People often ask me what my favorite metal is. For that, I
don’t really have an answer since I tend to think in terms of an application. However,
I will say my favorite microstructure is that of duplex stainless steels since
nothing else looks quite as striking when prepared and etched correctly.
Duplex stainless steels are a unique variety of stainless
steels. Stainless steels include the typical austenitic corrosion resistant
stainless steels (CRES), ferritic stainless steels, duplex stainless steels,
precipitation hardened, and martensitic stainless steels. The duplex stainless
steels are named after their structure which contains both austinite and
ferrite (ideally in exact proportions). Duplex stainless steels such as alloy
2205 are used when both strength and corrosion resistance are required. Also,
the lack of expensive alloying elements such as nickel and higher strength which
requires less metal in most designs provides a significant cost savings. Also,
the alloy is more corrosion resistant in comparison to some austenitic grades
of stainless steel such as 316 while being weldable making it a superior choice
for many marine and oil/gas applications.
Below are photomicrographs of a 2205 duplex stainless steel
at three magnifications (100X, 200X, and 500X). The sample was prepared from a
piece of 2205 bar stock which was ground and polished to a sub-micron final polish.
The sample was etched using an electrolytic etching technique. The electrolyte used
was potassium hydroxide (40%) with a platinum cathode at 5 volts for approximately
5-6 seconds. The sample microstructure consists of islands of austinite in a
ferrite matrix.
KOH Solution:
40 grams Potassium Hydroxide, 100 mL Water
