PT89910A - Heat treating metal workpieces in plasma flame - with temp. control by infrared sensor - Google Patents

Abstract

A metal workpiece is heat treated by moving it in the path of a plasma flame. It is rotated at a fast enough speed to prevent significant cooling between the point of heating in the plasma flame and a point where the surface temperature is measured. It is preferably moved alone the axis of rotation. Surface temperature is measured by a non-contact means, e.g. an infrared sensor. Soaking temperature is preferably maintained at about 600-1000 deg.C. The plasma flame is extinguished, and the workpiece quenched by spraying cooling fluid, e.g. liquefied gas onto the rotating surface.

Description

Description relating to the patent of invention of L'AIR LIQUIDE, SOCIETE ANONYME POUR L 'ETUDE ET L' EXPLOITATION DES PROCEDES GEORGES CLAUDE, French, industrial and commercial, established at 75, Quai d 'Orsay, 75007 Paris, France, (inventor : Frederick W. Giacobbe, Resident E, UA), " PROCESS FOR HEAT TREATMENT OF METALS OR METALLIC ALLOYS IN A THERMAL PLASTIC FLAME. &Quot;.

DESCRIPTION

BACKGROUND OF THE INVENTION Heat treatment of metals and metal alloys is a very old and important industrial processing technology which is required for the production of a very wide range of products required. In particular, a major part of this technology relates to surface hardness and / or full hardening techniques which give rise to hard surfaces, wear resistant and overlapping fatigue (in the case of surface hardening) or completely (in the case of of complete hardening) in a hard but relatively soft or alloy steel flat object. Common methods of currently obtaining these results involve the use of layer hardening, flame hardening,

chemical properties, or the use of induction hardening. See for example " Principie of Heat Treatment " by Georges KRAUS - ASM - 1980 - Chapter 10 - or "Practical Heat Treating" by Howard E. Boyer ASM 1984 Chapter 11. Layer hardening is typically used for the purpose of hardening only the surface layers of a particular metal or metal alloy object. The specific object is usually immersed in a heating medium such as molten lead or a molten salt bath. This step is followed by filling which gives rise to a hard outer layer. This hardening method is greatly limited by the design of the part (i.e., it has to be heated to the entire part) and by inherent limitations of the temperatures of the heating bath. These limitations prevent extremely rapid and localized heating in specific areas or areas of an object to be heat-treated. Flame hardening involves the use of a flame produced by a chemical reaction between an appropriate combustible gas and oxygen. The commonly used fuel gases are hydrogen or acetylene. Using this technique, the objects to be selectively cured in the flame for an appropriate period of time are warmed up and then cooled rapidly. The main limitation of this technique is that the temperature of the chemical flame limits the rate of heat transfer to the metal object and thus the rate of increase of temperature of the sample is limited. This effect tends to allow the transmission to overheat some sections of the sample which are adjacent to the zone to be thermally treated.

When using the induction process, the uncured metal or metal alloy model is heated indirectly by means of currents induced in the model by an external electromagnetic induction coil. There are a number of significant disadvantages associated with this technology. For example, localized temperature control is difficult to achieve using the induction hardening technique, especially in parts with pronounced ends or varying thicknesses.

Since induction heating 2

depends on induced currents within specific metallic models, some objects (due to their geometric shape) can not be effectively hardened using this technique. In addition, objects of different shapes or sizes require specially designed induction coils for each of them so that the induction hardening process is optimized. In some cases, the costs associated with designing and manufacturing these special induction coils may be prohibitive, especially if the articles to be cured are not manufactured in large quantities.

SUMMARY AND OBJECTIVES OF THE INVENTION

Many of the above limitations can be overcome by the use of an appropriately designed and controlled thermal plasma heat treatment system. The present invention, described herein, describes such a system which can be used for the heat treatment of a wide variety of specific metal objects and / or metal alloys. This thermal mains sink system uses a " flame " of argon plasma or nitrogen at ambient pressure to facilitate surface hardening or complete hardening of specific metal and / or metal alloy models. This system has several advantages over the methods described above as well as advantages over conventional furnace hardening techniques.

One of these advantages involves the fact that a suitably designed and controlled thermal plasma system can be used to obtain a very precise temperature control and located on various types of metal objects that have to be treated; thermally. Another advantage of this technique involves extremely fast sample heating rates due to the extremely high temperatures that can be produced within the " flame " of plasma. For example, temperatures of " flame " of plasma exceeding 10,000 ° C whereas typical temperatures of molten salt baths or chemical flame temperatures rarely exceed 3,300 ° C. Due to the very high temperatures which can be reached in a " flame " of 3

plasma, metal or metal alloy objects can heat up extremely fast in a " flame " of plasma. However, the phenomenon of associated overheating and localized melting can be completely eliminated by reducing the temperature of the " flame " of plasma as soon as the metal or metal alloy object to be treated reaches its saturation temperature. This adjustment is easily done by manual or automatic adjustment of the electric power input to the " flame " of plasma. This saturation temperature is preferably maintained between about 600 ° C and 1000 ° C.

A very small number of modifications of the plasma system may also be used in order to facilitate the heat treatment of a large number of modes. with wide range of different geometric shapes and sizes. In addition, multiple plasma systems can be used simultaneously to efficiently heat-treat relatively large strips of material in large models. When multiple systems are used, variable thicknesses of material within the same model may be independently and simultaneously heated under the same or different temperature conditions.

Generally, extremely rapid heating of the controlled (only possible with a thermal plasma system) followed by appropriate cooling and / or annealing or quenching, allows the production of a wide range of suitable metal mechanical parts, which are difficult, if not impossible to treat efficiently by heat, using any other technique. Cooling of the workpieces may be carried out immediately after the plasma flame is extinguished, for example by spraying a liquid such as water or a liquefied gas, such as liquid nitrogen, argon or carbon dioxide, while the workpiece is still rotating and / or translating in its holder. Cooling may also take place after the rotation and / or translation of said part has stopped by immersing it in a liquid. The process according to this invention provides a means for heat treatment of metals and / or metal alloys in a " flame " of plasma for the purpose of. 4.

harden any predetermined thickness of the model or completely harden the model. An additional aspect of this process is that it can be used to heat suitable metal models very quickly, thereby minimizing harmful heating of adjacent regions in the same model from being conducted. The process is also capable of producing excellent temperature control in heated models as well as excellent depth control in the hardened layer.

In addition, this process is relatively easy to adapt to the various shapes and sizes of workpieces and is capable of providing excellent microstructural properties in selectively hardened areas of the workpieces.

SUMMARY OF THE INVENTION The process according to this invention involves the use of a high temperature thermal plasma system at ambient pressure which may be used to facilitate surface hardening and full hardening of specific test samples of alloy. The apparatus of this invention may be adapted to create a very high temperature thermal plasma within an argon gas stream. However, other pure gases or gas mixtures may be used instead of argon. During the use of this system, a " calls " of argon plasma at elevated temperature against the outer surface of a rotating alloy test pattern. The surface temperature of the test model is monitored by means of a non-contact infrared temperature sensor. The test temperature is controlled by varying the flow rate of inert gas through the plasma generator and / or by varying the input of the electric power in the plasma generator. This means that the temperature control can be used to obtain very precise setting of the model temperatures during the heating and / or saturation phase of any specific heat treatment operation.

According to a preferred embodiment, this invention provides a method of thermally treating a metal workpiece in a flame of 5%

plasma generated by means of plasma generation, comprises the steps of: - movement of the workpiece in the course of the plasma flame. rotation of the workpiece at a sufficiently rapid rate so that there is no significant cooling between the position in which said workpiece is heated in the plasma flame and the position required to measure the surface temperature by means of a temperature measurement non-contact surface; - ignition of the plasma generating medium in order to generate the plasma flame; - control of the plasma flame temperature and thus of the surface temperature of the workpiece by means of non-contact surface temperature measuring means in order to maintain the desired heating rate of the sample and the desired saturation temperature of the workpiece - extinction of the plasma flame; - cooling of the workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic side view with a cross-sectional view of the thermal plasma processing apparatus for use in surface hardening and full hardening according to this invention, Figure 2 is a view with a portion of the cross-section of the apparatus according to figure 1.

DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows a thermal plasma processing device according to this invention,

comprising a plasma flame generator means 100, a heat treatment receiver 200, an infrared temperature sensor 300 and the gas venting means 400. The plasma flame generator means 100 includes a cathode vessel 7 in which the cathode 11 is positioned and maintained by means of a support flange 9, which is connected through the gas injection ring of the plasma 8 to the vessel 7. The plasma gas inlet is made by the ring 8, which is not shown in the drawing. The carrier means 11 aligned coaxially with the container 7 determines an inner channel 30 aligned coaxially with the cathode 11 in which the plasma gas flows, and outer channels 31 and 32 respectively connected to the water inlets 4.40 and to the water 5,50 to cool said inner channel 30. The water inlet 41 and outlet 51 also serve to cool the cathode 11 internally.

An adapter flange connects the outlet 33 of said channel 30 to the heat treatment vessel 200, said flange being also connected to the anode cable 15 and thus being the high voltage anode. The plasma flame 21 is generated between the cathode 11 and the anode 6 and the workpiece 19 to be treated is extended which is shown as a hollow model. This pattern rotatably attaches to the support rings 18, 38 and is then placed in the path of the mains flame. The housing 200 includes a tube 56 connected to the cooling drain valve 16 which in turn is connected to the cooling fluid pipe 54, 55 in order to allow the workpiece to cool immediately after the flame is extinguished or later.

The temperature measuring means IV is located above the sample, almost in front of the tube 56. The temperature sensor 1 is connected to the remote energy supplier (not shown in the figure) by the cable 2. A cylindrical hollow 60 connects to the container 200 at one end and to a second hollow cylindrical coating 65 on the end of which is then attached to the IR sensor 1. A wrapping is provided with water 66 between the first and the second coating 60 , 65 to cool the gas between the recycle 7

and the I.V. sensor 1. FIG. 2 is the top view of the apparatus of FIG. 1 in which the same references indicate the same devices. The workpiece 19 (or template) rotates between two support anisols 18, 38 and is secured through the attachment means 22 to a variable speed DC motor 24, including means (not shown in the drawing) for varying the speed of rotation of the model in accordance with the needs of the process.

EXAMPLES The apparatus and process of this invention described herein were tested in various actual metal alloy models. These models were made of E 52100 steel since this type of steel can be effectively hardened by the induction process. EXAMPLE 1

The initial test models were run from hot-finished raw material in the form of 1.27 cm outer diameter spherical rings. Each of these test specimens were cut into 15.2 cm lengths. Approximately 0.02 cm of surface material of this model has been transformed, also in order to remove micro-structural imperfections due to the manufacturing process used to form this material. Several of these models were heat treated in a " flame " of pure argon plasma produced using the apparatus shown in Figures 1 and 2. However, the template support assembly was modified to attach the 1.27 cm OD bars performed for these assays.

During this evaluation, the 1.27 cm outer diameter test rods rotated to approximately 10 rev./sec. in order to guarantee the uniform heating of the model. 8 *

Pure argon was used, at a flow rate of approximately 100 SLPM (standard liters per minute), as plasma gas. An additional argon gas flow of about 12SLPM was used to purge the optical path between the infrared sensor and the surface of the rotating assay model. The electrical power requirements depended on the plasma gas flow rates, the system geometry, the losses of heat with the outside and the cooling water, and the fast heating rates chosen for this series of tests.

J

The initial plasma input power requirement was about 10 KW (100 amps at 100 volts). Under these conditions, samples of 1.27 cm outer diameter steel alloy showed a near linear temperature increase from room temperature to 850 ± 59 ° C for approximately 30 sec. The test samples were maintained or saturated at 850 ± 59 ° C by varying the time periods, manually decreasing the current input which kept the " flame " of plasma. The power requirement during the saturation phase was about 9 kW (90 amps to 100 volts). This electrical energy was required to maintain the sample temperature at 850 ± 5 ° C and simultaneously (compensate for the inherent heat losses associated with the experimental setup of the apparatus and the operating parameters. After each saturation period, the plasma energy supply and the hot and rotating test models were rapidly cooled in water. The results obtained from two of the models treated as described above are shown in Table 1. It can be seen that provided excellent and uniform hardening in these two models.The hardness through the heat affected zones in these two models was greater than 60 on the Rockwell Scale C. The microstructure in the heat affected areas was excellent and all physical properties were equivalent or better than those obtained using an induction type hardening process.

Larger models of the same 9

) steel from solid raw material with 2.54 cm of external diameter. These models were also hardened using the thermal plasma system described herein. Except for the electrical energy input used during the initial heating phases of the sample, most of the operating conditions used during the treatment of these models were approximately identical to the operating conditions used for heat treatment of the 1.27 cm diameter described above. However, the 2.54 cm models of the outside meter heated and saturated at 870 + 5 ° C. These models also took longer to reach the saturation temperature due to their larger masses. For example, one of these samples was heated from 300 to 870Â ° C in about 45 sec. Another similar model was heated from 370 ° to 870 ° C by about 81 sec. The main reason for the significant difference in these heating rates relates to the fact that different inputs of plasma electric energy have been used during the initial heating phases of these samples. The results of the tests obtained from two of these 2.54 cm external diameter outer models are shown in Table 2. It was found that excellent toughness was obtained through the thermally treated zones in these samples. However, complete hardening in the saturated sample at 870 ± 5 ° C was not obtained for only 10 sec, but in the other sample complete hardening was obtained. This clearly demonstrates that the time variable can be used to control the thickness of the cured layer when this process is suitably applied to appropriate models of metal alloys. This is an important aspect of this process. EXAMPLE 3

An additional series of hollow steel alloy models, 2.54 cm in OD were run and tested using the apparatus and process of this invention. These models were also run from the same E52100 steel bar used to make the solid models. 10

) The central section 6ca of these bars had an inner diameter (1.0) of 1.27cm. The length of this central hollow section extended beyond the centrally heated zone exposed to the "plasma heating flame." Figures 1 and 2 also show a good representation of the configuration of the apparatus used to treat these samples. The operating conditions were also almost identical to those described above with the exception of the electric power input of the plasma during the initial heating phases of these samples. Due primarily to these differences in the electrical energy input of the plasma, these samples were warmed from room temperature to 870 ± 5 ° C in about 100 sec. The test results obtained from two of these hollow test models are set forth in Table 3. These test results demonstrate that complete hardening can be obtained within a selected and well defined region in hollow models. The fact that neighboring regions can remain soft and resilient is another important advantage associated with this process.

11

TABLE 1

RESULTS OF THERMAL TREATMENT OF PLASMA USING SOLID MODELS OF STEEL ALLOY OF 1.27 CM OF EXTERNAL DIAMETER SAMPLE 1 SAMPLE 2 Saturation time 850 + 52C (sec) 10 30 Length of hardened outer surface area (cm) 1.97 1.97 Length of zone hardened at the center of the model (cm) 1.76 1.84 Hardness Inside the heat treated zone (RC Scale) &gt; 60> 60 External hardness of the thermally treated zone (RC Scale) 4 10 <10> 10 -

TABLE 2 RESULTS OF THERMAL TREATMENT BY PLASMA USING SOLID STEEL ALLOY MODELS OF D.E. 2.54 cm SAMPLE 1 SAMPLE 2 Saturation time 850 + 52C (sec) 10 300 Length of the hardened outer surface area (cm) 2.2 6.0 Hardened zone length at the center of the model (cm) 0.0 4.5 Hardened layer thickness at the center of the heated zone (cm) 0.5 1.3 Hardness Inside the thermally treated zone (RC Scale) &gt; 60 &gt; 60 External hardness of the thermally treated zone (RC Scale) 110 ^ 10

TABLE 3

RESULTS OF THERMAL PLASMA TREATMENT USING SOLID STEEL ALLOY MODELS OF 1.27 cm OF EXTERNAL DIMETER SAMPLE 1 SAMPLE 2 Saturation time at 850 + 59C (sec) 10 60 Length of hardened outer surface area (cm) 1.7 2.1 Length of Hardened Zone of Inner Diameter Model (cm) 1.7 2.1 Inner Hardness of Thermally Treated Zone (RC Scale) &gt; 55 &gt; 60 External Hardness of the Trematically Treated Zone (RC Scale) ^ 10 l 10

Claims (4)

1. A process for the treatment of a metal part in a plasma flame generated by plasma forming means, characterized in that: - the part is moved in the course of the plasma flame, - the part is rotated at a sufficiently high speed so that there is no significant cooling between the position in which said part is heated in said plasma flame and the position required to measure the surface temperature by means of non-contact surface temperature measurement means, the plasma forming means is used to generate the plasma flame, the plasma flame temperature and thereby the temperature of the surface of the workpiece are measured by means of measuring the temperature of said non-contact surface to maintain the heating rate of the sample and the desired impregnation temperature of the part, - extinguishing the plasma flame, - tempering the part. 2. A method according to claim 1, characterized in that said non-contact surface temperature measuring means are I: V means. (infra red). 3. A process according to claim 1 wherein the desired impregnation temperature is maintained between about 600 ° C and about 1000 ° C. 4. A process according to claim 1, Characterized in that a step of transferring the part along the rotational axis is further included. The method of claim 4, wherein a step of varying the translational speed of the workpiece according to its geometry is further included so as to apply the treatment at the same depth over approximately the entire workpiece. 6. A process according to claim 1 or claim 5, further characterized in that a step of varying the speed of rotation of the workpiece according to its geometry is included in order to apply the treatment with the same depth over the entire workpiece. 7. A process according to claim 1, characterized in that a heat treatment step of different zones of the same part is simultaneously included with a plurality of plasma generating means, each of which is associated with measuring means of the same. non-contact surface temperature. 8. A process according to claim 7, wherein each of the different zones is treated at a temperature different from the others. A method according to claim 7, characterized in that it is treated with one of the different zones at a different thickness from the others. 10. A process according to claim 1, wherein the plasma gas is selected from hydrogen, 16-helium, neon, argon, krypton, zenon, radon, nitrogen oxo, carbon dioxide or a mixture thereof. 11. A process as claimed in claim 1, wherein said temperature treatment is performed immediately after flame quenching. 12. A process according to claim 1, wherein said temperature step is performed by spraying a cooling fluid onto the workpiece surface. 13. The method of claim 12, wherein said temperature is achieved while the workpiece is still rotating. 14. The method of claim 12, wherein said temperature is achieved by spraying a liquefied gas on the surface of the part. Lisbon, March 6, 1989 17