Blacksmithing
and Cutlery
by Gérard HEUTTE
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Steel grain
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Bases
Steel is not a homogeneous matter! The page on
microstructure
already gives an outline.
Moreover, steel consists of juxtaposed grains. Each
grain can contain one or more components, in the form of plates, of
needles or pearls...
The structure inside a grain is in general rather
regular. The border between two grains is called "grain
boundary". It can be a simple change of orientation of the
crystal matrix or a space containing one or more phases of steel,
such as for example Cementite. See the opposite diagram.
For example, a hypereutectoid steel with 1% Carbon will have
grains of Pearlite (containing 0.77% of Carbon) and grain
boundaries with Cementite. Broadly the proportion of Carbon is
1% well. But on a macroscopic scale, the uniformity does not
exist! ! !
Why be worried about?
The grain size has a major influence on the
mechanical properties of steel in term of strength and cutting ability!
A good blade must have a fine grain.
Theoretical definition of the grain size
French reference: NF IN ISO 643 standard
Reference the USA: ASTM
| N° |
Average diameter (µ) |
General appreciation |
| -1 or 00 |
500 |
Very coarse |
| 0 |
360 |
| 1 |
250 |
Coarse |
| 2 |
180 |
| 3 |
125 |
| 4 |
90 |
Average |
| 5 |
65 |
| 6 |
45 |
| 7 |
32 |
Fine |
| 8 |
22 |
| 9 |
16 |
| 10 |
11 |
Very fine |
| 11 |
8.0 |
| 12 |
5.6 |
| 13 |
4.0 |
Ultra fine |
| 14 |
2.8 |
| 15 |
2.0 |
But all this information is not usable easily!
In practical
To make sure that the size of grain of the
blades is correct, it is necessary to make tests!
Make a blade with your usual working method. Then break
it (by tightening it in a vice and strongly striking it with
a hammer). The grain can then be observed in the zone of
rupture. If you can see it with eyes, it is too coarse.
If you distinguish it a little with a magnifying lens, it is
already well (but can be better). If you do not see it
with a magnifying lens it is a very fine grain!
Grain size growth
It is not a sought effect, but unfortunately frequent!
The grain tends to become bigger via two factors:
> High temperature.
> Long duration of heating.
Be careful, temperature is the critical parameter for grain growth! Time is a less
important parameter. A strong overheating has more impact than a blade kept in fire for a
few minutes.
The opposite diagram shows clearly what occurs:
> The rise in temperature does not produce an effect on the
grain until AC1 (blue part).
> Between AC1 and AC3, the grain size decreases (green part)!
> If you continue to increase the temperature or if you
maintain long enough a temperature higher than AC3, the grain grows
bigger (pink and orange parts)...
> Any later cooling from there will left a coarse grain.
Grain size reduction
The size of the grain could be reduced by heat
treatment.
The figure opposite shows the principle:
> Go up in temperature a little above AC3 (blue part).
> Go down quickly below AC1, before the size of
grain increase (green part).
> As soon as the temperature passed below AC1, no more
risk of enlargement of the grain.
> Then you can heat the steel to start again this operation.
The whole set of these operations is called
Normalization.
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