US2764515A

US2764515A - Method of spheroidizing steel stock

Info

Publication number: US2764515A
Application number: US445016A
Authority: US
Inventors: Orville E Cullen
Original assignee: Surface Combustion Corp
Current assignee: Surface Combustion Corp
Priority date: 1954-07-22
Filing date: 1954-07-22
Publication date: 1956-09-25
Anticipated expiration: 1973-09-25

Description

Sept. 25, 1956 o. E. CULLEN METHOD OF SPHEROIDIZING STEEL STOCK Filed July '22, 1954 I from Carbon Therma/ 'Cm'f/k-a/ Poi/1f;

Farce/72 Carbon we, Y /2 /:5 77/776 HO UK? V INVENTOR.- Orv/fie 15 C ATTOFrNE United States Patent Oflice 511136011 Corporation, Toledo, Ohio, a corporation of Application July '22, 1954, Serial No. 445,016

7 claims, (Cl. 148-415 This invention relates to spheroidi'zi'ng steel stock, and more particularly, to a com aratively rapid method for 'spheroidizirig cement'itiou's portions of steel stock.

In many instances, for example where severe forming operations must be performed on steel stock, either car'- hon steel or alloy steel, it is necessar to spheroidiz'e the cementitious portions thereof. Y Sphe'roidiZation has heretofore been achieved either by extremely long heating cycles where the work is first heated to a temperature within the austen'itiz'in'g range and then cooled slowly, or else cooled to about room temperature, either by uenching or by slow cooling, and then reheated to a temperature just below its Ac1 temperature, or the minimum temperature at 'whieh, upon heatin g austenite is formed. In the former method, extremely long heating eyes are required, for example 80 to 170 hours (depending on the size or the load and on the degree of spheroidization required) in the ease of A. I. S. I. 1030 steel, which is known to be hard to spheroidi'ze. The latter process is not generally used unless spheroidiz'atio'n is to be carried out at an intermediate rocessing. point, since it involves a double heating operation, and it is generally equally long.

The resent invention is based upon the discovery that, by proper control of'proeessing temperatures, spheroidization can be accomplished in a fraction of the time required by former techniques.

It is therefore, an object of the invention to provide an improved spheroidizlrig process.

I Other objects nd advantages will be ap arent from the descri tion which follows, reference being had to the attached drawing, or which Fig. l is a portion of the iron-carbon diagram showing thermal critical points as a function of carbon content, and Fig. 2 is" a typical timetemperature curve according to this invention.

According to the invention an improvement is provided in a method t rspheroidizing eetnehtitious ortions of steel tock which comprises substantially austeniti'zihg the stock by heating to a temperature at which sl'l'bstali tia-lly all eemehtite therein is converted to austehi'te, and cooling the stock f om such tem erature to a temperature sub tantially below the Air temperature for the stools. The im ro ement or the invention ons'ist 'i cooling he austenltize'd stock to a temperature below the Arr tem erature for that stock, but above about.

1600 1?. and preferabl not more than bout se F. below the Air temperature for a time sulfic'i ntlylong that a part of the austenite is converted to cementite nuclei, reheating the stock to a temperature between its Ac temperature and a temperature not more than about l0?" below its Ari temperature and preferably abo e it's Arr; temperature maintaining the stock at'a sphetoldiz As a first step in spheroidiz'ing steel according to the improved method of the invention it is necessary to heat the stock to a temperature at which cementitious portions thereof are substantially converted to austenite. In the case of a hypoeuteetoid steel this can be accomplished by heating above the A61 temperature, or to a point at which ferrite and austenite coexist, or by carrying the heating to a higher temperature where only austenite is the stable form. In the former instance little if any ehan e in position of cementite occurs, so that spheroidi'zed cementite appears in the finished stock in larger areas, but in approximately the same positions that the cementite deposits, in another form, were present in the original stock. In the latter case, substantially the entire stock becomes austenitic so that a redistribution of cementitious portions, after spheroidizing occurs. In the instance of hypereiltectioid steel, complete austenitizing occurs only upon heating above the Acm temperatu'r'e, which is a function or carbon content. In practice it is often referred to austenitize at temperatures somewhat below the Acm line, but siifiiciently' high to austenitiz'e substantially all the cementite. Since only ausfenite is the stable form at austeiiitiz'ing temperatures above the Acm line for hypereu'tect'oid steel, it will be apparent that substantial migration or carbon,- or FeaC, and therefore of the location of spheroidiied particles in the finished'pfodu'ct, can be expected ill this type of steel. The preferred range of austenit'iz'ing temperatures as a function of carbon content is represented for unalloyed, or carbon steel, by the shaded ortion of the attached drawing.

As is above stated, in practicing the method of the invention, the austenitized stock is cooled to a temperature below its Ari temperature, but not lower than about 1'00'0 F., as a second step. Although the invention is not limited to the following theoretical explanation, it is believed that, upon cooling to a temperature within the indicated range, c'fiie'ntit nuelei are precipitated from the stock and, upon subsequent reheating at a temperature near the A61 temperature, additional cementite precipitate'd from the austenitic phase f rms spherules around these nuclei. In this connection it has been ob served that maintaining the stock below the Ali tem erature for a longer poriod of time tends to produce smaller spherules than does maintaining the stock at such temperature for arelatively short period of time. This difference is believed to be attributable to the formation, in the former instaue'e, ot a lar er number of nuelei around which the pherules form, as compared with a smaller nufnbef of nuclei around which larger spherllle's ffifnl in thelatter instance. It has also been observed that eementite precipitation which occurs during this step of thorn-edited or": the invention should be terminated,- by reheating as hereinafter described in more detail,- before thest'dck is converted to a relatively stabilized pearlitic form. If the" precipitation of cementite is allowed to proeeed, durin this stage, untilthe stock contains sllbst an tial amounts or pearlite, it is found that ubsequent mg temperature within the las -indicated ra ge fo a time stl'fiiient'to form spheroidizedeementite, and cooling the stock. A substantiall completely s heroidized micro: structure is usually achieved by virtue of the above ros ess after from about four to about ten hours at the sphei'oidizing temperature;

spheroidizationproceeds onl with substantial dihieulty 'tiiid at a relatively low rate. It is preferred that this c'e meritite preeipitation step Of the method of the invention be terminated b'fo'r'e substantial equilibrium is reached, 61* While some austemte remains in the stock; If is also preferred that the sto'ek be quenchedfrom the austeriitii ing temperature, or at least from between the A01 and the Art temperatures after austenitizln to the temperature at which cementite preeipi-tated, asdescribed, because the presses can be substantially speeded-up by virtue of such queil ilig since the time required for such cooling 'is es sefitially lost time,- and since precipitation occurs more raoidly at comparatively low temperatures. However, as is hereinatter'deseribed in more detail, the entire splii' oidizing process according to the invention can be carried out in a single continuous furnace, if desired.

The final step in sphero'idizing steel stock according to the invention is reheating to a temperature between its A01 temperature and a temperature not more than about F. below its Ari temperature and preferably above its Ari temperature, and maintaining the stock within such range for from about four hours to about ten hours. When the stock has been cooled, prior to this reheating, as described in the preceding paragraph, spheroidization proceeds rapidly at temperatures within such range, with the result that even hard to spheroidize stock is substantially completely spheroidized within the indicated time.

The attached drawing shows one specific iron-carbon diagram whereon the differences between the various critical points upon heating and cooling are represented. It is 'known that differences between these critical points are a function of the composition of the particular metal being studied. In consequence, in order to illustrate the various critical temperatures of all alloy steels, or even for all grades of carbon steels, it would be necessary to prepare a separate diagram for each such steel. However, it is generally true that the A1 change, which term is used herein and in the appended claims in its usual and accepted sense, namely to indicate the minimum temperature at which austenite forms or disappears as a. stable constituent of a steel, occurs at a higher temperature upon heating (AC1) that upon cooling (Ari), for any given steel. A detailed discussion of the thermal critical points for steel will be found in The Metallography and Heat Treatment of Iron and Steel, pages 110l28 of the 1926 edition, Albert Sauveur. The specific temperatures shown on the attached drawing must be considered as exemplary only, however, because of the differences in these specific temperatures which occur with difierences in composition.

If the particular temperatures, especially the AC1 and the Ar temperatures, of a particular stock which it is desired to spheroidize according to the invention are not known, they can readily be determined according to procedures described in the Sauveur reference, cited above. In many cases, however, the temperatures can be approximated with sufiicient accuracy from the attached drawing.

The following example is presented solely for the purpose of further illustrating and disclosing the invention, and is in no way to be considered as a limitation thereon.

Example Samples of A. I. S. I. 1030 steel were continuously spheroidized in a furnace schematically illustrated on the last page of a reprint entitled Continuous Short-Cycle Anneals for Spheroidization of Cartride-Case Steel, originally published in Metal Progress, July 1953. The Ac temperature of this stock was about 1340? 1 while the Ari temperature was about 1250 "F. The furnace controllers were setso that the stock was heated to about 1380 F. in approximately 3% hours in an initial heating zone, maintained at about 1380" F. for approximately two hours in an austenitizing zone, cooled to about 1245 F. in about 1 /2 hours and maintained at about 1245 F. for approximately hour in a nucleating zone, reheated to about 1325 F. and maintained at such temperature for approximately eight hours in a spheroidizing zone, and then cooled to about 1100 F. in approximately 2% hours in a final cooling zone. The work was then discharged from the furnace. Samples, after'polishing and etching, were examined and found to show substantially completely spheroidized structures.

It is known that the displacement of the critical temperatures of steels from the theoretical equilibrium critical temperatures as currently published will vary both with the composition of the steel and the rate of heating or cooling. In Fig; 2 this has been illustrated by the curves of the converging A01 and Ari lines. The superimposed annealing cycle curve of Fig. 2 will infact be a wavy. line due to the practical impossibility of maintaining constant furnace temperatures. It has been demonstrated that carbides, in the form of pearlite or cementite, will dissolve less readily into austenite than they precipitate therefrom, the classic demonstration of this being the carburization of steel to up to 3 /2 or so per cent carbon at about 1700 F. in less than one hour for thin steel strips, in a carburizing atmosphere whose carbon potential is limited by phase rule considerations to the Acm line, or about 1.2% carbon. Considering these facts, it is apparent that on'the sub-critical annealing portion of this process, the apparent AC1 line for the steel treated in a given furnace maybe higher than the text-book figures, since it is necessary only to periodically drop the work being annealed to below the AC1 critical temperature to cause the spheroidizing to proceed. In such a case the cementite which is agglomerated as spherules below the AC1 line as the temperature wavers therabout will not substantially redissolve into the austenite during the moments the work is actually above the theoretical Ac1 line. For this reason the illustrated Aci and Ari lines may appear too widely spaced in Fig. 2, but in fact they represent a close approach to the practical temperatures with which one must work in industrial heating equipment.

it will be apparent that the method of the invention is not limited to a spheroidizing cycle carried out in a furnace or furnaces, as salt or other liquid bath-s could equally well be used for carrying out the various heating operations involved therein. It will also be apparent that other changes and modifications can be made from the specific details disclosed and described without departing from the spirit of the attached claims.

I claim:

1. In a method for spheroidizing cementitious portions of steel stock which comprises austenitizing the stock by heating to a temperature at which cementite therein is converted to austenite, and cooling the stock from such temperature to a temperature below the Ari temperature for the stock, the improvement which consists in cooling the austenitized stock to a temperature below its Ar1 temperature, but not lower than about 1000 P. for a time sufiiciently long that a part of the austenite is converted to cementite nuclei, but not sufliciently long for the formation of a relatively stable pearl-itic structure, reheating the stock to a temperature between its A0 temperature and its Ari temperature, maintaining the stock at a temperature within the last-indicated range for from about four hours to about ten hours, and cooling the stock, whereby aspheroidized microstructure is achieved. 1

2. In a method for spheroidizing cementitious portions of steel stock which comprises austenitizing the stock by heating to a temperature at which substantially all cementite therein is converted to 'austenite, and cooling the stock from such temperature to a temperature below the Ari temperature for the stock, the improvement which consists in cooling the austenitized stock to a temperature below its Ari temperature but above about 1000 F. for a time sufficiently long that a part of the austenite is converted to cementite nuclei, but not sufficiently long for the formation of a relatively stable pearlitic structure, reheating the stock to a temperature between its AC1 temperature and a temperature not more than about 10 F. below its Ari temperature, maintaining the stock at a tempera-ture within the last-identified range for from about four hours to about ten hours, and cooling the stock, whereby a substantially completely spheroidized microstructure is achieved.

3. In a method for spheroidizing cementitious portions of steel stock which comprises austenitizing the stock by heating to a temperature at which substantially all cementite therein is converted to austenite, and cooling the stockfrom such temperature to a temperature below the Art temperature for the stock, the improvement which consists in cooling the austenitized stock to a temperature between the Art temperature and the Ac1 temperature, rapidly cooling the stock to a temperature below its Ari temperature but above about 1000 F. for a time such that only a portion of the austenite is converted to cementite nuclei, reheating the stock to a temperature between its A01 temperature and a temperature not more than about F. below its Ari temperature, maintaining the stock at a temperature within the last-identified range for from about four hours to about ten hours, and cooling the stock, whereby a substantially completely spheroidized microstructure is achieved.

4. In a method for spheroidizing cementitious portions of steel stock which comprises austenitizing the stock by heating to a temperature at which cementite therein is converted to austenite, and cooling the stock from such temperature to a temperature below the AH temperature for the stock, the improvement which consists in cooling the austenitized stock to a temperature below its All temperature, but not more than about 50 F. therebelow, maintaining the stock at a temperature within the last-identified range for a time sufficiently long that at least a part of the austenite is converted to cementite nuclei, reheating the stock to a temperature between its AC1 temperature and a temperature not more than about 10 F. below its Ari temperature, maintaining the stock at a temperature within the last-above identified range until a substantially completely spheroidized microstructure is achieved.

5. In a method for spheroidizing cementitious portions of steel stock which comprises austen-itizing the stock by heating to a temperature at which cementite therein is converted to austenite, and cooling the stock from such temperature to a temperature below the Art temperature for the stock, the improvement which consists in quenching the austenitized stock to a temperature below its Ari temperature, but above about 1000 F., maintaining the stock at a temperature Within such range for a time such that a part only of the austenite is converted to cementite nuclei, reheating the stock to a temperature between its A01 temperature and a temperature not more than about 10 F. below its Ari temperature, maintaining the stock at a temperature within the lastabove identified range for from about four hours to about ten hours, and cooling the stock, whereby a substantially completely spheroidized microstructure is achieved.

6. In a method for spheroidizing cementitious portions of steel stock which comprises austenitizing the stock by heating to a temperature at which cementite therein is converted to austenite, and cooling the stock from such temperature to a temperature below its Ari temperature, the improvement which consists in cooling the austenitized stock to a temperature below its Ari temperature, but not lower than about 1000 F. for a time suificiently long that at least a part of the austenite is converted to cementite nuclei, reheating the stock to a temperature between its AC1 temperature and a temperature not more than about 10 F. below its All ternperature, maintaining the stock at a temperature within the last-indicated range until a substantially spheroidized microstructure is achieved, and cooling the stock.

7. In a method for s'pheroidizing cementitious portions of steel stock which comprises austenitizing the stock by heating to a temperature at which cementite therein is converted to austenite, and cooling the stock from such temperature to a temperature below its Ari temperature, the improvement which consists in cooling the austenitized stock to a temperature below its Ari temperature, but not lower than about 1000" F for a time sufficiently long that at least a part of the austenite is converted to cementite nuclei, but not sufiiciently long for the formation of a relatively stabilized pearlitic structure, reheating the stock to a temperature between its A01 tempera ture and a temperature not more than about 10 F. below its Ari temperature, maintaining the stock at a temperature within the last-indicated range until a substantially spheroidized microstructure is achieved, and cooling the stock.

References Cited in the file of this patent UNITED STATES PATENTS 2,181,947 McCarroll Dec. 5, 1939 2,188,155 Payson Jan. 23, 1940 2,368,418 McCarroll Jan. 30, 1945 2,376,454 Schneider May 22, 1945

Claims

1. IN A METHOD FOR SPHEROIDIZING CEMENTITIOUS PORTION OF STEEL STOCK WHICH COMPRISES AUSTENITIZING THE STOCK BY HEATING TO A TEMPERATURE OF WHICH CEMENTIE THEREIN IS CONVERTED TO AUSTENITE, AND COOLING THE STOCK FROM SUCH TEMPERATURE TO A TEMPERATURE BELOW THE AR1 TEMPERATURE FOR THE STOCK, THE IMPROVEMENT WHICH CONSISTS IN COOLING THE AUSTENTITIZED STOCK TO A TEMPERATURE BELOW ITS AR1 TEMPERATURE, BUT NOT LOWER THAN ABOUT 1000* F. FOR A TIME SUFFICIENTLY LONG THAT A PART OF THE AUSTENITE IS CONVERTED TO CEMENTITE NUCLEI, BUT NOT SUFFICIENTLY LONG FOR THE FORMATION OF A RELATIVELY STABLE PEARLITIC STRUCTURE, REHEATING THE STOCK TO A TEMPERATURE BETWEEN ITS AC1 TEMPERATURE AND ITS AR1 TEMPERATURE, MAINTAINING THE STOCK AT A TEMPERATURE WITHIN THE LAST-INDICATED RANGE FOR FROM ABOUT FOUR HOURS TO ABOUT TEN HOURS, AND COOLING THE STOCK, WHEREBY A SPHEROIDIZED MICROSTRUCTURE IS ACHIEVED.