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2012
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For the structures with abnormally coarse grains circled with
sufficiently homogeneous fine grains, there is a special standard,
ASTM Е 930 Methods for Assessment of the Largest Observed
Grains (ALA), which has a number of drawbacks and practically is
not used in industrial practice.
In the present-day practice of making products from high-alloy
steels and alloys, one of the main requirements determining quality
of the finished products is the grain size determined by the methods
established in the existing standards.
The following standards were most commonly used at the end of
the last century:
•
ASTM E l12-95: Standard Test Methods for Determining Average
Grain Size
•
DIN 50601: Determining Ferrite and Austenite Grain Size in Iron
Base Steels and Materials
•
ISO 643: Steel. Micrographic Determination of the Apparent
Grain Size
•
GОSТ 5639: Steels and Alloys. Methods for Determining Grain
Size
It should be noted that all ASTM standards set narrow scopes of
their competence. It ensures determining grain size in corresponding
(though rather conventional) units with a rather high reliability.
As regards standard GOST 5639, its authors’ attempt to establish
a universal normative document has resulted in a number of
uncertainties and, in some cases, in impossibility of a reliable grain
size determination in steels prone to anisotropy, namely the steels
for which anisotropy is the main quality index. This is why there were
cases of rejection of this standard application in industry.
The most serious drawbacks of GOST 5639 include the following:
•
setting limits of grain quantity in 1mm
2
for each grain number
•
introduction of a method for assessment of anisomerous structures
•
its obligatory use raised to the rank of a law
All three standards contain different quantities of whole numbers
conventionally defining grain sizes. For all that, ASTM Е112 permits
assessment in fractional numbers (eg G8.5). It should be pointed
out that average grain sizes with the same numbers are identical in
the three standards.
Standards DIN 50601 and GOST 5639 use a metric system in which
number of grains in 1mm
2
is determined and the average diameters
approximately correspond to numbers G0 tо G14 in ASTM E112 in
which another size scale in inches is additionally given.
Use of the metric system in European standards has brought
about a shift of the standard sequence into the field of negative
numbers having no physical sense. Common disadvantages of
all three standards include conventionality of units (ie numbers)
which complicates their conversion into the commonly used units
(millimetres) in design calculations.
For such method standard as GOST 5639, its obligatory use
is incorrect because it includes provisions allowing double
interpretation of the same quantity.
GOST 5639 contains a number of intolerable errors which were
discussed in detail in
[4-6]
.
Determination of average grain diameter by the method of grain
boundary intersection (it is unclear why it was called “conditional”)
is analogous to that of E112 but numerical value
d
L
(GOST 5639)
and
l
- (ASTM Е112) somewhat differ due to the difference between
British and metric systems of units.
Major doubts are raised by the method of counting the number
of grains in 1mm
3
in GOST 5639, ie counting an average number
of grains in the specimen volume not specified in any one of the
standards under consideration. The American standard specifies
determination of an average number of nonequiaxed grains in 1mm
2
and GOST 5639 in 1mm
3
which is totally baseless and can only be
done with the use of stereological reconstruction.
It would be quite another matter if it were a question of area. Then,
according to
[5]
, an equigranular in volume structure (ie a structure
consisting of grains of a same size) for which
D
max
= D
av
= D
min
gives distribution containing up to 28% of grains sizing from
D
min
→
0 to
D
max
in a random secant plane.
For comparison, Tables 1 and 2 list grain structure parameters and
methods for their determination in accordance with the three most
widespread standards to demonstrate basic differences between
them.
The final objective of development of the technology for manufacture
of products of special steels and alloys is formation of a specified
uniform grain size which in its turn ensures required design material
strength.
As such structures are mostly assessed in practice by the method of
visual comparison with photographic references from the functional
standards and in conventional units (numbers), such assessment
is subjective, open to serious measurement errors and methods
of assessment of grain size unhomogeneity are unavailable at all.
As the analysis of the existing standards shows, except the chord
method, they do not take into account real distribution of the grain
characteristics (chords, plain section diameters, diameters in a
volume). Therefore, it is inexpedient to use it for assessment of
high-duty products in which grain size is the main quality index,
and as SA Saltykov affirms in his book, “Standardised methods of
metallographic analysis… are mostly semiquantitative methods of
number-based assessment. Its main disadvantage is its subjectivity
and the resulted low accuracy and reliability”.
An objective assessment of grain structures in metals and alloys
can be done with the help of the function of distribution of three-
dimensional objects and its quantitative characteristics which
can be realised through a stereological reconstruction of the
actual structure images visible in microsections and by the use of
computerised methods of primary data processing
[5]
.
The known quantitative methods in the standards of developed
countries (ASTM Е112, DIN 50601, etc) allow to measure grain size
with identical errors.