Abstract:
A metal base alloy and methods for producing the alloy. The metal base alloy product includes the formula Me base  T a  Si b  Cr c  Mnj V e  Cf, wherein—Me base  is a metal base selected from the group having Fe, Co and Ni, in an amount ranging from about 45-75 w %. The metal base alloy product contains a substantially homogenous dispersion of separate precipitated carbide particles in an amount ranging from 10-65 percentages by volume and the precipitate carbide particles have an average diameter of 0.01-5 micrometers.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a metal base alloy and to methods for producing the alloy. More particular, the present invention relates to a high alloy metal base alloy product having advanced mechanical and physical properties as a result of its microstructure. 
       BACKGROUND OF THE INVENTION 
       [0002]    There is a great demand for metal based alloys, in particular iron based alloys, with advanced mechanical and physical properties. 
         [0003]    The properties of iron base alloys are decided by the aligning elements and the main phases are normally ferrite, austenite and carbides which are characterizing the different structures such as martensite, perlite and bainite. The distribution and fraction of carbides influences together with the grain size the mechanical properties. 
         [0004]    From  1960  towards today an impressive amount of research has been performed about rapid solidification of metal alloys to produce amorphous metals. During the years a large number of metal base alloys and in particular iron base alloys have been investigated. As a spin of from these investigations, new crystalline phases and new structures have been reported in a number of publications and patents for rapidly quenched iron based liquids. In this case the cooling rates has been 10 5 K/sec and higher. A featureless phase with high hardness was reported to be found in iron base alloys at cooling rates of 10 5 K/sec for example in U.S. Pat. No. 4,582,536B. A similar featureless phase formed at these high cooling rates was also reported in an article by Satheees Ranganathan et al, “Rapid solidification behavior of Fe-Cr-Mn-Mo-Si-C alloys”, from Metallurgical and Materials Transactions B, Volume 38B, December 2007. 
         [0005]    The rapidly solidified alloys described in the prior art are only possible to produce as rather small products or articles of less than 500 μm of thickness by using special casting technologies. This results in high production costs and also gives large problems to control the structure and properties. Some of the prior art metal base alloys show interesting microstructures but which are brittle and therefore difficult to make use of commercially. Further, the alloys known in the prior art may have good mechanical properties but which are difficult to control and to predict. 
       SUMMARY OF THE INVENTION 
       [0006]    It is an object of the present invention to provide a metal base alloy product having a microstructure resulting in advanced mechanical and physical properties. 
         [0007]    It is a further object of the present invention to provide a metal base alloy having outstanding mechanical and physical properties that can be produced in cost effective production processes and in an amount that provides a wide range of opportunities for articles to be produced from the alloy. 
         [0008]    At least one of the above objects is obtained by a metal base alloy product consisting of the formula Me base  T a  Si b  Cr c  Mn d  V e  C f , wherein
       Me base  is a metal base selected from the group consisting of Fe, Co and Ni, in an amount ranging from about 45-75 w %,   T a  is an alloying material selected from the group consisting of Mo, Nb and Ta in an amount a ranging from about 5-10 w %,   Si b  is a further alloying member in an amount b ranging from about 4-10 w %,   Cr c  is a further alloying member in an amount c ranging from about 8-30 w %,   Mn d  is a further alloying member in an amount d ranging from about 0-10 w %   V e  is a further alloying member in an amount ranging from about 0-10 w %, and   C f  is a further alloying member in an amount a ranging from about 2-4 w %,
 
and wherein the metal base alloy product contains a substantially homogenous dispersion of separate precipitated carbide particles in an amount ranging from 10-65 percentages by volume and wherein the precipitated carbide particles have an average diameter of 0.01-5 micrometer.
       
 
         [0016]    The metal base alloy in accordance with the present invention shows a microstructure with fine precipitated carbide particles in a relatively high volume fraction compared to available advanced materials of today. The volume fraction of carbides is directly proportional to the carbon content according to the principle of the level rule. The carbide particles are further relatively small compared to available advanced materials of today. 
         [0017]    The obtained structure having a large amount of small carbide particles is proven to give excellent strength and hardness to the material. For example, a hardness of 900-1300 Vickers and a yield stress of 1200 MPa may be obtained. In this context it may be interesting to mention that the so-called hard metals normally have a carbide size of about 5-10 micrometer and has a hardness of 1100-1400 Vickers. 
         [0018]    Fe-base alloys with this high carbon content normally give a cast structure of coarse carbides. An advantage of the present alloy is that it can be casted and solidified using conventional methods to form a featureless structure, defined as the η-phase. If the η-phase is preserved to room temperature the alloy or product will be very brittle. Therefore the cooling conditions after solidification has to be controlled to achieve the fine carbide structure as described above. 
         [0019]    A further advantage with the metal base alloy of the invention is that the microstructure may be further controlled through at least one heat treatment. Depending on the conditions during the solidification of the melt and the following cooling conditions after solidification, one or more suitable heat treatments may be performed. 
         [0020]    The combination of high volume fraction of precipitated carbide particles and the carbide particles having a small average diameter or size ranging from 0.01-5 micrometers gives the material several advantages both related to mechanical properties and to the manufacture possibilities. As a result, at least one of the above mentioned objects is achieved. 
         [0021]    According to an embodiment of the invention the precipitated carbide particles are substantially spherical carbides having an average diameter or size of 0.1-5 micrometer. The precipitation of fine, i.e. small, carbides of this size may be provided in the following ways. The alloy is casted or solidified using known methods. The cooling conditions after solidification are controlled in such a way that the η-phase is decomposed into carbides and austenite or ferrite. 
         [0022]    By performing a heat treatment of the solidified and cooled alloy the morphology is further controlled. The heat treatment can be performed at rather high temperatures, i.e. at 800-1000 degrees Celsius, whereby carbides having an average diameter or size of 0.5-5 micrometer may be obtained. An even more preferred temperature interval may be used, i.e. 850-950 degrees Celsius. The material may be treated from 0.1-50 hours or more preferably 2-10 hours. High temperature and long treatment time gives a softer and more ductile material than at low temperature and shorter time. This is compared with the properties of the featureless phase that is very hard and brittle. The austenite phase by its nature contains carbon and the carbides will form in moderate volume fractions, as much of the carbon is locked in the austenite phase. The longer the heat treatment may last, the more the carbides will grow. Further, the shape of the carbides becomes uniform and substantially spherical or rounded during the heat treatment. 
         [0023]    By performing a heat treatment of the alloy at medium high temperatures, i.e. at 650-800 degrees Celsius, carbides having an average diameter or size of 0.1-5 micrometer may be obtained. An even more preferred temperature interval may be used which is 700-750 degrees Celsius. The material may be treated from 1-30 hours or more preferably in the interval of 2-10 hours. This somewhat lower heat treatment temperature gives a harder and more brittle article with a higher yield stress. In this case the microstructure consists of a ferrite and carbides. If this heat treatment is chosen, a single heat treatment is performed. Thus, it is not followed by a second heat treatment. 
         [0024]    According to another embodiment of the invention a second heat treatment is performed after a heat treatment at high temperatures. In the second heat treatment carried out at low temperatures (200-600° C.) very fine carbides, i.e. of 0.01-0.5 in size are formed. These very fine carbides are nano-crystalline carbides showing an atomic lattice matching in the matrix. This means that they are preferably precipitated in the matrix in-between the larger carbides, which are formed during the high temperature (800-1000° C.) heat treatment. During the heat treatment at low temperatures for approximately 1-30 hours, some of the austenite may transform to ferrite. Ferrite has a low solubility of carbon and therefore carbides are formed in a large volume fraction. 
         [0025]    According to a further embodiment of the invention the precipitated carbide particles are a mixture of substantially spherical carbides having an average diameter of 0.5-5 micrometer and of nano-crystalline carbides having an average diameter of 0.01-0.5 micrometer. This mixture of fine carbides is obtained by controlling the cooling conditions after solidification followed by a heat treatment at elevated temperatures (800-1000 degrees Celsius) and eventually followed by a second heat treatment at lower temperatures (200-600 degrees Celsius). The mixture of many and very fine nano-crystalline carbides with carbides having a slightly larger diameter and a different shape and matching in the matrix give the material of the invention unique properties. 
         [0026]    According to an embodiment of the invention the precipitated carbide particles are surrounded by a matrix of ferrite and/or austenite depending on the chosen heat treatment(s). Accordingly, the matrix may consist of austenite after the high temperature heat treatment and of ferrite after the temperature heat treatment at 650-800 degrees Celsius. 
         [0027]    According to an embodiment of the invention the alloy has a tensile strength of at least about 800 MPa. The excellent mechanical properties of the material are a result of its microstructure. 
         [0028]    According to an embodiment of the Invention the Me base  is Fe present in an amount from about 45-75 w %. 
         [0029]    According to an embodiment of the invention the T a  is Mo present in an amount from about 5-10 w %. 
         [0030]    According to an aspect of the present invention it is provided a method to produce a metal base alloy product consisting of the formula Me base  T a  Si b  Cr c  Mn d  V e  C f , wherein
       Me base  is a metal base selected from the group consisting of Fe, Co and Ni, in an amount ranging from about 45-75 w %,   T a  is an alloying material selected from the group consisting of Mo, Nb and Ta in an amount a ranging from about 5-10 w %,   Si b  is a further alloying member in an amount b ranging from about 4-10 w %,   Cr c  is a further alloying member in an amount c ranging from about 8-30 w %,   Mn d  is a further alloying member in an amount d ranging from about 0-10 w %   V e  is a further alloying member in an amount ranging from about 0-10 w %, and   C f  is a further alloying member in an amount e ranging from about 2-4 w %, wherein the method comprises the steps of:   melting said metal base together with chosen alloying components using different melting processes such as high frequency or arc melting   casting said alloy providing segregation free solidification conditions forming a featureless structure of said alloy   controlling cooling conditions after solidification of said alloy, and   heat treating said alloy in at least a first heat treatment step whereby said metal base alloy product obtains a substantially homogenous dispersion of separate precipitated carbide particles in an amount ranging from 10-65 percentages by volume and said precipitate carbide particles have an average diameter of 0.01-5 micrometer.       
 
         [0042]    To be able to produce articles with an amorphous or featureless structure by more conventional methods, lower cooling rates are desirable. Casting of the alloy provides the possibility to make larger articles with reliable properties at a low cost and in a process that is easy to control. The featureless structure in iron base alloys formed at casting starts to decompose under controlled cooling conditions. Cooling rates as low as 10 −2  (0.01) K/sec may be obtained. At low cooling rates, most of the featureless phase decomposes into ferrite and precipitated carbides during the cooling after solidification. Depending on the obtained structure after casting and cooling, a suitable heat treatment is chosen. In some cases the final material or product is obtained after casting and cooling. However, in most cases the material undergoes an initial heat treatment at high temperatures and then eventually a second heat treatment at low temperatures. Heat treatment of the alloy after casting and controlled cooling is here used to change and control the properties of the articles. The different procedures are described in more detail below. 
         [0043]    According to an embodiment of the invention the method comprises the further step of heat treating the alloy in an inert atmosphere or under vacuum at 800-1000 degrees Celsius for 1-10 hours wherein said precipitate carbide particles are formed as substantially spherical carbides having an average diameter of 0.5-5 micrometer. Depending on the result after casting, the exact time and temperature is chosen. Generally, longer time at high temperatures of the interval gives larger carbides than shorter times in the first part of the temperature interval. 
         [0044]    According to an embodiment of the invention the method comprises the step of heat treating the alloy in an inert atmosphere or under vacuum at medium high temperatures of 650-800 degrees Celsius for 2-10 hours wherein said precipitate carbide particles are formed in a matrix of ferrite having an average diameter of 0.1-5 micrometer. 
         [0045]    Depending on the outcome of the casting, it may be preferable to perform a single heat treatment at medium high temperatures only. 
         [0046]    After a heat treatment at high temperatures a second heat treatment may be performed to improve the microstructure and thus the properties of the material further. For this purpose a heat treatment at low temperatures (200-600° C.) may be performed. In the heat treatment at low temperatures very fine carbides are precipitated. If a first heat treatment at high temperatures is followed by a heat treatment at low temperature, a mixture of carbide particles having an average diameter of 0.5-5 micrometer and nano-crystalline carbide particles having an average diameter of 0.01-0.5 micrometer is obtained. The result is a ductile material but which is at the same time hard with a high yield stress. 
         [0047]    Heat treatment of the cast structure consisting of a decomposed featureless structure in iron base alloys may be used to change and control the properties of the articles. During heat treatment the microstructure is changed and a substantially uniform dispersion of carbide particles is formed in an amount ranging from 10-65 percentages by volume, or even more preferably 35-50 percentages by volume and the carbide particles have an average diameter of 0.01-5 μm. 
         [0048]    According to a further aspect of the present invention, it is provided a method to produce a metal base alloy product according to claim  1 , wherein the method comprises the steps of:
       melting the metal base together with chosen alloying components using different melting processes such as high frequency or arc melting   powder processing of the alloy to produce powder with a particle size of 0.1-200 μm size&#39;   heating the obtained powder to control the transformation of a featureless structure into carbides and austenite/ferrite,   keeping the powder at a constant temperature during heating to ensure that the carbides become small and rounded,   pressing the obtained powder using high isostatic pressing (HIP) at a pressure of 60-150 MPa at a temperature of 800-1050° C.,   cooling the product to approximately room temperature (20° C.), and   heat treating the alloy in an inert atmosphere or under vacuum, to further control the precipitate carbide particles to an average diameter of 0.01-5 micrometer.       
 
         [0056]    The low temperature during heating of the powder, also mentioned as the transformation heat treatment of the powder, before the high temperature high isostatic pressure treatment, leads to the decomposition of the η-phase to the initial carbide structure as is also seen in the solidified casted material after controlled cooling conditions. Suggested temperatures for the transformation heat treatment are 200 to 800 degrees Celsius. 
         [0057]    The high temperature heat treatment for a cast material is for a HIP processed material substituted by the heat treatment during the sintering process at 800-1050° C. The HIP processed material may then optionally be heat treated at a low temperature of 200-600° C. for approximately 1-30 hours. 
         [0058]    Using this method to produce powder by atomization and by high isostatic pressing which makes it possible to produce articles of very large size with a good homogenous structure and with properties close to hard metal. 
         [0059]    When performing the HIP process on the metal base powder, the temperature is elevated and corresponds to a heat treatment at high temperatures as described above. During the pressing step a carbide rich structure in austenite is formed. The carbides are substantially spherical or rounded and of a size of 0.5-5 μm in a volume fraction of 20-50%. Therefore, a single heat treatment at low temperatures may be sufficient for an article produced through the powder method in order to obtain the desired properties of the material. 
         [0060]    Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. Those skilled in the art realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention, as defined by the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0061]    The present invention will hereinafter be further explained by means of non-limiting examples with reference to the appended figures where; 
           [0062]      FIG. 1  is a temperature/time diagram showing the controlled solidification and cooling process to obtain a cast alloy product according to an embodiment of the invention, 
           [0063]      FIG. 2  is a diagram illustrating the fractional change of the structure from the η-phase to the formation of carbides and austenite depending on the cooling rate during the controlled cooling process for the cast alloy ( FIG. 1 ) or the transformation heat treatment in the powder alloy ( FIG. 5 ), 
           [0064]      FIG. 3   a  shows an example of a cast microstructure where all the featureless η-phase is transformed into ferrite and carbides, 
           [0065]      FIG. 3   b  shows an example of a cast structure where only some of the featureless η-phase is transformed into ferrite and carbides, 
           [0066]      FIG. 3   c  shows the microstructure of an example of a heat treated cast alloy product according to the invention, treated at 700° C. for 4 hours, 
           [0067]      FIG. 3   d  shows the microstructure of an example of a heat treated cast alloy product according to the invention, treated at 900° C. for 8 hours, 
           [0068]      FIG. 4  is a flowchart showing the different techniques that can be used to obtain the structure of the alloy product 
           [0069]      FIG. 5  is a temperature/time diagram of the powder and HIP processing to obtain the powder alloy product, and 
           [0070]      FIG. 6  shows an example of the microstructure of a HIP-material, after high isostatic pressing at 950° C. with a pressure of 120 MPa. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0071]    The invention will be described using examples of embodiments. It should however be realized that the example embodiments are included in order to explain principles of the invention and not to limit the scope of the invention, defined by the appended claims. 
         [0072]      FIG. 1  shows an illustration of the temperature/time process during casting and controlled cooling. The featureless structure is formed during the solidification process at around 1100° C. After solidification, the featureless structure decomposes during the cooling process. The carbide formation is controlled by keeping the temperature at a constant level. It should be noted that  FIG. 1  shows an example of such an experiment and that the temperature could vary depending on the desired microstructure. 
         [0073]    In  FIG. 2  it is shown how the cooling rate influences the fraction of decomposed featureless phase. The crystalline featureless structure that has been observed in the metal base alloy of the invention is defined as a η-phase. The featureless η-phase has been found to be very hard and has a high yield stress level. In fact, much harder than in any other observed structure in iron base alloys except for an iron based structures containing carbides. However, the η-phase with these properties is very brittle. The η-phase is formed from the liquid at high temperatures. During a following cooling it decomposes to austenite and precipitated carbide particles. The rate of decomposition is determined by the cooling rate, as is illustrated in  FIG. 2 . At a high cooling rate, and thus short cooling time, the decomposition is so slow that no η-phase is transformed. In-between high and low cooling rates only fractions of the η-phase are transformed to austenite and carbides. At lower cooling rates, down to 0.01 K/s, and thus longer cooling time, all η-phase is transformed to austenite and carbides. 
         [0074]    The volume fraction as well as the size and distribution of the precipitated carbide particles determine the properties of the article. An alloy with a small carbide size, but a high volume fraction gives a much harder material with better yield stress. Smaller volume fraction and larger carbide sizes gives lower hardness with a lower yield stress. Thus, the smaller the carbide size, the better the toughness of the material. 
         [0075]    In cases where the η-phase still exists at room temperature the decomposition can be controlled and completed by heat treatment. Therefore, after casting the alloy may be subjected to heat treatment to further improve the microstructure of the material and thereby the mechanical properties. The time and temperature for the heat treatment is selected depending on the desired properties for the article. 
         [0076]      FIG. 3   a  shows the microstructure where approximately 100% of the featureless phase is transformed to ferrite and carbides during cooling of the cast component. The structure shows a cellular structure of fine plate like and rounded carbides in a matrix of ferrite. To obtain the structure in  FIG. 3   a  the cooling rate has been close to 0.01 K/s such that all featureless phase decomposed into ferrite and carbides during the cooling in the cast mould. 
         [0077]    In  FIG. 3   b , a microstructure is shown in which only parts of the featureless η-phase is transformed or decomposed into ferrite and carbides. Accordingly, in this case the cooling rate was higher than the cooling rate for the microstructure of  FIG. 3   a . The featureless phase may be seen in  FIG. 3   b  as large white areas mostly In the upper middle of  FIG. 2   b.    
         [0078]      FIG. 3   c  shows the microstructure in a cast sample with the structure shown in  FIG. 3   a  but heat treated at 700° C. during 4 hours. The coarser and rounded carbides are recognizable in a matrix of ferrite. 
         [0079]      FIG. 3   d  shows the microstructure in a sample with the microstructure shown in  FIG. 3   a , but heat treated at 900° C. during 8 hours. The figure shows a network of rounded carbides in a matrix of austenite. 
         [0080]      FIGS. 3   a ,  3   b ,  3   c  and  3   d  show examples of the microstructure of the material obtained before and after heat treatment. Generally, higher temperature and longer time gives a softer and more ductile material. At higher heat treatment temperatures, in the interval of 800-1000° C. coarser, larger and spherical or rounded carbide particles (0.5-5 μm) are formed in a matrix of austenite. In order to improve the properties even further, the samples are rapidly quenched and then heat treated a second time at lower temperatures. Lower temperatures give a harder and more brittle article with a higher yield stress. At heat treatment temperatures, in the interval of 650-800° C. a small carbide particle size (0.1-5 μm) is formed in a matrix of ferrite. 
         [0081]      FIG. 4  is a flowchart showing different process steps to produce the metal base alloy according to the invention. A metal base alloy product according to an embodiment of the invention is preferably produced by casting the alloy, which is the process way to the left in  FIG. 4 . First, the metal base component together with the alloying elements, are melted. Melting of the alloy may be performed in high frequency equipment with a vacuum chamber. After melting in vacuum the alloy was casted in a mould made out of a ceramic material. The step of casting the alloy is performed in a sand, a ceramic or a metal mould. Casting in a ceramic mould or sand mould provides the possibilities to make larger products, less expensive and a process that is easier to control. The pressure in the vacuum chamber was increased after cooling by an inert gas to fill the mould completely. The mould was produced using the wax-melting method, wherein a wax model is made of the product and imbedded in ceramic material and finally burned out leaving behind a cavity wherein the metal is casted. This is done under a constant temperature to control the structure of the alloy. Further, the cooling rate must be controlled to obtain the desired properties of the material. Preferably, the cooling rate is low. The cooling rate may be as low as 0.01 K/sec. 
         [0082]    After the casting the metal base alloy may be heat treated. A single heat treatment or a double heat treatment may be chosen depending on the required end result. 
         [0083]    As illustrated in  FIG. 4  a metal base alloy product according to an embodiment of the invention may also be produced by a powder forming process, which is the process way to the right in  FIG. 4 . The method of powder formation starts with pulverization of the alloy. Gas or water atomization is used to produce a powder from a melt of the metal base alloy. An inert gas or water jet is penetrating a stream of the liquid metal alloy, splitting it into small droplets. The liquid droplets cool rapidly and solidify into a powder with a featureless structure. The size of the powder particles vary between 1 μm up to 200 μm. The powder is pressed to a green body or a steel container is filled with the powder, evaporated and sealed. The green body or the steel container is then placed into a closed chamber with a furnace. The temperature is increased to between 200-800° C., heat treated and later the temperature is increased further to 900 and 1000° C. and the pressure is increased to more than 100 MPa, causing the powder to be inferred together,  FIG. 5 . The featureless structure decomposes to austenite or ferrite and carbide particles during the initial transformation heat treatment at low temperature. Where the transformation heat treatment gives a corresponding carbide structure in the powder alloy as was given during the controlled cooling process of the cast alloy. The carbide particles have a size of 1-10 μm and are well distributed in a matrix of austenite as is shown in  FIG. 6 . To further improve the microstructure of the material and hence the mechanical properties, a heat treatment is performed. The heat treatment is preferably carried out at low temperatures, i.e. between 200-600 degrees Celsius, to increase the volume fraction of precipitated carbide particles and to precipitate carbide particles having an average diameter in the interval of 0.01-0.5 micrometers.
   An example of double heat treatment is performed as follows;   
 
         [0085]    A cast sample with fully transferred featureless structure, i.e. the featureless phase decomposed into ferrite and carbides is heat treated at 1000° C. for 8 hours. A coarse carbide structure in a matrix of austenite is formed. The sample is quenched to room temperature, parts of the austenite is preserved. The austenite keeps a high carbon content. The carbon content in austenite is high at high temperature and decreases with decreasing temperature. The decrease of the carbon content in austenite occurs by precipitation of carbides. To get those carbides as small as possible the sample is rapidly cooled to room temperature and then heat treated at 550° C. where small carbides are precipitated in the austenite matrix. The austenite might transform to ferrite. 
         [0086]    Generally, the casted metal base alloy is primarily suitable for small articles or products, whereas the powdered HIP method is primarily used when larger articles or products are produced. Small artides are for instance dental instruments like brackets and dental tools such as luxators, chisels and curettes. There are also several areas within the medical field where advanced material requirements are necessary. For example, in orthopaedic surgery several special tools are being used, such as drills, cutters, chisels, saws etc. In the workshop industry, small special tools for tough materials are produced, such as drills, bits, chisels. For larger articles, large cutters and workshop-tools can be produced. 
         [0087]    Example embodiments may be combined as understood by a person skilled in the art. Therefore, it is to be understood that the foregoing is illustrative of various example embodiments and is not to be limited to the specific embodiments disclosed and that modifications to the disclosed embodiments, combinations of features of disclosed embodiments as well as other embodiments are intended to be included within the scope of the appended claims.