How is the microstructure of steel differs when it's ferrite, pearlite, bainite or martensite?

Question

I like to know how each microstructure looks like or be determined under the microscope in terms of color, shape and texture, and which one is best in bending, welding and corrosion resistance.

Answer 1

So good to see other materials engineers here :). I also recommend the ASM handbook, as the type of etchant used can make the colour very different. The section taken also can affect the shape and sizes.

Ferrite is the most 'standard' one- consisting of mostly equiaxed grains without a whole lot of features. On grain boundaries you may see small clumps of cementite which looks whitish with a nital etchang. Grain size may vary quite a bit. It's the softest iron phase.

Pearlite is pretty easily to identify as it's an eutectic structure with narrow lines and lamellae of cementite in ferrite.

Martensite is the hardest phase and can have lath or plate form. Generally flat, sharp and angular, can look like needles, but depend on quench and temper methods. Forgot about bainite...sorry.

Generally metal working is done with all sorts of steels depending on application. Softer steel = easier and lower costs. Corrosion resistance does not depend too much on microstructure but rather on composition and protective mechanisms applied. Of course, with steels you can get carbon leached out, and steel reinforced concrete has a whole set of problems if not made correctly due to pH and 'concrete cancer'. I suggest you refer to websites and texts for more descriptive and accurate info.

Answer 2

The microstucture is different is because it depends on its heating & cooling manner. Ferrite shows the same phase for the whole structure, pearlite will shows alternating structure which consists of ferrite(white)& cememtite(black). Bainite- distribution of sphere shape cemetite in ferrite. Martensite- needle like cemetite in ferrite.
The best in bending is the one which is most ductile, martensite and bainite are structure with higher strength and hardness, so these two may not suitable for bending. I am not sure that which structure is the best in welding, but since welding involves melting between joining component, so i guess the structure which has lower melting will be more weldable.
For steel with high corrosion resistance must alloyed with some element like cromium.Stainless steel is a good example which has high corrosion resistance. This property is due to a thin, adherent, stable chromium oxide or nickel oxide that effectively protects the steel against corroding media.

Answer 3

Ferrite looks like white poligons with very defined black grain boundaries. Pearlite looks exactly like a dactilar print. Bainite and martensite are really similar and hard to distinguish under the optical microscope. They usually look like needles or plaques depending on the heat treatment.
Martensite is always the hardest, and then, in order, bainite, pearlite and ferrite, which is the softer, due to it's low carbon content.
Welding and corrosion properties are mostly determined by other elements of aleation.

Answer 4

Pearlite Microstructure

Answer 5

way over everyone head simplest answer is it reduces the level of carbon. Just a guess!