Heat treatment by plasma electron heating and solid/gas jet cooling

A workpiece is heated by first forming an ionized gas plasma around the workpiece. A positive potential is applied to the workpiece to accelerate electrons from the plasma into the workpiece. The workpiece is uniformly surface heated by the energy directed into the workpiece by the electrons. The workpiece is cooled by providing a flow of a pressurized liquid material such as carbon dioxide having a triple point. The liquid material is expanded through a nozzle to form solid particles that contact the surface of the workpiece and remove heat from it by subliming.

BACKGROUND OF THE INVENTION 
This invention relates to the heat treating of materials, and, more 
particularly, to techniques for rapidly and selectively heating and 
cooling in production heat treatment operations. 
One of the most important characteristics of many commercial metallic 
alloys is their capacity for property modification by heat treatment. The 
basis of heat treatment is the existence of various strength modifying 
mechanisms such as precipitation hardening and phase transformations such 
as the formation and modification of martensites. Workpieces made of such 
alloys may be modified throughout their volumes or only at their surfaces, 
as needed for particular applications. From a knowledge of the various 
mechanisms, the properties of such alloys may be modified over wide ranges 
by the choice of a heat treatment. 
Heat treatment generally involves controlled heating and cooling of a 
workpiece. The result of the heat treatment of a workpiece of a particular 
composition depends upon a number of parameters such as the temperatures 
selected, heating and cooling rates, times at temperatures, the use of 
multiple heating and cooling steps and cycles, and other process 
parameters. 
Some of the commercially most important heat treatments are performed to 
modify only the surface regions of the workpiece. For example, many steels 
are specially treated to preferentially harden their surfaces to improve 
their wear resistance, while maintaining an interior of high toughness and 
fracture resistance. Where only the surface of the workpiece is to be 
treated, the depth of penetration of the heated and cooled region into the 
workpiece and the temperature profiles during the heat treatment are of 
particular concern. These parameters are often determined at least in part 
by the shape of the workpiece and the presence of irregularities such as 
sharp points or recesses at its surface. 
Because heat treatment is so important to the most desirable utilization of 
many materials, many different types of heat treatment apparatus and 
methods have been developed. One commonly used approach is to heat the 
workpiece in a furnace heated by gas or electrical elements. After the 
workpiece has been at the required temperature for the required time, it 
may be cooled at any of several cooling rates, ranging from very slow 
furnace cooling to gas cooling to a rapid water quench. This technique 
processes the entire object at once, but can be very inefficient due to 
the slow heatup and/or cooling of the workpiece. 
In another heat treating approach, a heating beam such as a laser or an 
electron beam is directed at the surface of the workpiece to heat it 
rapidly. The surface may instead be induction heated. These techniques are 
efficient in that they do not heat the workpiece, but they require that 
the beam or induction coil be moved over the surface of the workpiece. 
This stepping action can be slow and may result in variability in the heat 
treatment as a function of location, particularly where there are surface 
irregularities on the workpiece. Plasma heating has also been tried, but 
it has not produced rapid heating rates. 
The cooling of a workpiece during surface treatments poses similar 
difficulties. If the workpiece is heated uniformly, rapid cooling is 
normally achieved by immersing the workpiece in a cooling medium such as 
water or oil. On the other hand, if the heating source must be stepped 
over the surface of the workpiece, it is difficult to achieve a uniform, 
controllable cooling. 
There is a need for an improved approach to the heat treating of materials. 
This need is particularly acute where the heat treatment is a surface heat 
treatment or where relatively rapid cooling is required, either uniformly 
over the entire surface of the workpiece or selectively over specific 
areas of the workpiece. Such treatments are widely utilized in industrial 
operations. The present invention fulfills this need, and further provides 
related advantages. 
SUMMARY OF THE INVENTION 
The present invention provides methods and apparatus for heat treating 
metals and other workpieces. The approach permits rapid, controllable 
heating that is particularly suitable for surface heat treating. The 
workpiece is heated uniformly over its entire periphery or selectively 
over specific areas, even in the presence of significant surface 
irregularities. The heating procedure is highly efficient, since only the 
workpiece is heated. Multiple workpieces can be heat treated to different 
temperatures at the same time and in the same chamber. Cooling according 
to the invention is accomplished without immersing the workpiece into any 
quenching medium. The workpiece is cooled at a higher rate than possible 
by high-pressure gas cooling or oil immersion, and there is no residue 
left on the workpiece. The surface of the workpiece can be cooled 
uniformly over its entire surface, or selectively in specific areas. 
In accordance with the invention, an apparatus for heat treating a 
workpiece comprises a vacuum chamber, means for controllably evacuating 
the vacuum chamber, and means for controllably supplying a partial 
pressure of an ionizing gas to the interior of the vacuum chamber. There 
is means for providing a plasma within the vacuum chamber. Heating is 
accomplished by means for applying a pulsing positive voltage to the 
workpiece relative to the plasma, so that electrons are accelerated from 
the plasma into the workpiece. The means for applying a pulsing positive 
voltage operates independently of the means for providing a plasma. The 
workpiece is held on an electrically isolated support in a position to be 
plasma electron heated. There is no need to manipulate the workpiece. 
In a corresponding process, a method of heat treating a workpiece comprises 
the steps of forming a plasma of an ionizing gas around the workpiece, and 
accelerating electrons from the plasma to the workpiece with a series of 
pulses of positive voltage, relative to the plasma, applied to the 
workpiece to heat the workpiece. The step of accelerating is continued 
until a preselected state of heat treatment has been reached in the 
workpiece. 
The plasma created by the plasma source surrounds the workpiece, and 
provides a reservoir of electrons on all sides of the workpiece. The 
applied pulsed positive voltage of the workpiece relative to the plasma 
extracts electrons from the plasma to the workpiece. The voltage 
accelerates the electrons, and upon impacting the workpiece the energy of 
the electrons is transferred to the workpiece to heat it. Since the 
electrons come from the plasma that completely surrounds the workpiece, 
the workpiece is heated on all sides from its surface and therefore need 
not be manipulated during the treatment. This approach is particularly 
advantageous where only the surface of the workpiece need be heated, and 
the heating is to be uniform. In another approach, the surface of the 
workpiece can be masked to achieve uniform heating of selected (unmasked) 
areas. 
The heat treating approach also extends to cooling of the workpiece. 
Further in accordance with the invention, apparatus for heat treating a 
workpiece comprises a support adapted to receive the workpiece to be heat 
treated. A nozzle means expands a flow of a liquid from a higher pressure 
to a lower pressure and directs the expanded flow toward a location of the 
workpiece when it is held on the support. There is a pressurized source of 
a liquid material having a thermodynamic triple point of equilibrium 
between its liquid, solid, and gaseous forms, and a conduit controllably 
extending from the source of the liquid material to a higher pressure side 
of the nozzle means. The liquid material is preferably pressurized carbon 
dioxide, but argon, nitrogen, and other suitable gases can also be used. 
In a related method, a method of heat treating a workpiece comprises the 
steps of heating the workpiece and directing a flow of solid particles at 
the workpiece. The flow is produced by the steps of supplying a 
pressurized liquid material having a thermodynamic triple point of 
equilibrium between its liquid, solid, and gaseous forms, expanding a flow 
of the liquid material from a higher pressure to a lower pressure to 
produce small clusters of solid particles of the material, and directing 
the expanded solid flow toward the workpiece. 
When the pressurized liquid material is expanded through the nozzle, it 
cools and transforms to the solid state. Small particles of the 
transformed solid penetrate through boundary layers and other obstacles at 
the surface of the workpiece. These particles absorb heat from the 
workpiece, and transform at the reduced pressure to the gaseous state. The 
gases leave the vicinity of the workpiece, allowing more solid coolant to 
reach the surface. The cooling by this approach has a higher rate than 
(corresponding gas cooling or oil quenching approaches because the 
limitations of boundary layers and film boiling phenomena on the heat 
transfer coefficient are overcome. The particles of the solid coolant can 
be directed to a specific area of the surface to selectively cool that 
area, or they can be distributed everywhere around the periphery of the 
workpiece to achieve spatially uniform cooling, by using multiple nozzles 
and/or solid distributing techniques. Thus, for most applications 
manipulation of the workpiece is not required. 
The heating and cooling approaches of the invention may be used together or 
separately with other cooling and heating techniques, respectively. 
The approaches of the invention provide an important advance in the art of 
heat treatment. They permit articles to be heat treated in vacuum and 
cooled in a generally inert, low pressure atmosphere without contamination 
that must be later removed. Other features and advantages of the present 
invention will be apparent from the following more detailed description of 
the preferred embodiment, taken in conjunction with the accompanying 
drawings, which illustrate, by way of example, the principles of the 
invention.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 depicts a plasma electron heating apparatus 20. The apparatus 20 
includes a vacuum chamber 22 that is evacuated by a vacuum pump 24 through 
a conduit 26 having a controllable gate valve 28. An ionizing gas such as 
nitrogen is supplied to the interior of the vacuum chamber 22 from an 
ionizing gas source 30 through a conduit 32 having a controllable valve 
34. A pressure gauge 36 measures the pressure within the vacuum chamber 
22. In operation, the vacuum chamber 22 is evacuated by the vacuum pump 24 
with the gate valve 28 fully open. The gate valve 28 is then partially 
closed and the controllable valve 34 is partially opened to admit a 
partial pressure of the ionizing gas. The pressure measured by the gauge 
36 is observed, and the partial pressure of ionizing gas is maintained at 
a selected level by coordinating the relative flows through the valves 28 
and 34. 
A plasma 38 is created which fills the interior of the vacuum chamber 22, 
by any suitable approach. In FIG. 1, an exposed hot wire filament 40 is 
positioned in the interior of the vacuum chamber 22. The filament is 
electrically isolated from the walls of the vacuum chamber 22. The 
filament 40 is heated to thermionic temperatures using a power supply 41 
connected across the leads of the filament. The ionizing gas is ionized 
using a power supply 42, which has its negative pole connected to one lead 
of the filament 40 and its positive pole connected to the vacuum chamber 
walls 22. The filament 40 is therefore the cathode and the vacuum chamber 
22 is the anode for forming the plasma 38. The gas molecules are ionized 
by electrons emitted from the filament 40, producing a plasma comprised of 
equal numbers of free electrons and positively charged gas ions. The 
plasma that is produced diffuses to completely fill the interior of the 
vacuum chamber 22, thereby immersing any workpiece within the plasma. The 
plasma that is produced is at the potential of the vacuum chamber walls, 
which are at ground potential. 
As shown, two workpieces 44 rest on electrically insulating supports 46 in 
the vacuum chamber 22. The supports are covered with shielding to protect 
them from the plasma that fills the chamber. The free electrons and gas 
ions of the plasma 38 surround the workpieces 44, with a plasma sheath 45 
whose dimensions are dependent upon the properties of the plasma such as 
gas pressure, plasma density, and electron temperature. The apparatus 20 
is suitable for treating one or more workpieces at a time, and two 
workpieces are shown for illustration. The plasma may be produced from 
multiple filaments, and may be tailored or shaped using known techniques 
for manipulating plasmas. Specifically, magnets 47 placed within the 
interior of the vacuum chamber 22 can be used to shape the plasma for 
specific applications. The plasma density and plasma electron temperature 
can be adjusted by varying the gas pressure, filament temperature, and/or 
electron energy. The apparatus shown in FIG. 1 can be scaled to any 
required size in order to treat any size or number of parts. 
A bipolar pulsing power source 48 is connected at its positive pole to the 
workpieces 44, and at its negative pole to the walls of the vacuum chamber 
22. In this case, both workpieces are pulsed with the same voltage and 
waveform. The walls of the vacuum chamber 22 are grounded, as is the 
plasma 38 surrounding the workpieces. 
When the workpieces 44 are pulsed to a positive voltage relative to the 
plasma 38, the plasma becomes a cathode with respect to the workpieces, 
which are each an anode. The anode workpieces 44 are separate and distinct 
from the vacuum chamber anode 22 used to create the plasma 38. The applied 
positive voltage develops across the plasma sheath 45, so that free 
electrons from the plasma are accelerated to the workpieces, as indicated 
at numeral 50. The energized electrons impact the workpieces 44, and their 
energy is transferred to the workpieces 44. 
The electrons penetrate into the workpieces only a small distance, which is 
dependent upon the applied voltage and the material of the workpieces. In 
most cases the penetration is at most a few micrometers. Only this surface 
region of the workpiece is heated directly, although the underlying 
regions are heated by thermal diffusion from the surface regions. 
The plasma electron heating can be conducted in two different and distinct 
modes. The first mode consists of rapid, burst heating to achieve rapid 
and preferential heating of the surface of the workpiece without 
significantly heating the interior of the workpiece. The second mode 
consists of a slow, continuous heating to achieve both surface and 
interior heating of the workpiece. In either heating mode, manipulation of 
the workpiece is not required. 
An important advantage of plasma electron heating for either mode is that 
heating is not dependent upon the surface properties of the workpieces. A 
further important advantage is that the heating is uniform around the 
entire periphery of the workpieces, even including most types of surface 
irregularities. The uniform heating results from the plasma sheath 45 
which envelops the workpieces. By tailoring the plasma properties, the 
plasma sheath can be made to conform to many distinct surface 
irregularities of the workpiece. Because of this conformal feature of the 
plasma sheath, electrons are provided from all sides of the workpieces, 
not just from a single fixed location as in conventional electron beam 
heating. 
For the rapid, burst heating mode, the surface heating is nearly 
independent of the thermal mass of the workpiece. For either operating 
mode, the heating of the workpieces by the plasma electron bombardment is 
also independent of furnace components such as heaters and insulation, if 
such components are used in conjunction with the present invention. 
As an alternative to the uniform heating of the entire surface of the 
workpiece 44, the plasma electron heating process can be utilized in a 
manner that allows for preferential heating of specific areas of the 
workpiece surface. To accomplish this result, those surfaces of the 
workpiece 44 that are not to be heated are covered with a mask of an 
operable material such as a removable insulating coating or a metallic 
foil that is loosely placed on the workpiece surface to ensure poor 
thermal contact with the workpiece surface. For example, in FIG. 1 the 
ends of the generally cylindrical workpieces 44 are shown as being covered 
with a foil mask 49, so that electrons cannot penetrate those surfaces. 
Heating is thereby selectively accomplished only on the curved side 
surfaces of the cylinders, which are exposed to the plasma 38. The mask 
can be patterned, so that only a small portion of a large covered surface 
is exposed to the plasma and heated. 
The plasma that surrounds the workpiece includes positively charged gas 
ions, negatively charged electrons, and un-ionized gas. In the typical 
low-pressure (10.sup.-4 -10.sup.-5 Torr) plasma that is preferred, the 
ion-to-neutral-gas atom fraction is typically 1-10 percent. The plasma is 
therefore predominantly neutral, un-ionized gas atoms. These gas atoms 
impinge upon the workpiece from all directions uniformly, since they are 
not repelled by the voltage applied to the workpiece. 
By proper selection of the ionizing gas, the impingement of the un-ionized 
gas can be utilized to attain other advantageous results simultaneously 
with the plasma heating process. Argon is chemically inert, and its use as 
the ionizing gas provides only the capability to plasma electron heat the 
workpiece. Nitrogen, on the other hand, can form nitrides in steel. The 
use of nitrogen as the ionizing gas can result in not only heating but 
also formation of a hard nitride layer at the surface of the workpiece. 
This type of surface nitriding is distinct from conventional plasma 
nitriding in that there is no ion bombardment of the workpiece surface. 
The heating of the workpieces by the plasma electron heating apparatus can 
continue as long as necessary to achieve the desired heated state in the 
workpiece. For surface treatments, it is necessary only to heat the 
surface of the workpiece for the time required to reach a preselected 
temperature and/or achieve a preselected microstructural state. If 
through-thickness heat treatment is desired, the heating can continue 
until the internal temperature state is reached by thermal diffusion. 
Extended periods of electron heating are not damaging to the material, as 
the positive ions in the plasma are repelled from the workpieces. 
Typical operating conditions for the plasma electron heating apparatus 20 
are an ionizing gas pressure of from about 10.sup.-5 to about 10.sup.-4 
Torr. For gas pressures below about 10.sup.-4 Torr, the full applied 
voltage provided by the power supply 48 is sustained across the plasma 
sheath 45 surrounding the workpiece. This allows the plasma electrons to 
bombard the workpiece surface with the full applied voltage to efficiently 
heat the surface. However, for pressures above about 10.sup.-4 Torr, the 
applied voltage cannot be sustained across the plasma sheath surrounding 
each part. The plasma electrons cannot bombard the workpiece surface with 
the full applied voltage, and minimal heating of the surface results so 
that the process is less efficient. 
In the preferred approach, the power source 48 is pulsed at a rate of from 
about 10 Hertz to about 3000 Hertz, with each pulse lasting from about 1 
to about 50 microseconds. The voltage applied to the workpieces is from 
about 1 kilovolt to about 100 kilovolts. The pulses may be in the form of 
a continuous pulse train, or they can be provided in short-duration bursts 
of short-duration pulses. A typical short-duration burst would be from 
about 0.1 to about 3 seconds, and would contain about 100-5000 pulses, 
each pulse of width about 10-30 microseconds. These values are not 
limiting of the invention, but are intended to be exemplary of preferred 
operating conditions. 
The use of a pulsed accelerating voltage applied to the parts 44 is to be 
distinguished from the use of a constant or DC voltage applied to the 
parts. Pulsing permits high voltages of 50-100 kilovolts to be applied to 
an anode workpiece to achieve preferential surface heating or slow, 
through-thickness heating. A 50-100 kilovolt DC voltage cannot be 
sustained across the plasma sheath 45 of a workpiece operated at an anodic 
potential. Furthermore, arcing would result, which can damage the surface 
of the part. By way of contrast, a 50-100 kilovolt pulse can be sustained 
across the plasma sheath of a workpiece operated at an anodic potential, 
but only at the pressures stated herein. 
With the use of pulsing, care is taken to select the positions of the parts 
44 in relation to each other and the walls of the vacuum chamber. During 
each pulse, the plasma sheath expands, and during the off time between 
pulses the sheath contracts back to its original size. To maintain good 
conformal electron bombardment of the parts, the parts should be placed in 
the vacuum chamber such that adjacent sheaths do not overlap during a 
pulse. For the typical 1-50 microsecond pulse widths, the sheath does not 
expand to a dimension of more than about 6 centimeters. The use of 
longer-duration applied voltage pulses would result in larger sheath 
sizes, limiting the manner in which parts are closely packed in the 
apparatus during heat treatment. Longer duration voltage pulses provided 
by the power supply 48 also would adversely affect the conformality of the 
sheath and the plasma and would prevent the conformal treatment of parts 
having surface features finer than the plasma sheath dimension. 
FIG. 2 depicts a variation of the apparatus 20, a plasma electron heating 
apparatus 20'. The apparatus 20' primarily utilizes components like those 
of the apparatus 20. Corresponding components have been numbered in FIG. 2 
as in FIG. 1, and the prior description is incorporated here. (In FIG. 2, 
the end masks 49 have been omitted, so that the plasma electron heating 
occurs over the entire surface of the workpiece, to illustrate this 
possibility.) 
A difference between the apparatus 20' and the apparatus 20 is that 
workpieces 44 and 44' are heated differently in the two cases. A single 
power source 48 is used, but its positive side is connected to the 
workpieces 44 and 44' through ballast resistors 52 and 52', respectively, 
of different resistances. The effective positive voltages applied to the 
workpieces 44 and 44' are therefore different. As a result, the energies 
of the electrons accelerated into the workpieces 44 and 44' are different, 
so that the respective workpiece surfaces are heated differently. 
It is therefore possible to heat treat the workpieces 44 and 44' 
differently in a single operational run of the apparatus 20'. Because the 
walls of the vacuum chamber are not heated except incidentally, and there 
are no resistance-type heating elements, multiple types of simultaneous 
heat treatments in the same chamber are readily conducted. This principle 
can be extended to the use of multiple power sources 48, the use of 
switching arrangements to further control the heating, or the use of 
capacitors and inductors to replace the ballast resistors, for other heat 
treatments in a single chamber and a single operational run. This 
capability is an advantage of the invention, allowing more complete and 
economical utilization of the apparatus and less interruption to work 
schedules. Thus, for example, in some businesses where small numbers of 
parts must be specially heat treated, it is sometimes the practice to 
delay the heat treating of each type of part until a full furnace load can 
be accumulated. The present approach permits different types of heat 
treatments to be accomplished at once, avoiding the need for such delays. 
It has been found advantageous to use a low voltage (e.g., 20 kilovolt) and 
either a continuous or rapid burst mode heating of the workpiece prior to 
conducting the actual heating of the workpiece surface in order to clean 
or desorb any residue or films that are present on the workpiece prior to 
its treatment. This pre-cleaning process has been found to be beneficial 
in preventing arcing during the subsequent plasma electron heat treatment 
of the workpiece. Typically, the pre-cleaning procedure takes about 5 
minutes, depending upon the degree of cleaning desired. 
The plasma electron heating modes and techniques that have been described 
can be used with or without the addition of a separate forced cooling 
process to cool the heated workpiece. No manipulation of the work piece is 
required in either case. In the case where no separate, forced cooling is 
employed, the workpiece is cooled by two distinct modes. In the first, 
rapid self-quenching of the workpiece is achieved after rapid, burst-mode 
heating of the workpiece surface. In this case, the surface is cooled by 
conduction to the interior of the workpiece. In the second mode, slow 
radiative cooling of the surface is achieved after slow, continuous 
heating of the workpiece that heats both the surface and the interior of 
the workpiece. 
Where a separate forced cooling process is used in conjunction with the 
plasma electron heating modes, cooling of the workpiece is achieved after 
the desired temperature and/or microstructural state of the workpieces has 
been obtained. For example, if relatively rapid cooling of workpieces is 
performed, particularly in ferrous metallurgy, a particular phase 
transformation can be suppressed or particular phases can be formed, 
depending upon the rate of cooling. 
Specifically, it is often desirable to cool the surface of the 
vacuum-heated workpiece relatively rapidly. In conventional practice, a 
workpiece heated in a vacuum by radiation, for example, is cooled at a 
relatively high rate by high pressure gas cooling. In this approach, the 
vacuum chamber is rapidly backfilled, typically in 10-20 seconds, with a 
gas such as helium to a pressure of 1-20 atmospheres. The cooling gas 
contacts the surface of the workpiece and carries away heat. The heat 
removal rate is usually limited by a boundary layer at the surface of the 
workpiece, and the convection of the cooling gas. 
The present invention provides an alternative approach to cooling the 
vacuum-heated workpieces, termed solid/gas-jet cooling ("SGJC"). FIG. 3 
illustrates a solid/gas-jet cooling apparatus 60. The apparatus 60 
includes a converging/diverging nozzle 62 that expands a high-pressure 
flow that is input to the nozzle. 
The apparatus 60 further includes a source 64 of a liquid material having a 
thermodynamic triple point of equilibrium between its liquid, solid, and 
gaseous forms. The liquid material is preferably the heteroatomic species 
liquid carbon dioxide (CO.sub.2), which can exist as a liquid in a 
pressurized state. At one atmosphere pressure, carbon dioxide exists only 
as a gas or a solid ("dry ice"). The liquid material is conveyed from the 
source 64 to the nozzle 62 through a conduit 66 having a controllable 
valve 68 therein. The valve 68 permits a controllable flow of the liquid 
material to pass to the nozzle 62. The valve may be actuated manually or 
remotely, and may be placed near the source 64, depending upon the system 
design and system performance that is desired. Other liquid materials that 
may be used include diatomic species such as nitrogen and noble species 
such as argon. 
FIG. 4 schematically illustrates the pressure-temperature phase diagram for 
the liquid material having a triple point equilibrium between solid, gas, 
and liquid. The material is stored in the source 64 under pressure. In the 
preferred case, the carbon dioxide is stored in the liquid state at a 
pressure of about 835 pounds per square inch (psi). As a liquid, the 
carbon dioxide can flow along the conduit 66, and the flow rate can be 
readily controlled by the valve 68. 
As the liquid carbon dioxide expands through the diverging portion of the 
nozzle, its pressure and temperature are reduced. The liquid carbon 
dioxide transforms to a solid as it passes into the solid phase region of 
the phase diagram. Small, snow-like particles 70 of solid carbon dioxide 
are formed downstream of the nozzle 62. Depending upon the operating 
conditions, the particles may be as small as 1000 molecules in size or may 
be much larger. 
The small carbon dioxide particles 70 travel through free space and impact 
upon a surface 72 of the workpiece 44. The solid carbon dioxide snow 
absorbs heat from the surface 72 and is vaporized by sublimation. Carbon 
dioxide molecules diffuse away from the surface, as indicated 
schematically at numeral 74, carrying the heat of the workpiece 44 with 
them. Because the liquid carbon dioxide cannot exist thermodynamically at 
one atmosphere pressure, there is no liquid remaining on the surface 
following the cooling treatment, an important advantage. 
The solid/gas-jet cooling technique has been found to give superior results 
to ordinary high pressure gas cooling for several reasons. First, each 
carbon dioxide particle absorbs not only the heat required to increase its 
temperature, but also the heat of sublimation as it transforms to the 
vapor phase. Thus, its effective heat capacity is greater than that of a 
molecule that always remains in the gaseous phase. Second, during cooling 
a boundary layer 76 of heated gaseous molecules develops immediately 
adjacent to the surface 72. It is difficult to force other gaseous 
molecules through this boundary layer from the outside, resulting in a 
decreased surface heat transfer coefficient when high pressure gaseous 
cooling is used. The relatively massive solid particles 70 can easily 
penetrate through the boundary layer 76 to reach the surface 72, avoiding 
the adverse effects of the boundary layer upon the heat transfer 
coefficient. 
The fraction of solid particles entrained in the flow of particles from the 
nozzles can be tailored for advantageous results. By adjusting the 
temperature or pressure of the source 64, the entrained flow can be made 
to be predominantly solid particles or predominantly gas. A temperature of 
the source of 70.degree. F. or below results in predominantly solid 
particles and a temperature about 90.degree. F. results in predominantly 
gaseous particles produced upon expansion through the nozzle 62. The 
fraction of gas and solid particles can be controlled by adjusting the 
pressure within the carbon dioxide bottle source 64. The fraction of gas 
and solid particles can also be controlled directly by diluting the 
solid-gas stream with the addition of more gas. The dilution can be 
accomplished by two different methods. In the first method, an aerosol is 
formed in the liquid phase by injecting a dissimilar gas directly into the 
conduit 66, the liquid phase of the cooling gas. The second method dilutes 
the solid entrained in the gas stream with more gas of the same species at 
nozzle 62. Cooling of the workpiece can also be controlled by 
simultaneously passing different gases through different nozzles and 
directing the resulting solid/gas mixture flows against the workpiece. 
Cooling of the workpiece can also be controlled by varying the location 
and/or orientation of the nozzle or nozzles with respect to the workpiece. 
One of the characteristics of the plasma electron heating technique is that 
it can be used to uniformly heat an entire workpiece surface or uniformly 
heat only selective areas of the workpiece without manipulation of the 
workpiece. Similarly, the SGJC process can be used to uniformly cool the 
entire workpiece surface or uniformly cool selected areas of the workpiece 
surface, also without manipulation of the workpiece. The SGJC nozzle-based 
cooling technique can be used to selectively cool specific areas of a 
workpiece by using a plurality of nozzles aimed at different areas and 
possibly operating with different operating conditions and/or coolant 
materials. Alternatively, selective cooling may be achieved by masking 
specific areas of the workpiece. 
SGJC is a low-pressure cooling process. The vacuum chamber is continuously 
pumped by the vacuum pump. During SGJC, the chamber pressure is increased 
by the cooling gas, but the valve 28 may be throttled to maintain a 
selected vacuum that is typically in the range of 100 micrometers to 1 
atmosphere. Alternatively, the vacuum valve 28 can be closed during 
cooling to permit the interior of the vacuum chamber to be pressurized by 
the cooling gas. A vent or relief valve can optionally be provided to 
prevent pressurization above a preselected level such as one atmosphere. 
In some instances, cooling of a single surface is preferred. In other 
instances, it is preferred to achieve more omnidirectional cooling of the 
entire workpiece surface. 
A modification of the SGJC apparatus 60 that achieves more nearly spatially 
uniform cooling is shown in FIG. 5. To cool a workpiece 44 on all sides, 
two or more of the SGJC apparatus 60 are arranged at various locations 
around the workpiece. These SGJC apparatus 60 direct a flow of particles 
70 at the workpiece 44. The number, cooling medium, and operating 
parameters of the various SGJC apparatus 60 can be adjusted as necessary. 
A variation of this technique uses dispersing jets 78 (shown in FIG. 5) 
provided at various locations relative to flows from the nozzles of the 
SGJC apparatus 60. The dispersing jets 78 are conduits 80 having apertures 
82 therein directed inwardly toward the workpiece 44. 
Pressurized gas streams 84 of the same (or a different) gas flowing in the 
conduit 82 escape inwardly toward the workpiece 44 through the aperture 
82, and force particles 70 toward the sides of the workpiece that are 
otherwise not as accessible to the SGJC apparatus 60. In practice, since 
it is usually desirable to reach all sides of the workpiece 44, the 
conduits 80 may be made part of a single toroidal conduit, whose remaining 
portions are out of sight in the view of FIG. 5. Other shapes of the 
conduits 80 can be devised for achieving spatially uniform cooling of 
other shapes of workpieces, as necessary. Any combination of SGJC 
apparatus 60, operating in the same or different operating conditions, and 
dispersing jets 78 can be used as needed for a particular cooling 
requirement. 
FIG. 6 shows another utilization of the SGJC technique. Some types of 
workpieces are typically heated in air, as for example by flame heating or 
induction heating, and then rapidly cooled to form a hard surface layer of 
martensite or other phase. The SGJC technique can be employed to advantage 
in such processes using an apparatus 120 such as that shown in FIG. 6. 
The apparatus 120 includes a heat treating head 122 that is movable with 
respect to a workpiece 124. In the illustrated case, the heat treating 
head 122 is movable and the workpiece 124 is stationary, but this could be 
reversed. Here, the heat treating head 122 is supported on an arm 126 from 
a drive structure 128. The drive structure 128 includes a drive motor 130 
that moves the arm 126, and thence the heat treating head 122, via a drive 
arm 132. In the illustrated embodiment, the heat treating head 122 is 
moved to the right, as indicated by the arrow 133. 
The heat treating head 122 includes a flame hardening nozzle 134 through 
which a flammable gas from a source line 136 is passed. The flammable gas, 
such as acetylene, is burned at the outlet of the nozzle 134. The 
resulting flame heats a surface 138 as the heat treating head 122 is 
passed over the surface 138. 
The heat treating head 122 also includes a SGJC nozzle 140 operating in the 
manner discussed previously in relation to the SGJC apparatus 60. A 
liquefied gas from a source line 141 is passed through the nozzle 140 and 
expands to produce a flowing stream 142 that may be a mixture of solid 
particles and gas. The nozzle 140 is directed so that the flowing stream 
impinges upon the surface 138 of the workpiece 124 a short distance behind 
the flame of the nozzle 134. The SGJC stream 142 thereby quenches the 
surface 138 of the workpiece 124 to produce a heat treated, 
quench-hardened zone 144. An alternative to the motorized movement shown 
in FIG. 6 is to manually manipulate the nozzle head 122. 
The approach of FIG. 6 has the same advantages over prior techniques as 
discussed previously. It requires no cleanup of the surface, as compared 
with oil or water quenches. It uses non-hazardous quenchant materials, and 
the workpiece need not be moved. 
The quench rate using the SGJC approach is superior to that of an oil 
quench. In a series of studies, an apparatus 120 using the SGJC quench 
approach to quench 5130 steel plugs achieved Rockwell-C hardnesses of 
57-60. By contrast, a comparable apparatus using an oil quench achieved 
Rockwell-C hardnesses of only 52-53. 
FIG. 7 depicts an apparatus 20" that is similar to the apparatus 20 and 20' 
discussed previously in relation to FIGS. 1 and 2, except that both plasma 
electron heating and solid/gas-jet cooling are incorporated into the same 
heat treating system. Corresponding components have been assigned the same 
numbering as in the prior discussion, and that discussion is incorporated 
here. Since all of the components and their functions have been discussed 
previously, no further description is required. In the illustrated case, 
two of the SGJC apparatus 60 are used, and they are aimed at a single 
surface of the workpiece 44. In this example, only that surface is to be 
rapidly cooled. In operation of the apparatus 20", the workpiece 44 is 
heated by the plasma electron heating components in the manner previously 
described. After the required heating temperature and time have been 
achieved, the plasma electron heating is discontinued. The SGJC cooling 
using the nozzles 62 is then commenced, usually immediately to achieve the 
fastest cooling of the surface of the workpiece. (In other cases it may 
not be necessary to commence cooling immediately, in which case there 
would be a delay in the cooling process following the heating process.) 
Thus, the SGJC cooling is commenced in vacuum, and the vacuum is gradually 
degraded as coolant enters the system through the cooling process. If 
desired, the resulting coolant gas could be removed using the vacuum pump 
24, but normally the cooling is not adversely affected by its presence. 
The apparatus 20", utilizes plasma electron heating and SGJC cooling 
together. These techniques can be used separately as well. Plasma electron 
heating can be used with other cooling techniques such as high-pressure 
gas cooling, or without any forced cooling of the workpiece. SGJC cooling 
can be used with other heating techniques. 
FIG. 8 depicts the heating aspect of heat treating a workpiece according to 
the approach of the invention as described previously. A plasma of an 
ionizing gas is formed around the workpiece, numeral 90. Electrons are 
accelerated from the plasma to the workpiece with pulses of positive 
voltage applied to the workpiece, numeral 92, thereby heating the 
workpiece. The heating is continued until a preselected temperature state 
of the surface of the entire thickness of the workpiece is achieved, 
numeral 94. The workpiece may thereafter be cooled, by SGJC cooling or any 
other technique. 
FIG. 9 depicts the cooling aspect of heat treating a workpiece according to 
the approach of the invention as described previously. The workpiece is 
heated, numeral 100, either by plasma electron heating as described 
previously, or otherwise. The workpiece is thereafter cooled by directing 
a flow of solid cooling particles at the workpiece. The flow is produced 
by supplying a pressurized liquid material having a thermodynamic triple 
point of equilibrium between its liquid, solid, and gaseous forms, numeral 
102. A flow of the liquid material is expanded from a higher pressure to a 
lower pressure to produce the solid particulate form of the material, 
numeral 104. The particles are directed toward the workpiece, numeral 106. 
The plasma electron heating and SGJC cooling of the invention have been 
implemented in an apparatus of sufficient size to demonstrate commercial 
operability. The apparatus had a vacuum chamber that was four feet in 
diameter and eight feet in length, and a power source 48 with a power 
output of 100,000 watts. This permitted the heat treating of 
commercial-sized workpieces. 
The following examples are presented to illustrate aspects of the invention 
and its practice, but should not be taken as limiting of the invention in 
any respect. 
EXAMPLE 1 
Automotive pinion gear blanks made of type 5130 steel were "normalize" heat 
treated using plasma electron heating with the apparatus according to the 
invention. Normalizing heat treatment does not require any accelerated 
cooling of the workpiece, and therefore provides an indication of the 
plasma electron heating approach. Each gear blank had a diameter of 2 
inches and weighed about one pound. Nine of the gear blanks were supported 
in the apparatus. The gear blanks were identically treated. The apparatus 
operating parameters were an applied voltage of the power source 48 of 30 
kilovolts, an average pulsed current of 150 amperes, an average current 
from the power supply to the workpieces of about 0.25 amperes, a pulse 
repetition frequency of 200 Hertz, a pulse width of 10 microseconds, and 
argon ionizing gas at a working pressure of 3.times.10.sup.-5 Torr. The 
gear blanks were heated to a surface and bulk temperature of 900.degree. 
C. in about 5 minutes and maintained at that temperature for 15 minutes. 
After the 15 minute heating period was complete, the power was 
discontinued, and the gear blanks were allowed to cool by radiation in the 
vacuum. No forced cooling, such as SGJC or high-pressure gas cooling, was 
used in the normalizing treatment. 
The microstructures of the normalized gear blanks were studied. One 
question of concern was whether all nine gear blanks processed at once 
would have similar structures, and whether those structures would be 
similar to those of gear blanks that were conventionally normalized in a 
gas furnace. The studies showed that the microstructures of all nine gear 
blanks were similar, and that their microstructures were comparable to 
those of conventionally normalized gear blanks. This result demonstrated 
that the plasma enveloped the nine gear blanks uniformly. It also showed 
that the plasma electron heating process could duplicate the 
microstructures of conventionally normalized gear blanks, but in a time 
reduced due to the faster plasma electron heating time of 5 minutes as 
compared with the conventional heating time of 30 minutes. 
EXAMPLE 2 
Samples of the gear blanks normalized as discussed in Example 1 were 
carburized and quench hardened in a commercial facility. Gears that were 
normalized in a conventional furnace were similarly treated, and the 
property results compared. The surface hardness of the plasma electron 
normalized and carburized/quench hardened gears was 82-83 R.sub.a, and the 
surface hardness for the conventionally processed gears was 81-82 R.sub.a. 
The core hardness of each type of gear remained at about 47-48 R.sub.a. 
The case depth of the plasma electron processed gears was 0.48 
millimeters, and the case depth of the conventionally processed gears was 
0.45 millimeters. 
These results demonstrate that the plasma electron heating process is as 
controllable as conventional heat treating. However, it has the advantages 
over conventional heat treating discussed elsewhere herein. 
EXAMPLE 3 
Gear samples of type 5130 gear material were treated by rapid burst pulse 
plasma electron heating to achieve preferential surface heating without 
significant interior heating. The samples were heated using 100 kilovolt 
pulses, 10 microseconds pulse duration, a burst time of 0.5 seconds, and a 
total of 500 pulses per burst. The power density on the surface of the 
samples was about 1.5 kilowatts per square centimeter. 
The temperature of the surface of the gear material was measured with an 
infrared optical pyrometer having a response time of 0.1 seconds. During 
one burst of pulses, the surface of the gear reached 1000.degree. C. and 
then reduced to 500.degree. C. in less than 3 seconds by self quenching of 
the surface by the underlying mass of interior metal. 
Examples 1 and 2 demonstrate the use of slow, continuous heating of a 
workpiece using plasma electron heating with no forced cooling. This 
Example 3 demonstrates the rapid burst heating mode with no forced 
cooling. 
EXAMPLE 4 
To demonstrate SGJC cooling, an apparatus like that of FIG. 3 was 
constructed. The coolant material wag carbon dioxide, maintained in the 
source at 835 pounds per square inch. The nozzle of the cooling apparatus 
was sized such that the throughput of carbon dioxide under these 
conditions was about 100 pounds per minute. 
A steel workpiece was heated to 1000.degree. C. It was then cooled using 
the SGJC apparatus, and the temperature of the workpiece measured during 
cooling. FIG. 10 shows the temperature at a distance of about 2.5 
millimeters below the surface of the workpiece as a function of time, as 
measured by an embedded thermocouple. The effective heat transfer 
coefficient was estimated from these measurements to be about 5500 watts 
per square meter-degree K. 
For comparison, the same steel workpiece was again heated to 1000.degree. 
C. and cooled by immersion into oil quenchant. FIG. 10 shows that the 
temperature reduction as a function of time for the oil quench is slower 
than for the SGJC quench. 
From the technical literature, it is known that the heat transfer 
coefficient for high-pressure gas cooling using helium at a pressure of 20 
atmospheres is about 1000 watts per square meter-degree K, and for 
hydrogen at a pressure of 20 atmospheres about 2200 watts per square 
meter-degree K. The heat transfer coefficient for unagitated oil is about 
1500 watts per square meter-degree K, and for agitated oil about 2200 
watts per square meter-degree K. The heat transfer coefficient for 
agitated water is about 3500 watts per square meter-degree K. 
Thus, the heat transfer coefficient for SGJC cooling is superior to that of 
the previously most preferred approach for vacuum heated workpieces, 
high-pressure gas cooling, by a factor of 2-5. It is even superior to 
traditional liquid quenchants such as oil and water by a factor of 1.5-2. 
The SGJC process has the important advantage that it leaves no residue on 
the surface of the cooled workpiece and is environmentally preferred as 
compared with quenching in oil, polymers, salt solutions, and the like. 
EXAMPLE 5 
Gear material was surface hardened by heating, using plasma electron 
heating, and subsequent accelerated cooling, using carbon dioxide SGJC. 
Test samples were made from type 5130 steel, the same steel used to make 
the gears processed in Example 1. The samples were plasma electron heated 
to a temperature of about 900.degree. C. The plasma electron heating 
parameters were an applied voltage of 55 kilovolts, peak pulsed current of 
30 amperes, burst pulses with 700-800 pulses per burst, pulse repetition 
frequency of 1000 Hertz, pulse width of 10 microseconds, nitrogen ionizing 
gas, and working pressure of less than 3.times.10.sup.-5 Torr. With these 
parameters, the samples required 5 minutes to reach temperature, and were 
maintained at temperature for 5 minutes thereafter. After the heating was 
complete, the power was discontinued. 
A surface of each of the samples was thereafter cooled using the SGJC 
apparatus. 
After cooling, the samples were sectioned for microstructural examination. 
The microstructure showed a martensitic phase in the surface regions, the 
desired result. 
EXAMPLE 6 
The processing of Example 5 was repeated, except using actual gears. To 
achieve cooling on all sides of the gears, a toroidal dispersing jet 
device like that discussed in relation to FIG. 5 was used. The gears were 
successfully heated using the same plasma electron heating parameters 
discussed for Example 5, and cooled using the SGJC apparatus with the 
dispersing jet device. 
EXAMPLE 7 
Example 6 was repeated using a sequence of six identical gears, and one 
gear was retained in the untreated state as a control. The six gears were 
heated to different maximum temperatures by plasma electron heating, and 
cooled by SGJC using the toroidal dispersing jet device. 
After cooling, the microstructures were examined and the hardnesses of the 
surface regions were measured. The hardnesses were as follows: 
TABLE 
______________________________________ 
Sample No. Max. Temp (deg F.) 
Hardness (R.sub.a) 
______________________________________ 
1 1700 69 
2 1800 71 
3 1900 72 
4 2000 75 
5 2000 75 
6 2000 74 
Control -- 50-51 
______________________________________ 
The microstructure of Sample No. 2, heated to 1800.degree. F., was judged 
to have the fine-grained, martensitic structure and hardness that are 
preferred for the surface hardened gears of this type. Samples 4, 5, and 6 
demonstrate good repeatability of the treatment process, as indicated by 
the same (or nearly the same) hardness that is reached in each of the 
three runs. 
The present approach thus provides a method and apparatus for plasma 
electron heating and solid/gas jet cooling of workpieces, having the 
advantages over prior approaches as discussed herein. Although particular 
embodiments of the invention have been described in detail for purposes of 
illustration, various modifications may be made without departing from the 
spirit and scope of the invention. Accordingly, the invention is not to be 
limited except as by the appended claims.