US2322777A - Heat treatment of hardenable steel - Google Patents

Description

June 29, 1943. c. G. PURNELL HEAT TREATMENT OF HARDEN ABLE STEEL Filed July 18, 1940 2 Sheets-Sheet l hweizlar: (#42455 & PUB/V541,

Patented June 29,1943

Charles G. Parnell, Pittsburgh, Pa., assignor to Carnegie-Illinois Steel Corporation, a corporation of New Jersey Application Jilly 18, 1940, Serial No. 346,224

3 Claims. (Cl. 148-21) 3 This invention relates to the hardening heat treatment of hardenable steel.

Of the prior art methods, the one most widely used is to heat the steel above its critical tem-,

perature so that its microscopic structure transforms to austenite, it then being quenched at a rate at least equaling its critical cooling rate to a temperature causing its austenite to transform to martensite and then tempered as required to develop the physical properties daired. This method is objectionable in that it, is diflicult or impossible to quench certain steel shapes at a rate as fast as the critical cooling rate of the steel to sufficiently low temperatures by prior art methods of quenching, the danger of quenching cracks is always present and the hardened steel is lacking in toughness and ductility.

A relatively recent metho is to heat the steel to transform its structure to austenite and to then quench the steelata rate at least equaling its critical cooling rate to a temperature causing its austenite to directly transform t the structure providing the physical properties desired the temperature of the steel being held ubstanject of the present invention was to improve on these prior art hardening heat treatments of hardenable steel to the end of developing a means for adjusting the physical properties of such steel without involving the objections of the more common prior art method or the limitations of the more recently developed method. The invention that will now be described successfully attains this object.

According to the invention, hardenable steel is first heated to a temperature above its critical temperature or, in other words, it is heated so that its structure transforms wholly to austenite. As in all heat treatments, this heating should be done as uniformly as possible while the maximum temperature an time factors depend largely on the grain structure it is desired to impart the steel.

The steel is then quenched at a rate at least cooling rate to a temperature materially below that causing its austenite to transform to the structure providing the physical properties desired. However, the steel should not be cooled to a temperature causing its austenite to transform to martensite at the rate the steel is cooling.

It is to be understood that if the quenched steel is allowed to remain at its quenched temperature sufllciently long, its austenite will ultimately transform completely to martensite or some other structure than the ultimate structure providing the exact physical properties desired. However, before any austenitic transformation, the steel is reheated to a temperature causing its austenite to transform to the structure providing th physical properties desired, the steel being held, preferably constant, at this temperature until transformation of its austenite is complete.

Since the treatment is a hardening one, the temperature where the austenitic transformation.

is effected is other than causes austenite transformation to coarse pearlite, and since martensite is neither tough nor ductile, the temperature is other than causes austenite transformation .to

martensite. This involves all the advantages of the prior art method of effecting isothermal austenite transformation but is not subject to its limitations, the quenching of the steel to a temperature materially lower than that where austenite transforms to the structure providing the desired properties conditioning the austenite in some manner so that it transforms directly to this structure much more rapidly than is possible equaling, and preferably exceeding, its critical when the steel is placed in a quenching bath maintained at the isothermal transformation temperature. Also it is possible to use water or light oil as a quenchingmedium in conjunction with an ordinary reheating furnace, the practice being economical in this respect. In the case of steel having thick and thin sections, the quenching to a lower temperature than the austenite transformation temperature producing the ultimate structure desired, removes heat most rapidly from the thin section, and since during the reheating heat is put back into this thin section most rapidly, an equalizing effect is obtained, the thicker section cooling more slowly but reheating more slowly. By keeping the quenching temperature above that causing the production of martensite, quenching cracks cannot develop, it being understood that it is preferable to quench the steel to a temperature closely approaching but not reaching that causing quenching cracks. In any event, the quenching temperature should always be materially below the temperature at which the steel is quickly reheated so that its austenite may transform to the ultimate structure desired.

It is to be understood that when the steel has a. physical shape providing both relatively large and small masses, that the invention cannot be simultaneously applied to both masses. That is to say, if the principles of the invention are ap plied to the large mass, martensite may not be avoidable in the small mass during the quench; and if they are applied to the small mass; the

large mass may not be adequately quenched so as to produce the desirable acceleration in the austenite transformation rate. In instances where the invention is applied to the large mas, the small mass may be quenched and tempered in the prior art manner, the tempering occurring during the. time the austenite of the large mass is transforming in the desired manner.

The accompanying drawings illustrate heat treating apparatus that has been developed to carry out the method that has been described, the various figures being as follows:

Figure l, a plan;

Figure 2. an elevation in section on the line -IIII in Fi u e 1;

Figure 3. a horizontal III-III in Figure 2;

Figure 4. a modification of Figure 2;

Figure 5. a chart graphically showing an example of the invention;

Figure 6, the article treated as shown by Figure 5.

More specifically these drawings show a heating furnace having a heating chamber I and a soaking or temperature adJusting chamber 2, it being in this furnace that the steel is rendered fully austenitic in as even a manner as ispossihle. This furnace is of the continuous type having an entrance 3 and an exit I, with a conveyor 5 for continuously conveying steel articles through it. The furnace should be equipped with an adequate temperature controlling system.

A declining chute 6 leads from the exit I into a tank 'I for containing the quenching liquid. This liquid may be brine, water or a light oil; that is to say, van oil of lowviscosity. The best quenching effect has been found to be obtained by a petroleum oil having a viscosity of 100-115 at 100 F., a flash point of from 310-33051, .a pour test of ap roximately 40 at 0 F. and a fire point of from 350-360" F.

The chute 6 has a perforate portion i below the normal liquid level in the tank I and a propeller 8 is positioned opposite this perforate porsection on the line tion to force the quenching liquid through its holes. This propeller is mounted on a drive shaft 9 powered by a motor Ill that is mounted so it can tilt. the angularity of the propeller and its height being adjustable by a mounting II which raises or lowers the drive shaft 9. Furthermore, the mounting II and the motor II are arranged so that they can be slid back and forth overithe top of the tank I so that the propeller 8 can bespaced any distance desired from the perforate portion 6 of the chute i.

The motor I0 is extremely powerful and the propeller 8 is large and of relatively heavy pitch, the arrangement being such that the propeller can move the quenching liquid with substantially the same force that it would be moved. by

the propeller of a 30 foot motor boat capable of driving this boat at least 30 miles per hour. In

the liquid in the tank I at terrific velocity and with extreme force. I

As the steel articles move from the exit 4 down the chute i and particularly down the perforate portion 8' of the chute, the quenching action occurs. Since this must exceed the critical cooling rate of the steel and, since martensite must not be produced during the quenching, it is necessary to vary the cooling action, and this is done by adjusting the spacing of the propeller I from the perforate portion 6 and the angularity .of this propeller, as well as the propeller speed. the motor It being a variable speed motor to obtain the speed adjustment. The rate at which the articles fall down the declining chute 0 also may be adjusted, this being done by adiusting the angularity of the chute, it being hinzcd at its top and provided at its bottom with adjustable supports I! for this purpose. In all cases a very violent quenching action is necessary so that in all instances the steel will be quenched at a rate exceeding or at least equaling its critical cooling rate.

A further feature is the provision of bailles l3 forming a channel extending longitudinally of the propeller 8 and on the outside of which the quenching liquid reversely passes after it has once been driven through the holes of the perforateportion 6'- of the chute 6. A liquid cooler ll receives the liquid from the tank I by way of anexit l5 positioned in the end of the tank opposite the chute so as to receive the quenching liquid traveling outside the bailles ll, the cooled liquid leaving the cooler I 4 and entering the tank I by an entrance I6 directly behind the propeller I. The cooler l4 may be any of the conventional types but must. of course, have sufficient capacity to keep the liquid sufficiently cool to'properly perform its quenching function.

Since the propeller 8 moves such a terrific volume of liquid it is impossible for the exit ii to handle driving the liquid over I A continuous conveyor ll having a loading I portion beneath the chute 6 catches the articles dropping from the end of the chute and quickly carries them continuously from the tank, this conveyor having an unloading portion outside the tank for dropping them on the receiving end of the conveyor ll! of a continuous reheating furnace H. In this fashion the quenched steel is quickly transferred to the reheating furnace IS. the latter being operated at a temperature causing the temperature of the steel to rise to that where its austenite quickly transforms directly to the structure providing the physical properties desired. The furnace ill need not be made unduly long since the transformation of the austenite proceeds extremely rapidly after the quenching phase of the method. The steel leaving the furnace. is. drops into a cooler 20.

The modification shown by Figure 4 is mainly featured by the use of a short -conveyor 2| between the chute, 6 and the conveyor ll, this prolonging the quenching period to allow time for proper cooling of sections which might be so heavy that they could not be adequately otherwise. Normally this modification is not needed, the extreme quenching action obtained by the violent driving of the quenching other words, the propeller 8 is capable of moving liquid around the steel articles and through the holes of the perforate portion 6 being adequate in most instances.

Before concluding the description of this apparatus, attention is called to the fact that one of the things which retards the quenching of steel is scale on the steels surface. Due to the tumbling action of the articles falling down the chute 6, a large amount of the scale is dislodged. it falling to the bottom of the tank 1. To aid in this action the atmosphere of the furnace having the chambers l and 2 may consist of a suitable gas producing a more easily dislodged scale, such as an oxidized atmosphere. In the actual operation of apparatus similar to that disclosed herein, the hardened'steel articles leaving the furnace l9 are very nearly scale free, this being obviously an advantage in connection with further processing as well as indicating that quenching of the articles was not retarded by scale on their surfaces during much of the time they were in the quenching liquid.

Since the critical cooling rate of steel, its austenite transformation temperature, etc. vary according to the chemical composition of the steel and other factors, the use of specific temperature and time factors have been avoided in this specification. However, the chart graphically shows a specific example of the invention as it has been commercially applied.

Figure represents the cooling and heating curve for hardening fine-grain open-hearth plain-carbon steel of from .90 to 1.00% carbon content in accordance with the principles of the present invention. The articles treated are rail anchor springs such as illustrated by Figure 6, the section being 1% inch x 1?; of an inch and the microscopic structure produced being finely dispersed carbide in alpha iron with a hardness of Rockwell 045. In most instances, quenching temperatures of from 250 to 450 F. are best, the steel being immediately reheated to higher temperatures.

I claim:

1. A method of transforming hardenable steel structures to structures other than coarse pearlite or martensite, comprising heating the structures to convert them to austenite, quenching the austenite at a rate at least equaling its critical cooling rate to temperatures of from about 250 to 450 F., and prior to transformation of any austenite at these temperatures reheating it to higher temperatures transforming it to structures other than coarse pearlite or martensite and allowing time for the austenite to transform at the last named temperatures.

2. A method of quenching a hardenable steel mass, comprising immersing the mass heated to a temperature above its critical temperature in a liquid of low viscosity such as water or a light quenching oil while flowing the liquid at sufficient velocity over the mass to quench it at a rate at least equaling its critical cooling rate to temperatures of from about 250 to 450 F., removing the mass from this liquid and, prior to the transformation of any austenite formed during the heating of the mass-to a temperature above its critical temperature, placing the mass in a reheating furnace maintained at a temperature materially above its quenched temperature and materially below the critical temperature of the mass, and finally removing the mass from the furnace and cooling it.

3. A method of transforming hardenable steel structures to structures other than coarse pearlite or martensite, comprising heating the structures to convert them to austenite, quenching the austenite at a rate at least equaling its critical cooling rate to relatively low temperaturesnot exceeding about 450 F., and prior to transformation of any austenite reheating it to higher temperatures transforming-it to structures other than coarse pearlite or martensite and allowing time for the austenite to transform at the last named temperatures.

CHARLES G. PURNELL.

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