US1810612A - Method of heat treating metal - Google Patents

Description

June 16, 1931. A. TfKATHNER METHOD OF HEAT TREATING METAL Filed Feb. 9, 1929 Nom, MSSN @25u00 u u u N Patented June 16, 1.9.31 'r UNITED STATES .ARTHUR T. KATHNER, OF PITTSBURGH, PENNSYLVANIA METHOD or HEAT Tammo `METAL Appncanon mea February v9, 1929. sermn. 338,780.

This invention relates to a method of heat treatin metal and more particularly to a method o continuously heat treating thin gau e metal. y

T e primary object of the invention is to reatly or materially reduce the time cycle 1n processing the metal, for example, heretofore in box annealing two to three days per box havesbeenrequired to properly anneal, whereas with the present invention an equal amount of annealing dias been accomplished in two hours.

An important oldject of' the invention is to reduce the time and temperatures employed in the subsequent processes of heat treating whereby to obtain a softer sheet and to Vreduce the number of processing steps through the mill.

In the drawings .y Fig. l is a longitudinal sectional view of one type of furnace in which. the invention maly be practiced, and l ig. 2 is a diagrammatic view showin the varying temperatures in the heating an cooling zones. a

In proceeding in accordance with the present' invention, a furnace is' provided: v which isformed with a heating chamber 1 and a cooling chamber 2, the two chambers having suitable conveyors, such as shaft conveyors 3, therein, by means of which "the metal is continuously moved in the ydirect1on of travel of the heated. gases, through the entire furnace. The heatinglchamber, as shown in the dialgram of Fig. 2, has a `preheating zone, a

eating zone proper, and a soakmg zone,

which are in the ratio of 10% for the preheatlng zone, 20% for the yheating zone 40 proper, and 20% for the soaking zone, theA cumulative ytotal being not more than th length ofthe cooling zone.

Tn this regard it will be understood that the pre-heating, heating and soaking zones, or d1v1s1ons of the heating chamber proper,

, various 'is continuously(moved'through the furnace and is discharged therefrom at what might be termed an approximately black heat.

The cooling zone ifY desired can be made longer than the heatin" and soaking zones, i

for exam le, a ratio o four feet of cooling to three eet of heating has been in practice found satisfactory. However, it is to be expressly understood that other ratios can be used if desired.

It is of course evident to those skilled in the art that in' this range Vof temperatures and speeds of travel ofthe metal through the furnacefor a combination of the two,

deinite physical characters have been achieved whlch it has not been possible to achieve in the old practice of box annealing.

By'my invention I am enabled tol dispense with the first box annealing step bysubstil tuting Iin its place a continuous normalizing heat treatment which not only relieves rolling strains in the hot rolled steel but also brings about a recrystallization of the grains. rlhis 'is followed by the arrest of the grain so growth to any desired grain size after which the steel may be cooled V`in a` controllable manner which will prevent. further grain growth and will impart to the steel a hardness value lower than that heretofore obtained in similar heat treating furnaces wherein the material is normalized.

Reference to Figure 2 shows that when such metal is so treated, it is quickly reduced in temperature to below` its upper critical range in the first part of the cooling zone. It will be understood that the temperature in the heating and soaking zones of the furnace 'is sufciently high to normalize the metal, that is, to bring about a recrystallization of the grains of the steel and produce a fine grained structure. Growth of the fine grains is arrested by the quick drop in temperature of the metal in the rst part of the cooling zone while the continued and slower cooling brings about a substantially uniform rate of metal cooling resulting i'n uniform surface and physical properties.

The quick drop in temperature in the first part of the'cooling zone may be controlled y varying the amount of heat admitted into the cooling chamber from the heating chamber, as for example, by varying the size of the passage between the two chambers. When a much lower temperature is desired in the cooling zone the passage may be restricted more and when a higher temperature is required it may be attained -for example by increasing the size of the passage, The fact that heated gases are flowed from the heating chamber into and' through the cooling chamber makes it possible to control the rate of cooling of the metal. That is, more heated gases may be passed through the cooling chamber when slow cooling is desired or less when faster cooling Iis desired. By keeping the Howing gases around and in close contact with the metal oxidation of the metal is minimized and the uniformity of cooling of the metal is controlled.

In instances where the sheets are to be normalized and improved, the metal 'is initially heated' to a predetermined degree in excess of the upper critical temperature of the metal and is then cooled to a degree wherein the crystalline structure of themetal has been set to a point where there will be no further growth of such structure and is further cooled in a manner to improve the surface, physical properties and microstructural properties of the metal and is then vdischarged from the cooling zone.

In practice, this phase of the invention the sheet metal is uniformly heated to a temperature above its upper critical range,

" which for .10% carbon steel is about 1600 degrees Fahrenheit. It is then passed into the coolingfzone and discharged therefrom at what might be termed a black heat,, namely one around 1000 degrees Fahrenheit. The length of the cooling zone, the time required for the sheets to pass therethrough and the conditions existing therein during the coolingv of the. sheets are so controlled as to result in imparting to the sheets the above mentioned improvedf properties. These conditions involve maintaining waste ases in the presence of the cooling sheets on a major portion of the length of the cooling zone and the dissipation of heat uniform from the sheets, in part, by way o f the gradually and uniformly cooling gases for a considerable portion 0f the coolin chamber lbe inning at the heating cham er end thereo The control of the conditions in the cooling chamber is facilitated by the construction of the cooling chamber of the furnace of Figure 1 hereof. That chamber has ,a smaller cross sectional area, due to its lower roof, than" the heatingchamber has.l The waste gases are thus confined or maintained in the presence of the steel for the major part of the length of the cooling chamber and are cooled 1n part by contact with the 80 cooling chamber walls`and extract heat from the steel? uniformly and progressively.

It will be understood that the rate of travel of the metal through the cooling zone, is determined by t-he particular character of structure and its physical properties of the metal which is to be obtained, and accordingly same will vary in accordance with the predetermined lcharacter and physical properties of such structure.

As a result of the immediate influence of 'the heat upon the metal and the rapid forward movement, such a uniform heating and cooling is accomplished as has not been possible in any previously tried method. Should the metal lbe subjected to such a high. telnperature by using annealing boxes, the sheets when hot would stick together and render the surface of the metal unusable. On the other hand, should the metal be placed" in open furnaces, as are in use, and given a sudden cooling under the immediate influence of the atmosphere, it would be oxidized or scaled. Moreover, even where such suddenly cooled metal is not immediately discharged into the atmosphere but is discharged from a short cooling chamber at a dull red heat that is above about 1300 degrees Fahrenheit and without proper regard to the conditions existing in thecoolin chamber as above mentioned, the steel will have scaly surfaces, high hardness, low ductility and micro structural properties greatly inferior to sheets treated according to my method.

Another advantage to be gained by the use of this new method is the possibility of handling metals of different physical properties,.a thin which up to this time was possible only y the use of a specially built furnace for every quality of metal. With this invention, it is only necessary, depending upon the length and construction of furnace employed, to Vary the speed according to the quality of material handled and to makethe corresponding adjustment of temperature.

Fig. 2 shows diagrammatically by means of' dash line curves, the maximum and minimum temperatures of the furnace, while the Lemma solid line curves indicate the maximum and minimum'temperatures followed by the metal in its passage through the furnace.

It can be seen from 'the diagram that thek temperatures are purposely regulated so that the metal is Apassed through the preheating Zone which approximates 10%; through the heating zone, 20%; through the equalizing or soaking zone, 20%, thus entailing a total heating zone or cycle of approximately 50%, and finally passes through the cooling zone, which latter approximates 50% of the whole length of the furnace.

This application claims methods which may be practiced in furnaces of my copending applications, Serial No. 270,637 filed April 17, 1928, Patent No. 1,725,398, issued Aug. 20, 1929, and Serial No. 362,094 filed May 10,

1929, Reissue Patent N o. 17,413, issued Aug.

20, 1929, for reissue of United States Patent No. 1,669,902.

What is claimed is 1. rIhe herein describedI method of heat treating steel which resides in passing low carbon steel continuously and in the presence of non-oxidizing gases through communicating heating and cooling zones of a furnace, heating the steel in the heating zone to a temperature above the upper critical temperature range ofthe steel and. between about 1600 F., and 1950 F., cooling the steel in the cooling zone to about a dull red heat, and discharging the steel from the furnace at such a tem erature that the steel will have reached a out a black heat when discharged into the open air, the time consumed by the steel in passing thru the cooling zone being not less than the time consumed in heating the steel to its maxium attained temperature.

2. The herein described method of heat treating steel which resides in passing low carbon steel continuously and in the presence of non-oxidizing gases through communicating heating and coolin zones of a furnace, heating the steel in theeating zone to a temperature above the upper critical temperature range of the steel and between about 1600 F., and 1950o F., cooling the steel in the cooling zone to about a dull red heat, discharging the steel from the furnace at such a temperature that the steel will have reached about a black heat when discharged into the open air, and controlling the speed of travel of the steel thru the said Zones so that the time consumed by the steel in passing thru the cooling zone is approximately one half of the time consumed in passing thru the furnace.

3. The herein described method of heat treating steel which resides in passing the steel continuously yand in the presence of non-oxidizing gases through communicating heating and cooling zones of a furnace, heating the steel in the furnace to above the upper critical temperature range of the steel, cooling the steel in the furnace from the maximum temperature attained to about a dull red heat, and,I discharging the steel from the furnace at such temperature that the steel reaches about a black heat when it contacts with the open air, the time of cooling in the furnace being not less than about the heating time.

In testimony whereof I aix In si nature.

ARTHUR T. KA H ER.

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