Metal fusion bonding – Process – Diffusion type
Reexamination Certificate
2000-11-29
2002-11-12
Elve, M. Alexandra (Department: 1725)
Metal fusion bonding
Process
Diffusion type
C228S195000
Reexamination Certificate
active
06478214
ABSTRACT:
FIELD OF THE RELATED ART
The present invention relates to a diffusion welding assembly process between a martensitic stainless steel and a copper alloy, and in particular, to an assembly process of a martensitic stainless steel component with a component comprising copper.
Diffusion welding is a technique used to assemble solid phase components, i.e. without melting through the simultaneous application of a high temperature and pressure.
This technique may be used for example to produce a bimetal part designed to operate at high temperatures. The martensitic stainless steel component, or part, plays a role in the mechanical strength and/or wear and corrosion resistance of the part, and the copper or copper alloy component, or part, acts as a thermal well enabling the cooling of said part. Therefore, said two components must be assembled to ensure optimum thermal transfer from one component to the other and excellent thermomechanical resistance of the assembly.
Such a bimetal part is useful for example in the glass industry to form a mould, in metallurgy to form a continuous metal strip casting roll, to produce a non-cyclic or single-pole fluid-cooled generator, to produce an X-ray tube, etc.
In addition, the present invention also relates to a process to produce a thermomechanically resistant bimetal part comprising a component made of martensitic stainless steel assembled with a component comprising copper, and a bimetal part that can be obtained using said process.
STATE OF THE RELATED ART
Known steel and copper welding assembly techniques use an intermediate layer or a film of alloy, for example, based on nickel, placed between the steel and the copper to be assembled. Indeed, nickel forms a solid solution with copper and is fully compatible with steels. It ensures a good metal transition between the two materials and forms a barrier preventing the diffusion of copper in the steel. In this way, it reinforces the assembly formed.
In this way, the patent application EP-A-0 001 173 by Mitsubishi Jukogyo Kabushiki Kaisha describes a diffusion welding assembly process between a stainless steel and copper. Said process consists of placing, between the steel and copper to be assembled, an intermediate layer composed of Cr or an Ni—Cr, Cr—Ni or Cr alloy, containing not more than 140 ppm of O
2
.
The patent application EP-A-0 490 800 describes a diffusion welding assembly process between an 18% Ni GS austenitic cast iron material or 304 type austenitic stainless steel and a copper alloy material.
The document “Journal of Nuclear Materials”, 212-215 (1994) 1585-1589 by H. Nishi, Y. Muto and K. Sato describes a welding assembly process between a 316 steel and a copper alloy. 316 steel is an austenitic steel comprising 10.25% of Ni by weight and 16.29% of chromium by weight. Chromium is described in this document as an element which weakens the assembly formed.
These documents do not describe a diffusion welding assembly between a copper alloy and a martensitic stainless steel.
The precise microstructure of the assembly, or join, depends on the type of steel and copper used.
In addition, to obtain a martensitic steel, it is necessary after assembly to perform a quench hardening treatment from a high temperature so as to induce the martensitic conversion of the steel. During said conversion, a change of volume takes place in the steel, which induces stress in the copper. This is not the case with austenitic steels. Said stress is added to the differential thermal expansion stress between the two assembled materials, which is much higher in the case of austenitic steels as indicated by the thermal expansion values of the materials presented in table 1 below.
TABLE 1
Thermal expansion values for copper, an austenitic
steel and a martensitic stainless steel.
Austenitic
Martensitic
Material
Copper
steel
stainless steel
Grade
pure copper
types 304L, 316
types 430, 416,
420
Composition
—
16-19% Cr,
13-17% Cr
6-14% Ni
Extension
1.366%
1.322%
0.899%
between 20° C.
and 727° C.
Linear thermal
22.4 × 10
−6
K
−1
21.1 × 10
−6
K
−1
13.9× 10
−6
K
−1
expansion
coefficient at
727° C.
Therefore, in the case of the martensitic stainless steel, the assembly is subject to very severe mechanical, “thermal shock” type, strain.
Therefore, the dissipation of said stress without damaging the assembly requires that said assembly is sufficiently resistant.
DESCRIPTION OF THE INVENTION
The present invention relates to an assembly process of a martensitic stainless steel component with a component comprising copper, said process comprising in the following order:
a step consisting of degreasing and stripping the surfaces of the components to be assembled,
a step consisting of placing the degreased and stripped surfaces of the components to be assembled in direct contact, and
a step consisting of diffusion welding assembly of the surfaces of the components in contact.
Those skilled in the art easily understand that the process according to the invention can be used to assemble one or more martensitic stainless steel components with one of more components comprising copper, respectively.
According to the invention, the martensitic stainless steel component preferentially comprises a nickel concentration less than or equal to 4% by weight, and/or, advantageously and, a chromium concentration greater than or equal to 10% by weight. For example, it may comprise 0.1 to 1% by weight of C, up to approximately 4% by weight of Ni, 10 to 18% by weight of Cr, up to 2% by weight of Mo or W, up to 0.5% by weight of V, up to 1.5% by weight of Mn, up to 1.5% by weight of Si, up to 0.1% by weight of P, up to 0.2% by weight of S, the remainder being composed of iron and impurities.
An example of a martensitic stainless steel which may be used as this component is Z20 CN17-02 steel which comprises 0.2% by weight of C, 1.25 to 2.5% by weight of Ni, 15 to 17% by weight of Cr, 1% by weight of Mn, 1% by weight of Si, 0.04% by weight of P, 0.03% by weight of S, the remainder being composed of iron and impurities.
According to the invention, the component comprising copper may be for example composed of a copper alloy, the matrix of which is not alloyed, i.e. only comprising copper and impurities, according to the standard NF A 51050 not less than 99.8% by weight of copper, or a copper alloy reinforced by oxide dispersion, for which the matrix is preferentially not alloyed.
According to the invention, the components that can be assembled according to the process according to the invention may be of any size or shape. They may, for example, have a volume greater than one dm
3
and a thickness of up to a few centimeters. In this way, they may easily have a thickness greater than or equal to 1 mm on their surface to be assembled.
In the process according to the invention, the surface degreasing and stripping step may consist of conventional degreasing and stripping treatments for metal surfaces. The purpose of this step is to obtain clean surfaces, free of grease and oxidation. The grease may be removed from said surfaces, for example, using a solvent or conventional metal degreasing detergent. The stripping may consist of chemical or mechanical stripping, it may for example by performed by means of an acid or basic solution, or by grinding or polishing. According to the invention, the stripping technique may consist of chemical stripping followed by rinsing with water during which the material surface is scrubbed using an abrasive pad with an alumina fibre base, for example. Said treatment may be repeated several times, with the possibility of performing the last rinse with demineralised water.
This list is not exhaustive and it is possible to choose any technique enabling the elimination of traces of pollution and oxidation on the surfaces of the components to be assembled.
For the surface of the martensitic stainless steel component to be assembled, the degreasing solvent may be an organic solvent, for example, a ketone, ether, alcohol, alkane or chlorinated alkene such as trichloroethy
Burlet Hélène
Rigal Emmanuel
Burns Doane Swecker & Mathis L.L.P.
Commissariat a l'Energie Atomique
Elve M. Alexandra
Johnson Jonathan
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