阅读说明：本技术 一种提高球墨铸铁件硬度均匀性的加工方法 (Processing method for improving hardness uniformity of nodular iron castings ) 是由 梁楚鹏 陈国辉 余荣杰 冯志军 曾亮 于 2020-11-24 设计创作，主要内容包括：本发明公开了一种提高球墨铸铁件硬度均匀性的加工方法,其包括以下步骤：低温石墨化步骤：将球墨铸铁坯件装于炉体中在740～760℃的温度下加热4～6h；降温淬火步骤：经过低温石墨化处理后的球墨铸铁坯件在850～860℃的温度下加热1～2h,然后迅速放入水槽中水冷淬火；回火步骤：经过降温淬火步骤处理后的球墨铸铁坯件在560～570℃的温度下加热3～4h,然后置于空气中冷却。本发明的加工方法先通过低温石墨化处理使得分布不均匀的碳化物转变为石墨,获得铁素体+小于5％球粒状珠光体,再进行降温淬火和回火,使得表里硬度差大大缩小,提高了硬度的均匀性,而且硬度比常规的均匀化退火高。(The invention discloses a processing method for improving the hardness uniformity of nodular iron castings, which comprises the following steps: a low-temperature graphitization step: placing the nodular cast iron blank in a furnace body and heating for 4-6 h at the temperature of 740-760 ℃; cooling and quenching: heating the nodular cast iron blank subjected to low-temperature graphitization treatment at the temperature of 850-860 ℃ for 1-2 h, and then quickly putting the nodular cast iron blank into a water tank for water cooling quenching; tempering: and heating the ductile cast iron blank treated by the cooling and quenching step at the temperature of 560-570 ℃ for 3-4 h, and then cooling in air. The processing method of the invention firstly converts the carbides with uneven distribution into graphite through low-temperature graphitization treatment to obtain ferrite and spherical pearlite less than 5 percent, and then carries out cooling quenching and tempering, so that the difference of surface hardness and inner hardness is greatly reduced, the uniformity of hardness is improved, and the hardness is higher than that of the conventional homogenization annealing.)

1. A processing method for improving the hardness uniformity of nodular iron castings is characterized by comprising the following steps:

a low-temperature graphitization step: placing the nodular cast iron blank in a furnace body and heating for 4-6 h at the temperature of 740-760 ℃;

cooling and quenching: heating the nodular cast iron blank subjected to low-temperature graphitization treatment at the temperature of 850-860 ℃ for 1-2 h, and then quickly putting the nodular cast iron blank into a water tank for water cooling quenching;

tempering: and heating the ductile cast iron blank treated by the cooling and quenching step at the temperature of 560-570 ℃ for 3-4 h, and then cooling in air.

2. The processing method for improving the hardness uniformity of ductile iron castings according to claim 1, wherein in the low-temperature graphitization step, the ductile iron blanks are heated and then cooled in a furnace.

3. The processing method for improving the hardness uniformity of nodular iron castings according to claim 2, wherein in the low temperature graphitization step, the nodular iron blanks are furnace cooled to below 500 ℃.

4. The processing method for improving the hardness uniformity of ductile iron castings according to claim 1, wherein in the low-temperature graphitization step, the heating temperature is 740 ℃ and the heating time is 5 hours.

5. The processing method for improving the hardness uniformity of ductile iron castings according to claim 1, wherein in the step of cooling and quenching, the heating temperature is 860 ℃ and the heating time is 1 h.

6. The processing method for improving the hardness uniformity of ductile iron castings according to claim 1, wherein in the step of quenching at reduced temperature, the ductile iron blanks are rapidly water-cooled to room temperature.

7. The processing method for improving the hardness uniformity of ductile iron castings according to claim 1, characterized in that the ductile iron blanks have the designation QT 500.

Technical Field

The invention relates to the technical field of nodular cast iron manufacturing, in particular to a processing method for improving the hardness uniformity of a nodular cast iron piece, which is a processing method for carrying out heat treatment on a finished product rough blank after nodular cast iron is molded.

Background

At present, ductile cast iron gradually replaces forged steel to be widely applied to parts such as crankshafts, rear axles and the like. At this time, the ductile cast iron can meet the performance requirements of the parts in terms of strength, elongation, plasticity and toughness, and compared with forged steel, the manufacturing process of the ductile cast iron has the advantages of low cost and simple process. High-strength ductile iron, for example, high-strength ductile iron such as QT500, QT600 has a higher carbon content in the center of the workpiece than in the vicinity of the outer surface due to the higher carbon equivalent, the pearlite + ferrite in the matrix structure, and the influence of the casting process and the size of the workpiece. The uneven carbon content causes uneven hardness of the surface and the inside of the workpiece, the phenomenon of inconsistent finish degree during machining can occur, and the uneven residual stress also causes subsequent nitriding deformation or deformation of the workpiece during service.

In the prior art, in order to obtain uniform 190-HB hardness and 240-HB hardness, the difference of the front hardness and the back hardness is over 40HB through single-step heat treatment (normalizing or tempering), while the hardness obtained by low-temperature graphitization annealing and high-temperature annealing is uniform, but the finally obtained hardness is lower than 180HB, and the hardness can not reach the requirement.

Disclosure of Invention

The invention aims to overcome the defects that the heat treatment method of the nodular iron castings in the prior art has high difference value of surface hardness and internal hardness and poor uniformity of the surface hardness and the internal hardness; or the technical problem that the surface hardness uniformity is good but the surface hardness is low is solved, and the processing method for improving the hardness uniformity of the ductile iron castings is provided. The processing method comprises the steps of firstly converting carbides which are distributed unevenly into graphite through low-temperature graphitization treatment to obtain ferrite and spheroidal pearlite less than 5%, heating for a short time within a eutectoid transformation temperature range to avoid excessive dissolution of the carbides, obtaining low-carbon austenite, and then quenching and tempering, so that the large hardness difference between the surface and the inside is greatly reduced, the uniformity of hardness is improved, and the hardness is high.

In order to achieve the purpose, the invention adopts the following technical scheme:

a processing method for improving the hardness uniformity of nodular iron castings comprises the following steps:

a low-temperature graphitization step: placing the nodular cast iron blank in a furnace body and heating for 4-6 h at the temperature of 740-760 ℃;

cooling and quenching: heating the nodular cast iron blank subjected to low-temperature graphitization treatment at the temperature of 850-860 ℃ for 1-2 h, and then quickly putting the nodular cast iron blank into a water tank for water cooling quenching;

tempering: and heating the ductile cast iron blank treated by the cooling and quenching step at the temperature of 560-570 ℃ for 3-4 h, and then cooling in air.

In the prior art, in order to obtain the hardness of 190-240HB for the nodular iron casting, the heat treatment method is as follows: quenching and tempering are carried out, wherein the quenching temperature is higher than the eutectoid transformation temperature range of the nodular iron casting. For example, for nodular cast iron with the mark of QT500, the eutectoid transformation temperature range is 810-880 ℃, the existing quenching temperature is usually heated at the temperature of more than 880 ℃, the quenching temperature is 880-920 ℃, and the heating and heat preservation time is 1-2 h; the tempering temperature is 580-600 ℃, and the time is 4 hours; quenching is carried out by adopting an oil cooling mode to cool to room temperature after heating, and tempering is carried out by adopting air cooling after heating. Through detection, the hardness of the finally obtained nodular iron casting in the conventional heat treatment mode can reach 190 plus 240HB, but the difference between the outer ring hardness and the core hardness of the nodular iron casting is more than 35HB, so that the finish degree is inconsistent during mechanical processing, and the subsequent nitriding deformation or the deformation of the workpiece in service can be caused. The inventor finds out through long-term research and research that carbides which are unevenly distributed in a nodular cast iron blank can be converted into graphite by heating at the temperature of 740-760 ℃, at the moment, the nodular cast iron blank is quenched within the eutectoid transformation temperature range, and then tempered at a temperature lower than the tempering temperature in the prior art, so that the nodular cast iron with high hardness and small surface-inside hardness difference can be obtained. The ductile iron castings processed by the heat treatment method have consistent finish in subsequent mechanical processing, and cannot deform in service.

Further, in the step of low-temperature graphitization, the nodular cast iron blank is cooled along with the furnace after being heated.

Further, in the step of low-temperature graphitization, the nodular cast iron blank is cooled to below 500 ℃ along with the furnace.

Further, in the low-temperature graphitization step, the heating temperature is 740 ℃, and the heating time is 5 hours.

Further, in the step of cooling and quenching, the heating temperature is 860 ℃ and the heating time is 1 h.

Further, in the step of cooling and quenching, the ductile cast iron blank is rapidly cooled to room temperature by water.

Further, in the tempering step, the heating temperature is 560 ℃ and the heating time is 3 h.

Compared with the prior art, the invention has the beneficial effects that:

1. heating the ductile iron blank at 740-760 ℃ to convert carbides which are not uniformly distributed on the ductile iron blank into graphite, quenching at a temperature lower than the quenching temperature in the prior art, and tempering at a temperature lower than the tempering temperature in the prior art to obtain the ductile iron blank with high hardness and small surface-inside hardness difference, wherein the surface-inside hardness temperature difference is within 20HB, and the hardness uniformity is greatly improved.

2. The carbon content of high-temperature austenite is lower than that of the conventional process by adopting the reduced quenching heating temperature, the quenching stress can be reduced, and the quenching water can not crack; the quenching and tempering of the conventional process need oil quenching, and the method of the application is more economical and more environment-friendly than the oil quenching by adopting a water quenching mode.

3. The ductile iron castings obtained by the processing method have consistent finish in subsequent mechanical processing, and cannot deform in service.

Drawings

FIG. 1 is an excircle organization chart of a nodular cast iron blank observed under a metallographic microscope of 100 times before processing;

FIG. 2 is an inner hole structure diagram of a nodular cast iron blank observed under a metallographic microscope of 100 times before processing;

FIG. 3 is an excircle organization chart observed under a metallographic microscope of 100 times after a nodular cast iron blank is treated by adopting the heat treatment mode of the comparative example 1;

FIG. 4 is a diagram of an inner hole structure observed under a metallographic microscope of 100 times after a nodular cast iron blank is treated by adopting the heat treatment mode of the comparative example 1;

FIG. 5 is an excircle structure diagram observed under a 500-time metallographic microscope after a nodular cast iron blank is treated by adopting the heat treatment mode of the comparative example 1;

FIG. 6 is a diagram of an inner hole structure observed under a 500-time metallographic microscope after a nodular cast iron blank is treated by the heat treatment mode of the comparative example 1;

fig. 7 is an excircle organization chart observed under a metallographic microscope of 100 times after the ductile iron blank in embodiment 1 of the present invention is subjected to a low-temperature graphitization step;

fig. 8 is an inner hole structure diagram observed under a metallographic microscope of 100 times after the nodular cast iron blank in embodiment 1 of the invention is treated by a low-temperature graphitization step;

FIG. 9 is an excircle structure diagram observed under a metallographic microscope of 100 times after the tempering treatment of the ductile iron blank according to embodiment 1 of the present invention;

FIG. 10 is a diagram of the internal pore structure of a nodular cast iron blank in example 1 of the invention observed under a metallographic microscope of 100 times after tempering treatment;

FIG. 11 is an excircle structure diagram observed under a 500-fold metallographic microscope after the tempering treatment of the ductile iron blank according to embodiment 1 of the present invention;

fig. 12 is an inner hole structure diagram observed under a 500-fold metallographic microscope after the tempering treatment of the ductile iron blank according to example 1 of the present invention.

Detailed Description

The technical solutions in the embodiments of the present invention will be described clearly and completely with reference to the accompanying drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments obtained by a person skilled in the art without making any inventive step are within the scope of the present invention.

Example 1

A processing method for improving the hardness uniformity of nodular iron castings comprises the following steps:

a low-temperature graphitization step: the nodular cast iron blank is put into a furnace body and heated for 5 hours at the temperature of 740 ℃, and then cooled to below 500 ℃ along with the furnace. The grade of the ductile cast iron blank is QT 500.

Referring to fig. 1, the outer circle of the nodular cast iron blank before machining is observed under a metallographic microscope of 100 times, and the structural diagram is shown in fig. 1, wherein the pearlite content of the outer circle is about 5%. The internal pores are observed under a metallographic microscope with a magnification of 100 times, the structural diagram of the internal pores is shown in figure 2, and the pearlite content of the internal pores is about 20 percent.

After low-temperature graphitization treatment, the excircle of the nodular cast iron blank is observed under a metallographic microscope of 100 times, and the organizational chart is shown in figure 7; the internal pores are observed under a metallographic microscope with a magnification of 100, and the tissue diagram is shown in FIG. 8. After low-temperature graphitization treatment, the pearlite content of the inner hole is less than 5%.

After low-temperature graphitization treatment, cooling and quenching treatment are carried out on the nodular cast iron blank, and the method comprises the following specific steps: heating at 860 deg.C for 1h, quickly quenching in water tank, and cooling to room temperature.

Tempering: and heating the ductile cast iron blank treated by the cooling and quenching step at the temperature of 560 ℃ for 3h, and then cooling the ductile cast iron blank in air.

After tempering treatment, observing the excircle of the nodular cast iron blank under a metallographic microscope with the power of 100 times, wherein the organization chart is shown in figure 9; the microstructure of the sample was observed under a 500-fold metallographic microscope, and the structure thereof was as shown in FIG. 11. Observing the inner hole under a metallographic microscope with the magnification of 100 times, wherein the tissue diagram is shown in figure 10; the microstructure of the sample was observed under a 500-fold metallographic microscope, and the structure thereof was as shown in FIG. 12.

After temperature reduction quenching and tempering treatment, the metallographic structures of the inner hole and the outer circle are basically consistent.

The hardness of the ductile iron blanks before, during and after the machining was tested and the results are shown in table 1 below.

TABLE 1 hardness of spheroidal graphite cast iron blanks before, during and after working treatment

Hardness of	Before processing	After low-temperature graphitization	After cooling and quenching	After tempering
					Outer circle of the circle	180HB	175HB	45HRC	220HB
Inner bore	200HB	180HB	45HRC	223HB

Comparative example 1

Quenching treatment: and (3) placing the nodular cast iron blank in a furnace body, heating for 2 hours at the temperature of 920 ℃, and then cooling to room temperature in an oil cooling mode. The black cast iron blank is given the designation QT 500.

Tempering treatment: and heating the quenched ductile cast iron blank at 580 ℃ for 4h, and then placing the blank in air to cool to room temperature.

Referring to fig. 1, the outer circle of the nodular cast iron blank before machining is observed under a metallographic microscope of 100 times, and the structural diagram is shown in fig. 1, wherein the pearlite content of the outer circle is about 5%. The internal pores are observed under a metallographic microscope with a magnification of 100 times, the structural diagram of the internal pores is shown in figure 2, and the pearlite content of the internal pores is about 20 percent.

After quenching and tempering, the excircle of the nodular cast iron blank is observed under a metallographic microscope of 100 times, and the organization chart is shown in figure 3; the microstructure of the sample was observed under a 500-fold metallographic microscope, and the structure thereof was as shown in FIG. 5. Observing the inner hole under a metallographic microscope with the magnification of 100 times, wherein the tissue diagram is shown in figure 4; the internal pores are observed under a 500-fold metallographic microscope, and the structure diagram is shown in FIG. 6.

The hardness of the ductile iron blanks before and after the machining was tested and the results are shown in table 2 below.

TABLE 2 hardness of spheroidal graphite cast iron blanks before, during and after working treatment

Hardness of	Before processing	After quenching treatment	After tempering
				Outer circle of the circle	180HB	50HRC	200HB
Inner bore	200HB	55HRC	235HB

Example 2

A processing method for improving the hardness uniformity of nodular iron castings comprises the following steps:

a low-temperature graphitization step: the nodular cast iron blank is put into a furnace body and heated for 4 hours at the temperature of 760 ℃, and then cooled to below 500 ℃ along with the furnace. The black cast iron blank is given the designation QT 500.

After low-temperature graphitization treatment, cooling and quenching treatment are carried out on the nodular cast iron blank, and the method comprises the following specific steps: heating at 850 deg.C for 2h, quickly quenching in water tank, and cooling to room temperature.

Tempering: and heating the ductile cast iron blank treated by the cooling and quenching step at the temperature of 570 ℃ for 4h, and then cooling the ductile cast iron blank in air.

The hardness of the ductile iron blanks before, during and after the machining was tested and the results are shown in table 3 below.

TABLE 3 hardness of spheroidal graphite cast iron blanks before, during and after working treatment

Hardness of	Before processing	After low-temperature graphitization	After cooling and quenching	After tempering
					Outer circle of the circle	180HB	170HB	42HRC	210HB
Inner bore	200HB	180HB	44HRC	220HB

Example 3

A processing method for improving the hardness uniformity of nodular iron castings comprises the following steps:

a low-temperature graphitization step: the nodular cast iron blank is put into a furnace body and heated for 5 hours at the temperature of 750 ℃, and then cooled to below 500 ℃ along with the furnace. The black cast iron blank is given the designation QT 500.

After low-temperature graphitization treatment, cooling and quenching treatment are carried out on the nodular cast iron blank, and the method comprises the following specific steps: heating at 855 deg.C for 1h, rapidly placing into water tank, water-cooling, and cooling to room temperature.

Tempering: and heating the ductile cast iron blank treated by the cooling and quenching step at the temperature of 570 ℃ for 3.5 hours, and then cooling the ductile cast iron blank in air.

The hardness of the ductile iron blanks before, during and after the machining was tested, and the results are shown in table 4 below.

TABLE 4 hardness of spheroidal graphite cast iron blanks before, during and after working treatment

Hardness of	Before processing	After low-temperature graphitization	After cooling and quenching	After tempering
					Outer circle of the circle	180HB	173HB	44HRC	215HB
Inner bore	200HB	177HB	45HRC	222HB

The above description is only for the preferred embodiment of the present invention, but the present invention should not be limited to the embodiment and the disclosure of the drawings, and therefore, all equivalent or modifications that do not depart from the spirit of the present invention are intended to fall within the scope of the present invention.