US3795551A - Case hardening steel - Google Patents

Abstract

A METHOD OF CARBURIZING AND CASE HARDENING STEELS WHICH REQUIRE SUBSEQUENT HEAT TREATMENT AT A TEMPERATURE ABOVE THE CARBURIZING TEMPERATURE, WHICH WOULD NORMALLY DIMINISH SURFACE CARBON AND SURFACE HARDNESS AND INCREASE CASE DEPTH BY DIFFUSION. THE PRESENT METHOD GAS CARBURIZES THE STEEL AT ABOUT 1700*F., FURNACE COOLS TO ABOUT 1450* F. WITH CONTINUED CARBURIZATION, QUENCHES IN OIL, TEMPERS, HARDENS AT ABOUT 1850*F., AND MARTEMPERS IN SALT.

Description

United States Patent O 3,795,551 CASE HARDENING STEEL Alan R. Swirnow, Brooklyn, N.Y., assignor to Curtiss-Wright Corporation No Drawing. Filed May 12, 1972, Ser. No. 252,720 Int. Cl. C21d 1/46; C23c 11/12 US. Cl. 148-15 16 Claims ABSTRACT OF THE DISCLOSURE A method of carburizing and case hardening steels which require subsequent heat treatment at a temperature above the carburizing temperature, which would normally diminish surface carbon and surface hardness and increase case depth by diffusion. The present method gas carburizes the steel at about 17 F., furnace cools to about 1450 F. with continued carburization, quenches in oil, tempers, hardens at about 1850 F., and martempers in salt.

BACKGROUND OF THE INVENTION This invention pertains to carburizing and heat treating steels to provide a hardened case, particularly steels which require heat treatment at very high temperatures. The carburizing of low alloy steels such as AISI 9310 to secure a hardened case is well known in the art. The practice with such low alloy steels generally has been to place semi-finished machined parts into a furnace and bring the furnace and the parts up together to the carburizing temperature of about 1650 F. to 1700 F. The parts are then gas carburized at that temperature for an appropriate length of time, depending on their configuration and the depth of case required. When carburization is complete the parts are withdrawn and pit cooled to room temperature, then heat treated for hardening at about 1450 F. to 1550 F., and finally tempered at about 300 F.

This procedure works reasonably well for ordinary low alloy steels, but for steels requiring heat treatment above the carburizing temperature it has certain weaknesses which render it unsuitable. For instance, there is some decarburization during the period of cooling after carburizing, amounting in elfect to outgassing or oxidation of surface carbon. With such loss of some of the surface carbon, and the subsequent diffusion of further surface carbon into the part during the period of heat treatment, the finished parts have improper case depth and some loss of surface hardness.

Although as stated, this procedure has been followed with ordinary low alloy steels, it is wholly inadequate with new steels requiring heat treatment at a temperature higher than the carburizing temperature, for instance, at about 1850 F. With loss of surface carbon during cooling, and with the very high rate of diffusion of surface carbon into the part at the high temperature of heat treatment, the finished parts would be unsatisfactory in case depth and might have substantially no enhancement of surface hardness.

SUMMARY OF THE INVENTION The introduction of new steels having higher tempering temperatures and higher heat treating temperatures than those previously known in the art has provided materials which have greater scufling resistance and which hold their hardness at higher operating temperatures than previous low alloy steels. Such higher quality steels require heat treatment well above the carburizing temperature, so that effective carburizing of them has heretofore been impracticable. The present invention provides an inte grated process of carburizing, tempering, and heat treating by means of which such steels can be consistently,

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DESCRIPTION OF THE PREFERRED EMBODIMENT Although the present method can be used with any steel which is heat treated at a temperature above the carburizing temperature, it will be described specifically as practiced on the steel sold under the trade designation of Vasco 1000-2 (also known as X-2), having the following nominal composition:

Percent Carbon 0.20

Silicon 0.90 Manganese 0.30 Chromium 5.00

Molybdenum 1.30 Vanadium 0.50 Iron Balance EXAMPLE I In the present example the workpiece is a transmission gear requiring high scufiing resistance and the ability to retain its hardness at higher operating temperatures than ordinary low alloy steel would resist. It is desired that it should have a final case depth between .035" and .045", and preferably about .040", after hardening, and a surface layer of approximately .005" thickness containing about .80% to .90% carbon, preferably about .85 after hardening, with a surface hardness of Rockwell C 60-64, preferably about R 62. To achieve this result it is desirable to achieve a case depth before hardening from about .023" to about .027", preferably about .025", with surface carbon in excess of .85%, preferably about 1.00% to 1.05%.

Following the method of the invention, the carburizing furnace is stabilized at 1700 F. (:10" F.) and at the desired carburizing potential, which in this case is 1.25% to 1.35% carbon in the form of carbon monoxide in the carburizing atmosphere. The carburizing atmosphere is of the conventional type, comprising endothermic carrier gas enriched by natural gas. The percentage of available carbon therein is determined and automatically controlled by an automatic infrared analyzer monitoring the carbon dioxide component of the carburizing atmosphere. Owing to equilibrium reactions in the gases, the proportion of carbon monoxide to carbon dioxide is constant at any given temperature; therefore, infrared monitoring of carbon dioxide readily provides a check and control of the carbon monoxide content.

When the furnace has been stabilized at the selected temperature and carbon potential, the workpieces at room temperature are charged directly into the furnace. The furnace is then allowed to restabilize at the selected temperature and the selected carbon potential, which means that the work will also have reached furnace temperature. carburizing time then begins. This procedure allows the work to be introduced directly into a carbon-rich atmosphere, avoiding the oxidizing potential which is inherent in retort furnaces during heating to the carburizing temperature.

The time of carburizing varies with several factors, such as goemetry of the piece, the desired case depth, and the desired percentage of carbon remaining at the surface after hardening. With case depth and carbon percentage kept constant, the geometry of the piece is an important factor varying carburizing time, which is determined by test pieces which are fractured and measured for surface carbon and case depth.

For the gear of this example, and with the desired parameters set forth above, the carburizing time, at about 1700 F. and a carburizing potential of 1.25% to 1.35% of available carbon, is from about 60 minutes to about 90 minutes, the actual time being determined by a test piece fractured and measured, and being in this case about 75 minutes.

The furnace control is then set down to 1450 F. and allowed to remain at that value until the work reaches thermal equilibrium with the furnace temperature, generally about one hour per half inch of thickness of the heaviest cross-section of the workpiece. During this furmace-cooling period the feed of carburizing gas is continued, maintaining the carbon dioxide setting at the same value as for the primary carburization, thereby increasing the carburizing potential of the furnace atmosphere and enriching the surface of the work with additional carbon, since at the lower temperature the proportion of carbon monoxide to carbon dioxide will increase, owing to the equilibrium reactions mentioned above. When the work has reached about 1450" F. (i F.) it is directly quenched into oil at a temperature of 160 F. to 210 F., after which it may be cooled to room temperature.

The foregoing procedure avoids the decarburization associated with slow air cooling, and allows the carburized layers to harden. Test specimens may be fractured at this point and analyzed for case depth and surface carbon to determine Whether the result after hardening will be according to specification, and any necessary adjustments in the carburizing procedure can be made at this time. Air cooled specimens as in the prior art could not be visually analyzed for case depth at this point, owing to lack of differentiation between the carburized layer and the core. In the present method, quenching at this point hardens the carburized layer.

Tempering may be commenced as soon as the parts have been quenched, but in any case should begin within not more than one hour from quenching. This is a socalled snap temper, carried out at a temperature of 300 F. to 325 F. for a minimum of two hours, and stress relieves the work so that the pieces can be handled without any danger of cracking, which might otherwise occur from stresses introduced during the oil quench. Test specimens may be analyzed at this time for case depth and surface carbon to determine whether adjustments should be made in the austenitizing time to follow.

The parts are allowed to air cool after tempering, and may be heat treated for hardening at any time thereafter. The workpieces are charged into a furnace at 1850 F. (i- 10 F.) and allowed to reach thermal equilibrium, which requires about one hour per half inch of thickness of the heaviest cross-section. The austenitizing time begins after thermal equilibrium is reached, and may vary somewhat depending on the results observed in fractured test specimens after carburizing.

Specimens which exhibit the optimum preliminary parameters after carburizing, that is, case depth from .023" to .027 and about 1.05% of surface carbon, will require about one hour of austenitizing at approximately 1850 F. to provide a final case depth of about .040" (i .005") and surface carbon of about .85%. Variations of case depth of less than i .005" in the preaustenitized specimens may be disregarded. However, for each .005" variation in the preaustenitized case depth a corresponding adjustment of about 1.5 minutes must be made in the austenitizing time. That is, if the preaustenitized case depth should be .005 less than nominal, austenitizing time must be increased about 15 minutes, and for a case depth .005" greater than nominal, austenitizing time may be decreased by about 15 minutes. Such adjustments cannot be carried very far, and if variation of the preaustenitized case depth exceeds .010" from the nominal value the parts should be recarburized.

After austenitizing, the parts are martempered according to conventional martempering procedure, that is, they are quenched from the austenitizing temperature in a molten salt bath at a temperature of about 450 F., held in the quenching medium until the temperature throughout the steel is substantially uniform, and then cooled in air.

After cooling to room temperature the parts are given a final temper as if they had been conventionally quenched, at a temperature from 600 F. to 625 F. for a minimum of two hours and cooled in air. This may be repeated in order to temper any retained austenite transformed during the previous temper.

EXAMPLE II In this example it is desired that the parts should have a final case depth after hardening between .025" and .035", preferably about .030", instead of the nominal case depth of .040" in Example I. Surface carbon and surface hardness after hardening are to be the same as in Example I, that is, about .85% and about R 62, respectively.

The steps of the procedure are carried out in the same order as before, but with some adjustment of the processing parameters to achieve the modified result. The carburizing temperature is the same as in Example I, 1700" F. (1- 10 F.), but the carburizing potential is higher, 1.35% to 1.45% of available carbon; The carburizing time is from about 35 minutes to about 55 minutes, preferably about 45 minutes. This produces a case depth of about .018" to .022" and surface carbon of about 1.0% to 1.05 after the carburizing step, the parameters being checked by fractured test specimens.

The Work is snap tempered as before, and austenitized at 1850 F. 10 F.). The austenitizing time, however. is between about 35 minutes and 55 minutes, preferably about 45 minutes, with adjustment one way or the other according to the measurements observed in the test specimens. This produces a final case depth in the workpieces of about .030", with surface carbon of approximately .85 After austenitizing the parts are marte'mpered, and then tempered or double-tempered as before, resulting in parts having a surface hardness of about R 60-64, preferably about R 62.

EXAMPLE III In this example the workpieces are to have a final case depth after hardening between .055" and .065, preferably about .060, with surface carbon of about .85% and surface hardness of about 'R 62, as before.

The carburizing temperature is the same as in the previous examples, but the carburizing potential is about 1.15% to 1.25% of available carbon, lower than that of Example I. The carburizing time is from about 2 hours to 3 hours, preferably about 2 /2 hours, producing a case depth after carburizing from about .035" to .045", preferably about .040", with surface carbon at about 1.0% to 1.05%. Test specimens are again fractured and examined.

The work is snap tempered, and austenitized at 1850 F. (:10" F.). The austenitizing time is approximately 2 hours, with a decrease of about /2 hour if the preaustenitized case depth is about .005" greater than nominal, or an increase of about /2 hour if the preaustenitized case depth should be about .005" less than nominal. The parts are then martempered, and given a final temper or double temper, exhibiting a final case depth of about .060", surface carbon of about .85 and surface hardness of R 60-64, preferably about R 62.

It will be seen that the foregoing procedures provide a method of predictably carburizing and hardening steel which requires heat treatment at a temperature considerably above the carburizing temperature. The method allows the production of cases of specified depth, and surface carbon layers to a specified percentage. Surface hardness of Vasco 1 000-2 steel processed in the foregoing manner is R 6064. Such results were not attainable by prior art processes, owing to the very rapid diffusion of carbon at the elevated temperatures of heat treating, resulting in increased case depth and loss of surface carbon, with insufficient surface hardness.

What is claimed is:

1. A method of case hardening alloy steel, comprising gas carburizing a workpiece at a selected carbon potential in a furnace at a temperature of at least about 1600 F., furnace cooling the workpiece to a lower temperature while continuing carburization at a higher carbon potential than the first selected carbon potential to enrich the surface carbon of the workpiece, quenching the workpiece in oil, austenitizing the workpiece at a temperature substantially above the first carburizing temperature, and martempering the workpiece by quenching it from the austenitizing temperature in a molten salt bath to stabilize the workpiece to a uniform temperature throughout the workpiece.

2. The method recited in claim 1,, wherein the first gas carburizing temperature is between about 1690 F. and about 1710 F., and the austenitizing temperature is between about 1840 F. and about 1860" F.

3. The method recited in claim 2, wherein after the first carburizing period at about 1690 F. to 1710 F. the workpiece is furnace cooled to about 1440 F. to about 1460 F. with continued car'burization during the cooling period.

4. The method recited in claim 3, wherein the oil quenching temperature is between 160 F. and 210 F.

5. The method recited in claim 4, wherein the workpiece is tempered at a temperature from about 300 F. to about 325 F. for a minimum of two hours beginning within one hour after quenching.

6. The method recited in claim 5, wherein the gas carburizing atmosphere contains from about 1.25% to about 1.35% available carbon during the first carburizing period, and the time of the first carburizing period is from about 60 minutes to about 90 minutes.

7. The method recited in claim 6, wherein after tempering the workpiece is charged into a furnace maintained at a temperature from about 1840 -F. to about 1860 F., allowed to reach equilibrium temperature with the furnace, then austenitized for about one hour, then martempered in a molten salt bath at a temperature between about 440 F. and about 460 F., and then air cooled.

8. The method recited in claim 7, wherein after martempering and air cooling the workpiece is given at least one temper at a temperature from about 600 F. to about 625 F. for a minimum of two hours.

9. The method recited in claim 5, wherein the gas carburizing atmosphere contains from about 1.35% to about 1.45% available carbon during the first carburizing period is from about 35 minutes to about minutes.

10. The method recited in claim 9, wherein after tempering the workpiece is charged into a furnace maintained at a temperature from about 1840 F. to about 1860 F., allowed to reach equilibrium temperature with the furnace, then austenitized for about 35 minutes to about 55 minutes, then martempered in a molten salt bath at a temperature between about 440 F. and about 460 F., and then air cooled.

11. The method recited in claim 10, wherein after martempering and air cooling the workpiece is given at least one temper at a temperature from about 600 F. to about 625 F. for a minimum of two hours.

12. The method recited in claim 5, wherein the gas carburizing atmosphere contains from about 1.15% to about 1.25% available carbon during the first carburizing period, and the time of the first carburizing period is from about two hours to about three hours.

13. The method recited in claim 12, wherein after tempering the workpiece is charged into a furnace maintained at a temperature from about 1840 F. to about 1860 F., allowed to reach equilibrium temperature with the furnace, then austenitized for about two hours, then martempered in a molten salt bath at a temperature between about 440 F. and about 460 F and then air cooled.

14. The method recited in claim 13, wherein after mar tempering and air cooling the workpiece is given at least one temper at a temperature from about 600 F. to about 625 F. for a minimum of two hours.

15. A steel workpiece case hardened in accordance with the method of claim 1.

1.6. A workpiece as recited in claim 15, wherein the workpiece is formed of Vasco 1000-2 steel.

References Cited UNITED STATES PATENTS 3,356,541 12/1967 Cullen 148-165 2,260,249 10/ 1941 Harder 148-165 2,266,565 12/1941 Ormsby 148-19 2,654,683 10/1953 McMullan 148-315 3,231,433 1/1966 Van Mater et al. 148-19 X FOREIGN PATENTS 745,013 2/ 1956 Great Britain 148-19 1,191,401 4/1965 Germany 148-165 OTHER REFERENCES Metals Handbook, 1948 ed., pp. 682-685. Alloy Digest; SA-62, October 1957, Vasconet 1000.

CHARLES N. LOVELL, Primary Examiner U.S. Cl. X.R. 148-165, 31.5, 134

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 795 ,551 Dated March 5 1974 Inventor(s) Alan R. Swirnow It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

IN THE SPECIFICATION:

Column 4., line 7; --about-- should be inserted after "exceeds' IN THE CLAIMS:

Column 6, claim 9; between lines 3' and 4 should be inserted -period, and the time of the first carburizing--.

Signed and sealed this 30th day of July 197A.

(SEAL) Attest: MCCOY M. GIBSON, JR. 0. MARSHALL DANN Attesting Officer Commissioner of Patents Us COMM-DC 60378-P69 w v.5 GOVERNMENT PRINTING DFFICE is" mass-3:4,

FORM PO-105O (IO-69)