Grain boundaries

[rinkman11]

Hello all,
After grinding a 12" 25/20 centrifugal casting to a 10 Ra, the surface appearance of this tube looks uneven in color(light and dark areas). It has been explained to me that this is a reflection of the grain boundaries? Is this true because of the steel mixture being less homogeneous.

Thank you in advance for your answers.
Doug

 

[arunmrao]

Is it a copper base alloy?

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"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[Metalguy]

Grain orientation, not their boundaries.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[monkeydog]

When I look in ASM, micrographical section, for X15CrNiSi25-20 cast Stainless Steel, there are different colors/shades of the grains.

 

[EdStainless]

You may also have removed different amounts of stock, the material near the surface may have different grain size/orientation that gives it a different look.

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Plymouth Tube

 

[Guest102023]

Ed is correct, a thin surface layer found in these castings will give a different colour when machined if the amount of stock removed is not uniform.