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The main characteristic of stainless steel is its corrosion resistance, which can be enhanced by adding specific alloying elements. These alloying elements also have further beneficial effects on other material properties, such as toughness and oxidation resistance.
For example, niobium and titanium enhance intergranular corrosion resistance because they absorb carbon to form carbides, and sulfur enhances machinability because it forms small amounts of manganese sulfides, which produce shorter chips.
Stainless steel plays an important role in aircraft, chemical, medical and food industries, professional kitchens, construction and even jewelry due to its corrosion resistance and excellent surface finish. Stainless steel is also frequently used in automotive applications.
Metallography of stainless steel is an important part of the overall quality control process in many production environments. The main metallographic tests are as follows:
Measurement of grain size
Study of general structure, including the amount of martensite, ferrite, pearlite or austenite
Identification of delta ferrite and sigma phases
Evaluation of carbides and their distribution
Study of welds
In addition, metallography is used in failure analysis to study corrosion/oxidation mechanisms.
Figure 1: Electrolytic etching of duplex steel using 40% sodium hydroxide in water, showing brown austenite and blue ferrite. Bright field.
The main characteristic of stainless steel is its corrosion resistance, which can be enhanced by adding specific alloying elements. These alloying elements also have further beneficial effects on other material properties, such as toughness and oxidation resistance.
For example, niobium and titanium can enhance intergranular corrosion resistance because they absorb carbon to form carbides; sulfur can enhance machinability because it forms small amounts of manganese sulfide, which produces shorter chips.
Figure 1: Electrolytic etching of duplex steel using 40% sodium hydroxide aqueous solution shows brown austenite and blue ferrite. Bright field.
Electrolytic polishing and etching of stainless steel (grinding on silicon carbide foil/sandpaper 220#, 500# and 1000#):
Electrolyte: A2
Area: 5 cm²
Voltage: 35 V
Flow rate: 13
Time: 25 seconds
External etching with stainless steel etching disk:
10% oxalic acid in water
Voltage: 15 V
Time: 60 seconds
Grinding
For soft and ductile stainless steels, it is strongly recommended to avoid very coarse grinding foils/sandpapers and high pressures as these can produce deep deformations.
In general, coarse grinding should be done with the finest abrasive consistent with the sample area and surface roughness.
Polishing
If any deformation introduced in the first grinding step is not removed by fine grinding, it will leave marks. These marks can be removed by final polishing, but this is time consuming.
Fine grinding should be done with diamond on a rigid grinding disc (MD-Largo) or (as an alternative for some types of stainless steel) on an MD-Plan or MD-Sat polishing cloth.
After fine grinding, a thorough polishing with diamond on a medium hard polishing cloth should be performed, followed by a final polishing with colloidal silica (such as OP-S) or aluminum oxide (OP-A) to remove any minor scratches. This final step should be very thorough and may take several minutes. A good final polishing increases the chances of improved contrast.
Preparation method for stainless steel samples with a diameter of 30 mm mounted on a semi-automatic Tegramin with a diameter of 300 mm.
As an alternative to DiaPro, Polycrystalline P can be used with green/blue lubricants.
Preparation method for stainless steel samples (65x30 mm) mounted or unmounted using Struers MAPS or AbraPlan/AbraPol with a diameter of 350 mm and stone-ground cold mounted.
As an alternative to DiaPro, Polycrystalline P can be used with green/blue lubricants.
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13472638080 Clara Xu
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