388 Prakt. Met. Sonderband 38 (2006) 
This ultrahigh strength type of steel has highly anticorrosive properties due to existence of C 
considerable amounts of Cr, Ni and Mo in their composition. Also its weldability, castability, © 
and machinability are suitable too [1-5]. ; 
Sandvik stainless steel is semiaustenitic type of steel. They are essentially austenitic in the 
annealed condition but they can be transformed to martensitic under other thermal or | 
thermomechanical treatments [6]. 
Microstructure and distribution of alloy elements in the microstructure of these steels 
developed under various thermomechanical conditions have been studied by different 
techniques such as APFIM' [1,2,4], FIM? [3,4], TEM/EDX%[1], TEM/3Dap*[1-3], TAP°[1-4], : 
EFTEM®[2]. Therefore a great data of electron-microscopy information are available about 
these steels, but there is only a little available information about their optical 
metallography. Since optical metallography has some unique advantages, it seems logical 
trying to see the effects of various parameters affecting the revelation of optical 
microstructure features such as time, temperature, kind and concentration of electrolyte 
solutions, etc. Thus this research is an attempt to reveal the microstructure of Sandvik 
1RK91 stainless steel through optical microscope, using various etching techniques and 
different etchants. 
During etching process non-equilibrium high energy locations within the free surface of the 
samples to be etched undergoes a weak corrosion by which a structural contrast at the 
surface will be created, so that this helps one to recognize features such as grain 
boundaries, twin boundaries, slip bands, secondary phases, inclusions, etc. 
In Stainless steels such as Sandvik 1RK91, due to existence of high percentage of 
Chromium a passive layer of chromium oxide at the free surface is formed which prevents 
corrosion during etching process [7-8]. In addition, the existence of 4wt% Mo within the 
composition of these steel, renders it to one of the most anticorrosive stainless steels 
against chlorine ions. 
On the other hand, one can increase the density of dislocation in this type of steel by 
subjecting it to working processes such as cold work. The higher the amount of reduction, 
the higher will become the density of dislocations. These are associated with higher 
energy relative to their adjacent locations. Therefore, as the energy of free surface 
increases by cold work the energized locations on the surface become more vulnerable to 
corrosion when subjected to etching. However the form of response of various locations 
having different level of energy is different to different kind and techniques of etching. 
2. Experimental works 
The composition of bars of Sandvik stainless steel casted in a 10 kg inductive furnace is 
shown in table 1. The mould was metallic and the dimension of bars was 70*40*30 mm. 
The bars were homogenized and then subjected to hot rolling process, so that their 
thickness reduced to 10 mm. The rolling samples were soulution annealed at 1000 °C for 1 
hr. Samples then cooled in furnace or quenched in water up to room temperature. Finally, 
the thickness at the samples coded 8 and 13 in table 1 were reduced by different amounts 
i.e. 20%, 40%, 60% and 80%. These samples were recoded as shown in table 5 in order 
to show their amounts of cold roll reduction. 
' Atom probe Field ion microscopy 
? Field ion microscopy 
* Transmission electron microscopy/Energy Dispersive X-ray R 
: Transmission electron microscopy/3 dimensional atom probe te 
* Tomographic Atom Probe S 
" Energy filter transmission electron microscopy