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Патент USA US2406534

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Patented Aug. 27, 1946
; 2,406,534
Luther D. Fetterolf, Palmerton, Pa., assignor ‘to
C _ The ‘New Jersey Zinc Company, New York, N. Y.,
I a corporation of New Jersey
Application March 10, 1944, Serial No. 525,972
4‘ Claims. (01. 266—19)
This invention relates to the protection of iron
and steel against oxidation when subjected to
high temperatures such as 1000 to 1300° C., and
has for its object the provision of a protective
ceramic coating on iron and steel, and more par
ticularly on externally heated retorts made of iron
or steel. The invention‘ is particularly applicable
' of the iron or steel retort is provided with a
ceramic coating that/effectively protects the iron
or steel fromoxidation and corrosion by the hot
to iron or steel retorts in the production of mag
' the single ?gure of the accompanying drawing
nesium by pyrometallurgical reduction or smelt
ing, and that application of the invention is here
in emphasized, but without thereby limiting the
vided with the composite ceramic coating of the
scope of the invention.
‘ heating gases.
illustrating in section an alloy steel retort pro
tecting ferrous articles, that is articles made of V_
Magnesium reduction or smelting is custom
arily carried out in externally heated metal re
an internal diameter of about 10 inches and a
wall thickness of about 11/8 inches.
In, its broad aspect the invention involves pro
torts, usually alloy steel tubes or cylinders having
Steel retorts so protected have
much longer life than unprotected steel retorts.
The invention will be best understood from the
following description taken in conjunction with
iron or steel, against oxidation at elevated-tem
peratures (e. g. 1000 to 1300’ C.) by a composite
ceramic coating comprising a base layer of rea
fractory material nonreactive with iron oxide and
The retorts
a covering layer of a vitreous material ?rmly
adhering to the base layer and forming an im
torts are mounted in a furnace structure with 5 20 pervious ?lm thereover. It is now my preferred
practice to use a magnesiferous refractory, such
feet or more of their length in the heated zone.
as magnesite or periclase, as the base layer,
The reducing charge consists of a briquetted
other suitable refractory materials for the base
mixture of calcined dolomite (or equivalent mag
are now customarily made of a steel alloy con
taining 25% chromium and 15% nickel.
The re
nesiferous material) and ferrosilicon (‘or equiv
layer include chromite-magnesite, chrome, for
Feldspar is ad
mirably adapted as the vitreous material of the
covering layer. Feldspar fuses to a glass attem
peraturesaround 1150-1250° C. If the operating
(2) connecting the retorts to a vacuum line ca
temperature to which the ferrous article is to be‘ '
pable of maintaining within the retorts a vacuum
of 200-250 microns of mercury during the initial 30 subjected is substantially lower than 1150° (3.,
a ?ux can be added to the feldspar to lower its
stage of the reduction and of 50-100m'i'crons of
alent reducing agent). The operation is inter
25 sterite, alumina and the like.
mittent or batch, and the operating cycle ‘com
prises (l) charging the briquets to the retorts,
mercury during the ?nal stage of the reduction,
(3) heating the charge' to a temperature of
1150-1250° C. for 7 to 8 hours, (4) removing the
fusion temperature correspondingly. Ceramic,
glaze compositions, common glass compositions of
the soda-limeesilica type, and the like can be used
condensed magnesium vapor from the cold ex vcl: an as the vitreous covering layer where such com
positions are adjusted to melt or fuse to an imtension of th'e'retort, and (5) discharging the
pervious ?lm at the contemplated elevated oper-:
Worked-off briquets or spent residue from the re
ating temperature.
torts. The metal retorts are thus subjected to
In practicing the invention, the surface of the
atmospheric pressure on the outside and to a
relatively high vacuum on the inside during the 40 ferrous article to be protected, e. g. the exterior.
surface of a steel magnesium reducing retort, is
reducing stage of the operating cycle. This re
covered with a relatively thick (e. g. 315 to 1':
sults in a considerable compressive stress on the
inch) base layer of the refractory material, and
retort wall. The retort ultimately fails by grad
the base coat is then covered by a relatively thin
ually collapsing under this long continued com- '
cover coat of a vitreous material adapted to fuse
pressive stress. In the heretofore customary
to form an impervious ?hn at the elevated oper
practice, the retorts collapse and must be re
ating temperature to which the ferrous article is
placed after an operating period of from 60 to 90
to be subjected. The covering layer of vitreous
material ?rmly adheres ‘to the base coat and
Attempts to replace the aforementioned alloy
steel retorts with iron or ordinary steel retorts 50 forms an impervious ?lm thereover. Preferably,
the vitreous material slightly penetrates the base
have heretofore failed because of the rapid oxida
layer to form a vitreous bond therewith.
tion of the iron or steel on the outside by the
The base layer of refractory material may be
heating gases, usually the hot products of com
applied to the ferrous article in any appropriate
bustion of solid, liquid or gaseous fuel. In ac
cordance with my present invention, the outside 55
Thus, for example, the refractory ma- ,
' 4
terial may be made into a vslurry with water, and
The feldspar is prepared for coating by milling a‘
theslurry appliedto the surface to be protected
by dipping, spraying, or the like. The base layer
may be dried before'applying the covering layer,
in Water to a slurry with the addition of a small
amount of clay. In view of the thinness of the '
covering'layer, it is not necessary to add an elec-'
trolyte.» The slurry may be milled to a ?neness
of through 100 mesh, though such a degree of
?neness is not ordinarily necessary,
or the covering layer may be applied to the still
wet base layer. The covering layer may be ap
plied to the base layer in any. appropriate man
ner,'but when'applied to the still wet base layer it
The functionv of the covering layeris twofold:
' is advantageous to apply the covering layer by »
(1) to form an impervious ?lm over the base layer
spraying a slurry of the vitreous material and '10.. .to prevent access of oxidizing gases to the ferrous
water. The covering layer should be thinly ap- ’ " article; and (2) to slightly penetrate the base
plied, since an excess amount of thisilayer may: 1 layer and form’a- vitreous bond therewith. The '
penetrate through the base layer andreact with . magnesia baselayer has no bond strength at all
the ferrous article. ‘The layers should be slowly
> aside .irom'that contributed by its small clay con
‘.tent, and a limitedipenetration by the'covering
layer'is' desirabl'e'rin order to increase’ the met
‘ 7 dried and ?red to prevent disintegration by ‘too
rapidrexpulsion of water.‘ The ?ring may be ef; '
fectedby exposing the dried'coatingto'the e1e-" ~ . chanical durability of the composite coating.
vated temperature of its contemplatedplaceof -
use, as for example in the case of an ordinary‘steel
magnesium reducing retort by installingthe re
tort,j'coatedlas herein described, in the magnesium
reductionlfurn-ace. However e?ected, ?ring fuses
1. A ferrous article protected against oxidation
20 at elevated temperature by a ceramic coating
comprising a relativelythick base layer of ' re
fractory material non-reactive with iron oxide ‘ '
or melts the covering layer and covers or encloses
selected from the group-consisting of magnesite,
the refractory layer with an impervious vitreous
peri'clase, chrome-magnesite, chromite, fo'rsterite
25 and alumina; and a relatively thin‘ covering layer
Byway of example, the coating of ‘the ferrous
of a vitreous material slightly penetrating the base
' article with a base layer of magnesiteor periclase
layerto form a vitreous boncltherewith and form.
and‘a covering layer of feldspar will be particu.‘ ‘ ing an impervious i?lm thereov-er, said vitreous
larlyT described as follows :fThe magnesiteior peri
material being selected from they group consisting
clasei-slmade up in slurry form by‘milling with 30 of feldspar, ceramic glaze compositions and glass
Water and a small amount of clay in order to
maintain the magnesia particles in ‘water sus-‘
compositions characterized -by=_melting to ‘an ‘im
pervious ?lm at temperatures ‘of from 1900' to
pension? About 5% of clay generally needed,
though this depends upon the suspensive prop
2. LA ferrous article in accordance with claim 1,
erties-of the clay' ~ The clay has the further funce 35 in which the-base layer is principally of magnesite
tion of contributing strength to vthe dried and ?red
to T16» inch thick-rand the covering layer
coating. Magnesia grain, whether in the form
is an impe-rvious-?lmof fusedfeldspar.
Ofm-agnesite or periclase, has no bonding (prop
S. -.A steel retort‘ having its exterior surface pro
- er'ties itself and is too refractory to develop any
tected' against? oxidation at elevated temperature '
?red strength at a temperature as low as 1200° C. 40 by ‘a ceramic coating comprising a re'lativelythick,
' ?--'~I‘he consistency of the slurry is adjusted, by the
‘base ‘layer ore-refractory materially non-reactive
addition of water, to the desired method ‘of apé
with iron oxide selected from the group consist
plying'the»layer—either spraying or dipping. It
may be desirable to adjust the yield value of the
slurry by adding some ‘electrolyte, such as, for
example,» a few tenths of one per cent of- mag-nee
ing of magnesite, pericl-ase,"chromite'emagnesite;
chrome, -';forsterite and‘ alumina,’ and a ‘relatively
l> thin covering layer of “a 'vitreous'material slightly
penetrating t-he'baselayerto form’ a vitreous bond
sium-sulfate, torthe slurry. The addition Orlan
appropriate amount of an electrolyte‘ will permitv
the depOsitionofa coating of the idesired'thickej
néss without'running or drainage of the coating;
The base‘ coating should be applied to a thicknessv
offrom~s% to 1% of an inch;
In'qnany ‘cases it' is desirable to mill the ‘mag
nesiteLer v‘periclase till the ?nal slurry is through
l00~meshf In other cases, however, 'it is desire
able to mill ‘only ?nelyenough to permit spraying,‘
whenlthe coating is to ‘be applied by» spraying.
Relatively coarse milling reducesthe shrinkage
of- the coating and thus preventslcracking that
might otherwise occur'on large surface areas.
therewith and forming an" imperviousl?lm there
ov-erggsaid vitreous material being selected ‘from
the group “consisting [of feldspar,v ceramic glaze
compositions’ and glass compositions»charactere~
ized by melting to "an impervious ?lm' at tem?
peratures of from 1000170 1300“ C.
4. A steel retort in accordance with claim 13,
in which, the base layer is principally of magnesit'e‘ '
from 3% to {zinc-h thick and the covering player'is'"
an impervious ‘?lm of fused feldspar.
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