Therefore
when a sufficiently thick layer of the cold metal has been heated the
interior is torn asunder by the expansion of the outer skin, and the
specific gravity of the whole mass is diminished. (See Mr Wrightson's
paper " On Iron and Steel at High Temperatures ", with discussion, Journal of the Iron and Steel Institute, No. 1 for 1880.)
We
may therefore safely conclude that when iron is suddenly cooled, the
only compressive bulk pressure that is brought to bear on the interior
is that arising from the contraction of the outer layers after setting,
and with highly carburised iron this can only be small because of the
low tensile strength of the metal.
Gases ejected from cast iron on setting
As
bearing upon the question of the possibility of the occluded gases
playing a part, Moissan was the first to observe that spherules or
small spheres of iron with cracks and geodes never contained diamond.
We have made experiments by pouring highly carburised iron, alloys and
mixtures on to iron plates, the cooling taking place from one side
only, and under such conditions no diamond results; in fact it only
occurs when the ingot or spherule is cooled on all sides nearly
simultaneously, so that an envelope of cold metal is formed all over
before the centre sets.
Since
my paper in 1907, the experiment of heating iron in a carbon crucible
and transferring it to a steel die and subjecting it to 11,200
atmospheres pressure has been repeated, and it has been found that if
the iron is allowed to set before the pressure is applied the amount of
diamond is much greater than if pressed when very hot and molten, and
that it is then about the same as when the crucible is cooled in water.
The only reason that suggests itself to account for this is, that when
pressure is applied while the iron is very hot some of the latter
permeates the carbon of the crucible, and because of the greater
specific heat and lesser con-ductibility of the carbon, the iron next
to and in the carbon remains molten after the ingot has been cooled by
direct contact with the steel cup on the face of the plunger. Thus,
when cooling, the occluded gases have a free exit from the ingot,
through the molten metal (which is pervious to gas) into the carbon of
the crucible, and are not retained in the ingot to the same extent as
when it is set and enclosed in an envelope of colder iron impermeable
to the gases before pressing.
The experiments of Baraduc Muller (Iron and Steel Institute, Carnegie Scholarship Memoirs, 1914, p. 216), on the extraction of gases from molten steel, showed that steel is permeable to gases down to 600° C.
