JPS62214123A - Manufacture of high strength ductile cast iron - Google Patents

Abstract

PURPOSE:To obtain the titled cast iron having by far larger elongation, impact value and fatigue limit than conventional in a relatively short time, by isothermally heat treating ductile cast iron having prescribed ratios of C, Mn respectively and component compsn. contained ordinarily. CONSTITUTION:Ductile cast iron having, by weight 2.5-3.6% C, <=0.3% Mn and the other ordinary component compsn. is treated as follows. Namely, the cast iron is firstly heated to 900+ or -50 deg.C for austenitizing, then subjected to isothermal heat treatment in which it is held in salt bath of 360-410 deg.C for 10-25min and the structure is converted to bainite. As the result, productivity and quality of gear, bracket and the other ductile cast iron product to which isothermal heat treatment is applied, having previously described characteristics can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for producing high-strength ductile cast iron, and more particularly to a heat treatment method for improving the characteristics of ordinary ductile cast iron to produce high-strength ductile cast iron.

<Prior Art> Conventional heat treatments for improving the properties of ductile cast iron include quenching/tempering, surface hardening, and austempering. In particular, the elongation characteristic of ductile cast iron,
Austempering is effective in improving toughness.

<Problems to be solved by the invention> The austempering of ductile cast iron is similar to that of steel, and after heat treatment to austenitize the structure, the austempering of ductile cast iron is approximately
A bainite structure is obtained by isothermal transformation in a salt bath at 00° C., but the isothermal treatment requires holding for 30 minutes or more in order to sufficiently complete the transformation.

However, from the viewpoint of productivity, it is always desired to shorten the processing time, and the above-mentioned holding time of 30 minutes or more is also a problem.

There have been various reports on the relationship between isothermal treatment temperature and effects in conventional austempering, but when holding time is considered as a problem as mentioned above, it is difficult to understand how the isothermal treatment temperature and holding time affect the properties. There was still some exploration left to do.

<Means for Solving the Problems> As described above, the present invention provides constant temperature treatment temperature and holding time in aus-tempering of ductile cast iron, various characteristics,
In other words, as a result of investigating the relationship between elongation, impact value, and fatigue limit, we succeeded in improving the above various properties more than before with a constant temperature treatment time of less than 30 minutes. , C in weight percentage: 2.5-3.6%, Mn=0.
Ductile cast iron having a composition of 3% or less and other commonly contained components is heated to 900°C ± 50°C for 1 to 2 hours, and then held in a salt bath at 360°C to 410°C for 10 to 25 minutes. The key point is that constant-temperature heat treatment is performed.

<Example> The ductile cast iron targeted by the present invention has a C: 2.5~
3.6%, Mn: 0.3% or less, and other components and compositions are normally included in the range, so-called normal ductile cast iron, and even though the composition is within the above range, the heat resistance can be improved by considering the composition. It should be emphasized that ductile cast iron for heat resistance is also a subject of the present invention, and a small amount of N, which is usually added to thick-walled castings etc. for hardenability etc.
Elements such as i are also so-called normally suppressed components.

Note that the limitation of C: 2.5 to 3.6% is that if it is less than 2.5%, it is difficult to manufacture ductile cast iron;
% or more, the amount of precipitated graphite increases, resulting in a decrease in quality.

In addition, Mn: 0.3% or less has the effect of reducing elongation in addition to deoxidizing and stabilizing pearlite, and as mentioned above, it is characteristic that ductile cast iron has a high elongation rate. This is a limitation to ensure a preferable elongation rate from this point of view.

Next, heat treatment will be explained.

First, the heating in the range of 900° C.±50° C. is for the purpose of austenitizing the structure, and this temperature range and heating time are also within the usual range and are not characteristic conditions.

The cast iron after austenitization is then subjected to a constant temperature heat treatment held in a salt bath at 360 to 410°C for 10 to 25 minutes to form bainite, and this process is the most distinctive feature of the present invention. The limitations of the constant temperature heat treatment temperature and holding time, as well as the limiting effects, will be explained with reference to the attached FIGS. 1 to 3.

First, to explain each figure, Figure 1 shows normal ductile cast iron samples (same type of samples) heated to 900℃,
Austenitizing treatment is performed by heating for 1 hour, then 4 hours.
The elongation rate was investigated for each sample treated in a salt bath at 00℃, 350℃, 300℃, and 250℃ for various holding times.The vertical axis shows the elongation rate, and the horizontal axis shows the constant temperature treatment time. , "Elongation constant temperature processing time" for each processing temperature
A diagram is shown.

The following Figure 2 shows the impact values (with V-notches) for each sample that is similar to the sample in Figure 1 and has been heat-treated in the same way.
The results of the investigation are shown in a "shock value vs. constant temperature treatment time" diagram for each treatment temperature, with the vertical axis representing the impact value and the horizontal axis representing the constant temperature treatment time.

In addition, Figure 3 shows a sample of 90% as in Figures 1 and 2 above.
After austenitization by heating at 0°C for 1 hour, 35
The fatigue limit was determined for each sample subjected to constant temperature treatment for 15 minutes and 60 minutes in a salt bath at 0°C and 375°C, respectively, and a "fatigue limit - constant temperature treatment time" diagram for each treatment temperature is shown. be.

It is noteworthy that among the above figures, FIG. 1 and FIG. 2 are diagrams showing similar trends.

In other words, both elongation and impact value are 1 when treated at a constant temperature of 300℃ or higher.
The highest value was shown in the case of holding for 5 minutes, and the higher the constant temperature treatment temperature, the better each characteristic value was.

Looking at the characteristic values for 15 minutes treatment at 400°C, they are significantly superior to the characteristic values for other holding times, and this "prominent tendency" is even more apparent in the 350°C treatment diagram. I'm starting.

Furthermore, what can be said through each of the graphs is that there is a tendency for the characteristic values to be approximately constant when the holding time is 30 minutes or more. This corresponds to the fact that conventionally the holding time was required to be 30 minutes or more. In other words, in the past, temporal conditions showing higher characteristic values were overlooked.

Also in FIG. 3, the fatigue limit is higher when the constant temperature treatment temperature is higher and when the holding time is 15 minutes.

As is clear from the above study results, it is advantageous to select the constant temperature treatment temperature in the high temperature range as much as possible, and considering the prominent tendency of the "characteristic value - constant temperature treatment temperature" diagram and Figure 3, 350 ° C. 360° C. is set as the lower limit in the present invention.

In addition, the upper limit of 410°C was specified for practicality.
Even if it is more than this, it is not practical.

Next, regarding the retention time, as is clear from Figures 1 to 3, excellent characteristic values were obtained for 10 minutes or more but less than 30 minutes, and it is clear that 30 minutes retention is already excessive. Therefore, in the present invention, 10
The duration is limited to ~25 minutes.

<Effects of the Invention> The present invention is as described above, and by subjecting ordinary ductile cast iron to constant temperature heat treatment at a specific constant temperature treatment temperature and treatment time according to the present invention, it can be performed in a shorter time than in the conventional same treatment, and This means that gears can be made with high strength, with much greater elongation, impact value, and fatigue limits than conventional gears.

It greatly contributes to improving the productivity and quality of brackets and other ductile cast iron products that undergo constant temperature heat treatment.

[Brief explanation of drawings]

Figure 1 shows ordinary ductile cast iron subjected to isothermal treatment at various temperatures (
Fig. 2 shows the "Impact value - constant temperature treatment time" diagram for each treatment temperature for ordinary ductile cast iron treated in the same manner as above. FIG. Figure 3 shows normal ductile cast iron at 350°C and 375°C.
It is a "fatigue limit - constant temperature treatment time" diagram for each treatment temperature for those subjected to constant temperature treatment (austempering) at °C.

Claims (1)

[Claims] (1) C: 2.5-3.6%, Mn: 0.
Ductile cast iron having a composition of 3% or less and other commonly contained components is heated to 900°C ± 50°C for 1 to 2 hours, and then held in a salt bath at 360°C to 410°C for 10 to 25 minutes. A method for manufacturing high-strength ductile cast iron, which is characterized by carrying out constant temperature heat treatment.