US3171759A

US3171759A - Method of heat treating high speed steels

Info

Publication number: US3171759A
Application number: US225333A
Authority: US
Inventors: Perry A Glenn
Original assignee: Ipsen Industries Inc
Current assignee: Ipsen Industries Inc
Priority date: 1962-09-21
Filing date: 1962-09-21
Publication date: 1965-03-02
Anticipated expiration: 1982-03-02

Description

March 2, 1965 P. A. GLENN METHOD OF HEAT TREATING HIGH swan srms Filed Sept. 21, 1962 \alezozzm 1005 OF DEGREE-5 F- lo :2, l4

TEMPERATURE 6 8 M v5: low-OM 5mm w 3 3 /1 t m 1 M u. u a u m H n P em m M M w W o BPDZQZ l NSC.

United States Patent Perry A. Glenn, Rockford, IlL, assignor to lpsen industries, Inc., Cherry Valley, Ill., a corporation of Illinois Filed Sept. 21, 1962, Ser. No. 225,333 4 Claims. (Cl. 148-46) This invention relates to the heat treating of high speed steels and, more particularly, to a heat treating method in which the work first is preheated at one temperature and then further heated at a higher temperature.

The general object of the invention is to provide a novel heat treating method which is rapid, inexpensive and capable of being performed automatically, which minimizes distortion of the work and which retains a bright surface on the work without scaling.

A more detailed object is to achieve the foregoing by performing the heat treating at subatmospheric pressures with the preheat taking place at a high degree of vacuum and the high heat taking place at an absolute pressure somewhat higher but substantially below atmospheric and in a non-oxidizing atmosphere.

Other objects and advantages of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings, in which FIGURE 1 is a schematic cross sectional view of a heat treating furnace suitable for carrying out the novel method of the present invention.

FIG. 2 is a chart of curves showing typical pressures and temperatures used in practicing the invention.

The present invention contemplates the provision of a novel method by which high speed steel such as tool steel may be heat treated while maintaining a bright surface on the workpiece. In general, this method utilizes both vacuum heating and heating in a controlled atmosphere with the work being preheated in a high degree of vacuum and the final heat taking place in the presence of a nonoxidizing gas at a pressure below atmospheric. Moreover, the work is cooled in a non-oxidizing atmosphere with the result that the surface of the work is substantially as bright after heat treating as it was before.

More specifically, the work is placed in a suitable chamber which may be selectively heated and which is arranged either to be evacuated or to contain a controlled atmosphere. With the Work in the chamber, the latter is evacuated and, after the absolute pressure within the chamber is at least as low as 1090 microns of mercury, the first heating step is begun. The evacuation continues until the pressure in the chamber is below 100 microns of mercury and preferably below 20 microns and, simultaneously, the temperature within the chamber is raised to the usual preheat temperature for the particular work. This temperature is the so-called critical temperature at which the steel becomes largely austenitic and the carbides begin to go into solution. The pressure and temperature are maintained at these values until the work is heated through.

Next, a non-oxidizing gas such as hydrogen is admitted to the chamber to raise the pressure to between 100 and 200 microns of mercury, the lower part of this range being preferred, and the temperature is raised to a point about 1000 degrees above the critical temperature. These conditions are maintained until the work is heated through at the new temperature and then the supply of heat is shut off and additional quantities of non-oxidizing gas are admitted to the chamber thereby raising the pressure to atmospheric. The supply of gas is continued until the work is cooled to about 208 degrees F. when it is removed from the chamber.

An apparatus suitable for performing the novel method 3,171,759 Patented Mar. 2, 165

is shown in FIG. 1 and includes a hollow, cylindrical vessel 10 which is supported on its side by a base 11 and is cooled by a peripheral water jacket 12. Within the vessel is a refractory bafiie 13 which defines the heat treating chamber 14 where the work 15 is supported on a roller platform 16. The upper and lower walls of the bafiie are formed with openings 1'7 and 18 respectively so that an atmosphere within the vessel 10 may be circulated across the work. To circulate the atmosphere, a centrifugal fan 19 is keyed to the shaft of a motor 29 mounted on top of the vessel with the shaft projecting vertically through the vessel wall. \Vith this arrangement, the atmosphere is discharged radially by the fan and follows the path indicated by the arrows, that is, down along the outside of the baflle 13, into the chamber 14 through the opening 18 and axially back to the fan through the opening 17.

In the chamber 14, the work is heated by suitable radiant heating elements 21 which may be of the electrical type and which extend horizontally across the chamber adjacent the upper and lower walls of the baffle 13. Radiation of heat through the openings 17 and 18 to the outside of the battle is prevented by

refractory deflectors

22 and 23 which are spaced from but cover the openings. Preferably, the outer surface of the side walls of the baffle 13 are cooled by water flowing through a tube 24 coiled around the battle.

To evacuate the interior of the vessel 10 and hence the chamber 14, a mechanical pump 25 and a suitable oil diffusion pump 26 are connected in series and communicate with the interior of the vessel. Thus, the diffusion pump is at one end of a conduit 2'7 whose other end projects through the wall of the vessel and the outlet of the diffusion pump is connected by a second conduit 28 to the inlet of the mechanical pump. Rough evacuation of the vessel 10 is effected by the mechanical pump 25 alone while the final evacuation is achieved by the diffusion pump acting with the mechanical pump. A suitable valve (not shown) may be disposed in the conduit 27 and controlled by a power actuator 29 to hold the vacuum in the vessel. The non-oxidizing gas, when required, is supplied from a suitable source 36 through a pipe 31 which projects into the vessel 10 and is controlled by a valve 32.

A typical example of the novel method is illustrated in FIG. 2 in which the solid line represents temperature in degrees Fahrenheit and the broken line represents absolute pressure in microns of mercury, both being plotted against time in minutes and a logarithmic scale being used for pressure. In this example, the work was an M-3 high speed steel with about a one-inch maximum cross section and, after it was placed in the chamber 14, the mechanical pump 25 was started with the vacuum valve open to initiate the the evacuation of the vessel 10. When the absolute pressure in the vessel was lowered to about 1000 microns, the heating elements 21 were energized to start raising the temperature of the work. At about the same time, the diffusion pump 26 started in operation so that, after about 20 minutes from the start of the operation, the absolute pressure in the vessel 10 was only a few microns. At that time, the temperature had been raised to 1550 degrees Fahrenheit and this pressure and temperature were maintained for approximately ten minutes after which the work was heated through.

Next, the valve 32 was opened to admit nitrogen to the vessel 10 and raise the pressure almost instantaneously to microns, the fan 1? having been started to circulate the nitrogen through the chamber 14 and across the work. At the same time, the heating elements 21 were further energized to raise the temperature of the work to 2225 degrees Fahrenheit and held until the work was heated through, this requiring a total of about fifteen minutes during which time the pressure was maintained at 100 microns. Finally, the valve 32 was opened enough to raise the pressure in the vessel to atmospheric and the heating elements were deenergized to permit the work to cool in the nitrogen atmosphere. In about fifteen minutes, the temperature of the work was reduced to about 300 degrees Fahrenheit after which it was removed from the vessel.

In practicing the invention, the preheating may be started any time the pressure in the vessel is 1000 microns or less but, where the temperature control is responsive to an atmospheric indicator which operates on a logarithmic scale, better control is achieved if the preheat is begun when the pressure is between and microns. For the preheat, the pressure should be lowered to at least 100 microns and held there during the balance of the preheat. The preferred practice, however, is to reduce the pressure further to a value below 10 microns.

As stated above, the preheat temperature is that at which the steel is largely austenitic and the carbides be gin to go into solution. This temperature varies for different steels and is determined from the conventional S curve for the particular composition of the work. The high heat temperature is the one normally used for heat treating each type of steel and usually is between 700 and 1000 degrees above the preheat temperature, the precise temperature depending upon the properties desired in the end product as is well known in the art. The pressure for the high heat is substantially below atmospheric and preferably does not exceed 200 microns, 100 microns being suitable for most applications.

In some instances, outgassing will occur, this being the removal of surface contaminants and the dissociation of unwanted oxides in the work, and it usually takes place after the pressure in the vessel 10 has been lowered below 1000 microns. Such outgassing momentarily raises the pressure in the vessel and, for this reason, it is desirable to have the heating elements controlled by the pressure indicator so that the heating of the work may be interrupted during this temporary increase in pressure.

With the foregoing method, high speed steels may be heat treated in one continuous operation and in a single zone. This provides accurate control over the process and minimizes distortion of the work. Moreover, there is no scaling of the work which retains a bright surface throughout the heat treating operation. The entire process may be performed rapidly, inexpensively and automatically.

I claim as my invention:

1. The method of heat treating high speed steel, said method comprising the steps of placing a workpiece in a chamber, evacuating said chamber to an absolute pressure on the order of 20 microns of mercury, heating'said chamber after the pressure therein is below 1000 microns of mercury to approximately the critical temperature of the steel, after the temperature of the workpiece has been raised to said critical temperature introducing nitrogen to said chamber to raise the pressure therein to approxi mately 100 microns of mercury and simultaneously raising the temperature in the chamber to a point substantially above said critical temperature, and, after the temperature of the workpiece has been raised, further introducing nitrogen to said chamber to raise the pressure to atmospheric and cool the workpiece.

2. The method of heat treating high speed steel, said method comprising the steps of placing a workpiece in a chamber, evacuating said chamber to an absolute pressure less han mi ons Q mer ry eat n sa Chamber after the pressure therein is below 1000 microns of mercury to a preheat temperature of approximately 1550 degrees B, after the temperature of the workpiece has been raised to said preheat temperature introducing a nonoxidizing gas to said chamber to raise the pressure therein to approximately microns of mercury and simultaneously raising the temperature in the chamber to a temperature of approximately 2200 degrees F., and, after the temperature of the workpiece has been raised to the temperature of the chamber, further introducing a non-oxidizing gas to said chamber to raise the pressure to atmospheric and cool the workpiece.

3. The method of heat treating high speed steel, said method comprising the steps of placing a workpiece in a chamber, evacuating said chamber to an absolute pressure on the order of 20 microns of mercury, heating said chamber after the pressure therein is below 1000 microns of mercury to approximately the critical temperature of the steel, after the temperature of the workpiece has been raised to said critical temperature introducing a nonoxidizing gas to said chamber to raise the pressure therein to approximately 100 microns of mercury and simultaneously raising the temperature in the chamber to a point substantially above said critical temperature, and, after the temperature of the workpiece has been raised, further introducing a non-oxidizing gas to said chamber to raise the pressure to atmospheric and cool the workpiece.

4. The method of heat treating high speed steel, said method comprising the steps of placing a workpiece in a chamber, evacuating said chamber to an absolute pressure of not more than 100 microns of mercury, heating said chamber after the pressure therein is below 1000 microns of mercury to approximately the critical temperature of the steel, after the temperature of the workpiece has been raised to said critical temperature introducing a non-oxidizing gas to said chamber while maintaining the pressure therein substantially below atmospheric and simultaneously raising the temperature in the chamber to a point substantially above said critical temperature, and, after the temperature of the workpiece has been raised, further introducing a non-oxidizing gas to said chamber to raise the pressure to atmospheric and cool the workpiece.

References Cited in the file of this patent UNITED STATES PATENTS 2,565,360 Du l g- 2 1.951

Claims

1. THE METHOD OF HEAT TREATING HIGH SPEED STEEL, SAID METHOD COMPRISING THE STEPS OF PLACING A WORKPIECE IN A CHAMBER, EVACUATING SAID CHAMBER TO AN ABSOLUTE PRESSURE ON THE ORDER OF 20 MICRONS OF MERCURY, HEATING SAID CHAMBER AFTER THE PRESSURE THEREIN IS BELOW 1000 MICRONS OF MERCURY TO APPROXIMATELY THE CRITICAL TEMPERATURE OF THE STEEL, AFTER THE TEMPERATURE OF THE WORKPIECE HAS BEEN RAISED TO SAID CRITICAL TEMPERATURE INTRODUCING NITROGEN TO SAID CHAMBER TO RAISE THE PRESSURE THEREIN TO APPROXIMATELY 100 MICRONS OF MERCURY AND SIMULTANEOUSLY RAISING THE TEMPERATURE IN THE CHAMBER TO A POUNT SUBSTANTIALLY ABOVE SAID CRITICAL TEMPERATURE, AND, AFTER THE TEMPERATURE OF THE WORKPIECE HAS BEEN RAISED, FURTHER INTRODUCING NITROGEN TO SAID CHAMBER TO RAISE THE PRESSURE TO ATMOSPHERIC AND COOL THE WORKPIECE.