Note: Descriptions are shown in the official language in which they were submitted.
11198:~Z `
Background of the Invention
This invention is directed to a method for bene-
ficiating scarfer spittings to produce a product which can be
used as size-graded metallic abrasives in machine or manual
blast cleaning the surfaces of metals and non-metals.
Scarfer spittings are produced during scarfing of
the surface of semi-finished steel products such as blooms,
slabs, billets and bars to remove defects. During scarfing,
the surface of the steel is heated to a molten temperature
by gas torches in order to eliminate surface defects. The
molten metal thus produced is customarily removed by high
pressure water jets which impinge upon the surface of the
workpiece immediately following passage of the gas torches.
The molten metal removed from the surface of the product
solidifies in the form of generally spherical-like particles
having a wide range of sizes, for example larger than 2 inches
(50.8 mm) to less than #100 sieve size. The solidified
particles, or scarfer spittings, are comprised of metallic
cores having substantially the same chemical composition as
the steel which had been scarfed, enclosed in brittle shells
which are substantially iron oxides. The scarfer spittings
are usually collected in a water bath. Scarfer spittings
have been used in the past as a portion of the charge to
sintering strands to reclaim the iron which they contain.
X
152;~
However, only the larger particles can be used in this manner.
The!refore a large portion of the finer particles must be
either stored or discarded. In recent years, increased
emphasis on the surface cleanliness of steel has resulted in
an increase in the use of automatic scarfing machines to scarf
the steel surfaces. Because of the use of automatic scarfing
machines, the volume of scarfer spittings produced in a steel
plant has increased. At the present time, the scarfer
spittings are a waste product for which no good use has been
found.
In accordance with the invention we provide a size-
graded steel abrasive material for blast cleaning metallic and
non-metallic surfaces, characterized by having a hardness
within the range of Rc 20 to 35, a microstructure comprised of
untempered lath-like martensite substantially free from inter-
granular and intragranular cracking, a grain size of about 3
to 4 and having animpactroughness equivalent to that of
higher carbon and alloy grades of metallic abrasives.
The size-graded steel abrasive material is produced
from scarfer spittings, a steel plant waste product. The
size-graded metallic abrasives of the invention are useful in
machine or manual blast cleaning of metallic and nonmetallic
surfaces.
We also provide in accordance with the invention a
method of producing size-graded steel abrasives from scarfer
spittings comprised of steel cores enclosed in shells of iron
oxides, the scarfer fitting being within a size range of plus
2 inches (50.8 mm) to minus #100 sieve size, characterized by
~ 8'~2
screening said scarfer spittings in a first screening step to
separate all plus l/4 inch (6.35 mm) spittings from all minus
1/4 inch (6.35 mm) spittings and to remove any foreign matter
contained therein, charging said minus 1/4 inch (6.35 mm) spit-
tings into a grinding mill, removing said shells of iron oxidesfrom around said cores of said minus lt4 inch (6.35 mm) spittings
in said mill to produce a mixture of steel cores and fragmented
shells of iron oxides, screening the mixture thus praduced in a
second screening step to separate said steel cores from the iron
lO ' oxide shells drying said steel cores, and screening said steeL
cores in a third screening step to produce a size-graded
metallic abrasive product.
In a specific a~pect o our method, the mixture of
metallic cores and the fine particles of 'the shells is screened
to separate the cores from the fine particles of shells and to make
a size separation on a ~35 mesh sieve size. The plus ~35 sieve
size fraction, comprised of metallic cores, is stored. The minus
~35 sieve size fraction, comprised o metallic cores and particles
- ~f shells, is screened on-a ~100 sieve to separate substantially
all the pius ~lOO sieve size cores from the minus ~100 sieve si~e
cores and particles of shells. The plus ~100 sieve size cores
are mixed with the plus ~35 sieve size cores. The minus ~lOO
sieve size cores and particles of shells are recycled to the
steel plant. The mixture of plus #35 sieve size and plus ~100
sieve size cores are dried in a rotary drier and are then
screened on a series of sieves into a plurality of sizes useful
as size-graded metallic abrasives.
- Objects and advantages of the invention will become
apparent from the following disclosure taken in conjunction with
the accompanying drawings, in which:
FIGU~E 1 is a reproduction of a photomicrograph taken at lOO m~gnifi-
cations of metallic cores prepared by the method of the invention.
~ ~ .
~ -4-
~ ~ .
1119822
FIGURE 2 is a reproduction of a photomicrograph
talcen at 100 magnifications of prior art metallic abrasives.
Preferred mbodiment of the Invention
It has been discovered that scarfer spittings,
which are a steel plant waste product having a wide range of
sizes and comprised of a dual struct~re of metallic cores
enclosed in shells of iron oxides can be beneficiated to
remove the shells and the freed cores can be used as size-
graded metallic abrasives to blast clean metallic and non-
metallic surfaces. The metallic cores are separated into
; various sizes on a series of sieves to produce size-graded
abrasives which meet the requirements of SAE Shot and Grit
Specifications J444.
In the preferred embodiment of the invention the
scarfer spittings, which range in size from plus 2 inches
(50.8 mm) to minus #100 sieve size, are screened in a first
screening step to separate substantially all the plus 1/4 inch
(6.35 mm) spittings and all foreign matter which had been col-
lected with the scarfer spittings, from substantially all the
minus 1/4 inch (6.35 mm) spittings. The plus 1~4 inch (6.35
mm) fraction of spittings are separated from the foreign
matter and are recycled in the steel plant to recover the iron
which they contain. The foreign matter is discarded. The
minus 1/4 inch (6.35 mm) fraction of spittings are charged
into a rotating continuous wet grinding mill which contains
a grinding media, such as steel, iron or ceramic balls or
pebbles ranging in size from 1/4 inch (6.35 mm) to 1-1/2
inches (38.1 mm) in diameter. The size and weight of the
X -5-
Z
~;
ll
grindlng medla selected for use ln the grlnding mlll is
o~ ~ufficlent slze and welght and of a type to fracture
the brlttle shell~ of lron oxldes into relatively fine
l¦partic].es, whlch break away from the metalllc cores, without
~¦materlally affecting the ~hapes or the ~urface~ of the
cores. The mlnu~ 1/4 inch (6.35 mm) ~pittin~a and the
grlnding medla are tumbled for a time, usually not le~a than
about eight minutes, to cause the brlttle shells of iron
oxlde to ~racture into flne particle~. About 20 welght
¦percent of the cores freed ~rom the ~hells have a generally
¦spherlcal shape and a relatively smooth surface. The remalnder
lor about 80 ~el~ht percen~ of the core~ ha~e irregular non-
~ angular ~urface~. The mixture of core~ and fine particles
¦ of lron oxides ~ormed in the mlll together with a portlon of-
¦ grlnding medla which breaks down durlng ~ervice, 15 rèmovedfrom the mlll through a dlscharge screen havlng appropriate
l ope~in~s, for example a #16 sieve size. The mixture is
I ¦screened in a second wet screenlng step to separate sub-
stantlally all the plus ~lO0 sieve slze cores ~rom the minu~
¦ #100 sleve size cores and partlcles o~ shell B and grlndlng
¦ medla. For ease of operation and to prevent overloadlng of
¦ the screens, the mlxture 18 pre~erably flrst screened on a
¦ #35 sleve slze to separate the plus ff35 ~leve ~ize cores
¦ ~hich compri~e about 40 welght percent o~ the feed to the
25 ¦ mill from the minus #35 sleve ~ize cores, particles of
~ shell~ and the grindlng medla. An lnsi~nlficant quantlty of
¦ grindlng media may remaln in the #35 sieve slze. The #35
~leve ~lze ~raction is ~tored in a storage bln or hopper.
¦ ~he minus #35 sieve size fraction of cores and partlcles of
lll9b2z
shells and any broken grlndlng media remalning in the
rraction are then ~creened on a #100 ~leve to make the
de~lred ~eparatlon at ~100 ~leve ~ize. All the plu~ #100
siève ~lze fraction Or core~ which comprlse about 35 welght
5 1l percent of the feed to the mill are mlxed wlth the plu8 ~35
sieve ~ize core~ in the stora~e bin or hopper. The minus
¦l#100 sleve 3ize fraction of particle~ Or shells and core~,
which compri~e about 25 we~ght percent of the feed are
¦re¢ycled to the ~intering plant to reclaim the iron which
1~ Ithey contain. Prior to separatlng the metallic core~ lnto
Ivarious ~12e~ of slze-graded metallic abraslve~ they are
~drled ln a ~uitable dryer, for example a fluld bed dryer or
a rotary dryer at a temperature of not le~ t`han about 300F
(149~). The drled metalllc cores are then separated or slze
graded on a seriea of ~creens into varlouR ~lze~ to produce
~i~e-graded metallic abraaive~ accordin~ to SAE Shot and
Grit Specificatlon J444. In the~e ~peclflcatlons and
Iclaim~ whenever ~creen or ~leve sizes are u~ed such ~creen
¦and ~ieve ~l~e~ are Unlted State3 Sleve Serie~. In the
-- 20 jmethod of the lnvention ln ~Ihich a contlnuou~ mill i3 u8ed
to bene~iclate the ~plttln~s described above, the minu~ 1/4
lnch (6.35 mm) ~pittlngs are continuously fed lnto the mlll
at a rate ~ufflcient to obtain maxlmum throughput. Grindlng
media are a~ded from time to tlme to maintain the propèr
ratlo be~ween the ~pittlng~ and ~rinding medla to malntaln
maximum efPlclency o~ grlndln~ and throughput.
¦ While a method for beneflclating ~carfer ~plttlngs
to produoe slze-graded metalllo abrQslves ln whloh Q wet
-7-
ll
. I .
1.'
1119~
continuoua grlndin~ mill u~ing ~teel or ceramic and the
llke balls or pebble~ has been described, lt ls withln the
scope o~ thl~ inventlon to u~e a batch-type grlnd1ng mlll in
llwhich the ~rlndlng media may be steel or ceramic balls or
5 1I pebbles or the like or to u~e an autogeneous mlll in which
the ~car~er ~pittln~s are used as both the material to be
llbene~lclated and the grindin~ medla requlred to bene~lciate
¦~the spitting~. In the batch method, a quantity o~ a mixture
containing desired amount~ of splttings and grinding medla
are charged into the mill. The mill is operated for a
period of time, not lesa than eight mlnutes. ~he mill is
stopped and the mixture or batch in the mlll i~ removed and
proce~sed a3 described above and another batch ls charged
lnto the mill.
In an autogenou~ mill, the ~plttlngs ~all upon
each other with sufficient force to fracture the brittle
shells of lron oxides. Dur1ng tumbling, the spittlngs rub
¦ agalnst each other and a~ a reault ~ome o~ the shells o~
I lron oxlde~ are removed by abrasion. The metallic core~ and
;20 the partlcles of shells are discharged ~rom the mill through~
a screen havlng suitable openlng~. The discharged material
is treated in the manner described above ~or the operatlon
¦ of the contlnuous wet mlll to separa~e the metallic core~ -
¦ ~rom the ~artlcles o~ shells and to produce slze-graded shot
¦ and ~rlt abra~ives.
¦ ~he spittin~s may be bene~iciated in an lmpact
mlll ln which the ~plttings are hurled agalnat a target wlth
¦ sufficient force to ~racture the shell~ in a one-polnt
111~8'~2
.
impact The ~pittings may be recycled to the mill a
suf~lclent number Gf tlmes to remove the shells or a series
~of mills may be used.
I Any Or the above mills may be used dry, however,
5 1l in a dry operation, copious amounts o~ du~t are generated.
~It ls therefore preferred to wet grlnd and wet screen to
¦~prevent e~ce~slve generation of dust.
It has been found that ln any of the above processes,
except the lmpact mill but in particular uhen an autogeneous
o ! mill ls used, small areas o~ iron oxides may adhere to the
~ur~aces of the smaller slze~ core3, that is, core~ whlch
are suf~l¢lently small to pass a #40 sieYe size. The smaller
cores are therefore preferably subJected to a flnal bene~lciating
~tep in whlch they are impacted by a one-point lmpact in an
lmpact mlll to remove any iron oxides which may adhere to
the 3urfaces of the cores. `
The metallic core3 have sub~tantlally the same
composlti~n a~ ~he ~teel which ha~ been scarfed. Such ~;
composition~ can be any AISI carbon or alloy ~rade of steel
but are generally Or the low carbon grades havlng a typical
chemlcal ¢ompositlon of 0.03 to 0. o8 wei~ht percent carbon,
.lO to .30 welght percent mangane~e, under .02 sul~ur, under
.01 phosphoru~, under .02 silicon and the remainder sub-
~tantlally lron and incidental lmpurltles usually assoclated
with such grade~ of steel. Slnce the parti¢les of scar~er
spittings are customarily quenched and cooled ln water whlle
theg are at a relatively hlgh temperature, the microstructures
and hardnQss o~ the particles are usually that o~ comparable
~rade~ o~ water quenched steeI.
1119~22
Turning to the FIGURES 1 and 2, a typical
microstructure of the metallic cores produced by the method
of the invention is shown in FIGURE 1 and a typical micro-
structure of a commercially available steel abrasive is
shown in FIGURE 2. The cores and abrasives have a size of
minus #20, plus #40 sieve size. The microstructures of the
metallic cores shown in FIGURE 1 comprise lath-like untempered
martensite substantially devoid of any retained austenite,
devoid of any intergranular or intragranular cracking and
having a grain size of between 3 and 4 as determined by ASTM
E112-63 "Estimating the Average Grain Size of Metals", plate 1.
The microstructures of the commercially available metallic
abrasive, shown in FIGURE 2, comprise plate-like tempered
martensite with areas of alloy segregation and carbides and
intragranular microcracks extending across the plates of
untempered martensite and a grain size of about 7-8.
Typical chemical compositions of the cores and
commercial abrasives are shown below:
ElementCores Abrasives
; 20 (Weight Percent)
Carbon .o6 .97
Manganese .10 .96
Phosphorus .014 .017
Sulfur .012 .031
Silicon less than .93
.01
Iron 96.5 95.8
P.emainder incidental impurities and oxygen.
-10-
~''
22
The hardness of the metallic cores was between
Rc 20 and 35 with segregate areas of about Rc 45~5 while
the commercially available steel abrasive had a hardness of
Rc 45 to 50. The intragranular microcracks in the com-
mercially available steel abrasives can act as stress pointscausing transverse cracking across the grains of the abrasives
leading to early failure of the abrasives when used in the
high stress process of blast cleaning metallic and non-
metallic surfaces.
The metallic cores prepared by the method of the
invention can be used to machine or manually blast clean the
surface of metallic and non-metallic material. The time
required to machine blast clean the surface of a ferrous
metal with the metallic cores is somewhat shorter than the
time required when using conventional steel shot or grit of
comparable size. The presence of both substantially smooth
surfaced spherical metallic cores and irregularly shaped
metallic cores results in a machine blast cleaned surface
which has a surface profile intermediate between the surface
profile formed by using steel shot or the surface profile
formed by using steel grit.
The breakdown rate or impact resistance of the
metallic cores prepared by the method herein described and a
commercially available metallic abrasive grit having a RC45-50
were compared as described in "Metallic Shot and Grit Mechanical
Testing - SAE 445A" appearing in the SAE Handbooks 1976, of
the Society of Automotive Engineers, dated 1976. The test
may be conducted on an Ervin Test machine as outlined and
shown in Bulletin 644 of the Alloy Metal Abrasives Division
--11--
X
11198ZZ
of Ervin Industries, 121 S. Division Street, Ann Arbor,
Michigan. In the test, a measured amount of a screened
metallic abrasive of known size is prepared. One hundred
grams of the abrasive is charged into the test machine. The
test machine has a throwing arm which rotates at 6900 revolu-
tions per minute, and an anvil and recirculating device
which rotate around the throwing wheel on the same axis at
25 revolutions per minute. Each particle of abrasive is
subjected to one impact each time the anvil and recirculat-
ing device rotate. The number of rotations are counted toshow the impacts the abrasive can absorb.
The 100 grams (0.22 pounds) of abrasives are sub-
Jected to a number of impacts. The machine is stopped and
the abrasive particles are removed from the machine and
carefully screened to remove all the fine particles from the
sample. The remaining abrasives are weighed and a sufficient
amount of fresh abrasive needed to bring the sample up to
100 grams (0.22 pounds) is added. The 100 gram (0.22 pounds)
sample is then subject to another known plurality of impacts
and the procedure is repeated until about 100 weight percent
; of the test abrasives have been replaced. The results of
the comparison tests on a G-40 grit are shown below:
-12-
No. of Cumulative Weight
Impacts Weight Percent Loss Percent Loss
Commer- Metallic Commer- Metallic
cial Cores cial Cores
250 16.8 4.6 16.8 4.6
500 lo.o 6.4 26.8 ll.o
750 8.7 7.5 35.5 18.5
8.3 g . o 43.8 27.5
1250 8.1 lo . l 51.9 37.6
lo1500 9.2 lo .0 61.1 47.6
1750 8.8 11.2 69.9 58.8
2000 9.7 11.5 79.6 70.3
2250 9.5 11.6 89.1 81.9
2500 lo . o 11.6 99.1 93.5
After 2500 impacts almost 100 weight percent of
the original amount of the commercial metallic grit abrasive
had been replaced whereas only 93.5 weight percent of the
metallic cores had been replaced, indicating that the
metallic cores herein described were more resistant to
0 impact than the same size commercially available steel grit.
In an example of the invention, 661ooo pounds
(29,937.1 kilograms) of scarfer spittings were screened on a
1/4 inch ( 6.35 mm) screen to separate plus 1/4 inch ( 6.35 mm)
spittings and foreign matter from minus 1/4 inch (6.35 mm)
25 spittings. Less than 1 weight percent of the total feed
was larger than 1/4 inch ( 6.35 mm). The remaining 65,500 pounds
(29.710.3 kilograms) passed through the screen. These
relatively finer particles were charged at a rate of 1500
pounds per hour (680.4 kilograms per hour) along with 40
30 gallons per hour (151.4 liters per hour) water into a wet
mill containing 1000 pounds ( 453.6 kilograms) of steel balls
822
whlch ran~ed in size from 1/4 lnch (6.35 mm) to one inch (25.4 mm
ln dlameter. ~he re~ultant mixture of spittln~s and water
~wa~ continuou~ly dl~charged from the mlll onto a ~35 sieve.
About 40 weight percent of the spittlngs feed was retained
on the aleve. The remalnlng 60 wel~ht percent pa~sed through
the sieve. The chemlcal analysea of the plu9 #35 sleve
product are a~ follow:
! Product Reta~ned Product Passing
'' on #35 Sieve ` Through ~35 Sieve
lO ,I Element Welght Percent Wei~ht Percent _
I, C 0-0S ~4
¦i Mn 0.10 0.49
P 0.017 0.013
I S 0.010 0.016
15 ll Sl <0. 01 0. ~3
llll Fe 96~3 78.8
Il
Remainder inclden~al lmpurltie~ and oxygen
The metalllc core~ retained on the #35 sieve were dried
lland sl~ed into typical SAE cast steel Bhot alze rangea. The
; 20 ill particles pasaing throu~h the #35 sieve were further screened
on a #lO0 ~ieve to remove the fine oxide shells. The produot
retalned on the #lO0 ~leve was drled and pas~ed through a
I Ijdry lmpact mlll to remove addltlonal oxldes from the finer
;metalllc cores. The product from the dr~ impact mill was
25 l¦ screened on a fi70 and #100 sleve.
! The chemistry of the products retalned on the #70
~and #lO0 aievea i8 as follow~:
~I
.,
. 11
1119822
Product Retained Product Passing
on #70 & #100 Through #100
Sieve Sieve
Element Weight Percent Weight Percent
C 0.05 0.09
Mn 0.10 0.53
P 0.017 0.013
S 0.010 0.016
Si <0.01 0.04
~e 96.3 74.9
Remainder incidental impurities and oxygen.
Microscopic examination of a representative sample of the
metallic cores was made at 100 diameters. The microstructure
consisted of untempered lath-like martensite. No evidence
of intragranular cracks was seen. The hardness was between
28 and 32 Rc.
The metallic cores were size~graded on a series of
sieves. The weight percent of the cores retained on each
sieve is shown below:
Sieve # Weight Percent
6 0.9
~ 8 1.6
- 12 2.1
9 0
14.7
7 8.4
100 4.1
The remaining 59.2 weight percent
consisted of particles of shells
~ 82
Representative ~amples of the me~alllc core~ were
subJected to a durability te~t in an Ervin breakdown test.
Sample~ of the metalllc cores were recycled between 2700 and
~3O00 tlme~ in the te~t. The re~ults compare favorably with
'Icommercially avallable steel shot and gr~t slnce only the
best grade~ of these abraslves gave slmllar value~ ln the
Ervin breakdown te~t.
The cleaning actlon of metallic cores produced by
I'beneficiating scarfer ~plttin~s wa~ compared to the cleanlng
action of standard SAE 280 grade steel ~hot. To compare the
cleanlng action of the sbraslve~, the ~urface o~ a ~teel
plate 4 feet by 8 feet by 3/8 lnch (1.22 meters by 2.44
meter~ by 9.5 mlllimeters) covered with mill scale and
patches o~ ru~t was divided into two equal parts. One part
wa~ cleaned wlth the metallic cores o~ the invention and the
other part was cleaned with the steel shot. The abra~ives
were impinged onto the surfaces to be cleaned throu~h a
hand-held compressed air nozzle having an opening of 1~4
inch (6.35 mm) at a pre~ure of 100 pounds per ~quare inch
(7.O3 kilo~rams per aquare centlmeter). The nozzle was held
about 8 to 12 inche~ ( 20.32 to 30.48 centimeters) away ~rom
¦the ~urfaces of the steel plate in a position perpendlcular
Ito the surfaces. The results of the cleaning action are
~shown below in Table I:
I ' .
..
~1
-16-
lll
1119822
X h h
! ol ~ P~^ ~
~D Sr'~ 1
o ~ 'ô ~
,,~o ~ ~
^
h ~ ~ b
~ bl~ ~o~
o ~ ~
N
X ' ::' C
~ O ~ C
' ' ' O U~ S; o
: ~ . ~a "
~.
--17--
111~8;2Z
Whlle the mean~ for e~fecting the inventlon
have been speclrlcally descrlbed hereinabove with some
speclrlclty~ it will be understood that other equlvalent
;means may be resorted to ln order to accompll h the same
¦re~ults.
,,1
I
-18-
