GB2208819A - Preparation of metallic interface - Google Patents

Abstract

A method of producing a metallic interface suitable for overlaying consists of exposing the area of an alloy combination to be overlayed to a temperature above its lower critical until an element reduced zone arises. This element reduced zone occurs by diffusion and more easily accepts element diffusion that takes place during overlaying. Accordingly, an improved interface, one that is more ductile and less prone to cracking, is produced.

Description

DESCRIPTION METHOD OF PRODUCING A METALLIC INTERFACE The present invention relates to a method of producing a desirable metallurgical bond when applying dissimiliar metals by such thermal processes as welding, hot isostatic pressing (HIP) and diffusion.

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 dissimiliar 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 dissimiliar metals immediately before bonding. The method may be thermal in nature, involving exposing a portion of one of the dissimiliar 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, products 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 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 2-1/2 Cr-lMo. 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 2-1/2 Cr-lMo alloy steel system. Such exposure is maintained until, as can be seen with further reference to the micrograph 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, becasuse 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 micrograph.

Because of being reduced in alloy content, the element reduced zone can more easily accept element diffusion that is invariabl & known to take place during overlaying, cladding and similiar 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 H2S type service.

Claims (7)

1. A method of producing an improved metallurgical bond between dissimiliar metals, the method comprising the step of causing an element reduced zone to arise between the dissimiliar metals immediately before bonding.

2. A method according to claim 1, wherein the element reduced zone is caused to arise thermally.

3. A method according to claim 2, wherein at least one of the dissimiliar metals comprises an alloy combination having a surface to which another dissimiliar metal is to be joined and wherein the reduced element zone is cause to arise by the step of heating the surface until surface diffusion of at least one of the constituent elements is caused.

4. A method of joining together dissimiliar metals of which at least one comprises an alloy and has a surface to which the other metal is to be applied, the method comprising the steps of heating the surface of the alloy until surface diffusion of at least one of the constituent alloy elements causes a surface zone reduced in alloy content to arise; and applying the other metal to the surface zone reduced in alloy content.

5. A method according to claim 4, wherein the step of heating the surface further comprises uniformly heating the surface so that the surface zone reduced in alloy content which arises is of uniform depth.

6. A method of preparing an alloy combination for overlaying, the alloy combination having a lower critical temperature and a surface to be overlayed, the method comprising the steps of exposing the surface of the alloy combination to a temperature above its lower critical; and maintaining exposure of the surface of the alloy combination to a temperature above its lower critical; and maintaining exposure of the surface of the alloy combination to a temperature above its lower critical until diffusion of at least one of the constituent alloy elements takes place throughout the entire surface.

7. A method of joining two metals, substantially as described with reference to the accompanying micrograph.