US2260249A - Case carburizing - Google Patents

Description

Oct. 21,1941. o. E. HARDER CASE CARBURIZING Filed Aug. 51, 1959 Fig. 1

Fig. 5

INVENTOR Oscar E. Harder? ATTORNEYS Patented Oct. 21, 1941 CASE CARBURIZINGv Oscar E. Harder, Columbus, Ohio, assignor to BattelleMemorial Institute, Columbus, Ohio, a

corporation of Ohio Application August 31, 1939, Serial No. 292,928

5 Claims.

My invention relates to case carburizing. It has to do with a method of case carbur'izing which will impart to case carburized steel a wear resistance superior to the wear resistance obtainable by methods known to the prior art. My method is particularly applicable to the carburizing of such articles as ball and roller bearings or gear and other machine parts, though it is not limited thereto.

In the prior art, relating to case carburizing and heat treating machine parts, it is common practice to carburize the work in a pack of carburizing material or' in a suitable gaseous atmosphere at a temperature of about 1600 to 1800 degrees F. for a period of time of about four to sixteen hours, depending upon the desired carbon content and the desired depth of case. The work may then be cooled slowly in the furnace or it may be quenched directly from the carburizing container. Slow cooling causes the separation of carbides at the boundaries of the austenite grains in cases containing carbon in excess of about 0.90 per cent. Carbides distributed in this manner tend to make the material brittle and such carbides are difficult to break up in later heat treatments. Quenching from the carburizing container introduces severe stresses which are likely to cause excessive Warpage and breakage. This is particularly true if the carburizing temperature is high.

If slow cooling is used, it is then necessary to reheat for grain refining and hardening. In some instances there is first a reheating to a relatively high temperature to refine the structure of the core and then to a lower temperature to refine the case, after which the work is quenched for hardening. In carrying out these heat treatments many problems are encountered and finally it is difficult, if not impossible, to secure spheroidized carbides embedded in a hard matrix for Wear resistance. This is also difcult, if not impossible, to obtain in the work which is quenched from the carburizing container.

In my method the steel parts are carburized at a relatively high temperature and under such conditions that a hypereutectoi-d case is produced. The depth of case is controlled by the carburizing time and t some extent by the temperature employed. I then lower the temperature of the carburizing container and the work so `as to precipitate out the carbides in excess of the eutectoid composition but still retaining the work in the carburizing container. When this reaction is completed the container and the which have precipitated. It is particularly desirable to break up and spheroidize the carbides which have precipitated in the grain boundaries. Thus reheating is also a continuation of the carburizing process, but generally at a lower temperature. This results in some diffusion of the carbon in solution in the austenite into the work being processed so that there is a more gradual gradation. in carbon content from the case to the core; the carbides are spheroidized; and se- Y lected carbon contents in the case are obtained by controlling such factors as the carburizing temperature' and the carburizing medium;

Instead of just cooling the container and the work to the eutectoid temperature which precipitates out the hypereutectoid carbide, it is advantageous for certain .purposes to cool to still lower temperature where fthe steel will be converted to pearlite, thus precipitating practically all of the carbides. This refines the. grain of the steel and provides more carbides for spheroidization on reheating. It will be understood by I those skilled in the art that when the steel is cooled below the transformation temperature bides will be redissolved but the amount of carbides dissolved can be controlled by selecting the reheating temperature and the time at that temperature. My process will be more readily understood from the examples which will be given hereinafter.

While my invention is applicable to the various carburizing steels, both plain carbon and alloy, the steps in the process and the results obtained y win be iuustratea by a steel known as s. A. E.

4620 which is one of the most extensively used carburizing steels. The typical composition of this steel is .15 to .25 per cent C, .40 to .70' per cent Mn. 1.65 to 2.00 per cent Ni and 0.20 to 0.30 40 per cent Mo.

VEau/imple 1 A specimen was carburized in a natural gas atmosphere at 1650 degrees F., cooled to 1200 degrees F. in forty minutes, held at that temperature for five minutes and then quenched in water. The specimen was then tested for hardness 'and' microstructure. The hardness was 62.5 Rockwell C and the structure showed essentially a continuous carbide network at the grain boundary, demonstrating that insuicient time had been used for spheroidization of the carbides. Example 2.-A specimen was processed as in Example 1, except that after cooling to 1200 degrees F. it was reheated to 1500 degrees F. and

work are reheated to spheroidize the carbides held at that temperature for two hours, after and then reheated some of the precipitated car` i which it was quenched in water and examined for hardness and microstructure. The hardness was 63.5 Rockwell C and the carbides were Well spheroidized. The structure seemed well suited to resist wear because of its hardness and the amount and distribution of spheroidized carbides.

Example 3.-A specimen was processed as in l Example 1, except after cooling to 1200 degrees l F.'to precipitate the excess of carbides it was heated to 1600 degrees F. held at that temperature for one-half hour,'quenched in Water and then examined for hardness and microstructure.

The hardness was 63.5 Rockwell C and the carbides were spheroidized but there had been too much resolution of the carbides in one-half I hour at 1600 degrees F: for what is considered the best structure for wear resistance in most types of service. f

Example 4.-A specimen of S.A.E. 4620 steel l was carburized at 1700 degrees F. sixteen hours,

l Rockwell C and still appeared to have a suflicient amount of spheroidized carbides to prol vide a good wear resisting surface. Cooling the steels of Example 4 to 800 degrees F. and holding th'em one-half hour at that temperature effected the austenite to pearlite transformation, thus precipitating practically all of the carbides.

While many other tests were made in the course of this research and development, the procedures and the results presented in Examples 1 to 4 are thought to give enough infomation to enable those skilled, in the art to practice my invention, important features of which are outlined in the following major steps:

1. Carburize the steel under such conditions that a hypereutectoid case is produced. By hypereutectoid case is meant a carbon content which on slow cooling will deposit outl excess carbide'before the'austenite to pearlite transformation takes place. Carburizing temperatures of 1550 to 1800 degrees F. may be'used but the range of 1650 to 1750 degrees is generally v preferable.

2. Cool in carburizing container to precipitate excess of carbides. In one method in step 2 the steel is not cooled into -the region in -which transformation fromA austenite to pearlite takes place, but to a temperature slightly above this point. This temperature willvary with the type of steel but for S. A. E. 4620 is about 1300 to 1100 degrees F. Good results were obtained on cooling to -1200 degrees F., but when the Specimens were cooled to 1000l degrees F. and held at that temperature for one-half 'hour 'some of the austenite to pearlite transformation took place.-

In this step .2, a non-oxidizing environment should be maintained. f

3. Step 2 may be modied so as to cool in the carburizing container to precipitate the excess of carbides and to effect the austenite to pearlite 4 transformation. This procedure jis preferred when it is desired to combine a grain rening treatment with the process' of producing-the spheroidized carbides in a hard matrix. The

temperature of the austenite to pearlite transformation depends upon the composition of the steel, the .temperature to which it has been heated and the time of holding at any given temperature. With the S. A. E. 4620 steel no transformation took place in five lminutes at 1200 degrees F., but there was partial transformation on cooling to 1000 degreesi F. and the transformation was practically complete on cooling to 800 degrees4 F. and holding one-half hour at that temperature. Step 3 is preferred when'it is necessary to effect the best grain refinement in the Work 'being processed. As in step 2, a non-oxidizing environment should be maintained.

4. Reheat the work in the carburizing container to spheroidize the carbides and to adjust the carbon content of the case. The factors involved are the composition-of the steel, the desired amount of spheroidized carbides and the carbon content of the matrix. The desired results are obtained by selecting the reheating temperature and the time of holding at Asuch temperature. With S. A. E. 4620 steel good results have been obtained by reheating at 1550 degrees F. for one-half to two hours. Heating to 1600 degrees F. tended to redissolve too much of the carbides whileheating twenty minutes at 1500 degrees F. did not result in satisfactory spheroidization of the carbides and they tended to outline the austenite grains. The reheating time to produce the desired structure decreases as the temperature increases. In general, the temperature to be used in reheating in this step 4 will be lower than that used in the original carburizing of step 1. This reheating step in the process serves to cause some diffusion of the carbon from the case into the core and can be made to effect a more gradual gradation in the carbon content from this outer case to the core which is generally advantageous. Carburization during the reheating can be adjusted to regulate the carbon content of the outer case within rather wide limits. It is generally desirable to maintain a carburizing environment during reheating.

5. Quench as in usual hardening practice.

The quenching operationr may be carried out in lvarious ways by means familiar to those skilled in the art. In general, it will be preferable to quenchv directly from vthe carburizing container and at the temperature used in the reheating in step 4 as described previously. Because of the relatively lower carbon content ofthe matrix iii-carrying out my invention, there is less danger of cracking and warping the steel than in the prior art practice, Yet by my process it. is possible to have suflicientcarbon in solution in the austenite at the time fof quenching to obtain full hardness and to have spheroidized carbides embedded in the matrixjof the case for improved wear resistance. y

6. Temper to desired hardness and mechanical properties. My process does not require a special means or method of tempering or drawing. Any well-known prior art methods may be used.

It will be seen from the above that I have pro-v `spheroidized carbides embedded in the matrix thereof. In preferred form, these spheroidized carbides are scattered throughout the grains and possibly in the matrix boundaries, though A it is novel with me to produce spheroidized carbides in the grain boundaries in contrast to stringers of carbides or a complete carbide envelope as in the prior art. An important aspect of my invention consists in the fact that I am able to produce a case carburized article which is devoid of continuous or substantially continuous carbide stringers in the grain boundary, which carbide stringers tend to make the material brittle yand to increase the diiculty of breaking up such carbides even by later heat treatments. Another important aspect of my invention is that it permits of the spheroidization of the carbides and the distribution thereof so as to produce a hard, wear resistant case on carburizing grades of steel. In order to illustrate the difference between prior art case carburized articles and case carburized articles made by my method, I am appending hereto photolithographed figures of photomicrographs of carburized cases whereinz' Figure 1 is a photomicrograph, at 500 diameters, of a specimen of S. A. E. 4620 steel which has been gas carburized at 1650 degrees F. and cooled in the furnace in forty minutes to 1200 degrees F. It was then held at 1200 degrees F. for five minutes, water quenched and subjected to a picric acid etch. The outer zone of the case ,showed a Rockwell C hardnessI of 62.5. This method is substantially identical with one of the prior art methods. This photomicrograph shows the separation of the carbides to the grain boundaries in a continuous network. Such a structure is known to be brittle because of the hard and brittle nature ofthe cementite surrounding the grain boundaries.

Figure 2 isa photomicrograph, taken at 500 diameters, of a steel of the same composition as that shown in Figure 1. However, this steel has been gas carburized at 1650 degrees F., cooled in forty minutes to 1200 degrees F. and held at 1200 degrees F. for ive minutes. It was then heated in t-wenty minutes to 1500 degrees F. and held at 1500 degrees F. for one hour, being then water quenched and subjected to a picric acid etch. The outer zone of the case showed a Rockwell C hardness of 63.5. However, it will be noted that the carbides in the grain boundaries have been broken up and spheroidized to a considerable extent and some of the carbide particles now appear as rounded masses and there is discontinuity of the envelope, which will decrease the brittleness and increase the wear resistance of the case. This is a product of one form of my method, though -not of the preferred form.

Figure 3 is a photomicrograph, at 500 diameters, of a steel of the same composition as those shown in Figures 1 and 2 which has been gas carburized at 1650 degrees F., cooled in forty minutes to 1200 degrees F., held at, 1200 degrees F. for ve minutes and heated in twenty minutes to 1500 degrees F. However, it was then held at 1500 degrees F. for two hours, water quenched and subjected to a picric acid etch. The outer zone of the case shows a Rockwell C hardness of 63.5. This is illustrative of the type of carburized case which can be made by my preferred form of method and it will lle` notedthat the carbides in the grain boundaries are entirely broken up and spheroidized and that spheroidized carbides occur throughoutthe grains themselves. (Because of the fact that the carbides are discontinuous, well distributed and in rounded form they do not cause the material to be brittle. but serve in an effective way to resist wear.)

1550 degrees F. in forty minutes.

Figure 4 a photomicrograph, at 500 diameters, of a piece of steel ot the same composition as that shown in Figure 1 which has been gas carburized sixteen hours at 1700 degrees F., to

produce a hypereutectoid case, cooled to 800 degrees F. in two hours, held at'800 degrees F. for one-half hour. This represents the procedure of cooling the specimen down below the temperature at which the austenite is mostly transformed to pearlite. The specimen was then heated to It was then held at 1550 degrees F. for two hours and water quenched. It :showed a Rockwell C hardness of 63 as quenched.` This figure shows the microstructure after the specimen has been tempered for one hour at 600 degrees F. so as to darken the matrix which is tempered martensite and to make the carbide particles stand out, in relief as the white areas. This is another representation of the type of structure to be producedby this invention. The dark matrix has adequate hardness but relatively good toughness. The carbide particles are well distributed and in more or less round form. They are definitely not continuous as in Figure 1 but are discontinuous as in Figure 3 and this is a structure which is desired for relatively good toughness associated with good wear resistance which are objects of this invention.

Thus, it will be seen that I have provided a novel method of case carburizing which results in breaking up and spheroidizing the carbides which under certain prior art methods appear as stringers in the grain boundaries and that my method can be utilized to distribute these spheroidized carbides throughout the grains as well as in the grain boundaries so that the carburized case is rendered more Wear resistant and less brittle.` Moreover, my method is such that a controlled diffusion of the carbon in solution in the austenite can be effected with a resultant gradual gradation in carbon from the case to the core. Likewise, as pointed out, slight modifications of my method may be utilized to' precipitate the excess of carbides and to eiect the austenite to pearlite. transformation by which a grain rening treatment is combined with the production of the spheroidized carbides in a hardmatrix.

In this specificationand in the appended claims the term carburizing container is used to designate the chamber in which the carburizing process is carried out and may be a carburizing box or pot, a retort, or a furnace in which the carburizing medium may be a gas, a solid, a fused bath or a combination of these.

Having thus described my invention, what I claim is:

1. A method of case carburizing steel, which comprises carburizing to produce a hypereutectoid case, cooling in the carburizing container to precipitate hypereutectoid carbides in combined form. in a matrix substantially free from graphite, reheating suiciently to spheroidize precipitated hypereutectoid carbides substantially without formation of graphite, and subsequently cooling, thereby forming a high-carbon case characterized by having a substantial portion of its carbon content in thc form of spheroidized carbides embedded in a matrix substantially free from graphite.

2. The method of claim 1, wherein said reheating is effected while the steel remains in the car` burizing'container and the final cooling is applied to the steel immediately after withdrawal therefrom.

3. The method of claim 1, wherein the final cooling is eiected byl quenching, to form a hard matrix having said spheroidized carbides embedded therein.

4. The method of claim 1, wherein the initial cooling precipitates eutectoid carbides with said hypereutectoid carbides, and wherein said eutectoid carbides are redissolved during the said reheating.

5. A method of case carburizing steel, which comprises carburizing to produce ahypereutectoid case, cooling in the carburizing container to precipitate substantially all of the hypereu- OSCAR E. HARDER.

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