WEAR-RESISTANT IRON-BASED ALLOY COMPOSITIONS COMPRISING CHROMIUM

Abstract

An iron-based alloy composition including: boron (B): 1.6-2.4 wt. %; carbon (C): 2.2-3.0 wt. %; chromium (Cr): 3.5-5.0 wt. %; manganese (Mn): below 0.8 wt. %; molybdenum (Mo): 16.0-19.5 wt. %; nickel (Ni): 1.0-2.0 wt. %; silicon (Si): 0.2-2.0 wt. %; vanadium (V): 10.8-13.2 wt. %; and balanced with iron (Fe). Further, an item including a substrate portion and a hardfacing coating bonded to the substrate portion, wherein the hardfacing coating is made by an overlay welding process using the iron-based alloy composition.

Claims

1-19. (canceled)

20. An iron-based alloy composition comprising: boron (B): 1.6-2.4 wt. %; carbon (C): 2.2-3.0 wt. %; chromium (Cr): 3.5-5.0 wt. %; manganese (Mn): below 0.8 wt. %; molybdenum (Mo): 16.0-19.5 wt. %; nickel (Ni): 1.0-2.0 wt. %; silicon (Si): 0.2-2.0 wt. %; vanadium (V): 10.8-13.2 wt. %; and balanced with iron (Fe) and unavoidable impurities; wherein the iron-based alloy composition is a powder composition.

21. The iron-based alloy composition according to claim 20, wherein the amount of silicon is 0.2-1.5 wt. %.

22. The iron-based alloy composition according to claim 20, wherein the amount of boron is 1.8-2.3 wt. %.

23. The iron-based alloy composition according to claim 20, wherein the amount of chromium is 3.5-4.5 wt. %.

24. The iron-based alloy composition according to claim 20, wherein a total amount of unavoidable impurities in the iron-based alloy composition is below 1 wt. %.

25. The iron-based alloy composition according to claim 20, wherein at least 95 wt. % of the powder composition has a particle size of up to 300 μm.

26. The iron-based alloy composition according to claim 25 wherein at least 95 wt. % of the powder composition has a particle size of at least 5 μm.

27. The iron-based alloy composition according to claim 20, wherein at least 95 wt. % of the powder composition has a particle size of at least 5 μm.

28. The iron-based alloy composition according to claim 20, wherein at least 95 wt. % of the powder composition has a particle size of up to 150 μm and wherein at least 95 wt. % of the powder composition has a particle size of at least 50 μm.

29. The iron-based alloy composition according to claim 20, wherein a total amount of unavoidable impurities in the iron-based alloy composition is below 1 wt. %, and wherein the unavoidable impurities comprise one or more of nitrogen (N), oxygen (O), sulfur (S), copper (Cu), and cobalt (Co).

30. The iron-based alloy composition according to claim 20, wherein the composition comprises 45-50 volume % hard phase particles.

31. An iron-based alloy composition consisting of: boron (B): 1.6-2.4 wt. %; carbon (C): 2.2-3.0 wt. %; chromium (Cr): 3.5-5.0 wt. %; manganese (Mn): below 0.8 wt. %; molybdenum (Mo): 16.0-19.5 wt. %; nickel (Ni): 1.0-2.0 wt. %; silicon (Si): 0.2-2.0 wt. %; vanadium (V): 10.8-13.2 wt. %; and balanced with iron (Fe) and unavoidable impurities; wherein a total amount of unavoidable impurities in the iron-based alloy composition is below 1 wt. %; wherein the iron-based alloy composition is a powder composition.

32. The iron-based alloy composition according to claim 31, wherein the amount of silicon is 0.2-1.5 wt. %.

33. The iron-based alloy composition according to claim 31, wherein the amount of boron is 1.8-2.3 wt. %.

34. The iron-based alloy composition according to claim 31, wherein the amount of chromium is 3.5-4.5 wt. %.

35. The iron-based alloy composition according to claim 31, wherein at least 95 wt. % of the powder composition has a particle size of up to 300 μm.

36. The iron-based alloy composition according to claim 35, wherein at least 95 wt. % of the powder composition has a particle size of at least Sum.

37. The iron-based alloy composition according to claim 31, wherein at least 95 wt. % of the powder composition has a particle size of at least 5 μm.

38. The iron-based alloy composition according to claim 31, wherein at least 95 wt. % of the powder composition has a particle size of up to 150 μm and wherein at least 95 wt. % of the powder composition has a particle size of at least 50 μm.

39. The iron-based alloy composition according to claim 31, wherein the composition comprises 45-50 volume % hard phase particles.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The invention will be described in more detail in the following by referring to examples and the appended drawings, wherein the drawings show in

[0043] FIG. 1 is a graph showing hardness of coatings produced by PTA and laser cladding using different alloy compositions;

[0044] FIG. 2 is a graph plotting impact energy as a function of number of strikes to achieve the first crack, for alloy composition Alloy 11 and the reference alloy REF;

[0045] FIGS. 3a-3c are SEM micrographs showing the microstructure of three different alloys processed into ingot samples;

[0046] FIGS. 4a-4b are SEM micrographs showing the microstructure of two different alloys processed into ingot samples;

[0047] FIG. 5 is a graph showing the influence of the addition of Si on the microstructure of processed alloys;

[0048] FIG. 6 has micrographs showing the microstructure of two different alloys processed into coatings by PTA welding.

[0049] FIGS. 7a-7h are energy dispersive SEM micrographs showing elemental mapping of V, Mo, Cr, Fe, Si, C, and B for one example of an alloy; and in

[0050] FIG. 8 is a schematic arrangement for testing impact wear resistance according to the ball drop method.

DETAILED DESCRIPTION

[0051] As mentioned above, one drawback of PTA welded and laser cladded coatings made using known iron-based alloy compositions or NiSiB mixes with tungsten carbides is an unsatisfactory wear resistance performance in scenarios of a combination of different wear mechanisms. This is due to a combined effect of microstructure and poor weldability resulting in pore and crack formation in the case of iron based coatings and cracks, sinking and dissolution of the tungsten carbides in the case of NiSiB coating with tungsten carbides.

[0052] By optimizing the amount of Silicon in iron-based alloy compositions containing selected amounts of Chromium, a surprisingly high hardness and resistance to both abrasive wear and impact wear can be achieved.

[0053] In the following, the invention is described by reference to exemplifying alloy compositions with systematically varied chromium (Cr) and silicon (Si) contents. Details of the alloy compositions are given in the MATERIAL section. Details of overlay welding procedures by plasma transfer arc (PTA) welding and laser cladding are given in the PROCESS section. Analysis techniques for characterizing the properties of the processed alloys are described in the EVALUATION section. Analysis results are presented in the RESULTS section, including a discussion of the influence of adding Cr and Si to the iron-based alloy compositions according to embodiments of the present invention.

EXAMPLES

[0054] Material

[0055] Alloy powders REF and 11 to 15 with the chemical composition reported in Table 1 were investigated. The alloys were gas atomized and sieved between 53-150 μm for compatibility with powder feeding devices of the overlay welding equipment.

TABLE-US-00001 TABLE 1 Chemical composition of investigated alloys C B Mo V Mn Ni Si Cr Fe Alloy wt. % wt. % wt. % wt. % wt. % wt. % wt. % wt. % wt. % REF Powder 2.6 2.0 17.5 12.0 0.5 1.5 0.7 Bal 11 2.7 2.0 17.6 11.7 0.6 1.8 0.7 3.7 Bal 12 2.6 2.0 18.0 11.8 0.6 1.7 1.1 3.8 Bal 13 2.6 2.0 18.3 11.9 0.5 1.6 1.4 3.9 Bal 14 2.5 1.9 17.7 11.9 0.5 1.5 0.9 4.8 Bal 15 2.4 1.9 17.6 11.9 0.5 1.5 0.9 5.6 Bal 26 Ingots 2.6 1.7 17.2 11.7 0.4 1.2 0.3 4.0 Bal 27 2.7 1.8 17.7 12.1 0.3 1.2 1.0 4.1 Bal 28 2.6 1.8 17.3 12.0 0.3 1.2 2.0 4.0 Bal 29 2.6 1.7 17.2 11.8 0.3 1.1 0.5 1.9 Bal 30 2.2 1.7 17.5 12.1 0.2 1.1 0.7 5.7 Bal

[0056] Process

[0057] a) PTA Welding

[0058] Alloy 11-15 in Table 1 were deposited on EN S235JR mild structural steel plates using a commercial PTA unit (Commersald 3001). One layer, single track clads were deposited on a substrate with size 125×40×20 [mm] using the welding parameters in Table 2. A mix of Argon and 5% H.sub.2 with flow rate of 16.5 l/min was used as shield gas to protect the melt pool from oxidation. Argon with flow of 2.0 l/min was used to transport the powder from the hopper to the melt pool. Pilot gas was 2.0 l/min. The samples coated with the parameters in Table 2 were used for measurements of coating hardness, dilution and microstructure.

TABLE-US-00002 TABLE 2 PTA welding parameters for coating of 125 × 40 × 20 mm substrates, one layer, single track Substrate Feed rate Speed Power Oscillation T(° C.) g/min cm/min A (mm/min) Cooling RT 25 8 125 10 Air

[0059] Clads consisting of two overlapping tracks were deposited on a substrate with size 220×60×30 [mm]. Overlap between the two adjacent clads was 3 mm and oscillation of the PTA torch 10 mm. The clads were deposited using the welding parameters in Table 3 on room temperature substrates. The coated samples were cooled in vermiculate. A mix of Argon and 5% H.sub.2 with flow rate of 16.5 l/min was used as shield. Argon, flow 2.0 l/min was used as transport gas. Pilot gas was 2.0 l/min. Blanks with size requested by ASTM G65 were cut out from these samples, plane grinded and tested for resistance to abrasive wear.

TABLE-US-00003 TABLE 3 PTA welding parameters for coating of 220 × 60 × 30 mm substrates, one layer, two overlapping tracks Substrate Feed rate Speed Power Alloy T(° C.) g/min cm/min A Cooling 11, 13, 15, RT 25 8 120 Air

[0060] b) Laser Cladding

[0061] Laser cladding was performed using an IPG 6 kW fibre coupled diode laser with a Coax 8 powder feed nozzle and a 5 mm round spot. The process window was typically determined using two laser travel speeds, 16 and 8 mm/s. Powder feed rate was designed to give approximately 1 mm thick coatings. The laser power was varied between 1000 to 2500 W. Argon, 15 l/min, was used as shielding gas. Argon, 6 l/min, was used as transport gas for the powder. The powders were deposited on EN S235JR mild steel substrates with size 100×35×10 mm pre-heated at 200° C. Six tracks were deposited with 50% overlap. Welding parameters investigated are summarized in Table 4. Cross section of the cladded samples were checked for degree of bonding to the substrate, interface porosity and dilution from the substrate by using optical microscopy. The samples with good bonding to the substrate and dilution <10%, were selected for evaluation of the coating properties.

TABLE-US-00004 TABLE 4 Laser cladding parameters used for coating of 100 × 35 × 10 mm EN S235JR substrate, 6 overlapping tracks Laser Robot Powder Power Speed flow Test [W] [mm/sec] [g/min] Comment A 1500 16 20 Poor bonding to the substrate B 2000 16 20 Good bonding, dilution <5% C 2500 16 20 Good bonding, dilution approx. 5-10% D 1000 8 13 Poor bonding to the substrate E 1500 8 13 Good bonding, dilution <5% F 2000 8 13 Good bonding, dilution approx. 5-10%

[0062] Pucks with size 80×80×30 mm were coated for production of abrasive wear test samples according to ASTM G65, procedure A. Two samples with size 58×25×30 mm were cut out from each puck. The samples were than plane grinded to fulfil the requirements for the abrasive wear test.

TABLE-US-00005 TABLE 5 Laser cladding parameters used for coating of 80 × 80 × 30 mm EN S235JR substrate Alloy Power [W] Speed [mm/sec] Powder flow [g/min] 11 2500 16 25 13 2500 16 25 15 2500 16 25

[0063] Evaluation

[0064] The clads were investigated for presence of cracks and other surface flaws. They were cleaned (CRC Crick 110) and then coated with a red dye (CRC Crick 120) penetrating into surface defects or cracks through capillary forces. After 10 minutes excess dye was removed from the surface and a white developer (CRC Crick 130) applied. The developer drew the penetrant out of crevices, cracks or other hollow imperfections communicating with the surface and coloured them in red.

[0065] Rockwell hardness HRC was measured using a Wolpert Universal hardness tester. The coatings were ground. Seven hardness indents were performed on the flat surface and the average was calculated.

[0066] For measuring dilution from the substrate the coated samples were sectioned perpendicular to the coating direction and then ground on SiC paper. The cross section was examined using a stereomicroscope and dilution was determined geometrically. Prior to measurement, the samples were etched in Nital 1% to attack the substrate material and in this way facilitate the detection of the coating. The as-grinded coating cross section were photographed using a Leica stereomicroscope. The total coating area (A.sub.coating+A.sub.substrate) and the area of the coating that used to be substrate prior to overlay welding (A.sub.substrate) were measured by image analysis. Dilution from the substrate material by cross-sectional area was thus calculated as defined in the following equation:

Dilution in %=((A.sub.substrate)/(A.sub.coating+A.sub.substrate))×100

[0067] For analysis of the coatings quality and microstructure and in some cases measurement of the geometrical dilution from the substrate the samples were than moulded in Bakelite, ground and polished using standard procedures for metallographic sample preparation. Oxide polishing with colloidal SiO.sub.2 was used as the final step of metallographic sample preparation. The coatings cross section was examined using a light optical microscope (Leica DM 6000) and a FEGSEM (Hitachi FU6600) equipped with a silicon drift detector (SDD) for EDS analysis (Quantax 800 Bruker). EDS maps for Mo and V were used to evaluate the volume fraction of phases present in the coatings by image analysis.

[0068] Low stress abrasive wear testing was performed according to ASTM G65 standard (ASTM G65: Standard Test Method for Measuring Abrasion Using the Dry Sand/Rubber Wheel Apparatus, 2010), procedure A, by using a commercial multiplex sand/wheel abrasion tribometer (Phoenix tribology TE 65). Five sample replicas per material were tested.

[0069] Impact wear testing was performed by using an in-house build test rig. A schematic of the set-up is shown in FIG. 8. Standard steel bearing balls of mass m drop from pre-defined heights onto the coated test specimen. The potential energy (Ep) of each ball is Ep=m h g, wherein m is the mass of the ball, h is the drop height, and g is the gravitational constant. By varying the mass of the steel balls and the height from which they are dropped different potential energies i.e. impact energies are simulated. A data point corresponds to the total number of ball strikes for a pre-defined height, i.e. impact energy, until occurrence of a first circular crack around the impact dent. This type of model impact wear testing is suitable to rank impact wear resistance of materials exposed to impact overloads at relatively low impact velocities. Operation conditions closest to the modelling in this testing can be exemplified by a first contact of the excavator bucket teeth with the ground; by filling of the excavator buckets by the dig-out material; by forwarding the dig-out material to the truck bed etc. Abrasive wear is removed from this testing in difference to a combined abrasive-impact wear testing.

[0070] Results

[0071] Dilution, abrasive wear resistance (AW) and hardness HRC of alloys 11-15 as processed into a hardfacing coating by PTA-welding and laser cladding are summarized in Table 6.

TABLE-US-00006 TABLE 6 Dilution, abrasive wear resistance and HRC of the alloys 11-15 as PTA welded and laser cladded PTA welding Laser cladding Dilution AW Dilution AW Alloy % HRC (mm.sup.3) % HRC (mm.sup.3) REF <10 68 8.0 <5 67 9.0 11 8 68 9.0 <5 67 7.8 12 8 67 <5 67 13 10 66 11.7 <5 63 10.9 14 5 65 <5 63 15 8 64 12.0 <5 59 17.5

[0072] In Alloy 11-14 with 3.5 to 5 wt. % Chromium abrasive wear resistance is below 12 mm.sup.3 and hardness HRC is above 65 units. This level of resistance to abrasive wear is comparable to NiSiB mixes with tungsten carbides, which are state of the art alloys in applications exposed to severe abrasive wear, but is achieved at lower material cost. This level of resistance to abrasive wear is also comparable to the reference alloy (REF). When increasing the amount of Cr to 6% both hardness and abrasive wear resistance drops.

[0073] By adequate additions of Chromium and Silicon according to embodiments of the present invention, a surprising combination of high hardness, abrasive wear resistance and impact resistance is achieved when the iron-based alloy compositions are processed into a hardfacing coating. This is e.g. shown by the hardness and abrasive wear resistance data for the alloys in Table 6 and the graph of FIG. 1 showing the hardness of PTA-welded coatings made from iron-based alloy compositions with different Chromium content. In particular, alloys with a Chromium content between 3.5 wt. % and about 5 wt. % and a Silicon content above 0.2 wt. %, such as above 0.5 wt. %, such as above 0.6 wt. % show a good combination of hardness and abrasive wear combined with significantly improved resistance to impact as illustrated in FIG. 2. While samples of the reference alloy (REF) without or low Chromium content and a corresponding Si content exhibit a lower impact wear resistance especially at low impact energies below 15 J, the combined addition of both Chromium and Silicon in selected amounts provides the above-mentioned surprising combination of wear resistance properties.

[0074] Impact resistance data is shown in FIG. 2. FIG. 2 shows the impact energy per strike as a function of the number of strikes needed to achieve the first crack in the coating. The graph shows data for the iron-based alloy composition Alloy 11 and the reference alloy REF. Each of the plotted lines is a linear regression to measurements obtained on at least two samples of the respective alloy, wherein measurement points have been collected for energies per strike of 30 J, 25 J, 20 J, 15 J, and 10 J. The corresponding regression data are given in Table 8 below. The best performing samples may show a so-called run-out behavior, where at the lowest impact energies per strike no crack formation is observed, or at least not observed in a reproducible manner, within a large number of strikes of up to 100 strikes. Data points showing such a run-out behavior were not included in the linear regression. The diagram shows that towards lower impact energies per strike the coatings made using alloy composition Alloy 11 can withstand considerably more accumulated impact energy as expressed in number of strikes before the first crack is formed when compared to the reference alloy (REF) without Chromium. For an impact-energy of 10 J, about 15 strikes are needed to form the first crack in the reference alloy (REF), while more than 25 strikes or even 30 strikes are needed for the alloy composition Alloy 11.

[0075] One important insight underlying the present invention relies on an analysis of the microstructure of the alloys when processed by melting and subsequent cooling to form a (re-)solidified coating, as further explained by way of example below. The microstructure analysis reveals that the skilled person may use the present invention to design an alloy composition optimized for a particular application by setting the Chromium content of the iron-based alloy composition, and further adding Silicon within carefully selected ranges allowing for tuning the distribution of different phases in the microstructure of the processed material, in order to achieve desired properties of combined wear resistance including combinations of hardness, abrasive wear, impact wear, and/or coating quality. Notably, Silicon was found to affect the amount of primary hard phase particles formed in the iron-based alloy compositions with Cr addition, more particularly the amount of primary boride particles as best seen in FIG. 5. A particularly advantageous range for the Silicon content for tuning the alloy properties was found to occur below 1.5 wt. %, or below 1.4 wt. %, or below 1.3 wt. %, or below 1.2 wt. %, or below 1.1 wt. %, or below 1 wt. %, and above 0.2 wt. %, or above 0.3 wt. %, or above 0.4 wt. %, or above 0.5 wt. %, or above 0.6 wt. %.

[0076] For a systematic implementation, the skilled person designing an alloy composition according to desired wear resistance properties may develop information on the phase formation properties of the alloy composition by producing a sample of processed alloy and analyzing the microstructure of the sample with respect to its phase composition, and advantageously with respect to the fractions of primary boride particles and of eutectic matrix material in the processed alloy material. For the purposes of analyzing different alloy compositions in a systematic implementation of the invention, the skilled person may e.g. prepare samples by melting the corresponding iron-based compositions and casting them into ingots that are polished for a microstructure analysis according to known metallurgical analysis techniques.

[0077] An example of such a microstructure analysis is given in the following. Alloys with a Cr content of 4 wt. % and a Si content varying between 0.2 wt. % to 2 wt. % were melted in an induction furnace and then poured in a copper mould. Furthermore, ingots with a Cr content of 1.9 wt. % and 5.7 wt. %, and with a Si content of 0.5 wt. % and 0.7 wt. %, respectively were prepared in the same manner. Chemical composition of the produced ingots was analyzed and the results are reported in Table 1 as alloys 26, 27, 28, 29, and 30. The microstructure was investigated using a SEM equipped with EDS detector for energy-dispersive X-ray spectroscopy. Examples of SEM micrographs are seen in FIGS. 3a-3c for alloy compositions Alloy 26, 27, and 28, and in FIGS. 4a and 4b for alloy compositions Alloy 29 and 30, respectively.

[0078] FIGS. 3a-3c show the microstructure of the ingots from alloy compositions 26-28 with 4 wt. % of Cr, as seen in SEM BSE (back scatter) micrographs, wherein alloy composition 26 has 0.2 wt. % Si (FIG. 3a); alloy composition 27 has 1 wt. % Si (FIG. 3b); and alloy composition 28 has 2 wt. % Si (FIG. 3c). FIGS. 4a-4b show the microstructure of the ingots from alloy compositions 29-30 as seen in SEM BSE (back scatter) micrographs, wherein alloy composition 29 has 1.9 wt. % Cr and 0.5 wt. % Si (FIG. 4a); and alloy composition 30 has 5.7 wt. % of Cr and 0.7 wt. % Si (FIG. 4b).

[0079] FIG. 6. shows the microstructure of coatings made by PTA-welding using alloy compositions Alloy 11 and Alloy 13, as seen in SEM BSE (back scatter) micrographs, wherein alloy composition 11 has 3.7 wt. % Cr and 0.7 wt. % Si; and alloy composition 13 has 3.9 wt. % of Cr and 1.4 wt. % Si.

[0080] The microstructure consists of primary carbides (PC, dark grey), primary borides (PB, white/light grey particles), eutectic structure consisting of Molybdenum rich borides and martensite as well as martensitic islands. An example of elemental mapping of V, Mo, Cr, Fe, Si, C, and B using EDS is shown in FIGS. 7a-7h for alloy composition 11.

[0081] Variations in the amount of primary borides (PB, open circles), primary carbides (PC, solid diamonds), and eutectic structure (Eutectic, solid squares) with increased Si content is shown in FIG. 5 for ingot samples made from alloy compositions 26-28. The volume fraction of primary carbide is similar for all four alloys and approx. 17 vol. %. The diagram shows that by increasing the amount of Si the volume fraction of primary borides (PB) increases, while the amount of eutectic structure (Eutectic) decreases. Most notably, Silicon was found to influence the amount of primary hard phase particles formed in the iron-based alloy composition with Cr additions when varied within ranges below 2 wt. % of Si, with advantageous ranges as given above. A particularly pronounced response is seen in the range around and below 1 wt. % Si. The amount of primary borides (PB) as compared to the amount of eutectic structure (Eutectic) affects the abrasive wear resistance of a clad. Controlling the Si content is therefore a most useful tool in determining the final microstructure of an alloy, and as a consequence the final properties of a clad.

[0082] Similar results were obtained on PTA welded coatings using alloy compositions 11 and 13, as summarized in Table 7.

TABLE-US-00007 TABLE 7 Volume fraction of phases present in PTA welded alloys with different Si content and abrasive wear (AW) resistance Cr Si PC PB Mart Eutectic AW Alloy wt. % wt. % vol. % vol. % vol. % vol. % (mm.sup.3) 11 4 0.7 17 4 5 74 9.0 13 4 1.4 17 10 11 65 11.7

TABLE-US-00008 TABLE 8 Linear regression data for impact wear measurements using the ball drop method Slope Intercept Alloy [J/strike] [J/strike] R{circumflex over ( )}2 11 −0.55 26.6 0.66 REF −1.0 26.9 0.77