SE539667C2 - A wear resistant alloy - Google Patents

Description

A WEAR RESISTANT ALLOY TECHNICAL FIELD The invention relates to a wear resistant Fe- and/or Ni-based alloy. The alloy is alloyed with boron in order to form hard phase particles.

BACKGROUND OF THE INVENTION Nitrogen and vanadium alloyed powder metallurgy (PM) tool steels attained a considerable interest because of their unique combination of high hardness, high wear resistance and excellent galling resistance. These steels have a wide range of applications where the predominant failure mechanisms are adhesive wear or galling. Typical areas of application include blanking and forming, fine blanking, cold extrusion, deep drawing and powder pressing. The basic steel composition is atomized, subjected to nitrogenation and thereafter the powder is filled into a capsule and subjected to hot isostatic pressing (HIP) in order to produce an isotropic steel. A high performance steel produced in this way is described in WO 00/79015 Al.

Although the known steel has a very attractive property profile there is a continuous strive for improvements of the tool material in order to further improve the surface quality of the products produced as well as to extend the tool life, in particular under severe working conditions, requiring a good resistance against galling and abrasive wear at the same time.

DISCLOSURE OF THE INVENTION The object of the present invention is to provide powder metallurgy (PM) produced alloy having an improved property profile for advanced forming applications.

Another object of the present invention is to provide a powder metallurgy (PM) produced alloy having a composition and microstructure leading to improvements in the surface quality of products produced by the use of the alloy in tools and moulds.

The foregoing objects, as well as additional advantages are achieved to a significant measure by providing an alloy having a composition and microstructure as set out in the claims.

The invention is defined in the claims.

DETAILED DESCRIPTION The present invention relates to an alloy comprising a hard phase consisting mainly of multiple borides containing Fe and/or Ni in a Fe- and/or Ni-based matrix. Preferably, the matrix is hardenable. The double boride is of the type M2MB2, where M and M' stand for metals of the multiple boride. Said boride forming elements are generally selected from Cr, Mo, W, Ti, V, Nb, Ta, Hf and Co. In the present case M is Mo and M' is Fe and/or Ni. However, the boride may contain substantial amounts of one or more of the other boride forming elements. However, in the following the double boride will be referred to as Mo2FeB2for the Fe-based alloys although the boride also may contain Ni and one or more of the above mentioned boride forming elements. Similarly, in the Ni-based alloys the double boride will be referred to as M02MB2. The size of the hard phase particles may be determined by microscopic image analysis. The size thus obtained is the diameter corresponding to the diameter of a circle with the same projected area as the particle, the Equivalent Circle Diameter (ECD).

The importance of the separate elements and their interaction with each other as well as the limitations of the chemical ingredients of the claimed alloy are briefly explained in the following. All percentages for the chemical composition of the steel are given in weight % (wt. %) throughout the description. The upper and lower limits of the individual elements may be freely combined within the limits set out in claim 1.

Carbon(0 - 2.5 %) Carbon need not be present in Ni-based alloys. However, in Fe-based alloys carbon is to be present in a minimum content of 0.1 % or 0.2 %, preferably 0.3 % or 0.35 %. The upper limit for carbon is 2.5 %. Carbon is important for the formation of carbides and for the hardening. Preferably, the carbon content is adjusted in order to obtain 0.4-0.6 % C dissolved in the matrix at the austenitizing temperature. In any case, the amount of carbon should be controlled such that the amount of carbides of the type M23C6, M7C3, MeC and MC in the steel is limited. The upper limit may therefore be set to 2.1 %, 1.5 %, 1.3 %, 1.0 %, 0.8 %, 0.6 %, 0.5 % or 0.45 %.

Chromium(< 25 %) Chromium is an optional component, which is commonly present in Ni- and Fe-based alloys. The lower limit maybe 0.1 %, 0.3 %, 0.5 %, 1 %, 1.5 % or 2 %. However, Cr is for most applications for Fe-based alloys present in contents of at least 2.5 % in order to provide a sufficient hardenability. Cr is preferably higher for providing a good hardenability in large cross sections during heat treatment. If the chromium content is too high, this may lead to the formation of undesired carbides, such as M7C3. In addition, this may also increase the propensity of retained austenite in the microstructure. The lower limit may be 2.8 %, 3.4 % or 4.2 %. The upper limit may be 6.0 %, 5.4 %, or 4.6 %. On the other hand, chromium contents of more than 10 %, preferably more than 12 % are used for stainless applications.

Molybdenum(8 - 35 %) Mo is the main element forming the hard boride. In the present invention, a high amount of Molybdenum is used in order to obtain a desired precipitation of the boride Mo2FeB2in an amount of 5-35 vol. %. Preferred ranges include 12-30 % and 15-25%. Mo is also known to have a very favourable effect on the hardenability is essential for attaining a good secondary hardening response. For this reason it is preferred that the amount of Mo remaining in the matrix after quenching form 1100°C is 1.5-2.5 %.

Boron(0.5 - 3 %) Boron, which is the main hard phase-forming element, should be at least 0.5 % so as to provide the minimum amount of 5 % hard phase Mo2FeB2. The amount of B is limited to 3 % for not making the alloy to brittle.

Tungsten(< 22 %) Tungsten may be present in an amount of up to 22 % because high contents W are often used in Ni-based alloys, high speed steels (HSS) and in T-type tool steels. The effect of tungsten is similar to that of Mo. However, for attaining the same effect it is necessary to add twice as much W as Mo on a weight % basis. Tungsten is expensive and it also complicates the handling of scrap metal. In Fe-based alloys the maximum amount may therefore be limited to 3 %, preferably 1%, more preferably 0.3 % and most preferably W is not deliberately added at all.

Vanadium(< 15 %) Vanadium forms evenly distributed primary and secondary precipitated carbides of the type MC. In the inventive steel M is mainly vanadium but Cr and Mo may be present to some extent. The maximum addition of V is restricted to 15 % and the preferred maximum amount is 5 %. However, in the present case V is mainly added for obtaining a desired composition of the steel matrix before hardening. The addition may therefore be limited to 2.0 % or even to 0.5 %. A preferred range is 0.1-0.5 % V.

Niobium(< 15 %) Niobium is similar to vanadium in that it forms MC. However, for attaining the same effect it is necessary to add twice as much Nb as V on a weight % basis. Nb also results in a more angular shape of the MC. Hence, the maximum addition of Nb is restricted to 15 % and the preferred maximum amount is 5 %. Preferably, no niobium is added.

Silicon(0.1-2.5%) Silicon may be used for deoxidation. Si also increases the carbon activity and is beneficial for the machinability. Si is therefore preferably present in an amount of 0.1 - 2.5 %. For a good deoxidation, it is preferred to adjust the Si content to at least 0.1 %. The lower limit may be set to 0.2 %, 0.3 %, 0.35 % or 0.4 %. However, Si is a strong ferrite former and should be limited to 2.5 %. The upper limit may be set to 1.5%, 1 %, 0.8 %, 0.7 % or 0.6 %. A preferred range is 0.2 - 0.8 %.

Manganese(0-15 %) Mn is an austenite former and increases the solubility for nitrogen in the alloy. Mn may therefore be present in amounts of up to 15 %. Manganese contributes to improving the hardenability of steel and together with sulphur manganese contributes to improving the machinability by forming manganese sulphides. Manganese may therefore be present in a minimum content of 0.1 %, preferably at least 0.2 %. At higher sulphur contents manganese prevents red brittleness in the steel. The upper limit may be set to 10 %, 5 %, 2.5 %, 1.5 %, 1.2 %, 1.0 %, 0.8 % or 0.6%. However, preferred ranges are 0.2 - 0.8 % and 0.2 - 0.6 % in Fe-based alloys.

Nickel Nickel may be used as balance to make Ni-based products having M02MB2 as the dominating hard phase. However, in the Fe-based alloys Ni is optional and may preferably be present in an amount of not more than 15 %. It gives the steel a good hardenability and toughness. Because of the expense, the nickel content of the steel should be limited. Accordingly, the upper limited may be set to 5 %, 2 %, 1.0 % or 0.3 % in the Fe-based alloys.

Iron Iron may be used as balance to make Fe-based products having Mo2FeB2as the dominating hard phase. However, in the Ni-based alloys Fe is optional and may be present in an amount of not more than 15 %. The upper limit may be 8 %, 5 % or 3%.

Copper(< 1.0%) Cu is an optional element, which may contribute to increasing the hardness and the corrosion resistance of the steel. The upper limit may be 0.9 %, 0.7 %, 0.5 %, 0.3 % or O.P/o. However, it is not possible to extract copper from the steel once it has been added. This drastically makes the scrap handling more difficult. For this reason, copper is normally not deliberately added.

Cobalt(< 20 %) Co is an optional element, which may be present in an amount of not more than 20 % %. Co dissolves in iron (ferrite and austenite) and strengthens it whilst at the same time imparting high temperature strength. Co increases the Ms temperature. Co can substitute mainly Fe in the Mo2FeB2boride. A preferred maximum content is 2 %. However, scrap handling will be more difficult. For this reason, Co need not be deliberately added Ti, Ta, ZrandHf These elements are boride and carbide formers and may be present in the alloy in the claimed ranges for altering the composition of the hard phases. However, normally none of these elements are added.

Phosphorous P is an impurity element and a solid solution strengthening element. However, P tends to segregate to the grain boundaries, reduces the cohesion and thereby the toughness. P is therefore normally limited to < 0.05 %.

Sulphur(< 0.5%) S contributes to improving the machinability of the steel. At higher sulphur contents there is a risk for red brittleness. Moreover, a high sulphur content may have a negative effect on the fatigue properties of the steel. The steel shall therefore contain < 0.5 %, preferably < 0.03 %.

Nitrogen(< 0.5%) Nitrogen is an optional component. N can be present in solid solution but may also be found in the hard phase particles together with B and C. The upper limit may be 0.4%, 0.3 %, 0.2 %, 0.15 %, 0.1 %, 0.05 % and 0.03%.

EXAMPLE (not part of the invention) kg of an alloy having the composition given below was melted in a laboratory furnace and subjected to N2-gas atomizing.

Image available on "Original document" The powder was sieved to < 500[ im,filled in steel capsules having a diameter of 63 mm and a height of 150 mm. HIPing was performed at a temperature of 1150 °C, the holding time was 2 hours and the pressure 110 MPa. The cooling rate was < 1 °C/s. The material thus obtained was forged at 1130 °C to the dimension 20x30 mm. Soft annealing was performed at 900 °C with a cooling rate of 10 °C/h down to 750 °C and thereafter cooling freely in air. Hardening was performed by austenitizing at 1100 °C for 30 minutes followed by quenching in water followed by tempering. The result of the hardness testing after tempering is given in Table 1.

The amount of the hard phase was found to be 24 vol. % and the borides were found to have a small size. The area fraction of borides in different size classes is given in Table 2 below.

Image available on "Original document" Image available on "Original document" Table. 2. Size distribution of the borides.

The microstructure is shown in Fig. 1. The high area fraction and the uniform distribution of the Mo2FeB2 borides results in a material having excellent anti-galling properties such that it would be possible to dispense with surface treatments like PVD, CVD and TD.

INDUSTRIAL APPLICABILITY The alloy of the present invention is particular useful in applications requiring very high galling resistance.

Claims (13)

1. An alloy produced by powder metallurgy and having a non-amorphous matrix, the alloy consists of in weight % (wt.%): Image available on "Original document" balance Fe and/or Ni apart from impurities, wherein the Chromium content is at least 3.0 % when the alloy is balanced with Fe and the Nickel content is < 5, wherein the alloy comprises 5-35 volume % hard phase particles, the hard phase particles comprises at least one of borides, nitrides, carbides and/or combinations thereof, at least 90 % of the hard phase particles have a size of less than 5[ im and at least 50 % of the hard phase particles have a size in the range of 0.3 - 3 um.

2. An alloy according to claim 1, wherein the alloy fulfils at least one of the following conditions the alloy comprises 8-30 volume % hard phase particles, at least 90 % of the hard phase particles have a size of < 3 um, at least 80 % of the hard phase particles have a size in the range of 0.3 - 3 um, at least 60 % of the hard phase particles consist of Mo2 FeB2 or M02MB2, the alloy has a theoretical density (TD) of > 98 %.

3. An alloy according to claim 1 or 2, wherein the alloy is balanced with Fe and fulfils at least one of the following conditions Image available on "Original document" at least 80 % of the hard phase particles consist of Mo2 FeB2 .

4. An alloy according to any of the preceding claims, wherein the Ni-content is < 5 and the alloy fulfils at least one of the following conditions Image available on "Original document" at least 90 % of the hard phase particles consist of Mo2 FeB2 .

5. An alloy according to any of the preceding claims, wherein the alloy the alloy is balanced with Fe and fulfils at least one of the following conditions Image available on "Original document"

6. An alloy according to any of claims 1-4, wherein the alloy the alloy is balanced with Fe and fulfils at least one of the following conditions Image available on "Original document"

7. An alloy according to any of the preceding claims, wherein the alloy fulfils at least one of the following conditions Image available on "Original document" Image available on "Original document"

8. An alloy according to any of claims 1-4, wherein the alloy is balanced with Fe and the metallic matrix fulfils the following requirements after quenching from 1100 °C Image available on "Original document"

9. An alloy according to any of the preceding claims, wherein the alloy comprises 15 - 25 volume % hard phase particles and wherein the size of the hard phase particles is <4 um.

10. An alloy according to claim 1, wherein the alloy is in the form of a pre-alloyed powder obtained by atomizing an melt comprising Image available on "Original document"

11. An alloy according to any of claims 1-9, wherein the alloy has been subjected to atomizing and hot isostatic pressing resulting in that the alloy is isotropic.

12. Use of an alloy according to any of the preceding claims for making solid objects by the use of any of hot isostatic pressing, powder extrusion and additive manufacturing.

13. Use of an alloy according to any of claims 1-9 and 11 as a tool for punching, forming, blanking, fine-blanking, extrusion, deep drawing, powder pressing or in a part or mould used for die casting or plastic moulding.