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 me- chanical 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 prop- erties, 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“).