CN114318117A - Hypereutectic high-chromium cast iron for heavy abrasive wear - Google Patents
Hypereutectic high-chromium cast iron for heavy abrasive wear Download PDFInfo
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Abstract
The invention relates to hypereutectic high-chromium cast iron for heavy abrasive wear, which comprises the following processing steps: s1, charging, namely putting carbon, chromium and molybdenum into a cooling furnace, wherein the weight ratio of chromium is ten times that of carbon, and adding 12.5%; s2, heating at a speed of less than 50 ℃/h, heating to 950-. The invention has the advantages that: the wear resistance of the casting is improved, and the carbide stress is increased. The invention has the advantages that: the wear resistance and the corrosion resistance of the casting are improved, and the service life of the casting is prolonged.
Description
Technical Field
The invention relates to the technical field of metal materials, in particular to hypereutectic high-chromium cast iron for heavy abrasive wear.
Background
The high-chromium cast iron is the third generation of white cast iron developed following ordinary white cast iron and nickel hard cast iron, and has much higher toughness, high-temperature strength, heat resistance, wear resistance and the like than ordinary cast iron due to the characteristics of the metal structure of the high-chromium cast iron, so that the high-chromium cast iron is known as the best wear-resistant material in the present day and is widely applied. The high-chromium cast iron has more practical value for application under normal temperature or high temperature impact wear conditions.
At present, a lot of manufacturers produce high-chromium iron castings, but a lot of enterprises simply produce the high-chromium iron castings according to specified chemical compositions, the characteristics of the high-chromium iron castings are poorly known, and particularly, the production research on large high-chromium iron castings is relatively less. Therefore, it is necessary to make an intensive analysis and study on some characteristics and process points of the high-chromium cast iron and to provide a heat treatment process suitable for mass production.
Disclosure of Invention
The invention aims to solve the technical problem of providing hypereutectic high-chromium cast iron for heavy abrasive wear.
In order to solve the technical problems, the technical scheme provided by the invention is as follows: hypereutectic high chromium cast iron for heavy abrasive wear comprising the following processing steps:
s1, charging, namely putting carbon, chromium and molybdenum into a cooling furnace, wherein the weight ratio of chromium is ten times that of carbon, and the addition amount of chromium is 12.5%, and the formula is as follows:
Cr(％)≥10×C％+12.5％；
s2, heating at a speed lower than 50 ℃/h to 950 ℃ and 1050 ℃,
s3, preserving heat at 950-1050 ℃, preserving heat for 1 hour and at least 3 hours when the wall thickness of the casting is increased by 25mm in the period,
s4, adopting blowing or spraying methods to accelerate cooling, cooling all parts of the casting evenly,
s5, tempering the cooled casting at the temperature of 400-500 ℃, preserving heat for 3-4 hours,
s6, annealing at 740-780 ℃, wherein the heat preservation time during annealing can be 4-8 hours, then furnace cooling is carried out to below 300 ℃, and a machinable casting is obtained,
and S7, hardening the casting after machining.
Preferably, the weight ratio of the carbon is 3.25%, the weight ratio of the chromium is 18.2%, and the weight ratio of the molybdenum is 0.58%.
Preferably, the weight ratio of chromium is ten times of that of carbon and is added by 12.5%, and the formula is as follows: and replacing Cr (%) > 10 XC% + 12.5% by the weight part ratio of the chromium to the carbon of 10: 1.
Preferably, the method further comprises a high-temperature treatment of toughness, wherein the high-temperature treatment of toughness comprises the steps of slowly heating the casting in a vacuum furnace to 1150-1180 ℃, preserving heat for about 8 hours, and then blowing nitrogen into the vacuum furnace to rapidly cool the casting.
Preferably, the molybdenum is replaced by one or more of nickel, manganese and copper.
After adopting the structure, the invention has the following advantages: austenite around eutectic carbides, low carbon content, MSThe temperature is increased, the cast iron is basically completely changed into martensite or bainite, primary austenite dendritic crystal edges generate a small amount of pearlite, and the ultrahigh-carbon ultrahigh-chromium alloy cast iron with refined structure and excellent wear resistance is obtained, and has high volume content of carbide and an as-cast martensite matrix. The macro hardness is above 60 HRC. The wear resistance is 1.5-2 times of that of the conventional high-chromium cast iron.
Drawings
FIG. 1 is a schematic diagram of the ternary phase diagram (Cr 17% cross section) of Fe-C-Cr according to the present invention.
FIG. 2 is a view showing the distribution of carbon, chromium and silicon in primary austenite dendrites according to the present invention.
FIG. 3 is the microstructure of carbides in the as-cast hypoeutectic high chromium cast iron of the present invention.
FIG. 4 is a microstructure of carbides in the as-cast eutectic high chromium cast iron of the present invention.
FIG. 5 is the microstructure of the carbides (x 200) in the as-cast hypereutectic high chromium cast iron of the present invention.
FIG. 6 shows the microstructure of the high-chromium cast iron of the present invention after high-temperature tempering.
FIG. 7 shows the microstructure of the high-chromium cast iron of the present invention after high-temperature tempering and hardening treatment.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings.
With reference to all the figures, a hypereutectic high chromium cast iron for heavy abrasive wear comprises the following processing steps:
s1, charging, namely putting carbon, chromium and molybdenum into a cooling furnace, wherein the weight ratio of chromium is ten times that of carbon, and the addition amount of chromium is 12.5%, and the formula is as follows:
Cr(％)≥10×C％+12.5％；
s2, heating to 950-1050 ℃ at a speed of less than 50 ℃/h, preserving heat at the temperature to reduce the stability of austenite, precipitate secondary carbides, reduce the contents of carbon and chromium in the austenite around the carbides, increase the martensite transformation starting temperature Ms, and easily transform into martensite in the subsequent cooling process,
s3, preserving heat at 950-1050 ℃, preserving heat for 1 hour and at least 3 hours during the period when the wall thickness of the casting is increased by 25mm, so as to fully precipitate secondary carbide in austenite
S4, adopting blowing or spraying to accelerate cooling, cooling each part of the casting evenly, and adopting oil quenching cooling method,
s5, tempering the cooled casting at 400-500 ℃, preserving heat for 3-4 hours, eliminating internal stress, further reducing retained austenite during martensite transformation of the casting with martensite as the main structure, and not generating pearlite transformation as long as the temperature is not more than 500 ℃,
s6, annealing at 740-780 ℃, wherein the heat preservation time during annealing can be 4-8 hours, then furnace cooling is carried out to below 300 ℃ to obtain a machinable casting, after the treatment, the matrix structure is mainly pearlite, the Vickers hardness is about 350-400, the processing is convenient,
and S7, hardening the casting after machining.
Preferably, the weight ratio of the carbon is 3.25%, the weight ratio of the chromium is 18.2%, and the weight ratio of the molybdenum is 0.58%.
Preferably, the weight ratio of chromium is ten times of that of carbon and is added by 12.5%, and the formula is as follows: and replacing Cr (%) > 10 XC% + 12.5% by the weight part ratio of the chromium to the carbon of 10: 1.
Preferably, the method further comprises a high-temperature treatment of toughness, wherein the high-temperature treatment of toughness comprises the steps of slowly heating the casting in a vacuum furnace to 1150-1180 ℃, preserving heat for about 8 hours, and then blowing nitrogen into the vacuum furnace to rapidly cool the casting.
Preferably, the molybdenum is replaced by one or more of nickel, manganese and copper.
In the specific implementation of the invention, 3.25% of carbon is selected; 18.2 percent of chromium; 0.58 percent of molybdenum.
The casting is firstly tempered at the high temperature of 600 ℃ for 13 hours and then cooled in air. The microstructure after treatment is shown in FIG. 6.
After high temperature tempering, eutectic carbides and a small amount of secondary carbides are visible in the microstructure. In the matrix structure, there are a large amount of pearlite colonies and a small amount of ferrite in addition to austenite.
And (3) carrying out high-temperature tempering on the casting, then carrying out instability at 970 ℃, and carrying out air cooling after heat preservation for 3 hours. The resulting microstructure is shown in FIG. 7.
Because the original structure has high pearlite content, the carbon concentration is easy to reach the equilibrium state during the destabilization treatment, and the precipitated secondary carbides are mainly M with higher hardness7C3Type, and secondary carbides are also precipitated at the edges of austenite dendrites. Thus, although there is some pearlite in the structure, the hardness of high chromium cast iron as a wholeIt is still a great improvement.
High temperature treatment to improve toughness
This is a special toughening process. The casting is slowly heated to 1150-1180 ℃ in a vacuum furnace and is kept at the temperature for about 8 hours. And then blowing nitrogen into the vacuum furnace to rapidly cool the casting.
This treatment results in a reduction in the continuity of the original eutectic carbides and the precipitation of larger secondary carbides in a stable austenite matrix.
The working principle of the invention is as follows:
first, high chromium cast iron is a multi-component alloy in which chromium and carbon are the basic components and molybdenum, nickel, manganese and copper are the common alloying elements.
1. In high-chromium cast iron, chromium is a strong carbide-forming element, and the influence of the content on the carbide morphology is crucial:
when the chromium content is below 8%, the carbide is M3C type (wherein M represents Fe, Cr and other alloy elements, the same holds true below), not only has low hardness (about 1000 Vickers hardness), but also is in continuous net distribution, and the cast iron is easy to brittle fracture;
with increasing chromium content, the carbides in the cast iron gradually move towards M7C3And (4) carrying out type conversion.
Above 12%, the carbide is mainly M7C3And (4) molding. The carbide is in a rod shape or a sheet shape, has poor continuity and high hardness (about 1600 Vickers hardness), and the wear resistance and the toughness of the cast iron are greatly improved.
The chromium content is between 8 and 10 percent, and then the carbide has M3Type C, also having M7C3And (4) molding. Due to the existence of M7C3Type carbide, interfere with M3The continuity of C-type carbide and the toughness of the cast iron are still obviously higher than those of low-chromium cast iron;
in the case of high chromium and low carbon, M, which has a low hardness, may also appear23C6A type carbide. If the chromium content is above 12%, but not too high, and the carbon content is low, the Cr/C ratio is above 8, it is also possible for M to occur23C6A type carbide.
In the heat treatment, M is generally precipitated when the local carbon content is low because secondary carbides are not sufficiently diffused in austenite in the process of precipitation of austenite23C6A type carbide.
If the cast iron is used under the working condition of corrosion and abrasion, and the cast iron is required to be both wear-resistant and corrosion-resistant, the chromium content of the cast iron is calculated according to the following formula:
Cr(％)≥10×C％+12.5％
the starting point for the above formula is: in the high-chromium cast iron, the electrode potential of carbide is higher than that of a matrix structure, and the key point of improving the corrosion resistance is to improve the corrosion resistance of the matrix, so that the chromium content in the matrix is ensured to be more than 12.5 percent; in addition, the chromium content of carbides in high-chromium cast irons is approximately 10 times the carbon content.
Cast iron containing chromium of 35% or more has good corrosion resistance and heat resistance, but its carbide is M23C6The matrix structure is mainly ferrite, which can not strongly support carbide, and is only suitable for working conditions of low stress abrasion.
The chromium in the high-chromium cast iron may also be such that the martensitic transformation starting temperature M isSThis is mainly because chromium is a strong carbide former, and a large amount of chromium combines with carbon to reduce the concentration of dissolved carbon in austenite, and reducing the concentration of carbon increases MSIs much greater than the effect of the other alloying elements.
2. The carbon content in high-chromium cast iron is a major factor affecting the amount of carbides in the cast iron. Generally, the carbon content is increased, the carbide content is increased, and the cast iron has better wear resistance but enhanced brittleness.
The content of eutectic carbon in the Fe-Cr-C ternary alloy is important data, and is an important basis for judging the microstructure of high-chromium cast iron, adjusting the components of the cast iron and controlling the quality of castings. Since it is a ternary alloy, eutectic carbon content (C)E) Of course, depending on the chromium content, this can be roughly calculated as follows, or roughly estimated with reference to Table 1.
Eutectic carbon content CE＝4.4－0.054(％Cr)
TABLE 1 relationship between eutectic carbon content and chromium content of high chromium cast iron
TABLE 1
During the solidification process of hypoeutectic high-chromium cast iron, austenite dendrite is firstly separated out, and then eutectic transformation is carried out to form austenite-carbide eutectic. The greater the carbon content is below the eutectic carbon content, the greater the amount of austenite dendrites.
High chromium cast iron, which is close to eutectic composition, substantially does not precipitate austenite dendrites and is entirely transformed into a eutectic.
When the hypereutectic high-chromium cast iron is solidified, primary carbides are firstly precipitated and then eutectic transformation occurs, and the primary carbides are large in crystal grains and are very brittle, so that the advantages of the high-chromium cast iron cannot be embodied. Therefore, hypereutectic high chromium cast iron is rarely used in practice and is used only under special conditions for the working conditions of abrasive low stress wear.
Generally, hypoeutectic high chromium cast iron is used for manufacturing wear resistant parts. In the hypoeutectic range, not only does the eutectic carbide increase by increasing the carbon content, but also if the secondary carbide can be precipitated from the austenite structure by heat treatment, the carbide increases, and after the secondary carbide is precipitated, the carbon content in the austenite surrounding the carbide further decreases, and the martensite transformation start temperature M is increasedsThe martensite in the matrix structure can also be increased. Therefore, the effect of increasing the carbon content to increase the hardness is manifold, and the limiting factor is increased brittleness.
3. Other alloying elements
Although high chromium cast iron has a high chromium content, it is mainly present in eutectic carbides and secondary carbides, and only a small portion of the total amount dissolves in the matrix and acts to strengthen the matrix, and cast iron does not have good "hardenability". From the viewpoint of improving the "hardening ability" (i.e., the ease with which martensite is obtained) of cast iron, it is not sufficient to rely solely on chromium enrichment. For example: high-chromium cast iron containing 2.7% of carbon and 15-20% of chromium, wherein the as-cast matrix structure is mainly pearlite; the chromium content is increased to 27%, the cast matrix structure is mainly ferrite, the effect of supporting hard carbide is insufficient, and the wear resistance of cast iron is unsatisfactory.
Therefore, in order to improve the hardening capacity of the casting, other alloy elements such as molybdenum, nickel, manganese, copper and the like are often added.
There are also alloying elements, such as silicon, that have to be added during smelting.
The alloying elements added to high-chromium cast iron often have a dual role: elements capable of inhibiting the formation of pearlite, and promoting the formation of pearlite when the elements are not properly applied; elements which promote the precipitation of secondary carbides, and which inhibit the precipitation thereof; elements that promote the formation of martensite may also act in the opposite sense.
For producing thick-walled castings, alloying elements such as molybdenum, nickel, copper, manganese and the like are generally required to be added to improve the hardening capacity of the castings. In this case, the amount of the alloy to be used is determined appropriately according to the wall thickness of the casting and the cooling conditions thereof. If the amount of alloy added is too large, retained austenite may increase, hardening may become difficult, and wear resistance may be adversely affected.
Molybdenum is an important alloying element in high-chromium cast iron. Molybdenum is a strong carbide former, mainly present in M7C3In addition, M with high hardness may be formed when molybdenum-containing carbide precipitates from the liquid phase during solidification2Type C, M6Type C carbides, but in lesser amounts. The carbide with high hardness can be formed, and is the main reason that the hardness of the high-chromium cast iron can be improved and the wear resistance can be improved by adding molybdenum.
Meanwhile, molybdenum forms stable carbide in high-chromium cast iron, so that the carbon content in austenite can be reduced, the martensite transformation starting temperature Ms can be increased, the hardening capacity of the cast iron can be enhanced, and the formation of pearlite can be inhibited.
Molybdenum may also be dissolved in austenite in small amounts, but its effect on the reinforcement matrix is not too great.
The nickel has low solubility in carbide, is basically completely dissolved in austenite, and has the function of strengthening matrix, thereby improving the hardening capacity of the high-chromium cast iron. However, since nickel is an element for enlarging the austenite region and has a strong ability to stabilize austenite, the addition of nickel increases the retained austenite amount in cast iron, resulting in a decrease in wear resistance. The research results show that: in high chromium cast iron containing 5.5% nickel, retained austenite is large and it is difficult to transform it by heat treatment.
When the content of nickel and manganese in cast iron is high, AC3The temperature is lowered, and the heating temperature during the hardening treatment is preferably lowered to avoid an excessive retained austenite amount after the treatment. However, the negative effect of this is that the resulting martensite has a lower carbon content and therefore the cast iron has a lower hardness and wear resistance.
Most of manganese is dissolved in austenite, and a part of manganese enters carbide, so that the effect of enhancing the hardening capacity of the high-chromium cast iron is achieved. However, manganese, like nickel, also has a strong effect of stabilizing austenite, resulting in an increase in the retained austenite amount.
Copper also has little solubility in carbides, and is soluble in austenite, but not so high (about 2%). In high-chromium cast iron, copper can improve the hardening capacity and has better effect when being matched with molybdenum. Copper also increases the retained austenite content of the cast iron.
The silicon in the high-chromium cast iron is mainly added for deoxidation in the smelting process, and the silicon is insoluble in carbide and mainly exists in a matrix. A silicon content of 1% or less promotes the austenite transformation, and the retained austenite amount is reduced. However, silicon has a strong effect of promoting pearlite precipitation, and is disadvantageous in wear resistance of cast iron. Therefore, the silicon content should be strictly controlled.
Microstructure of second, high chromium cast iron
The properties of high-chromium cast iron are closely related to its microstructure. The type, morphology and amount of carbides of course affect the wear resistance and toughness of the cast iron; it is also important that the matrix structure supports the hard carbides so firmly that they do not flake off under the stress and impact conditions of abrasive wear.
In order to better understand the microstructure of high-chromium cast iron, the basic law of phase transformation needs to be understood. Considering that the chromium content in the common high-chromium cast iron is approximately between 15% and 20%, one corner of the section where Cr is 17% in the Fe-C-Cr ternary phase diagram is listed in fig. 1 for reference, where α -ferrite; gamma-austenite; l-liquid.
High-chromium cast iron generally selects hypoeutectic components, primary austenite dendritic crystals are precipitated during solidification, and eutectic transformation occurs after the primary austenite dendritic crystals are cooled to eutectic temperature, so that eutectic consisting of eutectic austenite and eutectic carbide is formed.
Austenite is stable at high temperatures. If cooled to the eutectoid transformation temperature (around 800 ℃) under equilibrium conditions, the austenite transforms into ferrite and carbides, that is, pearlite or bainite is present in the microstructure of the cast iron.
The casting is cooled in the mold at a rate which is, of course, much higher than in equilibrium conditions and which varies with the wall thickness of the casting.
When a high-chromium cast iron casting is cooled in a casting mold, primary austenite dendrites and eutectic austenite in the cast iron may remain to room temperature due to different chemical compositions and different cooling rates of the cast iron, and a part of the primary austenite dendrites and eutectic austenite may be transformed into martensite or ferrite and fine carbides (pearlite or bainite, and ferrite and M may be added)23C6A mixture of type carbides). The thin wall of the cast iron (high cooling rate), the high Cr/C ratio of the cast iron, and the addition of alloy elements such as nickel, manganese, molybdenum and the like in the cast iron may be factors for promoting the retention of austenite.
1. Matrix structure
The matrix structure referred to herein means a structure in which primary austenite dendrites, eutectic austenite, and both austenite are transformed during cooling.
The matrix structure of hypoeutectic high-chromium cast iron is an important factor for determining the wear resistance and the mechanical property of the hypoeutectic high-chromium cast iron.
(1) Austenite phase
The high-chromium cast iron has high content of alloy elements and low carbon content, and the martensite start temperature M can be caused by the alloy elementsSThe temperature of the mixture is reduced to be below 0 ℃,the stability of austenite is high. In the case of cast or heat-treated high-chromium cast iron, the matrix structure generally contains a large amount of austenite. In general, the austenite content (volume fraction) of the structure of the finally treated casting is mostly around 30%. The cooling rate may be more than 70% in thin-walled castings. When the retained austenite content is high, it is difficult to reduce the retained austenite content to 10% or less even by cold treatment at-196 ℃. We will refer to "retained austenite" herein to distinguish it from a small amount of "retained austenite" in quenched steel.
High chromium cast iron, whether as-cast or heat treated, retains considerable austenite, both primary austenite dendrites and eutectic austenite, in its matrix structure. How well is the content of this "retained austenite"? Many studies have shown that retained austenite (V) corresponding to the highest hardness value after hardening treatment is used for each grade of high-chromium iron casting commonly usedγ) The content is about 20%.
VγAt a content of 20% or less, the cast iron structure generally has a large amount of pearlite, which is accompanied by VγThe increased content reduces pearlite and increases martensite transformed from austenite, thereby increasing hardness.
VγWhen the content is about 20%, VγThe content and the martensite content are in the optimal balance state, and the hardness value is the highest.
VγAfter the content exceeds 20%, V is addedγThe amount of martensite is relatively continuously reduced and the hardness is continuously reduced due to the increase of the content.
The retained austenite content in the casting is one of important factors affecting the service performance of the casting. The austenite content in high-chromium cast iron is difficult to measure under an optical microscope, and can be measured by a computer-corrected X-ray diffraction analyzer at present, so that the problem about quantitative control is not involved at present in consideration of the fact that the measurement cannot be carried out by general production enterprises.
The high-chromium cast iron with the matrix structure mainly comprising austenite has good toughness, and on the surface impacted in the casting operation process, the austenite can be processed and hardened, the hardness is improved, and the wear resistance of the operation surface can be improved. The austenite on the operation surface of the high-chromium iron casting is processed and hardened under the action of external force, and the hardness can reach HV 900-950. However, such a work hardened layer is shallow, austenite below the surface is soft, and eutectic carbides cannot be sufficiently supported, and therefore, the wear resistance is lower than that of a high-chromium cast iron containing a large amount of martensite in the matrix.
The high-chromium cast iron has high austenite content of carbon and chromium, and generally contains alloy elements such as manganese, nickel, molybdenum and the like, so that the stability of the high-chromium cast iron is quite high. In order to make the structure of high-chromium cast iron have more martensite, measures are taken to reduce the stability of austenite so as to make the structure of the high-chromium cast iron easy to transform.
Hypoeutectic high-chromium cast iron has a large amount of primary austenite dendrites and is relatively coarse. The distribution of various alloying elements in the austenite dendrites is not uniform, and here, the approximate distribution of carbon, chromium and silicon in the primary austenite dendrites will be described with reference to fig. 2, taking a high-chromium cast iron containing 2% carbon, 0.48% silicon, 3.73% manganese, 17.6% chromium and 3% molybdenum as an example.
The core of the austenite dendrite is crystallized from the liquid phase first, and the contents of carbon and chromium are low. Then, during the growth of the dendrites, the content of carbon and chromium is gradually increased. When eutectic transformation occurs and eutectic carbide is precipitated, carbon and chromium in the dendrite rapidly diffuse into the eutectic carbide, and as a result, the carbon and chromium content in the portion adjacent to the carbide is low. The martensite transformation starting temperature M is just because of the low content of carbon and chromium in austenite at the interfaceSThe temperature is increased and only then during cooling is it readily transformed into martensite.
Since silicon is not dissolved in carbide and exists only in the matrix, and silicon is discharged when carbide is precipitated, the concentration of silicon in austenite in the vicinity of carbide is extremely high.
Such austenite dendrites, which later undergo a structural transformation during cooling, are dependent on the cooling conditions. In the case of thin castings with high cooling rates, the austenite will remain substantially, at most only in the vicinity of the eutectic carbides, and will be transformed to martensite or pearlite due to the low carbon and chromium content. Secondary carbides may also precipitate in the local austenite if the casting is thick, has a low cooling rate, or is "destabilized".
The eutectic austenite adjacent to the eutectic carbide is fine and contains much less carbon and chromium than the dendrite edges due to diffusion of carbon and chromium into the eutectic carbide during the eutectic transformation. Such austenite has a lower stability and a martensite start temperature MSAnd higher, more readily to fine pearlite or martensite during later cooling.
The higher the carbon content in high-chromium cast iron, the less primary austenite dendrites and the more eutectic austenite, the less retained austenite content in the cast iron structure.
(2) Martensite
By controlling the composition and performing appropriate heat treatment, the matrix structure of the high-chromium cast iron can contain a large amount of martensite. The high-chromium cast iron containing more martensite has the advantages of high hardness, good wear resistance, high strength, strong support to carbides, and lower toughness than cast iron with a matrix structure mainly containing austenite.
(3) Pearlite and bainite.
In high chromium cast iron containing mainly austenite or much martensite in the matrix, stress is required to reduce pearlite or bainite in the matrix structure, including ferrite and M23C6Mixed structure formed by carbide. Because the matrix structure reduces the wear resistance and mechanical properties of the cast iron.
Of course, if the casting requires cutting, the casting should be annealed before machining to convert the martensite in the matrix to pearlite and reduce the hardness to around HB 400. And hardening the processed product.
2. Carbide(s) and method of making the same
In the solidification process of hypoeutectic high-chromium cast iron, primary austenite dendritic crystals are firstly precipitated, and then eutectic transformation is carried out to form eutectic clusters. Eutectic carbides are distributed in clusters in the eutectic cluster, and the eutectic cluster in an as-cast structure is shown in fig. 3, wherein the eutectic cluster contains 9.9% of a-chromium, 3.12% of carbon, 16.5% of b-chromium and 3.01% of carbon. The middle is fine grained carbide, followed by granular and strip-like carbide extending to the periphery, and the edge of eutectic is coarser fine grained carbide. Eutectic austenite is between carbides, and austenite dendrites are outside the eutectic group.
High chromium cast iron close to the eutectic composition is substantially free of austenite dendrites and primary carbides, and has only a eutectic structure. The carbides in the eutectic cell are shown in fig. 4, wherein the a-chromium content is 9.7%, and the carbon content is 3.88%; b-chromium content 16.0%, carbon content 3.45%.
During the solidification process of hypereutectic high-chromium cast iron, coarse primary carbides are firstly separated out, and then eutectic transformation occurs to form eutectic clusters. The as-cast microstructure of the carbide is shown in FIG. 5, in which the a-chromium content is 15.5% and the carbon content is 3.99%; b-chromium content 18.2%, carbon content 3.69%.
M7C3The crystal grains of the type carbide are hexagonal rod-shaped and have a sheet structure. The study with an electron microscope showed that M7C3Type carbides tend to have structural defects of crystallographic faults, the effect of which is to be further investigated.
After the primary carbides are formed, they are not changed by the heat treatment. Eutectic carbides, during subsequent heat treatment, do not dissolve completely, but may change in shape over extended periods of time.
High chromium cast irons can precipitate fine secondary carbides from the austenite during slow cooling, or under "destabilization" conditions. When secondary carbides are precipitated from the eutectic austenite of high-chromium cast iron, the carbon content is lower than the edge of austenite dendrites, the carbon diffusion is not fast, the local Cr/C ratio is high, and M with low hardness is mainly used23C6And (4) molding. Secondary carbides do not precipitate in the vicinity of eutectic carbides, but are formed in the form of individual particles in the eutectic austenite, and secondary carbides may also precipitate at the edges of austenite dendrites.
M23C6The type carbide may be reconverted to M during the heat treatment7C3And (4) molding.
Given the chromium content, the carbon content in high-chromium cast iron is a key factor in controlling the wear resistance and toughness. Increasing the carbon content increases the eutectic carbides in the structure, which is also of good wear resistance and poor toughness.
Maratray, f.m. of Climax Molybdenum corporation, carbide M in high chromium cast iron7C3The volume fraction of the content can be calculated as follows:
CV(％)＝12.33(％C)+0.55(％Cr)－15.2
in the formula: CV — volume fraction of carbide content (%);
% C — carbon content (mass fraction,%);
% Cr-chromium content (mass fraction,%).
Some people have verified the above formula by using an electrolytic extraction experimental method, and confirmed that the formula is applicable.
The present invention and its embodiments have been described above, but the description is not limitative, and the actual structure is not limited thereto. In summary, those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiments as a basis for designing or modifying other structures for carrying out the same purposes of the present invention without departing from the spirit and scope of the invention as defined by the appended claims.
Claims (5)
1. A hypereutectic high-chromium cast iron for heavy abrasive wear, characterized in that: the method comprises the following processing steps:
s1, charging, namely putting carbon, chromium and molybdenum into a cooling furnace, wherein the weight ratio of chromium is ten times that of carbon, and the addition amount of chromium is 12.5%, and the formula is as follows:
Cr(％)≥10×C％+12.5％；
s2, heating at a speed lower than 50 ℃/h to 950 ℃ and 1050 ℃,
s3, preserving heat at 950-1050 ℃, preserving heat for 1 hour and at least 3 hours when the wall thickness of the casting is increased by 25mm in the period,
s4, adopting blowing or spraying methods to accelerate cooling, cooling all parts of the casting evenly,
s5, tempering the cooled casting at the temperature of 400-500 ℃, preserving heat for 3-4 hours,
s6, annealing at 740-780 ℃, wherein the heat preservation time during annealing can be 4-8 hours, then furnace cooling is carried out to below 300 ℃, and a machinable casting is obtained,
and S7, hardening the casting after machining.
2. A hypereutectic high chromium cast iron for regrind wear according to claim 1, wherein: the weight ratio of the carbon is 3.25%, the weight ratio of the chromium is 18.2%, and the weight ratio of the molybdenum is 0.58%.
3. A hypereutectic high chromium cast iron for regrind wear according to claim 1, wherein: the weight ratio of the chromium is ten times of that of the carbon and is added by 12.5 percent, and the formula is as follows: and replacing Cr (%) > 10 XC% + 12.5% by the weight part ratio of the chromium to the carbon of 10: 1.
4. A hypereutectic high chromium cast iron for regrind wear according to claim 1, wherein: and the high-temperature treatment of toughness is carried out, wherein the high-temperature treatment of toughness comprises the steps of firstly slowly heating the casting in a vacuum furnace to 1150-1180 ℃, preserving heat for about 8 hours, and then blowing nitrogen into the vacuum furnace to rapidly cool the casting.
5. A hypereutectic high chromium cast iron for regrind wear according to claim 1, wherein: the molybdenum is replaced by one or more of nickel, manganese and copper.
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