MadSci Network: Engineering

Re: What is significant about varying the grain lengths in steels.

Date: Mon Jan 31 09:55:42 2000
Posted By: Bob Novak, Other (pls. specify below), Sr Process Research Engineer, Carpenter Technology
Area of science: Engineering
ID: 947866744.Eg

Hi Adam,

Lets take your questions in reverse order. 

Metals have crystaline structures. A grain is a single crystal in a 
metallic material. The grains form when the metal solidifies from a liquid.
Metals have a tendancy to fracture or break along the grain boundaries. 
Metals with large elongated grains are usually not as strong as metals 
with smaller and more spherical grains. 

ASTM method E 112 provides a detailed explanation of how grains are 
measured. The following images are examples of different size grains with 
ASTM size ratings. The sample is polished and viewed through a microscope 
at 100 X magnification. Typically, an etch (chemical solution) is used to 
enhance the contrast between the grains and the grain boundaries. One 
method for measuring grain size is to draw a line of known length across a 
portion of the image and count the number of times a grain boundary 
crosses the line. The average grain diameter is the line length divided by 
the number of grain boundaries. 
ASTM method E 112 has a table for converting average grain diameter to 
grain size.

Notice that the larger grains have a smaller ASTM grain size.
If you require a more detailed explanation, I recommend an introductory 
course to metallurgy.

Bob Novak
Specialist -Process R&D
Carpenter Technology

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