BS EN ISO 17781:2017
BSI Standards Publication
Petroleum, petrochemical and natural gas industries
- Test methods for quality control of microstructure
of ferritic/austenitic (duplex) stainless steels
BS EN ISO 17781:2017
EN ISO 17781
EUROPEAN STANDARD
NORME EUROPÉENNE
EUROPÄISCHE NORM
August 2017
ICS 75.180.01
English Version
Petroleum, petrochemical and natural gas industries - Test
methods for quality control of microstructure of
ferritic/austenitic (duplex) stainless steels (ISO
17781:2017)
Industries du pétrole, de la pétrochimie et du gaz
naturel - Méthodes d'essai de contrôle de la qualité de
la microstructure des aciers inoxydables (duplex)
austénitiques/ferritiques (ISO 17781:2017)
This European Standard was approved by CEN on 1 July 2017.
Erdöl-, petrochemische und Erdgasindustrie Prüfverfahren für die Qualitätslenkung von
Microstrukturen von austenitisch/ferritisch
nichtrostendem Duplexstahl (ISO 17781:2017)
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2017 CEN
All rights of exploitation in any form and by any means reserved
worldwide for CEN national Members.
Ref. No. EN ISO 17781:2017 E
BS EN ISO 17781:2017
EN ISO 17781:2017 (E)
European foreword
This document (EN ISO 17781:2017) has been prepared by Technical Committee ISO/TC 67 “Materials,
equipment and offshore structures for petroleum, petrochemical and natural gas industries” in
collaboration with Technical Committee CEN/TC 12 “Materials, equipment and offshore structures for
petroleum, petrochemical and natural gas industries” the secretariat of which is held by NEN.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by February 2018, and conflicting national standards
shall be withdrawn at the latest by February 2018.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.
Endorsement notice
The text of ISO 17781:2017 has been approved by CEN as EN ISO 17781:2017 without any modification.
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BS EN ISO 17781:2017
ISO 17781:2017(E)
Contents
Page
Foreword ........................................................................................................................................................................................................................................ iv
Introduction..................................................................................................................................................................................................................................v
1
2
3
4
5
Scope ................................................................................................................................................................................................................................. 1
Normative references ...................................................................................................................................................................................... 1
Terms, definitions and abbreviated terms ................................................................................................................................ 2
3.1
Terms and definitions ....................................................................................................................................................................... 2
3.2
Abbreviated terms ............................................................................................................................................................................... 3
Sampling of test specimens ....................................................................................................................................................................... 4
4.1
General ........................................................................................................................................................................................................... 4
4.2
Casting test blocks................................................................................................................................................................................ 6
4.3
Welds in the as welded condition ........................................................................................................................................... 6
Test methods............................................................................................................................................................................................................. 7
5.1
General ........................................................................................................................................................................................................... 7
5.2
Microstructural examination ...................................................................................................................................................... 7
5.2.1
General...................................................................................................................................................................................... 7
5.2.2
Preparation of specimen ........................................................................................................................................... 7
5.2.3
Etching of specimens.................................................................................................................................................... 7
5.2.4
Microstructural evaluation of test specimens ........................................................................................ 8
5.3
Ferrite content measurement ................................................................................................................................................. 10
5.3.1
Test standard and conditions............................................................................................................................. 10
5.3.2
Acceptance criteria ..................................................................................................................................................... 11
5.3.3
Reporting ............................................................................................................................................................................. 11
5.4
Charpy V-notch impact toughness test ............................................................................................................................ 11
5.4.1
Test standard and conditions............................................................................................................................. 11
5.4.2
Acceptance criteria ..................................................................................................................................................... 11
5.4.3
Reporting ............................................................................................................................................................................. 12
5.5
Corrosion test........................................................................................................................................................................................ 12
5.5.1
Test standard and conditions............................................................................................................................. 12
5.5.2
Preparation of test specimens........................................................................................................................... 13
5.5.3
Acceptance criteria ..................................................................................................................................................... 13
5.5.4
Reporting ............................................................................................................................................................................. 13
Annex A (informative) Chemical compositions of duplex stainless steels .................................................................14
Annex B (informative) Preparation and etching for microstructural examination .........................................16
Bibliography ............................................................................................................................................................................................................................. 18
© ISO 2017 – All rights reserved
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BS EN ISO 17781:2017
ISO 17781:2017(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO’s adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
URL: www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 67, Materials, equipment and offshore
structures for petroleum, petrochemical and natural gas industries.
iv
© ISO 2017 – All rights reserved
BS EN ISO 17781:2017
ISO 17781:2017(E)
Introduction
The aim of this document is to establish common test methods for quality control of microstructure of
ferritic/austenitic (duplex) stainless steels for the oil and gas industry, enabling the manufacturers to
apply the same test procedures for their clients.
Duplex stainless steels have a dual phase microstructure consisting of ferrite and austenite. Ideally,
these phases are present in equal proportions; although in alloys which are commercially available, the
ferrite phase volume fraction can vary between 35 % and 65 % for products in the solution annealed
condition. They are characterized by high-chromium (19 % to 33 %) and low-nickel contents compared
with austenitic stainless steels.
Duplex stainless steels are prone to precipitation of intermetallic phases, carbides and/or nitrides
possibly causing embrittlement and reduced corrosion resistance. The formation of intermetallic
phases such as Sigma, σ, and Chi, χ, occurs depending on exposure time in the approximate temperature
range 590 °C to 1 000 °C (1 094 °F to 1 832 °F) and decomposition of ferrite to Alpha Prime occurs in the
range 300 °C to 540 °C (572 °F to 1 004 °F).
The microstructure of components or fabrication welds is affected by amongst others the thermalmechanical history associated with hot working, solution annealing and with subsequent forming
and welding. The destructive test methods with acceptance criteria specified herein are considered
relevant to verify that exposure time at above stated temperature ranges have been within acceptable
limits and to ensure that desired corrosion resistance and mechanical properties are obtained in final
products.
© ISO 2017 – All rights reserved
v
BS EN ISO 17781:2017
INTERNATIONAL STANDARD
BS EN ISO 17781:2017
ISO 17781:2017(E)
Petroleum, petrochemical and natural gas industries —
Test methods for quality control of microstructure of
ferritic/austenitic (duplex) stainless steels
1 Scope
This document specifies quality control testing methods and test conditions for the characterization
of microstructure in relation to relevant properties in ferritic/austenitic (duplex) stainless steel
components supplied in the solution annealed condition and fabrication welds in the as welded
condition.
This document supplements the relevant product and fabrication standards with respect to destructive
testing methods including sampling of test specimens, test conditions and test acceptance criteria to
show freedom from deleterious intermetallic phases and precipitates in duplex stainless steels. In
addition, this document specifies the documentation of testing and test results by the testing laboratory.
NOTE 1
This document is based upon experience with duplex stainless steels in offshore oil and gas industry
applications including topside and subsea hydrocarbon service, sea water service, as well as structural use.
NOTE 2
The austenite spacing is relevant to the susceptibility of duplex stainless steels to hydrogen-induced
stress cracking (HISC) in subsea applications where cathodic protection is applied. This falls outside the scope of
this document. Reference is made to DNV/GL RP-F112[4].
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 148-1, Metallic materials — Charpy pendulum impact test — Part 1: Test method
ISO 15614-11), Specification and qualification of welding procedures for metallic materials — Welding
procedure test — Part 1: Arc and gas welding of steels and arc welding of nickel and nickel alloys)
ASTM A 370, Standard test methods and definitions for mechanical testing of steel products
ASTM A 1058, Standard test methods and definitions for mechanical testing of steel products — Metric
ASTM A 1084, Standard test method for detecting detrimental phases in lean duplex austenitic/ferritic
stainless steels
ASTM E 3, Standard practice for preparation of metallographic specimens
ASTM E 562, Standard test method for determining volume fraction by systematic manual point count
ASTM E 1245, Standard practice for determining the inclusion or second-phase constituent content of
metals by automatic image analysis
ASTM G 48, Standard test methods for pitting and crevice corrosion resistance of stainless steels and
related alloys by use of ferric chloride solution
1)
For the purpose of this document, the following documents are considered equivalent: ASME Boiler and
pressure vessel code, section IX Welding and brazing qualifications[2].
© ISO 2017 – All rights reserved
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ISO 17781:2017(E)
3 Terms, definitions and abbreviated terms
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http://www.electropedia.org/
— ISO Online browsing platform: available at http://www.iso.org/obp
NOTE
For limitations in chemical composition of each specific material grade of duplex stainless steel,
reference is made to the appropriate product standards or UNS number. For nominal chemical composition of
duplex stainless steels and grouping of different types, used within this document, reference is made to Annex A.
3.1 Terms and definitions
3.1.1
centreline intermetallic stringer
group of intermetallic phases (3.1.4) aligned within the mid-thickness area due to alloy segregation
Note 1 to entry: They can be observed as both continuous and discontinuous precipitates.
3.1.2
fabrication
building of structures or equipment by cutting, bending, and assembling processes such as welding,
riveting, threaded fasteners or other joining methods
3.1.3
ferritic/austenitic (duplex) steel
stainless steel (3.1.8) with a high-chromium mass fraction (19 % to 33 %) with or without molybdenum
additions up to 5 %, and a nickel mass fraction intermediate to those of ferritic and austenitic
stainless steels
3.1.4
intermetallic phase
solid-state compounds, containing two or more metallic elements, whose ordered structure differs
from that of its constituents
Note 1 to entry: In duplex stainless steel, the most relevant phases are identified as σ-phase, χ-phase and R-phase.
3.1.5
lot
finite quantity of products from the same heat (or melt), same manufacturing process steps and same
heat treatment load
Note 1 to entry: For continuous and semi-continuous furnaces, the lot definition should comply with the
applicable product standards.
3.1.6
non-metallic precipitate
solid-state compounds, containing two or more elements, whose ordered structure differs from that of
its constituents
Note 1 to entry: In duplex stainless steel, the relevant non-metallic precipitates are chromium carbides and
nitrides.
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BS EN ISO 17781:2017
ISO 17781:2017(E)
3.1.7
pitting resistance equivalent number
PREN
number indicating the resistance of stainless steel (3.1.8) to pitting corrosion related to chemical
composition and derived from one of the equations PREN = % Cr + 3,3 % Mo + 16 % N or PREN = % Cr +
3,3 × % (Mo + 0,5W) + 16 × % N (mass fraction)
Note 1 to entry: All PREN limits are absolute limits based upon the heat analysis. The calculated value is not to be
rounded.
3.1.8
stainless steel
steel with at least 10,5 % mass fraction or more chromium, possibly with other elements added to
secure special properties
3.1.9
type 20Cr duplex Group A
ferritic/austenitic stainless steel (3.1.8) alloys with 24,0 ≤ PREN < 28,0
3.1.10
type 20Cr duplex Group B
ferritic/austenitic stainless steel (3.1.8) alloys with 28,0 ≤ PREN < 30,0
3.1.11
type 22Cr duplex
ferritic/austenitic stainless steel (3.1.8) alloys with 30,0 ≤ PREN < 40,0 and Cr ≥ 19 % (mass fraction)
3.1.12
type 25Cr duplex
ferritic/austenitic stainless steel (3.1.8) alloys with 40,0 ≤ PREN < 48,0
3.1.13
type 27Cr duplex
ferritic/austenitic stainless steel (3.1.8) alloys with 48,0 ≤ PREN ≤ 55,0 and Cr ≤ 33,0 % (mass fraction)
3.2 Abbreviated terms
For the purposes of this document, the following abbreviated terms are used.
ASTM
American society for testing and materials
HIP
hot isostatically-pressed
CVN
NA
OD
PREN
QL
T
UNS
Charpy V-notch
not applicable
outside diameter
pitting resistance equivalent number
quality level
ruling section thickness
unified numbering system
© ISO 2017 – All rights reserved
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BS EN ISO 17781:2017
ISO 17781:2017(E)
4 Sampling of test specimens
4.1 General
The test samples shall be made from a sacrificial product or from a prolongation/extension of a
product in the final solution annealed condition with location of test specimens as defined in Table 1
representing the thickest product within the lot. Alternatively, a representative test block may be used
when agreed with the purchaser.
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BS EN ISO 17781:2017
ISO 17781:2017(E)
Table 1 — Sampling of test specimens dependent of product
Product
Product
dimension
Test method
All
CVN
Transverse
Mid-thickness
Prolongation or All
welded extension piece
CVN
Transverse
Mid-thickness
Test sample
Plates, and
Prolongation of
seamless tubes, the product
pipes and fittings
Welded pipes
and fittings
Bars and long
solid forgings
without weld
end
Prolongation
OD or section
thickness
<50 mm (2 in)
OD or section
thickness
≥50 mm (2 in)
Flanges and
Sacrificial prod- Alld
other hollow
uct or prolongacontour shaped tion at weld end
forgings with
weld end including tees
HIP products
with weld end
HIP products
without weld
endb
Castings
a
Sacrificial prod- All
uct or prolongation at weld end
with greatest
wall thickness
Sacrificial product or prolongation at cross
section with
greatest wall
thickness
Sacrificial
product or test
block (see 4.2)
Section thickness <50 mm
(2 in)
Section thickness ≥50 mm
(2 in)
Test block
thickness
<50 mm (2 in)
Test block
thickness
≥50 mm (2 in)
Corrosion,
Microstructure
Test directiona
Thickness location
Transverse
Full thicknessb
Corrosionc,
Transverse
Full thicknessb
Corrosion,
Microstructure
Transverse
Surface to centreb
Microstructure
CVN
Longitudinal
Mid-thickness
CVN
Longitudinal
CVN
Both longitudinal Mid-thickness
and tangential to weld end
centre bore
Corrosion,
Microstructure
Corrosion,
Microstructure
Transverse
1/4 thickness
Surface to centreb
Longitudinal or
tangential
Full thicknessb
weld end
Any direction
Full thicknessb
weld end
CVN
Any direction
CVN
Any direction
Mid-thickness
CVN
Any direction
1/4 thickness
Corrosion,
Microstructure
Corrosion,
Microstructure
Corrosion,
Microstructure
CVN
Corrosion,
Microstructure
CVN
Corrosion,
Microstructure
Any direction
Any direction
Mid-thickness
weld end
Surface to centreb
Surface to centreb
Any direction
Mid-thickness
Any direction
Within hatched
area (see Figure 1)
For definition of test directions, reference is made to ASTM A 370/ASTM A 1058.
b
For products with large sections, the corrosion test specimen shall be taken transverse to the longitudinal axis with
dimensions of approximately 6 mm × 25 mm (1⁄4 in × 1 in) by thickness. For very large sections, the thickness dimension of
the specimen can be cut so that one-half to two-thirds of the product thickness is tested.
c
For welded products, the test specimens for corrosion testing and microstructure examination shall include weld
metal and the heat affected zone of parent metal. For products with wall thickness exceeding 25 mm, more than one (1)
specimen can be taken to cover full thickness. In such a case, all specimens shall fulfil the specified criteria.
d
When flange body thickness <50 mm (2 in) or weld end OD ≤100 mm (4 in), test specimens may be taken from the
flange body mid-thickness in tangential direction.
For all products, the mid-length of the test specimens shall be located one T or minimum 50 mm to any
second surface provided this is feasible within the size of the test sample/sacrificial product.
CVN testing is required when the wall thickness is ≥6 mm, wherever geometry permits.
For all products, the notch axis of CVN test specimens shall be positioned perpendicular to the closest
outer surface.
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ISO 17781:2017(E)
For welded products, two (2) sets of three (3) CVN impact toughness test specimens shall be taken from
mid-thickness of the component, one (1) with notch located in base material and the other notched in
the weld metal.
For forgings, HIP products and fittings, dimensioned sketches shall be established showing type, size
and orientation of test specimens to be taken from a prolongation of product or a sacrificial product.
4.2 Casting test blocks
Test blocks shall be integral or gated with the casting(s) they represent and shall accompany the
castings through all heat treatment operations. During any heat treatment of products, which the test
block represents, the test blocks shall be tack welded onto the casting and shall accompany the castings
through all heat treatment operations. Alternatively, a sacrificial product may be used as a test sample.
The thickness of the test block shall be equal to the thickest section of the casting(s) represented. For
flanged components, the largest flange thickness should be used as the ruling section.
Dimensions of test blocks and location of test specimens within the test blocks are shown in Figure 1 for
integral and gated test blocks, respectively. All test specimens shall be taken within the cross hatched
area. When the thickness T of the test block is ≤50 mm, the longitudinal axis of test specimens shall be
located in the centre of the test block.
Figure 1 — Integral and gated test block for castings
4.3 Welds in the as welded condition
Test specimens for weld procedure qualification shall be taken in accordance with the requirements of
ISO 15614-1 or applicable design code.
The test specimens for microstructural examination shall comprise a cross section of the weld metal,
heat affected zones and base metal of parts joined in full thickness.
The ferrite content shall be determined in the weld metal root and the last bead of the weld cap 2 mm
(0,08 in) below the surface. For welds with thickness less than 5 mm (0,2 in), the ferrite content shall be
determined through the full thickness.
In total, three sets of CVN test specimens shall be taken from the following positions from the weld
cap area:
— weld metal;
— fusion line;
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© ISO 2017 – All rights reserved
BS EN ISO 17781:2017
ISO 17781:2017(E)
— fusion line +2 mm (0,08 in).
When the weld thickness exceeds 25 mm (1,0 in), two (2) additional sets of specimens shall be taken
from the weld root area 2 mm from the internal surface, one (1) from the weld metal and one (1) from
the fusion line +2 mm (0,08 in).
The corrosion test specimen shall include the external and internal surface and a cross sectional surface
including the weld zones in full wall thickness. The test specimens shall have a dimension of full wall
thickness by 25 mm (1,0 in) along the weld and 50 mm (2,0 in) across the weld. For products with great
wall thickness, e.g. exceeding 25 mm (1,0 in), more than one (1) specimen can be taken to cover full
thickness. In such a case, all specimens shall fulfil the specified criteria.
5 Test methods
5.1 General
Laboratory test methods for evaluation of the microstructure of duplex stainless steel are as follows:
a)
metallographic microstructural examination;
— check for presence of intermetallic phases and non-metallic precipitates;
— determine ferrite content;
b) CVN impact toughness testing;
c)
ferric chloride corrosion test.
There are several parameters of the material microstructure that can influence the test results and the
use of all three test methods, as far as practical, is necessary to demonstrate acceptable quality.
This document specifies how test results should be documented by the test house. Material certification
reporting requirements are outside the scope of this document.
5.2 Microstructural examination
5.2.1
General
Microstructural examinations shall be carried out by trained and experienced technicians.
5.2.2
Preparation of specimen
Metallographic preparation of duplex stainless steels shall be in accordance with ASTM E 3 and shall be
polished to a metallographic finish suitable for light optical examination at minimum 400 × (200 × for
casting) after etching.
With mechanical polishing, a 1 μm diamond final polish or equivalent shall be used as minimum.
Preferably, a final oxide polish (e.g. colloidal silica/alumina) should be used.
Electro polishing may be used as an alternative to mechanical polishing.
The use of conductive mounts can affect the subsequent etching. Samples should consequently be
mounted into a resin (thermo-setting or cold-setting).
5.2.3
Etching of specimens
The base case etchant to detect intermetallic phases and the ferrite balance electrolytic etching should
be performed after final polishing using the following solution:
— 20 % to 40 % NaOH or KOH solution.
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BS EN ISO 17781:2017
ISO 17781:2017(E)
In the case that it is of interest to detect nitrides/carbides, the following combination of two (2) solutions
in sequence with examination in between should be used (or using two test specimens):
1) 10 % oxalic acid solution or V2A etchant, followed by
2) 20 % to 40 % NaOH or KOH solution or, alternatively, see 5.3.
NOTE
For guidance on effect of different etching solutions, reference is made to Annex B. Exact etching
parameters (e.g. time, voltage, etc.) are typically established by trial and error, and can depend on factors such as
cathode material and area, specimen surface area and alloying content.
5.2.4
5.2.4.1
Microstructural evaluation of test specimens
General
The microstructure shall be examined by optical microscopy at a magnification sufficient to observe
the presence of any intermetallic phases and/or nitride/carbide precipitates. The entire metallographic
specimen shall be examined in the as-etched condition; starting at low magnification and followed by
progressively higher magnifications (screening).
The assessment of the microstructure shall be made with a magnification of minimum 400 × for all
product forms, except for castings where a 200 × magnification shall be used.
The presence and location of intermetallic phases and/or nitride/carbide precipitates shall be noted
and recorded with micrographs representative of the location(s) with the highest concentration of
intermetallic phases and the locations with the highest concentration of nitride/carbide precipitates. If
intermetallic phases and nitride/carbide precipitates are not detected, a representative micrograph at
or near mid-wall thickness shall be recorded. Micrographs shall be reported at the actual magnification
and shall include a scale bar.
5.2.4.2
Acceptance criteria
The following acceptance criteria for intermetallic phases and precipitates shall apply.
a)
intermetallic phases
The microstructure shall basically be free from intermetallic precipitates. However, discrete/standalone intermetallic precipitates can be tolerated provided these are located in isolated areas and not
homogeneously distributed throughout the test specimen, and that the maximum dimension of each
precipitate does not exceed 10 µm.
Micrographs included in Figure 2 are showing examples of microstructures representative of wrought
and cast products containing such isolated areas with intermetallic precipitates to a level that can be
considered borderline acceptable.
In case centreline intermetallic stringers are present, this shall be reported and acceptance of the
product shall be based upon CVN and corrosion test results.
For welds tested in the as welded condition, the microstructure shall be free from intermetallic phases.
Discrete/stand-alone intermetallic particles are acceptable given the maximum dimension of these is
≤10 µm, alternatively, the acceptance criteria of ISO 15156-3 shall apply.
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ISO 17781:2017(E)
a) Wrought microstructure etched in 20 % NaOH shown at 500 × magnification
b) Cast microstructure etched in 40 % KOH shown at 200 × magnification
Figure 2 — Micrographs showing examples of borderline acceptable discrete/stand-alone
intermetallic particles for wrought products and castings
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ISO 17781:2017(E)
b) non-metallic precipitates
If carbide or nitride precipitates are observed, the findings shall be reported and documented, and the
acceptance shall be based upon the corrosion and CVN impact toughness test (see Table 2 and 5.5).
NOTE
Nitrides precipitate along grain boundaries and within the ferrite phase (see References [5], [6], [7]
and [8]). Such precipitation can decrease resistance to hydrogen embrittlement in components under cathodic
protection. Avoidance of hydrogen embrittlement subsea is principally achieved by appropriate design to limit
service stresses/strains (see DNV/GL RP-F112[4]), but excessive nitride precipitation is to be avoided. While
determination of the nitride phase balance is impractical, the presence of nitrides in the ferrite phase reduces
the CVN impact toughness property, increases micro hardness in the ferrite phase and reduces the breakthrough
pitting corrosion potential (see Reference [7]). Quality control to avoid excessive nitrides is therefore best
achieved by meeting the impact and corrosion properties given in this specification and applying appropriate
design criteria where hydrogen embrittlement is a credible threat (see DNV/GL RP-F112[4]).
5.2.4.3
Reporting
The microstructural examination test report issued by the test laboratory shall, as a minimum, include
the following information:
— reference to examination procedure;
— identification of the test specimens;
— location and orientation of extracted specimen(s);
— specimen size;
— etching method(s) used;
— magnification used for examination;
— relevant micrographs;
— statement of findings/presence of intermetallic phases and precipitates. If intermetallic phases
or precipitates are identified, a micrograph with a scale bar from a view with the most densely
populated area shall be included together with a description of their distribution over the examined
specimen (s);
— statement whether the acceptance criteria are fulfilled;
— name/signature/identification of technician.
5.3 Ferrite content measurement
5.3.1
Test standard and conditions
The ferrite content shall be determined by either point counting according to ASTM E 562 or by image
analysis according to ASTM E 1245.
The ferrite content measurement shall cover the section thickness as stated in Table 1 at views equally
spaced over the section thickness and the mean ferrite content value shall be reported.
The relative accuracy shall be 20 % maximum.
To expose the ferrite phase, the specimen shall be etched in 20 % to 40 % NaOH or KOH solution,
Beraha’s etchant (ASTM E 407, number 212[3]) or an alternative acceptable etchant as mentioned in
Annex B.
10
© ISO 2017 – All rights reserved
BS EN ISO 17781:2017
ISO 17781:2017(E)
5.3.2
Acceptance criteria
The ferrite content, in percentage by surface/volume fraction, shall be as stated below for all duplex
grades:
— 35 % to 60 % in base material;
— 35 % to 65 % in solution heat treated weld metals;
— 30 % to 70 % for welds in the as welded condition.
5.3.3
Reporting
The ferrite content examination test report issued by the test laboratory shall, as a minimum, include
the following information:
— procedure reference and method of counting;
— location of extracted specimen;
— specimen size and etching agents used;
— magnification used during examination;
— digital micrographs with scale bar showing typical microstructure including their location;
— ferrite content; mean value of all examined views;
— the type and size of grid used, if applicable, number of fields examined and relative accuracy;
— statement whether the acceptance criteria are fulfilled;
— name/signature/identification of technician.
5.4 Charpy V-notch impact toughness test
5.4.1
Test standard and conditions
CVN impact toughness testing shall be carried out according to ISO 148-1 (KV8), ASTM A 370 or
ASTM A 1058 at a temperature specified in Table 2 or lower.
NOTE
5.4.2
The use of a hammer with smaller notch radius is considered conservative and therefore acceptable.
Acceptance criteria
The minimum absorbed energy of full size specimens is specified in Table 2. Two quality levels are
defined. Selection of the quality level shall be made by the end user or purchaser.
NOTE
Quality level II is in line with the acceptance level applied to the type 22/25 Cr duplex SS grades for
more than 20 years and is the recommended level for most oil and gas applications. Quality level I is intended
for special applications only. Selection of Quality level I can limit the availability of products with large cross
sections.
© ISO 2017 – All rights reserved
11
BS EN ISO 17781:2017
ISO 17781:2017(E)
Table 2 — Minimum absorbed energy requirements for Charpy V-notch impact toughness testing
Type of duplex Temperaturea
20Cr Group A
Room
temperature,
maximum of
28 °C/82 °F
Product
Bars
Forgings,
castings and
HIP products
Welds in as
welded
condition
–46 °C/
–50 °F
Bars
Forgings,
castings and
HIP
Welds in as
welded
condition
Transverse
Longitudinal
Transverse
105
70
80
55
NA
QL I
QL II
QL I
QL II
QL I
QL II
QL I
QL II
105
85
70
45
65
85
65
NA
80
60
65
NA
50
45
60
45
55
35
50
NA
65
50
NA
45
35
45
35
NA
NA
40
35
27
a
Testing at lower temperature than specified should be considered acceptable.
c
This includes welded pipes and associated weld metal in the solution annealed condition.
b
Minimum single
absorbed energy
(J)b
Longitudinal
Plates, pipesc
and fittings
Plates, pipesc
and fittings
20Cr Group B/
22Cr/25Cr/27Cr
Quality
level
Minimum mean
absorbed energy
(J)b
The reduction factors of energy requirements for sub-size specimens shall be 5/6 and 2/3 for 7,5 and 5 mm wide
specimen respectively.
5.4.3
Reporting
The CVN test report from the test laboratory shall, as a minimum, include the following information:
— standard of testing;
— test temperature;
— test location, direction and notch orientation of test specimen and specimen size;
— absorbed energy values;
— statement whether the acceptance criteria are fulfilled.
5.5 Corrosion test
5.5.1
Test standard and conditions
A ferric chloride corrosion test in accordance with ASTM G 48, Method A shall be carried out for
type 22Cr duplex and higher alloyed grades. For leaner duplex grades, an inhibited FeCl3 + NaNO3 test
solution in accordance with ASTM A 1084 shall be used. The test method and test temperature for the
different duplex grades base material and welds in the as welded condition shall be as stated in Table 3.
The test exposure time shall be 24 h for both tests methods.
Test temperature tolerance shall be ±1 °C (±1,5 °F).
12
© ISO 2017 – All rights reserved
BS EN ISO 17781:2017
ISO 17781:2017(E)
Table 3 — Corrosion testing for different grades of duplex stainless steel
Material grade
Test standard
Type 20Cr Group A
ASTM A 1084
Type 25Cr
ASTM G 48
Type 20Cr Group B
Type 22Cr
5.5.2
Type 27Cr
ASTM A 1084
Minimum test
temperature base
material
(°C/°F)
Minimum test
temperature welds in
as weld condition
(°C/°F)
25/77
22/72
ASTM G 48
ASTM G 48
Preparation of test specimens
18/64
30/86
50/122
Not applicable
Not applicable
60/140
35/95
40/104
The whole specimen shall be pickled before being weighed and tested. Pickling shall be performed in
an acid solution following the guidance of ASTM A 380. An example for such a procedure is for 5 min at
60 °C in a solution of 20 % HNO3 + 5 % HF (volume fraction).
Where it is agreed to test non-pickled specimens, the test conditions are considered more aggressive
and the test should be accepted if passing the acceptance criteria.
5.5.3
Acceptance criteria
All surfaces of the test specimens shall be considered in the test.
The acceptance criteria shall be as follows.
— No pitting at 20 × magnification.
— The weight loss shall be less than 1,0 g/m2 for products in the solution annealed condition.
— The weight loss shall be less than 4,0 g/m2 for welds in the as welded condition.
Penetrant testing is recommended to judge if pitting is present or not.
5.5.4
Reporting
The corrosion test report from the test laboratory shall, as a minimum, include the following
information:
— reference standard of testing;
— location of extracted specimen;
— specimen size and surface treatment/condition;
— test temperature and duration of test;
— magnification used during examination;
— weight of specimen before and after testing and weight loss in g/m2;
— statement on presence of pitting;
— statement whether the acceptance criteria are fulfilled;
— name/signature/identification of technician.
© ISO 2017 – All rights reserved
13
BS EN ISO 17781:2017
ISO 17781:2017(E)
Annex A
(informative)
Chemical compositions of duplex stainless steels
Table A.1 shows the chemical compositions of duplex stainless steels. Only the alloying elements are
included in Table A.1. Maximum levels of impurity elements are not included, except for carbon. The
grouping of alloys is based upon their nominal compositions and the group definitions in this document.
This means that some alloys may also belong to another group dependent of the actual alloying content.
Table A.1 — Chemical compositions of duplex stainless steels
UNS
No. a
EN No. a
S32001
1.4482
S82011
—
0,03 20,5 to 23,5 1,0 to 2,0
—
0,03 19,5 to 22,5
3,0 to 4,0
0,03 19,0 to 22,0
2,0 to 4,0
Cb
Crb
Nib
Mob
1,0 to 3,0
0,6
0,03 19,5 to 21,5
S32202 1.4062d 0,03 21,5 to 24,0 1,0 to 2,8
S32003
S32304
S31500
S81921
J92205
1.4362
1.4424
S31803
1.4462
S32404
—
S32205
S31200
S32506
S32808
1.4462
—
—
—
Mnb
Type 20Cr Group A duplex, 24,0 ≤ PREN < 28,0
S32101 1.4162d 0,04 21,0 to 22,0 1,35 to 1,7
1.4655
Nb
Composition range given in mass percent
0,03 22,0 to 24,0 3,5 to 5,5
0,05 to 0,17 4,0 to 6,0
c
0,5
c
c
c
2,00
1,0 to 3,0
2,5
0,05 to 0,60
c
c
c
1,50
1,0
c
2,00
c
c
0,18 to 0,26
2,00
0,1 to 1,0 0,15 to 0,27 2,0 to 3,0
1,5 to 2,0 0,14 to 0,20
0,05 to 0,6 0,05 to 0,20
0,03 18,0 to 19,0 4,25 to 5,25 2,5 to 3,0
0,05 to 0,1
2,00
c
1,0 to 2,0 0,14 to 0,20 2,0 to 4,0
c
c
Type 22Cr duplex, 30,0 ≤ PREN < 40,0 and Cr ≥ 19,5 %
0,03 21,0 to 23,5 4,5 to 6,5
2,5 to 3,5 0,10 to 0,30
c
0,03 21,0 to 23,0 4,5 to 6,5
2,5 to 3,5 0,08 to 0,20
2,00
0,04 20,5 to 22,5 5,5 to 8,5
2,0 to 3,0 0,20 to 0,30
2,00
1,0 to 2,0
1,0
c
0,03 22,0 to 23,0 4,5 to 6,5
0,03 24,0 to 26,0 5,5 to 6,5
0,03 24,0 to 26,0
0,03 27,0 to 27,9
5,5 to 7,2
7,0 to 8,2
3,0 to 3,5 0,14 to 0,20
1,2 to 2,0 0,14 to 0,20
3,0 to 3,5 0,08 to 0,20
0,8 to 1,2 0,30 to 0,40
1,0 to 2,0
c
2,00
1,00
1,10
0,03 20,5 to 23,5 1,0 to 2,0
0,10 to 1,0 0,15 to 0,27 2,0 to 3,0
S82551
0,03 24,5 to 26,5 4,5 to 6,5
0-10 to 0,35
0,03 20,5 to 21,5 1,5 to 2,5
0,60
0,75 to
2,00
0,15 to 0,20 2,0 to 4,0
1,5
c
Chemical element not voluntarily added and with commonly obtained residual level.
d
14
Single values denote a maximum allowable.
c
c
0,5
c
2,0 to 3,0
c
c
0,05 to 3,0
0,5 to 1,5
The chemical composition given by the UNS Nos. and EN steel Nos. are similar, but not equivalent.
c
-
c
a
b
c
0,1 to 0,6 0,05 to 0,20
0,45
0,08 23,0 to 28,0 2,5 to 5,0
S82122
1,0
0,1 to 0,8
S32900 1.4460
S82011
Wb
0,1 to 0,8 0,20 to 0,25 4,0 to 6,0
Type 20Cr Group B duplex, 28,0 ≤ PREN < 30,0
0,03 21,5 to 24,5 3,0 to 5,5
Cub
2,1 to 2,5
c
c
These grades are patented.
© ISO 2017 – All rights reserved
BS EN ISO 17781:2017
ISO 17781:2017(E)
Table A.1 (continued)
UNS
No. a
J93404
J93380
S31260
EN No. a
—
S32950
S39274
S39277
S33207d
Nb
Mnb
Cub
Wb
4,0 to 5,0 0,10 to 0,30
1,50
c
c
3,0 to 4,0 0,20 to 0,35
1,50
3,0 to 5,0 0,24 to 0,32
1,20
Type 25Cr duplex, 40,0 ≤ PREN < 48,0
0,03 24,0 to 26,0 6,0 to 8,0
0,03 24,0 to 26,0 6,5 to 8,5
3,0 to 4,0 0,20 to 0,30
1.4507
0,04 24,0 to 27,0
4,5 to 6,5
2,9 to 3,9 0,10 to 0,25
1.4477
0,03 28,0 to 30,0
5,8 to 7,5
1.4410
S32906
Mob
2,5 to 3,5 0,10 to 0,30
S32750
S32760
Nib
5,5 to 7,5
1.4507
S32550
Crb
0,03 24,0 to 26,0
S32520
S32505
Cb
1.4501
—
0,03 24,0 to 26,0 5,5 to 8,0
0,03 24,0 to 27,0
4,5 to 7,0
0,03 24,0 to 26,0 6,0 to 8,0
0,03 24,0 to 26,0 6,0 to 8,0
0,03 26,0 to 29,0 3,5 to 5,2
2,9 to 3,9 0,25 to 0,30
3,0 to 4,0 0,20 to 0,30
0,5 to 1,0
0,5 to 1,0
1,50
1,50 to 2,50
c
1,00
0,5 to 1,0
0,5 to 1,0
0,2 to 0,8
1,5 to 2,5
1,0
c
1,00
1,50
1,5 to 2,6 0,30 to 0,40 0,80 to 1,5
1,0 to 2,5 0,15 to 0,35
0,03 24,0 to 26,0 6,0 to 8,0
2,5 to 3,5 0,24 to 0,32
0,03 29,0 to 33,0
3,0 to 5,0 0,40 to 0,60
0,025 24,0 to 26,0 6,5 to 8,0
1,0
3,0 to 4,0 0,23 to 0,33
2,00
1,0
c
Type 27Cr duplex, 48,0 ≤ PREN ≤ 55,0 and Cr ≤ 33,0 %
1.4485
S32707d 1.4658
0,03 26,0 to 29,0
6,0 to 9,0
5,5 to 9,5
4,0 to 5,0 0,30 to 0,50
1,5
1,50
0,2 to 0,8
0,5 to 2,0
1,5 to 2,5
c
0,8
c
1,2 to 2,0
a
The chemical composition given by the UNS Nos. and EN steel Nos. are similar, but not equivalent.
c
Chemical element not voluntarily added and with commonly obtained residual level.
b
d
Single values denote a maximum allowable.
1,0
0,1 to 0,5
c
c
c
c
c
0,8 to 1,2
c
These grades are patented.
© ISO 2017 – All rights reserved
15
BS EN ISO 17781:2017
ISO 17781:2017(E)
Annex B
(informative)
Preparation and etching for microstructural examination
Examples of suitable etching methods are given in Table B.1. These methods are generally applicable for
all commercially available duplex stainless steel alloys. The specified etchants are based on the ability
to reveal phases of interest, achieve contrast between constituents, consistency and ease of use. Exact
parameters are typically best established by trial and error and may depend on factors such as cathode
material and area, geometry of the electrochemical cell, and specimen surface area and alloying content.
The electrolytic etching techniques described are performed as direct current electrolysis with
the specimen as anode. Cathode material may be, for example, stainless steel or platinum. Electrical
connection to the sample surface can be made, for example, by touching it with a platinum (or other
noble material) wire. Electrolytic etching generally provides more reproducible and consistent results
than chemical and colour etching alternatives. Uneven etching of the specimen surface is, however,
often encountered, generally with heavier etching near edges.
For further information of specific etchants, reference is made to ASTM E 407[3].
Identification and evaluation of non-metallic inclusions, cracks, pores, etc. is best performed in the aspolished condition. Metallographic specimens should consequently also be examined in the as-polished
condition prior to etching and after each step in etching.
16
© ISO 2017 – All rights reserved
BS EN ISO 17781:2017
ISO 17781:2017(E)
Table B.1 — Examples of suitable etching methods for duplex stainless steels
Method
ASTM E 407
Etchant No.
Composition
Application of etching
Electrolytic etching. Typically at 5 V to 7 V for 5 s to 60 s.
10 % oxalic
13
10 g oxalic acid
100 ml H2O
Lower voltages at longer times may also be used. Excellent for
delineating microstructure.
Recommended for detecting nitride precipitates. May overetch intermetallic precipitates and small segregations quite
rapidly. If a two-step etching procedure is performed, a repolishing or another specimen is recommended.
Poor contrast between austenite and ferrite phases.
Electrolytic etching. Typically at 1,5 V to 3,0 V for 5 s to 10 s.
20 % NaOH
V2A etchant
60 % HNO3
Murakami’s
Beraha’s
220
104 mod
219
98
212
20 g NaOH
100 ml H2O
5 ml HNO3
50 ml HCl
50 ml H2O
60 ml HNO3
40 ml H2O
10 g K 3Fe(CN)6
10 g KOH
100 ml H2O
0,5 g to 1 g K 2SO5
20 ml HCl
100 ml H2O
Colours ferrite and intermetallic phases.
Recommended for ferrite measurements and the detection of
intermetallic precipitates. Considered not applicable for detection of nitrides.
Other solutions containing varying content of NaOH or KOH
may also be used with similar results. The conditions for
etching should be adjusted and qualified to verify acceptable
results.
To be used fresh and never be stored.
Etching performed at room temperature and up to 60 °C.
Recommended etchant for detecting intermetallic phases and
nitride precipitates.
Poor contrast between austenite and ferrite phases. Not
suitable.
Electrolytic etching. Typically at 1,0 V to 2,0 V for 10 s to 120 s
(with stainless steel cathode).
Highly sensitive to voltage and cathode material.
Good for delineating microstructure. Reveals intermetallic
phases and nitride precipitates. Darkens ferrite slightly.
Colour etching. To be used fresh.
Various modifications of etchant composition may be used.
At 20 °C: Reveals carbides after short times. May reveal intermetallic phases (vaguely) after approx. 3 min.
At 75 °C to 100 °C: May reveal intermetallic phases clearly.
Etches/colours ferrite. Considered not applicable for detection
of nitrides.
Colour etching. To be use fresh.
Immerse at room temperature until sample colouration is
observed.
HCl content may be varied depending in corrosion resistance
of alloy.
Good contrast between austenite and ferrite phases.
Not applicable for detection of intermetallic phases and
nitrides.
NOTE These recommendations only cover a limited selection of etchants. Numerous other etching procedures suitable for
duplex stainless steels can be found in published literature or acquired from other sources.
© ISO 2017 – All rights reserved
17
BS EN ISO 17781:2017
ISO 17781:2017(E)
Bibliography
[1]
ISO 15156-3, Petroleum and natural gas industries — Materials for use in H2S-containing
environments in oil and gas production — Part 3: Cracking-resistant CRAs (corrosion-resistant
alloys) and other alloys
[2]
ASME Boiler and pressure vessel code, section VIII Pressure vessels
[4]
DNV/GL RP-F112, Design of duplex stainless steel subsea equipment exposed to cathodic protection
[3]
[5]
[6]
[7]
[8]
18
ASTM E 407, Standard practice for microetching metals and alloys
Nilsson J.O., & Chai G. The physical metallurgy of duplex stainless steel. Sandvik Materials
Technology, R&D Centre, S-81181 Sweden
Nickel
Development
Institute.
High-performance
stainless
steels.
ht t p:// w w w .nickelinst it ute .org/ en/ Technic a lLiterat ure/ Reference %20Book %20
Series/11021_HighPerformanceStainlessSteels.aspx
Byrne G. R. Francis and G. Warburton. Variation in Mechanical Properties and Corrosion
Resistance of Different Alloys within the Generic Designation UNS S32760. Presented at Duplex
2000, Houston, TX, USA, February 2000
Mentz J. M. Frommert, K. Mahjoub, J Siekmeyer and J. Konrad. Correlation of nitride
precipitates and mechanical properties in 25%Cr – SDSS super duplex stainless steel. Salzgitter
Mannesmann Forschung GmbH
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