GB2036806A

GB2036806A - Spring steel and the production thereof

Info

Publication number: GB2036806A
Application number: GB7846621A
Authority: GB
Original assignee: Neturen Co Ltd
Current assignee: Neturen Co Ltd
Priority date: 1978-11-30
Filing date: 1978-11-30
Publication date: 1980-07-02
Also published as: GB2036806B

Abstract

The present invention relates to a high-strength spring steel which is obtained by heating the surface of a steel to over the AC3 transformation point by high-frequency induction heating or the like, stopping the heating and then decreasing the surface temperature of said material steel preferably to below the Ar1 transformation point and by self- quenching, this short-time heating of the surface being repeated to secure heating throughout the entire steel body or a condition close to it. The steel is then quenched, whereby the crystal grains in the steel become increasingly finer from the core to the surface layer of the steel, the crystal grain size of the metal in the surface layer being particularly fine.

Description

SPECIFICATION Spring steel and the production thereof The present invention relates to a high-strength spring steel and to a process for its production.

It is a desirable characteristic that materials for coil springs, torsion bars and the like should have high fatigue strength, especially high torsion fatigue strength.

The bending or twisting stresses occurring in this kind of spring material during use increases while moving toward the surface thereof from its neutral axis and the maximum stress usually occurs in the surface region of the spring.

In one conventional method of manufacturing a coil spring, a spring steel which has been drawn and then oil-tempered to increase its strength is cold-formed into a spring. In another method a spring steel which has been coiled is quenched and tempered to increase its strength. In either method, it is intended to obtain a uniform quenched and tempered structure of the steel over the whole section through a routine heat treatment. Thus, such conventional methods of manufacturing a spring steel cannot produce a spring with a strength distribution matching the stress distribution which develops in the spring under service conditions.Moreover, in the conventional method of manufacturing the coil spring or in the conventional quench-tempering process of the spring steel wire, in which the whole section of the steel wire is heated to the core only once to over AC3 transformation point and then immediately quenched, no particular consideration is given to making the crystal grains finer.

One means of making the crystal grains of steel finer is known as the "Repeated Quenching Process", for example as described in U.S. Patent No. 3,1 78,324. However, this method is not applied to the manufacture of steel springs. According to this process, the material steel is heated over its entire section to over the AC? transformation point and then forcibly cooled to render its structure martensitic, this cycle being repeated more than two times in succession.

If the above known process were applied to quench a coil spring or a spring steel, fine crystal grains may be produced, but since the effect of making the grains fine takes place uniformly over the entire section of the steel, this process is also hardly able to produce a spring with a strength distribution matching the stress distribution which develops in the spring under its service conditions.

It is an object of the present invention to provide a new and advantageous spring steel, as well as a process for producing such steel.

According to one feature of the present invention we provide a spring steel wherein the fineness of the crystal grains in the steel increases in one or more directions from the core of the steel to the surface thereof, which steel is produced by subjecting a steel to at least two heating steps, each heating step comprising heating the surface of the steel by high frequency induction or the like, the surface of the steel being cooled between consecutive heating steps, the number and frequency of the heating steps, as well as the degree of heating and cooling to which the steel is subjected, being such as to effect heating of the steel throughout substantially its total volume, and quenching the steel after the last heating step.

Preferably the above springs are tempered by high frequency induction or the like after the quenching step.

In the production of the above spring steels, the surface of the steel is advantageously heated to a temperature above the AC3 transformation point of the steel during each heating step while the surface is preferably cooled to below the Ar1 transformation point of the steel between consecutive heating steps.

According to a further feature of the present invention we provide a process for producing a spring steel which comprises subjecting a steel to at least two heating steps, each heating step comprising heating the surface of the steel by high frequency induction or the like to a temperature above the AC3 transformation point of the steel, the surface of the steel being cooled to a temperature below the Ar1 transformation point of the steel between consecutive heating steps, the number and frequency of the heating steps being such as to effect heating of the steel throughout substantially its total volume; and quenching the steel after the last heating step.

In the production of the above spring steels the steel is advantageously heated throughout substantially its total volume to a temperature above the AC3 transformation point of the steel.

We have produced spring steels in accordance with the invention which have a high strength, a high resistance to fatigue and particularly fine crystal grains in its surface layer as well as a strength distribution matching the distribution of bending or twisting stresses which may develop in the spring during use.

As described above, conventional coil spring manufacturing methods aim at attaining a uniform quenched-and-tempered structure of the steel over its entire section through the routine heat treatment and accordingly such conventional methods could hardly be expected to produce a spring having a strength distribution matching the stress distribution which may develop in the spring during use; and since, in said heat treatment, the steel over its entire section to the core is heated only once up to the AC3 transformation point and immediately thereafter quenched, fine crystal grains cannot be obtained.

As also pointed out above, a method of repeated quenching is also known, though not for the manufacture of springs, as a means of obtaining fine crystal grains. According to this method, the steel is rapidly heated to over the AC3 transformation point over its entire section and then forcibly cooled to the ambient temperature, this thermal cycle of rapid heating and cooling being repeated to improve the fineness of the crystal grains in the steel thereby improving the strength, particularly the fatigue strength of the steel.

The features distinguishing the present invention from the above prior art are as follows: 1) Whereas in the above prior art, i.e. in the method of manufacturing springs as well as in the method of improving the fineness of the crystal grains in the steel, rapid heating is used to heat the steel over its entire section to over the AC3 transformation point, according to the present invention only the surface layer of the steel is heated to above the AC3 transformation point.

2) Whereas in the prior method of improving the fineness of the crystal grains in the steel.over its entire section, the steel is repeatedly quenched to make the crystal grains finer, according to the present invention only a surface heating of the steel is effected and cooling between the repeated heating steps is effected by virtue of its own heat conductivity.

3) Whereas in the prior methods of either manufacturing a spring or improving the fineness of the crystal grains in the steel, the steel, which has been heated over its entire section to above the AC3 transformation point, is forcibly cooled down to the ambient temperature, according to the present invention only the surface layer of the steel is heated to over the AC3 transformation point, except in the final stage of repeated heating. Therefore, after the heating is stopped, the steel can be cooled in a short time to below the Ar, transformation point by its self-cooling action due to its own heat conductivity without resorting to a forcible cooling.Thus, in the prior art, after cessation of rapid heating, the steel over its entire section is forcibly cooled to the ambient temperature, but in the present invention after cessation of rapid heating, the steel in the surface layer is cooled in a short time to below the Ar, transformation point (not at ambient temperature) without resorting to a forcible cooling, this thermal cycle being repeated, whereby the core of the steel is gradually heated to attain a steel thoroughly heated throughout the entire volume of the steel or a condition close to it, followed by rapid cooling to quench the steel.

Accordingly, the above prior art method of rendering the crystal grains in the steel finer uniformly over the entire section of the steel is quite dissimilar to the present invention wherein the crystal grains are made increasingly fine from the core to the surface layer and in which the grains in the surface layer are particularly fine.

It will be appreciated that the AC3 transformation point and the Ar, transformation point, mentioned above, depend on the steel grade and its composition.

In the practice of the present invention, either a plurality of induction heating coils may be disposed at specific intervals and the wire is sent through these coils for repetitions of the thermal cycle; our a short piece of steel is fixed and submitted to similar repetition of a thermal cycle in such coils. Any method may be employed, provided it is capable of subjecting the steel to the above-mentioned thermal cycles.

In the thermal cycle according to the present invention, the heated surface of the wire is generally cooled by virtue of the heat conductivity of the wire itself and accordingly, the thermal energy consumed does not constitute any loss.

As explained before, a difference in the energy consumption is evident from the case of repeating the thermal cycle with an externally forced cooling. According to the present process, only about one third (1/3) of the power used in a conventional process is employed. This is thus a highly economical and important energy-saving process which is of particular importance in this age of scarce fuel resources.

An embodiment of the present invention will now be described with reference to the accompanying drawing, which illustrates the relation of temperatures in the core and on the surface of a steel high-frequency induction heated according to the present invention, the ordinate being the temperature and the abscissa the time.

L1-L4 represent the high-frequency induction heating coils disposed in series along the travelling path of the steel wire. The wire W travelling in the arrow direction is submitted to the thermal cycle of the present invention in the process of the wire passing through said high-frequency induction heating coils L1-L4. It will be appreciated that the size of the wire and the frequency of induced electric power should be appropriately related to each other.Moreover, the thermal cycle in said induction heating coils L1-L4 can be so designed that the surface layer of the wire may have the temperature rise characteristic A and the core of the wire may have the temperature rise characteristic B, as illustrated in the drawing, by appropriately setting the variables such as the numbers of induction heating coils L1-L4 disposed along the wire travelling path, the lengths 1-4 of respective coils, the intervals d1-d4 of said coils and the power P1-P4 supplied to respective coils, as related to the wire travelling speed.

Thus, the surface temperature of the wire W rises to over the AC3 transformation point in the heating of t, seconds in the first thermal cycle by the coil L1, but during t,, seconds of air-cooling from the time the wire goes out of the coil L, to the time it goes into the coil L2, the surface temperature of the wire drops to below the Ar, transformation point. In the heating oft2 seconds in the second thermal cycle by the coil L2, the surface layer of the wire again attains a temperature exceeding the AC3 transformation point and in the air-cooling oft2 seconds, after the wire W leaves the coil L2, the surface layer attains a temperature below the Ar transformation point. Thereafter, a similar thermal cycle is repeated.

Meanwhile, the core of the wire is still close to ambient temperature, while it is in the first thermal cycle by the coil L1, but as the thermal cycle is repeated, the temperature steadily rises and, for instance, when the cycle by the coil L4 is finished, a temperature above the ACa transformation point is attained.

In this stage, the surface temperature does not drop to below the Arl transformation point by t4' seconds of air-cooling and in consequence, the same effect as in through heating is brought about; and the wire is quenched by rapid cooling in this stage.

After the quenching is finished, and the work heated, if it is a wire, it is successively tempered; and if it is a rod of definite length, it is successively tempered or tempered on a separate line by known methods, such as high-frequency induction heating, to impart to the steel the desired mechanical properties.

The following Examples illustrate the present invention.

EXAMPLE 1 1) Test Conditions (1) Test Conditions Diameter 10 mm Chemical composition conforming to Japanese Industrial Standard JIS G 4801 as SUP 6: C 0.550.65% P less than 0.035% Si 1.501.80% S less than 0.035% Mn 0.701.00% (2) Disposition of Induction Heating Coils: As illustrated in the accompanying drawing, the coils L1-L4 were disposed at specified intervals along the wire travelling path.

(a) Length of Coils: L1 -L3=30 mm L4=180 mm (b) Coil Intervals: d1d2:120mm d3:360mm (3) Heating Conditions: (a) Power supplied to respective coils: L1 L2 L3 L4 20 15 15 14 (kW) (b) Wire travel speed 120 mm/sec.

2) Test Procedure Under the above test conditions, the test piece was submitted to a repeated thermal cycle according to the present invention and, upon conclusion of the fourth thermal cycle, it was quenched with cooling water.

The duration of induction heating in each thermal cycle was 0.25 sec for t1-t3 and 1.5 to t4 and in this heating, the test piece attained a surface temperature of 8800C-9000C. The quenching of the test piece was immediately followed by tempering at 5000C for 2 seconds by high-frequency induction heating.

3) Test Results A comparison of the sectional crystal grain size was made between the test piece thus-treated and one of the same chemical composition and diameter as the former, which had been heated only once for 3 seconds to 8800C-9000C by high-frequency induction heating, followed by quenching and the same tempering as the former. The results are summarized in Table 1 below.

TABLE 1

Cyclically heated Four times Heated once Cyclically heated Four times Crystal grain size (ASTM Number) Crystal grain Hardness size (ASTM No.) Surface layer (1 mm 10 13 46RC from the skin) Mid layer (3 mm deep from the skin) 9 11 45 RC Core 9 9 44 RC EXAMPLE 1) Test Conditions (1) Test prece:: Diameter 10 mm Chemical composition Same as in Example 1 2) Test Procedure The same test piece of Example 1 was subjected, as in Example 1, to a heating of four thermal cycles, followed by quenching and tempering. The tensile strength and the completely reversed fatigue strength of this test piece was compared with those of a test piece with the same chemical composition and diameter as the former which had been induction-heated for 3 seconds to 8800C-9000C, followed by quenching and then the same tempering as the former as well as those of a test piece which had been subjected to routine tempering with oil. The results are summarized in Table 2 below.

TABLE 2

Tensile strength Completely- reversed bending fatigue strength (Kg/mm2) (Kg/mm2) Oil-tempered piece 155 44 Routine induction quenched-tempered piece 161 57 Piece treated according to the present invention 163 66 According to the results of other tests which we have carried out, similar excellent results can be obtained even with a wire having a C-content more or less than 0.3%, if the Mn- and B-contents in it are respectively set at over 1% and 0.001K or a quenchable wire as shown in Table 3 is submitted to the repeated thermal cycles of this invention.

TABLE 3

C ( /0) Si ( wÓ) Mn (%) P ( / ) S (%) 0.18-0.24 0.15-0.35 1.35-1.65 0.04 0.05 0.17-0.23 0.15-0.35 t.20-1.50 0.03 0.03 It will be appreciated that the number of thermal cycles is not limited to four as in the above examples. If desired, it is possible to use external means to assist air cooling and attain an appropriate surface temperature, for example when the thermal cycle is suspended.

As seen from the above test results, it is possible to obtain a wire with its crystal grains increasingly fine from the core to the surface and having extraordinarily fine structure in the surface layer. The wire thus-obtained provides a spring steel characterized by high toughness and with a strength distribution matching the stresses, such as bending or twisting, which may develop in the spring under service conditions.

Claims

1. A spring steel wherein the fineness of the crystal grains in the steel increases in one or more directions from the core of the steel to the surface thereof, which steel is produced by subjecting a steel to at least two heating steps, each heating step comprising heating the surface of the steel by high frequency induction or the like, the surface of the steel being cooled between consecutive heating steps, the number and frequency of the heating steps, as well as the degree of heating and cooling to which the steel is subjected, being such as to effect heating of the steel throughout substantially its total volume; and quenching the steel after the last heating step.

2. A spring steel as claimed in claim 1 which has been tempered after the said quenching step.

3. A spring as claimed in claim 2 wherein tempering is effected by high frequency induction.

4. A spring steel as claimed in any of the preceding claims in which during production thereof the surface of the steel is heated to a temperature above the AC3 transformation point of steel during each heating step and the said surface is cooled to a temperature below the Ar, transformation point of the steel between consecutive heating steps.

5. A spring steel as claimed in any of the preceding claims in which, during production thereof, the steel is heated throughout substantially its total volume to a temperature abovesthe AC3 transformation point of the steel.

6. A spring steel as claimed in any of the preceding claims wherein the steel has a carbon content of more than 0.3%.

7. A spring steel as claimed in any of the preceding claims in the form of a wire.

8. A spring steel substantially as herein described in either of the Examples.

9. A process for producing a spring steel which comprises subjecting a steel to at least two heating steps, each heating step comprising heating the surface of the steel by high frequency induction or the like to a temperature above the AC3 transformation point of the steel, the surface of the steel being cooled to a temperature below the Ar1 transformation point of the steel between consecutive heating steps, the number and frequency of the heating steps being such as to effect heating of the steel throughout substantially its total volume; and quenching the steel after the last heating step.

10. A process as claimed in claim 9 wherein the steel is tempered after the said quenching step.

11. A process as claimed in claim 10 wherein tempering is effected by high frequency induction.

12. A process as claimed in any of claims 9 to 11 wherein the steel is heated throughout substantially its total volume to a temperature above the AC3 transformation point of the steel.

13. A process as claimed in any of claims 9 to 12 wherein the steel is subjected to four heating steps.

14. A process for producing a spring steel substantially as herein described in either of the Examples.

1 5. A process for producing a spring steel substantially as herein described with reference to the accompanying drawings.

1 6. A spring steel whenever produced by a process as claimed in any of claims 9 to 1 5.

17. A spring steel as claimed in claim 1 6 in the form of a coil spring or torsion bar.