Metallographic preparation of stainless Application

Corrosion resistant contain at Notes least 11% and are collectively known as “stainless steels“. Within this group of high 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 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- 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 ; nitrogen increases strength main metallographic tests are grain size and sulphur increases machinability, measurement, identification of delta because it forms small sul- ferrite and sigma 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 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 me- chanical and physical properties are produced. Hot forming Hot Hot 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 , 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% . 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 for chemical, environmental forming. Properties: high ductility, high and offshore industries, corrosion resistance, resistant to oxidizing architecture. acids, alkalis, very good cold forming prop- erties, easy to work and machine. Difficulties in Grinding the preparation of Step PG FG stainless steels Surface Foil/Paper 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 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 can be steels 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 mag- netic 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. is formed through rapid cooling and properties can then be opti- Type Diamond Diamond Silica/Alumina Abrasive mised by subsequent treatment. Size 3 μm 1 μm 0.04 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 tem- pered 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 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 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 , which increases - coarse abrasives for plane grinding the susceptibility to 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 deforma- tion 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 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 ApS Pederstrupvej 84 DK-2750 Ballerup, Denmark Phone +45 44 600 800 Fax +45 44 600 801 [email protected] www.struers.com

Application Notes AUSTRALIA & NEW ZEALAND NETHERLANDS Struers Australia Struers GmbH Nederland Metallographic preparation of stainless steel 27 Mayneview Street Zomerdijk 34 A Milton QLD 4064 3143 CT Maassluis Elisabeth Weidmann, Struers A/S, Copenhagen Australia Telefoon +31 (10) 599 7209 Anne Guesnier, Struers A/S, Copenhagen Phone +61 7 3512 9600 Fax +31 (10) 5997201 Fax +61 7 3369 8200 [email protected] Bill Taylor, Struers, Ltd., Glasgow, UK. [email protected] NORWAY BELGIUM (Wallonie) Struers ApS, Norge Acknowledgements Struers S.A.S. Sjøskogenveien 44C We wish to thank Böhler Edelstahl GmbH, Kapfenberg, 370, rue du Marché Rollay 1407 Vinterbro F- 94507 Champigny Telefon +47 970 94 285 Austria, for generously supplying information, sample sur Marne Cedex [email protected] material, and the permission to reproduce the photo Téléphone +33 1 5509 1430 Télécopie +33 1 5509 1449 AUSTRIA of parts on page 1 and the diagram of production [email protected] Struers GmbH flow on page 2. Special thanks go to J. Hofstätter for Zweigniederlassung Österreich the co-operation and to A. Dreindl for contributing BELGIUM (Flanders) Betriebsgebiet Puch Nord 8 Struers GmbH Nederland 5412 Puch micrographs Figs. 1, 2, 6, 9 and 10. Zomerdijk 34 A Telefon +43 6245 70567 We thank Dr. H. Schnarr, Struers GmbH, Willich, 3143 CT Maassluis Fax +43 6245 70567-78 Telefoon +31 (10) 599 7209 [email protected] Germany, for micrographs Figs. 4 and 7. Fax +31 (10) 5997201 [email protected] POLAND Struers Sp. z o.o. Bibliography CANADA Oddział w Polsce Schumann, VEB Deutscher Verlag für Struers Ltd. ul. Jasnogórska 44 7275 West Credit Avenue 31-358 Kraków Grundstoffindustrie, Leipzig 1968 Mississauga, Ontario L5N 5M9 Phone +48 12 661 20 60 Werkstoffkunde und Werkstoffprüfung, W. Domke, Phone +1 905-814-8855 Fax +48 12 626 01 46 Verlag W. Giradet, Essen, 1977 Fax +1 905-814-1440 [email protected] [email protected] Handbook, Desk Edition, ASM, Metals Park, ROMANIA Ohio, 44073, 1985 CHINA Struers GmbH, Sucursala Struers Ltd. Bucuresti Color Metallography, E. Beraha, B. Shpigler, ASM, No. 1696 Zhang Heng Road Str. Preciziei nr. 6R Metals Park, Ohio, 44073, 1977 Zhang Jiang Hi-Tech Park 062203 sector 6, Bucuresti Shanghai 201203, P.R. China Phone +40 (31) 101 9548 Handbuch der metallographischen Ätzverfahren, Phone +86 (21) 6035 3900 Fax +40 (31) 101 9549 M.Beckert, H. Klemm, VEB Deutscher Verlag für Fax +86 (21) 6035 3999 [email protected] [email protected] Grunstoffindustrie, Leipzig, 1976 SWITZERLAND Metallography, Principles and Practice, George F. CZECH REPUBLIC & SLOVAKIA Struers GmbH VanderVoort, McGraw-Hill Book Company, 1984 Struers GmbH Organizační složka Zweigniederlassung Schweiz vědeckotechnický park Weissenbrunnenstraße 41 Merkblatt 821, Edelstahl Rostfrei-Eigenschaften Přílepská 1920, CH-8903 Birmensdorf Informationsstelle Edelstahl Rostfrei, Düsseldorf, BRD CZ-252 63 Roztoky u Prahy Telefon +41 44 777 63 07 Phone +420 233 312 625 Fax +41 44 777 63 09 Metallographic instructions for color etching by Fax +420 233 312 640 [email protected] immersion, E. Weck, E. Leistner, Deutscher Verlag für [email protected] [email protected] SINGAPORE Schweisstechnik (DVS), Düsseldorf, 1983 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] Stainless steel weld Fax +39-02/38236274 etched according to [email protected] USA Struers Inc. Beraha II. JAPAN 24766 Detroit Road Marumoto Struers K.K Westlake, OH 44145-1598 20x 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] 28.02.2019 / 62140005