Duplex Stainless Steels - FAQ's during fabrication

The following questions reflect the experience of fabricators in the questions most typically asked during fabrication of duplex stainless steel. Answers are suggested but in these practical matters, there is a wide range of possibly "correct" answers. The answer given may not be applicable to all possible situations. For further reading on welding guidelines please visit the Fabrication and Welding page. Feel free to send us your own question to duplex.ssw@kci-world.com.

1. Although it is recommended to use plasma torches for back gouging of the root and defect removal, can a conventional carbon arc be used? What is a minimum grinding that should follow arc gouging in order to remove heat-affected layer?

Carbon arc back gouging has been successfully used in the construction of 2205 duplex stainless steel vessels, but care must be taken to minimise the heating and the potential for carbon contamination. When care is taken in the back gouging procedure, the minimum grinding is not burdensome. It would be appropriate to perform a weld procedure qualification in which the typically applied back gouging has been included as it will be used in the practical construction.

2. What is the maximum allowed thickness reduction resulting from cold forming before solution anneal/ water quenching treatment would be required?

A precise answer to this question has not been developed. However, it has been common in construction of 2205 duplex stainless steel vessels to apply the same limits that are applied to carbon steels by the ASME Code. This limitation, while possibly overly conservative, has not led to any problems in service. For vessels not being constructed to ASME Code, significantly more aggressive deformation has been permitted, with no reports of problems attributed to this cold worked condition.

3. What is a proper method of repairing small defects and metal tears on the process side (for example, caused by knocking off of the pre-cut ladder supports and lifting lugs - usual method of removal)?

The repair procedure most typically satisfactory is to open the defect by grinding, if necessary, and then to repair by GTAW with the typical matching filler. Because of the size of the weld involved, it is unlikely that small defects or tears will lead to excessive time at temperature for these repair welds. However, care must be taken to avoid too rapid cooling of the weld (with resulting excessive ferrite). Slight warming of the metal under shielded conditions using the weld torch before the filler is introduced will typically prevent too rapid cooling. Autogenous repairs are not recommended because of the likelihood of forming excessive ferrite in the weld.

4. Excessive heat input may result from weld repair of the defect discovered by post-weld NDT. Should such procedure be pre-qualified, and how?

It is appropriate to demonstrate that the weld repair has not damaged the material, i.e., to qualify the repair in much the same way as the procedure was qualified. So fabricators have qualified reasonably anticipated repair procedures in advance. Alternatively, the repair practice can be documented and simulated on a production runout tab, with the usual production test plate procedures then being applied to the repaired weld.

5. What is in fact the upper limit for weld heat input, provided base metal fully passed A-923 criteria?

Because the goal is to limit total time at temperature, it is generally better to complete a weld in fewer passes with relatively high heat input than many passes of lower heat input. The duplex stainless steels can tolerate relatively high heat inputs. It is not impossible to hot crack a duplex stainless steel during welding, but it is rare. The duplex stainless steels have relatively low thermal expansion and high thermal conductivity. The solidification of the duplex filler metals is not prone to hot cracking as is a fully austenitic solidification. Maximum heat input values as high as 65-100 kJ/mm have been found to be satisfactory, depending on the welding process.

6. Can heat input be allowed below the mentioned bottom value of 0.5 kJ/mm as long as the ferrite content does not exceed 70% (for example, due to the over-alloying of the base and electrodes)?

Exceedingly low heat input is permitted, provided that the result is demonstrated to meet the usual requirements for phase balance and corrosion resistance.

7. Does soda lime glass bead blasting provide an adequate finish for corrosive service, as an alternative to pickling and what is the recommended surface profile range?

Whether or not a glass blasting will be sufficient for corrosive service will depend on the degree and nature of the oxidized surface and the corrosivity of the service, including the tendency of the medium to adhere to the surface of the steel. While a pickled surface provides corrosion resistance to the maximum capability of the grade, a thoroughly blasted surface may be sufficient and economical. Scale and heat tint for the duplex stainless steels are especially adherent and resistant to both mechanical and chemical removal.

8. What is the best way to prepare weld/HAZ specimens for A 923 Method C testing?

The specimen should be removed by the method least disruptive of the metal condition. Cold cutting is recommended if possible. If a hot cutting method is applied, then there should be further cold cutting or grinding to remove all material that was affected by the hot cutting. In order to avoid weight loss during the test that could be associated with heat tint, it is a good practice to pickle the whole specimen before final grinding of the specimen surfaces. However, the surfaces that are actually tested should be as-ground without any subsequent pickling or other chemical treatment that might clear the surface of detrimental phases. It is permitted to leave the weld faces of the specimen in the as-pickled condition as long as the cross-sectional edges are tested in the ground condition. A slight chamfering of the specimen is helpful, but the should not be substantial rounding off of the edges. The presence of burrs on the edges will cause weight losses not related to the presence of intermetallic phases. Corrosion attack on the edges must be included in the limiting acceptance criterion. "Modified G 48" procedures that permit disregarding of edge corrosion are not correctly testing for the presence of detrimental intermetallic phases. If intermetallic phases are present, they are much more likely to occur within the metal, and therefore be exposed on the specimen edges, than on the faces of the product.

9. Is "modified G 48" testing the same thing as A 923 Method C?

ASTM G 48 Practice A and A 923 Method C are similar to the extent that they use similar equipment and laboratory procedures. However, they are substantially different in their application. ASTM G 48 is a description of laboratory procedure, but it does not specify the temperature of testing, the time of exposure, the technique of assessing corrosion, and an acceptance criterion. The "modified G 48" test indicated that the individual ordering specification was attempting to address these deficiencies, but few specifications addressed all of them. ASTM A 923 Method C specifically addresses each of these issues, and provides a basis for acceptance of the duplex stainless steels with regard to the absence of detrimental intermetallic phases.

One important difference is that G 48 permits the tester to disregard corrosion on the edges of the specimen. This permission is totally inappropriate for use of the test to demonstrate the absence of intermetallic phases in duplex stainless steels. It is unlikely that the intermetallic phases will occur in the faces of the plate or the faces of the weld, but rather will occur in the interior of the metal. Therefore, incidents of pitting on the edges of the sample should be considered indicative of a problem, and not ignored.

G 48 is usually a procedure performed at a series of temperatures, with the goal of identifying the critical pitting temperature. Accordingly, the time of exposure and the inspection for pitting on the surface are designed to detect subtle pitting initiation. The single test temperature for each grade in A 923 is chosen to be below the critical pitting temperature for material without intermetallic phases, and above the critical pitting temperature for material with intermetallic phases. The pitting, when it does occur, is readily visible. However, the weight loss is what is measured in order to remove the potential for debate over visual interpretation. That weight loss is converted to a corrosion rate in order to permit different sizes and geometries of specimens to respond to a single acceptance criterion.

An important issue is the surface preparation of the sample. The goal of the test is to detect intermetallic phases if present. Chemical treatment of the specimen surface (passivation or pickling) may reduce the exposure of intermetallic phases in the surface and thereby cause the test not to detect the presence of intermetallic phases. The specimen edges should be fine ground but not chemically treated for most effective use of the A 923 test. If there is concern that the faces of the specimen may contribute to the weight loss, the appropriate specimen preparation is to pickle the specimen before final grinding of the edges.

10. When you encounter a need to weld repair a structure of duplex stainless steel and you do not have a detailed history of the welding during construction, how do you decide how much welding is safe? What filler metal do you use?

The correct answer will depend on the nature of the weld, the conditions of application, and on the application itself, particularly whether or not the structure was built to ASME Code, or is being used in a situation of significant safety risk. The safest approach is to sample the fabrication weld and perform a qualification of the proposed repair. However, this approach imposes extra costs and opens the necessity to repair also the position of sampling. The value of good records in welding fabrication is amply demonstrated by this situation. It is appropriate to consult metallurgical engineers before making the weld repair.

The problem, it there will be one, will most likely occur in the HAZ of the original fabrication welds. The selection of the filler metal is unlikely to have any favorable effect on dealing with this part of the problem. The is no reason that the filler metal should not be the same filler metal that would be used with the duplex stainless steel in the original fabrication welds.

11. Are there any special problems in cleaning the heat tint of a duplex stainless steel?

Because of the relatively high chromium content and the relatively low thermal expansion of a duplex stainless steel, the oxide scale is typically thin and highly resistant to removal. It is desirable to remove any heat tint in order to get maximum corrosion resistance, but there are some applications where the process itself will remove the heat tint. Grinding to clean bright metal is effective. Blasting can also be effective but, depending on the scale and the blasting medium, may not be as effective as grinding for removing the oxide. Pickling, by solution or by paste, is effective, but longer times or more aggressive pickling chemistries are required for duplex grades than are typically required for austenitic grades.

Passivation, in the sense of removing free iron (from tooling contact, etc.), is no different than for austenitic stainless steels. It is appropriate to confirm the effectiveness of a passivation treatment by testing such as that listed in ASTM A 967.

It should be noted that the complete removal of heat tint may not always be necessary, depending on the application. For example, removal of all heat tint is not required for exposure to kraft liquor, but is desirable for service in acid sulphite liquors.

12. When is post weld heat treatment beneficial, and what treatments should be used?

There are no heat treatments in the 315-980° C (600-1800° F) range that are beneficial to duplex stainless steels. Postweld stress relief heat treatments are used with steels that are capable of forming martensite, but duplex stainless steels do not form martensite. The metallurgical condition of a duplex stainless steel will be severely damaged if it is exposed to the stress relief treatment applied to a carbon or alloy steel (a consideration in dissimilar welds).

If the duplex stainless steel for whatever reason is exposed to conditions that lead to the formation of intermetallic phases, then the appropriate remedy is to heat treat the whole structure. The only heat treatment that works for duplex stainless steel is a full anneal above the minimum temperature listed in ASTM A 240, (1040°C (1900° F) in the case of 2205) and quench. When the construction cannot be annealed and quenched, the only remaining alternatives are to scrap the whole construction, or to cut out and replace the affected parts of the metal.

13. When is preheating useful or required?

Preheating the duplex stainless steel before welding is useful in two situations. If the part is damp, as from condensation, heating uniformly to a maximum of about 95° C (200° F) will avoid the problems associated with moisture in the weld. Preheating is one alternative for avoiding welds that are excessively ferritic as a result of too rapid quenching. Examples include spot resistance welds, superficial surface repair, and welding of thin material to heavy sections (sheet liners, tube-to-tubesheet welds). As with the suggested interpass temperature, 150° C (300° F) is an appropriate maximum temperature for preheating.

14. What is the correct design for a runout tab?

The fact that the purpose of the runout tab is produce a sample of weld that is identical to the production weld dictates the design of the tab. Ideally, the plate of the tab is from the same heat and thickness as the workpiece. It should be of a size that will produce neither unusual heating or unusual cooling. It should be large enough to readily supply the samples necessary for the qualification tests selected. Experience indicates that tabs from 6x6xt to 12x12xt inches finished size have been satisfactory.

Sample material can be obtained from the plate itself when there are manways or nozzles to be cut, but this source of samples may not always be available. When a bill-of-materials order is made for a large project construction, with special sizes of plate being rolled, there may not always be off-cuts from the plates for the sample material. It is a good idea to obtain the sample material with the purchase of the plate in order to assure the availability of matching sample material.

15. How significant is the selection of the temperature for Charpy tests, comparing the -40° C (-40° F) of A 923 and the ASME minimum design metal temperature?

ASTM A 923 and ASME UHA 51 have in common only that they both use Charpy tests. However, the purpose on the tests for the two procedures are quite different. The purpose of A 923 was to demonstrate that the heat treatment applied to a duplex stainless steel mill product had eliminated the intermetallic phases. The Charpy test was chosen because it was familiar to producer and user. As shown in the appendix of ASTM A 923, an acceptance criterion of 40 ft-lb at -40° C (-40° F) was found to correlate with the appearance of the intermetallic phase in a metallographic examination and a loss of corrosion resistance. Impact energy was selected as the acceptance criterion because of its intuitive meaning and the fact that it is so readily measured in an impact test. A 923 was not intended to demonstrate suitability for use at this temperature. The test was chosen to demonstrate the absence of the intermetallic phase. The high impact energy and low test temperature were necessary in order to get a meaningful indicator for the extremely tough annealed mill product. A 923 states that it is not applicable to a welded structure.

In comparison, ASME UHA 51 is designed to demonstrate suitability for use. The temperature is minimum design metal temperature, a factor of design specific to each installation. The standard test of three specimens is performed using the lateral expansion measurement to confirm results. Impact strength well below 40 ft-lb is accepted as suitable for use. It is applicable to the whole construction, whether base metal, weld metal, or HAZ. Where appropriate, it is permitted to use the more demanding test conditions of ASTM A 923, but with the number of specimens and measurements of both impact energy and lateral expansion, to qualify for ASME UHA 51, and so reduce testing costs.

16. Why is 20 ft-lb impact energy sufficient for a weld when the specification for the plate requires 40 ft-lb at -40° C (-40° F)?

The ASME has determined that 20 ft-lb is an adequate toughness for service in a particular class of applications. This level of toughness is not high enough to correlate well with the observance of intermetallic phase in the microstructure and the associated loss of corrosion resistance in a duplex stainless steel mill plate. The duplex stainless steel plate structure is tough enough that it may still show significant impact energy even after significant intermetallic phase formation. On the other hand, a weld metal may occasionally have toughness less than 40 ft-lb even when no intermetallic phase is present. For example, weld toughness is particularly affected by the presence of oxygen in the weld, as may occur with flux-shielded welds.

17. Why is 25% ferrite enough for a weld, when higher ferrite content is specified for the base metal?

The base metal is specified with a composition that, for a fully annealed and quenched structure, will lead to about 40 to 50% ferrite, essentially the equilibrium structure. This chemistry is found to return rapidly to almost that balance after the thermal cycle that occurs in the HAZ during welding, retaining corrosion resistance and toughness. It is known that the oxygen associated with flux shielding reduces the toughness of the weld metal. Therefore, the compositions of the filler metal for flux-shielded welds have been adjusted to produce the highest austenite that can be accepted while still retaining the benefits of the duplex structure. If there is significant dilution from the base metal, then the weldment will have slightly more ferrite. The 25% ferrite represents the minimum that will be achieved in there is essentially no dilution, as in a capping pass.

18. Is it necessary to water quench after every heat treatment of a duplex stainless steel?

It is necessary to water quench after the final anneal of a mill product or of a constructed and heat-treated component such as a head, fitting, or forging. However, it may be convenient to air cool the piece during intermediate processing and then perform the final anneal and quench as a separate operation. The air-cooled piece will not have optimal toughness and corrosion resistance in that condition, but it is sufficient for further processing. The part will be brought to maximum toughness and corrosion resistance by the final heat treatment with its water quench.

19. Are there temperature limits, low and high, on the use of duplex stainless steels?

The toughness of the duplex stainless steel mill plate does not undergo an abrupt ductile-brittle transition. Rather it decreases gradually from its high shelf energy to a very low impact energy as temperature decreases from about ambient to temperatures in the range of -45 to -75° C (-50 to -100° F). So the minimum application temperature is determined in accordance with the tough of the duplex stainless steel. To date, there have been few applications with minimum design metal temperature below -40° C (-40° F).

The maximum temperature for ASME Code applications is 315° C (600° F). The temperature was chosen because it represents the lowest temperature for the transformation curve for 475° C (885° F) embrittlement. Below that temperature, the steel will not be embrittled by this reaction in many years of exposure. In non-Code applications, it would be possible to consider use of 2205 in applications where there are limited excursions in the range just slightly above the limiting temperature. However, the embrittling reaction is real and exceptions to the 315° C (600° F) limit should not be undertaken without full knowledge and evaluation.

20. How do the properties of duplex stainless steels affect wall thickness, thermal expansion, and heat transfer in comparison to austenitic stainless steels?

Although it is generally correct to say that the yield strengths of the duplex stainless steels are twice that of the common austenitic stainless steels, that relationship does not imply that the thickness of the duplex stainless steel will be simply half that of the austenitic stainless steel in the same design. The higher strength of the duplex grades is reflected in higher allowable design stresses in the ASME Code. Depending on the shape of the construction, it is possible to reduce significantly the thickness of the material required when using duplex stainless steel, an opportunity for cost savings.

The thermal expansion of a duplex stainless steel is intermediate to that of carbon steel and austenitic stainless steels. This difference can be an advantage in structure with cyclic heating because there is less necessity to accommodate the large expansions associated with the austenitic materials. On the other hand, using duplex stainless steel within a construction of austenitic stainless steel may create problems when the duplex steel does not expand to the same extent. The combination of high strength and lower expansion may mean that the duplex stainless steel will impose high stresses at the point where it is joined to the austenitic structure.

Because the duplex stainless steel has a ferritic matrix, it is more efficient in heat transfer than the austenitic stainless steels. This property, combined with the thinner material that results from economical use of the higher strength of the duplex grades, can be used to significant advantage in heat transfer applications.

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