Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
1999-01-28
2001-01-30
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S594000
Reexamination Certificate
active
06179936
ABSTRACT:
This application is based on Japanese Patent Applications HEI 10-16492 filed on Jan. 29, 1998, HEI 10 - 367639 filed on Dec. 24, 1998 and HEI11-1010 filed on Jan. 6, 1999, the content of which is incorporated into the present application by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for heat-treating a hollow cylindrical workpiece. The hollow cylindrical workpiece includes, for example, a bushing which is one of members used for an endless track mounted to construction vehicles, but the hollow cylindrical workpiece is not limited to the bushing only.
2. Description of Related Art
For a hollow cylindrical work such as a bushing used for an endless track, hardness is required at an inside surface, an outside surface, and portions adjacent thereto so as to ensure a high wear resistance, and toughness is required at a core portion of a wall so as to prevent cracks from propagating from the surfaces.
In order to satisfy those two quality requirements at the same time, the following heat-treatment methods have been proposed:
(1) A method as proposed in Japanese Patent Publication No. SHO 59 - 77979, as illustrated in
FIG. 6
, includes a first step in which a workpiece
111
is subjected to high-frequency quench-hardening from an outside surface of the workpiece
111
to a part of an inside effective quench-hardened layer, and a second step in which, after the first step, an inside portion of the workpiece
111
is subjected to high-frequency quench-hardening, while a core portion of a wall of the workpiece
111
is tempered.
In
FIG. 6
, a reference numeral
112
denotes a tool for setting a workpiece, reference numeral
113
denotes a heating coil, reference numeral
114
denotes a cooling jacket, reference numeral
115
denotes a heating coil, and reference numerals
116
and
117
denote cooling jackets. In the first step, the workpiece
111
is conveyed in a vertical direction, and quench-hardening is conducted by heating the workpiece
111
from the outside surface, followed by cooling the workpiece
111
from the outside surface. In the second step, the workpiece
111
is conveyed in a vertical direction, and quench-hardening is conducted by heating the workpiece
111
from the inside surface, followed by cooling the workpiece
111
from the outside surface and from the inside surface at the same time.
FIG. 7
illustrates a hardness distribution in the workpiece
111
after each step has been conducted.
(2) A method as proposed in Japanese Patent Publication No. HEI 9-143564 (Japanese Patent Application No. HEI 7-299997), as illustrated in
FIG. 8
, includes a first step in which high-frequency quench-hardening is conducted to an entire cross-section of a wall from an outside surface to an inside surface of a workpiece
120
, and a second step in which, after the first step, an inside portion of the workpiece
120
is subjected to high-frequency quench-hardening, while a core portion of the wall is tempered.
In
FIG. 8
, reference numeral
121
denotes a heating coil, reference numeral
122
denotes a cooling jacket, reference numerals
123
and
124
denote rotatory rollers, reference numeral
125
denotes a heating coil, and reference numeral
126
denotes a cooling jacket. In the first step, the workpiece
120
is conveyed in a horizontal direction, and quench-hardening is conducted by heating the workpiece
120
from the outside surface, followed by cooling the workpiece
120
from the outside surface after a temperature of the workpiece
120
is made uniform, while the workpiece
120
is conveyed from the heating coil
121
to the cooling jacket
122
which are spaced from each other. In the second step, the workpiece
120
is conveyed in a vertical direction including an oblique direction inclined with respect to the vertical direction, and quench-hardening is conducted by heating the workpiece
120
from the inside surface, followed by cooling the workpiece
120
from the outside surface.
FIG. 9
illustrates a hardness distribution in the workpiece
120
after each step has been conducted.
(3) A method as proposed in Japanese Patent Publication No. HEI 1-75629 includes a first step in which high-frequency quench-hardening is conducted to an entire cross-section of a wall from an outside surface to an inside surface of a workpiece, and a second step in which after the first step, while an inside portion of the workpiece is cooled, the workpiece is high-frequency quench-hardened from the outside portion, and a core portion of the wall is tempered.
In the second step, a cooling jacket is inserted within an inside surface of the workpiece so as to cool the inside portion of the workpiece.
However, in the above conventional methods, it is not possible to conduct a continuous heat-treatment in an integrated line including the first step and the second step.
More particularly, with the first conventional method (1), since the workpiece is conveyed in a vertical direction in both the first step and the second step, heat-treating the workpieces is not conducted continuously, but conducted intermittently one by one.
With the second conventional method (2), in the first step, the workpiece is conveyed in a horizontal direction, which enables a continuous heat-treatment in the first step. However, since the workpiece is conveyed in a vertical direction in the second step, heat-treating the workpieces is conducted intermittently one by one in the second step.
With the third conventional method (3), since the cooling jacket is inserted within the inside surface of the workpiece in the second step, heat-treating the workpieces is conducted intermittently one by one.
Accordingly, the above conventional methods have the following problems:
(a) A total cycle time is determined by a cycle time of the intermittent heat-treatment step, so that productivity is not further improved beyond that.
(b) A handling apparatus for inserting the heating coil and the cooling jacket within the inside surface of the workpiece needs to be provided, resulting in an increase in an initial cost.
(c) Since a temperature difference is caused between opposite end portions and a central portion of an entire length of the workpiece in the intermittent heat-treatment, the magnitude of deviation in a dimension of the workpiece after heat-treatment is increased.
SUMMARY OF THE INVETION
An object of the present invention is to provide a method for heat-treating a hollow cylindrical workpiece which enables continuous heat-treatment conducted in an integrated line.
In the method of the above-described object, the hollow cylindrical workpiece has an outside surface and an inside surface, which jointly define the outer and inner surface of a wall. The wall has an outside portion defined between the outside surface and a position spaced from the outside surface by a distance greater than one fourth of a thickness of the wall and less than one half of a thickness of the wall. An inside portion is defined between the inside surface and a position spaced from the inside surface by a distance less than one half of a thickness of the wall, and a core portion defined between the outside portion and the inside portion.
The above-described object is achieved with the method of quench-hardening. In a first step of quench-hardening, a workpiece is continuously conveyed, one workpiece after another in a substantially horizontal direction without a space between adjacent workpieces. While the workpieces are being continuously conveyed one after the other, they are simultaneously induction-heated across an entire cross-section of the wall of the workpiece to a temperature equal to or higher than an Ac
3
, transformation temperature and equal to or lower than a temperature 200° C. higher than the Ac
3
transformation temperature, but only from the outside surface of the workpiece. The temperature of the workpiece is made substantially uniform in a longitudinal direction and in a thickness direction of the workpiece by using the time required to move the workpiece to a co
Goto Kimio
Nakajima Masahiro
Takeno Hiroyuki
Yoshida Isao
Morillo Janell Combs
Pillsbury Madison & Sutro LLP
Topy Kogyo Kabushiki Kaisha
Wyszomierski George
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