Chromium cast steel with enhanced wear resistance

Abstract

The subject of the invention is a chromium cast steel with enhanced wear resistance, comprising 0.2- 2.1 wt% C, 0.3-3.0 wt% Mn, 0.2- 2.0 wt% Si, 3-25 % Cr, <3.0 wt% Ni, < 2.0 wt% Mo, 0.02 - 0.05 wt% Al, < 0.05 wt% P, < 0.05 wt% S and a carbide forming element selected from a group comprising Ti, Nb and Zr, which participation equals for Ti 4×wt% C, for Nb 8×wt% C, and for Zr 8×wt% C, while remaining comprises Fe and unavoidable impurities.

Description

[0001] The present invention relates to chromium cast steel with enhanced wear resistance.

[0002] Chromium is one of the most commonly used alloying elements in cast steel. It provides heat resistance and thermal strength, but most importantly, it enhances corrosion resistance. Chromium cast steels are utilized in the production of machine components, structural elements, and tools designed for operation in extreme conditions, where they are exposed to Wear-resistant chromium cast steel is predominantly used for manufacturing crusher jaw plates and hammers, track links for crawler vehicles, excavator components, and similar applications. These elements and tools operate under conditions where compressive or impact forces induce significant material stresses, while friction leads to continuous surface abrasion, resulting in a gradual loss of functional properties over time. Furthermore, industrial environments, particularly the aggressive operating conditions in the extraction and processing industries, contribute to the accelerated corrosion of machinery and tools. Therefore, it is essential to develop cast steel with high resistance to mechanical stresses induced by compression and impact, exceptional wear resistance, especially against friction-induced degradation, and superior corrosion resistance under industrial conditions.

[0003] The decisive factor determining the functional properties of a casting is its microstructure, which is influenced by both the heat treatment of the alloy and its chemical composition. As such for instance, the carbon content significantly affects the impact toughness of cast steel. A low carbon content reduces susceptibility of an alloy to cracking but considerably decreases the achievable hardness and wear resistance. Wear resistance is obtained through the addition of chromium, which exhibits a strong tendency to form carbides, resulting in a microstructure composed of a ferritic matrix with chromium carbides precipitated along grain boundaries. Chromium carbides of the M7C3 type enhance wear resistance; however, such material is only suitable for operation under low dynamic loads. Chromium carbides are highly stable, form a continuous network, and reduce both crack resistance and impact toughness. They are very difficult to dissolve, even when heated to the austenitizing temperature of 1150 °C. Moreover, the binding of chromium into carbides significantly reduces the amount of free chromium in the matrix, leading to a decline in corrosion resistance.

[0004] In the state of the art, known are solutions that aim to improve the microstructure of chromium cast steels by reducing or controlling the amount of chromium carbide precipitates and optimizing their properties through the addition of elements with strong carbide-forming tendencies or through specialized heat treatment.

[0005] For example, patent application publication US4536232A discloses white cast iron containing primarily 1.2-2 wt% carbon, 1-4 wt% nickel, 1-4 wt% molybdenum, 24-32 wt% chromium, up to 1 wt% copper, and up to approximately 1 wt% of a microalloying element selected from the group consisting of titanium, zirconium, boron, niobium, and rare earth elements, with the remainder being iron. This cast iron has a matrix composed essentially of tempered martensite, in which primary chromium-rich carbides and ferrite islands are present.

[0006] Patent JP4648094B2 discloses high-chromium cast iron with the following composition: 2.5-3.5 wt% C, 0.2-1.0 wt% Si, 0.6-2.0 wt% Mn, 13-22 wt% Cr, 1.0-3.0 wt% Mo, 0.01-0.15 wt% N, with the ratios [Cr] / [C] = 4.5-6.5 and [Mn] × [Mo] = 1.8-2.5, with the remainder consisting of Fe and unavoidable impurities. Carbon forms high-hardness carbides (types MC, M 7 C 3 , M 23 C 6 , M 3 C, etc.) with Ti, V, Zr, Nb, Cr, Mo, or Fe and creates a solid solution in the matrix. It plays a key role in controlling the transformation of austenite into high-hardness martensite (achieving a martensitic structure) through quenching (air cooling) of the cast iron and is a crucial element in ensuring the required hardness. In the known solutions concerning both chromium cast steels and their production methods, both publications only limit the intensity of chromium carbide precipitation but do not mention their complete elimination.

[0007] Patent PL233674 discloses chromium-nickel cast steel with enhanced wear resistance, containing a maximum of 2.0 wt% C, a maximum of 2.5 wt% Mn, a maximum of 1.50 wt% Si, a maximum of 0.05 wt% P, a maximum of 0.05 wt% S, 15.0-21.0 wt% Cr, 7.0-13.0 wt% Ni, a maximum of 2.0 wt% Mo, niobium, with the remainder being Fe and unavoidable impurities, where the niobium content ranges from 5.4-10.0 wt%. The publication concerns casting alloys with a relatively narrow chromium content range and does not provide any indication regarding the applicability of this solution to chromium cast steels with a chromium content lower than 15 wt% or greater than 21 wt%.

[0008] Patent application PL440950 discloses a high-hardness tool casting alloy containing 0.6-4.6 wt% C, 0.5-2.0 wt% Mn, below 2.0 wt% Si, below 0.05 wt% P, no more than 0.05 wt% S, 1.0-3.0 wt% Cr, 0.5-2.0 wt% Ni, 0.3-1.5 wt% Mo, 0.02-0.05 wt% Al, with the remainder being Fe and unavoidable impurities, wherein part of the iron is substituted by at least one element from the group comprising titanium, niobium, vanadium, tungsten, zirconium, hafnium, and tantalum, with the total combined mass of these elements ranging from 1.0-15.0 wt% of the alloy. The alloy may advantageously contain titanium in an amount four times the weight percentage of carbon in the alloy, reduced by 0.6-0.8%, vanadium in an amount 5.5 times the weight percentage of carbon, reduced by 0.6-0.8%, zirconium in an amount eight times the weight percentage of carbon, reduced by 0.6-0.8%, niobium in an amount eight times the weight percentage of carbon, reduced by 0.6-0.8%, tungsten in an amount 15 times the weight percentage of carbon, reduced by 0.6-0.8 %, tantalum in an amount 15 times the weight percentage of carbon, reduced by 0.6-0.8%, and hafnium in an amount 15 times the weight percentage of carbon, reduced by 0.6-0.8%. The publication does not provide any indication regarding the potential application of this solution to medium- and high-chromium cast steels with a chromium content exceeding 3 wt%. Such a low chromium content does not confer corrosion resistance nor significantly enhance wear resistance, as atmospheric corrosion resistance requires at least 11-12 wt% chromium. In alloy PL440950, chromium dissolves in the matrix, strengthening it via solid solution hardening and promoting the formation of a martensitic structure after quenching, thereby improving the hardenability of the alloy. However, when chromium content exceeds 4 wt%, the inevitable precipitation of secondary chromium carbides or the formation of alloyed cementite (Fe,Cr) 3 C in the form of needles and plates at grain boundaries occurs, leading to a decrease in impact toughness. For comparison, tool cast steel containing 1 wt% C and 6 wt% Cr exhibits twice the wear resistance of tool cast steel containing 0.7 wt% C and 2 wt% Cr, but its impact toughness decreases several times, reaching values as low as 3-5 J / cm 2< .

[0009] Although known solutions improve the microstructure of cast chromium alloys, they remain insufficient because they do not completely eliminate plate-like and needle-like chromium carbides, which deplete the matrix of chromium. On the other hand, they are often economically unfeasible due to the high-energy heat treatment required, frequently exceeding temperatures of 1000°C and even reaching 1150°C, in an attempt to dissolve the chromium carbide network as much as possible, yet without achieving fully satisfactory results. Additionally, in state-of-the-art solutions, obtaining the desired mechanical properties and corrosion resistance often necessitates a substantial technological chromium excess to ensure a sufficient residual chromium concentration in the matrix, which is not only economically inefficient but also environmentally unfavourable. Another significant drawback of known medium- and high-chromium cast iron alloys is their restriction to relatively narrow carbon and chromium content ranges, which limits their applicability.

[0010] The basis of the invention is to provide a chromium cast steel that combines heat resistance, high-temperature strength, and wear resistance with high corrosion resistance, making it suitable for use in equipment and tools operating in aggressive environments and exposed to severe wear.

[0011] According to the present invention, there is provided a chromium cast steel with enhanced wear resistance comprising 0.2- 2.1 wt% C, 0.3-3.0 wt% Mn, 0.2- 2.0 wt% Si, 3-25 % Cr, <3.0 wt% Ni, < 2.0 wt% Mo, 0.02 - 0.05 wt% Al, < 0.05 wt% P, < 0.05 wt% S and a carbide forming element selected from a group comprising Ti, Nb and Zr, which participation equals for Ti 4xwt% C, for Nb 8×wt% C, and for Zr 8×wt% C, while remaining comprises Fe and unavoidable impurities.

[0012] The invention enables a simple, effective, and economical method to completely bind the carbon present in the cast steel into primary carbides solely through the selection of chemical composition, preventing the formation of a chromium carbide network while ensuring that the entire initial amount of chromium remains in the matrix, which ensures the castings to achieve the desired wear resistance, heat resistance, and high-temperature strength while maintaining high corrosion resistance. The proposed solution involves chromium cast steel, in which chromium carbides are completely replaced by primary Ti, Nb, or Zr carbides, which appear as evenly distributed individual precipitates throughout the matrix. The wide range of carbon and chromium contents expands the potential applications of the cast steel in equipment and tools operating in aggressive environments and exposed to severe wear.

[0013] According to the invention, a chromium cast steel is provided, where chromium carbides are entirely replaced by primary carbides of Ti, Nb, or Zr, which occur as uniformly dispersed precipitates throughout the matrix. Regardless of the chromium-to-carbon ratio, the microstructure of the cast steel according to the invention does not contain chromium carbide precipitates, while instead, all carbon is bound into fine, evenly distributed primary carbides, while all chromium in the alloy remains dissolved in the matrix, ensuring the desired corrosion resistance without the need for an excessive technological chromium excess.

[0014] Unexpectedly, it was found that in chromium cast steel with a medium to high chromium content, the addition of just one strong carbide-forming element selected from a group comprising Ti, Nb, or Zr, in an amount defined as a function of the carbon content in the alloy, enable a complete binding of carbon into primary carbides while keeping all of the original chromium in the matrix, without the need for adding of an excessive amounts of costly carbide-forming elements like chromium and carrying time- and energy-intensive heat treatment to dissolve secondary chromium carbides, as these carbides are practically absent in the final structure of the cast steel. The secondary carbides (chromium carbides) that are formed during solidification and cooling of castings, are instead replaced by primary carbides that precipitate directly in the liquid steel, which primary carbides show a higher affinity for carbon than chromium, effectively binding all available carbon into finely dispersed precipitates with a favourable morphology.

[0015] In the chromium cast steel according to the invention, primary carbides precipitate first in the liquid alloy and remain evenly distributed throughout the entire volume of the alloy after solidification, rather than forming only along grain boundaries, as is the case with chromium cast alloys known in the prior art. The strict correlation between the content of element selected from a group comprising Ti, Nb, or Zr and the carbon content enables the formation of the desired beneficial microstructure across a wide range of chromium content in alloy, increasing the versatility of the invention's applications. formation of primary metal carbides directly in the liquid alloy is simple and allows for the direct casting of alloys into molds while maintaining an advantageous microstructure.

[0016] The final properties of the cast steel are achieved through heat treatment, specifically quenching and tempering. Any small amount of alloy cementite (Fe,Cr) 3 C that may remain in the matrix after casting will dissolve during the heat treatment soaking process. This process does not require complex, time-, and energy-intensive treatments that take into account the intricate crystallization process of castings, chemical composition inhomogeneity (segregation), or the risk of unforeseen precipitation reactions.

[0017] In the chromium cast steel according to the invention, all carbon is bound into primary carbides, which allows for a reduction in chromium content while maintaining corrosion resistance, improves crack resistance and reduces the susceptibility of the finished casting to wear under intense abrasive conditions. This is achieved by completely eliminating chromium carbides and replacing them with harder Ti, Nb, or Zr carbides, whose hardness values are presented in the table below: Carbide Microhardness [µHV] ZrC2920TiC2850÷3200NbC2000(Fe,Cr) 3 C1600Fe 3 C860

[0018] The cast steel according to the invention enables a reduction in manufacturing costs for castings that require the desired corrosion resistance, heat resistance, or high-temperature strength, because chromium is no longer bound in secondary carbides, which would otherwise reduce its content in the matrix and consequently deteriorate its properties. Based on a cost analysis of the mentioned elements, it can be estimated on average that reducing chromium content by 1% allows for the introduction of 1% Ti without increasing production costs. Prices of raw materials in January 2024, PLN / kg FeCr HC9.70FeCr LC19.40Metallic Cr59.90Niobium173.00Titanium39.95

[0019] The subject of the invention is presented in exemplary embodiments in the drawing, on which: Fig. 1illustrates a comparison of the microstructure of chromium cast steel according to the prior art and the chromium cast steel according to the invention, wherein: Fig. la shows the microstructure of chromium cast steel according to the prior art, containing 1.1 wt% C and 7.2 wt% Cr, with a visible white network of carbides and alloyed cementite precipitated along ferrite grain boundaries; Fig. 1b shows the microstructure of chromium cast steel according to the invention, containing approximately 1.1 wt% C and 12 wt% Cr with the addition of 4.5 wt% Ti, displaying primary carbide precipitates uniformly distributed within a ferritic matrix; Fig. 2illustrates a comparison of wear curves for chromium cast steel containing 1.1 wt% C and approximately 12 wt% Cr and a cast steel of similar chemical composition with titanium carbides formed according to the invention. Example 1

[0020] In induction furnace with a 10 kg crucible capacity, a melting process of chromium tool cast steel was carried out, in which the carbon content was approximately 1.1 wt%, while the chromium content was around 7 wt%. During the metallurgical process of cast steel melting, 4.4 wt% Ti was introduced.

[0021] As the charge material, 9227 g of in-house chromium cast steel scrap metal was used, with its chemical composition determined based on analysis, with the remainder consisting of Fe, other alloying elements, and unavoidable impurities. The cast steel scrap metal was loaded at the bottom of the crucible before switching on the furnace, along with 36 g of electrolytic nickel. After melting the charge and heating the metal to approximately 1600 °C, followed by stirring and temperature equalization, the metal was deoxidized using 10 g of aluminium, which was added in portions to prevent a drop in the bath temperature. The remaining alloying additions were then introduced: 250 g of fine metallic chromium (99.9% Cr) and 123 g of fine Fe-Mn (80% Mn). After melting the alloying additions, stirring the molten steel, and equalizing the temperature, the metal was deoxidized again using 10 g of aluminium. Next, the carbide forming element in the form of fine Fe-Ti72 was added in an amount of 659 g. The Fe-Ti was introduced in portions to avoid lowering the molten bath temperature. After adding the final portion of ferroalloy, the molten metal was held in the furnace for approximately 10 minutes to ensure chemical composition uniformity and achieve a temperature of around 1600 °C. Subsequently, the metal was poured into a ladle, with 10 g of aluminium placed at the bottom, and a "Y"-shaped test casting was made in a sand mold with a wall thickness of approximately 35 mm. Table 1 The composition of the charge material for the smelting of chromium steel with TiChemical Composition [wt%] FeCSiMnCrNiMoTiTotal mass [g]Scrap metal 183.501.470.700.706.001.500.104682Scrap metal 290.000.900.800.805.001.801.504545Cr100.00250Ni100.0036Mn80.00123Ti7270.00659Composition77.711.070.671.637.36421.830.70774.4810295

[0022] In this way, a steel casting with the chemical composition shown in Table 2 was obtained with remining comprises Fe and unavoidable impurities. Table 2. Chemical composition of the resulted chromium steel with TiChemical Composition [wt%] CMnSiPSCrNiMoTiAl1.11.50.70.020.037.21.80.74.40.02 Example 2

[0023] In induction furnace with a 10 kg crucible capacity, a melting process of chromium tool cast steel was carried out, in which the carbon content was approximately 0.5 wt%, while the chromium content was around 12 wt%. During the metallurgical process of cast steel melting, 3.9 wt% Nb was introduced. In Table 3 shows charge material summary for smelting of chromium tool cast steel with Niobium.

[0024] As the charge material, 9220 g of in-house chromium cast steel scrap metal was used, with known chemical composition shown in table 3 with the remainder consisting of Fe, other alloying elements, and unavoidable impurities. The cast steel scrap metal was loaded at the bottom of the crucible before switching on the furnac e. After melting the charge and heating the metal to approximately 1600 °C, followed by stirring and temperature equalization, the metal was deoxidized using 10 g of aluminium, which was added in portions to prevent a drop in the metal bath temperature: 455 g of metallic chromium (99.9% Cr) and 123 g of fine Fe-Mn (80% Mn). After melting the alloying additions, stirring the molten steel, and equalizing the temperature, the metal was deoxidized again using 10 g of aluminium, followed by addition of the carbide forming element in the form of fine Fe-Nb60 in an amount of 682 g. The Fe-Nb was introduced in portions to avoid lowering the molten bath temperature. After adding the final portion of ferroalloy, the molten metal was held in the furnace for approximately 10 minutes to ensure chemical composition uniformity and achieve a temperature of around 1600 °C. Subsequently, the metal was poured into a ladle, with 10 g of aluminium placed at the bottom, and a "Y"-shaped test casting was made in a sand mold with a wall thickness of approximately 35 mm. This way a cast steel was made that has chemical composition as shown in Table 4 wherein the remaining consists of Fa and unavoidable impurities. Table 3. The composition of the charge material for the smelting of chromium steel with NbChemical Composition [wt%] FeCSiMnCrNiMoNbTotal mass [g]Scrap metal 183.500.400.700.7013.000.800.104680Scrap metal 290.000.700.800.805.001.80001.504540Cr100.00455Mn80.00123Fe-Nb6060.00682Composition76.030.480.661.5912.26351.13020.69243.910523 Table 4. Chemical composition of the resulted chromium steel with Nb Chemical Composition [wt%] CMnSiPSCrNiMoNbAl0.471.30.50.020.0312.11.10.73.80.02

Claims

1. Chromium cast steel with enhanced wear resistance, characterized by comprising 0.2 - 2.1 wt% C, 0.3-3.0 wt% Mn, 0.2- 2.0 wt% Si, 3-25 % Cr, <3.0 wt% Ni, < 2.0 wt% Mo, 0.02 - 0.05 wt% Al, < 0.05 wt% P, < 0.05 wt% S and a carbide forming element selected from a group comprising Ti, Nb and Zr, which participation equals for Ti 4xwt% C, for Nb 8×wt% C, and for Zr 8×wt% C, while remaining comprises Fe and unavoidable impurities.

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