US2619439A

US2619439A - Method of hardening rolls

Info

Publication number: US2619439A
Application number: US167163A
Authority: US
Inventors: James S Rennick
Original assignee: United States Steel Corp
Current assignee: United States Steel Corp
Priority date: 1950-06-09
Filing date: 1950-06-09
Publication date: 1952-11-25
Anticipated expiration: 1969-11-25

Description

Nov. 25, 1952 J. s. RENNlCK METHOD OF HARDENING ROLLS 2 IS'HEETSSHEET 1 Filed June 9, 1950 FIEA- FIEE- Nov. 25, 1952 Filed June 9, 1950 FIEJZ'L J. s. RENNICK 2,619,439

METHOD OF HARDENING ROLLS 2 SHEETSSHEET 2 8 s g I a 2- \1 Lu gs? & hwentm 3 R E 14/1455 5 Em/v/(z, a g 4 AMJ Patented Nov. 25, 1952 UNITED STATES PATENT OFFICE METHOD 'oF'HARDEN-ING-RoLLs New J 'ersey Application 31111159, 1950, Serial'No. 167,163

2 Claims. 1

This invention relates to improved methods of hardening steel rolls that have a central bore. I define the term roll or steel roll when used herein without further qualification as including steel back-up r011 sleeves, as well as "actual steel Work rolls and back-up rolls.

This application is a continuation-in-part of my earlier application Serial No. 12,493, filed March 2, 1948, now abandoned.

Steel rolls commonly are formed of hypercutectoid steel and contain alloying constituents, such as vanadium, molybdenum and chromium. The outer surface regions of such rolls are hardened to a substantially martensitic structure to resist abrasive forces encountered durin rolling operations, while the inner regions near the bore preferably are kept soft and tough to resist mechanical shocks. In steel work rolls the outer regions should be hardened at least to 58 Rockwell C, while in steel back-up rolls and sleeves these regions should be hardened at least to 40 Rockwell C. This hardness is obtained by heating the entire roll body at once for a sufiicient period for only the outer surface regions to reach the critical temperature for transforming the steel into austenite, and then quenching the entire roll body to transform the austenitized regions to a substantially martensitic structure. In order to furnish the necessary hardness it is essential that the quenched outer regions contain no appreciable amount of austenite nor its higher temperature transformation products, such as sorbite, troostite and bainite. It is essential also that the entire roll body be heated at once and quenched at once so that the hardened-layer is of uniform depth. Any non-uniformity causes a roll to wear unevenly and thus necessitatesits early replacement. Since the inner surface regions are kept soft, such regions must not be heated to the critical temperature.

In the usual steel roll hardening practice with which I am familiar, the roll is maintained at its critical temperature for only a relatively short period, commonly about minutes. This period is brief to prevent overheating the inner surface regions, but it is insufficient for any appreciable depth of the outer surface to reach the critical temperature for hardening. Usually the maximum depth of the hardened layer has been about inches, and I have observed in many instances this depth is insuificient to furnish a satisfactory roll life.

An object of the present invention is to provide improved methods for hardening steel rolls wherein the depth of the hardened layer at the outer 12 surface is increasedover that obtained by prior practices and yet the inner regions remain soft and'tough.

A further object of the invention is to provide improved methods for heat treating steel rolls wherein the roll bores are cooled artificiallywith gas-streamsin order that the outer reg-ionef the roll can be maintained above the critical temperature for a longer period, thus increasing the depth of the hardened region.

In accomplishing these and other objects of the present invention, I have provided improved details of structure, preferred forms of which areshown in the accompanyingdrawings where- Figure 1 is a vertical sectional View through a heating furnace illustratin the manner in which the method of the present invention can be applied to a steel work roll;

Figure'2 is a vertical sectional view through a heating furnace similar to Figure 1, butillustrating the method applied to a steel back-up roll sleeve; and

Figure 3 is a graph which-compares the hardness the method of the :present invention produces in a steel work r011 with-the hardness usual prior art methods produce. v

Figure -1 shows a heating furnace l0, whichcan be of any -conventional type for heating steel shapes above their critical temperature, and a steel work roll l2 therein undergoing heat treatment. Said roll comprises a body I3, shoulders I4 and necks l5, and has a centralbore [5. The outer surface region of body I 3 is to be hardened, while the rest of the roll is kept soft and tough. During the heat treatment the outside of the shoulders and necks are shielded with insulating material, preferably asbestos covered :with sheet metal covers H to prevent their being "heated to the critical temperature. The work roll issupported in a stationary positionon stools 18 that preferably engage the'shielded necks.

In accordance with 'the present invention, a pipe l9 enters the furnace through an opening 20 in the furnace wall and extends throug-h bore l6 of the work roll, being in spaced relati'on to the bore surface. Said pipe has a multiplicity of orifices 2i throughout the portion of its length that is Within said bore. A stream of cooling gas passes through said pipe and impinges on the inner surface of the roll at the bore. The cooling gas conveniently can be compressed air.

The furnace heats the entire outer surface of the roll body at one time uniformly beyond the critical temperature, which in hypereutectoid alloy steel used in rolls is around 740 C. The streams of cooling gas maintain the bore surface close to the critical temperature but at all times below. I have found that the outer surface can be held indefinitely at temperatures above the critical while the bore is maintained at any desired lower temperature. By proper adjustment of the gas streams, the bore temperature can be controlled very accurately. The very long heating periods possible according to the present invention contrast with the 10 minute period possible when the bore is not artificially cooled. I prefer to heat the outside to temperatures of about 925 C. since in alloy steels used for rolls the amount of carbon which goes into solution to form austenite is a function of temperature. Higher temperatures than about 925 C. are undesirable since they tend to form austenite so high in hardenability that it does not transform to martensite upon subsequent quenching, but is retained as austenite. In a typical operation according to the invention the entire outside of the roll body is maintained at about 925 C. for about 105 minutes, while the bore temperature is maintained at about 740 C.

Following the heating just described, the entire steel roll body is quenched cold at one time, preferably with water, whereupon the outer regions, which have been heated beyond the critical temperature, are transformed to a substantially martensitic structure. In work rolls this outer region has a hardness of at least 58 Rockwell C, and with a heating period of 105 minutes is about inches in depth. The hardened layer is free of austenite and its higher temperature transformation products such as sorbite, troostite and bainite. The inner regions including the bore surface remain soft and tough to resist mechanical shocks.

Figure 3 shows graphically the benefits obtained by use of the present invention as applied to steel work rolls. This figure shows curves which compare roll hardness produced by the method of the present invention and that produced by prior art methods, such as I have described. Line A indicates the lowest usable hardness for steel work rolls, 58 Rockwell C. This showing demonstrates that the method of the present invention increases the depth of usable hardness from about inches to inches, while leaving the interior soft and tough.

Figure 2 shows the method of the present invention applied to a steel back-up roll sleeve. This figure shows a similar furnace i0, and a steel back-up roll sleeve 12a therein undergoing heat treatment. In this instance the sleeve ends are shielded, as indicated at Ila, and the sleeve is supported on stools I8. The arrangement of a pipe [9 which has orifices 2| for admitting cooling gas is the same as described in connection with Figure 1. The invention is practiced in the same fashion as with the work roll, except that such sleeves can be of a lower carbon steel and the outer layers of the sleeve are not used at as great a hardness. They are tempered to a hardness of about 40 to Rockwell C.

Specific examples of roll hardening according to the methods of the present invention are as follows:

Example 1 The roll was a steel work roll of the following analysis:

0:.90 Mn:.45 P:.004%

4 S=.016% Si=.37% Ni=.09% Cr=l.80% M0:.21% V=.18% balance iron and incidental impurities The roll was placed in a furnace as illustrated in Figure 1. The furnace was brought to a temperature of 925 C. in 2'70 minutes and held at this temperature for 105 minutes. Meanwhile the bore was held at a temperature of 725 C. by impinging streams of compressed air against its surface. The roll then was removed from the furnace, quenched with a Water spray, and was drawn or tempered at 170 C. for 18 hours. The outer surface had a hardness of -97 Shore, which is in excess of 58 Rockwell C. It consisted of approximately 90% martensite.

Example 2 The roll was a steel back-up roll sleeve of the following analysis:

02.55 M11 25 P::.U05% S:.018% Si:.23 Ni:2.31% Cr=3.58

balance iron and incidental impurities The roll was placed in a furnace as illustrated in Figure 2. The furnace was brought to a temperature of 875 C. in 17% hours and held at this temperature for 120 minutes. Meanwhile the bore was held at a temperature of 735 C. by impinging streams of compressed air against its surface. The roll then was removed from the furnace, quenched with a water spray, and was drawn or tempered at 490-495 C. for 10 hours. The outer surface had a hardness of 62-63 Shore or 42, 43 Rockwell C. It consisted of approximately 90% tempered martensite.

From the foregoing description, it is seen that the present invention affords a simple but effective method for increasing the depth of the hardened outer layer of a steel roll, and thereby increasing roll life.

While I have shown and described only certain preferred embodiments of the invention, it is apparent that modifications may arise. Therefore, I do not wish to be limited to the disclosure set forth but only by the scope of the appended claims.

I claim:

1. A method of uniformly hardening the outer surface regions of steel rolls, which are of a hypereutectoid steel capable of hardening when heated to a critical temperature and quenched and which include a cylindrical body and have a central bore, comprising supporting such a roll within a furnace, heating the entire outside of the roll body at one time within the furnace with the furnace and the outside of the roll body being maintained at a temperature between the critical and 925 C. for a minimum period of approximately minutes, controlling the bore temperature so that it remains close to the critical but at all times below by impinging streams of cooling gas on the bore surface, and quenching the entire roll at one time to produce on the roll body a hardened outer layer which has a hardness of at least 40 Rockwell C, a uniform depth greater than inch and a substantially martinsitio structure, the inner regions of the bore including the bore surface remaining soft and tough to resist mechanical shock.

2. A method as defined in claim 1 in which the roll is a work roll and the hardness produced in the outer layer is at least 58 Rockwell C.

JAMES S. REN'NICK.

REFERENCES CITED The following references are of record in the file of this patent:

Number Number 6 UNITED STATES PATENTS Name Date Unger Aug. 19, 1913 Schonemann Apr. 27, 1915 Scott Nov. 18, 1924 Kinzel Feb. 17, 1942 FOREIGN PATENTS Country Date Great Britain of 1887

Claims

1. A METHOD OF UNIFORMLY HARDENING THE OUTER SURFACE REGIONS OF STEEL ROLLS, WHICH ARE OF A HYPEREUTECTOID STEEL CAPABLE OF HARDENING WHEN HEATED TO A CRITICAL TEMPERATURE AND QUENCHED AND WHICH INCLUDE A CYLINDRICAL BODY AND HAVE A CENTRAL BORE, COMPRISING SUPPORTING SUCH A ROLL WITHIN A FURNACE, HEATING THE ENTIRE OUTSIDE OF THE ROLL BODY AT ONE TIME WITHIN THE FURNACE WITH THE FURNACE AND THE OUTSIDE OF THE ROLL BODY BEING MAINTAINED AT A TEMPERATURE BETWEEN THE CRITICAL AND 925* C. FOR A MINIMUM PERIOD OF APPROXIMATELY 105 MINUTES, CONTROLLING THE BORE TEMPERATURE SO THAT IT REMAINS CLOSE TO THE CRITICAL BUT AT ALL TIMES BELOW BY IMPINGING STREAMS OF COOLING GAS ON THE BORE SURFACE, AND QUENCHING THE ENTIRE ROLL AT ONE TIME TO PRODUCE ON THE ROLL BODY A HARDENED OUTER LAYER WHICH HAS A HARDNESS OF AT LEAST 40 ROCKWELL C. A UNIFORM DEPTH GREATER THAN 3/8 INCH AND A SUBSTANTIALLY MARTINSTIC STRUCTURE, THE INNER REGIONS OF THE BORE INCLUDING THE BORE SURFACE REMAINING SOFT AND TOUGH TO RESIST MECHANICAL SHOCK.