Method of Producing a Metallic Interface

1. Field of the Invention

The present invention relates to a method for producing a more desirable metallurgical bond when applying dissimilar metals by such thermal processes as welding, hot isostatic pressing (HIP) and diffusion.

2. Description of the Prior Art

Many alloy combinations used in various bonding processes tend to produce interfaces that have undesirable metallurgical and mechanical characteristics. A primary cause of these undesirable characteristics is element diffusion between alloy systems. Such element diffusion causes a zone of varying thickness, including diffused elements from both systems, to arise between involved alloy systems.

The present invention overcomes the shortcomings of the prior art by providing a method of producing a compatible metallic interface between dissimilar metal systems being thermally joined. In its broadest terms, the present invention comprises the step of causing a reduced element zone to arise between the dissimilar metals immediately before bonding. In a more narrowly defined embodiment, the method of the present invention is thermal in nature, involving exposing a portion of one of the dissimilar metals, an alloy combination, to a temperature above its lower critical. Such exposure is maintained for a period of time sufficient to cause surface diffusion of at least one of the constituent alloy elements. This, in effect, produces a surface zone reduced in alloy content that can more easily accept element diffusion which is known to take place during bonding. Production of such a surface zone leads directly to an interface with less total alloy, less residual stresses, increased ductility and decreased proneness to cracking.

The invention will now be more fully described, by way of example, with reference to the accompanying drawing in which the sole figure is a microphotograph of a cross-section of an interface produced by the method of the present invention.

A typical example of the method of the present invention would be HIP cladding of Inconel 625 to an alloy steel system such as 21/2 Cr-1Mo. The figure shows such a steel system after execution of the method of the present invention. Three clearly defined areas can be seen in the figure. The darkest area, on the far right, is the surface of the steel system to which Inconel 625 could be HIP clad. In accordance with the teachings of the present invention, that surface is exposed to a temperature above the known lower critical temperature for a 21/2 CR-1Mo alloy steel system. Such exposure is maintained until, as can be seen with further reference to the figure, an element reduced zone (the light rectangular area) is caused to form. Of course, uniform exposure of the surface of the steel system causes such a uniform deep zone to form, which, because of such factors as predictability of strength, would be desirable in most cases. This element reduced zone, because of the surface diffusion of at least one element which has taken place therein, has reduced alloy content compared to the remaining portion of the steel system, the rectangular area to the left in the figure. Because of this reduction in alloy content, the element reduced zone can more easily accept element diffusion that is known to invariably take place during overlaying, cladding and similar bonding operations. Desirable characteristics of such an interface have been found to include increased ductility and decreased proneness to cracking. Further, less residual stresses have been found within such interfaces and heat treatability is more predictable, making interface hardness easier to control and more suitable for H.sub.2 S type service.

The invention is not limited to the example set forth above. The scope of the invention is defined by the following claims.