GB/T 39077-2024Test methods for detecting detrimental phase in austenitic-ferritic (duplex) stainless steels (English PDF)

Foreword Foreword

The document is drafted in accordance with the rules given in GB/T 1.1-2020, Directives for standardization—Part 1: Rules for the structure and drafting of standardizing documents.

It replaces GB/T 39077-2020, Test methods for detecting detrimental phase in lean austenitic-ferritic duplex stainless steels. Apart from structural adjustments and editorial changes, the main technical changes are: the terms and definitions have been changed (Clause 3, against Clause 3 of the 2020 edition); sampling requirements for large-section products, for flanges and for welded joints have been added (Clause 4); electrolytic etching reagents and electrolytic parameters have been added to the microstructural examination method (5.2.3); a corrosion solution has been added to the ferric chloride corrosion test method (5.4.3.2.2); and methods for the examination of detrimental phases and for the assessment of the result have been added for the 22Cr duplex type, the 25Cr duplex type and the 27Cr duplex type (5.2.4, 5.2.5.2, 5.3.2, 5.3.3, 5.4.4 and 5.4.6). In this list of changes the 25Cr duplex type is printed as 25r duplex type.

The document was proposed by the China Iron and Steel Association and is under the jurisdiction of the National Technical Committee on Steel of Standardization Administration of China (SAC/TC 183). It was first issued in 2020 as GB/T 39077-2020, and the present edition is the first revision.

1 Scope

The document lays down the test conditions, the test methods and the acceptance requirements for detecting detrimental phases in austenitic-ferritic (duplex) stainless steels. The test methods are the microstructural examination method, the Charpy impact test method and the ferric chloride corrosion test method.

It applies to the examination of detrimental phases precipitated in austenitic-ferritic (duplex) stainless steel during production and manufacture, such as heat treatment or welding, or during long service at a given temperature. It does not apply to a loss of toughness or of corrosion resistance in austenitic-ferritic (duplex) stainless steel arising from other causes.

A note refers to Annex A for the product grades to which the document mainly applies.

2 Normative references

The following documents are cited normatively in the text and their content constitutes indispensable provisions of the document. For dated references only the edition cited applies; for undated references the latest edition, including all amendments, applies.

GB/T 229 Metallic materials—Charpy pendulum impact test method (GB/T 229-2020, ISO 148-1:2016, MOD).

GB/T 2650 Destructive tests on welds in metallic materials—Impact tests (GB/T 2650-2022, ISO 9016:2022, MOD).

GB/T 13298 Inspection methods of microstructure for metals.

GB/T 17897-2016 Corrosion of metals and alloys—Method of ferric chloride pitting corrosion test for stainless steels.

GB/T 30067 Metallographic terms.

3 Terms and definitions

The terms and definitions given in GB/T 30067 and the following apply.

3.1.1 Austenitic-ferritic (duplex) stainless steel: a stainless steel whose matrix is made up of the two phases austenite and ferrite, the lesser phase amounting to at least 25 percent, and which can be strengthened by cold working. A note records that, for ease of examination, the document divides austenitic-ferritic (duplex) stainless steels according to their chromium content and their pitting resistance equivalent number PREN into the 20Cr duplex type group A, the 20Cr duplex type group B, the 22Cr duplex type, the 25Cr duplex type and the 27Cr duplex type; in that note the 25Cr duplex type is printed as 25r duplex type.

3.1.2 Type 20Cr duplex group A: an austenitic-ferritic (duplex) stainless steel whose pitting resistance equivalent number lies between 24.0 and 28.0.

3.1.3 Type 20Cr duplex group B: an austenitic-ferritic (duplex) stainless steel whose pitting resistance equivalent number lies between 28.0 and 30.0.

3.1.4 Type 22Cr duplex: an austenitic-ferritic (duplex) stainless steel whose pitting resistance equivalent number lies between 30.0 and 40.0 and whose chromium content is not less than 21.0 percent by mass.

3.1.5 Type 25Cr duplex: an austenitic-ferritic (duplex) stainless steel whose pitting resistance equivalent number lies between 40.0 and 48.0.

3.1.6 Type 27Cr duplex: an austenitic-ferritic (duplex) stainless steel whose pitting resistance equivalent number lies between 48.0 and 55.0 and whose chromium content is not greater than 33.0 percent by mass.

3.2.1 Pitting resistance equivalent number, PREN: a parameter based on the chemical composition that quantifies the resistance of a stainless steel to pitting, expressed as a mass fraction in percent. Note 1 gives PREN = w(Cr) + 3.3 w(Mo) + 16 w(N), or, for steels containing tungsten, PREN = w(Cr) + 3.3 x [w(Mo) + 0.5 w(W)] + 16 x w(N). Note 2 states that PREN values are absolute values based on the ladle analysis.

3.3.1 Detrimental phase: the intermetallic phases and non-metallic precipitates formed in an austenitic-ferritic (duplex) stainless steel at a given temperature that have an adverse effect on the in-service performance of the steel.

3.3.2 Intermetallic phase: a compound or intermediate solid solution containing two or more metals. A note records that the intermetallic phases precipitated in austenitic-ferritic (duplex) stainless steels are mainly the sigma phase, the chi phase and the R phase.

3.3.3 Non-metallic precipitate: a non-metallic compound precipitated in an austenitic-ferritic (duplex) stainless steel. Note 1 excludes the sulfide and oxide non-metallic inclusions produced during smelting. Note 2 records that the non-metallic precipitates in these steels are mainly chromium carbide, Cr23C6, and nitride, Cr2N.

3.3.4 Boundary percentage content of the detrimental phases, P: the percentage of the total phase-boundary area occupied by the detrimental phases. A note records that P reflects the degree to which the detrimental phases have precipitated on the phase boundaries and bears directly on the toughness and the corrosion resistance of the duplex stainless steel.

4 Sampling method

4.1 General. The test piece is to be cut from the finished product or from the final test coupon accompanying the furnace charge. Cutting is not to affect the microstructure; where the sample is cut by flame cutting or another thermal cutting method, the melted zone, the plastically deformed zone and the heat-affected zone are to be removed completely.

4.2 Sampling from products. The sampling position and the number of samples follow the product standard or the agreement between the two parties. Where these are not laid down, sampling as given in Table 1 is recommended; with the agreement of the purchaser, representative test samples may also be chosen.

Table 1 sets, for each product form, the way the sample is cut, its size in millimetres, the test method, the direction of examination and the sampling position. The product forms covered are plate and strip, discs, seamless tube, pipe and fittings; welded tube and fittings; bar and forgings, with separate lines for an outside diameter or section thickness below 50 mm and one of 50 mm and above; flanges and other hollow-profile forgings with welded ends, including tees; and castings, with separate lines for a section thickness below 50 mm and one of 50 mm and above. For each of these, the three test methods are listed with their direction of examination, given as longitudinal, transverse, tangential or any direction, and their sampling position, stated as the full wall thickness, one half or one quarter of the thickness, from the surface to the core, or the central region of the wall. A note records that the product thicknesses in the table are the thickness of the thickest part of the batch.

Footnote a to Table 1: for large-section products the test piece for the corrosion test method is to be cut perpendicular to the longitudinal axis, with a size of 6 mm by 25 mm by the product thickness. Where the section is very large, the product thickness in that size may be reduced, and one half to one third of the product thickness may then be taken for the test.

Footnote b: where the thickness of the flange body is less than 50 mm or the outside diameter of the welded end is not greater than 100 mm, the test piece may be taken tangentially at the mid-thickness of the flange body.

Footnote c: for bar, the outside diameter or section thickness range is 50 mm to 200 mm.

4.3.1 Sampling from welded joints. A test piece for the microstructural examination method is to include the weld metal, the heat-affected zone and the parent metal. Where the parent metal is thicker than 25 mm, several test pieces may be taken to cover the full thickness of the product. All test pieces are to comply with the product standard or with the agreement between the two parties.

4.3.2 For the Charpy impact test method, where the parent metal is not thicker than 25 mm three sets of test pieces are to be taken, from the weld metal, from the fusion line and from the fusion line plus 2 mm respectively. Where the parent metal is thicker than 25 mm, two further sets are to be taken from the weld root region 2 mm from the inner surface, one from the weld metal and one from the fusion line plus 2 mm.

4.3.3 A test piece for the ferric chloride corrosion test method is to include the weld metal, the heat-affected zone and the parent metal. Where the parent metal is not thicker than 25 mm, the size of the test piece is to be 25 mm in the direction of the weld by 50 mm perpendicular to the weld by the full wall thickness. Where the parent metal is thicker than 25 mm, several test pieces may be cut to cover the full thickness of the product. All test pieces are to comply with the product standard or with the agreement.

5 Test methods

5.1 General. Three test methods are given for assessing the detrimental phases in austenitic-ferritic (duplex) stainless steel, called duplex stainless steel below: the microstructural examination method, for detecting the presence of detrimental phases; the Charpy impact test method, for detecting their presence and their effect on impact properties; and the ferric chloride corrosion test method, for detecting their presence and their effect on corrosion resistance. The three methods serve only to detect the presence of the detrimental phases and their effect on the impact properties and the corrosion resistance of the steel, and cannot establish the particular structure of a detrimental phase. Which method is used is settled by the product standard or by agreement between the two parties. The microstructural examination method suits every application environment and every shape of sample, and suits in particular the assessment of special sections and of components in service.

5.2.1 The microstructural examination is to be carried out by trained and experienced technical staff. A note records that the degree of etching of the microstructure depends appreciably on the experience of the operator, and that an experienced operator can hold the results steady.

5.2.2 Test piece preparation. The size of the test piece follows the product standard or the agreement between the two parties. Where these are not laid down, the examined face of the test piece should be not less than 400 square millimetres. Preparation is to follow GB/T 13298.

5.2.3 Etching. After polishing, the test piece should be etched electrolytically; the recommended etchants and electrolytic parameters are given in Table 2, and further etchants, parameters and etching effects are given in Annex B. After etching, the test piece should be washed with hot water, absolute ethanol or acetone and then blown dry.

Table 2, Electrolytic etching reagents and electrolytic parameters, has two entries. Reagent 1 is a 20 percent to 40 percent aqueous solution of sodium hydroxide, at 1 V to 5 V for 5 s to 60 s; the intermetallic phases and the ferrite are coloured and darkened, and the reagent is recommended for observing the distribution of the intermetallic phases and of the ferrite but is not suited to the analysis of nitrides. Reagent 2 is a 10 percent aqueous solution of oxalic acid, at 5 V to 7 V for 5 s to 60 s; it suits the display of the microstructure and is recommended for observing the distribution of nitrides or carbides, may cause rapid over-etching of the intermetallic precipitates, and gives poor contrast between austenite and ferrite. The table adds that the composition of the etchant and the electrolytic parameters are determined by the cathode material and area, the surface area of the test piece and the alloy content.

5.2.4 Microstructural examination. After etching, the whole of every examined face of the metallographic sample is to be observed under the optical microscope. Observation is to begin at low magnification and be raised step by step. For the assessment, the magnification is to be 200 times for castings and 500 times for all other products, and a higher magnification may be used in addition where needed. Attention is to be paid to whether intermetallic phases and non-metallic precipitates are present in the structure, and the micrograph of the field with the highest content of intermetallic phases and non-metallic precipitates is to be kept. Where none is found, a typical micrograph at the mid-wall position is to be kept. The actual magnification of the micrograph is to be reported and a scale bar included.

5.2.5.1 For the 20Cr duplex type group A and the 20Cr duplex type group B, Annex C lays down two ways of assessing the detrimental phases: the comparison method and the quantitative calculation method. The assessment turns mainly on the percentage of the phase boundaries occupied by the detrimental phases. Although precipitation also occurs inside the ferrite phase, the detrimental phases precipitated on the phase boundaries have a more marked effect on the impact properties, so the quantitative calculation rests on the percentage of the phase-boundary content held by the detrimental phases and disregards the detrimental phases precipitated inside the ferrite.

5.2.5.2.1 Intermetallic phases in the other types of duplex stainless steel. Unless the product standard or the agreement provides otherwise, the microstructure is to be free of continuous intermetallic phases; scanning electron microscopy with energy-dispersive analysis may be used to identify them. Discrete or isolated intermetallic phases are permitted, but they are not to be spread evenly over the whole test piece and the largest dimension of each is not to exceed 10 µm. Figure 1 shows micrographs of the critical level of discrete or isolated intermetallic phases acceptable in forged and cast products, and Figure 2 shows typical micrographs of intermetallic phases in S22053 and S25073 duplex stainless steels. Where a centreline band of intermetallic compounds is present, this is to be recorded and reported, and acceptance of the product is then to rest on the results of the Charpy impact test method and of the ferric chloride corrosion test method. A note defines the centreline band as an accumulation of intermetallic phases caused by alloy segregation and arranged as a segregation band in the mid-wall region.

5.2.5.2.2 Non-metallic precipitates. The microstructure is to be free of continuous non-metallic precipitates, that is of carbide Cr23C6 or nitride Cr2N; scanning electron microscopy with energy-dispersive analysis may be used to identify them. Where carbide Cr23C6 or nitride Cr2N precipitates are observed, as shown in Figure 3, this is to be reported and recorded, and acceptance rests on the results of the Charpy impact test method and of the ferric chloride corrosion test method. A note records that nitride Cr2N precipitates along the grain boundaries within the ferrite phase and lowers the resistance of a component to hydrogen embrittlement under cathodic protection; that subsea hydrogen embrittlement is prevented mainly by design that limits the service stress or strain, so as to avoid excessive nitride precipitation; that determining the phase-equilibrium point of the nitride is not practicable; that the presence of nitride in the ferrite lowers the impact toughness, raises the microhardness of the ferrite and lowers the pitting potential; and that where hydrogen embrittlement is possible the best way to control excessive nitride precipitation in the ferrite is for the duplex stainless steel to meet the impact and corrosion-resistance requirements of the document.

5.3.1 Charpy impact test, test piece preparation. Preparation follows GB/T 229. Where the shape and size of the product do not allow a full-size Charpy test piece to be machined, a sub-size test piece may be taken by agreement between supplier and purchaser. For welded structures and other products of irregular shape, particular attention is to be paid to the position of the V notch; for welded structures the notch position is to comply with GB/T 2650.

5.3.2 Test equipment and test method. The equipment is to comply with GB/T 229. The Charpy impact test is carried out in accordance with GB/T 229 at the temperature laid down in Table 3, or at a lower temperature.

Table 3, Absorbed energy for the Charpy impact test method, gives for each type the unified numerical codes, the test temperature in degrees Celsius, the part examined and the minimum absorbed energy in joules. For the 20Cr duplex type group A, codes S20013 and S21014, the temperature is 23 °C +/- 5 °C, the parent metal is to reach 70 J and the weld and heat-affected zone are by agreement. For the 20Cr duplex type group B, codes S20033 and S20113, the temperature is 23 °C +/- 5 °C, the parent metal is to reach 100 J and the weld and heat-affected zone are by agreement. For the 22Cr duplex type, codes S22253 and S22053, the temperature is -40 °C +/- 5 °C, the parent metal and the heat-affected zone are to reach 54 J and the weld 34 J. For the 25Cr duplex type, codes S25073 and S27603, and for the 27Cr duplex type, code S27073, the temperature is -40 °C +/- 5 °C and the parent metal, weld and heat-affected zone are by agreement.

Footnote a to Table 3: the test may be carried out at a temperature lower than the one laid down. Footnote b: the mean of a set of three test pieces is not to fall below the value in the table, but one test piece may fall below it provided its value is not lower than 70 percent of the value in the table.

5.3.3 Assessment and retest. Unless the product standard or the agreement provides otherwise, the results of the full-size impact test are to comply with Table 3; Annex D gives the absorbed energies for duplex stainless steels used in the petroleum, petrochemical and natural gas industries. For sub-size test pieces the minimum absorbed energy may be reduced from Table 3 in proportion to the ratio of the test-piece area to the full-size area. Where the mean of a set of three test pieces falls below the value in Table 3, or a single value falls below 70 percent of that value, a double set is to be taken from the same product and the test repeated once; where all the retest results exceed the value in Table 3 the product is regarded as conforming, and otherwise it does not conform. Where one product does not conform, two further products are to be taken from the batch and three test pieces tested from each; if all results meet the requirement the batch conforms, apart from the non-conforming product, and otherwise the batch does not conform. Where the test result does not conform, the duplex stainless steel may be given a fresh solution treatment and re-batched for examination.

5.4.1 Ferric chloride corrosion test, general. Since the result on welded pieces depends on the chemical composition of the weld filler metal, supplier and purchaser are to agree the acceptance criteria before testing.

5.4.2 Test piece preparation. After cutting, the structure affected by the high temperature is to be removed fully by grinding or machining; sharp corners and burrs are to be removed so that edges and corners run smooth, and all surfaces are to be smooth. The surface roughness is to be uniform over all faces and the roughness Ra is not to exceed 0.8 µm; the machining is to be cooled so that the surface does not overheat. For pipe products the inner surface is to be left as it is, while the cut face and the outer surface are to be uniformly smooth with Ra not exceeding 0.8 µm. For sections of special shape such as reinforcing bar, the test piece is to keep its original surface because of its actual shape, while the cut face is to be uniformly smooth with Ra not exceeding 0.8 µm. The polished surface of the test piece is not to be treated with a chemical passivating agent such as nitric acid, citric acid or phosphoric acid, nor pickled with a mixture of nitric and hydrofluoric acid or with other acids. The test piece is to be rinsed clean with water, rinsed with alcohol or acetone and blown dry; it is to be weighed to better than 0.0001 g and kept in a dry vessel before the test.

5.4.3 Apparatus and preparation of the corrosion solution. The recommended apparatus is a 1000 mL glass beaker or a 1000 mL tall-form or conical flask or another suitable vessel, together with a thermostatic water bath or salt bath.

5.4.3.2.1 For 20Cr type duplex stainless steel the corrosion solution is 55.1 g of ferric chloride hexahydrate and 6.6 g of sodium nitrate dissolved in 600 mL of distilled water, giving mass fractions of about 5 percent ferric chloride and 1 percent sodium nitrate; it is filtered through glass fibre or filter paper to remove undissolved particles. 5.4.3.2.2 For the 22Cr, 25Cr and 27Cr duplex types the corrosion solution is prepared by method A of GB/T 17897-2016. 5.4.3.2.3 Before the test begins, hydrochloric acid or sodium hydroxide is added as needed to bring the pH of the corrosion solution to about 1.3. The volume of solution is to be not less than 150 mL, or 20 mL per square centimetre of test-piece surface, whichever is the greater.

5.4.4 Test method. The test follows GB/T 17897-2016. Unless the product standard or the contract provides otherwise, the test temperature is as laid down in Table 4 and the corrosion time is 24 h. At the end of the test the piece is taken out and the corrosion product removed thoroughly with a soft bristle brush under running water, then immersed in acetone or alcohol, taken out and blown dry; where the corrosion is heavy, ultrasonic cleaning may be used.

Table 4, Acceptance requirements for the ferric chloride corrosion test method, gives for each type the unified numerical codes, the test temperature and the maximum corrosion rate for the parent metal, and the test temperature and the maximum corrosion rate for the welded joint. The corrosion rate is expressed in milligrams per square decimetre per day; the unit in the heading of the last column is printed incomplete. For the 20Cr type group A, codes S21013 and S20013, and for the 20Cr type group B, codes S23043 and S20033, the parent metal is tested at 25 °C +/- 1 °C with a rate of not more than 10, and the welded joint is by agreement. For the 22Cr duplex type, codes S22253 and S22053, the parent metal is tested at 25 °C +/- 1 °C and the welded joint at 22 °C +/- 1 °C, with a rate of not more than 10 in both cases. For the 25Cr duplex type, codes S25073, S27603, S25554 and S25203, and for the 27Cr duplex type, code S27073, the parent metal is tested at 40 °C +/- 1 °C with a rate of not more than 10, and the welded joint is by agreement.

5.4.5 Test result. The dimensions of the test piece are measured, the total exposed area calculated, and the corrosion rate, which expresses the mean degree of corrosion in unit time, calculated from formula (1): mdd = (m1 - m2) x 1000 / (S x t), where mdd is the corrosion rate in milligrams per square decimetre per day, m1 the mass of the test piece before the test in grams, m2 its mass after the test in grams, S the total area of the test piece in square decimetres and t the test time in days.

5.4.6 Assessment and retest. Unless the product standard or the contract provides otherwise, the mean of three test pieces is taken as the result and is to comply with Table 4. One test piece may exceed the value in Table 4, but not by more than 30 percent of it. Annex E gives the maximum corrosion rates permitted for duplex stainless steels used in the petroleum, petrochemical and natural gas industries. Where the corrosion rate exceeds the value in Table 4, a double set of test pieces is to be cut from the same batch of duplex stainless steel for retest; where their mean does not exceed the permitted maximum, the retest is regarded as passed. Where the test result does not conform, the material may be given a fresh solution treatment and the test repeated.

6 Test report

6.1 The test report is to contain at least the unified numerical code or grade of the steel and the heat number; the sampling position; the heat-treatment condition; the test method chosen; a statement of the test result; and the signature of the technician.

6.2 Beyond 6.1, the report is also to contain one of the following. Where the microstructural examination method is used, the result is to include at least the etching method, the magnification used, the boundary percentage content of the detrimental phases and at least one representative micrograph of the detrimental phases. Where the Charpy impact test method is used, the result is to include the test temperature and the absorbed energies measured, judged as conforming or not against the acceptance criteria given in Table 3. Where the ferric chloride corrosion test method is used, the result is to include the test temperature, the duration of the test, the mass loss measured and the corrosion rate, judged as conforming or not according to 5.4.6; for a non-conforming test piece the position of the corroded areas is to be recorded in detail, whether a face, an edge or both, together with any other unforeseen corrosion behaviour.

Annex A Annex A (informative) Grades and chemical composition of austenitic-ferritic (duplex) stainless steels

Table A.1 gives the grades and chemical composition of the 20Cr duplex type group A, the 20Cr duplex type group B, the 22Cr duplex type, the 25Cr duplex type and the 27Cr duplex type duplex stainless steels. For each grade the table lists the unified numerical code, the ISC code, the UNS code, the Chinese grade designation, and the composition limits as mass fractions in percent for carbon, silicon, manganese, phosphorus, sulfur, chromium, nickel, molybdenum, nitrogen, copper and tungsten. The composition limits themselves are not reproduced here.

The grades are grouped under headings that repeat the PREN band of each type: 20Cr duplex steel group A with 24.0 <= PREN <= 28.0, covering S21013 and S20013; 20Cr duplex steel group B with 28.0 <= PREN <= 30.0, covering S23043 and S20033; 22Cr duplex steel with 30.0 <= PREN <= 40.0 and Cr >= 19.5 percent, covering S22253, S22053 and S29008; 25Cr duplex steel with 40.0 <= PREN <= 48.0, covering S22553, S22583, S25073, S25554, S27603, S22582 and S22584; and 27Cr duplex steel with 48.0 <= PREN <= 55.0 and Cr <= 33.0 percent, covering S27073.

A note records that, except where a range is stated, the figures in the table are maximum values. Footnote a records that ISC stands for iron and steel code and comes from GB/T 20878; footnote b records that UNS stands for unified numbering system and comes from ASTM A959.

Annex B Annex B (informative) Preparation for microstructural examination and etching methods

B.1 Table B.1 gives examples of suitable etching methods, which are generally applicable to duplex stainless steels. The etchants were selected for their ability to reveal the phases, for the contrast they give between the constituents, for their consistency and for their ease of use. The parameters are usually established by repeated trial, but they also depend on such factors as the cathode material and area, the geometry of the electrochemical cell, the surface area of the test piece and the alloy content.

B.2 The electrolytic etching technique is a direct-current electrolytic process in which the test piece serves as the cathode; the cathode material may be stainless steel or platinum, and electrical contact with the surface of the test piece may be made with a platinum wire or a wire of another noble metal. Compared with chemical etching and tint etching, electrolytic etching generally gives better reproducibility and consistency; uneven attack of the surface of the test piece is however often met with, and the attack is usually heavier near the edges.

B.3 Non-metallic inclusions, cracks, pores and the like should be identified and assessed in the polished condition.

Table B.1 lists four etching methods with their composition, technique and application. The V2A etchant is 5 mL nitric acid, 50 mL hydrochloric acid and 50 mL water; it is prepared fresh and cannot be stored, and the etching temperature runs from room temperature to 60 °C; it serves as an etchant for determining intermetallic phases and nitride precipitates, and gives poor contrast between austenite and ferrite, for which it is unsuited. The 60 percent nitric acid etchant is 60 mL nitric acid and 40 mL water, used electrolytically at 1.0 V to 2.0 V for 10 s to 120 s with stainless steel as the cathode; it is highly sensitive to the voltage and to the cathode material, suits the delineation of the microstructure and the display of intermetallic phases and nitride precipitates, and darkens the ferrite slightly. The Murakami method uses 10 g potassium ferricyanide and 10 g potassium hydroxide or sodium hydroxide in 100 mL water as a tint etch prepared fresh, and its composition may be modified in several ways; at 20 °C a short etch reveals the carbides and after about 3 min the intermetallic phases appear faintly, while at 75 °C to 100 °C the intermetallic phases are shown clearly. The Beraha method uses 0.5 g to 1 g of the potassium salt printed in the table as K2SO5, 20 mL hydrochloric acid and 100 mL water; the table calls it an electrolytic etch prepared fresh in which the test piece is immersed at room temperature until it is seen to tint, and states that the hydrochloric acid content is to be varied according to the difference in corrosion resistance of the alloy; it gives good contrast between austenite and ferrite and is not suited to determining intermetallic phases and nitrides. A note adds that the etching methods for duplex stainless steel are not limited to those recommended.

Annex C Annex C (normative) Method for the microstructural examination of detrimental phases in 20Cr duplex type group A and 20Cr duplex type group B austenitic-ferritic duplex stainless steels

C.1 Comparison method. Figures C.1 and C.2 are schematic drawings of the segmentation of the phase boundaries for the detrimental phases in the 20Cr duplex type group A and the 20Cr duplex type group B; Figure C.1 shows the form of the detrimental phases as black phase boundaries, and Figure C.2 the boundaries after binary segmentation, with the boundaries where the detrimental phases have precipitated in green and those where they have not in red.

C.1.2 The rating charts for the detrimental phases fall into two series, I and II: series I, Figure C.3, is used for the 20Cr duplex type group A and series II, Figure C.4, for the 20Cr duplex type group B. The most severely affected field on the examined face is chosen and compared with the rating charts of Figures C.3 and C.4 in order to rate the amount of detrimental phase precipitated. Where the amount lies between two charts, the nearest chart is used. Where P is not greater than 2 percent, the detrimental phases are regarded as very few or absent. Each series is reproduced at 500 times and at 1000 times for the steps P <= 2 percent, P <= 10 percent, P <= 20 percent, P <= 30 percent, P <= 40 percent, P <= 50 percent, P <= 60 percent and P <= 70 percent.

C.1.3 For welded pieces, where the chemical composition of the weld filler metal is the same as that of the parent metal, the rating chart corresponding to the pitting resistance equivalent number of the parent metal is used. Where the filler metal is a duplex stainless steel whose composition differs from that of the parent metal, the pitting resistance equivalent number of the weld metal is to be calculated and the corresponding chart chosen. Where the filler metal is not a duplex stainless steel, the rating charts do not apply to the assessment of the amount of detrimental phase in that weld.

C.2 Quantitative calculation method. Five fields are chosen at random on the examined face, each of an area not less than 0.0065 square millimetres, and their mean is taken as the result. The boundary percentage content of the detrimental phases P is calculated from formula (C.1), P = S1 / S2 x 100 percent, where S1 is the percentage of the total field area held by the phase boundaries carrying detrimental phases and S2 the percentage of the total field area held by all the phase boundaries. Twin boundaries revealed by the etch do not enter the calculation and are to be deducted. The quantitative calculation method governs in the event of arbitration.

C.3 Worked correspondence between the boundary percentage content of the detrimental phases and the Charpy absorbed energy. Figure C.5 shows the detrimental phases present in the microstructure of an S21013 plate 10 mm thick of the 20Cr duplex type group A, with a boundary percentage content of 10 percent and a corresponding Charpy absorbed energy KV2 of 70 J. Figure C.6 shows the detrimental phases in an S23043 plate 10 mm thick of the 20Cr duplex type group B, with a boundary percentage content P of 40 percent and a corresponding Charpy absorbed energy KV2 of 100 J.

Annex D Annex D (informative) Charpy impact test on austenitic-ferritic (duplex) stainless steels for the petroleum, petrochemical and natural gas industries

Table D.1 gives the two quality levels used for the acceptance of the Charpy impact test on austenitic-ferritic (duplex) stainless steels for the petroleum, petrochemical and natural gas industries. Quality level II is the level recommended for most oil and gas applications, and quality level I applies only to particular applications; choosing quality level I may restrict the suitability of large-section products.

The table is arranged by material type, test temperature, product or part examined, quality level QL, minimum average absorbed energy in joules and minimum individual absorbed energy in joules, the last two each split into a longitudinal and a transverse column. The material types are the 20Cr duplex type group A, tested at 23 °C +/- 5 °C, and the 20Cr duplex type group B, the 22Cr duplex type, the 25Cr duplex type and the 27Cr duplex type taken together, tested at -46 °C +/- 5 °C. The products or parts examined are plate, tube and fittings; bar; forgings, castings and hot isostatically pressed products; and welded joints. For the 20Cr duplex type group A the quality level column is marked as not applicable.

The energy values themselves are not reported here: in the extracted text the longitudinal and transverse columns run together on the forgings and castings line and on the welded joints line, and the pairing of the four value columns to those two lines could not be verified.

Footnote a: testing below the temperature laid down is acceptable. Footnote b: the minimum absorbed energy for sub-size test pieces is agreed between supplier and purchaser. Footnote c: the plate and tube line includes welded tube in the solution-annealed condition and the associated weld metal.

Annex E Annex E (informative) Ferric chloride corrosion test method for austenitic-ferritic (duplex) stainless steels for the petroleum, petrochemical and natural gas industries

Table E.1 gives the maximum corrosion rates for the ferric chloride corrosion test method applied to austenitic-ferritic (duplex) stainless steels for the petroleum, petrochemical and natural gas industries. After the test, the test piece observed at a magnification of 20 times is to show no pitting on its surface.

The table gives, for each material type, the test temperature and the maximum corrosion rate of the parent metal and the test temperature and the maximum corrosion rate of the welded joint, the rate being expressed in milligrams per square decimetre per day. For the 20Cr group A the parent metal is tested at 18 °C +/- 2 °C and for the 20Cr group B at 30 °C +/- 2 °C, in both cases with a rate of not more than 10 and with the welded joint by agreement. For the 22Cr type the parent metal is tested at 25 °C +/- 2 °C and the welded joint at 22 °C +/- 2 °C; for the 25Cr type at 50 °C +/- 2 °C and 35 °C +/- 2 °C; and for the 27Cr type at 60 °C +/- 2 °C and 40 °C +/- 2 °C. In each of these three types the maximum rate is 10 for both the parent metal and the welded joint.

Bibliography Bibliography

The bibliography lists GB/T 20878 on stainless and heat-resisting steel grades and chemical composition; ISO 15156-3 on materials for use in hydrogen sulfide containing environments in oil and gas production, Part 3 on cracking-resistant corrosion-resistant alloys and other alloys; ISO 17781:2017 on test methods for quality control of the microstructure of ferritic and austenitic duplex stainless steels; Section VIII of the ASME Boiler and Pressure Vessel Code on pressure vessels; ASTM E407 on microetching metals and alloys; DNV/GL RP-F112 on the design of duplex stainless steel subsea equipment exposed to cathodic protection; ASTM A959 on specifying harmonized standard grade compositions for wrought stainless steels; and four papers and conference contributions on the physical metallurgy of duplex stainless steel, on the variation of mechanical properties and corrosion resistance within UNS S32760, and on the correlation between nitride precipitates and mechanical properties in 25 percent chromium alloys.