JPH02163313A - Manufacture of cast iron parts - Google Patents

Abstract

PURPOSE:To manufacture cast iron parts having superior fatigue strength and nearly free from surface roughness by austempering cast iron parts, ion-nitriding the resulting spheroidal graphite cast iron parts at a specified temp. and shot- peening the surfaces of the parts. CONSTITUTION:Raw material for cast iron is melted and cast in casting molds to manufacture cast iron parts. These parts are anstempered and the resulting spheroidal graphite cast iron parts having a structure consisting of 30-60vol.% retained austenite and the balance bainite are ion-nitrided at a low temp. of <=420 deg.C to form hardened surface layers and the surfaces of the cast iron parts are shot-peened to produce high residual compressive stress in the surface parts. Cast iron parts having considerably improved fatigue strength and nearly free from surface roughness are manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for manufacturing cast iron parts that can be austempered to improve fatigue strength.

[Prior Art] In recent years, materials for automobile parts have been made to have higher strength and toughness in order to reduce their weight and improve their performance. Austempering is also performed on cast iron parts as one of the means for increasing the strength and toughness, and at present, a considerable number of cast iron parts are manufactured by this austempering process.

When applying this austempered cast iron part to a member that requires high fatigue strength, it is possible to obtain high fatigue strength by rolling processing or, for example, shot peening treatment as shown in JP-A No. 56-116853. There are known techniques for obtaining .

[Problems to be Solved by the Invention] However, in the past, shot peening conditions for upper bainite, which contains a large amount of retained austenite, among cast irons that have been austempered in this manner have not been sufficiently studied. . Further, the inventor of the present application conducted a detailed study and found that under normal shot peening conditions, the skin was severely roughened, sufficient compression pressure was not generated, and sufficient fatigue strength was not obtained.

This invention was made in view of the above-mentioned problems, and an object of the invention is to provide a method for manufacturing cast iron parts that has less surface roughness and can obtain sufficient fatigue strength.

[Means for Solving the Problems] In order to solve the above-mentioned problems and achieve the objectives, a method for manufacturing cast iron parts according to the present invention involves melting and casting a cast iron material, subjecting it to austempering treatment, and removing residual austenite. A first step of obtaining a spheroidal graphite cast iron casting having a bainite structure, a second step of nitriding this spheroidal graphite cast iron part by an ion nitriding method at a temperature of 420° C. or lower, and a shot peening of the nitrided cast iron part. It is characterized by comprising a third step of.

[Function] In the method for manufacturing cast iron parts calibrated as described above, the cast iron parts austempered in the first step are
Ionic nitridation occurs at temperatures below 0°C. As a result,
Decomposition of retained austenite is prevented and retained well. Then, by ion nitriding, the surface part is moderately hardened, and even if it is subsequently shot peened, sufficient compressive stress is formed without cracking on this surface part, and the desired fatigue strength is achieved. will be obtained.

[Example] Below, the configuration of an example of the method for manufacturing cast iron parts according to the present invention will be described in detail with reference to the accompanying drawings.

That is, in this method of manufacturing cast iron parts, first, a cast iron material is melted and cast, and the cast iron part is austempered. And through this austempering process,
A spheroidal graphite cast iron part with a bainite structure having 30 to 60% retained austenite by volume is made.

Here, the amount of residual austenite formed by this austempering process needs to be at least 30% or more in order to ensure workability and fatigue strength of the cast part after the austempering process. In other words, if the amount of retained austenite is less than 30%, the hardness of the base material of the component will increase, the processing quality will deteriorate, and it will be difficult to adopt the processes after austempering as described above. becomes. In addition, this retained austenite has the function of suppressing the propagation of cracks, so it is
If it is less than %, the fatigue strength will decrease significantly. In this way, the lower limit of the amount of retained austenite is set at 30%.

On the other hand, when 1 of retained austenite becomes 60% or more,
From the perspective of fatigue strength, there is no point in going beyond this point.
On the contrary, in terms of manufacturing, it becomes necessary to input a large amount of alloying elements, and the heat treatment temperature must be set high, which is not advantageous in terms of cost. In this way, the upper limit of the amount of retained austenite is set at 60%.

Then, the member containing a large amount of retained austenite is subjected to ion nitriding treatment at a temperature of 420° C. or lower to form a hardened layer on the surface. 42 like this
By adopting a temperature condition of 0° C. or lower, retained austenite is not decomposed in this temperature range and is well maintained. In other words, the retained austenite becomes ferrite and Fsa at a temperature of 420°C or higher.
This results in a significant decrease in fatigue strength. In this way, in order to harden the surface, instead of simply nitriding, ion nitriding, which can be performed at low temperatures, is employed.

With the surface of the cast iron component thus hardened, shot peening treatment is performed. This shot peening treatment causes less roughness on the surface, creates high residual compressive stress on the surface, and significantly improves fatigue strength.

A specific example of manufacturing a cast iron member using this manufacturing method will be described below.

First, when melting and casting a cast iron material, the composition was set as shown in Table 1 below.

Table 1: Weight% Here, during the above-mentioned ion nitriding, it is preferable to add Cr, V, etc. as an alloying element that facilitates the formation of a hardened layer.

Moreover, two types of casting shapes were set: a smooth material with a diameter of 12 mm for use as an Ono rotary bending test piece, and a prismatic material of 80 x 12 x 20 (mm) as a test piece for stress measurement.

On the other hand, the processing conditions for austempering are as follows:
'C for 2 hours, and then at 380°C for 2 hours. In addition, the ion nitriding conditions were 1 at 410°C.
The test was carried out continuously for 0 hours. The shot peening conditions were HRC45 using a steel ball with a diameter of 0.6 mm.

The cast member (i.e., test piece) manufactured as described above and the following two types of comparative examples were evaluated for four points: surface roughness, degree of surface hardening, residual compressive stress, and fatigue strength. A comparative study was conducted.

First Comparative Example> A cast part was formed using only conventional austempering treatment.

Second Comparative Example> After the cast parts were subjected to conventional austempering treatment, shot peening treatment was performed. That is, in this second comparative example, the ion nitriding treatment at a temperature of 420° C. or lower, which is a feature of the present invention, was not performed.

Regarding the surface roughness in these first and second comparative examples and one example, as shown in FIG. Although a relatively good surface roughness was obtained, the second comparative example showed a considerably rough surface roughness. this is,
As pointed out in the prior art, since the surface is excessively hardened by the austempering treatment, so-called cracks occur due to shot peening, resulting in a rough surface.

Regarding fatigue strength, the fatigue limit is shown in Table 2 below.

Table 2 As is clear from Table 2, the fatigue limit of the cast parts of this example was confirmed to be improved by about 20% or more compared to the first and second comparative examples.

Further, regarding the surface hardness, a comparison is shown in FIG. The hardness of the surface shown in FIG. 2 is constant regardless of the distance from the surface when only austempering is performed, whereas as in this example, It was confirmed that when the ion nitriding treatment was performed following the austempering treatment, the hardness of the surface portion was increased.

Further, regarding the residual compressive stress, a comparison thereof is shown in FIG. The residual compressive stress shown in FIG. 3 appears in the deep part of the cast part when shot peening is applied in addition to austempering treatment, and in this example, it appears in the surface part. was confirmed.

As detailed above, by manufacturing a cast part based on the manufacturing method of this embodiment, a good cast part can be provided in which the fatigue limit is increased and the residual compressive stress is strongly expressed in the surface portion. That will happen.

It goes without saying that this invention is not limited to the configuration of the one embodiment described above, and can be modified in various ways without departing from the gist of the invention.

[Effects of the Invention] As detailed above, the method for manufacturing a cast part according to the present invention includes the steps of melting and casting a cast iron material, austempering the same, and obtaining a spheroidal graphite cast iron casting having a bainitic structure with residual austenite. 1, a second step of nitriding this spheroidal graphite cast iron part by an ion nitriding method at a temperature of 420'C or less, and a third step of shot peening this nitrided cast iron part. It is characterized by

Therefore, according to the present invention, there is less roughness on the surface,
A method for manufacturing cast iron parts that can obtain sufficient fatigue strength will be provided.

[Brief explanation of the drawing]

Figure 1 shows the difference in surface roughness between an embodiment of the method for manufacturing cast iron parts according to the present invention and two comparative examples. Figure 2 shows the difference in surface roughness between cast iron parts before and after ion nitriding. FIG. 3 is a diagram showing the distribution of microhardness on the surface of the cast part; and FIG. 3 is a diagram showing the stress distribution within the cast part. 6 history'()JHV>

Claims (1)

[Claims] A first step of melting and casting a cast iron material and austempering it to obtain a spheroidal graphite cast iron casting having a bainitic structure with residual austenite;
The second step is nitriding using the ion nitriding method at a temperature of 20°C or lower, and the third step is shot peening the nitrided cast iron parts.
A method for manufacturing cast iron parts, characterized by comprising the steps of: