Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
1998-07-29
2001-04-10
Ip, Sikyin (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S693000
Reexamination Certificate
active
06214136
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light-alloy casting heat treatment method and, more particularly, to a method of heat-treating a cylinder head consisting of an aluminum alloy.
2. Description of the Related Art
In the heat treatment of a light-alloy casting, a T
6
process complying with JIS (Japanese Industrial Standard) is generally employed to largely increase the material strength. In the T
6
process, a light-alloy casting (to be referred to as a work hereinafter) is heated at about 500° C. and held at this temperature for several hours. After this, the work is hardened in water at room temperature or warm water and held at about 180° C. for several hours.
However, since residual stress is generated in the work in hardening by the T
6
process, the work cracks due to fatigue under a severe use environment.
The residual stress is traced back to temperature differences among different portions of the work. For example, during hardening, the cooling rate is high outside the work while it is low inside the work, and a temperature difference is generated between outside and inside the work. When the thermal stress due to this temperature difference exceeds the material proof strength of the work, residual stress is generated. Especially, because castings have complex shapes, the temperature readily varies locally, and the residual stress becomes high.
To reduce the residual stress, various methods have been proposed.
A polymer solution is used as a hardening refrigerant (Japanese Patent Application No. 2-62247).
The work shape or refrigerant circulation is improved to promote refrigerant supply to the work (Japanese Patent Laid-Open No. 4-136141).
The refrigerant temperature in hardening is increased (Sumitomo Light Metal Industries, Ltd., Technical Report Vol. 31, No. 2, 1990 (pp. 28-44).
The temperature in tempering is increased (Aluminum, Vol. 3, ASM (1967), 355).
Vibration is applied to the work after heat treatment (Papers of Japan Society of Mechanical Engineers, Vol. 52, No. 477, 1986, May).
However, these prior arts cannot sufficiently reduce the residual stress. Even if the residual stress can be reduced, the material strength lowers.
SUMMARY OF THE INVENTION
The present invention has been made to solve the above problem, and has as its object to provide a light-alloy casting heat treatment method capable of reducing residual stress in heat treatment without lowering the material strength of a light-alloy casting.
In order to solve the above problem and achieve the above object, according to the present invention, there is provided a light-alloy casting heat treatment method of heating a light-alloy casting to a predetermined hardening temperature and cooling the light-alloy casting using a refrigerant, comprising the steps of maintaining film boiling of the refrigerant at least to a temperature at which a proof strength of the casting exceeds thermal stress is reached.
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patent: WO 94/26939 (1994-11-01), None
Shin Tsuchida, et al., Heat Treatment of Aluminum, pp. 28-45, (Apr. 1990).
Yasuhide Maekawa, et al., The Behavior of Surface Residual Stress Relief and Lattice Defect Density with Mechanical Vibration in FC25 Cast Iron, No. 85-1078 B, pp. 1295-1299, 1986.
Kent R. Van Horn, Aluminum, vol. III, Fabrication and Finishing, pp. 353-382, 1967.
Bates, C.E., “Selecting Quenchants to Maximize Tensile Properties and Minimize Distortion in Aluminium Parts”, Journal of Heat Treating, New York, NY, USA, (1987), vol. 5, No. 1, pp. 27-40.
Abe Ryoji
Fukuda Yukiyoshi
Ishimura Fuminori
Kodama Hiroshi
Miyatani Toshio
Ip Sikyin
Mazda Motor Corporation
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