JP7803776B2 - Softening heat treatment method for high chromium cast iron - Google Patents

Description

The present invention relates to a method for softening heat treatment of high-chromium cast iron.

High chromium cast iron is used as a wear-resistant material, and its main constituent elements are iron (Fe), carbon (C), and chromium (Cr). As the C and Cr content of high chromium cast iron increases, its wear resistance improves, but its machinability deteriorates. One of the factors that determines the machinability of high chromium cast iron is its hardness; the lower the hardness, the better the machinability. To improve the machinability of high chromium cast iron, heat treatment to reduce its hardness, i.e., softening heat treatment, is required.

For high-chromium cast iron with low Cr and C contents, the heat treatment conditions are disclosed in Non-Patent Document 1, for example.

P. Ortega-Cubillos et. al, Wear resistance of high chromium white cast iron for coal grinding rolls; Revista Facultad de Ingenieria, Universidad de Antioquia, No. 76pp. 134-142, 2015.

In order to improve the wear resistance and corrosion resistance of high-chromium cast iron, it is necessary to increase the C and Cr contents. However, if at least one of the Cr and C contents is increased, softening heat treatment cannot be performed under known heat treatment conditions.

The present invention was made in light of these circumstances, and aims to provide a softening heat treatment method that can improve the machinability of high-chromium cast iron, especially when the high-chromium content is at least one of the Cr and C contents.

The inventors conducted extensive research to provide a method for softening high chromium cast iron that can achieve a Rockwell hardness HRC of 40 or less after heat treatment and improve the machinability of high chromium cast iron. As a result, they discovered that by performing heat treatment at a specific furnace temperature for a specific period of time, it is possible to achieve a Rockwell hardness HRC of 40 or less after heat treatment and improve the machinability of high chromium cast iron, leading to the present invention.

That is, according to the present invention,
The following softening heat treatment method for high chromium cast iron is provided.

[1]
A softening heat treatment method for high chromium cast iron containing, by weight%, C in the range of 2.1% to 4.0% and Cr in the range of 30.0% to 40.0%,
a heating step of heating and holding the high chromium cast iron at a furnace temperature of 700°C or higher and 750°C or lower for a holding time of 10 hours or longer;
a slow cooling step of slowly cooling the high chromium cast iron to a furnace temperature of 600°C at a cooling rate of 100°C/hour or less,
The total time required for the heating step and the slow cooling step is 20 hours or more;
A softening heat treatment method for high chromium cast iron, wherein the high chromium cast iron after heat treatment by the softening heat treatment method has a Rockwell hardness HRC of 40 or less.
[2]
A softening heat treatment method for high chromium cast iron containing, by weight%, C in the range of 4.0% to 7.0% and Cr in the range of 20.0% to 40.0%,
a primary heating step of heating and holding the high chromium cast iron at a furnace temperature of 900°C or higher and 1100°C or lower for a holding time of 10 hours or longer;
a secondary heating step of further heating and holding the high chromium cast iron at a furnace temperature of 700°C or higher and 750°C or lower for a holding time of 10 hours or longer after the primary heating step;
a slow cooling step of slowly cooling the high chromium cast iron to a furnace temperature of 600°C at a cooling rate of 100°C/hour or less,
A softening heat treatment method for high chromium cast iron, wherein the high chromium cast iron after heat treatment by the softening heat treatment method has a Rockwell hardness HRC of 40 or less.
[3]
The softening heat treatment method for high chromium cast iron according to the above [1] or [2],
The method for softening heat treatment of high chromium cast iron, wherein the high chromium cast iron further contains, by weight, Si in the range of 0% to 2.0%, Mn in the range of 0% to 5.0%, Fe in the range of 40.0% to 77.9%, and unavoidable impurities in the range of 0% to 0.1%.
[4]
The softening heat treatment method for high chromium cast iron according to any one of the above [1] to [3],
A softening heat treatment method for high chromium cast iron, wherein the high chromium cast iron further contains one or more elements selected from the group consisting of Mo, W, Nb, V, Co, Ni, Cu, Ti, and rare earth elements.
[5]
The softening heat treatment method for high chromium cast iron according to any one of the above [1] to [4],
After the slow cooling step,
a processing step of processing the high chromium cast iron;
After the processing step, the high chromium cast iron is heated and held at a furnace temperature of 850°C to 1200°C for a holding time of 1 hour to 12 hours, and then cooled at a cooling rate of 50°C/min or more to perform a quenching step.
The method for softening heat treatment of high chromium cast iron further comprises:
[6]
The softening heat treatment method for high chromium cast iron according to the above [5],
The method for softening high chromium cast iron, wherein the high chromium cast iron after the quenching step has a Rockwell hardness HRC of 55 or more.
[7]
The softening heat treatment method for high chromium cast iron according to the above [5] or [6],
The method for softening heat treatment of high chromium cast iron, wherein the high chromium cast iron after the quenching step is a part of a pump or a part of a crusher.

The present invention provides a softening heat treatment method for high chromium cast iron that can reduce the Rockwell hardness of the high chromium cast iron to HRC 40 or less after heat treatment and improve the machinability of the high chromium cast iron.

FIG. 1 is a diagram showing the relationship between the temperature and time of heat treatment in a heating step and a slow cooling step. FIG. 1 is a diagram showing the relationship between the temperature and time of heat treatment in a heating step and a slow cooling step. FIG. 10 is a diagram showing the relationship between the temperature and time of heat treatment in the quenching step.

Embodiments of the present invention will be described below with reference to the drawings. Please note that the drawings are schematic and do not reflect actual dimensional proportions. Unless otherwise specified, "~" between numbers in the text indicates "above" or "below."

The softening heat treatment method of this embodiment is a method for softening high chromium cast iron containing, by weight, 2.1% to 4.0% C and 30.0% to 40.0% Cr, and includes a heating step of heating and holding the high chromium cast iron at a furnace temperature of 700°C to 750°C for 10 hours or more, and a slow cooling step of slowly cooling the high chromium cast iron to a furnace temperature of 600°C at a cooling rate of 100°C/hour or less, the total time required for the heating step and the slow cooling step being 20 hours or more. The high chromium cast iron after heat treatment by this softening heat treatment method has a Rockwell hardness HRC of 40 or less.
FIG. 1 shows the relationship between the temperature and time in the heating step and the slow cooling step in the softening heat treatment method for high chromium cast iron according to this embodiment.

As mentioned above, when high-chromium cast iron with a Cr content of 30.0% by weight or more or a C content of 2.1% by weight or more is heat treated under conventional conditions, it typically has a Rockwell hardness (HRC) of 40 or more, making it difficult to machine. The softening heat treatment method for high-chromium cast iron in this embodiment includes a heating step in which the high-chromium cast iron is heated and held at a furnace temperature of 700°C to 750°C for a holding time of 10 hours or more, and a slow cooling step in which the high-chromium cast iron is slowly cooled to a furnace temperature of 600°C at a cooling rate of 100°C/hour or less. By setting the total time required for the heating step and the slow cooling step to 20 hours or more, the Rockwell hardness (HRC) of the high-chromium cast iron after heat treatment decreases to 40 or less, thereby improving the machinability of the high-chromium cast iron.

The reason for this is unclear, but it is thought that heating and holding high-chromium cast iron at a furnace temperature of 700°C to 750°C for a holding time of 10 hours or more causes the carbon contained in the ferrite phase of the high-chromium cast iron to precipitate. This makes it possible to achieve both improved wear resistance and corrosion resistance due to the inclusion of chromium, as well as machinability.

The composition of the high-chromium cast iron in this embodiment may be the raw material blending ratio at the time of production, or the composition measured by component analysis after production. Component analysis can be performed using known analytical methods, such as energy dispersive X-ray spectroscopy (SEM-EDS), optical emission spectroscopy (OES), and inductively coupled plasma analysis (ICP).

In this embodiment of the softening heat treatment method for high chromium cast iron, the furnace temperature in the heating step is 700°C or higher and 750°C or lower, preferably 710°C or higher and 750°C or lower, and more preferably 720°C or higher and 750°C or lower. By keeping the furnace temperature within this range, the machinability of the high chromium cast iron can be improved.

In the softening heat treatment method for high chromium cast iron according to the present embodiment, the lower limit of the holding time in the heating step is 10 hours or more, preferably 12 hours or more, and more preferably 15 hours or more. By setting the lower limit of the holding time to be equal to or greater than the above lower limit, the machinability of the high chromium cast iron can be improved.
Moreover, the upper limit of the holding time is preferably 48 hours or less, more preferably 36 hours or less, and even more preferably 24 hours or less. By making the upper limit of the holding time equal to or less than the upper limit, it is possible to more suitably improve the productivity of high chromium cast iron.

In the softening heat treatment method for high chromium cast iron of this embodiment, the cooling rate of the high chromium cast iron in the slow cooling step is 100° C./hour or less, preferably 50° C./hour or less, and more preferably 30° C./hour or less. By keeping the upper limit of the cooling rate below the upper limit, the machinability of the high chromium cast iron can be made more suitable.
The lower limit of the cooling rate is not particularly limited, but may be, for example, 5° C./hour or more, 10° C./hour or more, or 20° C./hour or more.
The cooling method at this time is not particularly limited, and any known cooling method can be used, but slow cooling in a furnace is preferred.

In the softening heat treatment method for high chromium cast iron of this embodiment, the total time required for the heating step and the slow cooling step is 20 hours or more, preferably 25 hours or more, and more preferably 30 hours or more. By making the total time required for the heating step and the slow cooling step equal to or greater than the lower limit, the hardness of the high chromium cast iron is reduced, and machinability can be improved.
Furthermore, the upper limit of the total required time is not particularly limited, but may be, for example, 60 hours or less, 50 hours or less, or 40 hours or less.

The softening heat treatment method for high chromium cast iron according to this embodiment includes a primary heating step in which the high chromium cast iron is heated and held at a furnace temperature of 900°C to 1100°C for at least 10 hours. The secondary heating step further heats and holds the high chromium cast iron at a furnace temperature of 700°C to 750°C for at least 10 hours. The secondary heating step also includes a slow cooling step in which the high chromium cast iron is slowly cooled to a furnace temperature of 600°C at a cooling rate of 100°C/hour or less. The high chromium cast iron after the softening heat treatment has a Rockwell hardness (HRC) of 40 or less. This improves the machinability of the high chromium cast iron.
FIG. 2 shows the relationship between the temperature and time in the heating step and the slow cooling step in the softening heat treatment method for high chromium cast iron according to this embodiment.

The reason for this is unclear, but it is thought that the carbon contained in the austenite phase precipitates when high-chromium cast iron is heated and held at a furnace temperature of 900°C to 1100°C for a holding time of 10 hours or more in the primary heating process. Then, when the high-chromium cast iron is heated and held at a furnace temperature of 700°C to 750°C for a holding time of 10 hours or more in the secondary heating process, the carbon contained in the ferrite phase of the high-chromium cast iron precipitates. This reduces the hardness of the high-chromium cast iron and improves its machinability.

In this embodiment of the softening heat treatment method for high chromium cast iron, the furnace temperature in the primary heating step is 900°C or higher and 1100°C or lower, preferably 950°C or higher and 1100°C or lower, and more preferably 950°C or higher and 1050°C or lower. By keeping the furnace temperature within this range, the machinability of the high chromium cast iron can be improved.

In the softening heat treatment method for high chromium cast iron of this embodiment, the upper limit of the cooling rate when transitioning from the primary heating step to the secondary heating step is preferably 100°C/hour or less, more preferably 50°C/hour or less, and even more preferably 30°C/hour or less. By setting the upper limit of the cooling rate to the above upper limit or less, the machinability of the high chromium cast iron can be improved.
Furthermore, the lower limit of the cooling rate when transitioning from the primary heating step to the secondary heating step is not particularly limited, but is, for example, 5°C/hour or more, 10°C/hour or more, or 15°C/hour or more.

In this embodiment of the softening heat treatment method for high chromium cast iron, the furnace temperature in the secondary heating step is 700°C or higher and 750°C or lower, preferably 710°C or higher and 750°C or lower, and more preferably 720°C or higher and 750°C or lower. By keeping the furnace temperature within this range, the machinability of the high chromium cast iron can be improved.

In the softening heat treatment method for high chromium cast iron according to this embodiment, the lower limit of the holding time in each of the primary heating step and the secondary heating step is 10 hours or more, preferably 12 hours or more, and more preferably 15 hours or more. By setting the lower limit of the holding time to be equal to or greater than the above lower limit, the machinability of the high chromium cast iron can be improved.
Moreover, the upper limit of the holding time is preferably 48 hours or less, more preferably 36 hours or less, and even more preferably 24 hours or less. By making the upper limit of the holding time equal to or less than the upper limit, the machinability and productivity of the high chromium cast iron can be made more suitable.

In the softening heat treatment method for high chromium cast iron according to this embodiment, the cooling rate of the high chromium cast iron in the slow cooling step is 100° C./hour or less, preferably 50° C./hour or less, and more preferably 30° C./hour or less. By keeping the upper limit of the cooling rate below the upper limit, the machinability of the high chromium cast iron can be improved.
The lower limit of the cooling rate is not particularly limited, but may be, for example, 5° C./hour or more, 10° C./hour or more, or 20° C./hour or more.
The cooling method at this time is not particularly limited, and any known cooling method can be used, but slow cooling in a furnace is preferred.

In the softening heat treatment method for high chromium cast iron of this embodiment, it is preferable that the high chromium cast iron further contains, by weight %, 0% to 2.0% Si, 0% to 5.0% Mn, 40.0% to 77.9% Fe, and 0% to 0.1% unavoidable impurities.
The contents of these elements can be appropriately determined depending on the application of the high chromium cast iron according to this embodiment.

Each ingredient is explained in detail below.

Si: 0% or more and 2.0% or less (weight%)
Silicon is an element that improves the fluidity of molten high-chromium cast iron and acts as a deoxidizer during melting. Therefore, the Si content is preferably 0% to 2.0% by weight, more preferably 0.2% to 1.5%, and even more preferably 0.3% to 1.0%. By setting the Si content within the above numerical range, the strength of the martensite phase that is formed when the high-chromium cast iron is quenched can be increased.

Mn: 0% or more and 5.0% or less (weight%)
The Mn content is preferably 0% to 5.0% by weight, more preferably 0.3% to 3.0%, and even more preferably 0.5% to 2.0%. By setting the Mn content within the above range, the strength of the martensite phase formed when high chromium cast iron is quenched can be increased, and the corrosion resistance of the quenched product can be improved.

Fe: 40.0% or more and 77.9% or less (weight%)
The Fe content is preferably 40.0% to 77.9% by weight, more preferably 45.0% to 77.5%, and even more preferably 50.0% to 77.0%. By setting the Fe content within the above range, it is possible to improve the machinability of the high chromium cast iron without reducing the wear resistance and corrosion resistance.

Inevitable impurities: 0% to 0.1% (wt%)
The inevitable impurities refer to components that cannot be completely removed from the raw materials used in the production of high chromium cast iron. Examples of such inevitable impurities include P, S, B, Al, Pb, and Zn.
The content of unavoidable impurities is preferably 0% to 0.1% by weight, more preferably 0% to 0.05%, and even more preferably 0% to 0.01%. By keeping the content of unavoidable impurities within the above numerical range, it is possible to prevent performance degradation, including embrittlement of the high chromium cast iron and a decrease in corrosion resistance.

In the softening heat treatment method for high chromium cast iron of this embodiment, it is preferable that the high chromium cast iron further contains one or more elements selected from the group consisting of Mo, W, Nb, V, Co, Ni, Cu, Ti, and rare earth elements. By including the above elements in the high chromium cast iron of this embodiment, the corrosion resistance and toughness of a quench-formed product using the high chromium cast iron of this embodiment can be further improved, and cracking during quench-forming can be prevented.
The contents of these elements can be determined appropriately depending on the application of the high chromium cast iron according to this embodiment, but for example, the total of the above elements is 0% or more and 1.0% or less by weight.

Although various elements have been exemplified above, other elements may be added in addition to the elements exemplified above as long as the effect of this embodiment is not impaired.
The composition of the various elements may be the raw material blending ratio during production, or may be a composition measured by component analysis after production. Component analysis can be performed using known analytical methods, such as energy dispersive X-ray spectroscopy (SEM-EDS), optical emission spectroscopy (OES), and inductively coupled plasma analysis (ICP).

The softening heat treatment method for high chromium cast iron of this embodiment preferably further includes, after the slow cooling step, a processing step in which the high chromium cast iron is processed, and, after the processing step, a quenching step in which the high chromium cast iron is heated and held at a furnace temperature of 850°C to 1200°C for a holding time of 1 hour to 12 hours, and then cooled at a cooling rate of 50°C/min or more to perform quenching.

(Processing process)
In the softening heat treatment method for high chromium cast iron according to the present embodiment, the high chromium cast iron is finally processed into a desired shape in the processing step. Processing can be performed by a conventionally known method, such as lathe processing, milling, cutting such as drilling, grinding, polishing, and electric discharge machining.

(Quenching process)
In the softening heat treatment method for high chromium cast iron according to this embodiment, the quenching step is a step in which the high chromium cast iron processed in the processing step is further heated and cooled to harden the high chromium cast iron. As shown in Fig. 3, the temperature conditions in the quenching step are preferably such that the high chromium cast iron is heated and held at a furnace temperature of 850°C to 1200°C for a holding time of 1 hour to 12 hours, and then cooled at a cooling rate of 50°C/min or more.

In this embodiment of the softening heat treatment method for high chromium cast iron, the furnace temperature during the quenching step is preferably 850°C or higher and 1200°C or lower, more preferably 900°C or higher and 1150°C or lower, and even more preferably 950°C or higher and 1100°C or lower. By keeping the furnace temperature within this range, the hardness of the quenched high chromium cast iron product can be improved without reducing its wear resistance and corrosion resistance.

In the softening heat treatment method for high chromium cast iron according to this embodiment, the lower limit of the holding time in the quenching step is preferably 1 hour or more, more preferably 2 hours or more, and even more preferably 4 hours or more. By setting the lower limit of the holding time to be equal to or greater than the above lower limit, it is possible to improve the hardness of the quenched product of high chromium cast iron without reducing the wear resistance and corrosion resistance.
The upper limit of the holding time is preferably 12 hours or less, more preferably 10 hours or less, and even more preferably 8 hours or less. By keeping the upper limit of the holding time at or below the upper limit, the wear resistance and corrosion resistance of the quench-formed product of high chromium cast iron can be made more favorable.

In the softening heat treatment method for high chromium cast iron according to the present embodiment, the lower limit of the cooling rate of the high chromium cast iron in the quenching step is preferably 50°C/min or more, more preferably 60°C/min or more.
When the lower limit of the cooling rate is equal to or greater than the above lower limit, the hardness of the quench-formed product of high chromium cast iron can be improved without reducing the wear resistance and corrosion resistance.
The upper limit of the cooling rate is preferably 150°C/min or less, more preferably 140°C/min or less, and even more preferably 130°C/min or less. By keeping the upper limit of the cooling rate below the upper limit, the wear resistance and corrosion resistance of the quench-formed product of high chromium cast iron can be made more favorable.
The cooling method at this time is not particularly limited, and any known cooling method can be used, but forced air cooling is preferred.

In the softening heat treatment method for high chromium cast iron of this embodiment, the lower limit of the Rockwell hardness HRC of the high chromium cast iron after the quenching step is preferably not less than 55, more preferably not less than 57, and even more preferably not less than 60. By having a Rockwell hardness HRC not less than the above lower limit, the wear resistance and corrosion resistance of the quenched product of high chromium cast iron can be made more favorable.
The upper limit of the Rockwell hardness HRC is not particularly limited, but is, for example, 100 or less.

In the softening heat treatment method for high chromium cast iron of this embodiment, due to its characteristics, it is preferable that the high chromium cast iron after the quenching process be one that operates while in contact with other parts or substances such as liquids, solids, or liquid-solid mixtures. In particular, it is preferable that it be a part of a pump or a crusher.

The above describes embodiments of the present invention with reference to the drawings, but these are merely examples of the present invention, and various configurations other than those described above can also be adopted.

The present embodiment will be described in detail below with reference to examples and comparative examples. Note that the present embodiment is not limited in any way to the descriptions of these examples.

The methods for melting, casting, heat treating, quenching, and measuring the hardness of high-chromium cast iron to confirm the effects of this embodiment are described in detail below.

[Melting and casting of high chromium cast iron]
High-chromium cast iron was melted using a high-frequency vacuum furnace. Fe, C, Cr, Si, Mn, and other elements with a purity of 99.9% (by weight) were blended in the composition shown in Table 1 to obtain a total weight of 2.8 kg. The blend was then poured into a magnesia crucible and heated and melted under an argon atmosphere. After melting, the molten liquid was cast into a sand mold to produce high-chromium cast iron samples measuring 140 mm long, 49 mm wide, and 12 mm high.

[Annealing heat treatment of high chromium cast iron]
The high chromium cast iron samples produced in the above-mentioned "Melting and casting of high chromium cast iron" were annealed and heat treated using an electric furnace under the air atmosphere under the furnace temperature program shown in FIG. 1 or FIG. 2 and the temperature conditions shown in Table 1, thereby obtaining high chromium cast iron samples after annealing and heat treatment.

[Quenching heat treatment of high chromium cast iron]
The high chromium cast iron samples after the annealing heat treatment prepared in the above [Annealing heat treatment of high chromium cast iron] were machined by electric discharge machining to have dimensions of 20 mm length × 20 mm width × 6 mm height. Thereafter, the high chromium cast iron samples after the annealing heat treatment prepared in the above [Annealing heat treatment of high chromium cast iron] were subjected to quenching heat treatment in an air atmosphere using an electric furnace under the temperature program shown in Figure 3 and the temperature conditions shown in Table 2, thereby obtaining high chromium cast iron samples after quenching heat treatment.

[Hardness measurement of high chromium cast iron]
The high chromium cast iron samples prepared in the above [Annealing Heat Treatment of High Chromium Cast Iron] and [Quenching Heat Treatment of High Chromium Cast Iron] after the annealing heat treatment and the quenching heat treatment were machined by polishing into test pieces measuring 20 mm in length, 20 mm in width, and 5.5 mm in height. The hardness of the test pieces was measured at nine points using a Rockwell (HRC) hardness tester under conditions of a load of 150 kgf and a load holding time of 15 seconds. Of the nine measurement data points, the maximum and minimum hardness data were excluded, and the average value of seven points was used as the hardness data for each test piece.

Example 1
As shown in Table 1, high-chromium cast iron Fe61.1C2.75Cr35.0Si0.4Mn0.75 (wt%) was heat-treated in an electric furnace at a furnace temperature of 750°C for a holding time of 10 hours, and then slowly cooled to room temperature at a cooling rate of 15°C/hour. As a result, the Rockwell hardness HRC was 39.9. In Example 1, the total time required at a furnace temperature of 600°C to 750°C was 20 hours.

(Comparative Example 1)
As shown in Table 1, high-chromium cast iron Fe61.1C2.75Cr35.0Si0.4Mn0.75 (wt%) was heat-treated in an electric furnace at a furnace temperature of 750°C for a holding time of 2 hours, and then slowly cooled to room temperature at a cooling rate of 15°C/hour. As a result, the Rockwell hardness HRC was 42.7. In Comparative Example 1, the total time required at a furnace temperature of 600°C to 750°C was 12 hours.

Example 2
As shown in Table 1, high-chromium cast iron Fe61.1C2.75Cr35.0Si0.4Mn0.75 (wt%) was heat-treated in an electric furnace at a furnace temperature of 750°C for a holding time of 24 hours, and then slowly cooled to room temperature at a cooling rate of 15°C/hour. As a result, the Rockwell hardness HRC was 36.5. In Example 2, the total time required at a furnace temperature of 600°C to 750°C was 34 hours.
Thereafter, the specimen was again quenched in an electric furnace under the conditions of a heating rate of 350°C/hour, a furnace temperature of 1050°C, and a holding time of 4.5 hours, as shown in Table 2, and then rapidly cooled to room temperature at a cooling rate of 3000 to 6000°C/hour. As a result, the Rockwell hardness HRC after quenching was 64.6.

(Comparative Example 2)
As shown in Table 1, high-chromium cast iron Fe61.1C2.75Cr35.0Si0.4Mn0.75 (wt%) was heat-treated in an electric furnace at a furnace temperature of 850°C for a holding time of 2 hours, and then slowly cooled to room temperature at a cooling rate of 15°C/hour. The resulting Rockwell hardness HRC was 44.1. In Comparative Example 2, the total time required at a furnace temperature of 600°C to 750°C was 10 hours.

Example 3
As shown in Table 1, high-chromium cast iron Fe59.6C2.75Cr35.0Si0.4Mn0.75Ni0.5Mo1.0 (wt%) was heat-treated in an electric furnace at a furnace temperature of 750°C for a holding time of 24 hours, and then slowly cooled to room temperature at a cooling rate of 15°C/hour. The resulting Rockwell hardness HRC was 38.6. In Example 3, the total time required at a furnace temperature of 600°C to 750°C was 34 hours.
Thereafter, the specimen was again quenched in an electric furnace under the conditions of a heating rate of 350°C/hour, a furnace temperature of 1050°C, and a holding time of 4.5 hours, as shown in Table 2, and then rapidly cooled to room temperature at a cooling rate of 3000 to 6000°C/hour. As a result, the Rockwell hardness HRC after quenching was 64.1.

(Comparative Example 3)
As shown in Table 1, high-chromium cast iron Fe59.6C2.75Cr35.0Si0.4Mn0.75Ni0.5Mo1.0 (wt%) was heat-treated in an electric furnace at a furnace temperature of 850°C for a holding time of 4 hours, and then slowly cooled to room temperature at a cooling rate of 15°C/hour. The resulting Rockwell hardness HRC was 43.7. In Comparative Example 3, the total time required at a furnace temperature of 600°C to 750°C was 10 hours.

Example 4
As shown in Table 1, high-chromium cast iron Fe59.06C2.79Cr35.0Si0.4Mn0.75Ni0.5Mo1.0Nb0.5 (wt%) was heat-treated in an electric furnace at a furnace temperature of 750°C for a holding time of 24 hours, and then slowly cooled to room temperature at a cooling rate of 15°C/hour. The resulting Rockwell hardness HRC was 37.9. In Example 4, the total time required at furnace temperatures of 600°C to 750°C was 34 hours.
Thereafter, the specimen was again quenched in an electric furnace under the conditions of a heating rate of 350°C/hour, a furnace temperature of 1050°C, and a holding time of 4.5 hours, as shown in Table 2, and then rapidly cooled to room temperature at a cooling rate of 3000 to 6000°C/hour. As a result, the Rockwell hardness HRC after quenching was 64.6.

(Comparative Example 4)
As shown in Table 1, high-chromium cast iron Fe59.06C2.79Cr35.0Si0.4Mn0.75Ni0.5Mo1.0Nb0.5 (wt%) was heat-treated in an electric furnace at a furnace temperature of 850°C for a holding time of 4 hours, and then slowly cooled to room temperature at a cooling rate of 15°C/hour. The resulting Rockwell hardness HRC was 40.6. In Comparative Example 4, the total time required at a furnace temperature of 600°C to 750°C was 10 hours.

Example 5
As shown in Table 1, high-chromium cast iron Fe66.1C2.75Cr30.0Si0.4Mn0.75 (wt%) was heat-treated in an electric furnace at a furnace temperature of 750°C for a holding time of 24 hours, and then slowly cooled to room temperature at a cooling rate of 15°C/hour. The resulting Rockwell hardness HRC was 39.7. In Example 5, the total time required at furnace temperatures of 600°C to 750°C was 34 hours.
Thereafter, the specimen was again quenched in an electric furnace under the conditions of a heating rate of 350°C/hour, a furnace temperature of 1050°C, and a holding time of 4.5 hours, as shown in Table 2, and then rapidly cooled to room temperature at a cooling rate of 3000 to 6000°C/hour. As a result, the Rockwell hardness HRC after quenching was 65.4.

(Comparative Example 5)
As shown in Table 1, high-chromium cast iron Fe66.1C2.75Cr30.0Si0.4Mn0.75 (wt%) was heat-treated in an electric furnace at a furnace temperature of 850°C for a holding time of 4 hours, and then slowly cooled to room temperature at a cooling rate of 15°C/hour. As a result, the Rockwell hardness HRC was 40.9. In Comparative Example 5, the total time required at a furnace temperature of 600°C to 750°C was 10 hours.

Example 6
As shown in Table 1, high chromium cast iron Fe65.2C4.5Cr28.0Si0.8Mn1.5 (wt%) was first subjected to a primary heat treatment in an electric furnace under conditions of a furnace temperature of 950°C and a holding time of 24 hours, and then slowly cooled to 750°C at a cooling rate of 30°C/hour. Thereafter, a secondary heat treatment was performed under conditions of a furnace temperature of 750°C and a holding time of 24 hours, and then slowly cooled to room temperature at a cooling rate of 30°C/hour. As a result, the Rockwell hardness HRC was 38.0.
Thereafter, the specimen was again quenched in an electric furnace under the conditions of a heating rate of 350°C/hour, a furnace temperature of 950°C, and a holding time of 4.5 hours, as shown in Table 2, and then rapidly cooled to room temperature at a cooling rate of 3000 to 6000°C/hour. As a result, the Rockwell hardness HRC after quenching was 69.1.

(Comparative Example 6)
As shown in Table 1, high-chromium cast iron Fe65.2C4.5Cr28.0Si0.8Mn1.5 (wt%) was heat treated in an electric furnace at a furnace temperature of 850°C for a holding time of 4 hours, and then slowly cooled to room temperature at a cooling rate of 15°C/hour. As a result, the Rockwell hardness HRC was 47.5.

Example 7
As shown in Table 1, high chromium cast iron Fe65.0C4.55Cr28.15Si0.8Mn1.5 (wt%) was first subjected to a primary heat treatment in an electric furnace under conditions of a furnace temperature of 950°C and a holding time of 24 hours, and then slowly cooled to 750°C at a cooling rate of 30°C/hour. Thereafter, a secondary heat treatment was performed under conditions of a furnace temperature of 750°C and a holding time of 24 hours, and then slowly cooled to room temperature at a cooling rate of 30°C/hour. As a result, the Rockwell hardness HRC was 37.1.
Thereafter, the specimen was again quenched in an electric furnace under the conditions of a heating rate of 350°C/hour, a furnace temperature of 950°C, and a holding time of 4.5 hours, as shown in Table 2, and then rapidly cooled to room temperature at a cooling rate of 3000 to 6000°C/hour. As a result, the Rockwell hardness HRC after quenching was 67.0.

(Comparative Example 7)
As shown in Table 1, high-chromium cast iron Fe65.0C4.55Cr28.15Si0.8Mn1.5 (wt%) was heat treated in an electric furnace at a furnace temperature of 850°C for a holding time of 4 hours, and then slowly cooled to room temperature at a cooling rate of 15°C/hour. As a result, the Rockwell hardness HRC was 41.5.

(Example 8)
As shown in Table 1, high chromium cast iron Fe63.7C4.5Cr28Si0.8Mn1.5Ni0.5Mo1.0 (wt%) was first subjected to a primary heat treatment in an electric furnace under conditions of a furnace temperature of 950°C and a holding time of 24 hours, and then slowly cooled to 750°C at a cooling rate of 30°C/hour. Thereafter, a secondary heat treatment was performed under conditions of a furnace temperature of 750°C and a holding time of 24 hours, and then slowly cooled to room temperature at a cooling rate of 30°C/hour. As a result, the Rockwell hardness HRC was 39.8.
Thereafter, the specimen was again quenched in an electric furnace under the conditions of a heating rate of 350°C/hour, a furnace temperature of 950°C, and a holding time of 4.5 hours, as shown in Table 2, and then rapidly cooled to room temperature at a cooling rate of 3000 to 6000°C/hour. As a result, the Rockwell hardness HRC after quenching was 67.1.

(Comparative Example 8)
As shown in Table 1, high-chromium cast iron Fe63.7C4.5Cr28Si0.8Mn1.5Ni0.5Mo1.0 (wt%) was heat treated in an electric furnace at a furnace temperature of 850°C for a holding time of 4 hours, and then slowly cooled to room temperature at a cooling rate of 15°C/hour. As a result, the Rockwell hardness HRC was 47.2.

As a result of heat treating the high chromium cast irons of Examples 1 to 8 using the softening heat treatment method of the present invention, the high chromium cast irons after softening heat treatment had a Rockwell hardness HRC of 40 or less, making them suitable for machining such as lathe processing, milling, drilling, grinding, polishing, etc. After machining, the high chromium cast irons were subjected to quenching heat treatment to have a Rockwell hardness HRC of 60 or more, giving them excellent wear resistance.
On the other hand, when the heat treatment conditions deviated from those of the present embodiment, as in Comparative Examples 1 to 8, it was not possible to soften the high chromium cast iron.

Claims (7)

A softening heat treatment method for high chromium cast iron containing, by weight%, C in the range of 2.1% to 4.0% and Cr in the range of 30.0% to 40.0%,
a heating step of heating and holding the high chromium cast iron at a furnace temperature of 700°C or higher and 750°C or lower for a holding time of 10 hours or longer;
a slow cooling step of slowly cooling the high chromium cast iron to a furnace temperature of 600°C at a cooling rate of 100°C/hour or less,
The total time required for the heating step and the slow cooling step is 20 hours or more,
A softening heat treatment method for high chromium cast iron, wherein the high chromium cast iron after heat treatment by the softening heat treatment method has a Rockwell hardness HRC of 40 or less. A softening heat treatment method for high chromium cast iron containing, by weight%, C in the range of 4.0% to 7.0% and Cr in the range of 20.0% to 40.0%,
a primary heating step of heating and holding the high chromium cast iron at a furnace temperature of 900°C or higher and 1100°C or lower for a holding time of 10 hours or longer;
a secondary heating step of further heating and holding the high chromium cast iron at a furnace temperature of 700°C or higher and 750°C or lower for a holding time of 10 hours or longer after the primary heating step;
a slow cooling step of slowly cooling the high chromium cast iron to a furnace temperature of 600°C at a cooling rate of 100°C/hour or less,
A softening heat treatment method for high chromium cast iron, wherein the high chromium cast iron after heat treatment by the softening heat treatment method has a Rockwell hardness HRC of 40 or less. 3. The softening heat treatment method for high chromium cast iron according to claim 1 or 2,
The method for softening heat treatment of high chromium cast iron, wherein the high chromium cast iron further contains, by weight, Si in the range of 0% to 2.0%, Mn in the range of 0% to 5.0%, Fe in the range of 40.0% to 77.9%, and unavoidable impurities in the range of 0% to 0.1%. 3. The softening heat treatment method for high chromium cast iron according to claim 1 or 2,
A softening heat treatment method for high chromium cast iron, wherein the high chromium cast iron further contains one or more elements selected from the group consisting of Mo, W, Nb, V, Co, Ni, Cu, Ti, and rare earth elements. 3. The softening heat treatment method for high chromium cast iron according to claim 1 or 2,
After the slow cooling step,
a processing step of processing the high chromium cast iron;
After the processing step, the high chromium cast iron is heated and held at a furnace temperature of 850°C to 1200°C for a holding time of 1 hour to 12 hours, and then cooled at a cooling rate of 50°C/min or more to perform a quenching step.
The method for softening heat treatment of high chromium cast iron further comprises: 6. The softening heat treatment method for high chromium cast iron according to claim 5,
The method for softening high chromium cast iron, wherein the high chromium cast iron after the quenching step has a Rockwell hardness HRC of 55 or more. 6. The softening heat treatment method for high chromium cast iron according to claim 5,
The method for softening heat treatment of high chromium cast iron, wherein the high chromium cast iron after the quenching step is a part of a pump or a part of a crusher.