US3704183A - Method for producing a low-cost hypereutectoid bearing steel - Google Patents

Abstract

METHOD FOR PRODUCING A LOW-COST HYPEREUTECTOID BEARING STEEL HAVING CHARACTERISTICS EQUAL TO OR SUPERIOR TO HIGHER PRICED STEELS OF THIS TYPE AND CONTAINING AS ESSENTIAL ALLOYING ADDITIONS .80-1.10% CARBON, .50-1.00% MANGANESE, .20-.60% CHROMIUM AND .05-.25% MOLYBDENUM. A STEEL OF THIS TYPE, USUALLY IN THE FORM OF WIRE, ROD, BAR, FORGINGS, RINGS OR TUBING, IS ANNEALED AT A MAXIMUM TEMPERATURE OF 1400*F. FOR THRITEEN HOURS TO PRODUCE AN ANNEALED MICROSTRUCTURE COMPRISING SPHEROIDIZED CARBIDES IN A MATRIX OF FERRITE. AFTER FORMING INTO BEARING RACES OR BALLS, THE FORMED PRODUCT IS QUENCHED FROM 1550*F. AND THEREAFTER TEMPERED TO PRODUCE A STEEL HAVING A ROCKWELL C HARDNESS OF ABOUT 60 OR GREATER AND EXCELLENT FATIGUE STRENGTH.

Description

c. P. WEIG EL 3,704,183

METHOD FOR PRODUCING A LOW-COST HYPEREUTECTOID BEARING! STEEL Nov. 28, I972 Filed May 26, 1971 m mm INVYENTOR. CHARLES I? WEIGEL A! rarnoys United States Patent O ice 3,704,183 METHOD FOR PRODUCING A LOW-COST HYPEREUTECTOID BEARING STEEL Charles P. Weigel, Canton, Ohio, assignor to The Timken Company, Canton, Ohio Filed May 26, 1971, Ser. No. 146,911 Int. Cl. B22d 25/ C21d 7/14 US. Cl. 148-126 2 Claims ABSTRACT OF THE DISCLOSURE Method for producing a low-cost hypereutectoid bearing steel having characteristics equal to or superior to higher priced steels of this type and containing as essential alloying additions .801.l0% carbon, .50l.00% manganese, .20.60% chromium and .05.25% molybdenum. A steel of this type, usually in the form of wire, rod, bar, forgings, rings or tubing, is annealed at a maximum temperature of 1400 F. for thirteen hours to produce an annealed microstructure comprising spheroidized carbides in a matrix of ferrite. After forming into bearing races or balls, the formed product is quenched from 1550 F. and thereafter tempered to produce a steel having a Rockwell C hardness of about 60 or greater and excellent fatigue strength.

BACKGROUND OF THE INVENTION As is known, AISI 52100 steel has been used extensively, and almost exclusively, for ball bearing applications. Such a steel contains as essential alloying additions, .98-1.10% carbon, .25-.45% manganese and 1.30-1.60% chromium. The relatively high amount of chromium is added to assist in hardenability; however this materially increases the cost of the product.

In the manufacture of bearings from AISI 52100 steel, tubular shapes, bars, forgings and rings (for forming bearing races) and wire or rod (for forming balls) are initially annealed at about 1450 F., maximum temperature, and furnace cooled at a controlled rate for twenty-two hours, followed by air cooling. In the heat treating process, spheroidized carbides are formed in a matrix of ferrite to produce a product having good machinability, cold heading and/or cold forming properties. After the machining, cold heading or forming operation, the balls and races are quenched in oil from about 1550 F. and then tempered to achieve a Rockwell C hardness of 60 or greater. Of course, the steel must also have good fatigue strength.

While various alloys with about .6 or .7% carbon have been proposed in the past as low-cost substitutes for AISI 52100 bearing steel, these generally result in greater difiiculty in obtaining spheroidized annealed microstructures or fatigue strengths inferior to those of AISI 52100.

SUMMARY OF THE INVENTION The present invention resides in the discovery that a high carbon bearing steel can be prepared which contain substantially less total alloy content than conventional bearing steels, without sacrificing the hardenability, fatigue strength and processing characteristics of higher alloy bearing steels. At the same time, the annealing time required to achieve adequate carbon diffusion to produce a spheroidized microstructure is materially reduced with the steel of the invention.

Specifically, it has been found that the carbon content Patented Nov. 28, 1972 DESCRIPTION OF THE PREFERRED EMBODIMENTS The bearing steel of the invention has the following broad and preferred compositions:

Analysis, percent by Welg 11 Broad Preferred Carbon -1.10 .90 Manganese 50-1. 00 50-. 80 Sulfur 005-. 06 008. 04 Silicon. 20-. 35 .20. 35Chromiu 20-. 60 25-. 40 Molybdenu 05-. 25 05-. 10 Nickel .35 .35 Iron Balance Balance 1 Maximum.

Nickel and silicon are relatively unimportant in achieving the desired characteristics of the alloy and can be classified as residual amounts. .20-.35% silicon is that amount found in most alloy steels. Similarly, a certain amount of sulfur is found in every steel; however, it is desired to deliberately include .008-.04% sulfur to improve machinability and fatigue strength. As was mentioned above, carbon, manganese, chromium and molybdenum are the essential additions, the chromium and molybdenum acting to increase hardenability. In the preferred analysis given above, percent by Weight of carbon is however it should be understood that this can vary plus or minus five points, in accordance with usual metallurgical practices.

The steel of the foregoing composition is melted, formed into tubing, forgings, rings, bars, rod, wire or other shapes, and then annealed at a maximum temperature of 1400 F. for thirteen hours in order to achieve a spheroidized microstructure comprising spheroidized carbides in a matrix of ferrite. In the annealing step, the steel is passed through eleven zones of a roller hearth furance at a constant speed, the time in each zone being about oneeleventh of the total time of thirteen hours. The temperatures of the respective zones are shown in the following Table I:

TABLE I.ANNEALING TEMPERATURES IN ELEVEN ZONE ROLLER HEARTH FURNACE Zone l 2 3 4 5 Temperature, F

with the required annealing time of twenty-two hours at v,

3 a maximum temperature of 1450" F. for AISI 52100 steel. Furthermore, the shorter annealing time reduces the cost of the product and results in less scaling and less decarburization.

The annealed product is then cold headed or cold formed into balls and machined or formed into races which are thereafter heated to a temperature of 1550 F., followed by quenching in oil. In this condition, the product has a Rockwell C hardness of about 63-66. The final step in the process is to temper the product at 300 F. for about 1 to 2 hours to achieve a Rockwell C hardness of 60 or greater.

The resulting product has a density of 0.283 pound per cubic inch, a specific gravity of 7.83, a modulus of elasticity of 29 l0 pounds per square inch and a modulus of rigidity of 12x10 pounds per square inch. In addition, the product has excellent fatigue strength, comparable to that of the higher alloy AISI 52100 bearing steel and much better than AISI 5160 which is sometimes used as a low-cost substitute for hearing steels. This is shown in the accompanying drawing wherein curve 10 is that for the steel of the invention; whereascurve 12 is that obtained from fatigue tests conducted on AISI 5160 steel. The steel of the invention has an L fatigue life (i.e., a probability of 10% failures, or 90% survival) after about 10 million stress cycles; whereas AISI 5160 steel has an L fatigue life after only 6.5 million stress cycles. Note that the improvement in fatigue life persists at 50% survival. Thus, the steel of the invention has an L fatigue life of 45 million stress cycles as compared with 25 million stress cycles for AISI 5160.

Typical hardenability of the steel of the invention, determined with the Jominy hardenability test, is shown in the following Table II:

TABLE II.'IYPICAL HARDENABILITY (NO RMALIZE 1,656

F., ANNEAL, QUENOH 1,ao F.)

J distance from quenched end in) Me inch Rockwell C hardness atcorrespending point 65 65 65 61 47 43 43 43 43 41 Norm-Minimum hardness on 0.5 inch section heated to 1,525 F.=l= for 20 minutes and oil quenched, 63.0 Rockwell C.

The effect of the austenitizing temperature on the steel of the invention is shown in Table III:

TABLE IIL-HEAT TREATED PROPERTIES EFFECT OF AUSTENITIZING TEMPERATURE Note that the austenitizing temperature has very little effect on the as-quenched hardness. Finally, the eifect of the tempering temperature on the steel of the invention is shown in Table IV:

TABLE IV.-EFFECT OF TEMPERDNG TEMPERATURE (1,550 F. QUENOH) Hardness Rockwell C after- Iempering One- Twotemperature hour hour F.) l temper temper As-quenehed hardness; 65.0 Rockwell 0.

tion.

I claim as my invention:

1. In the method for forming a bearing element from an alloy steel consisting essentially of .80-1.10% carbon, .50l.00% manganese, .20.60% chromium and .05.25% molybdenum, the steps of: melting said alloy steel and forming it into a desired shape, annealing said alloy steel at a maximum temperature of 1400 F. for thirteen hours to produce a spheroiclized microstructure comprising spheroidized carbides in a matrix of ferrite, thereafter heating said article to a temperature of about 1550" F. and quenching, and finally tempering said article at a temperature of about 300 F. to achieve a Rockwell C hardness of about at least 60.

2. The method of claim 1 wherein said steel is annealed starting at a temperature of 1250 F. which increases during said thirteen hour annealing time to 1400 F. and thereafter decreases to a low of 1300 F. during said annealing time.

References Cited UNITED STATES PATENTS V HYLAND BIZOT, Primary Examiner U.S. Cl. X.R.

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