US3167460A - Method of surface-hardening steel workpieces in the form of bodies of revolution - Google Patents

Description

United States Patent 0 M 3,167,460 METHOD OF SURFACE-HARDENING STEEL WORKPIECES IN THE FORM OF BODIES 0F REVOLUTION Gerhard Seulen, Remscheid, Germany, assignor to Deutsche Edelstahlwerke Aktiengeseilschaft, Krefeld, Germany, and Allgemeine Elcktricitats-Qesellschaft, Berlin-Grunewald, Germany No Drawing. Filed Aug. 6, 1962, Ser. No. 214,846 Claims priority, application Germany, Aug. 18, 1961, D 36,838 4 Claims. (Cl. 148-143) The present invention relates to a method of surfacehardening steel workpieces in the form of bodies of revolution.

A well known process is that of inductively heating workpieces made of steel, such as bolts, bearings for shafts, rolls, rollers and like objects, which have the form of bodies of revolution, and of then quenching them for the purpose of hardening. The inductors employed for this purpose are either of a kind comprising one or more turns which completely embrace the workpiece or the section of the workpiece that is to be hardened, or they merely cover parts of the workpiece. When using the first kind of inductor rotation of the workpiece is not a necessity, but the latter kinds of inductor require that the workpiece should be rotated about its axis to permit a strip of the desired width around the entire peripheral surface to be raised to hardening temperature.

Inductive heating has the particular operational advantage over other methods of heating, such as by gas jets, by immersion in baths or in furnaces, that heating can be accomplished within the shortest possible time by a high local concentration of power. As known, currents are induced inside the workpiece at a depth which primarily depends upon the nature of the material and upon the frequency that is employed. When heating begins, this depth is shallow because the depth of penetration is reduced by the high permeability of the material and by its low electrical resistivity at room temperature. The depth at which heat is generated does not increase until the Curie temperature has been exceeded at which the permeability, as known, becomes unity. Despite the above mentioned advantage of inductive heating, this phenomenon involves a drawback from the point of view of subsequent hardening because it is a matter of considerable difiiculty to achieve the greater hardening depths which are needed for instance in the case of steel or cast cold rolls, working and backing rolls. Moreover, when hardening the rear axle shafts of motor vehicles the same trouble arises. In all these cases it is desirable that a sufi'icient depth, i.e. a sufficiently thick surface layer, should be raised to a temperature above the A0 point without at the same time undesirably raising the temperature of the core. Since the required depths of penetration could in the past be achieved only by prolonging the heating time it was unavoidable that the transfer of heat during these longer times also raised the temperature of the core to undesirably high levels.

It is the object of the invention to avoid these drawbacks of inductive heating when surface hardening steel workpieces which have the form of bodies of revolution and which are raised to hardening temperature by heating the whole of their peripheral surface by rotating the workpiece about its axis under an inductor which covers only part of the surface of the rotating workpiece which is then quenched. According to the invention this can be achieved by controlling the depth of penetration of the heat and hence the hardening depth by varying the relative speed between inductor and workpiece during rotation in the course of the heating process, said variation 3,167,460 Patented Jan. 26, 1965 being in the direction of an acceleration. More partcularly, assuming a given power density of the inductor, the speed of rotation is at first kept sufiiciently low for the surface area located directly below the inductor to be raised at least to the critical transformation temperature.

This temperature is then kept constant within close limits by increasing the speed of rotation. The treatment proceeds roughly as follows:

The part of the rotating workpiece surface which emerges from underneath the inductor can cool whilst turning through the remaining path of revolution and until it re-enters the range of the inductor by conductively transferring some of its heat to more inwardly located parts of the workpiece. When re-entering the range of the inductor the next thermal pulse raises the temperature to the former level again or to a level slightly above it. This process repeats itself until the whole of the surface has uniformly accepted the hardening temperature, when quenching may immediately follow. Shortly before quenching takes place the speed of rotation should reach a maximum to produce a distribution of temperature of optimum uniformity.

In the course of progressive hardening an increase in the speed of rotation is indicated when the period of time a portion of the workpiece is underneath the inductor tends to become too long from the point of view of the overall temperature level already achieved, and therefore likely to give rise to undesirably high temperature levels. It is therefore most desirable to raise the speed of rotation continuously to between 30 and 300 revolutions per minute before quenching takes place.

The effect of the proposed method is due to the fact that the heat sources inductively generated inside the workpiece are distributed throughout a much greater depth at temperatures above the Curie point. Since a temperature above the Curie point is reached underneath the inductor when the speed of rotation is low, the production of a very penetrative heating effect is ensured even when relatively high frequencies are employed. In the method as hitherto practised the distribution of the heat sources is confined to a layer which is extremely thin until the Curie point has been reached over the whole of the surface. The transfer of heat to regions located further inside is exclusively by conduction. On the other hand, the method'according to the invention generates the heat sources in every surface section at a depth as if the workpiece had already been austenitic from the commencement of heating.

The method proposed by the present invention has yet another advantage over hitherto known methods of hardening. As known, the short period of heating in an inductive heating process often makes it extremely difficult to bring the carbides into solution in such a way that a satisfactory martensite is formed after quenching. This applies more particularly to low or higher alloyed steels requiring a certain period of time for transforma tion, which is not made available in inductive heatng because of its rapidity. In the method according to the invention the carbides are dissolved each time the surface areas pass underneath the inductor. These consecutive thermal pulses give rise to a particularly fine grained martensite structure when the workpiece is finally quenched.

For steels which are liable to develop hair cracks in low temperature regions it is proposed in one embodiment of the invention to use a high speed of rotation at the beginning of the heating process, first to raise the surface region which is subsequently to be hardened uniformly around the circumference to a temperature between 200 and 500 C. This temperature level is chosen according to the composition of the steel that it to be hardened and particularly with reference to its sensitivity to rapid temr3 perature rises. The speed of rotation is then reduced to raise each of the surface areas passing under the inductor from this temperature of 200 to 500 C. to a temperature level of say 850 C. and then during its further rotation outside the inductor to cool the same to a temperature above the initial temperature by transfer of heat to the interior. In order to prevent the final hardening temperature from being exceed whilst the supply of energy remains constant the speed of rotation must be progressively accelerated also in this variant of the proposed method with a view to progressively shortening the time of action of the inductor on each surface section.

It has been found that the method of heating in this way ensures a considerable hardening depth, even when currents of high frequency are used, without any risk of the surface being overheated. The resultant hardened structure is a uniform fine grained martensite.

The method is performed with inductors which only partly cover the workpiece. Preferably hairpin-shaped or rod-shaped inductors are used extending axially along the rotating work or section of work. It is also feasible to provide several such inductors around the workpiece periphery.

For controlling the speed of rotation of the workpiece a timing controller may be used which operates on the basis of empirically determined values and which progressively increases the speed of the motor. Alternatively, control can be by reference to a temperature feeler located where the surface emerges from underneath the inductor and arranged to prevent a selected temperature, say 850 C., from being exceeded at the point of emergence from the inductor by raising the speed of rotation when the temperature rises excessively.

According to one embodiment of the invention a roll of a steel with 1% carbon and l.52% chromium and of and inches diameter is treated, using a loop-type inductor generating a power density of 0.3-1.5 kw. per square inch. The roll is rotated at a speed of 2-4 revo lutions per minute until the surface area located directly below the inductor has been raised to the critical transformation temperature, i.e., 860 C. The speed of rotation of the roll is then continuously increased until it has a speed of -40 revolutions per minute, and then it is immediately quenched by a dipping, spraying or other usual process.

According to another embodiment, where the object is of a steel liable to develop hair cracks, e.g., a steel with 04-05% carbon, 06-09% manganese, 02-04% silicon, 0.8-l.2% chromium and 0.5-1% nickel, in low temperature regions, the object is rotated first at a high speed, e.g., at a speed of 200 revolutions per minute until the surface region which is subsequently to be hardened is raised to a temperature between 200 C. and 500 C., e.g., 350 C. The speed of rotation is then reduced to 2-4 revolutions per minute until the temperature level of 850 C. is reached. The object then rotates outside the influence .of the inductor to cool the object to a temperature of said 450 C. by transfer of heat to the interior of :the object. Then with the object again under the influence of the inductor, its speed of rotation is progressivelyv accelerated to 300 revolutions per minute to bring the object to the'hardening temperature whilst the supply of energy by the inductor remains constant, and the object is finally quenched.

What I claim is:

1. A method of surface-hardening a steel workpiece having the form of a body of revolution which can be hardened by heating to at least its critical temperature and then quenching, comprising the steps of: subjecting said workpiece to the heating influence of induction heating means having an inductive heating influence over only a portion of the workpiece section to be hardened; eflecting relative rotation between-said workpiece and said induction heating means to thereby subject the entire Peripheryof the workpiece section to be hardened to induction heating, said relative rotation being at a speed and for a time suflicient to heat substantially only the surface portion of said workpiece section .to be hardened to at least said critical transformation temperature; thereafter increasing the speed of relative rotation between said work piece and said induction heating means to thereby maintain the tem erature of said surface portion substantially constant while increasing the depth of heating of said workpiece until said heated depth is at least at said critical temperature; then quenching said workpiece to thereby form the desired surface hardened layer.

2. A method of surface-hardening a steel workpiece having the form of a body of revolution which can be hardened by heating to at least its critical temperature and then quenching, comprising the steps of: subjecting said workpiece to the heating influence of induction heating means having an inductive heating influence over only a portion of the workpiece section to be hardened; effecting relative rotation between said workpiece and said induction heating means to thereby subject the entire periphery of the workpiece section to be hardened to induction heating, said relative rotation being at a speed not exceeding thirty revolutions per minute and for a time sufficient to heat substantially only the surface portion of said workpiece section to be hardened to at least said critical transformation temperature; thereafter increasing the speed of relative rotation between said workpiece and said induction heating means to a speed which is at least seven times the first-mentioned speed of relative rotation to thereby maintain the temperature of said surface portion substantially constant while increasing the depth of heating of said workpiece until said heated depth is at least at said critical temperature; then quenching said workpiece to thereby form the desired surface hardened layer.

3. A method of surface-hardening a steel workpiece having the form of a body of revolution which can be hardened by heating to at least its critical temperature and then quenching, comprising the steps of: subjecting said workpiece to the heating influence of induction heating means having an inductive heating influence over only a portion of the workpiece section to be hardened; effecting relative rotation between saidworkpiece and said induction heating means to thereby subject the entire periphcry of the workpiece section to be hardened to induction heating, said relative rotation being at a speed such that the relative surface speed between said workpiece and said induction heating means does not substantially exceed the order of three-hundred inches per minute and for a time suiiicient to heat substantially only the surface portion of said workpiece section to be hardened to at least said critical transformation temperature; thereafter increasing the speed of relative rotation between said workpiece and said induction heating means so that the relative surface speed between said workpiece and said induction heating is at least seven times the first-mentioned relative surface speed to thereby maintain the temperature of said surface portion substantially constant while increasing the depth of heating of said workpiece until said heated depth is at least at said critical temperature; then quenching said workpiece to thereby form the desired surface hardened layer.

4. A method of surface-hardening a steel workpiece having the form of a body of revolution which can be hardened by heating to at least its critical temperature and then quenching, comprising the steps of: subjecting said workpiece to the heating influence of induction heating means having an inductive heating influence over only a portion of the workpiece section to be hardened; effectin relative rotation between said workpiece and said induction heating means to thereby subject the entire periphery of the workpiece section to be hardened to induc tion heating, said relative-rotation being at a speed and for a time sufficient to uniformly heat the surface portion of said workpiece section to be hardened to a temperature in the range of 200 C. to 500 C.; thereafter continuing relative rotation between said workpiece and said induction heating means at a speed and for a time sufficient to heat substantially only the surface portion of said workpiece section to be hardened to at least said critical transformation temperature; thereafter progressively increasing the speed of relative rotation between said workpiece and said induction heating means to between thirty and three-hundred revolutions per minute to thereby maintain the temperature of said surface portion substantially constant while increasing the depth of heating of said work- References Cited by the Examiner UNITED STATES PATENTS 1,783,764 12/30 Adams 21910.4l 2,643,325 6/53 Body et al 219-10.41 2,935,433 5/60 Pribyl 148l44 DAVID L. RECK, Primary Examiner.

Claims (1)

1. A METHOD OF SURFACE-HARDENING A STEEL WORKPIECE HAVING THE FORM OF A BODY OF REVOLUTION WHICH CAN BE HARDENED BY HEATING TO AT LEAST ITS CRITICAL TEMPERATURE AND THE QUENCHING, COMPRISING THE STEPS OF: SUBJECTING SAID WORKPIECE TO THE HEATING INFLUENCE OVER ONLY A PORTION OF THE WORKPIECE SECTION TO BE HARDENED; EFFECTING RELATIVE ROTATION BETWEEN SAID WORKPIECE AND SAID INDUCTION HEATING MEANS TO THEREBY SUBJECT THE ENTIRE PERIPHERY OF THE WORKPIECE SECTION TO BE HARDENED TO INDUCTION HEATING, SAID RELATIVE ROTATION BEING AT A SPEED AND FOR A TIME SUFFICIENT TO HEAT SUBSTANTIALLY ONLY THE SURFACE PORTION OF SAID WORKPIECE SECTION TO BE HARDENED TO AT LEAST SAID CRITICAL TRANSFORMATION TEMPERATURE; THEREAFTER INCREASING THE SPEED OF RELATIVE ROTATION BETWEEN SAID WORKPIECE AND SAID INDUCTION HEATING MEANS TO THEREBY MAINTAIN THE TEMPERATURE OF SAID SURFACE PORTION SUBSTANTIALLY CONSTANT WHILE INCREASING THE DEPTH OF HEATING OF SAID WORKPIECE UNTIL SAID HEATED DEPTH IS AT LEAST AT SAID CRITICAL TEMPERATURE; THEN QUENCHING SAID WORKPIECE TO THEREBY FORM THE DESIRED SURFACE HARDENED LAYER.