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Home , Forge welding, Metallography

Metallography of Welds Application

Up until the end of the 19th century, sec- Notes tions of were joined together by a heating and hammering process called forge . Today, a variety of differ- ent welding processes are available, such that welding is extensively used as a fab- rication process for joining materials in a wide range of compositions, part shapes and sizes.

Many types of manufacturing industries make use of a wide variety of welding processes: Shielded metal • Aircraft and aerospace industries e.g. wings and fuselages. • Shipbuilding and marine industries e.g. panels for decks and super- structures. • Land transportation / automotive industries. • Oil and petrochemicals industries e.g. off shore production platforms number and size of passes, depth of and pipelines. penetration, extent of HAZ (Heat affected Polished and colour etched section through a multi pass • Domestic e.g. white goods and zone), and any defect such as pores and austenitic stainless weld. Colour etched according to Lichtenberger and Bloech. Bright field, 6.5x. metal furniture. cracks on representative work pieces. As welds are produced in many differ- For process control, research and failure ent materials it is essential to select an analysis, metallography is used to check appropriate metallographic preparation many different aspects in a weld e.g. the method.

Difficulties during metallographic preparation Solution The introduction of any thermal damage during the cutting process must be avoided It is important to select the correct cut- as it can alter the and properties in the welded joint. off wheel with the appropriate cutting A thorough understanding of the preparation process is necessary to deal with the parameters such that thermal damage of difficulties presented by the variations in the material properties in and across the the sample is avoided. welded joint, if flatness between microstructural features with different The preparation method should be se- values is to be achieved. lected according to the material types of a weld and optimized to minimise the risk of relief between hard and soft phases in the weld, heat affected zone and parent material.

Thermal damage on a steel weld. Polished and etched transverse section shows depth to which thermal damage penetrates. Bright field, 50x. Joining Processes Welding involving heat Processes

Some of the more common welding processes The processes which are available for the Metal Inert Gas are shown in the diagram purpose of joining and alloys are (MIG) Metal Active Gas • Soft * Consumable (MAG) • Braze welding** Mixed gas shielded CO gas shielded • Welding Electron beam 2 Arc weld covered Fusion Arc welding electrode Features which distinguish welding from Laser welding Sub-arc soldering and are: • Soldering (Fig.1) and brazing (Fig. 2) Tungsten Inert Gas Non-consumable type in-volve melting a material with lower melt- (TIG) Welding Plasma welding ing point between the work pieces to form Spin welding a bond between them, without melting the work pieces. Spot resistance • The materials to be welded are raised above Projection welding Pressure Seam welding their in the vicinity of the joint, Lap resistance welding in order for fusion to take place. Resistance welding • As a consequence of the above, complex Butt resistance welding chemical and metallurgical changes occur in the materials in the proximity of the welded joint. • These microstructural changes can have a profound influence upon the joint properties Generally, the weld is regarded as a junction objective is to examine the overall integrity and suitability for service. between two or more pieces of metal in which of the weld or examine the microstructure/ their surfaces have to be raised to a plastic property relationship or to identify the nature (e.g. friction welding Fig. 3) or liquid state by and origin of defects. It follows then, that the application of heat with or without added the accuracy of microstructural analysis and *Soft soldering: Soldering is the process in which two metals are joined together by means of a third metal or metal and with or without the application of interpretation will depend on the production of having a relatively low melting point. Soft soldering pressure. prepared specimens, free from any artefacts is characterized by the value of the melting point of the which may have been introduced at any stage third metal or alloy, which is below 450°C (840°F). The third metal or alloy used in the process is called solder. Each of these processes has their own unique in the preparation process. characteristics e.g. penetration, speed of wel- **Braze Welding: Braze welding takes place at the ding, generation, heat input, properties of melting temperature of the filler (e.g., 870 °C to 980 °C weld, etc. and this in turn can have a consider- or 1600 °F to 1800 °F for alloys) which is often considerably lower than the melting point of the base able influence on the resultant microstructural material (e.g., 1600 °C (2900 °F) for mild steel). detail.

Consequently, any study on the effects of a particular welding process will require careful metallurgical examination of representative weld samples, irrespective of whether the

Robot welding

Fig.1: As polished section through solder ball connec- Fig. 2: As polished section through a alloy Fig. 3: Fusion interface on pressure friction welded low tion between copper conducting tracks in PCB. brazing joint in austenitic . Differential alloy steel showing heat affected zone and associated Bright field, 100x. Interference Contrast, 100x. plastic deformation. Bright field, 25x. Taking Test Sections from Welds Two levels of metallographic inspection Metallographic principles and practices can The examination of metallographic sections be applied to the examination of welded sec- through welded joints is commonly carried tions to satisfy a number of objectives; the out at two levels of inspection: more common of which are listed below: Macro: Where magnifications up to 50x are •  Approval Testing employed with stereomicroscopes. In this type of test, an individual welder welds an appropriate test piece, under Micro: Where examination is at higher specified conditions. This test piece is then magnifications (up to 1000x) using optical examined by measurement, visual inspec- microscopes. tion, and examination of a prepared section through the weld. If the weld reaches the Macro examination is commonly carried agreed standard, that welder is approved to out on unmounted cross sections through weld the same type as the test weld. welded joints and simply involves cutting and coarse/fine grinding techniques. The resultant • Procedure Approval Testing finish is adequate for , followed by Cross section through electron beam weld in nickel In this type of test, it is a welding procedure an examination of the macro features of the base alloy showing a welded columnar microstructure in a particular material with a particular joint weld joint. with few scattered gas pores. Bright field, 50x. configuration which is being approved. The completed weld is examined by a variety of For micro examination techniques and hard- means, one of which includes a prepared ness traverse, the provision of a polished, section through the welded joint. A hard- optically flat surface will be required. This ness traverse across parent material, heat involves cutting, mounting and grinding and affected zone and weld metal is normally polishing. One has to be aware from the carried out. outset, that artefacts can be introduced at any stage of the preparation process. This is • Production Quality Control particularly true of welded sections because In this type of test a representative number not only do microstructural variations occur of welds are sectioned and examined as over relatively short distances but welds can part of a production process. also involve joints between dissimilar metals having widely different properties. • Failure Analysis Pipe welds • Research and Development

Slag Porosity Toe crack Hot (solidification) cracks Heat Affected Zone Undercut (HAZ) Hot tears

Unaffected Unaffected base metal base metal

Subcritical-HAZ Lamellar tears Underbead cracking Intercritical-HAZ Root crack Grain-refined- Micro-fissures Intercritical-HAZ Grain-coarsed-HAZ Lack of fusion Drawing showing the various regions of the HAZ in a single-pass weld and the possible defects. Recommendations for the preparation of weld microsections

CUTTING MECHANICAL PREPARATION For most welder approval tests it is suggested that macro sections are cut in the transverse Macro sections direction through weld stop/start positions. It Traditionally, welded sections for macro is at these locations where any lack of skill on examination are prepared manually on suc- the part of the welder will result in the forma- cessively finer grades of carbide Foil/ tion of weld defects. Paper to a 1200 grit finish. This is usually sufficient for hardness traverse For and other studies the section through parent material, heat affected zone, must be truly representative. Often, flame and weld metal, as well as being suitable Ground and etched macro section through a MIG (Metal cutting is used as a primary cutting technique for macro etching to facilitate weld macro Inert Gas) weld in steel, etched with 4% Nital. e.g. to remove a more manageable welded examination. SiC Foil is limited in respect Bright field, 6.5x section from a larger fabrication. It is impor- of its cutting life (1.0-1.5 mins) and this is tant in these cases that the macro/micro sec- exacerbated with increasing section size. As tion is cut by an wet cutting process an alternative grinding/fine grinding media features is minimal and all microstructural and is sectioned well away from the influence for manual preparation the Struers MD-Piano elements are retained. In this respect, semi- of any thermal damage from a primary ther- discs offer a number of advantages: automatic or automatic preparation equip- mal cutting operation. ment is preferred as it provides a consistency • A longer cutting life. and reproducibility of polish which facilitates In order that deformation from cutting is mini- • A constant removal rate over a longer accurate microstructural analysis. Prepara- mised and the risk of thermal damage on the time period. tion methods for the wide range of welded cut surface is avoided, it is important that: • Suitable for a wide range of materials materials which can be experienced cannot hardness (HV150-2000). be covered in this document. • The correct type of abrasive cut-off wheel • Less waste. There are, however, four methods detailed is selected. in the following which cover the more com- • An appropriate feed speed is used. MD-Piano discs are resin bonded monly used welded materials. • There is an adequate level of coolant discs which have been developed for coarse supplied during cutting. and fine grinding of materials in the hardness Carbon and low alloy steel welds

range HV150-2000 and they are available Step PG FG DP 1 DP 2 MOUNTING in comparable grain size to SiC Foil/Paper Surface MD-Piano MD-Allegro MD-Plus MD-Nap Normally, macro sections for procedural 80,120, 220, 600, and 1200. 220 testing are prepared unmounted because of Abra- Type time constraints, and because a finely ground Micro sections sive Size finish is usually adequate for macro exami- Weld specimens can involve wide variations Suspension / Water DiaPro DiaPro DiaPro nation. If semi-automatic preparation is an in material hardness across the specimen Lubricant Allegro/Largo 9 Plus 3 NapB 1 option, then there are a number of specimen either because of a phase changes during Rrpm 300 150 150 150 holders which will accommodate unmounted welding, or because the joint incorporates Force (N)/ 150 210 180 150 cross sections from welded joints. dissimilar metals. The weld metal may con- specimen If mounting is required then there is the op- tain hard precipitates or some indigenous Time 1 min. 4 min. 4 min. 1 min. tion of hot compression mounting or cold weld defect. As a consequence, it is important Valid for a total sample area of approximate 40 cm2. mounting. It is not uncommon, however, that the preparation method should ensure Holder need to be balanced. As an alternative to DiaPro, in weld examination to have relatively large that polish relief between microstructural DP-Suspension P, 9 µm, 3 µm and 1 µm can be used together with DP-Lubricant blue. cross sections. In this case, section sizes up to 120 x 60 x 45 mm can be accommodated in Struers FlexiForm, rectangular moulds for cold mounting.

Rectangular mounts of various welds. Polished and etched micro section through MAG (Metal Active Gas) welded . Microstructure consists of acicular and primary ferrite. Etched with 2% Nital. Bright field, 200x Stainless steel welds

Step PG FG DP 1 DP 2 Surface SiC #220 MD-Largo MD-Dac MD- Chem

Abra- Type sive Size

Suspension / Water DiaPro DiaPro OP-A Lubricant Allegro/Largo 9 Dac 3 Rrpm 300 150 150 150 Force (N)/ 150 180 150 120 specimen Time 1 min. 5 min. 4 min. 2 min. Weld-base metal interface on Duplex stainless steel. Same material. Weld microstructure consists mainly Mechanically prepared; electrolytically etched in 40% of delta ferrite and . Valid for a total sample area of approximate 40 cm2. aqueous sodium hydroxide solution. Bright Field, 200x. Holder need to be balanced. MD-Largo can be replaced Bright Field, 25x. by MD-Plan. As an alternative to DiaPro, DP-Suspension P, 9 µm and 3 µm can be used together with DP-Lubricant blue. MD-Chem with OP-A can be replaced by MD-Nap with DiaPro NapB.

Titanium welds

Step PG FG OP Surface SiC #220 MD-Largo MD-Chem

Abra- Type sive Size

Suspension / Water DiaPro OP-S* Lubricant Allegro/Largo 9 Rrpm 300 150 150 Force (N)/ 150 180 120 specimen Time 1 min. 5 min. 5-10 min. Weld area. Mechanically polished weld section of full Same material. Parent-heat affected zone interface. 2 Valid for a total sample area of approximate 40 cm . penetration weld in grade 1 . The weld micro- Polarised light with a sensitive tint plate (λ1/4-plate), Holder need to be balanced. As an alternative to DiaPro, structure consists of Widmanstätten α phase. 50x. DP-Suspension P, 9 µm, can be used together with Polarised light with a sensitive tint plate (λ1/4-plate), DP-Lubricant blue. 50x. *Note: OP-S with an addition of 10-30% by volume peroxide (30%).

Aluminium Welds (Friction stir welding)

Step PG FG DP OP Surface SiC #220 MD-Largo MD-Mol MD-Chem

Abra- Type sive Size

Suspension / Water DiaPro DiaPro OP-S Lubricant Allegro/ Mol R 3 NonDry Largo 9 Rrpm 300 150 150 150 Force (N)/ 120 150 120 100 specimen Time 1 min. 5 min. 4 min. 1-2 min.

Valid for a total sample area of approximate 40 cm2. Holder need to be balanced. As an alternative to DiaPro DP-Suspension P, 9 µm and 3 µm can be used together with blue and red lubricant.

Aluminium friction stir weld, etched externally according to Barker, using 15V for 3 min. Polarised light with a sensitive tint plate (λ1/4-plate), 25x Etching

ELECTROLYTIC POLISHING/ETCHING Some of the more common chemical and electrolytic etching reagents for welded joints in a It is not uncommon, in shop floor produc- variety of materials are listed below. tion control applications, to find electrolytic Material Etchant Comment polishing/etching being used as a method for obtaining prepared weld cross sections Carbon and low 100 ml ethanol (95%) or methanol (95%) Good general purpose reagent; can be increased to 15 ml alloy 1-5 ml nitric acid nitric acid for macro etching. for macro examination. Here the sections are (Nital) cut on an abrasive cut off machine, then after 100 ml distilled water Good macro etching a single grinding stage, the specimens are 10 g ammonium persulphate electrolytically polished and etched to provide Stainless steels 480 ml distilled water Macro etching a section suitable for macro examination. The 120 ml hydrochloric acid (32%) advantages of this technique are: 50 g (III) choride, - Its speed. 100 ml distilled water Electrolytic etching 4-6 volts for a few secs. - Its ease of operation. 10 g oxalic acid - Minimises user contact with acidic etchants. 100 ml distilled water Electrolytic etching 2-4 volts for a few secs - A more suitable option for a wide range of 5 ml sulphuric acid (95-97%) stainless steel types and other metals Nickel alloys 100 ml distilled water Electrolytic etching 3-6 volts for a few secs. difficult to etch just chemically. 5 ml sulphuric acid (95-97%) Copper alloys 100 ml distilled water Use freshly made For applications where detailed microstructural 10 ml ammonium hydroxide (25%) with a few drops of aqueous hydrogen peroxide (3%) analysis is required the specimens for electro- lytic polishing and etching should be ground to Aluminium alloys 100 ml distilled water Macro etching 15 g sodium hydroxide 1000 grit (or more for softer metals).

It is important to follow the recommended safety precautions when handling chemical reagents and when using chemical etchants.

METALLOGRAPHY Micro sections Some of the more common metallographic Macro sections tests carried out on welded joints are detailed Etched macro sections allow the identifica- below: tion of the boundaries of the weld metal, heat affected zone, fusion boundary, grain growth Area fraction of a constituent - identifica- and the individual runs in multi-run welds. In tion of individual phases and determination addition weld defects such as cracks, pores/ of area fraction by point counting, e.g. delta- voids, lack of fusion, and lack of penetration ferrite in austenitic stainless steel welds. can be identified.

Mechanically polished and electrolytically etched sec- tion through micropulse Tungsten Inert Gas weld in Inconel 625 alloy. Etched in 10% oxalic acid; 10 Volts for 10 secs. Bright field, 2.5x.

Bright field, 100x

Bright field, 500x Same material. Microstructure in the weld consists of Macro section through deep penetration fillet weld in Polished and etched section of austenitic primary solid solution and fine irresolvable secondary carbon steel. Section was etched with 5% Nital. stainless steel weld showing islands phase. Section shows pronounced grain coarsening of Bright field, 2.5x. of delta ferrite and small area of pearlite. base material in proximity of fusion face. Bright field, 10x. Grain size / grain size measurements of Hardness survey - normally a hardness / grain coarsened / grain refined regions in microhardness traverse across parent weld metal and heat affected zone. material heat affected zone and weld metal is carried out to ensure whether weld and heat affected zone properties are satisfactory.

Aluminium weld showing assortment of in weld, base metal and heat affected zone. Etchant: 100 ml distilled water + 2 ml hydrofluoric acid. Bright field, 100x.

Microstructure type / morphology / identi- fication of microstructural transformation products in weld metal and heat affected Steel welds zone.

Heat affected zone in duplex stainless steel weld. Etched electrolytically with 40% aqueous sodium hydroxide solution. Bright field, 200x.

Defect analysis / identification and charac- terisation of indigenous weld defects. Weld with hardness indentations (according to standard DIN EN 1043).

Sub surface gas porosity in auto- matically MAG (Metal Active Gas) welded steel components. Ground and etched with 10% Nital. Bright field, 2x.

Curves showing the hardness differences measured on Duramin-A300.

Heat affected zone crack below fillet weld in low alloy Pores in an austenitic stainless steel weld. steel. Bright field, 5x. Bright field, 100x. Summary

Struers ApS Pederstrupvej 84 DK-2750 Ballerup, Denmark Phone +45 44 600 800 Fax +45 44 600 801 [email protected] www.struers.com

Application Note The application of good metallographic Metallography of Welds AUSTRALIA & NEW ZEALAND NETHERLANDS Struers Australia Struers GmbH Nederland practices as applied to welded joints has Bill Taylor, Anne Guesnier, Struers A/S 27 Mayneview Street Zomerdijk 34 A Copenhagen, Denmark Milton QLD 4064 3143 CT Maassluis been outlined. An appropriate preparation Australia Telefoon +31 (10) 599 7209 Phone +61 7 3512 9600 Fax +31 (10) 5997201 method needs to be selected according to the Acknowledgements Fax +61 7 3369 8200 [email protected] physical properties of weld material(s). It is [email protected] We wish to thank Mrs Erika Weck and Mrs Elisabeth Leistner NORWAY important that care is taken at all stages of for permission to reproduce the fourth picture on the front BELGIUM (Wallonie) Struers ApS, Norge Struers S.A.S. Sjøskogenveien 44C the preparation process to obtain a correct page “Polished and colour etched section through a multi 370, rue du Marché Rollay 1407 Vinterbro pass austenitic stainless steel weld. Colour etched according F- 94507 Champigny Telefon +47 970 94 285 analysis of the microstructure and properties to Lichtenberger and Bloech. Bright field, 6.5x”. sur Marne Cedex [email protected] in the weld region. Téléphone +33 1 5509 1430 We thank Mrs Brigitte Duclos, Struers S.A.S. for three Télécopie +33 1 5509 1449 AUSTRIA [email protected] Struers GmbH micrographs on page 7: Zweigniederlassung Österreich BELGIUM (Flanders) Betriebsgebiet Puch Nord 8 Aluminium weld showing assortment of microstructures in Struers GmbH Nederland 5412 Puch weld, base metal and heat affected zone. Zomerdijk 34 A Telefon +43 6245 70567 3143 CT Maassluis Fax +43 6245 70567-78 Heat affected zone in duplex stainless steel weld. Telefoon +31 (10) 599 7209 [email protected] Fax +31 (10) 5997201 Pores in an austenitic stainless steel weld. [email protected] POLAND Struers Sp. z o.o. CANADA Oddział w Polsce Bibliography Struers Ltd. ul. Jasnogórska 44 7275 West Credit Avenue 31-358 Kraków Metallographic instructions for colour etching by immersion, Mississauga, Ontario L5N 5M9 Phone +48 12 661 20 60 Part 2: Behera colour etchants and their different variants”, Phone +1 905-814-8855 Fax +48 12 626 01 46 Erika Weck and Elisabeth Leistner. Fax +1 905-814-1440 [email protected] [email protected] Process and Physical ; J. E. Garside ROMANIA CHINA Struers GmbH, Sucursala 2rd Edition 1967. Struers Ltd. Bucuresti No. 1696 Zhang Heng Road Str. Preciziei nr. 6R Metallographic and Materialographic Specimen Preparation, Zhang Jiang Hi-Tech Park 062203 sector 6, Bucuresti K. Geels, ASTM, 2007. Shanghai 201203, P.R. China Phone +40 (31) 101 9548 Phone +86 (21) 6035 3900 Fax +40 (31) 101 9549 Metallographic etching. 2nd Edition, G. Petzow, ASM, Fax +86 (21) 6035 3999 [email protected] Metals Park Ohio, 1999. [email protected] SWITZERLAND CZECH REPUBLIC & SLOVAKIA Struers GmbH Struers GmbH Organizační složka Zweigniederlassung Schweiz vědeckotechnický park Weissenbrunnenstraße 41 Přílepská 1920, CH-8903 Birmensdorf CZ-252 63 Roztoky u Prahy Telefon +41 44 777 63 07 Phone +420 233 312 625 Fax +41 44 777 63 09 Fax +420 233 312 640 [email protected] [email protected] [email protected] SINGAPORE Struers Singapore GERMANY 627A Aljunied Road, Struers GmbH #07-08 BizTech Centre Carl-Friedrich-Benz-Straße 5 Singapore 389842 D- 47877 Willich Phone +65 6299 2268 Telefon +49 (0) 2154 486-0 Fax +65 6299 2661 Fax +49 (0) 2154 486-222 [email protected] [email protected] SPAIN FRANCE Struers España Struers S.A.S. Camino Cerro de los Gamos 1 370, rue du Marché Rollay Building 1 - Pozuelo de Alarcón F-94507 Champigny CP 28224 Madrid sur Marne Cedex Teléfono +34 917 901 204 Téléphone +33 1 5509 1430 Fax +34 917 901 112 Télécopie +33 1 5509 1449 [email protected] [email protected] FINLAND HUNGARY Struers ApS, Suomi Struers GmbH Hietalahdenranta 13 Magyarországi Fióktelepe 00180 Helsinki 2040 Budaörs Puhelin +358 (0)207 919 430 Szabadság utca 117 Faksi +358 (0)207 919 431 Phone +36 2380 6090 [email protected] Fax +36 2380 6091 Email: [email protected] SWEDEN Struers Sverige IRELAND Box 20038 Struers Ltd. 161 02 Bromma Unit 11 Evolution@ AMP Telefon +46 (0)8 447 53 90 Whittle Way, Catcliffe Telefax +46 (0)8 447 53 99 Rotherham S60 5BL [email protected] Tel. +44 0845 604 6664 Fax +44 0845 604 6651 UNITED KINGDOM [email protected] Struers Ltd. Unit 11 Evolution @ AMP ITALY Whittle Way, Catcliffe Struers Italia Rotherham S60 5BL Via Monte Grappa 80/4 Tel. +44 0845 604 6664 20020 Arese (MI) Fax +44 0845 604 6651 Tel. +39-02/38236281 [email protected] Fax +39-02/38236274 [email protected] USA Struers Inc. JAPAN 24766 Detroit Road Marumoto Struers K.K Westlake, OH 44145-1598 Takanawa Muse Bldg. 1F Phone +1 440 871 0071 3-14-13 Higashi-Gotanda, Fax +1 440 871 8188 Shinagawa [email protected] Tokyo 141-0022 Japan Phone +81 3 5488 6207 Fax +81 3 5488 6237 [email protected] 08.03.2019 / 62140409