TECHNICAL FIELD

The present invention relates to a composite material based on reinforced manganese steel, a wear part made thereof and a method for making the same.

BACKGROUND

A particular category of wear resistant steels are typically referred to as manganese steel or Hatfield steel. These materials are suitable for applications where a high toughness and a moderate abrasion resistance are required including for example use as wear parts for crushers that are subjected to strong abrasion and dynamic surface pressures due to the rock crushing action. Abrasion results when the rock material contacts the wear part and strips-off material from the wear part surface. Additionally, the surface of the wear part is subjected to significantly high surface pressures that cause wear part fatigue and breakage.

Manganese or Hadfield steel is typically characterised by having an amount of manganese, usually above 11% by weight. However, the problem with manganese steel is that it is typically too ductile for wear parts in modern crushers that are subject to extreme operating conditions, meaning the at the lifetime of the wear parts is reduced and the maintenance costs are increased. Therefore, the problem to be solved is to provide a manganese steel with enhanced wear resistance.

A known solution is to reinforce at least part of the manganese steel with particles having an increased hardness. W020200222662 discloses a composite material, however the problem with this material is that is not provide an optimal balance between wear resistance and impact resistance and an even more significant problem is that there is poor bonding between the reinforcing particle and the manganese steel matrix and poor bonding between reinforced and nonreinforced zones, which leads reduced wear resistance and premature failure of the wear parts.

Therefore, the problem to be solved to provide a composite material that can be used for wear parts having an optimal balance between wear resistance and impact resistance, wherein there is improved bonding between the reinforcing particles and the manganese matrix and the bonding between the reinforced and non-reinforced zones in order to reduce defects and cracking that would lead to premature failure of the wear parts. DEFINITIONS

A "catalysis" is a metal powder or mixture of metal powders which during the reaction in the selfpropagating high temperature synthesis (SHS) undergo melting and form a matrix of the composite zone. The fundamental role of catalysis is to reduce the amount of dissipated energy in the SHS process.

A "compact" is a densified powder composition.

SUMMARY OF INVENTION

It is an objective of this invention to provide a novel and improved composite material for wear parts. The objective is achieved by providing a composite material comprising: at least one reinforcing zone comprising Niobium carbide (NbC) and a manganese steel matrix; a manganese steel zone that surrounds each of the reinforcing zones; and an interface layer positioned between each of the reinforcing zones and the manganese steel zone; characterized in that: the average grain size of the NbC particles in each of the reinforcing zone(s) is between 2-5 pm, preferably between 2- 4 pm.

Advantageously, this produces a composite material that has both increased wear resistance and structural integrity. Therefore, when the material is used on areas of wear parts that are highly exposed to wear the lifetime of the parts is increased. If the average grain size of the NbC grains is too large, then the composite material will be too brittle. If the average grain size of the NbC grains in too small the wear resistance will be reduced. Further, if the grain size is < 2pm or >5pm then pores and / defects appear in the interface and / or reinforcing zones which decreases the wear resistance of the material. Using NbC in the reinforcing zone results in high hardness and high wear resistance.

Preferably, the composite material comprises between 60-90 wt% of NbC in each of the reinforcing zones. Preferably between 70-90 wt%, even more preferably between 80-90 wt%. Advantageously, this provides the optimal balance between wear resistance and impact resistance. If the wt% of NbC in each of the reinforcing zones in too high the composite material will be too brittle and more prone to failure. If the wt% of the NbC in each of the reinforcing zones is too low, then composite material will have low hardness and therefore it will not have sufficient wear resistance.

Preferably, the composition of the manganese steel in manganese steel zone has the chemical composition by weight of: carbon: 0.5 to 2.0%; manganese: 11 to 22%; silicon: 0.2 to 1.0%; chromium: 1 to 2%; nickel: up to 0.6%; molybdenum: up to 0.5%; and a balance of Iron. Advantageously, this steel composition is characterized by the addition of micro-alloying elements such as chromium, nickel and molybdenum in good amounts which induce high yield strength and high hardness resulting in increase in wear resistance of manganese steel.

Preferably, the Vickers hardness of the reinforcing zones is between 600-1000 HV1 and the hardness of the manganese steel zone is between 200 - 300 HV1 before work hardening. Advantageously, the increased hardness in the reinforcing zones leads to a more wear resistant material.

Preferably, the thickness of each of the interface layer is between 90-300 pm, preferably between 130-200, even more preferably between 250-300 pm. Advantageously, this thickness of interface layer or thickness of contact area between manganese and composite zone is an indication of an increase in the reaction propagation rate and the amount of heat generated due to the high combustion temperature taking place at the contact between the molten manganese steel and the insert. A high combustion temperature leads to the precipitation of large grains at the interface. If the thickness is too large the heat conductivity increases in the composite zone which results in a faster heat dissipation towards the inside of the composite zone resulting in high nucleation rate of NbC particles. If the thickness is too small the heat conductivity is less which favours growth, consequently less nucleation of NbC particles.

Preferably, the interface layer is free of defects. Advantageously, the absence of any defects in the interface layer means that there is good bonding between the manganese steel zone and each of the reinforcing zones and consequently the structural integrity of the composite material is improved, meaning that the lifetime of the wear parts that the materials is used in is increased. Further, the absence of the presence of any pores is an indication that the composition has the ability to absorb the excess heat and gases from the SHS process and so therefore signifies that the synthesis reaction has been successful.

Preferably, the wettability between the NbC grains and the manganese steel in the reinforcing zone (s ) is >99%, preferably >99.5%, even more preferably >99.9%. Advantageously, good wettability induces an excellent bonding between the composite zone and manganese steel preventing defects such as pores and cracks to form and consequently the wear resistance increases.

Preferably, the each of the reinforcing zones has a volume of between 30-75 cm ³ . Advantageously, this size provides the optimal balance between wear resistance and impact resistance.

Preferably, at least 90% of the NbC grains in the reinforcing zones have a square prismatic shape.

Advantageously, the square prismatic shape of NbC will contribute to crack deflection and stop crack propagation increasing the ductility and high wear resistance of the reinforcing zone. A square prismatic shape is defined as having two square faces and two rectangular faces, 12 edges and 8 vertices. To calculate the percentage of the NbC grains having a square prismatic shape a SEM fracture surface image is taken, then the number of grains having the square prismatic shape is counted, and the total number of grains is counted. The percentage of square prismatic grains can then by calculated from "number of grains having square prismatic geometry / total number of grains x 100". Preferably, the distance between two neighbouring reinforcing zones is between 1-5 mm, preferably between 1-3 mm, more preferably between 1-2 mm. Advantageously, this provides the optimal balance between wear resistance and impact resistance. If the reinforcing zones are spaced too far apart then the wear resistance will not be high enough. If the reinforcing zones are spaced to close together then the impact resistance will not be high enough.

Another aspect of the present invention relates to a wear part comprising the composite material as described hereinbefore or hereinafter. Advantageously, the presence of the reinforcing zones within the manganese zone will improve the wear resistance and therefore the lifetime of the wear parts which in turn increases profitability.

Another aspect of the present invention relates to a method of producing the composite material as described hereinbefore or hereinafter comprising the steps of: a) mixing together 50-90 wt% niobium, 6-20t% carbon and 0-40 % catalysis powders; b) compacting the mixed powders together to form at least one compact; c) positioning and optionally fixing at least one compact into the interior of a mold; d) pouring molten casting manganese steel into the mold to surround the at least one compact to initiate a self-propagating high temperature synthesis (SHS) reaction to produce a cast; e) heat treating the cast; f) quenching the cast; characterized in that: in step b) the powders are compacting with a pressure of between 450-700 MPa, preferably between 500-650 MPa, more preferably between 550-650 MPa.

Advantageously, if this pressing pressure is used the compacts have a low density which enables the manganese steel to more easily infiltrate between the NbC grains and consequently results in improved bonding between the NbC grains and the manganese steel. Further it avoids the creation of defects which would lead to premature failure of the wear parts that the composite material is used in.

Preferably, the catalysis is selected from Fe, Mn, Ni, Mo, Cr, W, Al, or a mixture thereof.

Advantageously, he addition of a catalysis in a specific amount will contribute to a strong stabilization to austenite phase within the microstructure in addition to good mechanical properties and high wear resistance. The catalysis addition will also act as a grain growth inhibitor which results in a fine microstructure.

BRIEF DECRSIPTION OF DRAWINGS

Figure 1: Shows a line drawing of the composition of the composite material.

Figure 2: Shows an SEM image taken of the reinforced zone with low magnification on the left and high magnification on the right.

Figure 3: Shows an SEM image taken of the interface layer with low magnification on the left and high magnification on the right.

Figure 4: Shows an SEM image of the composite material

Figure 5: Shows a perspective drawing of a wear part.

Figure 6: Shows an SEM image of a comparative sample having pores in the interface layer (left picture) and reinforced zone (right picture).

DETAILED DESCRIPTION

Figure 1 shows a composite material 2 comprising at least one reinforcing zone 4 comprising niobium carbide (NbC) and a manganese steel matrix; a manganese steel zone 6 that surrounds each of the reinforcing zones 4 ; and an interface layer 8 positioned between each of the reinforcing zones 4 and the manganese steel zone 6. In each of the reinforcing zones, the NbC acts to reinforce the manganese steel matrix.

The average grain size of the NbC particles in each of the reinforcing zone(s) (4) is between 2-5 pm, preferably between 2-4 pm, most preferably between 2-3pm.

The average grain size of the NbC grains is measured by Scanning Electron Microscopy (SEM) analysis where several and different areas from the samples were analysed and particle sizes were measured. Then, the average particle size was calculated.

Each interface layer 8 comprises NbC and manganese steel and can be distinguished from the reinforcing zones 4 as the shape and size of the NbC grains are different. The interface layer(s) 8 can be distinguished from the reinforcing zone(s) 4 can either: comparing the geometry and / or comparing the average grain size. If the geometry is being compared, the reinforcing zone(s) 4 comprise >90% NbC grains having a square prismatic geometry whereas the interface layer(s) 8 comprise <5% NbC grains having a square prismatic geometry. A NbC grain is considered to have square prismatic geometry if the grains have 4 sharp edges. If the grain size is being compared the average NbC grain size of in the interface layer(s) 8 is at least 5% less than the average NbC grain size on the reinforcing zone(s) 4.

Figure 2 shows a Scanning Electron microscope image using MIRA3 TESCAN equipment. A secondary electron detector (SE) with a high voltage of 15 KV and a working distance of 9 mm configuration were used. SEM image of the NbC grains in the reinforcing zone 4. Figure 3 shows an SEM image of the NbC grains in the interface layer 8. The different NbC grain geometry and size can be clearly be seen when comparing these two figures.

In one embodiment the wt% of NbC in each of the reinforcing zones 4 is between 60-90 %, more preferably between 80-91 %, even more preferably between 80-90%.

In one embodiment, the composition of the manganese steel in manganese steel zone 6 has the chemical composition by weight of: carbon: 0.5 to 2.0%; manganese: 11 to 22%; silicon: 0.2 to 1.0% ; chromium: 1 to 2%; nickel: up to 0.6%, molybdenum: up to 0.5% and a balance of Fe.

In one embodiment, the chemical composition of the manganese steel in each of the reinforcing zones 4 has the chemical composition by weight of: 1-1.5 %C, 11-14 % Mn, 0.4-0.8 % Si, 1.3-2.0 % Cr, 0.6 % Ni, 0.065 % P.

In one embodiment, the hardness of the reinforcing zones 4 is between 600-1000HV1, preferably between 750-1000. The hardness of the manganese steel zone 6 is between 200 - 300 HV1.

Hardness is measured using Vickers hardness mapping on polished samples using a 1 kg. and a holding time of 15 seconds. A micro-hardness tester, Matsuzawa, model MXT was used. Hardness measurement profiles are performed starting from the non-reinforce zone, moving to the interface layer and then to the reinforced zone.

In one embodiment, the interface layer 6 is between 90-300 pm wide, preferably between 130-200. Figure 4 shows an SEM image taken at 15.0 kV, 563 magnification of the reinforced zone 4, the manganese steel zone 6 and the interface layer 8. The width of the interface layer 6 is measured from a start point 10, which is defined as being adjacent to the manganese steel zone 6 and the point at where the NbC grains are present. The end point 12 for measuring where the interface layer 8 ends, and therefore where the reinforcing zone 8 starts is considered to be where the average grain size of the NbC grains has increased by 20% compared average NbC grains measured at the start point 10 and / or where the percentage of NbC grains having a square prismatic shape increases above 90%. In one embodiment, the interface layer 8 is free of defects. Defects are considered to be cracks or pores.

In one embodiment, the wettability between the NbC grains and the manganese steel in the reinforcing zones 4 is >99%, preferably >99.5%, more preferably >99.9%, most preferably 100%. Wettability is measured by a Scanning Electron Microscope where the contact area and the bonding between the NbC grains and the manganese steel have been evaluated.

In one embodiment each of the reinforcing zones 4 has a volume of between 30-75 cm ³ . For example, but not limited to the reinforcing zone(s) 4 could have a length of between 100-200 mm, preferably between 100-150 mm, a width of between 20-30 mm, preferably between 20-25 mm and a thickness between 15-30 mm, preferably between 15-25 mm.

In one embodiment >95%, preferably >98%, more preferably >99% of the NbC grains in the reinforcing zones 4 have a square prismatic shape. Preferably, the NbC grains are uniformly distributed in the manganese steel in the reinforcing zone(s).

In one embodiment, there are a plurality of reinforcing zones 4 with its interface zone 8 and the distance between two neighbouring reinforcing zones 4 with its interface layer 8 is between 1-5 mm, preferably between 1-3 mm, more preferably between 1-2 mm.

Figure 5 shows an example of a wear part 14 comprising the composite material 2 as described hereinabove or hereinafter. For example, the wear part 2 could be, but not limited to, a cone crusher or a stationary jaw crusher or a mobile jaw crusher that is configured to crush material or other material/rock processing unit. The reinforcing zone(s) 4 are positions on the wear parts 14 in the locations that are most subjected to high wear, for example on a crushing zone 18 of a cone crusher 16.

The method for producing the composite material 2 as described hereinbefore or hereinafter comprising the steps of: a) Mixing together 50-90 wt%, preferably 77-90 niobium; 6-20 wt%, preferably 10-20 wt% carbon and 0-40 wt%, preferably 0-10 wt% catalysis powders; b) compacting the mixed powders together to form at least one compact using a compacting pressure of between 450-700 MPa, preferably 500-650 MPa more preferably 550-650 MPa ; c) positioning and optionally fixing at least one compact into the interior of a mold; d) pouring molten casting manganese steel into the mold to surround the at least one compact to initiate a self-propagating high temperature synthesis (SHS) reaction to produce a cast; e) heat treating the cast; and then f) quenching the cast. Preferably, the cast is treated at a temperature of between 1400-1500°C, the cast is quenched using water. Preferably, the catalysis is selected from Fe, Co, Ni, Mo, Cr, W, Al, or a mixture thereof. Carbon could be added in the form of graphite, amorphous graphite, a carbonaceous material or mixtures thereof. The compacts could for example be held in place using me a metallic fixation system to hold them in place during casting.

EXAMPLES

Example 1 - Samples

Sample A is a comparative sample of non-reinforced manganese steel having the composition 1- 1.5 %C, 11-14 % Mn, 0.4-0.8 % Si, 1.3-2.0 % Cr, 0.6 % Ni, 0.065 % P. Samples B-H are samples of composite materials produced by mixing together powders of niobium, carbon and a catalysis powder. The compacting the mixed powders to form compacts which were then positioned in a mold and then molten manganese steel having a composition of 1- 1.5 %C, 11-14 % Mn, 0.4-0.8 % Si, 1.3-2.0 % Cr, 0.6 % Ni, 0.065 % was poured into the mold to surround the compacts which initiated a SHS reaction, the cast was then heat treated at a temperature of 1450 °C and then quenching with water. Table 1 shows a summary of the reinforced samples:

Table 1: Summary samples

It can be seen if the compacting pressure is not high enough then the wettability is reduced.

Example 2 - Hardness

Vickers hardness was measured by a micro-hardness tester, Matsuzawa, model MXT using 1 kgf and a holding time of 15 seconds. Hardness measurement profiles are performed starting from the nonreinforce zone, moving to the interface layer and then to the reinforced zone.

The hardness measurement results are shown in Table 2 below:

Table 2: Hardness measurement

It can be seen that the inventive samples have an increased hardness in reinforced zones compared to the comparative samples.

Example 3 - Wear test

Wear was tested using a standard wear test using a lab jaw crusher. The wear test procedure consists on using fixed amount of rocks from 1 Ton up to 4 Ton of rocks. Four plates, two stationary and two moving, were placed inside the jaw crusher. Reference plates were also mounted in both positions. The reference plates are based on Weldox type of material.

The calculation of wear is based on the difference in volume loss of the test plates compared to the reference plates. All plates were weighed before and after wear test. Then volume loss is calculated using the density of 7.85 g/cm ³ and 7.6 g/cm ³ for the reference and test plates respectively. The total wear ratio (WR) is calculated according to ASTM G81-97a(2013).

The wear test results are shown in table 3 below:

Table 3: Wear test results

It can be seen that the wear rate for the inventive samples is low. Example 4 - Defects

Table 4: Defects

Defects were assessed by using Scanning Electron microscopy analysis where cracks and pores are identified. It can be seen that the inventive samples are free of defects. It can be seen that the comparative sample contains pores, whereas the inventive samples do not. Figure 6 shows the pores in sample F, whereas figure 4 shows the absence of any pores from sample B.