FIELD OF THE INVENTION

The present invention relates to a powder mixture of three different pre-alloyed metal based powders, intended to be used in surface coating of metal parts. The powder mixture is deposited using e.g. laser cladding or plasma transfer arc welding (PTA), or thermal spray (e.g. HVOF). The powder mixture is useful for reducing friction and improving wear reducing properties of the deposited coating. Such coatings may also improve machinability. As friction or wear reducing component, inclusions of manganese sulphide or tungsten sulphide in the pre-alloyed powder may be used.

BACKGROUND

Thermal surfacing i.e. thermal spray coating and overlay welding powder grades are widely used for coating of component surfaces against wear and corrosion. Fe-, Ni- and Co- based grades are known to radically improve life time of wear- and/or corrosion exposed components. However, there is still a large number of applications where component life times need to be improved. In addition, high prices and limited availability of Ni and Co on the world market also calls for longer life time

improvement. Finally, development of new coating deposition methods like laser cladding, cold spraying and high velocity spraying open new possibilities for alloying, more accurate control of coating process and higher automation, thereby calling for additional types of powders.

One approach to improve friction and wear properties may be to incorporate solid lubricant to thermal surfacing grades so that the deposited coating includes friction and wear reducing substances while maintaining acceptable levels of corrosion resistance and hardness.

Solid lubricants are soft solid phase materials which are capable of reducing friction and wear between two surfaces sliding against each other without the need for a liquid media. Materials to be considered as solid lubricants need to meet at least the following criteria: adhere contacting surfaces - stickiness: low shear strength - low intrinsic friction; low hardness - low abrasivity and thermochemical stability for the intended environment. Examples of solid lubricants are; talc, graphite, manganese sulphide (MnS), molybdenum disulphide (M0S2), or tungsten disulphide (WS2). Use of solid lubricants may provide advantages in: stability at extremely low or high temperatures; stability in extreme environments, such as cold or hot environments, or environments having high radiation levels; mechanical design issues (lighter design, reduced critical velocity) or able to carry extreme loads.

For a long time, the use of solid lubricants in thermal surfacing has been a difficult proposition, the reason being that numerous solid lubricants are metal sulphides and that even trace amounts of sulphur in welds can lead to cracking and/or corrosion.

Skarvelis et al; ASME J. Tribol. 132 (2010) 031302-1-031302-8, Surf. & Coat. Techn. 203 (2009) 1384-1394, and Trib. Int. 42 (2009) 1765-1770 describe the use of mixing MnS powder with a metal powder and using the resulting powder mix in e.g. PTA (plasma transferred arc welding).

An additional example of using metal powder in conjunction to MnS as solid lubricant is disclosed in Senad et al; WO2014090922.

Solid lubricants, however may have high friction coefficient compared to that of oil or grease; finite wear life for solid lubricant films when renewal is not possible; no or limited cooling capacity compared to oil or grease, or tendency to clogging caused by debris and residual particles.

SUMMARY OF THE INVENTION It may be possible to add e.g. manganese and sulfur as individual components in a metal powder, i.e. as separate powder particles. These components will then (when the metal powder melts) form a so-called solid lubricant (in this case MnS). There are, however, drawbacks of having e.g. Mn and S as individual components, such as severe dusting, and formation of inhomogenous inclusions of MnS in the final surface coating.

The inventor of the present invention has now found that it may be advantageous to add each of the components of the solid lubricant to separate metal powders and then mixing the metal powders either concurrently with carrying out the surface coating procedure, or prior to carrying out the surface coating procedure. In short, three powders are mixed; one metal powder containing manganese or tungsten; one metal powder containing sulfur; and one iron based powder to enable proper ratios between the various components. Mn, W, and S are pre-alloyed in their respective powder particles. These three metal powders are then mixed together and used in a surface coating procedure, wherein the metal particles are melted, and MnS or WS inclusions are formed in the melt (also termed melt pool).

In the case of MnS, because of MnS formation in the melt pool, the slag cannot be easily removed from the top of the melt. The slag is left on the top or sides of the surface coating, such as an overlay welding seam. If on the sides, the next seam will cover the slag and the slag will not have time enough to move to the seam top. Because of this, the microstructure of the resulting hard face includes both fine-dispersed MnS but also slag-MnS.

Surprisingly, the inventor has noticed that the powder mixture according to the present invention can be used in applications with high tolerance with regard to surface quality (such as surface finish, slag formation, or dimensional variability). The resulting hard face is thus suitable for use in heavy outdoor equipment, such as rails, wheels in rail- and tram-ways, mining-, agriculture-, oil-, gas-, and construction- tools. FIGURES

Figure 1 Wear rate vs. sliding velocity for S-powder clad pin and carbon steel pin for Hertzian max. contact pressure of 500 MPa.

Figure 2 Wear rate vs. sliding velocity for S-powder clad pin and carbon steel pin for Hertzian max. contact pressure of 1000 MPa.

Figure 3 SEM micrograph of S-powder clad, top of the micrograph is wear test surface.

Figure 4 SEM micrograph of S-powder clad, top of the micrograph is wear test surface.

DETAILED DESCRIPTION

All percentages herein, and in the claims are % by weight.

The invention is a powder mixture containing;

i) atomised metal powder having the following composition; C, 0.05- 0.5%; Si, 2.0-4.0%; B, 0.8-1 .3%; Cr, 2-10%; Fe, 3-15%; Al, 0.3-0.5%; Mn, 5-15%; the balance being Ni;

ii) atomised metal powder having the following composition; C, 0.05- 0.2%; Si, 2.2-2.9%; B, 0.8-1 .3%; Cr, 2.8-3.45%; Fe, 1 .4-2.3%; Al, 0.3- 0.5%; S, 3-13%; the balance being Ni;

iii) atomised metal powder having the following composition; C, 0.2- 0.27%; Si, 3.5%; B, 1 .6; Fe, 2.5; Cr, 7.5; the balance being Ni. Further, the invention is a powder mixture according to the above, wherein the ratio between the powders are such that the amount of MnS is 4-15%.

Further, the invention is a metal powder according to the above, wherein the particle size of the prealloyed powder is from 45μηη to 200mm, or from 50-150μηη.

The invention is also a method for surface coating metal parts, by way of laser cladding or PTA (plasma transferred arc), with a metal powder according to the above, thereby producing a metal coated component.

It is previously known that solid lubricants such as MnS or WS are useful in the field of surface coating, whereby a hard phase is formed on the surface of a substrate. MnS or WS function as a so-called solid lubricant. The present inventor has shown that a mixture of metal powders can be used in a surface coating procedure, such as plasma transfer arc, and by choosing the right components in the individual metal powders, the solid lubricant can form in the resulting surface coating or hard phase. The metal powders may be nickel, cobalt, or iron based.

Three atomised metal powders are used in the mixture according to the invention; In one embodiment, Powder M may have the following composition; C, 0.05-0.5%; Si, 2.0-4.0%; B, 0.8-1 .3%; Cr, 2-10%; Fe, 3-15%; Al, 0.3-0.5%; Mn, 5-15%; the balance being Ni. The powder was prepared by atomisation of a melt containing the elements above in said amounts. The resulting powder contains Mn as inclusions in a matrix of metal alloy. This powder is herein denoted "Powder M";

Powder S may have the following composition; C, 0.05-0.2%; Si, 2.2-2.9%; B, 0.8- 1 .3%; Cr, 2.8-3.45%; Fe, 1 .4-2.3%; Al, 0.3-0.5%; S, 3-13%; the balance being Ni. The powder was prepared by atomisation of a melt containing the elements above in said amounts. The resulting powder contains S as inclusions in a matrix of metal alloy. This powder is herein denoted "Powder S"; and the third powder is 1540 - a standard grade. This powder is herein denoted "Powder MP" Powder S, Powder Mn and powder P are mixed, in order to achieve 4-15 % MnS in the final melt pool which forms in the below mentioned cladding methods. This powder mixture is herein denoted "Mixture PM".

The Mixture PM is especially well suited for weld cladding methods, such as laser cladding or PTA. In addition, thermal spray, e.g. flame spray, HVOF, HVAF, coldspray, plasma spray, and the like may also be suitable applications.

The prealloyed nickel, iron, or cobalt based powder is preferably produced by water or gas atomization of a melt which includes Mn, W, or S and other alloying elements chosen from the group consisting of C, Si, B, Cr, Fe, Al, Ni, Co, and V.

The particle size of the pre-alloyed powder alloy is typically from 10μΐη to δΟΌμηη, or from 10μΐη to 20Όμηη, or preferably from 15-150μηη, or 50-150μηη.

In one aspect, the invention provides a method for surface coating metal parts, by way of deposition techniques such as laser cladding or PTA (plasma transferred arc); thermal spray methods such as HVOF (high velocity oxy fuel spray), HVAF (high velocity acetylene fuel spray) or plasma spray; or by slurry methods such as centrifugal casting, with the above mentioned metal powder.

In a further aspect, the invention also provides metal parts produced by the above mentioned suitable for coating by the powder according to the invention for dry friction contacts in machinery, such as e.g. industrial valves, sheet metal forming (SMF) tools, transport rollers in iron works, paper knives, and glass moulds.

EXAMPLES

Example 1 Preparation of pre-alloyed powder

A metal powder with the following composition; C, 0.05-0.5%; Si, 2.0-4.0%; B, 0.8- 1 .3%; Cr, 2-10%; Fe, 3-15%; Al, 0.3-0.5%; Mn, 5-15%; the balance being Ni, was prepared by atomisation of a melt containing the elements above in said amounts. The resulting powder contains Mn as inclusions in a matrix of metal alloy. This powder is herein denoted "Powder M"

An additionalmetal powder with the following composition; C, 0.05-0.2%; Si, 2.2- 2.9%; B, 0.8-1 .3%; Cr, 2.8-3.45%; Fe, 1 .4-2.3%; Al, 0.3-0.5%; S, 3-13%; the balance being Ni, was prepared by atomisation of a melt containing the elements above in said amounts. The resulting powder contains S as inclusions in a matrix of metal alloy. This powder is herein denoted "Powder S"

1540 - a standard grade This powder is herein denoted "Powder M

P"

Powder S, Powder Mn and powder P are mixed, 3MA powder mix, in order to achieve 4-15 % MnS.

Example 2

Application of powder by deposition using PTA

Pre-alloyed or pre-mixed powder was applied to test samples as follows; Powder A was deposited onto S235JRG (base structural steel) substrate plates by PTA

(plasma transfer arc) with parameters set to allow for a dilution of 5-15%.

Example 3

Powder S was spread by hand on substrate as a powder before fusing with the substrate. How was the powder fuwed? Example 4

Powder according to the invention was also applied to substrate by laser cladding. The coating from Powder S appears to result in finer inclusion sizes of MnS than when applied by PTA

Example 5

Block on ring wear testing was performed, and shows the beneficial effects of 3MA powder mix in a metal surface coating layer or clad. The specimens were rectangular blocks 10x1 Ox 50 mm where the base metal was commonly used low carbon structural steel (EN S235 JRG, ASTM A570 Gr.36) and the surface layer was at least 0.5 mm thick in the as finished measure. The test surface had a ground finish with surface roughness of Ra 0.3-0.4 μιτι, prepared by grinding. The counter rings

06O/RI 00x020x16 mm were made of UIC 900A rail steel. The test was unlubricated i.e. dry, and the test samples were carefully cleaned and then degreased by ethanol prior to testing. The testing was performed as a wear mechanism mapping trial. The test normal load was 5 and 42 N what correspond 500 respective 1000 MPa in max. Hertzian contact pressure. Sliding velocity was 0.045, 0.13, 0.37, 1 .1 and 2.9 m/s. The total sliding distance was 800 m. Results are shown in Figure 1 and Figure 2 for contact pressures of 500 respective 1000 MPa. Figure 3 and Figure 4 illustrate microstructure of S-powder laser clad.