Pre-forge aluminum oxide blasting of forging billets as a scale resistance treatment

A ferrous forging billet is treated before heating for forging by grit blasting it with aluminum oxide grit having a pre-blasting particle size in the range of from about 100 microns to about 180 microns, at a pneumatic pressure in the range of from about 60 psi to about 160 psi, and for a time period in the range of from about 30 seconds to about 60 seconds, so as to remove pre-existing scale from the surface of the billet stock, to roughen the cleaned billet surface and to embed particles of aluminum oxide in the cleaned and roughened billet surface, thereby to form a tightly adhering deposit of aluminum oxide particles which are dispersed over the billet surface and cover at least 10% thereof. This substantially inhibits the formation of oxides on the surface of the billet during heating for forging.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to methods of surface treatment of ferrous 
materials, commonly referred to as forging billet stock, and particularly, 
relates to methods for surface treatment of such stock to inhibit the 
formation thereon of oxides during heating for forging. 
2. Description of the Prior Art 
Many products, including hand tools, such as wrenches and the like, are 
formed by forging. In this process, a hot-rolled bar formed of a suitable 
ferrous alloy, such as a suitable steel, which bar constitute forging 
billet stock, is heated to a forging temperature, typically in the range 
of from about 1500.degree. F. to about 2300.degree. F., and forged in 
forging dies. Then, the forged billet passes through a trim station to 
remove excess material, thereby forming the forged part or "forging." The 
forged part then typically undergoes a number of finishing processes to 
produce the desired quality of surface finish on the part. It is important 
that the surface finish quality and consistency of the as-forged part be 
as good as possible to minimize the extent of post-forging finishing 
operations. To this end, trim punches are carefully designed to eliminate 
marks created during trimming and the forging dies may be polished to 
eliminate the transfer of machining marks on the dies to the forged part. 
Typically, hot rolled billet stock, as received from the mill, has surface 
oxidation scale which can adversely affect the surface condition of the 
forged part. This scale can be removed by a number of known methods, 
including centerless grinding with abrasive wheels or belts, steel grit 
and steel shot blasting, glass beading, slag blasting, drawing or bending 
and machining. However, even though mill scale may be cleaned from the 
billet stock by these methods, the heating of the cleaned billet in air to 
a forging temperature results in the rapid growth of additional thick 
oxide surface layers, typically characterized by high variability of 
thickness and non-uniform adherence. These oxidation layers require 
additional processing to remove them from the forged part. Thus, the 
oxidation layers may be forged into the surface of the part, to varying 
depths, resulting in an uneven and inconsistent surface finish. In order 
to bring the surface of the forged part to a required quality and 
consistency, hand and machine polishing are necessary and may result in 
the removal of as much as 0.01 inch of stock per surface. This type of 
heavy stock removal is tantamount to regrinding and regenerating the 
entire shape of the part, altering the complex three-dimensional, 
as-forged geometry, thereby negating the painstakingly achieved precision 
geometry of the forging dies. 
Furthermore, because of the variable adherence of the oxidation layers to 
the billet surface, some of the oxide layer will flake off. Some of these 
flakes may be impacted randomly into the forged part by the forging 
pressures, as described above, further aggravating the surface 
irregularity problem. Other scale residue may be left in the forging dies. 
Such residue can be driven into the next forging billet, creating surface 
irregularities therein. In order to avoid this accumulation of oxide scale 
in the forging dies, expensive automatic blow-off and vacuum collection 
systems have been necessitated. 
Efforts have previously been made to inhibit the formation of oxide scale 
during the heating of forging billet stock to forging temperature by 
treatment of the billet stock prior to heating it to the forging 
temperature. Thus, commercially available scale preventive coatings, such 
as glass/alcohol coatings and graphite/water coatings, have been utilized 
on billet stock. However, such commercially available coatings have, 
generally, been found to be ineffective in preventing scale formation 
during heating for forging. Also, they are relatively difficult to apply 
to billet stock, require drying or curing after application, and can be 
relatively easily rubbed off by handling. 
SUMMARY OF THE INVENTION 
It is a general object of the invention to provide an improved method of 
treatment of ferrous forging billet stock to inhibit the formation of 
oxidation scale thereon during heating for forging, which method avoids 
the disadvantages of prior techniques while affording additional operating 
advantages. 
An important feature of the invention is the provision of a method of the 
type set forth which has a relatively high benefit to cost ratio and is 
easy to perform. 
In connection with the foregoing feature, another feature of the invention 
is the provision of a method of the type set forth, which does not involve 
the use of liquid coatings on the billet stock. 
Another feature of the invention is the provision of a method of the type 
set forth, which substantially inhibits the formation of oxide scale on 
the surface of the billet stock. 
In connection with the foregoing feature, a further feature of the 
invention is the provision of the method of the type set forth, which 
minimizes the thickness of oxide layers formed on the surface of the 
billet stock. 
A still further feature, in connection with the foregoing features, is the 
provision of a method of the type set forth, which results in a consistent 
and uniform oxide thickness on the surface of the billet stock. 
Yet another feature of the invention is the provision of a method of the 
type set forth, which results in increased adherence of the oxide layer 
formed on the surface of the billet stock. 
A further feature of the invention is the provision of a method of the type 
set forth, which results in a forged part with improved surface quality 
and uniformity. 
Certain ones of these and other features of the invention are attained by 
providing a method of treating ferrous material to be forged, which may 
have oxidation scale on the surface thereof to inhibit the formation of 
oxides during heating for forging, the method comprising: removing 
oxidation scale from the surface of the material to form a cleaned 
surface, and forming on the cleaned surface a tightly adhering deposit of 
aluminum oxide. 
Further features of the invention are attained by providing a method of 
treating ferrous material to be forged, which may have oxidation scale on 
the surface thereof to inhibit the formation of oxides during heating for 
forging, the method comprising: removing oxidation scale from the surface 
of the material to form a cleaned surface, roughening the cleaned surface, 
and embedding particles of aluminum oxide in the cleaned and roughened 
surface of the material. 
A still further feature of the invention is attained by providing a method 
of treating ferrous material to be forged, which may have oxidation scale 
on the surface thereof to inhibit the formation of oxides during heating 
for forging, the method comprising: grit blasting the material by 
impacting on the surface thereof a pneumatically propelled stream of 
aluminum oxide grit, the stream of aluminum oxide grit being propelled at 
a pressure and for a time period sufficient to remove oxidation scale from 
the surface of the material and to form thereon a tightly adhering deposit 
of aluminum oxide particles. 
The invention consists of certain novel features and a combination of parts 
hereinafter fully described, illustrated in the accompanying drawings, and 
particularly pointed out in the appended claims, it being understood that 
various changes in the details may be made without departing from the 
spirit, or sacrificing any of the advantages of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The method of the present invention fundamentally comprises grit blasting 
the surface of the ferrous forging billet stock with an aluminum oxide 
grit, a grit medium not heretofore used for mill scale removal. More 
specifically, referring to FIG. 1, the ferrous forging billet stock is 
supported in a blast chamber 10 by suitable means and the surface thereof 
is then impacted with a pneumatically-propelled aluminum oxide grit medium 
from a grit reservoir 11. Spent particles of the aluminum oxide grit 
medium are then collected from the blast chamber 10 and recycled, as at 
12, to the grit reservoir 11. 
A significant aspect of the present invention is the use of an aluminum 
oxide grit blasting medium for the removal of mill scale from ferrous 
forging billet stock, and, more particularly, use of such grit blasting as 
a treatment of the surface of the billet stock to inhibit the formation of 
further oxidation scale thereon as a result of subsequent heating of the 
forging billet to forging temperature. Thus, it has been found that grit 
blasting of the surface of ferrous forging billet stock with an aluminum 
oxide grit medium, under the conditions of the present invention, results 
in a significant inhibition of the formation of oxide scale on the surface 
of the billet stock during subsequent heating for forging. The method of 
the present invention involves the following physical changes in the 
billet stock: 
(a) removal of any existing mill scale from the surface of the billet; 
(b) abrasion or roughening of the surface of the billet with consequent 
increase in the surface area thereof; and 
(c) embedment of a dispersion of coarse and fine aluminum oxide particles 
in the roughened surface of the billet to form an aluminum oxide deposit 
thereon. 
More specifically, the removal of existing mill scale cleans the surface of 
the billet stock and permits the abrasion and roughening thereof as a 
result of cold working by the impacts of the aluminum oxide particles. 
This roughening, in turn, enhances the embedment of aluminum oxide 
particles in the cleaned ferrous surface of billet stock, resulting in a 
firmly adhering deposit of dispersed aluminum oxide particles on the 
surface of the billet. This deposit has been found to result in 
significantly reduced formation of iron oxide scale on the surface during 
heating to forging temperature. Thus, while some scale still does form 
during heating, the scale layer is substantially thinner, of substantially 
more uniform thickness, and has a substantially greater adherence, than is 
the case with scale formed on billet stock which has not been subjected to 
the treatment of the present invention. 
It has been found that the method of the present invention achieves 
satisfactory results over a relatively wide range of process parameters. 
The aluminum oxide grit medium may be any of a number of commercially 
available media, such as a brown, 96.15% aluminum oxide medium of the type 
sold by Sinclair Mineral & Chemical Co. under the trade name "EXOLON 
FASAST," or a white, 99.75% aluminum oxide grit medium of the type sold 
by Sinclair Mineral & Chemical Co. under the trade name "EXOLON WP." Good 
results are obtained with grit sizes in the range of from about 80 grit to 
about 54 grit (corresponding to the particle thicknesses in the range of 
from about 100 microns to about 180 microns). Within this range it does 
not appear that grit size has a significant effect on the efficacy of the 
treatment. Other grit sizes may be used, but their effectiveness may be 
diminished. Significant improvement in oxide formation has been achieved 
with grit blast pressures ranging from about 60 psi to about 160 psi, but 
the preferred blast pressure is substantially 120 psi. Significant 
improvement in oxide formation has been achieved with grit blasting for 
time periods ranging from 30 seconds to 60 seconds, but the preferred time 
period is substantially 60 seconds. In general, it has been found that an 
effective blast time is inversely proportional to the blast pressure. 
It has also been found that the temperature to which the billet is heated 
and the rate at which it is heated significantly affect the amount of 
oxide scale formation, irrespective of the particular process parameters 
used. Thus, in general, the amount of scale formation is directly 
proportional to the temperature to which the billet is heated and, at 
higher temperatures, the amount of oxide formed is proportional to the 
heating time. Accordingly, in general, it is desirable to forge at the 
lowest practical temperature, to heat the billet to that temperature as 
rapidly as possible and to minimize the length of time the billet is at 
that temperature in order to maximize the scale inhibition effect of the 
method of the present invention. However, regardless of the heating 
conditions, use of the treatment of the present invention with parameters 
anywhere within the above-listed ranges, invariably results in improved 
scale resistance as compared to untreated stock. 
Sedimentation analysis of the grit medium reveals that a large number of 
fine particles are generated during the grit blasting operation. This 
indicates rounding or splintering of particles on impact. Since the spent 
grit medium is recycled, this results in a bimodal distribution of fine 
and coarse grit as the grit blasting operation proceeds and, therefore, 
grit particles of a variety of sizes are deposited on the surface of the 
billet to form the aluminum oxide deposit thereon. The particles making up 
the deposit are dispersed substantially randomly over the treated surface 
of the billet stock. 
FIGS. 2A and 2B are backscatter images of 200.times. scanning electron 
micrographs (SEM) of the surfaces of two billets respectively treated with 
different versions of the method of the present invention, with the dark 
areas indicating the aluminum oxide particles making up the deposit on the 
surface of the billet stock. Measurement reveals that the aluminum oxide 
deposit makes up 18.2% of the surface area of the sample of FIG. 2A and 
20% of the surface area of the sample of FIG. 2B. It is believed that, for 
effective results, the aluminum oxide deposit should cover at least 10% of 
the treated surface area of the billet stock. The sample of FIG. 2A was 
blasted with an aluminum oxide grit having an initial 54 grit size, while 
the sample of FIG. 2B was blasted with aluminum oxide particles having an 
initial 80 grit size. Both were blasted at a blast pressure of 120 psi for 
60 seconds. 
FIG. 3 is a backscatter image of a 1000.times. SEM of a transverse 
cross-section of the surface of billet stock after having been treated 
with the method of the present invention, with the large, generally 
V-shaped feature just left of center illustrating an aluminum oxide 
particle embedded in the steel substrate. The large particle has inhibited 
diffusion of the oxide scale as indicated by oxide growth (light-colored 
regions) being only partially around and through cracks in the particle. 
The white line 15 is a scale reference line and measures 10 microns. It 
can be seen that the aluminum oxide particle is embedded over 20 microns 
(approximately 0.0008 inch) into the surface of the steel substrate. It 
will be appreciated that the depth of embedment of aluminum oxide 
particles will vary with the size of the particles and with the blast 
pressure and can range from about 1 to about 50 microns. It is a 
significant aspect of the invention that this embedment results in a very 
tightly adhering deposit of aluminum oxide on the billet surface, which 
deposit cannot easily be removed and which will survive normal handling. 
A number of ferrous forging billets were mounted in a blast chamber and 
subjected to grit blasting in accordance with various versions of the 
present invention. Each forging billet, in the nature of an elongated 
cylindrical bar 11/16-inch in diameter, was mounted in the blast chamber 
for rotation about its longitudinal axis while being traversed 
longitudinally by a single grit blasting nozzle. However, it will be 
appreciated that other mounting and blasting arrangements could be used, 
as long as substantially uniform exposure of the entire surface of the 
billet to the grit blasting medium is achieved. The following examples 
illustrate the process of the present invention and are directed to 
describing its preferred aspects relating to the treatment of a ferrous 
forging billet. However, these examples are not intended to unduly limit 
the broad scope of the present invention. 
EXAMPLE I 
Six ferrous forging billets meeting the AISI 50B44 alloy steel chemical 
requirements were subjected to the treatment of the present invention, as 
illustrated in FIG. 1, by grit blasting with aluminum oxide grit having a 
pre-blasting 54 grit size meeting ANSI B74.12 size requirements. The blast 
medium was pneumatically impacted on the surface of each billet with a 
blast pressure of 120 psi for 60 seconds. FIG. 2A is a photomicrograph of 
the surface of the billet after this treatment in accordance with the 
present invention. FIG. 3 is a photomicrograph of a transverse 
cross-section of the surface of this billet after this treatment in 
accordance with the present invention and after subsequent induction 
heating to 1850.degree. F. 
EXAMPLE II 
This example is the same as Example I, except the billets were grit blasted 
for 30 seconds. 
EXAMPLE III 
This example is the same as Example I, except that the billets were grit 
blasted at a blast pressure of 80 psi. 
EXAMPLE IV 
This example is the same as Example III, except that the billets were grit 
blasted for 30 seconds. 
EXAMPLE V 
This example is the same as Example I, except that the grit medium was an 
aluminum oxide grit having a pre-blasting 80 grit size. The 
photomicrograph of FIG. 2B illustrates the surface of the billet after 
this treatment in accordance with the invention. 
EXAMPLE VI 
This example is the same as Example V, except that the billets were grit 
blasted for 30 seconds. 
EXAMPLE VII 
This example is the same as Example V, except that the billets were grit 
blasted at a blast pressure of 80 psi. 
EXAMPLE VIII 
This example is the same as Example VII, except that the billets were grit 
blasted for 30 seconds. 
After treatment of the billets in accordance with Examples I-VIII above, 
the six treated billets from each Example were heated to three different 
forging temperatures at two different heating rates in a 12-station 
induction heating unit. In particular, for each Example, one billet was 
heated to 1700.degree. F. at a fast rate of 7 seconds per station, one 
treated billet was heated to 1700.degree. F. at a slow heating rate of 9 
seconds per station, two treated billets were heated to a temperature of 
1850.degree. F., respectively at the fast and slow rates, and two treated 
billets were heated to 2000.degree. F., respectively at the fast and slow 
rates. The billets thus treated with the method of the present invention 
and heated were compared with six untreated hot rolled billets, as 
received from the mill, and six centerless ground billets, which were 
subjected to the same pre-forge heating regimens. 
It was found that all billets that were blasted with aluminum oxide have 
thinner oxide layers than hot rolled or centerless ground units, 
irrespective of grit-blasting parameters or the induction heating 
parameters. The lowest average oxidation level (0.0002-0.0010 inch) was 
achieved with billets blasted with 50 grit or 80 grit aluminum oxide and 
heated to either 1700.degree. F. or 1850.degree. F. The oxide layers that 
were formed on the billets untreated with the aluminum oxide blasting had 
an average oxide thickness of 0.0022-0.00258 inch. Also, the variability 
of the thickness of the oxide layers was found to be greater in the 
un-blasted billets than with those subjected to the aluminum oxide 
blasting of the present invention. In general, as the temperature is 
increased, the oxide thickness increases. Billets blasted with aluminum 
oxide in accordance with the present invention do not show a significant 
change in oxidation between 1700.degree. F. and 1850.degree. F., but 
increasing the temperature to 2000.degree. F. dramatically increases 
oxidation. The induction heating rate did not significantly affect oxide 
levels of aluminum oxide blasted billets between 1700.degree. F. and 
1850.degree. F. However, a significant reduction in oxide thickness was 
achieved by fast induction heating aluminum oxide-blasting billets as 
compared to slow induction heating to the 2000.degree. F. temperature. The 
oxide formed on aluminum oxide-blasted at 1700.degree. F. and 1850.degree. 
F. is not only reduced in thickness, but also has a finer structure as 
compared to the oxide formed on the non-blasted billet stock. 
The reason why, or the mechanism by which the aluminum oxide blasting 
treatment of the invention achieves the significantly reduced oxide 
formation described above, is not fully understood. It is believed that 
one operative mechanism may be that the cleaning and roughening of the 
surface of the billet stock changes the nucleation mechanism for the 
growth of oxides. The roughening as a result of cold working of the 
surface by the aluminum oxide particles significantly increases the 
numbers of nucleation sites. The oxides of iron grow epitaxially on the 
billet surface and the greater number of nucleation sites may help pin 
oxide grain boundary movement, which movement would otherwise break the 
epitaxial relationship and reduce scale adhesion. The finer, more dense 
oxide formed may also inhibit diffusion or, alternatively, the increase in 
diffusion which would normally be expected to follow from finer oxide 
structure may be temporarily restrained by the pinning effect of increased 
nucleation sites. In any event, the fine-grained thinner oxide produced 
should have greater plasticity, thereby improving scale adhesion. 
Also, the particles of aluminum oxide residing on and embedded in the 
surface of the billet as a result of the grit-blasting treatment, may 
provide localized diffusion barriers for the oxidation and may also help 
pin oxide grains, restraining oxide growth. 
Additionally, the aluminum oxide particles deposited on the billet surface 
may also enhance oxide adherence by a vacancy sink mechanism. Atomic 
vacancies due to metal outward diffusion cause void formation at the 
metal/scale interface and resultant flaking or spalling of the oxide after 
extended time at temperature. The blasting with aluminum oxide imparts 
dislocations and particles into the surface, which increase the number of 
vacancy "sinks," or areas to absorb vacancies and resultant voids. 
From the foregoing, it can be seen that there has been provided an improved 
method for the treatment of ferrous forging billet stock which 
significantly inhibits the formation of oxide scale on the surface of the 
billet stock during heating for forging, the method consisting essentially 
of grit blasting the surface of the billet with aluminum oxide grit under 
conditions such that any pre-existing oxidation scale is removed and a 
tightly-adhering deposit of aluminum oxide particles is formed on the 
cleaned billet surface.