Stainless Steel Corrosion There are five main types of stainless steel: ferritic, martensitic, austenitic, precipitation hardening and duplex. The ferritic and martensitic grades are so named because of their crystal structures. Both are iron-chromium-based alloys and were the type of stainless steel first developed in the early 1900’s. The ferritic and martensitic stainless steels are magnetic. The martensitic stainless steels can be hardened by a heat treatment similar to that used to harden ordinary steel, namely, heating to a high temperature, quenching, then reheating to an intermediate temperature (tempering) to achieve the desired balance of hardness and ductility.

Stainless and heat resisting steels possess unusual resistance to attack by corrosive media at atmospheric and elevated temperatures, and are produced to cover a wide range of mechanical and physical properties for particular applications.

Along with iron and chromium, all stainless steels contain some carbon. It is difficult to get much less than about 0.03 % and sometimes carbon is deliberately added up to 1.00% or more. The more carbon there is, the more chromium must be used, because carbon can take from the alloy about seventeen times its own weight of chromium to form carbides. Chromium carbide is of little use for resisting corrosion. The carbon, of course, is added for the same purpose as in ordinary steels to make the alloy stronger.

Stainless steels are mainly used in wet environments. With increasing chromium and molybdenum contents, the steels become increasingly resistant to aggressive solutions. The higher nickel content reduces the risk of SCC. Austenitic steels are more or less resistant to general corrosion, crevice corrosion and pitting, depending on the quantity of alloying elements. Resistance to pitting and crevice corrosion is very important if the steel is to be used in chloride containing environments. Resistance to pitting and crevice corrosion typically increases with increasing contents of chromium, molybdenum and nitrogen.

Corrosion resistance of stainless steels is a function not only of composition, but also of heat treatment, surface condition, and fabrication procedures, all of which may change the thermodynamic activity of the surface and thus dramatically affect the corrosion resistance. It is not necessary to chemically treat stainless steels to achieve passivity. The passive film forms spontaneously in the presence of oxygen. Most frequently, when steels are treated to

improve passivity (passivation treatment), surface contaminants are removed by pickling to allow the passive film to reform in air, which it does almost immediately. Most of the ferritic and martensitic stainless steels have limited corrosion resistance in marine environments, but some of the newly developed ferritic grade s (often called “superferritics”) have excellent marine corrosion resistance and are widely used in applications such as tubes for power plant condensers.

Pickling and Passivation

Stainless steel can corrode in service if there is contamination of the surface. Both pickling and passivation are chemical treatments applied to the surface of stainless steel to remove contaminants and assist the formation of a continuous chromium-oxide, passive film. Pickling and passivation are both acid treatments and neither will remove grease or oil. If the fabrication is dirty, it may be necessary to use a detergent or alkaline clean before pickling or passivation. (reference)

Stainless Steel Weld Decay

This type of intergranular corrosion can occur in the heat-affected zone of welded components and also in cast components of stainless steel due to precipitation, during cooling, of chromium carbides at the grain boundaries (and hence loss of chromium in the immediately-adjacent zone). The local loss in corrosion resistance arises because the chromium is crucial in promoting the formation of a Cr-rich passive film on the surface of stainless steels. The susceptibility to weld decay can be counteracted by carrying out a suitable post-weld heat treatment to restore a uniform composition at the grain boundaries but this is clearly often not a practicable proposition. Consequently the usual strategy in combating weld decay is by the choice of stainless steel with either of the two following features:

a. specification of a stainless steel containing a small amount of either titanium or niobium; which have a higher affinity than does chromium for carbon: hence carbides of these elements tend to form instead of chromium carbides, thus avoiding the Cr-depletion problem: such steels are usually termed “stabilised stainless steels”

b. specification of a stainless steel with low carbon content (< 0.03%); this will clearly decrease the likelihood of carbide formation in the steel. Such low-carbon grades of stainless steel are often designated by a “L” in their code; for instance the “316” grade of steel (18%Cr/10Ni/2.5Mo) is designated as “316L” when its carbon content has been limited in this way.

Stress corrosion cracking (SCC)

Austenitic stainless steels suffer from stress corrosion cracking in hot solutions containing chloride. A high chloride concentration is required, although relatively small amounts of chloride are sufficient at heated surfaces, where chloride concentration can occur, or where chloride is concentrated by pitting or crevice corrosion, and problems can be experienced in tap water.

The temperature usually needs to be above 70°C, although SCC can occur at lower temperatures in some situations, notably more acid solutions. The cracking continues at low stresses and commonly occurs as a result of residual stresses from welding or fabrication. The cracking is normally transgranular, although it may switch to an intergranular path as a result of sensitization of the steel.

Stainless Steel Rouging

Rouging is a thin film, usually reddish-brown or golden in color, of iron oxide or hydroxide, typically on stainless steels. The contrast between this film and shiny metal accentuates this aesthetics problem. The rouge film typically wipes off easily with a light cloth (Figure 1), but it reforms while the process fluid is in contact with the stainless steel. This problem is most chronic in the pharmaceutical industry on the interior surfaces of high purity water (i.e., water for injection, WFI) distillation units, storage tanks, distribution systems (piping, valves, pump housings, fittings, etc.) and process vessels. (reference)

Corrosion Theory

Humans have most likely been trying to understand and control corrosion for as long as they have been using metal objects. The most important periods of prerecorded history are named for the metals that were used for tools and weapons (Iron Age, Bronze Age). With a few exceptions, metals are unstable in ordinary aqueous environments. Metals are usually extracted from ores through the application of a considerable amount of energy. Certain environments offer opportunities for these metals to combine chemically with elements to form compounds and return to their lower energy levels.

Corrosion is the primary means by which metals deteriorate. Most metals corrode on contact with water (and moisture in the air), acids, bases, salts, oils, aggressive metal polishes, and other solid and liquid chemicals. Metals will also corrode when exposed to gaseous materials like acid vapors, formaldehyde gas, ammonia gas, and sulfur containing gases.

Corrosion specifically refers to any process involving the deterioration or degradation of metal components. The best known case is that of the rusting of steel. Corrosion processes are usually electrochemical in nature, having the essential features of a battery. When metal atoms are exposed to an environment containing water molecules they can give up electrons, becoming themselves positively charged ions, provided an electrical circuit can be completed. This effect can be concentrated locally to form a pit or, sometimes, a crack, or it can extend across a wide area to produce general wastage. Localized corrosion that leads to pitting may provide sites for fatigue initiation and, additionally, corrosive agents like seawater may lead to greatly enhanced growth of the fatigue crack. Pitting corrosion also occurs much faster in areas where microstructural changes have occurred due to welding operations.

Corrosion is the disintegration of metal through an unintentional chemical or electrochemical action, starting at its surface. All metals exhibit a tendency to be oxidized, some more easily than others. A tabulation of the relative strength of this tendency is called the galvanic series. Knowledge of a metal's location in the series is an important piece of information to have in making decisions about its potential usefulness for structural and other applications.

The corrosion process (anodic reaction) of the metal dissolving as ions generates some electrons, as shown here, that are consumed by a secondary process (cathodic reaction). These two processes have to balance their charges. The sites hosting these two processes can be located close to each other on the metal's surface, or far apart depending on the circumstances. This simple observation has a major impact in many aspects of corrosion prevention and control, for designing new corrosion monitoring techniques to avoiding the most insidious or localized forms of corrosion. (more advanced reading)

The electrons (e- in this figure) produced by the corrosion reaction will need to be consumed by a cathodic reaction in close proximity to the corrosion reaction itself. The electrons and the hydrogen ions react to first form atomic hydrogen, and then molecular hydrogen gas. If the acidity level is high (low pH), this molecular hydrogen will readily become a gas as it is demonstrated by exposing a strip of zinc to a sulfuric acid solution.

As hydrogen forms, it could inhibit further corrosion by forming a very thin gaseous film at the surface of the metal. This "polarizing" film can be effective in reducing water to metal contact and thus in reducing corrosion. Yet it is clear that anything which breaks down this barrier film tends to increase the rate of corrosion. Dissolved oxygen in the water will react with the hydrogen, converting it to water, and destroying the film.

High water velocities tend to sweep the film away, exposing fresh metal to the water. Similarly, solid particles in the water can brush the hydrogen film from the metal. Other corrosion accelerating forces include high concentrations of free hydrogen ions (low pH) which speed the release of the electrons, and high water temperatures, which increase virtually all chemical reaction, rates. Thus a variety of natural and environmental factors can have significant effects on the corrosion rate of metals, even when no other special conditions are involved.

Corrosion Rate ConversionThe following charts provide a simple way to convert data between the most common corrosion units in usage, i.e. corrosion current (mA cm-2) , mass loss (g m-2 day-1) and penetration rates (mm y-1 or mpy) for all metals or for steel

  mA cm-2 mm year-1 mpy g m-2 day-1 mA cm-2 1 3.28 M/nd 129 M/nd 8.95 M/n mm year-1 0.306 nd/M 1 39.4 2.74 d mpy 0.00777 nd/M 0.0254 1 0.0694 d g m-2 day-1 0.112 n/M 0.365 /d 14.4 /d 1

← where: ← mpy = milli-inch per year ← n = number of electrons freed by the corrosion reaction ← M = atomic mass ← d = density

Note: you should read the Table from left to right, i.e.:1 mA cm-2 = (3.28 M/nd) mm y-1 = (129 M/nd) mpy = (8.95 M/n) g m-2

day-1

For example, if the metal is steel or iron (Fe), n =2, M = 55.85 g and d = 7.88 g cm-3 and the Table of conversion becomes:

  mA cm-2 mm year-1 mpy g m-2 day-1 mA cm-2 1 11.6 456 249 mm year-1 0.0863 1 39.4 21.6 mpy 0.00219 0.0254 1 0.547 g m-2 day-1 0.00401 0.0463 1.83 1

 Note: you should read the Table from left to right, i.e.:

1 mA cm-2 = 11.6 mm y-1 = 456 mpy = 249 g m-2 day-1

Controlling Stress Corrosion Cracking (SCC)

In order for SCC to occur, we require a susceptible material, an environment that will cause cracking of that material and a high enough stress or stress intensity factor. There are, consequently, a number of approaches that we can use to prevent SCC, or at least to give an acceptable lifetime. In an ideal world a stress corrosion cracking control strategy will start operating at the design stage, and will focus on the selection of material, the limitation of stress and the control of the environment. The skill of the engineer then lies in selecting the strategy that delivers the required performance at minimum cost. In this context we should appreciate that a part of the performance requirement relates to the acceptability of failure. For the primary containment pressure vessel in a nuclear reactor we obviously require a very low risk of failure. For the pressed brass decorative trim on a light switch, the occasional stress corrosion crack is not going to be a serious problem, although frequent failures would have an undesirable impact on product returns and the corporate image. (reference)

Selection and control of material

The first line of defence in controlling stress corrosion cracking is to be aware of the possibility at the design and construction stages. By choosing a material that is not susceptible to SCC in the service environment, and by processing and fabricating it correctly, subsequent SCC problems can be avoided. Unfortunately, it is not always quite that simple. Some environments, such as high temperature water, are very aggressive, and will cause SCC of most materials. Mechanical requirements, such as a high yield strength, can be very difficult to reconcile with SCC resistance (especially where hydrogen embrittlement is involved). Finally, of course, Murphy’s Law dictates that the materials that are resistant to SCC will almost inevitably be the most expensive (and that they will be found to be

susceptible to SCC in your environment as soon as you have used them!).

Control of stress

As one of the requirements for stress corrosion cracking is the presence of stress in the components, one method of control is to eliminate that stress, or at least reduce it below the threshold stress for SCC. This is not usually feasible for working stresses (the stress that the component is intended to support), but it may be possible where the stress causing cracking is a residual stress introduced during welding or forming.

Residual stresses can be relieved by stress-relief annealing, and this is widely used for carbon steels. These have the advantage of a relatively high threshold stress for most environments, consequently it is relatively easy to reduce the residual stresses to a low enough level. In contrast austenitic stainless steels have a very low threshold stress for chloride SCC. This, combined with the high annealing temperatures that are necessary to avoid other problems, such as sensitization and sigma phase embrittlement, means that stress relief is rarely successful as a method of controlling SCC for this system.

For large structures, for which full stress-relief annealing is difficult or impossible, partial stress relief around welds and other critical areas may be of value. However, this must be done in a controlled way to avoid creating new regions of high residual stress, and expert advice is advisable if this approach is adopted.

Stresses can also be relieved mechanically. For example, hydrostatic testing beyond yield will tend to ‘even-out’ the stresses and thereby reduce the peak residual stress. Similarly shot-peening or grit-blasting tend to introduce a surface compressive stress, and are beneficial for the control of SCC. The uniformity with which these processes are applied is important. If, for example, only the weld region is shot-peened, damaging tensile stresses may be created at the border of the peened area.

Control of environment

The most direct way of controlling SCC through control of the environment is to remove or replace the component of the environment that is responsible for the problem. Unfortunately, it is relatively rare for this approach to be applicable. If the active species is present in an environment over which we have some control, then it may be feasible to remove the active species, although even then it

may be difficult. For example, chloride stress corrosion cracking of austenitic stainless steels has been experienced in hot-water jackets around chocolate pipes (that is to say, pipes carrying molten chocolate) in the food industry. In this situation we can’t easily change the material or the temperature, and it is virtually impossible to eliminate the residual stresses associated with welding and forming of the stainless steel. However, we can remove the chloride from the water by an ion exchange process, and, with proper control and monitoring, this approach could be successful. Of course if we were dealing with hot tomato ketchup, which has a low pH and may contain enough chloride to cause SCC, we have a far more difficult problem!

In the latter situation, where the species responsible for cracking are a required component of the environment, the environmental control options consist of adding inhibitors, modifying the electrode potential of the metal, or isolating the metal from the environment with coatings.

To take another example of chloride SCC of austenitic stainless steels, tube and shell heat exchangers are frequently constructed using stainless steel tubes (since these must be thin-walled and corrosion cannot be tolerated) with carbon steel tube plates and shell (since these can be made much thicker to provide a corrosion allowance). Chloride SCC is rarely experienced with this construction. However, it is quite common for an enthusiastic engineer to decide that the replacement heat exchanger should use an “all-stainless” construction to avoid the unsightly corrosion of the carbon steel. The result is frequently a rapid failure of the heat exchanger by SCC or pitting corrosion. This is because the carbon steel adopts a relatively low electrode potential that is well below that required to cause SCC or pitting of austenitic stainless steel, which is thereby protected. When the all-stainless construction is adopted, this unintentional electrochemical protection is lost and failure occurs.

Corrosion inhibitors are chemicals that reduce the rate of a corrosive process. They are generally rather specific to a particular alloy system, and they typically also have specific requirements in terms of the composition of the environment. Inhibitors may be effective at controlling SCC, although the requirements are rather different from those for the inhibition of general corrosion. Indeed chemicals that inhibit general corrosion may create the necessary conditions for stress corrosion cracking (e.g. hydroxides, carbonates and nitrates for carbon steel). Even when inhibitors are effective against SCC, higher concentrations may be required than for the inhibition of general corrosion.

Metallic coatings isolate the metal from the environment, and can, thereby, prevent SCC. However, the possibility of the coating being penetrated by imperfect application or by mechanical damage in service must be taken into account. For this reason zinc is a popular coating for carbon steel. The normal corrosion potential for zinc is relatively low, and if any of the underlying steel is exposed, this will be cathodically protected. However, the low electrode potential will also encourage hydrogen evolution, and this may lead to hydrogen embrittlement. Hydrogen embrittlement may also occur as a result of the hydrogen evolution during the initial electroplating operation, as noted above. Consequently, zinc plating must be used with care on strong steels. Cadmium adopts a rather more positive potential, and produces a much lower risk of hydrogen embrittlement, while still protecting the underlying steel. Unfortunately the toxicity of cadmium compounds means that it is essentially banned as a coating material.

Paints and other polymeric coatings protect the underlying metal largely by virtue of their high electrical resistance, which restricts the passage of current from the anode to the cathode (both oxygen and water diffuse relatively easily through most polymers, so paints don’t, as is often thought, work by isolating the metal from the environment). Paints may be effective at restricting SCC, particularly where they incorporate inhibitors that can inhibit any solution that does find its way to the metal. However, as with metallic coatings, it is important to think about what will happen if the coating is removed by mechanical damage.

Welding Stainless Steel

The stainless properties of stainless steels are primarily due to the presence of chromium in quantities greater than roughly 12 weight percent.  This level of chromium is the minimum level of chromium to ensure a continuous stable layer of protective chromium-rich oxide forms on the surface.  The ability to form chromium oxide in the weld region must be maintained to ensure stainless properties of the weld region after welding.  In commercial practice, however, some stainless steels are sold containing as little as 9 weight percent chromium and will rust at ambient temperatures.

Stainless steels are generally classified by their microstructure and are identified as ferritic, martensitic, austenitic, or duplex (austenitic and ferritic).  The microstructure significantly affects the weld properties and the choice of welding procedure used for these stainless steel alloys.  In addition, a number of precipitation-hardenable (PH) stainless steels exist.  Precipitation-hardenable stainless steels have martensitic or austenitic microstructures.

Iron, carbon, chromium and nickel are the primary elements found in stainless steels and significantly affect microstructure and welding.  Other alloying elements are added to control microstructure or enhance material properties.  These other alloys affect welding properties by changing the chromium or nickel equivalents and thereby changing the microstructure of the weld metal.  Generally, 200 and 300 series alloys are mostly austenitic and 400 series alloys are ferritic or martensitic, but exceptions exist.

Stainless steels are subject to several forms of localized corrosive attack.  The prevention of localized corrosive attack is one of the concerns when selecting base metal, filler metal and welding procedures when fabricating components from stainless steels.

Stainless steels are subject to weld metal and heat affected zone cracking, the formation of embrittling second phases and concerns about ductile to brittle fracture transition.  The prevention of cracking or the formation of embrittling microstructures is another main concern when welding or fabricating stainless steels. 

Welding Austenitic Stainless Steels

Ideally, austenitic stainless steels exhibit a single-phase, the face-centered cubic (fcc) structure, that is maintained over a wide range of temperatures. This structure results from a balance of alloying additions, primarily nickel, that stabilize the austenite phase from elevated to cryogenic temperatures. Because these alloys are predominantly single phase, they can only be strengthened by solid-solution alloying or by work hardening.  Precipitation-strengthened austenitic stainless steels will be discussed separately below.

The austenitic stainless steels were developed for use in both mild and severe corrosive conditions.  Austenitic stainless steels are used at temperatures that range from cryogenic temperatures, where they exhibit high toughness, to elevated temperatures, where they exhibit good oxidation resistance. Because the austenitic materials are

nonmagnetic, they are sometimes used in applications where magnetic materials are not acceptable.

The most common types of austenitic stainless steels are the 200 and 300 series.  Within these two grades, the alloying additions vary significantly.  Furthermore, alloying additions and specific alloy composition can have a major effect on weldability and the as-welded microstructure.  The 300 series of alloys typically contain from 8 to 20 weight percent Ni and from 16 to 25 weight percent Cr.  

A concern, when welding the austenitic stainless steels, is the susceptibility to solidification and liquation cracking.  Cracks can occur in various regions of the weld with different orientations, such as centerline cracks, transverse cracks, and microcracks in the underlying weld metal or adjacent heat-affected zone (HAZ).  These cracks are primarily due, to low-melting liquid phases, which allow boundaries to separate under the thermal and shrinkage stresses during weld solidification and cooling.

Even with these cracking concerns, the austenitic stainless steels are generally considered the most weldable of the stainless steels. Because of their physical properties, the welding behavior of austenitic stainless steels is different than the ferritic, martensitic, and duplex stainless steels.  For example, the thermal conductivity of austenitic alloys is roughly half that of ferritic alloys.  Therefore, the weld heat input that is required to achieve the same penetration is reduced.  In contrast, the coefficient of thermal expansion of austenite is 30 to 40 percent greater than that of ferrite, which can result in increases in both distortion and residual stresses, due to welding.  The molten weld pool of the austenitic stainless steels is commonly more viscous, or sluggish, than ferritic and martensitic alloys.  This slows down the metal flow and wettability of welds in austenitic alloys, which may promote lack-of-fusion defects when poor welding procedures are employed.

Welding Ferritic Stainless Steels

Ferritic stainless steels comprise approximately half of the 400 series stainless steels.  These steels contain from 10.5 to 30 weight percent chromium along with other alloying elements, particularly molybdenum.  Ferritic stainless steels are noted for their stress-corrosion cracking (SCC) resistance and good resistance to pitting and crevice corrosion in chloride environments, but have poor toughness, especially in the welded condition.

Ideally, ferritic stainless steels have the body-centered cubic (bcc) crystal structure known as ferrite at all temperatures below their melting temperatures.  Many of these alloys are subject to the precipitation of undesirable intermetallic phases when exposed to certain temperature ranges.  The higher-chromium alloys can be embrittled by precipitation of the tetragonal sigma phase, which is based on the compound FeCr.

Molybdenum promotes formation of the complex cubic chi phase, which has a nominal composition of Fe36Cr12Mo10.  Embrittlement increases with increasing chromium plus molybdenum contents.  It is generally agreed that the severe embrittlement which occurs upon long-term exposure is due to the decomposition of the iron-chromium ferrite phase into a mixture of iron-rich alpha and chromium-rich alpha-prime phases.  This embrittlement is often called "alpha-prime embrittlement."   Additional reactions such as chromium carbide and nitride precipitation may play a significant role in the more rapid, early stage 885 °F embrittlement.

The ferritic stainless steels have higher yield strengths and lower ductilities than austenitic stainless steels.  Like carbon steels, and unlike austenitic stainless steels, the ferritic stainless alloys exhibit a transition from ductile-to-brittle behavior as the temperature is reduced, especially in notched impact tests.  The ductile-to-brittle transition temperature (DBTT) for the ultrahigh-purity ferritic stainless steels is lower than that for standard ferritic stainless steels.  It is typically below room temperature for the ultrahigh-purity ferritic stainless steels.  Nickel additions lower the DBTT and there by slightly increase the thicknesses associated with high toughness.  Nevertheless, with or without nickel, the ferritic stainless steels would need engineering review for anything other than thin walled applications as they are prone to brittle failure.

Welding Martensitic Stainless Steels

Martensitic stainless steels are considered to be the most difficult of the stainless steel alloys to weld.  Higher carbon contents will produce

greater hardness and, therefore, an increased susceptibility to cracking.

In addition to the problems that result from localized stresses associated with the volume change upon martensitic transformation, the risk of cracking will increase when hydrogen from various sources is present in the weld metal.  A complete and appropriate welding process is needed to prevent cracking and produce a sound weld.

Martensitic stainless steels are essentially alloys of chromium and carbon that possess a body-centered cubic (bcc) or body-centered tetragonal (bct) crystal structure (martensitic) in the hardened condition. They are ferromagnetic and hardenable by heat treatments.  Their general resistance to corrosion is adequate for some corrosive environments, but not as good as other stainless steels.

The chromium content of these materials generally ranges from 11.5 to 18 weight percent, and their carbon content can be as high as 1.2 weight percent.  The chromium and carbon contents are balanced to ensure a martensitic structure after hardening.  Martensitic stainless steels are chosen for their good tensile strength, creep, and fatigue strength properties, in combination with moderate corrosion resistance and heat resistance.

The most commonly used alloy within this stainless steel family is type 410, which contains about 12 weight percent chromium and 0.1 weight percent carbon to provide strength.  Molybdenum can be added to improve mechanical properties or corrosion resistance.  Nickel can be added for the same reasons.  When higher chromium levels are used to improve corrosion resistance, nickel also serves to maintain the desired microstructure and to prevent excessive free ferrite.  The limitations on the alloy content required to maintain the desired fully martensitic structure restrict the obtainable corrosion resistance to moderate levels.

Welding Duplex Stainless Steels

Duplex stainless steels are two phase alloys based on the iron-chromium-nickel system.  Duplex stainless steels usually comprise approximately equal proportions of the body-centered cubic (bcc) ferrite and face-centered cubic (fcc) austenite phases in their microstructure and generally have a low carbon content as well as, additions of molybdenum, nitrogen, tungsten, and copper.  Typical chromium contents are 20 to 30 weight percent and nickel contents are 5 to 10 weight percent.  The specific advantages offered by duplex stainless steels over conventional 300 series stainless steels are strength, chloride stress-corrosion cracking resistance, and pitting corrosion resistance.

Duplex stainless steels are used in the intermediate temperature ranges from ambient to several hundred degrees Fahrenheit (depending on environment), where resistance to acids and aqueous chlorides is required.  The weldability and welding characteristics of duplex stainless steels are better than those of ferritic stainless steels, but generally not as good as austenitic materials.

A suitable welding process is needed to obtain sound welds.  Duplex stainless steel weldability is generally good, although it is not as forgiving as austenitic stainless steels.  Control of heat input is important.  Solidification cracking and hydrogen cracking are concerns when welding duplex stainless steels, but not as significant for some other stainless steel alloys. 

Current commercial grades of duplex stainless steels contain between 22 and 26 weight percent chromium, 4 to 7 weight percent nickel, up to 4.5 weight percent molybdenum, as well as some copper, tungsten, and nitrogen.  Modifications to the alloy compositions have been made to improve corrosion resistance, workability, and weldability.  In particular, nitrogen additions have been effective in improving pitting corrosion resistance and weldability.

The properties of duplex stainless steels can be appreciably affected by welding.  Due to the importance of maintaining a balanced microstructure and avoiding the formation of undesirable metallurgical phases, the welding procedures must be properly specified and controlled.  If the welding procedure is improper and disrupts the appropriate microstructure, loss of material properties can occur.

Because these steels derive properties from both austenitic and ferritic portions of the structure, many of the single-phase base material characteristics are also evident in duplex materials.  Austenitic stainless steels have good weldability and low-temperature toughness, whereas their chloride SCC resistance and strength are comparatively

poor.  Ferritic stainless steels have good resistance to chloride SCC but have poor toughness, especially in the welded condition.  A duplex microstructure with high ferrite content can therefore have poor low-temperature notch toughness, whereas a structure with high austenite content can possess low strength and reduced resistance to chloride SCC.

The high alloy content of duplex stainless steels also makes them susceptible to the formation of intermetallic phases from extended exposure to high temperatures.  Significant intermetallic precipitation may lead to a loss of corrosion resistance and sometimes to a loss of toughness.

Duplex stainless steels have roughly equal proportions of austenite and ferrite, with ferrite being the matrix.  The duplex stainless steels alloying additions are either austenite or ferrite formers.  This is occurs by extending the temperature range over which the phase is stable.  Among the major alloying elements in duplex stainless steels chromium and molybdenum are ferrite formers, whereas nickel, carbon, nitrogen, and copper are austenite formers.

Composition also plays a major role in the corrosion resistance of duplex stainless steels.  Pitting corrosion resistance can be adversely affected.  To determine the extent of pitting corrosion resistance offered by the material, a pitting resistance equivalent is commonly used.

Welding Precipitation-Hardenable Stainless Steels

Precipitation-hardening (PH) stainless steels are iron-chromium-nickel alloys.  They generally have better corrosion resistance than martensitic stainless steels.  The high tensile strengths of the PH stainless steels is due to precipitation hardening of a martensitic or austenitic matrix.  Copper, aluminum, titanium, niobium (columbium), and molybdenum are the primary elements added to these stainless steels to promote precipitation hardening.

Precipitation-hardening stainless steels are commonly categorized into three types martensitic, semiaustenitic, and austenitic based on their martensite start and finish (Ms and Mf) temperatures and the resulting microstructures.  The issues involved in welding PH steels are different for each group. 

It is important to understand the microstructure of the particular type of alloy being welded.  Some of the PH stainless steels solidify as primary ferrite and have relatively good resistance to hot cracking.   In

other PH stainless steels,  ferrite is not formed, and it is more difficult to weld these alloys without hot cracking.

If your company is experiencing these or other welding problems you can retain AMC to improve your weld processing.  Hire AMC to act as your welding specialist.