US3573999A

US3573999A - Mechanical strength of metals

Info

Publication number: US3573999A
Application number: US594348A
Authority: US
Inventors: Isao Gokyu
Original assignee: Nippon Kokan Ltd
Current assignee: JFE Engineering Corp
Priority date: 1965-10-30
Filing date: 1966-10-26
Publication date: 1971-04-06
Anticipated expiration: 1988-04-06
Also published as: DE1508400A1; GB1166477A

Abstract

A METHOD FOR IMPROVING THE MECHANICAL AND PHYSICAL PROPERTIES OF ALPHA IRON STEEL BAR AND ROD ARTICLES BY PLASTICALLY TWISTING THE ARTICLES WITHOUT DIMENSIONAL DEFORMATION BETWEEN ABOUT 200*C. AND ABOUT 400*C. THE STEEL BAR AND ROD ARTICLES ARE REQUIRED TO HAVE INTERSTITIAL SOLUTE ATOMS SELECTED FROM THE GROUP CONSISTING OF CARBON AND NITROGEN IN AN AMOUNT OF AT LEAST 0.02% CARBON AND AT LEAST 0.008% NITROGEN.

Description

Vickers Hardness (Hv) Vickers Hardness (Hv) April 6, 1971 ISAO GOKYU 3,573,999

MECHANICAL STRENGTH OF METALS Filed 001;. 26, 1966 '60 Outer Surfeee Inner Surface I 50 j l 40 Central Portion Inner Surface '20 Outer Surface I I0 *Central Portion Measured Positions IOO l Working Temperature (C) INVENTOR. IS A 0 G0 n Yo! United States Patent 3,573,999 MECHANICAL STRENGTH 0F METALS Isao Gokyu, Musashinoshi, Tokyo, Japan, assignor to Nippon Kokan Kabushiki Kaisha, Tokyo, Japan Filed Oct. 26, 1966, Ser. No. 594,348 Claims priority, application Japan, Oct. 30, 1965, 40/ 66,312 Int. Cl. C21d 7/10 US. Cl. 148-42 Claims ABSTRACT OF THE DISCLOSURE A method for improving the mechanical and physical properties of alpha iron steel bar and rod articles by plastically twisting the articles without dimensional deformation between about 200 C. and about 400 C. The steel bar and rod articles are required to have interstitial solute atoms selected from the group consisting of carbon and nitrogen in an amount of at least 0.02% carbon and at least 0.008% nitrogen.

The present invention relates to a method of treating metals so as to improve the mechanical strength thereof.

In particular, the method of the invention relates to the treatment of steel by warm-working thereof.

The expression warm working is intended to signify plastic working of metals and alloys at an elevated temperature which is substantially higher than room temperature while being substantially lower than the recrystallization temperature thereof, so that the Warm-working temperature is within a warm temperature range.

It is known that the mechanical strength of steel can be increased by subjecting it to plastic deformation, such as, for example, during rolling or die drawing, with the temperature being in the neighborhood of 300 0, although the precise reasons for this phenomenon have not, as yet, been completely explained. An important drawback of conventional methods of this type resides in the fact that there is a considerable deformation, in the form of a reduction in the thickness or cross-sectional area of the material, which necessarily accompanies the practice of the method. This disadvantage is so great that methods of this type have not become very popular with manufacturing industries because of the possibility of unavoidable complications in production or of increased difficulties in maintaining the dimensional stability of the product.

Thus, it is a primary object of the present invention to provide a method of increasing the strength of metals, without, however, giving rise to any substantial dimensional changes, alterations, or deformation thereof. Also, the present invention contemplates industrial applications of the method which can be readily integrated into current mass-production procedures. Other objects included in the scope of the invention will be described in part or will be easily surmised by those Who are skilled in the art.

The inventor of the present invention came to realize, after many tests and investigations, that in order to achieve the same, or even better, results from the abovementioned warm-working, those types of plastic working which are accompanied by dimensional changes or alterations are not at all necessary. In fact, it was found that such relatively simple procedures as straightening, levelice ling, or twisting, which do not result in any substantial dimensional change, but instead restore flatness or straightness, can be satisfactorily used in the method of the invention provided that certain conditions are met and observed. Thus, it was established by hardness measurements and by the use of an electron microscope as well as other pertinent mechanical and metallurgical tests that there are four major factors involved in the straightening process, of which two are controllable and the other two are not controllable to the same extent.

The first of these factors is the temperature range at which the working takes place, this temperature range being from approximately 200 C. to approximately 400 C., and preferably being in the neighborhood of 300 C. The second factor is the presence of interstitial solut atoms, such as, for example, carbon or nitrogen. The third factor is the nature of the working which must be capable of generating a large number of dislocations in the alpha iron matrix. The fourth factor is the state of the dislocations or dislocation configuration at the end of the plastic working, this state of dislocations or dislocation configuration being required to be such that an effective immobilization of dislocations results.

The invention is described in detail with reference to the accompanying drawings in which:

FIG. 1 is a diagram showing hardness measurements of a steel pipe prior to and after the application of one embodiment of a method according to the present invention;

FIG. 2 is a schematic end view of the above-mentioned steel pipe, FIG. 2 showing the locations where the hardness measurements were taken; and

FIG. 3 is a diagram showing hardness changes in alpha iron according to different temperatures of warm-working, this working taking place according to another embodiment of a method of the present invention.

EMMPLE 1 A welding pipe, such as that shown in FIG. 2 having the welded seam W, having a chemical composition of 0.09% carbon, a trace of silicon, 0.30% manganese, 0.030% phosphorus, 0.025% sulphur, and having been shaped to predetermined dimensions according to which the pipe has a diameter of 27.3 mm., a thickness of 2 mm., and a length of 1 m. by a conventional method, was straightened by passing a pair of oblique levelling rolls at a temperature of 300 C. No measurable dimensional deformations occurred. The Vickers hardness, measured at the

locations

1, 2, 3 and 4 indicated in FIG. 2, where the welded seam W is shown situated between the

locations

2 and 3, for the inner, middle and outer portions of the pipe wall, clearly indicate an appreciable strengthening, on the order of 25 in terms of the Vickers hardness numbers, as is indicated in-FIG. 1.

EXAMPLE 2 1 kg. of alpha iron of a composition listed below in Table l was vacuum melted and cast into an ingot having a diameter of 60 mm. and a length of mm. The ingot was hot-rolled in a conventional manner to form a rod of a diameter of 6 mm., cooled to room temperature, then cold-drawn to a wire of 3 mm., diameter, and finally annealed in an evacuated atmosphere for 24 hours at 850 C.

3 TABLE 1 Element, percent:

Mn Trace The sample was divided into two portions. Specimens from one of the portions were twist-worked at room temperature, 100" C., 200 C., 300 C., and 400 C. to surface strains of 4%, 13%, and 20%, respectively. The other portion of the sample was twisted at room. temperature and subsequently annealed at 300 C. for thirty minutes. This is one form of a known low temperature strain tempering method and was carried out in order to provide a reference. In no case was a dimensionchanging deformation observed.

The change in Vickers hardness for 4% surface strain is illustrated in FIG. 3 with respect to the temperature of warm-working. Considering that the hardness as annealed was 62 and the hardness as cold-worked was 72, the effect of warm-working is seen to be appreciable at a temperature as low as 100 C., reaching the maximum at 300 C., but falling beyond 400 C. However, above a temperature of 500 C., for example, the hardness drops rapidly, as recrystallization commences.

The same observations were made for other surface strains, in addition to that of 4%, except that the hardness curves were shifted upwardly to a small extent with increasing strain at any given temperature. The elfect was achieved with strains as small as 1%, although the increment was not as great as 4%.

In the case of cold-working followed by annealing at 300 0., namely the known low temperature strain annealing referred to above, there was also an increase in hardness, but only to 80.

Therefore, the proper temperature range for warmworking of the method according to the invention is defined as approximately between 200 C. and 400 C., the preferred range being in the neighborhood of 300 C.

The exact mechanism by which this phenomenon is achieved is not, as yet, completely known, but it is be lieved certain that the interaction of dislocations with interstitial atoms, in the above-illustrated case, carbon atoms, and among themselves are responsible. In fact, in a study carried out by transmission electron microscopy, it was found that the density of dislocations was unusually high for a given strain as compared with coldworked or strain-tempered material, and that those dislocations were effectively immobilized in mutual entanglement or by newly formed precipitates of submicron sizes. Besides these, such atomic, and therefore unobservable, processes as Cottrell atmosphere formation or interaction with vacancies and voids were believed to have contributed. The rise and fall of the curve, and hence the presence of the optimum conditions, is probably the reflection of the balance between diffusion of the interstitial atoms and their reaction with dislocations.

Inasmuch as the present invention has been found to be readily practised in conjunction with such apparently non-deforming processes as twist-straightening or spinfinishing, as often employed in modern manufacturing as part of a predetermined sequence of operations, the method of the invention can be integrated into industrial mass-production without any difficulty, provided that the above conditions are observed.

EXAMPLE 3 A steel pipe having a thickness of 2 mm. and the composition listed below in Table 2, while cooling from the temperature of hot roll forming in still air, was fed to a roller levelling machine at a temperature of about 300 C. There was a slight, though insignificant, reduction in thickness of less than 1%. In addition to achieving the desired straightness, some 10 kg./mm. was gained in the tensile strength as compared to samples finished in the customary manner, namely during completion of the levelling at a temperature above the recrystallization temperature.

TABLE 2 Element, percent:

C 0.15 Si 0.35 Mn 1.06 P 0.018 N 0.008 S 0.025

While it is not known how much of the 0.15% total carbon was left as interstitial at the temperature of the warm-working or how much was rendered inefiicacious by forming stable carbide during cooling to those temperatures, practically the entire nitrogen must have existed as the interstitial solute, since there was no stable nitrideforming element, such as aluminum or vanadium. Therefore, the very notable increase in the strength can be considered as caused by interstitial carbon and interstitial nitrogen. Thus, the control factors of the method of the invention are temperature of the Warm-working of approximately 200 C. to approximately 400 C., preferably in the neighborhood of 300 C., whether these temperatures are reached from below, namely by heating, or from above, namely by cooling, being immaterial, and the presence of interstitial solute atoms, of which as little as 0.02% carbon or 0.008% nitrogen were shown to be sufficient, and in addition the use of a method of working where the surface strain of at least 1% Was achieved by twisting or repeated slight bending during roller levelling.

Furthermore, it will be understood that the present invention is applicable to metals and alloys other than iron and steel, as long as the basic principle of strengthening according to the method of the invention, namely that due to the interaction of dislocations with interstitial solute atoms, is universal.

Finally, it will be appreciated that the method of the present invention is basically distinct from known method of warm-working in that while the latter necessarily give rise to a substantial dimensional deformation, such as a reduction in cross section, no such complexities are involved with the method of the invention. Moreover, the method'of the invention is not the same as, or even similar to, the known method of low temperature strain tempering, as the straining and heating is not separate with the method of the invention, while the straining and heating are separate in the known method.

Having thus described the principles and some applications of the method of the invention, it will be immediately evident to those skilled in the art that the method of the invention is not limited to the few specific methods which were discussed above by way of example and the illustrations of the drawings, except as limited by the appended claims.

What is claimed is:

1. Method of improving mechanical and physical properties of alpha iron steel bar and rod articles of circular cross-section in both surface and inner layers thereof, which comprises subjecting such steel article containing interstitial solute atoms of an element selected from the group consisting of carbon and nitrogen in an amount of at least 0.02% carbon and at least 0.008% nitrogen to warm working by twisting with a surface strain of at least 1% at a temperature between about 200 C. and about 400 C., the plastic deformation by said twisting being in proportion to the cross sectional dimensions of said article, whereby the mechanical and physical properties of said article are improved without dimensional deformation thereof.

2. Method according to claim 1 and wherein said plastic deformation by twisting is carried out on said steel article while the same is cooling from previous hot working References Cited themf- UN D STATES PATENTS 3. Method according to claim 1 wherein a hot drawn ITE metal article is subjected to said plastic deformation by $33 2? twisting.

4. Method according to claim 1 wherein said plastic 5 3,102,060 8/1963 Schoch at 14812 deformation by twisting is accompanied by warm bend- 3,196,052 7/1965 ing of said article.

5. Method according to claim 2 wherein said plastic HYLAND BIZOT Pnmary Exammer deformation by twisting is accompanied by warm straight- 10 W. W. STALLARD, Assistant Examiner ening of said article.