Shank or back material for high speed steel tools

Abstract

A shank or back material for high speed steel tools, consisting of an alloy steel composed of 0.2-0.6% of carbon (C), 3.0-7.0% of chromium (Cr), 0.1-1.0% of vanadium (V), not more than 0.8% of silicon (Si), not more than 0.8% of manganese (Mn) and the remainder of iron (F), or said alloy further containing 0.02-0.3% of niobium (Nb). By using the alloy of the invention, a defect-free weld can be obtained, and a strength and toughness degradation can be avoided.

Parent Case Text

This is a division of application Ser. No. 164,298 filed July 20, 1971, now abandoned.

Description

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

This invention relates to shank or back materials for high speed steel tools of the type in which a blade element consisting of high speed steel and a shank or back are welded together by various welding methods.

2. DESCRIPTION OF THE PRIOR ART

In the production of cutting tools or the like utilizing high speed steel, it has been practiced, with a view to saving the quantity of expensive high speed steel, to weld a shank or back of a low grade steel to a cutting blade element which is made of high speed steel. For instance, the cutting blade portion of high speed steel and the shank or back of low grade steel are welded by the friction welding or butt welding in case of an end mill cutter, and by the electron beam welding in case of a metal band saw.

The shank or back material is required, as an essential condition, to be inexpensive, but in addition, it is required to satisfy such conditions (1) that decarburization or cementation does not occur at the welded portions of both materials during annealing which results from the activity difference of carbon contained in both materials, (2) that it will have a sufficient toughness when subjected to a heat treatment at elevated temperatures together with the high speed steel, (3) that it will have an HRC hardness of 40 or higher when subjected to a heat treatment under the same tempering condition as that for the high speed steel, and (4) that it has good weldability and gives a crack-free weld.

Speaking about the decarburization or cementation of the weld, when the cutting blade element of high speed steel and the shank or back of low grade steel are welded together and annealed at a temperature sufficiently high enough to cause a sufficient diffusion of carbon, the carbon diffuses from the shank or back material into the blade material at the welded portion due to the activity difference of carbon in both materials, even when the welding is carried out in a protective atmosphere and under such a condition as will not induce decarburization or cementation externally, no matter what welding method is employed. Namely, regardless of the amount of carbon contained in the shank or back material, the carbon diffuses from the shank or back material into the high speed steel material of the blade element, whereby the shank or back material tends to be decarburized and the high speed steel material tends to be cemented at the portions on both sides of the weld. Thus, the structural strength of the product tool is degraded. In the actual tools, it has been acknowledged that breakage of the tool occurs from the cemented portion of the high speed steel material adjacent to the weld, and in this view, it becomes necessary to add to the shank or back material such alloying elements which will restrain the diffusion of carbon at the welded portion, namely chromium, vanadium, etc., which are effective for lowering the activity of carbon.

The methods of heat-treating the welded tools are classified into two types, i.e. a method wherein the shank or back material and the high speed steel are concurrently subjected to quenching and tempering after they are welded together, and a method wherein the high speed steel only is subjected to quenching and tempering after the welding. Where the high speed steel blade and the shank or back are concurrently subjected to quenching and tempering, as in the case of metal band saw, the grain size of the crystals tends to become large, with the result that the tool has a degraded strength and insufficient toughness, and breakage of the shank or back frequently occurs during use of the tool. For avoiding such trouble, it is necessary to add a grain size refining element to the shank or back material.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a shank or back material for high speed steel tools of the type having a high speed steel blade element and a shank or back welded together, which will not cause decarburization or cementation to occur at the welded portion in the production of such tools.

Another object of the invention is to provide a shank or back material for high speed steel tools of the type having a high speed steel blade element and a shank or back welded together, which will have a sufficient toughness and hardness even after the entire tool is subjected to quenching and tempering subsequent to the welding in the production of such tools.

Still another object of the invention is to provide a shank or back material for high speed steel tools of the type having a high speed steel blade element and a shank or back welded together, which completely prevents the carburization and cementation of the welded portion, will have sufficient toughness and hardness even after the entire tool is subjected to quenching and tempering subsequent to the welding, has good weldability and gives a crack-free weld, in the production of such tools.

The shank or back material according to the present invention consists of an alloy composed of 0.2- 0.5 % of carbon (C), 3.0-7.0 % of chromium (Cr), 0.1-1.0 % of vanadium (V), not more than 0.8 % of silicon (Si), not more than 0.8 % of manganese (Mn) and the remainder of iron (Fe) and impurities, or said alloy further containing 0.02-0.3 % of niobium (Nb).

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1( a) and (b) are a set of microphotographs showing the grain size increasing tendency of the crystals during quenching;

FIGS. 2(a) and (b) are a set of microphotographs showing the structures of the blade material and the shank or back material at portions adjacent to the weld after both materials are welded together by the electron beam welding and subjected to a constant temperature annealing;

FIGS. 3(a-c) are a set of microphotograph showing the quenched structures of both materials at the welded portions; and

FIGS. 4-8 are graphic representations showing the hardness distribution actually measured of both materials at the portions adjacent to the weld.

PREFERRED EMBODIMENTS OF THE INVENTION

Examples of the composition of the steel material according to the invention are shown in Table 1, in which symbol A.sub.1 represents a high speed steel (equivalent to AlSl M2) used for the blade, A.sub.2 a steel similar to SAE 1055 which is commonly being used as a shank or back material, A.sub.3 and A.sub.4 comparative steels and B, C, D, E, F and G the shank or back materials of the invention.

Table 1 __________________________________________________________________________ Symbol C Si Mn P S Ni Cr Mo W V Co Cu Nb Remarks __________________________________________________________________________ A.sub.1 0.88 0.22 0.32 0.022 0.002 0.08 4.08 4.88 6.20 1.84 0.48 0.07 -- Blade material AISI M2 A.sub.2 0.59 0.20 0.35 0.009 0.008 0.09 0.01 0.02 -- 0.01 -- 0.08 -- Conventional steel equivalent to SAE 1055 A.sub.3 0.54 0.16 0.37 0.009 0.010 0.11 1.16 0.03 -- 0.32 -- 0.10 -- Comparative Steel B 0.40 0.19 0.31 0.008 0.008 0.10 3.01 0.02 -- 0.27 -- 0.09 -- Steel of the invention C 0.35 0.16 0.22 0.009 0.009 0.09 4.98 0.02 -- 0.26 -- 0.09 -- Steel of the invention D 0.31 0.14 0.30 0.009 0.010 0.07 6.88 0.03 -- 0.28 -- 0.08 -- Steel of the invention A.sub.4 0.30 0.16 0.29 0.008 0.010 0.07 9.92 0.02 -- 0.29 -- 0.08 -- Comparative steel E 0.35 0.12 0.26 0.009 0.009 0.09 4.97 0.02 -- 0.27 -- 0.09 0.09 Steel of the invention F 0.36 0.14 0.28 0.010 0.009 0.09 4.92 0.02 -- 0.28 -- 0.09 0.18 Steel of the invention G 0.35 0.14 0.27 0.010 0.009 0.08 4.95 0.02 -- 0.22 -- 0.08 0.30 Steel of the invention __________________________________________________________________________

A.sub.3, b, c, d and A.sub.4 are materials in which the concentration of Cr is varied, and E, F and G are materials in which the concentration of Nb is varied. The concentration of C is reduced by the addition of Cr because Cr has the effect of increasing the resistance to temper softening and an HRC hardness of 45-47 suitable for the shank or back material can be obtained even when the shank or trunk material is subjected to a heat treatment concurrently with the high speed steel. FIGS. 1(a) and (b) are a set of microphotographs of which FIG. 1(a) is the comparative steel A.sub.3 and FIG. 1(b) is the steel E of the present invention which were quenched at 1200.degree.C and 1250.degree.C respectively. The steel materials of the invention has the nature of preventing the grain size from becoming large and a remarkable difference is noted between them and the comparative steel A.sub.3 particularly after quenching at temperatures above 1200.degree. C which are used for high speed steels.

Table 2 __________________________________________________________________________ Symbol Quenched at 1200.degree.C., tempered at 560.degree.C. Quenched at 1250.degree.C., tempered at 560.degree. C. Breaking Maximum deflection Absorbed Hardness Breaking Maximum deflection Absorbed Hardness load (kg) (mm) energy (kg-m) (HRC) load (kg) (mm) energy (HRC)) __________________________________________________________________________ A.sub.2 189 1.08 0.131 45.2 182 0.93 0.092 44.9 A.sub.3 196 1.20 0.188 45.6 184 1.04 0.144 45.9 B 190 1.75 0.292 45.5 178 1.68 0.268 45.7 C 190 2.11 0.376 45.5 176 1.86 0.312 45.4 D 206 1.94 0.364 45.0 187 1.90 0.314 45.4 A.sub.4 203 1.93 0.350 45.1 186 1.83 0.303 45.0 E 203 2.33 0.436 46.6 195 2.13 0.402 46.8 F 202 2.84 0.532 46.9 176 2.86 0.468 47.0 G 203 2.66 0.499 46.4 183 2.65 0.451 46.4 __________________________________________________________________________

Table 2 given above shows the toughness of each material obtained by a transverse breaking test conducted on a test piece having a thickness of 3 mm, a width of 5 mm and a length of 30 mm. Each test piece was heat-treated by immersing it in a salt bath held at 1200.degree.C or in a salt bath held at 1250.degree.C, for 60 seconds, then quenched in oil, and tempered twice at 560.degree.C for 1 hour and cooled in air, and the transverse breaking test was conducted by using an Amsler universal testing machine by supporting the test piece at two points (the span being 10 mm).

It will be seen from Table 2 that the steel materials of the invention, particularly those containing Nb, are greater in toughness than the conventional and comparative steel materials. It will also be seen that the toughness increases as the concentration of Cr increases up to 5 %, but remains substantially the same over 5 %.

FIGS. 2(a) and (b) show microphotographic structures at the welded portion of a test piece comprising a blade-constituting high speed steel (AlSl M2) and a shank material (comparative steel A.sub.3) welded together by the electron beam welding method, shown in FIG. 2(a) and that of a test piece comprising the same high speed steel and the shank material of the invention (steel C) welded together by the same method, as seen in FIG. 2(b). Each test piece was prepared by shaping each material into a plate having thickness of 3 mm, a width of 30 mm and a length of 500 mm, welding the plates of the respective materials together by electron beam welding, allowing the welded plate to cool, heating it at 900.degree.C for 30 minutes, maintaining it at 700.degree.C for 5 hours and subjecting it to a isothermal annealing in air. As shown, a ferritic decarurized layer is formed in the comparative steel A.sub.3, FIG. 2(a) but such layer is not formed in the steel C of the present invention, FIG. 2(b). FIGS. 3(a-c) show micrographic structures at the boundary of the weld and the high speed steel of test pieces prepared in the manner described above by using the comparative steel A.sub.3 in FIG. 3(a), the steel B of the invention in FIG. 3(b) and the steel C of the invention in FIG. 3(c), which, after the annealing, were respectively immersed in a salt bath at 1250.degree.C and subjected to quenching in oil. In these microphotographs, the portions appearing in etched black color between the grains are intergranular fused layers. It will be seen that the intergranular fused layers are present in a large number in the comparative steel A.sub.3 of FIG. 3(a) but decrease as the concentration of Cr increases, and are extremely decreased in the steel B of the invention containing 3 % of Cr, FIG. 3(b) and not present at all in the steel C of the invention containing 5 % of Cr, FIG. 3(c).

It has been known that the emergence of intergranular fused layers in high speed steel has a particularly close relation with excess carbon, and this substantiates the cementation of the high speed steel.

FIGS. 4-8 show the measured hardness distributions at the welded portion of the conventional steel, the comparative steel and the steel of the instant invention after said respective steels are quenched at 1200.degree.C, maintained at 560.degree.C for 1 hour and tempered in air, and also quenched at 1250.degree.C and annealed in the same manner. In the diagrams, a curve a represents the hardness distribution of each steel which was tempered after the quenching at 1200.degree.C and a curve b represents the hardness distribution of the same which was tempered after quenched at 1250.degree.C. The hatched portion shows the welded portion of the blade material and the shank material, and the left side of said hatched portion is the blade material and the right side thereof the shank material.

FIG. 4 shows the hardness distribution curves when the conventional steel A.sub.2 is used as the shank material. It will be seen that the hardness increases at the portion of the high speed steel adjacent to the weld and decreases at the portion of the shank material adjacent to the weld. This apparently results from the cementation of the high speed steel and decarburization of the shank material during the annealing due to diffusion of carbon from the shank material into the high speed material. FIGS. 5-7 show the hardness distribution curves of the steels B, C and D according to the invention which contains 3 %, 5 % and 7 % of chromium respectively. The curves in FIG. 5 show less tendency of cementation and decarburization, and the curves in FIG. 7 show uniform hardness distributions and no tendency of cementation and decarburization. FIG. 8 shows the hardness distributions when the comparative steel A.sub.4 containing 10 % of chromium is used as the shank material. In this case, the activity of carbon in the shank material is excessively lower than that of carbon in the high speed steel, resulting in cementation of the shank material.

In order to compare the weldabilities of the respective shank materials with each other, with respect to the blade-constituting high speed steel, the transverse breaking load, the maximum deflection and the absorbed energy were measured on a test piece comprising the high speed steel and the respective shank material welded together by supporting said tests test on both sides of the weld and applying a concentrated load thereon. the results are shown in Table 3 given below.

Table 3 ______________________________________ Breaking Maximum Absorbed HRC hardness load deflec- energy Sample (kg) tion (kg-m) Shank Blade (mm) material material ______________________________________ Compar- ative 750 0.62 0.13 45.8 65.6 steel A.sub.3 Steel B of the 953 0.71 0.23 45.7 65.5 invention Steel C of the 923 0.60 0.24 45.5 65.6 invention Steel D of the 870 0.60 0.22 45.3 65.7 invention ______________________________________

From Table 3, it will be seen that the steels of the present invention also excel the comparative steel in weldability. Each test piece was prepared by concurrently quenching the blade material and the shank material and subjecting the same to tempering.

Now, the reasons for which the concentrations of the respective elements in the steel materials of the invention are restricted, will be explained.

Carbon partially forms carbides with chromium, vanadium and niobium, and partially forms a solid solution with the matrix to increase the strength of said matrix after quench and tempering. In the present invention, the carbon concentration is restricted in the range of 0.2-0.6 % because a concentration lower than 0.2 % results in an insufficient hardness of the shank or back material, while a concentration higher than 0.6 % results in an extremely low toughness of the same. Chromium partially forms a carbide and partially dissolved in the matrix improving the hardenability of the matrix. An increasing concentration of chromium tends to lower the carbon activity, so that the diffusion of carbon from the shank or back material into the high speed steel is lessened and accordingly the decarburization of the shank or back material and cementation of the high speed steel are lessened. This tendency appears from the chromium concentration of 3 % and the decarburization and cementation of the weld can be prevented at the chromium concentration of around 5 %. However, when the concentration of chromium exceeds 7 %, the self-hardening property of the weld is enhanced so highly that the hardness of the weld after the annealing increases, making the subsequent working difficult. At the same time, an instable retained austenite is formed at the weld by the quenching and tempering. Furthermore, because of the excessively low activity of carbon, diffusion of carbon from the high speed steel into the shank or back material takes place, with the result that said shank or back material tends to be carburized and the weldability becomes degraded. The concentration of chromium is specified to be within the range of 3.0-7.0 % in the present invention for the above reasons. Vanadium is an element effective for lowering the activity of carbon since it forms a stable carbide. This carbide is hardly soluble in austenite and prevents the growth of grains, and vanadium also increases the resistance to softening of the material. Therefore, the concentration of vanadium should at least be 0.1 %. However, a concentration of vanadium exceeding 1 % results in a lowering of hardness in relation with carbon and niobium. In addition, the material becomes expensive. Therefore, the vanadium concentration is restricted within the range of 0.1-1.0 %. Niobium forms fine special carbides having a high melting point and therefore effectively prevents the grains from becoming large in size. This effect of Niobium is noted from a concentration of 0.02 %, and the grain size becomes smaller and the toughness is remarkably improved as the concentration increases, but is decreased at concentrations higher than 0.3 %. Thus, the Niobium concentration is restricted within the range of 0.02-0.3 %. Silicon and manganese are used as deoxidizers. The concentrations of these elements are restricted to be not higher than 0.8 % because concentrations higher than the value adversely affect the weldability of the material.

As described hereinabove, the shank or back material according to the instant invention completely eliminate the decarburization and cementation of the weld between the blade material consisting of high speed steel and the shank or back material, by virtue of chromium and vanadium which serve to lower the activity of carbon, and has improved toughness and weldability by virtue of carbon or niobium incorporated therein at a suitable concentration.

Claims

What we claim is:

1. In a tool including a tool portion of known high speed steel material and a shank or back portion welded to said tool portion, the improvement comprising said shank or back portion being of a steel material consisting of 0.2-0.6% of carbon, 3.0-7.0% of chromium, 0.1-1.0% of vanadium, 0- 0.3% of niobium, not more than 0.8% of silicon, not more than 0.8% of manganese and the remainder of iron and inevitable impurities, wherein said steel material of said shank or back portion prevents decarburization or cementation in the weld between said shank or back portion and said tool portion respectively, said shank or back portion maintaining its toughness when the tool is subjected to heat treatment at high temperatures, and said steel material of the shank or back portion having an HRC hardness of more than 40 when subjected to said heat treatment under the same tempering conditions as for said high speed steel tool portion, said steel material of the shank or back portion having good, crack-free weldability with said high speed steel tool portion.

2. In a tool including a tool portion of a known high speed steel material and a shank or back portion welded to said tool portion, the improvement comprising said shank or back portion being of a steel material consisting of 0.2-0.6% of carbon, 3.0-7.0% of chromium, 0.1-1.0% of vanadium, 0.02-0.3% of niobium, not more than 0.8% of silicon, not more than 0.8% of manganese and the remainder of iron and inevitable impurities, wherein said steel material of said shank or back portion prevents decarburization or cementation in the weld between said shank or back portion and said tool portion respectively, said shank or back portion maintaining its toughness when the tool is subjected to heat treatment at high temperatures, and said steel material of the shank or back portion having an HRC hardness of more than 40 when subjected to said heat treatment under the same tempering conditions as for said high speed steel tool portion, said steel material of the shank or back portion having good, crack-free weldability with said high speed steel tool portions.

3. A tool according to claim 2, wherein said steel material has a composition consisting of 0.3-0.4% of carbon, 4.0-6.0% of chromium, 0.2-0.5% of vanadium, 0.02-0.3% of niobium, not more than 0.8% of silicon, not more than 0.8% of manganese and the remainder of iron and inevitable impurities.

4. A tool according to claim 2, wherein said steel material has a composition consisting of 0.3-0.4% of carbon, about 5% of chromium, 0.2-0.3% of vanadium, 0.09-0.3% of niobium, not more than 0.2% of silicon, not more than 0.3% of manganese and the remainder of iron and inevitable impurities.

5. A tool according to claim 1, wherein said steel material has a composition consisting of 0.3-0.4% of carbon, 3-7% of chromium, 0.2-0.3% of vanadium, not more than 0.2% of silicon, not more than 0.3% of manganese and the remainder of iron and inevitable impurities.