Metallic Materials Based On Martensitic Steel

Abstract

The material is constituted by a regular alternation of relatively thick layers of martensitic structure and thinner layers of austenitic structure. The product is useful for sheet metal exposed to the action of hydrogen.

Description

The invention relates to metallic materials based on martensitic steel, i.e., metallic materials constituted of a major portion of a steel of martensitic structure, the expression "metallic material" being taken here in a very general sense and including both semi-manufactured products (sheet metal for example) and more fabricated products which have reached their final form from the industrial point of view (bars or tubes e.g.).

The invention relates also to methods of manufacture of such metallic materials based on martensitic steel.

Before introducing the main feature of the invention and to enable the originality and advantage of this feature to be better appreciated, it would be opportune to briefly recall here certain concepts, well known to metallurgists, relating to steels with martensitic structures and to stainless steels with austenitic structure.

Steels with martensitic structure, especially "maraging" steels (generally containing nickel, cobalt and molybdenum, but no chromium), have high mechanical properties (breaking load capable of reaching 230 hectobars), but, on the other hand, they are subject to corrosion phenomena by external agents and, especially, to the deleterious phenomenon of fragilization by hydrogen.

On the other hand, stainless steels of chrome-nickel, even materials based on a nickel, have, in the austenitic phase, an excellent resistance to corrosion but properties distinctly less, from the mechanical point of view, than the steels of martensitic structure.

There will be appreciated, under these conditions, the advantage which could be offered by a metallic material reconciling the properties (recounted above) of martensitic steels and stainless steels (or materials based on nickel) and having good resistance to the deleterious phenomena of fragilization by hydrogen.

It is a specific object of the invention to provide a material having these properties, as well as a method for the manufacture of such a material.

It constitutes a practical and industrial development of fundamental researches which have shown that a material constituted of a steel of martensitic structure (especially maraging steel) could, whilst preserving its excellent mechanical qualities, be effectively protected, against corrosion and the risks of fragilization by hydrogen, by a superficial coating constituted by a layer of austenitic structure obtained by diffusion of chromium, the said layer playing the role of a diffusion barrier vis-a-vis the attacking hydrogen agent coming from the outside.

For convenience in the description, such a protective layer of austenitic structure will be denoted below by the expression "austenitic layer," whether it relates to a stainless steel (especially chrome-nickel) or to a material based on nickel, even pure nickel.

The metallic material according to the invention is a massive ferrous material constituted by a regular alternation of relatively thick layers of martensitic structure and thinner layers of austenitic structure, the thickness of any martensitic layer being preferably equal to at least 5 times (and preferably even, about 10 times) the average thickness of the adjacent austenitic layers situated respectively on both sides of the martensitic layer concerned, the massive metallic material thus constituted having, due to its mixed austeno-martensitic stratified structure with predominance of martensite by weight, on the one hand, high mechanical properties (breaking load greater than 150 hectobars) due to the predominance of the martensitic phase hardenable by intermetallic precipitation, on the other hand, an excellent resistance to corrosion and especially to fragilization by hydrogen, this resistance in depth of the material resulting from the presence of successive diffusion barriers constituted by the austenitic layers, and, on the other hand lastly, the qualities inherent in stratified materials with rigid layers (martensitic layers) separated by ductile layers (austenitic layers of which the ductility is approximately five times greater than that of the martensitic layers).

The martensitic layers can all have a same thickness and the austenitic layers also a same thickness (less than the preceding one).

But it would also be possible to give various thicknesses to certain at least of the martensitic layers and/or to certain at least of the austenitic layers, with the reservation only of preserving the preponderance to the martensitic layers.

Thus, for example, it would be advantageous to increase the size of the austenitic layers, playing the role of diffusion barriers, on the side of the face most exposed to the manufactured element if the said element has in effect a face more exposed than the other to attacking agents.

In any case, in the majority of cases, it would be convenient to attribute to each martensitic layer, as already indicated, a thickness equal to at least five times, and preferably of the order of ten times, the average thickness of the two austenitic layers situated on both sides of the martensitic layer concerned.

As for the method according to the invention, it consists, with a view to obtaining the metallic material with alternated martensitic layers and austenitic layers which has just been considered,

in manufacturing first, in a first operation conducted at a temperature comprised between 850.degree. and 1000.degree.C and preferably of the order of 900.degree. to 950.degree.C, a material with alternated layers of austenitic structure and of non-hardened martensitic structure, the latter layers being preponderant from the weight point of view,

and in then subjecting, the material resulting from this first operation, to a thermal hardening treatment of the martensitic layers, this hardening treatment being preferably of the type of conventional hardening methods for maraging steels (e.g. a treatment for three hours at 480.degree.C).

The invention will, in any case, be better understood with the aid of the supplementary description which follows, as well of the accompanying drawing, which supplement and drawing are given purely by way of illustrative but non-limiting example of the methods of application and of the preferred embodiments.

FIG. 1 shows, in section with parts removed and with a considerable enlargement, a stratified composite sheet metal according to the invention.

FIGS. 2 and 3 show, under the same conditions as FIG. 1, a stratified composite bar and tube according to the invention.

The composite sheet metal illustrated in FIG. 1 is formed of alternated layers of martensite M and austenite A. All the layers M have a same thickness E and all the layers A a same thickness e at least five times less than E and, preferably, of the order of one tenth of E.

There can then, advantageously, be adopted, for E, a value comprised between 1000 and 50 microns, e.g. of the order of 200 microns,

and for e, a value comprised between 100 and 10 microns, e.g. of the order of 20 microns.

From the operational point of view, it would be possible to manufacture such a composite sheet metal,

by welding edge to edge, e.g., in an oven with electronic bombardement, the constituent elements of an alternated stack of thin sheets of stainless steel containing nickel (even thin sheets of nickel) and of thicker sheet metal of maraging steel,

and then proceeding, at a temperature of 900.degree. to 950.degree.C, with a co-rolling of the stratified blank thus obtained, in order to obtain anchoring of the said leaves and sheets through their surfaces in contact, in the course of hot colaminating, the ratio of thicknesses of the two types of layers remaining constant,

and making the composite sheet metal thus obtained undergo a hardening treatment of the martensitic layers, e.g. by heating for three hours at 480.degree.C.

According to a variation, a composite sheet metal according to the invention can be obtained from a simple stack of sheets of maraging steel having a layer of nickel on their two faces.

In any case, the outer surfaces of the sheet metal are preferably formed by austenitic layers, i.e., by the layers more adapted to resisting corrosion.

FIG. 2 shows a bar or rod formed of alternated coaxial layers of martensite M and austenite A, the martensite being predominant from the weight point of view and the respective thicknesses E and e of the layers M and A being advantageously able to satisfy the dimensional criteria explained previously, in a general form, relative to the abovesaid thicknesses.

Such a bar could be used for the reinforcement of concrete and, whatever its application, it would be advantageous to make it include an austenitic outer layer A1 in order to protect it against corrosion by environmental agents.

FIG. 3 finally, shows a tube formed of alternated coaxial layers of martensite M and of austenite A, the respective thicknesses E and e of the said layers being able, this time again, to satisfy the abovesaid dimensional criteria.

Such tube could be suitable for channeling gas or destructive liquids under pressure and it would then be advantageous to make it include an inner wall Ao of austenitic structure, its outer wall A1 being also able to be austenitic, especially if there is risk of destruction from the outside.

In a general way, the fields of application of the composite elements according to the invention are multiple and there can be mentioned, especially, the fields of aerospace technology and of chemical and nuclear engineering.

Claims

We claim:

1. Massive ferrous metallic material having high mechanical strength and resistance in depth of the material to hydrogen embrittlement consisting essentially of a regular alternation of a plurality of layers of martensitic steel and of layers of austenitic structure having a resistance to corrosion higher and a mechanical resistance lower than those of said martensitic steel, the thickness of each martensitic layer being of a magnitude equal to at least five times the average thickness of the adjacent austenitic layers situated respectively on both sides of the martensitic layer concerned.

2. Massive ferrous metallic material according to claim 1 wherein the outer surface thereof comprises a layer of said austenitic structure.

3. Metallic material according to claim 1, wherein said magnitude is about ten times.

4. Metallic material according to claim 1, wherein each of the martensitic layers have the same thickness and each of the austenitic layers have the same thickness.

5. Metallic material according to claim 1, wherein the martensitic layers have different thicknesses, and the austenitic layers have different thicknesses.

6. Metallic material according to claim 1, wherein said material is a metal sheet with parallel layers.

7. Metallic material according to claim 1, wherein said material is a bar with coaxial layers.

8. Metallic material according to claim 1, wherein said material is a tube with coaxial layers.

9. Metallic material according to claim 1, wherein the thickness of the martensitic layers is between 1000 and 50 microns and the thickness of the austenitic layers is between 100 and 10 microns.

10. Iron base material according to claim 1, wherein said layers of austenitic structure are of a material selected from the group consisting of Cr. Ni. austenitic steels, nickel and nickel-base alloys.