JPS5931567B2 - Heat treatment method for spheroidal graphite cast iron parts - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for heat treatment of spheroidal graphite cast iron parts, and in particular, it reduces deformation of products during austempering, improves product precision, and improves mechanical efficiency. This aims to improve properties and wear resistance.

As is well known, austempering treatment of spheroidal graphite cast iron improves wear resistance and fatigue strength, so it is suitably used in gears, sprockets, cams, and other items that require high wear resistance and fatigue strength.

However, usually, austempering is performed after machining a product (part), and when the multi-part part is heated and cooled during this process, the part is deformed.

Therefore, when manufacturing products that require high precision, it is necessary to perform further machining or polishing after austempering treatment, but since the product has high hardness and high wear resistance, the machining and polishing operations described above are difficult, and This work is very time-consuming, which inevitably leads to lower productivity and higher costs.

Furthermore, if the temperature during isothermal transformation during austempering is low, the fatigue strength will be weaker and the amount of deformation will tend to be larger than when the temperature is high, which has the disadvantage that the range of vortex conditions during isothermal transformation is limited. be.

This invention eliminates the above-mentioned drawbacks, and focuses on the fact that most of the deformation during austempering occurs during heating and cooling in the heat treatment method for spheroidal graphite cast iron parts. Heat treatment is performed to preemptively remove the deformation of the part that occurs during austempering after the machining process, and as a result, the deformation that occurs in the part after austempering is kept below the allowable amount, making it possible to reuse the part after austempering. The feature is that the machining process is omitted.

Furthermore, this invention suppresses the deformation of the parts as much as possible through the above-mentioned heat treatment process, machining process, and austempering process, and also performs shot peening after the austempering process to dramatically increase the fatigue strength of the parts. The object of the present invention is to provide a heat treatment method for spheroidal graphite cast iron parts, which is characterized by expanding the vortex range during isothermal transformation during austempering.

In order to achieve the above object, the method of the present invention uses, as a basic spheroidal graphite cast iron material, a chemical composition of normal spheroidal graphite cast iron, Ni: 0.3 to 2.0 degrees, Mo: 0.1 to
1.5 (or more by weight) was added, and the material was heated and held at 750 to 950°C for 0.5 to 4.0 hours, and then gradually cooled, as described below. After processing, the material is heated and held at 850 to 1000°C for less than 4 hours, then rapidly cooled to 200 to 400°C, and then subjected to austempering treatment in which the material is heated and held at that temperature for 30 minutes or more.
Finally, shot peening is performed.

In addition, it is not necessarily necessary to perform shot peening processing,
This is especially done when manufacturing parts that require a high degree of fatigue strength.

Accordingly, the method of the present invention generally involves the following steps.

Hereinafter, the method of the present invention will be described in detail. As mentioned above, the basic spheroidal graphite cast iron material has the following chemical composition.

That is, C: 2.6 to 4.0% by weight (hereinafter referred to as weight percentage), Si: 1.5 to 3.5, Mn: 0
．． 1~1.0゜Mg: 0.02'-0,10, Ni
: 0.3-2.0, Mo: 0.1-1.5. The remainder has a chemical composition consisting essentially of iron.

The reason for adding Ni and Mo and the reason for limiting the components will be explained.

Note that C, Si 2 , Mn 2 , and Mg are in the same range as the generally known composition of spheroidal graphite cast iron, and the numerical values are also the same as those based on well-known reasons, so the explanation will be omitted.

Ni and Mo are elements that increase the sensitivity of the material to thermal changes, and facilitate complete isothermal transformation in the austempering process described below.

In addition, in order to improve productivity as a casting material, it is a condition that the tendency of melting and chilling is not promoted so much, and as a result of experiments conducted by the present inventors, it was found that Ni and Mo are the most suitable elements. Ta.

The reason why the amount of Ni added was limited to 0.3 to 2.0% by weight is that the effect does not appear when it is less than 0.3%, and when the amount is increased, the effect gradually becomes saturated at around 1.5%. 2.0%
Even if this is done above, the cost will only increase and no improvement in effectiveness can be expected.

The reason why Mo is set at 0.1 to 1.5% is that the effect of Mo is remarkable even in a small amount compared to Ni, and the effect appears when the amount added is 0.1% or more, and the effect is not effective when it is added at about 1.5%. This is because if it becomes saturated and exceeds that level, castability will be hindered and costs will increase.

After casting the above material into manufacturing parts, first, as described above, 7
A primary heat treatment is performed by heating to 50 to 950°C, holding for 0.5 to 4 hours, and then slowly cooling.

This primary heat treatment causes deformation during heating among the deformations that occur during heating and cooling for the subsequent austempering treatment,
The primary heat treatment transforms the base of the material into almost a ferrite structure, thereby causing deformation.

This deformation is removed in the next machining process, so that the deformation during the austempering process after the machining process is minimized, and the next machining process can be performed very smoothly as a ferrite base.

The temperature and time of the primary heat treatment are limited for the following reasons.

The reason why the temperature is 750 to 950°C is that 750°C is the temperature necessary to turn the base into ferrite, and 950°C is the temperature required to turn the base into ferrite.
If it exceeds C, the crystal grains become coarse and brittle.

The reason for setting the time to 0.5 to 4.0 hours is that 0.5 hours is the time required to turn the base into ferrite, and 4.
If the time exceeds 0 hours, the crystal grains become coarse and brittle.

After the above primary heat treatment process to cause deformation of the material,
Machining or polishing is performed using known methods to remove the deformation.

After the above machining, the parts are austempered.

That is, as shown in Fig. 1, the part is heated to 850 to 1000°C and maintained at a temperature of several minutes to 4.0 hours to form an austenitic structure; The material is quenched in a hot bath such as the above, and held at a constant temperature for 30 minutes or more to undergo isothermal transformation, making the matrix mainly a bainite structure.

This austempering treatment increases hardness and improves wear resistance and fatigue strength.

When heating to 850-1000℃ during this austempering process, large deformation occurred in the conventional method, but since the deformation during heating is caused by the primary heat treatment, almost no deformation occurs during the austempering process. .

Note that deformation occurs when rapidly cooling from 850 to 1000°C to 200 to 400°C, but this deformation is within an allowable range.

The temperature and time during the austempering treatment are limited for the following reasons.

First, the temperature during heating is set at 850 to 1000°C because 850°C is the temperature necessary to austenite the matrix, and if it exceeds 1000°C, the crystal grains will become coarse and the heating equipment will be expensive. Become.

The reason for setting the above heating time to be from several minutes to 4.0 hours is that a heating time of several minutes or less is not sufficient to austenite the base;
．． If the time exceeds 0 hours, the crystal grains become coarser and decarburization occurs.

The rapid cooling temperature after heating is set at 200 to 400°C because
200° C. is the temperature necessary to prevent the formation of martensitic structure, and 400° C. or lower is the temperature necessary to prevent the formation of pearlite structure and make the main component of the matrix to be bainite structure.

The reason why the constant temperature holding time of 200 to 400° C. is set to 30 minutes or more is because 30 minutes is required to generate a bainite structure.

Further, when a high degree of fatigue strength is required, shot peening treatment is performed by a well-known method after the austempering treatment.

This is because when spheroidal graphite cast iron is austempered, tensile residual stress is generated on the surface, which has the disadvantage of insufficient dynamic bending fatigue.Especially, during isothermal transformation during austempering (200 ~ The lower the temperature (maintained at 400°C for 30 minutes or more), the more pronounced the lack of fatigue strength becomes.
This is because the amount of deformation also increases.

Therefore, by performing shot peening, fatigue strength can be greatly increased, and even if the temperature during isothermal transformation is low, by compensating for the lack of fatigue strength, the humidity range during isothermal transformation can be expanded. be done.

Furthermore, plastic working of the surface by shot peening improves the hardness of the part, and if austenite remains in the matrix, this austenite is transformed into martensite by shot peening, resulting in improved strength and durability. This improves abrasion resistance.

Next, an example of the present invention will be described in comparison with a comparative example using a conventional method.

Five test pieces each as shown in FIG. 2 were heat-treated by the method of the present invention and the conventional method under the following conditions.

[Embodiments of the present invention]

Conditional components・”・C: 3.75, Si: 2.65
, Mn: 0.41, Mg: 0.037. Ni
: 0.50.

Mo: 0.30 (more than weight), remaining Fe primary treatment: heated at 900°C for 1.5 hours, slow cooling austempering treatment: heated at 900°C for 10 minutes, After rapid cooling and constant temperature holding at 300° C. for 2 hours, a test piece cast with the components under the above conditions was subjected to primary heat treatment, then machined, and then subjected to austempering treatment.

(However, shot peening was not performed.

) [Comparative Example I] Conditions Components: Same austempering treatment as in the above-mentioned examples of the present invention... A test piece cast with the above components was machined, and then austempering treatment was performed.

[Comparative example ■]

Conditional components: Same austempering treatment as in the above-mentioned examples of the present invention... 〃 Normalizing: Heated at 930°C for 1.5 hours, then air cooled A test piece cast with the above components was normalized under the above conditions. After that, it was machined and then austempered.

As shown in the figure, the test piece prepared as described above has a diameter of 15φ and a length of 25N, with a small diameter portion of 12φ and 30M provided in the center of both sides.

[Rotational Fatigue Test] - Each of the test pieces described above was rotated horizontally, and the difference between when the 12φ portion of the test piece swung to the top and to the bottom was measured as the amount of swivel.

The results are as shown in the graph of FIG.
The amount of runout is 0 μ or more, and the amount of runout is large, and the examples of the present invention have less rotational fatigue than the comparative examples.

Note that the permissible range of the amount of runout varies depending on the use of the product, but is preferably 40μ or less.

Next, the results of tests regarding the presence or absence of shot peening treatment in the method of the present invention and bending stress when changing the holding temperature during isothermal transformation during austempering treatment will be explained.

[Creation conditions]

The components, shape, primary heat treatment, and austempering treatment are the same as in the embodiment of the method of the present invention shown in FIG. 3 above.

However, during isothermal transformation conditions during austempering, -△-
and -mu- is 240℃ x 2 hours -〇- and -・- is 30
0°C x 2 hours 5-Ro and -11 are 360'C A total of 6 types were made, including those subjected to shot peening (-・muichi and --・-2-11 in the figure).

[Test force method]

A load was applied to both ends of the six types of test pieces, and the test pieces were rotated, and the relationship between the number of rotations and the stress when the test pieces broke was measured.

The results are shown in the graph of Figure 4, which shows that the force of shot peening is strong bending stress, and
It was confirmed that the force that increases the temperature during isothermal transformation during austempering treatment causes strong bending stress.

As is clear from the above description, according to the heat treatment method for spheroidal graphite cast iron according to the present invention, the deformation of the product during austempering treatment is reduced, a highly accurate product can be obtained, and the austempering treatment and The subsequent shot peening treatment dramatically improves wear resistance and fatigue strength, which has the advantage that it can be suitably used in the manufacture of parts that require high wear resistance and strength, such as gears, sprockets, and cams.

[Brief explanation of the drawing]

Figure 1 is a graph showing the temperature and time of austempering treatment, Figure 2 is a front view of a test piece prepared to test the effects of the present invention, and Figure 3 is a comparison between the method of the present invention and the conventional method. Graph showing the amount of deflection of the test piece due to the difference in the pre-process of the austempering treatment performed for the purpose, No. 4
The figure is a graph showing the relationship between bending stress when the temperature during austempering treatment is changed, when shot peening is performed, and when shot peening is performed.

Claims (1)

[Claims] IC: 2.6 to 4.0 weight percent, Si: 1.5 to 3.5 weight %, Mn: o, 1 to 1. o weight%, Ni: 0
．． 3-2.0% by weight, MO: 0.1-1.5% by weight,
Mg: 0.02 to 0.1 by weight, spheroidal graphite cast iron with the balance essentially consisting of Fe at 750 to 950°C, 0.5 to 4
．． Perform a primary heat treatment of holding heat for 0 hours and then slowly cooling it,
Spheroidal graphite cast iron parts are then machined, heated at 850 to 1000°C for less than 4 hours, rapidly cooled to 200 to 400°C, and subjected to austempering treatment by heating and held at this temperature for 30 minutes or more. heat treatment method. 2C: 2.6 to 4.0 weight percent, Si: 1.5 to 3.5 weight %, Mn: o, 1 to 1. o weight%, Ni:
0.3-2.0% by weight, Mo: 0.1-1.
Spheroidal graphite cast iron consisting of 5% by weight, Mg: 0.02-0.1% by weight, and the remainder substantially Fe is heated and held at 750-950°C for 0.5-4.0 hours, and then slowly cooled. First heat treatment, then machining, then 850 ~
200-40 after heating and holding at 1000℃ for less than 4 hours
A heat treatment method for spheroidal graphite cast iron parts, which comprises rapidly cooling to 0°C, performing an austempering treatment by heating and holding at that temperature for 30 minutes or more, and then performing shot peening.