of solid wire.
In order to ensure the corrosion resistance of duplex steels during the processes described above, using some root protection is recommended. One method is the aforementioned root protecting ceramic backing, the other is to use the right gas shielding on the root side to keep air away. Usually, the same gasses are used for root shielding as with austenitic steels [8]. Due to the risk of cold cracking formations, the gas shield’s hydrogen content should be avoided. For root protection, argon, nitrogen or their mixture can be used. Generally speaking, in order to avoid root-side oxidation and to increase corrosion resistance, the remaining oxygen content on the root-side should not exceed 30 ppm. As a rule, spot corrosion resistance grows as root-side oxygen content decreases. An oxygen content measuring tool is the best way to ensure this.
Last but not least, submerged arc welding (SAW/UP) can also be used to weld duplex steel. Due to the peculiarities of the method it can only be done in a horizontal position, but in addition to a high effectiveness, it can also be used exceptionally well with plates thicker than 10mm. Root passes are usually prepared with a different process. The melt through value is smaller than welding standard austenitic corrosion resistant steels. With the right flux, good mechanical properties can be achieved. For example, using a flux with a high basically, we can get the highest impact value even at low temperatures. The durability of welds prepared like this is great. The formation of a lot of welding pour and excessive mixing measures should be avoided. Heat input should be maximized at 3 kJ/mm-ben for 2205 and 2304 duplex types, 1 kJ/mm-ben for LDX2101 lean-duplex and 2507/P100 super duplex steel [7].
Sources:
[1]Yrjöla,P.: Stainless steel hollow sections handbook, Finish Constructional Steelwork Association (2008), Helsinki
[5]How to weld duplex stainless steels, Avesta Welding
[6]Young,H.P,Zin-Hyoung,L.: The effect of nitrogen and heat treatment on the microstructure and tensile properties of 25Cr-7Ni-1.5Mo-3W-xN duplex stainless steel castings, Materials Science and Engineering,A297 (2001), pp.78-84
Duplex steels get their names for their special two-phase, approximately equal ferrite to austenite structure. Their favorable properties are due to this special microstructure, which combines the advantages of the two different types of corrosion resistant steels.
On the one hand, we have the ferrite and martensite “chrome steels,” whose high – over 18% – Cr content gives them relativelygood toughness, while the structure ensures high strength and thus a good resistance to stress corrosion cracking in chloride containing environments. However, their resistance to spot corrosion is worse and their weldability is limited compared to austenitic corrosion resistant steels. In the event that the cooling speed during welding is high, these steels are in large part inclined to hardening, that is, they harden and become brittle due to the formation of martensite.
On the other hand, we have the austenitic corrosion resistant steels, which are easy to weld, ductile and have a high resistance to gap and crevice corrosion.
In duplex steels, the Cr content is between 20-26%, Ni content is between 3-8%, and these materials generally contain between 1.5-5-5% molybdenum. These alloys further increase spot corrosion resistance, which is clearly demonstrated in Diagram 1. The names, classifications and chemical compositions of the most commonly used duplex steels are listed in Table 1 [5].
Table 1. Chemical composition, properties and standardised symbols of various duplex steels
Weldability of Duplex Steels
Ideally, the micro-structure of duplex steels is close to 50% austenite and 50% ferrite (Diagram 2). This ideal microstructure can be achieved by annealing the steel for close to 5 minutes at 1020-1100 °C, and then cooling it (in water) in a “controlled condition.” Duplex steels appear in the middle of the “austenite + delta ferrite” area of the well-known Schaeffler diagram, which represents corrosion resistant material structures.
Diagram 2. The characteristic structure of duplex steel contains 50% ferrite in the raw material (left picture) which, after welding, is contained in the weld (right picture) [5].
After welding, the ideal structure cannot be achieved after “unregulated” heating. Temperature differences and cooling speeds during welding alter the complex micro-structure, resulting in a modified structure that differs from the base metal and even throughout the binding’s different parts (Diagram 2). The impact of fast heat cycles in the heat-affected and melted zone may cause significant differentiation of the austenite-ferrite ratio from the base metal. Of course, this can impact the weld’s mechanical and corrosive properties as well. There is usually more ferrite in the heat-affected zone, while the amount of austenite is greater in the “middle” of the weld. After welding, generally 20-70% amount of ferrite form in various parts of the joint.
After fusion, the solidification is ferritic, and because the ferrite-austenite transformation happens in a solid state, an increased cooling speed can limit austenite formation. At the same time the high heat temperatures in the heat-affected zone may cause the unmelted austenite structure to become coarser from longer heating, or trigger a transformation to ferrite from slow cooling. Because of these two processes, the joint’s ferrite content can grow considerably, both in the heat-affected zone and in the melted part.
In order to achieve a proper microstructural balance in the weld, it’s crucial to choose welding material appropriate to the base metal. As a general rule, the welding materials used with duplex steels are basically the same as the base metal itself. However, the Ni content of welding materials is characteristically 2-4% larger than that of the base metal. Nickel’s austenite creating effect ensures a well-balanced austenite-ferrite ratio in the weld. Table 2 demonstrates the most commonly used welding materials and their compositions [5].
In some cases, especially when welding the root weld of steels with 22% Cr-content, higher chrome content containing welding materials are used in order to increase pitting corrosion resistance. But these welding materials, similar to their corresponding base metal, are more sensitive to the formation of intermetallic phases. As a result, ductility may decrease, so welding parameters must be thoughtfully chosen and strictly maintained. If the duplex or super-duplex welded joints must satisfy the highest requirements of corrosion resistance and mechanical properties, then overalloyed welding materials are recommended. In this case, an Ni-based alloy (where PRE=55…60) ensures the highest resistance to stress and pitting corrosion.
Duplex steels are favored for their good corrosion resistance and exceptional strength, as well as their relatively easy weldability. This is explored in detail in Diagram 1 [1], which illustrates the various duplex-, austenitic-, and super austenitic- steels’ mechanical properties and corrosion resistance. This article will dive into the weldability of duplex steels. We’ll introduce the effect of the material’s chemical composition, its corrosion resistant and mechanical properties, the metallurgical processes that take place during welding, as well as the different welding processes and the effects of their technological parameters.
Diagram 1. Corrosion resistance and solidity of various steels
One of the most frequent types of corrosion is spot corrosion, such as pitting- or crevice corrosion. Corrosion resistant steels are often ranked by their chemical composition, in relation to the level of their corrosion resistance. The PRE (Pitting Resistance Equivalent) value is an empirically calculated number, which factors each alloy element together to define overall resistance to pitting corrosion. These elements are Cr, Mo, and N. The most commonly used formula for this is the Herbsleb [2] equation:
PRE = % Cr + 3.3 % Mo + 30 % N
The equation demonstrates that Nitrogen content has the biggest impact on the PRE value, which isn’t surprising, as Nitrogen is a very strong austenite-forming element.
There are many published equations for calculating PRE similar to the one above, which seek to provide a more exact approximation for specific steel types. Of these, one of the more common ones is:
PRE=%Cr+3.3 (%Mo+0.5x%W)+a %N
(where “a”=16: for duplex and super-duplex steels, and “a”=30: for super-austenitic steels [3])
In this equation, the impact of tungsten’s pitting corrosion resistance is also defined. This is notable in the case of tungsten alloy super-duplex steels like 1.4501. In most stainless steels, the amount of nitrogen compared to the main alloys is minimal, thus the impact of multiplying factor “a” on pitting resistance is insignificant.
Pitting resistance is not measured using the usual weight loss based corrosion measurements. Though the signs of corrosion are clear, weight loss is minimal. Naturally there is a regulated measurement process to define pitting corrosion: The samples are “dissolved” in an aggressive chloride-ionic environment, and their surfaces are analysed after cleaning. These analyses are run in various temperatures, and the lowest temperature in which corrosion appears is labelled the CTP (critical pitting temperature) [4]. The analyses clearly prove that as corrosion resistance grows, the temperature at which corrosion begins rises linearly. Thus, duplex and super-austenitic materials can be used more securely in higher temperatures than traditional corrosion resistant steels.
