US3130614A - Method for controlling residual stresses in metal - Google Patents

Description

April 28, 1964 s. NACHTMAN ETAL 3,130,614

METHOD FOR CONTROLLING RESIDUAL STRESSES IN METAL Filed Oct. 5, 1957 3 Sheets-Sheet l FIG-Z fggaz Roll 1 W F Roll 5 E i 3: LE J 5 INVENTORS 511702 5 7hc/ziman BY fiaialislifilnkowskz dziorneys United States Patent 3,130,614 METHUD FUR CGNTROLLDIG RESIDUAL STRESSES IN IVETAL Eliiut S. Nachtman, Park Forest, and Natalis H. Polaltowski, Wilmette, IlL, assignors to La Salle Steel Co., Hammond, 11111., a corporation of Delaware Filed Oct. 3, 1957, Ser. No. 688,047 4 Claims. (CI. 80-60) This invention relates to the improvement of the residual stress characteristics of metals during processing in a cold finishing operation and it relates more particularly to the control of residual stresses in steel by a rolling operation which can be employed in the cold finishing of such steels.

This invention has application to steels and other wrought metals of various structural forms, such as bars, rods, wires, tubes and the like, and of various structural shapes, such as rounds, squares, rectangles, triangles, hexagons, and the like. While the concepts of this invention include metals other than steel, the most important industrially is the application of the inventive concept to steels and this invention will hereinafter be described with reference thereto.

Before entering into a discussion of the invention, it should be pointed out that in the usual practice for the cold finishing of steels, as in a drawing, rolling or an extrusion operation, high residual stresses are left in the steels. These high residual stresses are detrimental in many of the applications and uses that are to be made of the steel. They manifest themselves in the warpage of parts that are formed thereof to make the part unfit for its intended use. These residual stresses are generally tensile at the surfaces of the steel and they may cause cracks to develop in the metal during subsequent machining or heading operations. When selected portions are removed from the steel, the stress balance in the steel is disturbed to the extent that the finished product may become seriously distorted.

Various attempts have been made in the industry either to reduce the intensity of the stresses existing in the steel or to control the intensity and character of he stresses developed in the steel during the cold reduction or other operational steps. The former is represented by the practice of heat treating or annealing the steel subsequent to cold finishing to reduce the stresses that have been developed in the steel. Such heat treating or annealing operations are costly especially from the standpoint of the time and labor consumed, the equipment and space required, and the large inventory required because of the tie-up of materials in process. The latter is represented by the very recently developed process of elevated temperature reduction as described and claimed in one of the applicants recently issued US. Patents 2,767,835, 2,767,836, 2,767,837, and 2,767,838.

The latter is also represented by the invention recently made by Kyle et al. as described in the copending appiication Serial No. 484,726, filed January 28, 1955, and now abandoned, wherein die shape and particularly a slight taper in the bearing portion of the die was found beneficially to affect the residual stress characteristics of bars or steel bars which are drawn or extruded therethrough.

lt is an object of this invention to provide a still different method for the reduction or control of residual stresses existing in metals and it is a related object to pro duce a new and improved steel product having new and difierent residual stress characteristics. More specifically, it is an object of this invention to provide a method which may be employed in the cold finishing of steel to minimize the residual stresses present in the steel and to conice trol the intensity and direction thereof thereby to produce a steel having new and improved physical and mechanical properties.

These and other objects and advantages of this invention will hereinafter appear and for purposes of illustration, but not of limitation, an embodiment of the invention is shown in the accompanying drawings in which- FIG. 1 is a schematic diagram of the arrangement of parts for carrying out the invention;

FIG. 2 is a pictorial representation of a series of steel test bars illustrating the results secured in the practice of this invention;

FIG. 3 is a pictorial representation of a series of bars of the type illustrated in FIG. 2 which further illustrates the results secured by the practice of this invention;

FIG. 4 is a pictorial representation of another series of bars illustrating the results capable of being achieved by this invention;

FIG. 5 is a curve relating flare to percent reduction in edge-rolled bars;

FIG. 6 is a curve which relates flare to percent reduction in fiat-rolled steel bars;

FIG. 7 is a curve similar to that of FIG. 5 relating warpage factor to percent reduction, and

FIG. 8 is a curve similar to that of FIG. 6 relating warpage factor to percent reduction.

It has been found that the high residual stresses developed in steel bars and rods during cold reduction, as in a drawing, extrusion or rolling operation, and which are generally tensile in character in the surface or peripheral portions of the bar, can be markedly reduced or eliminated or directionally modified by subsequently processing the steel in a rolling operation to take a final reduction in an amount greater than 0.01 percent up to about 0.20 percent and preferably Within the range of 0.02 to 0.14 percent.

It has been found further that reduction or elimination of residual stresses by the described rolling operation is directional in that the relief of stresses Will be in a plane perpendicular to the surface rolled with little, if any, effect on the residual stresses in planes parallel thereto. As a result, it becomes possible to achieve directional control of stresses to produce steels having completely new and improved characteristics.

It was found further that subsequent treatment of the steel in other planes can be employed to provide the same desirable modifications of residual stresses in the other planes without disturbing the stress relationships previously developed by the first pass or passes thereby to enable the principles of superposition to be employed for effecting control of stresses in a steel.

The concepts of this invention can best be described by reference to the results secured in stress tests which have been applied to steel squares and flats processed in accordance with the process of this invention.

As used herein, the term warping value is an indication of the concentration and character of the longitudinal stresses present in steel. In the test for warping, test pieces were slotted through their centers for a distance of about 4 inches. The length of the slot Was recorded and the dimension perpendicular to the slot was also measured. The difference between the dimension of the piece before slotting and after slotting represents the flare that is caused by the presence of residual stresses. The flare is considered positive, indicating tensile stresses predominating in the material, if the bar expands on slotting. The flare is considered negative, indicating the presence of predominating compressive stresses in the material, if the ends move towards the cut through the center. The warping values determined War in 100 originalthieknessXflare g I (slot length) 7 i The steel test bars having a crosswise dimension of 1 inch and a thickness of /2 inch were advanced linearly between a pair of rollers 10 with the fiat faces (x-x in FIG. 2) in engagement with the periphery of the rollers. The rollers were adjusted to different settings for applying different pressures to vary the amount of reduction taken during advancement of the steel through the rollers. The following is a tabulation of the percent reduction for each of the test bars as illustrated in FIG. 2 of the drawings:

Percent reduction Test bar: (in inches) A (As drawn) B 0.04

The test bars were slotted lengthwise through the center between the faces that were engaged by the rollers for a distance of 4 inches from one end. The deflection of the arms may be taken as illustrative of the residual stresses existing in the bar. A spread or increase in the spaced relationship between the ends is indicative of a predominance of tensile stresses and convergence of the ends may be taken as indicative of the predominance of compressive stresses in the peripheral layers of the steel bars.

It may be seen from FIG. 2 that the magnitude of the residual stresses gauged by the warpage or flare of the bars slit along their centers in the horizontal plane of symmetry of the roll pass, alters after a single pass between the rotating cylindrical rolls and that the change is characteristically related to the pressure to which the bars have been subjected in the roll pass or the percent reduction resulting therefrom.

From the degree of flare in the as drawn bar A, it will be apparent that all of the bars prior to the roll pass were characterized by high residual stresses at the surface which were predominantly tensile in character. Upon rolling with increased pressure to take correspondingly increased reductions in cross-section, the positive tensile stresses decreased, as evidenced by specimens B and C, until a nearly zero stress level was reached, as evidenced by the specimen D. When the pressures acting on the bar are further increased to efiFect a corresponding greater reduction, the (positive) longitudinal tensile stresses which are present at and below the bar surface become changed to a slightly compressive or negative stress, as evidenced by the convergence which takes place in specimen E. Thereafter, the stress characteristics again revert to positive tensile stresses, as evidenced by specimen G, after first passing back through a neutral zone, as evidenced by specimen F.

From the foregoing, it will be apparent that the stress levels and that the type of stresses in a bar can be changed by a superficial rolling operation materially to reduce the magnitude and distribution of residual stresses origi nally existing in the bar or substantially to elminate the predominance of the tensile stresses in certain portions thereof or change the tensile stress characteristics to compressive stresses for negative warpage value simply by the process of compressing the bar in a rolling operation to take a reduction within a predetermined narrow range in an amount from 0.01 to 0.2 percent, depending upon the residual stress levels and stress-strain characteristics desired.

By way of further illustration of the concepts of this invention, bars having their stress levels reduced to zero warpage value in one direction, as by taking a pass be tween the rolls with a setting to produce the conditions represented by bar D of FIG. 2, are passed edgewise between the cylindrical rolls at various settings to process the bars at various pressures and take corresponding reductions. The bars illustrated in FIG. 3 have been edge-rolled to take the following reductions:

A No reduction From the results illustrated in FIG. 3, it will be apparent that while the stresses have been neutralized in the xx direction, as represented by bar D in FIG. 2, tensile stresses still exist in the crosswise direction, as indicated by the spread that is illustrated in specimen A of FIG. 3. Application of pressure to take correspondreductions at first causes elimination of some of the tensile stresses, as illustrated by specimen B in FIG. 3, until a bar having the stresses neutralized is secured with a 0.04 percent reduction, as represented by specimen C in FIG. 3. Thereafter, as in the flat rolled bars, the stress levels again rise from neutral upon further increase in pressure and in the amount of reduction taken during the rolling operation until tensile stresses exceeding those originally present in the bar are developed, as illustrated by specimen E of FIG. 3.

The directional relationship between the stress modification and the direction of applied pressure for reduction is illustrated by taking bars corresponding to bar D of FIG. 2 by slitting the bar centrally between the side edges in a direction crosswise to the direction of roll. It will be seen from specimen A of FIG. 3 that a high stress relationship dominated by tensile stresses still exists in the crosswise direction while being reduced to about zero in the direction of the roll pass.

From these results it will be apparent that each rolling pass will affect essentially only the residual stresses in a plane perpendicular therewith to decrease therewith to decrease the stress levels to about zero followed by increase in the tensile stresses as the pressure or reduction exceeds the optimum for obtaining the zero stress levels. The results secured are similar to those secured with the specimens of FIG. 2 with the exception that the stresses affected are perpendicular thereto without corresponding effect upon the stress conditions existing in the longitudinal direction but acting on planes normal to the former.

Thus to reduce the stress levels in structural shape, such as flats or squares, it is essential to both edge-roll and flat-roll the steel. This is further illustrated by the square bars of FIG. 4 wherein the steel bars were passed between the rolls both fiatwise and edgewise. bar B, slotted diagonally, showed no flare as compared to the large flare in the as drawn bar A. Thus the initially large stresses in the steel bar have been reduced by the two passes at right angles to each other to produce a practically stress-free or dead bar.

A similar procedure has been applied to inch bar stock of hexagonal shape. To process the bar for production of stress levels in all directions, it was found necessary to advance the bar for roll passes on all three parallel faces by turning the bar through an angle of degrees between each pass. It was found that the magnitude and the size of the three residual stress systerns acting upon the planes parallel to the three faces will be interdependent. Rolling the bar on one pair of planes operates to modify the residual stress distribution in the other two sets of planes under 60 and 120 degrees to the plane being worked. By proper selection of the rolling pressure in the three successive passes, it is possible to reduce the initial residual stresses to a fraction of their original value thereby materially to improve the dimensional stability of the bars. By proper control,

The rolled it is possible to also end up with a bar having slight compressive stresses or negative warpage value.

The amount of pressure and proportional reduction in cross-section in the rolled direction for reduction of stresses to the point where tensile stress levels begin to rise again to undesirable levels will vary slightly from steels of one composition to another but, in any event, the pressure and the amount of reduction for efiecting the desired results will usually reside within the range for effecting a reduction of from 0.01 to 0.2 percent, and preferably from 0.02 to 0.14 percent.

FIGS. and 6 relate the amount of flare to percent reduction in 1018 steel bars of 1 x A2 inch which have previously been subjected to a cold reduction in a drawing operation. FIG. 5 charts the flare secured when the bars are rolled flatwise, as indicated by the included sketch, and FIG. 6 charts the flare as against percent reduction when the bars are rolled edgewise.

The curves of FIGS. 7 and 8 graphically support the visual presentation made in FIGS. 2-4 and the flare values given in FIGS. 5 and 6. Commencing at about 0.02 percent reduction in the flat rolled and in the edge rolled bars, the warpage factor begins to fall from a relatively high positive value, indicative of high tensile stresses, to lower values as the pressure is increased on the metal to increase the percent reduction. The warpage values charted against percent reduction in FIGS. 7 and 8 fall off rapidly to a minimum at about 0.045 per cent reduction in the edge rolled bars and at about 0.1 percent reduction in the flat rolled bars. Beyond this minimum, further increases in pressure with corresponding further increases in percent reduction causes the trend of the warpage values to reverse and rise again until the original warpage is exceeded at about 0.1 percent reduction in the edge rolled bars and at about 0.14 percent in the flat rolled bars.

It will be apparent that the warpage values can be made to approach the zero levels in the edge rolled bars and to drop below the zero level in the fiat rolled bars to produce bars having a predominance of compressive stresses. The ability to enter into the range of compressive stresses of negative warpage values is not dependent upon the directional sequence of the rolling operations but is dependent more upon the composition of the steel, the dimensional characteristics of the metal and the stresses originally present in the metal before rolling. What is intended to be established is the remarkable effect that a smfll reduction has, when achieved by a rolling operation, on the residual stress characteristics of the steel and the ability to make use of such small reductions in a rolling step to reduce or to control the residual stresses in the steel and the directional characteristics thereof.

As illustrated by the curves, the shape of the curve or the position of the curve can be shifted slightly to the left or to the right or varied depending upon the chemical composition of the metal, the dimensional characteristics of the metal and its previous heat treating and forming history. In general, however, the desired results will be secured by rolling to secure a reduction within the area of 0.01 to 0.2 percent reduction, as previously pointed out.

While the inventive concepts can be employed with metals other than steel, as represented by copper, brass, aluminum and alloys thereof, it will be understood that fullest response will be experienced with steels and especially non-austenitic steels characterized by having a pearlitic structure in a matrix of free ferrite.

From the foregoing, it will be apparent that a new and simple means has been provided for the control and reduction of warpage values in steel thereby to make steels available having improved stress and warpage characteristics. It will be evident further that, in addition to stress reduction, the concepts of this invention can be used to control the directional stresses by taking roll passes in one direction as compared to others or by taking an amount of reduction which will convert tensile stresses into a predominance of compressive stresses, thereby to produce steels having new and improved characteristics.

It will be understood that changes may be made in the details of the means and equipment for effecting the desired reduction by rolling without departing from the spirit of the invention, especially as defined in the following claims.

We claim:

1. In the processing of steel having a shape other than round and having at least one pair of parallel faces, said steel originally containing residual stresses, the modification of the intensity and direction of the residual stresses in said steel comprising the steps of advancing the steel with a pair of parallel faces in a cold finishing operation between parallel faces of a pair of rolls in a rolling operation, and continuously compressing the steel between the parallel faces of said roll dies during advancement of the steel therethrough to take a 0.02 to 0.14 percent reduction in cross-sectional area whereby the stresses normal to the roll passes are reduced, and advancing the steel between the parallel faces of the rolls in a rolling operation at circumferential angles with respect to the previous roll passes comprising a multiple of the number of faces divided into 360 degrees to take a 0.02 to 0.14 percent reduction in cross-sectional area to reduce the intensity and direction of residual stresses interdependently in the steel.

2. In the processing of an elongated steel of polygonal shape having at least one pair of parallel faces, said steel originally containing residual stresses, the modification of the intensity and the character of the residual stresses in the steel comprising the steps of advancing the steel with a pair of parallel faces between the parallel faces of a pair of rolls in a cold finishing operation, and continuously compressing the steel between the parallel faces of said rolls during advancement of the steel therethrough to take a 0.02 to 0.14 percent reduction, rotating the steel through an angle corresponding to 360 degrees divided by the number of faces, advancing the steel again with another pair of parallel faces between the parallel faces of the rolls to take a similar reduction, and continuing the rolling operation between the parallel faces of the rolls until the steel has been turned through an angle not greater than 360 degrees.

3. In the processing of an elongated steel of polygonal shape having at least one pair of parallel faces, said steel originally containing residual stresses, the modification of the intensity and character of the residual stresses in the steel comprising the steps of advancing the steel with a pair of parallel faces between the parallel faces of a pair of rolls in a cold finishing operation and continuously compressing the steel between the parallel faces of said rails during advancement of the steel therethrough to take a 0.02-0.14 percent reduction, rotating the steel through an angle of at least degrees but less than degrees, advancing the steel again with another pair of parallel faces between the parallel faces of the rolls to take a similar reduction, and continuing the rotation of the steel followed by the rolling operation between the parallel faces of the rolls until the steel has been turned through an angle not greater than 360 degrees.

4. In the cold finishing of steel having a shape other than round and having at least one pair of parallel faces, said steel originally containing residual stresses, the modification of the intensity and character of the residual stresses in the steel comprising the steps of advanc ing the steel in a final finishing operation with a pair of parallel faces between parallel faces of a pair of rolls in a rolling operation, and continuously compressing the steel between the parallel faces of said rolls during advancement of the steel therethrough to take a 0.02-0.14 percent reduction in cross-sectional area.

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS Williams, Feb. 17, 1891 Fisk June 20, 1933 Morris Dec. 19, 1944 Wise Feb. 20, 1945 Heller Feb. 22, 1949 Heller Aug. 1, 1950 Sims Mar. 18, 1952 Appel July 14, 1959 FOREIGN PATENTS Great Britain Feb. 11, 1935 8 OTHER REFERENCES Metals Handbook, 1948 edition, page 536. (Copy in Division 14.)

Making, Shaping, and Treating of Steel, by U.S. Steel, sixth edition, 1951, pages 582, 1100-1107. (Copy in Div. 13.)

The Journal of the Institute of Metals, No. 2, 1914, vol. XII, page 27. (Copy in Scientific Library, tn-l-I59).

Surface Stressing of Metals, ASM, 1947, pages 85-142 (article by O. J. Horger). (Copy in Scientific Library, TA460-A44S.

Nachtman, E. 8.: Residual Stresses in Cold-Finished Steel Bars, Mechanical Engineering, volume 77, October 1955, pages 886889.

Claims (1)

1. IN THE PROCESSING OF STEEL HAVING A SHAPE OTHER THAN ROUND AND HAVING AT LEAST ONE PAIR OF PARALLEL FACES, SAID STEEL ORIGINALLY CONTAINING RESIDULA STRESSES, THE MODIFICATION OF THE INTENSITY AND DIRECTION OF THE RESIDULA STRESSES IN SAID STEEL COMPRISING THE STEPS OF ADVANCING THE STEEL WITH A PAIR OF PARALLEL FACES IN A COLD FINISHING OPERATION BETWEEN PARALLEL FACES OF A PAIR OF ROLLS IN A ROLLING OPERATION, AND CONTINUOUSLY COMPRESSING THE STEEL BETWEEN THE PARALLEL FACES OF SAID ROLL DIES DURING ADVANCEMENT OF THE STEEL THERETHROUGH TO TAKE A 0.02 TO 0.14 PERCENT REDUCTION IN CROSS-SECTIONAL AREA WHEREBY THE STRESS NORMAL TO THE ROLL PASSES ARE REDUCED, AND ADVANCING THE STEEL BETWEEN THE PARALLEL FACES OF THE ROLLS IN A ROLLING OPERATION AT CIRCUMFERENTIAL ANGLES WITH RESPECT TO THE PREVIOUS ROLL PASSES COMRPISING A MULTIPLE OF THE NUMBER OF FACES DIVIDED INTO 360 DEGREES TO TAKE A 0.02 TO 0.14 PERCENT REDUCTION IN CROSS-SECTIONAL AREA TO REDUCE THE INTENSITY AND DIRECTION OF RESIDULA STRESSES INTERDEPENDENTLY IN THE STEEL.

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