Metallography: Principles and Practices - ASM International

Metallography: Principles and Practice (#06785G)
Author(s): George F. Vander Voort
48 METALLOGRAPHY
Copyright © 1984 ASM International ®
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Because the solubility of solid sulfur in iron is very low, nearly all the sulfur is
precipitated as sulfide inclusions. Sulfide distribution is influenced by the steelmaking
process, with the degree of deoxidation being a primary factor. In steels
with a very high oxygen content (low carbon content, no strong deoxidizers), most
of the sulfides are found in the central portion of the ingot. In steels with low
oxygen contents (medium or high carbon content, additions of silicon, aluminum,
etc.), the sulfides are more uniformly distributed.
The early history of the sulfur print method has been reviewed by Berglund
[2]. The method used today was developed by Baumann in 1906, but a similar
technique was developed by Heyn earlier in 1906. In Heyn's method, a piece of
silk is placed on the ground steel surface and is moistened with a solution
containing 10 g mercuric chloride, 20 mL hydrochloric acid, and 100 mL water.
The solution attacks the sulfide inclusions, producing hydrogen sulfide gas, which
reacts with the mercuric chloride, precipitating black mercuric sulfide on the silk.
This results in a mirror image of the sulfide distribution on the face of the ground
sample. It is claimed that the method also shows the distribution of phosphorus, but
this claim has never been proven conclusively.
Baumann's method, which is being used today in preference to Heyn's
method, employs ordinary photographic paper soaked in dilute sulfuric acid. The
photographic paper should be thin so that good contact is achieved with the
surface. While glossy papers produce sharper prints, they are slippery, and it is
difficult to prevent blurring of the image. Consequently, matte finishes are preferred.
Bromide-type papers are preferred to chloride-type papers.
To obtain good prints, a ground surface is required. Polished surfaces can be
used but do not produce improved results. Indeed, paper slippage resulting in
blurred images is more common with polished surfaces. The surface to be printed
must be carefully cleaned; otherwise, dark spots will result. Many solvents
commonly employed in the metallography laboratory are suitable for cleaning. It
is not uncommon to read recommendations in the literature suggesting cleaning
with gasoline, petroleum ether, or carbon tetrachloride. However, in the interest
of safety in the laboratory, use of solutions of this type should be avoided. Instead
cleaning with acetone, alcohol, or 1,1,1-trichloroethane is recommended.
Preetching the surface of steel samples with 10% nitric acid in water often
helps to obtain quality sulfur prints. The preetch improves the image intensity and
promotes contact adhesion. Steels with less than 0.010% sulfur give faint prints,
and preetching produces a more distinct print. Preetched surfaces can be printed
several times, whereas only one print can be obtained from ground surfaces of
steels with normal sulfur levels. However, with resulfurized steels, the first print is
usually so dark that all details are obscured, and a second print from the ground
surface (no preetch) produces a better rendition of the sulfur distribution.
The photographic paper is soaked in a solution containing 1 to 5% sulfuric
acid in water; a 2% solution is used most often. A more dilute solution can be used
for resulfurized steels or for printing very large surfaces where some time is
required to cover the sample with the paper.
In general, the paper is soaked for 1 to 5 min in the solution of dilute acid; a 3-