Metallographic Preparation of Stainless Steel Application Notes

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Metallographic preparation of stainless steel Application Notes Corrosion resistant steels contain at least 11% chromium and are collectively known as “stainless steels“. Within this group of high alloy steels four categories can be identified: ferritic, martensitic, austenitic, and austenitic-ferritic (duplex) stainless steels. These categories de- scribe the alloy’s microstructure at room temperature, which is largely influenced by the alloy composition.

The main characteristic of stainless steels is their corrosion resistance. This prop- High performance stainless steel parts for the erty can be enhanced by the addition of aircraft industry specific alloying elements, which have a further beneficial effect on other charac- chemical, medical and food industries, teristics such as toughness and oxidation in professional kitchens, architecture and resistance. even jewelry.

For instance, niobium and titanium in- Metallography of stainless steels is an crease resistance against intergranular important part of the overall quality corrosion as they absorb the carbon to control of the production process. The form carbides; nitrogen increases strength main metallographic tests are grain size and sulphur increases machinability, measurement, identification of delta because it forms small manganese sul- ferrite and sigma phase and the evalu- phides which result in short machining ation and distribution of carbides. In chips. Due to their corrosion resistance addition, metallography is used in failure and superior surface finishes stainless analysis investigating corrosion/oxida- steels play a major part in the aircraft, tion mechanisms. Fig.1: Duplex steel etched electrolytically with 150x 40% aqueous sodium hydroxide solution, showing blue austenite and yellow ferrite

Difficulties during metallographic preparation Solution: Grinding and polishing: Deformation and scratching in ferritic and austenitic stain- Thorough diamond polishing and final less steels. Retention of carbides and inclusions. polishing with colloidal silica or alumina.

Surface of stainless steel after 3µm polish, DIC 25x Insufficiently polished stainless steel after 100x showing deformation from grinding colour etching (Beraha II), showing deformation Production and application of stainless steel Fig.2: Austenitic steel, colour etched (Beraha II).

100x

The production process of high alloy steels Production Flow is a sophisticated process of melting and Melting Electric Arc remelting. A mixture of iron and well sorted Furnace scrap is first melted in an electric arc furnace and then cast into ingot form or continu- Refining Vacuum ously cast into bloom or billet. For many Induction Furnace applications these primary products can be further processed into bar, rod or plate form. Ladle Vacuum Vacuum For steels with higher quality demands, the Furnace Degas- Treatment sing primary pro-duct can be used as feedstock for a secondary steelmaking process. This secondary process can be a double or even Casting Ingot Continuous triple remelt by vacuum induction melting Casting Casting plus vacuum arc remelting or electroslag remelting, which can also be done under pressure and protective gases. Remelting Vacuum Induction Melting The main purpose of this secondary process Protective Electro- Vacuum Gas slag Arc Furnace is to reduce impurities such as oxides, sul- Electroslag Remel- phides and silicates so that with successive Remelting ting remelts the degree of cleanliness increases and homogenous ingots with excellent mechanical and physical properties are produced. Hot forming Hot Hot Rolling Forging Application The corrosion resistance of stainless steels is based on alloying chromium with iron, Heat and is dependant on the formation of a pas- treatment sive surface oxide layer, which rebuilds itself and spontaneously when mechanically damaged. machining

A variety of different types of corrosion can occur, such as pitting, stress, intercrystalline or vibrational corrosion. Improved resistance Applications: magnetic valves, razor blades, Applications: screws, bolts and implants, low against any specific form of attack can be car trim. temperature applications, vessels and pipes provided by adding alloying elements other in the chemical, pharmaceutical and food than chromium, for instance molybdenum, Martensitic stainless steels are heat industries, kitchen utensils. which improves resistance against pitting treatable alloys with medium carbon content, corrosion. The main alloys, properties and 12-18% chromium and 2-4% nickel. Austenitic-ferritic steels, (Duplex) have examples of applications of the four types of Properties: high corrosion resistance, and a low carbon content and generally higher stainless steels are briefly described: high temperature and creep resistance. Appli- chromium (21-24%) and lower nickel content cations: scalpels, knives, hooks and tweezers (4-6%) than austenitic steels, and 2-3% Ferritic stainless steels: are non heat in medical applications, drive systems and molybdenum. treatable alloys with a low carbon content high performance parts for airplanes. and 11-17% chromium. Properties: fatigue resistance in corrosive Austenitic stainless steels are not heat media, good resistance against stress Properties: magnetic, resistant to atmos- treatable, have 0.03-0.05 % carbon, main corrosion. pheric corrosion, moderate strength and alloying elements are chromium (17-24 %), toughness. nickel (8-25%) and molybdenum (2-4%); Applications: equipment titanium and niobium are added for carbide for chemical, environmental forming. Properties: high ductility, high and offshore industries, corrosion resistance, resistant to oxidizing architecture. acids, alkalis, very good cold forming properties, easy to work and machine. Difficulties in Grinding the preparation of Step PG FG stainless steels Surface SiC-Paper 220 MD-Largo

Type SiC Diamond Abrasive Size #220 9 μm

Suspension/ Water DiaPro Lubricant Allegro/Largo 9 Ferritic stainless steels are soft and Table 1 shows a preparation method for stain- austenitic steels are ductile. Both are prone less steel samples, 30 mm mounted, on the rpm 300 150 to mechanical deformation. Final polishing semi-automatic Tegramin 300 mm diameter. usually leaves these steels highly reflective, Table 2 shows a preparation method for 6 Force [N]/ 25 40 specimen however, if they are not thoroughly prepol- stainless steel samples, 65x30 mm, cold ished, deformation can reappear after etching mounted or unmounted using Struers MAPS (Fig. 3). or AbraPlan/AbraPol, 350 mm diameter. Time (min) As needed 5

Due to their hardness, martensitic steels are Electrolytic polishing Polishing relatively easy to polish. In general, care For research work or fast, general structure should be taken to preserve the carbides. check, electrolytical polishing and etching Step DP OP can be an alternative to mechanical polishing of stainless steels, as it does not leave any Surface MD-Dac MD-Chem mechanical deformation.

Type Diamond Silica/Alumina Electrolytic polishing gives excellent results Abrasive 3 μm 0.04/0.02 μm for checking the microstructure (Fig. 4), but Size it is not suited to identify carbides. They are Suspension/ DiaPro OP-S NonDry washed out or appear enlarged. Lubricant Dac 3 OP-A

Before electrolytic polishing, the samples rpm 150 150 have to be ground to 1000# on silicon carbide foil/paper. The finer the initial surface the bet- Force [N]/ 20 15 specimen Fig. 3: Austenitic steel insufficiently polished 500x ter the results of the electrolytical polish (see showing deformation after etching (Beraha II) preparation method below). Time (min) 4 2-3

Table 1: Preparation method for stainless steel samples, 30 mm Recommendations for diameter mounted, on the semi-automatic Tegramin, 300 mm diameter. the preparation of stainless As an alternative to DiaPro polycrystalline diamond suspension P, steels 9 µm, 3 µm and 1 µm can be used together with green/blue lubricant. It is strongly recommended that for the soft and ductile stainless steels the use of very coarse grinding foil/papers and high pres- Etching sures should be avoided, as this can result Etching stainless steels requires some experi- in deep deformation. As a general rule, ence and patience. The literature for etchants the finest possible grit, consistent with the is extensive, and it is recommended to try a sample area and surface roughness, should variety in order to set up an individual stock of be used for plane grinding. Fine grinding solutions appropriate for the particular materi- is carried out with diamond on a rigid disc al which is regulary prepared in the laboratory. (MD-Largo) or, as an alternative for some Fig. 4: Stainless steel weld, polished and etched types of stainless steels, on a MD-Plan cloth. electrolytically, DIC By virtue of the fact that stainless steels are Fine grinding is followed by a thorough dia- highly corrosion resistant, very strong acids mond polish on a medium soft cloth, and the are required to reveal their structure. Stand- final polish with colloidal silica (OP-S), or Electrolyte: A3 ard safety precautions have to be used when alumina (OP-A) removes the fine scratches. Area: 1cm² handling these etchants. In many laboratories This step should be very thorough and it can Voltage: 35 V the etchants mentioned in the literature will take up to several minutes. A good final pol- Flowrate: 13 be modified according to the material to be Time: 25 sec ish increases the chance for a better contrast etched or even out of personal preference. (see “Etching“). For good etching results, a sufficient final External etching with stainless steel etching dish: oxide polishing is essential. Following are Any deformation from the first grinding step, 10% aqueous oxalic acid some etchants which have proved successful which is not removed by fine grinding, will Voltage: 15V in every day, routine applications. leave its traces and can not be removed by Time: 60 sec final polishing. Preparation method for electrolytic polishing and etching of stainless steel. Grinding on SiC foil/paper 320#, 500# and 1000#. Grinding Grinding

Step PG FG

Surface Stone 3A36 MD-Largo

Type Al O Diamond Abrasive 2 3 Size #150 9 μm

Suspension/ Water DiaPro Lubricant Allegro/Largo 9

rpm 1450 150 Structure interpretation Ferritic stainless steels do not respond to Force [N]/ 50 50 heat treatment. Their properties however can specimen be influenced by cold working. They are magnetic at room temperature. The microstructure Time (min) As needed 9 in the annealed condition consists of ferrite grains in which fine carbides are embedded.

Polishing Ferritic steels used for machining contain a large amount of manganese sulfides to facili-

Step DP 1 DP 2 OP tate free cutting (Fig. 5).

Martensitic stainless steels respond to Surface MD-Mol APS MD-Nap MD-Chem heat treatment. Martensite is formed through rapid cooling and properties can then be opti- Type Diamond Diamond Silica/Alumina Abrasive mised by subsequent tempering treatment. Size 3 μm 1 μm 0.04/0.02 μm The alloys are magnetic. Depending on the thermal treatment the microstructure can range Suspension/ DiaPro DiaPro OP-S NonDry Lubricant Mol B 3 Nap B 1 OP-A from pure martensitic structure to fine tempered martensite. rpm 150 150 150 Different alloys and various dimensions of semi-finished products require complex heat treatment temperatures and times. Force [N]/ 50 25 25 specimen Delta ferrite (Fig. 6) is usually an unwanted Time (min) 6 4 2-3 phase, because long annealing times of steels with high chromium content, at temperatures Table 2: Preparation method for stainless steel samples, 65x30mm, cold between 700 and 950°C, can change the delta mounted or unmounted using Struers MAPS or AbraPlan/AbraPol, 350 mm diameter. ferrite into the hard and brittle iron-chromium intermetallic sigma phase. As an alternative to DiaPro polycrystalline diamond suspension P, 9 µm, 3 µm and 1 µm can be used together with green/blue lubricant.

Chemical etching 3) V2A etchant: Always follow the recommended Caution: 100 ml water safety precautions when working with chemi- 100 ml hydrochloric acid cal reagents. 10 ml nitric acid For martensitic steels Etch at room temperature or 925 ml ethanol up to 50°C 25 g picric acid Color etchant Beraha II: 50 ml hydrochloric acid Stock solution For austenitic steels 800 ml distilled water Fig. 5: Ferritic stainless steel with manganese 200x sulfides and strings of small carbides, etched 1) Swab etch: 400 ml hydrochloric acid electrolytically with 10% oxalic acid 500 ml distilled water 48 g ammonium biflouride 300 ml hydrochloric acid To 100 ml of this stock solution 200 ml nitric acid add 1-2 g potassium metabisulfite 50 ml of a saturated for etching. iron-III-chloride solution 2.5 g copper-II-chloride Electrolytic etching For austenitic-ferritic steels (Duplex) 2) 100 ml water 40% aqueous sodium hydroxide 300 ml hydrochloric acid solution 15 ml hydrogen peroxide (30%) All stainless steels: 10% aqueous oxalic acid Fig. 6: Tempered martensitic stainless 75x steel with delta ferrite, etched with picric acid complex carbides within the austenite grains. For a successful mechanical polish, This leads to an impoverishment of chromium it is suggested that in the austenite solid solution, which increases - coarse abrasives for plane grinding the susceptibility to intergranular corrosion or are avoided. oxidation. - fine grinding and polishing with diamond should be thorough By reducing the carbon to below 0.015% and and ensure removal of all adding small amounts of titanium or niobium, deformation from plane grinding. the risk of intergranular corrosion is reduced, - a final oxide polish with colloidal as these elements form carbides in preference silica or alumina should be carried to the chrome (Fig. 8). out to provide a deformation free surface. Fig. 7: Cold worked austenitic steel showing twinning, etched with V2A etchant. Delta ferrite can appear due to critical heat A four step procedure, completed on automatic treatment conditions in martensitic or cold preparation equipment, gives good and repro- working of austenitic steels (Fig. 9). ducible results. Stainless steels are difficult to etch chemically and the recommended Austenitic-ferritic stainless steels (Duplex) etchants are very corrosive and require careful consist of ferrite and austenite. Electrolytic handling. etching in a 40% caustic soda solution reveals the structure and the correct percentage of each Alternatively, electrolytical polishing and etch- phase can be estimated (see Fig.1, and Fig. 10 ing is recommended, which gives a deforma- below). These steels are ductile and are specifi- tion free surface, but does not retain carbides. cally used in the food, paper and petroleum industries. Fig. 8: Austenite with carbides and some 200x titanium carbon nitrides Summary Stainless steels are corrosion resistant steels with high chromium and nickel contents. Fer- ritic and stainless steels are soft, respectively ductile, and are prone to mechanical deformation and scratching during metallographic preparation. In addition, carbides can not always be retained.

Fig. 9: Austenitic steel with strings of delta 125x ferrite, showing microsegregations. Blue areas: depletion of alloying elements.

Heating up to 1050°C and subsequent quenching removes the sigma phase and with it the embrittlement.

Austenitic stainless steels do not respond to thermal treatment, instead, rapid cooling results in the production of their softest condition. In this state they are non- magnetic and their properties are influenced by cold working. The microstructure of these steels consists of austenite grains which may Fig.10: Forged duplex steel showing blue ferrite, white exhibit twinning (Fig. 7). Exposure of these austenite and fine needles of sigma phase, etched electrolytically with 40% aqueous sodium hydroxide. steels to elevated temperatures in the region of 600-700°C can result in the formation of 150x Struers A/S Pederstrupvej 84 DK-2750 Ballerup, Denmark Phone +45 44 600 800 Fax +45 44 600 801 [email protected] www.struers.com

AUSTRALIA & NEW ZEALAND NETHERLANDS Struers Australia Struers GmbH Nederland 27 Mayneview Street Elektraweg 5 Application Milton QLD 4064 3144 CB Maassluis Notes Australia Telefoon +31 (10) 599 7209 Phone +61 7 3512 9600 Fax +31 (10) 5997201 Metallographic preparation of stainless steel Fax +61 7 3369 8200 [email protected] [email protected] NORWAY Elisabeth Weidmann, Struers A/S, Copenhagen BELGIQUE (Wallonie) Struers A/S, Norge Anne Guesnier, Struers A/S, Copenhagen Struers S.A.S. Sjøskogenveien 44C Bill Taylor, Struers, Ltd., Glasgow, UK. 370, rue du Marché Rollay 1407 Vinterbro F- 94507 Champigny Telefon +47 970 94 285 sur Marne Cedex [email protected] Acknowledgements Téléphone +33 1 5509 1430 Télécopie +33 1 5509 1449 ÖSTERREICH We wish to thank Böhler Edelstahl GmbH, Kapfenberg, [email protected] Struers GmbH Austria, for generously supplying information, sample Zweigniederlassung Österreich BELGIUM (Flanders) Betriebsgebiet Puch Nord 8 material, and the permission to reproduce the photo Struers GmbH Nederland 5412 Puch of parts on page 1 and the diagram of production Elektraweg 5 Telefon +43 6245 70567 3144 CB Maassluis Fax +43 6245 70567-78 flow on page 2. Special thanks go to J. Hofstätter for Telefoon +31 (10) 599 7209 [email protected] the co-operation and to A. Dreindl for contributing Fax +31 (10) 5997201 micrographs Figs. 1, 2, 6, 9 and 10. [email protected] POLAND Struers Sp. z o.o. We thank Dr. H. Schnarr, Struers GmbH, Willich, CANADA Oddział w Polsce Germany, for micrographs Figs. 4 and 7. Struers Ltd. ul. Jasnogórska 44 7275 West Credit Avenue 31-358 Kraków Mississauga, Ontario L5N 5M9 Phone +48 12 661 20 60 Bibliography Phone +1 905-814-8855 Fax +48 12 626 01 46 Fax +1 905-814-1440 [email protected] Schumann, VEB Deutscher Verlag für [email protected] Grundstoffindustrie, Leipzig 1968 ROMANIA CHINA Struers GmbH Werkstoffkunde und Werkstoffprüfung, W. Domke, Struers Ltd. Sucursala Sibiu Verlag W. Giradet, Essen, 1977 No. 1696 Zhang Heng Road Str.Scoala de Inot, nr. 18 Metals Handbook, Desk Edition, ASM, Metals Park, Zhang Jiang Hi-Tech Park RO-550005 Sibiu Shanghai 201203, P.R. China Phone +40 269 244 558 Ohio, 44073, 1985 Phone +86 (21) 6035 3900 Fax +40 269 244 559 Color Metallography, E. Beraha, B. Shpigler, ASM, Fax +86 (21) 6035 3999 [email protected] [email protected] Metals Park, Ohio, 44073, 1977 SCHWEIZ Handbuch der metallographischen Ätzverfahren, CZECH REPUBLIC Struers GmbH Struers GmbH Zweigniederlassung Schweiz M.Beckert, H. Klemm, VEB Deutscher Verlag für Organizační složka Weissenbrunnenstraße 41 Grunstoffindustrie, Leipzig, 1976 Havlíčkova 361 CH-8903 Birmensdorf CZ-252 63 Roztoky u Prahy Telefon +41 44 777 63 07 Metallography, Principles and Practice, George F. Phone +420 233 312 625 Fax +41 44 777 63 09 VanderVoort, McGraw-Hill Book Company, 1984 Fax +420 233 312 640 [email protected] [email protected] Merkblatt 821, Edelstahl Rostfrei-Eigenschaften SINGAPORE Informationsstelle Edelstahl Rostfrei, Düsseldorf, BRD DEUTSCHLAND Struers Singapore Metallographic instructions for color etching by Struers GmbH 627A Aljunied Road, Carl-Friedrich-Benz-Straße 5 #07-08 BizTech Centre immersion, E. Weck, E. Leistner, Deutscher Verlag für D- 47877 Willich Singapore 389842 Schweisstechnik (DVS), Düsseldorf, 1983 Telefon +49 (0) 2154 486-0 Phone +65 6299 2268 Fax +49 (0) 2154 486-222 Fax +65 6299 2661 [email protected] [email protected]

FRANCE SPAIN Struers S.A.S. Struers A/S, Spain 370, rue du Marché Rollay Camino Cerro de los Gamos 1 F-94507 Champigny Building 1 - Pozuelo de Alarcón sur Marne Cedex CP 28224 Madrid Téléphone +33 1 5509 1430 Teléfono +34 917 901 204 Télécopie +33 1 5509 1449 Fax +34 917 901 112 [email protected] [email protected]

HUNGARY SUOMI Struers GmbH Struers A/S, Suomi Magyarországi Fióktelep Hietalahdenranta 13 Tatai ut 53 00180 Helsinki 2821 Gyermely Puhelin +358 (0)207 919 430 Phone +36 (34) 880546 Faksi +358 (0)207 919 431 Fax +36 (34) 880547 [email protected] [email protected] SWEDEN IRELAND Struers Sverige Struers Ltd. Ekbacksvägen 22 Unit 11 Evolution@ AMP 168 69 Bromma Whittle Way, Catcliffe Telefon +46 (0)8 447 53 90 Rotherham S60 5BL Telefax +46 (0)8 447 53 99 Tel. +44 0845 604 6664 [email protected] Fax +44 0845 604 6651 [email protected] UNITED KINGDOM Struers Ltd. ITALY Unit 11 Evolution @ AMP Struers Italia Whittle Way, Catcliffe Via Monte Grappa 80/4 Rotherham S60 5BL 20020 Arese (MI) Tel. +44 0845 604 6664 Tel. +39-02/38236281 Fax +44 0845 604 6651 Fax +39-02/38236274 [email protected] [email protected] USA JAPAN Struers Inc. Marumoto Struers K.K. 24766 Detroit Road Stainless steel weld Takara 3rd Building Westlake, OH 44145-1598 18-6, Higashi Ueno 1-chome Phone +1 440 871 0071 etched according to Taito-ku, Tokyo 110-0015 Fax +1 440 871 8188 Beraha II. Phone +81 3 5688 2914 [email protected] Fax +81 3 5688 2927 [email protected] 20x

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