EP0028213A1 - Metal alloys - Google Patents

Abstract

The composition of metal alloys expressed in percentages follow the following ponderal relationships: 3% //c chromium //c 20%; 7% //c molybdenum //c 40%; C //c 0.1%; Fe, together with its usual impurities and optionally Mn (:5% at the most), makes up the balance yet not smaller than 50%, the whole in a matrix, at least partially ferritic, and having separately or in admixture intermetallic compounds of the Laves and Ktypes. In these alloys, the resistance to corrosion increases together with the chromium content, which enhances the formation of the phase K. The molybdenum content is a direct function of the desired amount of intermetallic compounds. To give the alloys an improved hot resistance or in certain corrosive medium, up to half of molybdenum may be replaced by tungsten. The stability of the Laves and Kphases is improved by supplying silicium. These alloys exhibit simultaneously a good resistance to wear and corrosion and a low friction coefficient.

Description

The present invention relates to metal alloys having improved properties, in particular as regards their resistance to wear.

In most machines or devices comprising metal parts or elements moving in relation to each other, or pressing on each other, the longevity of said parts or elements is directly conditioned by their mechanical properties, this term being taken in its broadest sense and including in particular the breaking load, the elastic limit, the resistance to shocks, wear, fatigue, etc ...

In addition to the best values for the above-mentioned properties which promote a long service life, it is also interesting to obtain a coefficient of friction as low as possible (which reduces the heating caused by this friction), as well as a satisfactory resistance to corrosive atmospheres to which the devices under consideration may be exposed, whether or not they are in motion, and whatever the temperature of the medium in which they are to be used.

It is worth remembering here that certain conditions of use do not allow continuous lubrication of the surfaces of the parts in contact and in relative movement; these must therefore, at least for a certain time, be able to run dry, without incurring damage. It should be noted that in some cases, the lubrication possibilities may be poor.

The great possible diversity of stresses and operating conditions of devices comprising metal parts or elements in relative motion with respect to each other has led users to make a selection from among the materials available and to choose from those which, all by having properties to be considered satisfactory, having regard to the stresses envisaged, could present, at the same time as very variable compositions, intrinsic costs also very diverse. This means that the economic context was also a factor that could influence the choice to be made.

Among the solutions already considered and especially with regard to wear resistance, mention may be made of high carbon steels or alloy steels, which have a high hardness due to the presence of carbon, respectively of carbide generating elements. .

Ferrous alloys, high chromium cast irons and alloys such as Co-Cr-W-C have also been used. Surface hardening treatments have also been considered.

It has however been noted that, among the compositions mentioned above, it is ferrous and non-ferrous alloys, with a high carbon content, which have been most widely used to constitute parts or elements which, while satisfactorily resistant to wear, were the least expensive. These last compositions, which owe their hardness However, the presence of carbide precipitates has recently been challenged by alloys for which the hardness is due to the. formation of intermetallic phases, such as the Laves phase, which have a matrix that is both more ductile and more tenacious in which the intermetallic compounds precipitate. They also have, which is particularly interesting, a low coefficient of friction.

However, these alloys with intermetallic phases, based on cobalt and / or nickel, are particularly expensive to prepare, which makes their use for many applications unattractive.

This is an important drawback, which the object of the present invention aims precisely to overcome.

The present invention therefore relates to metal alloys of a particular composition, simultaneously having good wear resistance, good corrosion resistance, a low coefficient of friction, as well as a low cost price. Thanks to these properties verified simultaneously, these alloys are capable of a wider range of use, under economically advantageous conditions.

constitue le solde non inférieur toutefois à 50%,le tout dans une matrice, au moins partiellement ferritique et présentant isolément ou en mélange des composés intermétalliques des types Laves et VThe alloys which are the subject of the present invention are essentially characterized in that they verify, in weight proportions, the following relationships:

constitutes the balance not less than 50%, all in a matrix, at least partially ferritic and presenting individually or as a mixture of intermetallic compounds of the Laves and V types

In the alloys according to the invention, the corrosion resistance increases with the chromium content, which also promotes the formation of the χ phase.

The molybdenum content is a direct function of the quantity of intermetallic compounds desired (which can range from 30% to 90%). In the case where it is sought to give the alloys of the invention an increased resistance to heat or in certain corrosive environments, it is possible to replace up to half of the molybdenum with tungsten, the replacement being carried out on the basis of 'an atom of W for an atom of Mo.

The stability of the Laves phases is increased by the addition of silicon, the proportion of which cannot, however, exceed 4%, while being at least 1%.

Furthermore, the alloys of the invention may also advantageously contain from 0.1% up to 7% of Ti, from 0.1% up to 7% of Zr and from 0.1% up to 7% of V, which has the additional advantage of favoring certain intermetallic phases over others.

It is furthermore advantageous, according to the invention, to replace iron with a mixture (Co + Ni) in a weight proportion not exceeding 20% of the initial iron, in which case the latter could not fall below 40% of the total.

This replacement leads to the appearance of a partially austenitic matrix (cubic lattice with centered faces), while the other part of the matrix remains ferritic (cubic lattice centered).

This variant has the advantage of better resistance of the alloy to corrosion and better heat resistance.

A few examples will serve to illustrate the characteristics of the alloys.

The T 10 alloy containing 30% Mo 10% Cr and 2.5% Si, iron balance, produced in an induction furnace from Armco iron and ferro-alloys and cast in the form of an ingot mold in a square section of 80 mm side at the base and 55 mm at the top, present in the cast state + 80% of intermetallic phase containing in the order of decreasing contents Mo, Fe, Cr, Si, the matrix containing in the same order Fe, Cr, Mo , Si. The micro-hardness of the intermetallic phase and that of an alloy (T 400) titrating 62% Co, 8% Cr, 28% Mo, 2.5% Si are of the same order of magnitude.

La bonne tenue des alliages T 10 et T 11 découle immédiatement des données ci-dessus.We have also compared the wear resistance of two alloys corresponding to the invention to that of a known _C o-Cr-Mo-Si alloy. The tests were carried out by rotating a hardened AISI 52100 steel disc returned to 50 RC against an alloy plate under an initial pressure of 41.7 MPa. After 80,000 revolutions, the weight losses were measured and the following results were obtained.

The good performance of the T 10 and T 11 alloys immediately follows from the above data.

Regarding the corrosion resistance of these alloys, they were immersed in a solution containing 60% nitric acid at 66 ° C for a total of 72 hours. The alloys T 10 and T 11 and Co Cr - Mo Si have a weight loss of the order of 20 to 40 gr / m ² h.

In the case where an even higher corrosion resistance is desired, alloys of modified composition such as Fe can be used; Cr 15, Mo 15; If 2.5 whose loss by weight under the same test conditions is 5 gr / m ² .h.

The alloys are used by techniques which are well known per se. The production is carried out starting from either basic raw materials or from ferro-alloys in induction furnaces. The implementation in the form of cast parts is done by the techniques, for example, precision casting in lost wax, casting by the Shaw method or other method allowing parts to be obtained with virtually final dimensions. The implementation in the form of powders involves the development of the alloy as indicated above and its atomization under water or under gas, with a view to obtaining powders of particle size adequate for the intended uses. If necessary, the powders are subjected to heat treatments with a view to relieving the tensions or to other operations necessary for the final conditioning of their particle size. The use of powders is done by the techniques of powder metallurgy per se well known. After mixing the powders of the alloys which are the subject of this invention, with powders constituting the matrix, such as steel powders, powders of non-ferrous alloys or powders of stainless steels, we proceed to their compacting followed by sintering under vacuum or under a reducing atmosphere. Densification techniques by processes comprising deformation such as, for example, densification by extrusion or isostatic hot compaction can also be used.

• Alliage A : composition 30 % Mo - 10 % Cr - 2,5 % Si - solde Fe sous forme de poudre, et dans une matrice de AISI 316 L, dans une proportion de 10 %.
• Alliage B : matrice AISI 316 L avec 10 % en poids d'un alliage contenant 50 % Ni - 32 % Mo - 3 % Si - 15 % Cr.
  

As an example of an alloy obtained by the powder metallurgy technique, the following can be given:

• Alloy A: composition 30% Mo - 10% Cr - 2.5% Si - balance Fe in powder form, and in a matrix of AISI 316 L, in a proportion of 10%.
• Alloy B: AISI 316 L matrix with 10% by weight of an alloy containing 50% Ni - 32% Mo - 3% Si - 15% Cr.
  

The addition of the powdered alloy to the 316 L matrix produces the same influence as the addition of a material with similar properties and having the same Laves phase content. We can expect equivalent properties with regard to their resistance.

A drum wear test produced the following results.

It can be concluded from this that the material T ₁₀ has advantages both in the cast state and in the plasma spray coating state.

Claims (8)

1. Metal alloys, characterized in that their centesimal composition verifies the following weight relationships:

accompanied by its usual impurities, the whole having an at least partially ferritic matrix, as well as, individually or as a mixture, intermetallic compounds of the Laves and χ types. 2. Alloys according to claim 1, characterized in that they also comprise silicon, in a proportion of between 1% and 4%. 3. Alloys according to either of Claims 1 and 2, characterized in that they also comprise Ti and / or Zr and / or V, in a proportion which can reach from 0.1% to 7 % for each of these 3 elements. 4. Alloys according to either of claims 1 to 3, characterized in that at most 20% of the Fe taken up in the composition of claim 1 is replaced by a mixture of Ni and Co. 5. Alloys according to either of claims 1 to 4, characterized in that at most half of the Mo present is replaced by W, on the basis of a W atom for a Mo atom. 6. Alloys according to either of claims 1 to 5, characterized in that the iron contains at most 2.5% of Mn. 7. Alloys according to either of claims 1 to 6, characterized in that they comprise at least any two and preferably the 3 elements mentioned in claim 3. 8. Alloys and methods for obtaining them, as described above.