JPH024915A - Manufacture of cast member - Google Patents

Abstract

PURPOSE:To improve machinability in machining by applying shot peening treatment to the whole surface of an austempered cast member and then subjecting only the part to be machined to tempering treatment at a specific temp. CONSTITUTION:A cast member made of spheroidal graphite cast iron or cast steel is subjected to austempering treatment, by which a matrix structure as a whole is formed into a mixed structure of bainite and retained austenite to improve strength and toughness. Subsequently, shot peening treatment is applied to practically the whole surface of the cast member to induce martensitic structure in the surface layer, by which a metallic structure having the desired high hardness is formed. Then, only the part to be machined of the cast member is subjected to tempering treatment at 250-400 deg.C and the above martensitic structure in the surface layer is decomposed into troostite structure of low hardness. By this method, machining can be applied to the required part easily without influencing the internal matrix structure of the cast member.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing a cast member made of cast iron or cast steel.
In particular, the present invention relates to a method of improving the machinability of machining by tempering the part to be machined after shot peening.

[Prior art]

Recently, cast members made of spheroidal graphite cast iron have been found to have higher strength in areas such as between roads than cast members, to be able to design shapes freely, to be able to reduce weight variations at low cost, etc. For this reason, it is being widely applied to various mechanical parts.

For example, in JP-A No. 61-174332, a connecting rod made of spheroidal graphite cast iron is subjected to austempering treatment to form a mixed structure of bainite and retained austenite, and then machined to exact size, resulting in a connecting rod with excellent service life. A method of manufacturing a connecting rod is described.

Furthermore, in JP-A No. 60-106944, spheroidal graphite cast iron is subjected to austempering treatment, and after the austempering treatment, a metastable austenite structure that induces martensitic transformation is formed in the base at a volume ratio of 10. ~70%, and the remainder is a single structure of bainite or a mixed structure of haynite and martensite.
High toughness spheroidal graphite cast iron is described.

[Problem to be solved by the invention]

According to the technology disclosed in the above-mentioned publication, when manufacturing a connecting rod as a typical cast member that requires high strength and high toughness, a connecting rod made of spheroidal graphite cast iron is manufactured, and this connecting rod is subjected to an male tempering treatment to form a hayite and retained austenite, and the surface of the connecting rod is subjected to a top peening treatment to induce a martensitic structure in the base layer of the surface layer through deformation-induced transformation. It is possible to form an organization.

However, like ordinary cast members, connecting rods must be machined after the above shot peening treatment on the inner peripheral surface of the shaft hole and the end face perpendicular to the shaft center at the large and small ends. Since the martensitic structure is induced in the entire surface layer of the connecting rod, resulting in high hardness, the cutting process described above becomes extremely difficult and the cutting process cost increases significantly.

Therefore, although it is possible to perform shot pinning only on the parts of the connecting rod other than the parts to be machined, the shot peening process becomes complicated.

An object of the present invention is to provide a method for manufacturing a cast member that can decompose the martensitic structure in the surface layer of the portion to be machined without affecting the internal matrix structure of the cast member after the shot peening treatment. It is to be.

[Failure to solve the problem]

The method for producing a cast member according to the present invention includes austempering treatment of a cast member made of spheroidal graphite cast iron or cast steel so that the matrix structure becomes Heinai 1 and a mixed structure of retained austenite throughout the cast member. Next, shot pinning is applied to almost the entire surface of the cast member, and then only the parts of the cast member that are to be machined are tempered at 250 to 400°C to decompose the martensite, and then the casting This involves performing machining on parts.

[Effect]

In the method for producing a cast member according to the present invention, the base of the cast member is made into a mixed structure of haynite and retained austenite through austempering treatment, and when shot peening treatment is applied to substantially the entire surface of this cast member, processing-induced Due to transformation, a part of the retained austenite in the surface layer becomes martensite with high hardness, and the base of the surface layer on almost the entire surface of the cast member becomes a mixed Mi texture of bainite, retained austenite, and martensite 1, and the hardness of the surface layer is extremely high. This results in excellent fatigue strength.

Here, after the shikosoto beaning treatment described in Section 2, only the part of the cast member to be machined is tempered at 250 to 400°C to decompose the martensite. Since the cast member is decomposed into troostite, which has a significantly lower hardness, and then machined, the martensite in the surface layer of the part to be machined can be decomposed and cast without adversely affecting the internal metal structure of the entire cast member. The machinability of parts can be greatly improved.

By the way, when tempering at a temperature below 250°C, the decomposition of martensite hardly progresses and it is difficult to reduce the hardness, and when tempering at a temperature higher than 400°C, This is undesirable because the entire base of the tempered portion is decomposed and the hardness of the base increases due to the precipitation of cementite.

〔Effect of the invention〕

According to the method for manufacturing a cast member according to the present invention, only the portion of the cast member to be machined can be
Since the martensite is decomposed by tempering at 0 to 400"C, the martensite in the surface layer is decomposed and the hardness of the part to be machined is reduced without adversely affecting the entire internal base structure of that part. At the same time, the effects of shot peening can be maintained because the parts that are not machined are not tempered to 250 to 400"c. .

〔Example〕

Hereinafter, an embodiment in which the present invention is applied to a method for manufacturing a connecting rod for an automobile engine will be described based on one page.

In the first step, as shown in FIG. 1 (81), a mold 3 having an induction cavity 2 for casting the cylindrical mold 1'1 is prepared, and the mold 3 is poured into the casting cavity 2 from the sprue 3a of the cylindrical mold 3. Casting is performed by pouring a molten metal of spheroidal graphite cast iron.After the molten metal solidifies, mold milling is performed, and the casting member IA of the connecting slot (■) is cooled to room temperature in the air.The base structure of this casting member 1A is It is a 3-woven fabric mainly made of perlite.

In the second step, the cast member IA in FIG. 1 (as shown in BL) is placed in a sawing furnace, and the entire cast member IA is subjected to austempering treatment. As an austempering treatment, for example, the cast member IA is heated to A, transformation point (730°C).
After heating to 850°C and maintaining it at that temperature for about 2.0 hours to change it to an austenite matrix structure, the cast member IA was placed in a salt bath maintained at about 380°C and brought to that temperature. The product is held for about 2.0 hours to generate bainite, and the i&GJf construction member IA is taken out of the bath and placed in water to be cooled to room temperature. As a result of the above austempering treatment, the base of the ↓Nof structural member IA has a mixed structure of haynite and residual austenite I, resulting in a structure with excellent strength and toughness.

In the third step, as shown in FIG. 1 fc), substantially the entire surface of the cast member IA is subjected to shot peening treatment. At this time, a shot abrasive made of steel balls of 0.6 taIIφ is used, and the arc height is 0.451). The shot peening process not only increases the smoothness of the surface of the cast member IA, eliminates fine defects in the surface layer, and adds residual compressive stress to the surface layer, but also processes some of the retained austenite in the surface layer. The induced transformation results in extremely hard martensite.

In this way, by performing the shot peening treatment, the mechanical and metallic structure of the surface and surface layer of the cast member I can be changed to significantly improve the fatigue strength.

In the fourth step, as shown in FIG. 5
The large end 4 and small end 5 are heated to 300° C. and held for 1.0 hour while cooling the I-rod portion 6 between them using a cooling means. As a result of this tempering treatment, the martenzai 1 structure in the surface layer of the large end portion 4 and the small end portion 5 is decomposed into a true squid structure with much lower hardness than this, so the hardness of the surface layer portion is significantly reduced. , cutting performance during machining is greatly improved.

However, the above tempering treatment does not necessarily need to be carried out at 300°C, and may be carried out at a temperature in the range of 250 to 400°C, but if it is below 250°C, the decomposition of martensite will hardly proceed and the hardness will be sufficiently reduced. It is difficult to
A temperature higher than 00°C is not preferable because it affects not only the surface layer but also the metal structure of the entire internal base, and the precipitation of cementite increases the hardness of the base and causes a decrease in toughness. (
Furthermore, when the entire cast member IA is subjected to tempering treatment, the martensitic structure in the surface layer is decomposed over the entire cast member IA, and the residual compressive stress in the surface layer is reduced. Since the shot pinning process is released, the effect of the shot pinning process is almost completely lost, which is not preferable.

In the fifth step, as shown in FIG. 1(e), the large end portion 4 is divided by cutting at the middle part thereof, the cut surface is cut, and the divided large end portions 4 are connected. A bolt hole for the connecting port 7 is cut, and the inner peripheral surfaces of the shaft holes 4a and 5a of the large end 4 and small end 5 are cut, and the axes of the large end 4 and small end 5 are cut. Both end faces 4 perpendicular to
Cut b.5b.

Through the first to fifth steps described above, a connecting rod 1 with high strength and high toughness can be manufactured.

Next, the measurement results of the microhardness of the surface layer portion of the casting method IA will be supplementarily explained with reference to FIGS. 2 and 3.

As shown in Fig. 2, when the cast member IA is not subjected to shot peening treatment, it has a hardness of a mixed structure of bainite and retained austenite as shown by broken line A.
When shot peening is applied, the influence of martensite generated by deformation-induced transformation becomes more pronounced toward the surface, and the surface hardness reaches as high as 600 Vickers hardness.

As shown in FIG. 3, when the cast member IA is shot peened but not tempered, the polygonal line C (
This corresponds to broken line B in FIG. 2). As in the above example, when tempering is performed at 300°C for 1 hour after shot peening, the hardness of the internal base structure does not change as shown by the polygonal line, and the surface layer has a hardness of 0 to 0.3 degrees Celsius. The hardness in this region is significantly reduced.

On the other hand, after Shoso 1 to peening treatment, 500℃×
When tempering was performed for 1 hour, the hardness of the internal matrix structure generally increased, but this was due to the precipitation of cementite, and precipitation of cementite is undesirable because the toughness decreases.

In addition, although the above-mentioned embodiment has been described in the case of manufacturing a connecting rod 1 for an automobile engine, the present invention is similarly applicable to the manufacture of various other types of spheroidal graphite cast iron cast members and cast steel cast members. I can do it.

[Brief explanation of the drawing]

The drawings relate to embodiments of the present invention, and include FIG.
el is an explanatory diagram of each process when manufacturing a connecting rod, and FIGS. 2 and 3 are diagrams showing measurement results of the hardness of a connecting rod casting member, respectively. 1A... Casting member, 1... Connecting rod.

Claims (1)

[Claims] (1) A cast member made of spheroidal graphite cast iron or cast steel is austempered so that the base structure becomes a mixed structure of bainite and retained austenite over the entire surface, and then approximately the entire surface of the cast member is Shot peening treatment is applied to the cast member, and then only the parts of the cast member that will be machined are
A method for producing a cast member, which comprises tempering at 400°C to decompose martensite, and then subjecting the cast member to machining.