US2779698A - Method of improving machinability of steel - Google Patents

Description

United States Patent METHOD OF IMPROVING MACHINABILITY or STEEL it Robert W. Baughman, Youngstown, Ohio, and Raymond A. Grange, Livingston, N. J., assignors to United States Steel Corporation, a corporation of New Jersey No Drawing. Application November 4, 1955,

Serial No. 545,112

2 Claims. (Cl. 148-215) This invention relates to a method of improving the machinability of steel and more particularly to improving the machinability of carburizing grades of steel.

Many machine parts, such as gears, are made of low carbon steel containing less than 30% carbon and are carburized to produce a hard wear resistant surface with a tough low carbon core. The parts are ordinarily machined to final size or to approximate size prior to carburizing and afterwards finished to final size by grinding or lapping. Since the finished parts must be quite smooth, the smoothness of the surface following machining is of great importance since it determines the amount of expensive grinding or lapping required to achieve the de sired degree of smoothness.

Heretofore it has been common practice to normalize or normalize and temper these steels prior to machining. This frequently causes a gummy condition which results in a rough machined surface because the steel is quite soft and is striated, i. e. contains bands of ferrite.

We have discovered that the rough machined surface resulting from the foregoing treatment can be eliminated, and a smooth unpitted surface obtained by treating ferritic steel containing less than 3% carbon, with the ,balance iron and other elements in amounts which do not adversely affect the properties in the following manner prior to machining. Such steel in the as-rolled or as-forged condition is heated or austenitized at a temperature determined by the formula when TA is the furnace temperature, Ael the lower equilibrium temperature and Airs the upper equilibrium temperature. The material should be held in the furnace until the temperature of the workpiece is equilized throughout at suchtemperature. By using the temperature derived by the formula, the workpiece will have a structure of about.50% austenite -50% ferrite, 110%. This structure is essential since if less austenite than'this is formed the final structure will be too soft and machine in a gummy fashion; if more austenite than this forms, the final structure will be too hard. After the temperature has been equalized at the TA holding temperature, the workpiece may be cooled in any convenient way to the bainite formation range, i. e. about 500 to 1000 F. for steels containing up to 3% carbon. The steel must be coo-led slowly enough through this range to insure essentially complete transformation to bainite. In some steels, depending on alloy content, an air cool will suffice,

in others retarded cooling will be necessary. Such retarded cooling may be accomplished by moving the workpiece to a cooler section of the furnace, moving to a second furnace or by turning the furnace down to about 500 F, i. e. about the M5 point of the untransformed austenite and permitting the temperature of the workpiece to equalize thereat and hold for any additional time as may be required to complete the transformation to temperature in any convenient manner.

2,779,698 Patented J 29, .1957- A sample of the steel so treated was quenched to room temperature and had a microstructure of about 50% ferrite and 50% austenite. Following this, the bars were cooled in the furnace from 1410 to 500 F. and then removed and cooled in air to room temperature. During cooling from 1410 to about 1000 F., about half of the austenite formed at 1410" F. transformed to additional ferrite leaving comparatively small uniformly distributed volumes of austenite. The latter are greatly enriched in carbon by the rejection of ferrite, that is, most of the carbon of the steel is present in the austenite phase. On cooling from 1000 to 500 F. most of the austenite transformed to bainite leaving small, uniformly distributed colonies of bainite. The few small particles of what was either austenite or martensite which remained were too few and too small to appreciably influence machining.

Bars of SAE 4317 steel of the same composition as the foregoing were treated in the conventional way by normalizing from 1700 F. and then tempering at 1200 F. These bars had a severely banded structure and were much rougher after machining. i

It was also observed that the bars treated in accordance with this invention had more broken-up chips, especially, on the lighter cuts, and did not become hot indicating that less energy was required. They also sawed in about /6 less time than the normalized and tempered bar.

While we have described one specific embodiment of our invention, it will be understood that this embodiment is merely for the purpose of illustration and description and that various other forms may be devised within the scope of our invention, as defined in the appended claims.

We claim:

1. A method of improving the machinability of ferritic steels containing less than 3% carbon with the balance iron and other elements in amounts which do not adversely affect the properties, comprising heating said steel at a temperature determined by the formula TA ==Ae1+% (Ass-A21) wherein TA is the furnace temperature Aei is the lower equilibrium temperature Aes is the upperequilibrium temperature to partially austenitize it, cooling said steel to the bainite transformation range, cooling through'such range at a sufliciently slow rate to insure essentially complete transformation of the austenite to bainite and thereafter cool- TA =A1-l-%(A3Al) wherein i TA is the furnace temperature Aer is the lower equilibrium temperature A23 is the upper equilibrium temperature for sufiicient time to equalize the temperature of the steel References Cited in the tile of this patent at said TA temperature to produce a structure therein UNITED STATES PATENTS which is substantially 50% austenite and 50% ferrite, cooling said steel to below about 1000 F. and cooling grabs 1 said steel through its bainite transformation range at a 5 2664369 122: 1953 sufiiciently slow rate to insure essentially complete transformation of the austenite to bainite and thereafter cool- OTHER REFERENCES ing it to room temperature. Metal Progress, October 1949, pages 504-505.

Claims (1)

1. A METHOD OF IMPROVING THE MACHINABILITY OF FERRITIC STELLS CONTAINING LESS THAN .3% CARBON WITH THE BALANCE IRON AND OTHER ELEMENTS IN AMOUNTS WHICH DO NOT ADVERSELY AFFECT THE PROPERTIES, COMPRISING HEATING SAID STEEL AT A TEMPERATURE DETERMINED BY THE FORMULA