The invention relates to a method for preparing a wear-resistant multimaterial, particularly for preparing a bi-metallic, cast and hot-worked wear steel. In addition, the invention relates to the use of a multimaterial prepared by such a method.

Background of the invention

The wear-resistance, such as the abrasion resistance, of steels and iron-based materials is mainly increased by two principal methods:

(i) by increasing the hardness of the steel mostly through alloying and heat treatment, or (ii) by generating ceramic particles, mostly carbides, in the material by means of alloying and heat treatment.

The latter of these methods is more effective, because the particles in the metallic matrix which are very hard and harder than the abrasive particles most efficiently prevent the penetration of abrasive particles into the metal and the wearing of the metal by a cutting wear mechanism. The hardest carbides have a hardness of about 3000 HV (vanadium carbide), whereas purely by means of a heat treatment a hardness level of about 650 to 700 HV at most can be produced in the metal.

Increasing the wear resistance by means of carbides produced in the microstructure decreases toughness substantially, because the carbides crack through nucleation of cracks and materials loss cracks which are easily induced because of wear loads. Large carbides or carbide networks producing continuous networks for example on grain boundaries are therefore particularly detrimental. The embrittling effect of carbides can be reduced by preventing the formation of continuous carbide networks (typical i.a. for cast materials such as white cast iron). Another way of reducing the embrittling effect of carbides is to orientate the carbides so that their longitudinal dimension is perpendicular to the wear surface. In addition, by controlling the size distribution of the carbides the combination of toughness and wear resistance can be optimized. With respect to wear materials, it is crucial to avoid macroscopic cracking of the material. Detaching of large objects from wear parts and their passing to a subsequent stage of the process may cause serious damages and an interruption of use in the whole facility. There are many ways to prevent macroscopic breaks. One way is to prepare a component so that the whole component is not prepared from a crack-sensitive material and a breakage of a brittle wear-resistant material will only cause a limited break and the detachment of a piece.

One way of preparing high-alloy materials containing a very large amount of small carbides is powder metallurgy. In powder metallurgical methods a high- alloy powder is first prepared by gas atomization, which powder is compacted to a compact material by hot isostatic pressing. When needed, the hot isostatically pressed material can be hot- worked later to a desired dimension and shape. The disadvantage of the powder metallurgical manufacturing method is its high cost. In addition, the carbide size obtained by the method, typically an average size less than 5 μm, is not suitable for all heavy wear conditions.

Summary of the invention

The manufacturing method according to the present invention is able to remove or at least to essentially reduce the above presented disadvantages of the prior art.

By the method according to the invention it is possible to prepare highly wear-resistant materials relatively cost-efficiently, the toughness of which materials is good owing to the decomposition of the carbide network of the microstructure achieved through hot- working and to the tough material supporting and at least partly surrounding the billet.

Owing to the tough material surrounding the starting billet and to the process metallurgical method, such as for example electro slag remelting, vacuum arc remelting or spray forming, which is used for preparing the hard wear-resistant material, the material is hot-workable. The machinability of the tough material surrounding the billet is clearly better than that of the wear-resistant material, which facilitates the machining of the outer surface of the billet to precise mounting and connection dimensions. The orientation of the carbide network of the wear-resistant material along the longitudinal dimension of the billet during hot-working can be utilized in wear parts by using preferably the heavy-wear resistant cross-sectional surface as the wear surface.

Particularly for high-alloy materials it is preferred to use electro slag remelting (ESR), vacuum arc remelting (VAR) or spray forming for manufacturing the wear-resistant material, so that the homogeneity of the billet and the hot-working properties of the structure can be improved. For lower-alloy materials also traditional casting methods can be used.

More particularly, the method according to the invention is characterized by what is disclosed in the characterizing part of claim 1, and the use of the multimaterial prepared by the method according to the invention is characterized by what is disclosed in claim 10.

Description of the drawings

The invention is described below in more detail by way of example by referring to the appended Figures, in which

Figure Ia shows schematically a structure of a starting billet used in the method according to the invention,

Figure Ib shows the starting billet of Figure Ia after hot- working,

Figure 2a shows schematically, before hot-working, large carbides in a cast starting billet used in the method according to the invention,

Figure 2b shows the carbides of Figure 2a after hot- working,

Figure 3 a shows schematically as a side view the location of material layers in a material prepared by the method according to the invention, and Figure 3b shows from above the location of the material layers of the material of Figure 3a.

Detailed description of the invention

Figure Ia shows a starting billet 100 used in the method according to the invention, which billet comprises a tough cast material 101, a wear-resistant material 102, a mold plate 103 and a mold 104.

In the method according to the invention a billet consisting of two or more materials is prepared. The materials are chosen so that at least one of them is a wear-resistant iron- based (Fe > 50 wt-%) material and at least one is a tough mechanically robust iron-based material (Fe > 50 wt-%). After manufacturing the billet, it is hot-worked by means of pressure and temperature for example by hot forging, hot rolling or hot isostatic pressing.

The billet 100 can be prepared in a number of ways depending on the materials to be used. The materials to be used are chosen in accordance with the material constraints imposed by the object, whereby the choice is affected i.a. by the loads on the material, wear conditions etc.

In the method according to the invention a billet 100 is prepared by static casting in a casting-technical sense, i.e. by casting to a solid mold, whereby two different materials are cast in sections of the mold 104, which materials are cast in different sections of the mold so that they are not mixed with each other. This can be carried out for example by placing a metallic divider plate 103 in the mold 104 for separating the regions formed by the two different materials 101 and 102 from each other. During casting, both materials 101 and 102 melt the metallic divider plate 103 partly but not completely, however, whereby the materials 101 and 102 do not become mixed with each other. As the casting solidifies, a billet 100 is formed, which billet, after solidification, is ready for being hot- worked to its final shape, as shown in Figure Ib. This hot-working additionally provides the compaction of the billet 100 and the desired changes in the microstructure, i.a. the desired orientation of the microstructure and its properties. Naturally, it must be ensured in the method that particularly the wear-resistant and more brittle material is hot- workable in cast condition without being cracked and the component damaged.

An alternative way according to the invention for the above described method is especially to prepare billets containing particularly high-alloy materials so that the billet 100 is prepared by electro slag remelting, vacuum arc remelting or spray forming so that segregation of the high-alloy material is minimized, whereby its purity level remains as high as possible.

In electro slag remelting a starting billet is first prepared from desired starting materials, after which it is melted as tiny droplets through a hot slag, whereby its purity level is increased. At the same time it is possible to reduce segregation mostly owing to the longitudinal solidification of the billet, whereby the quality of all billet materials is homogeneous. Thanks to this, the material has a better hot-workability than a traditionally cast and solidified material, and its working characteristics are better. In vacuum arc remelting the starting billet is melted as tiny droplets without slag by means of an electric arc generated between the starting billet and the basic plate of a copper mold. In spray forming the molten metal is decomposed into small ^" partly molten droplets by means of the kinetic energy of a gas spray, which droplets are collected to a collector plate before they are completely solidified, on which collector plate a metallic billet is thus deposited. Because the solidification rate is fast in spray forming, the homogeneity of the structure is very good.

To improve the hot- workability of the billet, in the method according to the invention the billet 100 can be prepared in the above-described manner so that the tougher and less crack-sensitive material 201 surrounds the more brittle material 202 with an inferior hot- workability, as shown in Figures 2a and 2b. Even if cracks were formed in the more brittle material 202 which is surrounded by the tougher material 201, particularly in the initial stage of hot-working, they will not open up to the surface and become oxidized, whereby these cracks will heal up when hot-working is continued. The success of hot- working is also promoted by the fact that by preparing the billet by electro slag remelting its hot-workability will be better than that of a material prepared by static casting. The tougher billet surrounding the more wear-resistant material can be prepared separately for example by bending from a plate, by machining a hole in a billet or by preparing a cast billet. The surrounding material can also be sealed entirely by welding covers to both ends of the billet. In certain situations the billet can even be prepared by casting a tough material around a wear-resistant material.

When particularly high-carbon materials (C > 1 wt-%) and a sufficient amount of carbide-forming substances are used as the wear-resistant material 202, relatively large carbides 203 (with a size greater than 20 μm) are obtained in the structure. The advantage of the carbides of this size is their advantageous effect on heavy-abrasion resistance. By means of the hot- working according to the invention these carbides 203 become further oriented so that their longitudinal axis is parallel to the longitudinal axis of the hot- worked material, as shown in Figure 2b. If the material can be used so that the surface exposed to heavy abrasion is a surface perpendicular to the longitudinal dimension of the worked material, i.e. a cross-sectional surface with respect to the longitudinal axis of the formed billet, the microstructure of the wearable material may be particularly preferred.

Figure 3 a shows as a cross-section and as a side view an alternative embodiment according to the invention for the billet shown in Figures 2a and 2b. In the Figure, the wear-resistant material 202 is located as island-like regions surrounded by the tough material 201. Figure 3b shows the arrangement according to Figure 3 a from above. As is apparent from the mentioned schematic Figures, the distribution and orientation of the hard phases 203 of the wear-resistant material can be changed to more advantageous with respect to wear-resistance and often also with respect to mechanical properties in the forming stage.

In the method according to the invention the materials to be used shall be chosen so that they are suitable for the intended operating conditions with respect to mechanical properties and wear-resistance. The tougher material, which is supposed to surround the more brittle and more wear-resistant material, should have such a toughness and fatigue resistance that it will not break or fatigue in normal operating conditions. To ensure this, the carbide concentration of the material should be limited to less than 10 vol-%, in particularly demanding objects to less than 5 vol-%. With respect to the wear-resistant material, a weaker toughness can be accepted with respect to mechanical properties, because the tough material surrounding it is intended for preventing any break-induced detachment of pieces from the brittle material. On the other hand, it is however important that the hot- workability of the material is sufficiently good so that the hot-working can be carried out without breakage of the material. In manufacturing the wear-resistant material it is preferred to use electro slag remelting, particularly for high-alloy materials (C > 1.5 wt-%, Cr+Mo+V+W > 10 wt-%, preferably C > 1.5 wt-% and Cr+Mo+V+Nb+W > 12 wt-%), to ensure a sufficient hot-workability and mechanical properties. For lower-alloy materials even normal casting processes can be employed.

Both in the case of normal casting processes and when using electro slag remelting, vacuum arc remelting or spray forming, also pre-manufactured solid, either cast or formed materials can be used as a part of billet manufacturing. The billet can be prepared for example so that a pipe is manufactured from a thick steel plate, inside of which pipe a high-alloy wear-resistant material is cast.

The cast material of the starting billet can preferably be prepared for example by one of the following methods:

a) packaging of an electro slag remelted, vacuum arc remelted, spray formed or normal casting before forging into a thick-walled envelope, sealing the envelope at its both ends before forging, then hot-working to a desired cross-section, b) simultaneous casting of two castings by retaining a capsule between the castings, after which hot-working, c) casting a wear-resistant material in a tubular steel mold which functions thereafter as the material surrounding the wear-resistant material and which is hot-worked together with the wear-resistant material.

What is essential in the chosen method is that the wear-resistant material contains a sufficient amount of carbides to provide the wear-resistance required in the application, the wear-resistant material is surrounded by a tough easily formable material, and that the structure is finally hot-worked to orientate the structure, to compact the cast structure and to decompose eventual carbide networks by means of pressure and temperature for example by hot forging, hot rolling or hot isostatic pressing. The manufacturing method of the wear-resistant material must be chosen so that it is hot- workable.

In the solution according to the invention a high-alloy wear-resistant material is used at least as one of the cast materials, which high-alloy wear-resistant material contains at least 50 wt-% Fe, at least 0.8 wt-% C and additionally at least 10 wt-% of one or more of the carbide-forming substances: Cr, Mo, V, Nb and W.

In a preferred embodiment according to the invention at least one of the cast materials is a high-carbon and high-chromium material which contains over 1.5 wt-% C and over 12 wt-% Cr. With this type of a composition the microstructure of the wear-resistant material preferably, when the method according to the invention is used, becomes such that it is able to reduce wearing in view of several applications.

In an embodiment according to the invention, in the structure of the starting billet the wear-resistant cast material is surrounded at least partly by a tough Fe-, Ni- or Co-based material in which the amount of carbides is less than 10 vol-% so that preferably at most 40 % of the cross-sectional surface of the billet is tough material.

In a preferred embodiment according to the invention the starting billet has been hot isostatically pressed before hot-working to improve structural integrity and uniformity.

The embodiment according to the invention is suitable for preparing all kinds of wear surfaces.

The wear material prepared by the method according to the invention is preferably suitable for use in all kinds of wear materials and wear parts, such as crushing and wear parts of crushing and grinding machinery of mineral material which are used in the excavation and mining industry. The invention is not intended to be limited to the embodiments described above by way of example but it can be applied widely within the scope determined by the following claims.