JPS59573B2 - Heat treatment method for spheroidal graphite cast iron pipes - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is characterized by the fact that a spheroidal graphite cast iron pipe is subjected to heat treatment in which it is heated and cooled under specific conditions during the cooling process after casting. The present invention relates to a heat treatment method for spheroidal graphite cast iron.

Spheroidal graphite cast iron pipes contain pearlite in their structure in an as-cast state, and although they have excellent strength, they are extremely brittle materials.

Therefore, it is common practice to carry out ferritic annealing to ensure the toughness.

The conventional method for this purpose is to first cool this type of cast iron pipe to room temperature after casting, and then heat the pipe body, which has a so-called bull's eye structure, to 850°C.
It was customary to carry out heat treatment by holding at 50° C. for about 30 minutes and then slowly cooling.

However, in the case of such conventional heat treatment,
The heat treatment requires a large amount of thermal energy, which is converted to kerosene consumption of 40 to 501/T, and the time required for the heat treatment is approximately 100 minutes, so it is not necessarily a highly efficient treatment. Furthermore, since the tube body is once cooled to room temperature and then heat treated, the amount of shrinkage is large, and in addition, variations are likely to occur due to thermal expansion, thermal contraction, structural changes, etc. during reheating and recooling. This has resulted in drawbacks and problems such as increased errors in dimensional accuracy.

In view of the technical problems of the conventional heat treatment method, the present invention aims to reduce the amount of thermal energy supplied to the ferrite annealing process, shorten the processing time, and improve the dimensional accuracy of cast iron pipes. With this, we aim to provide a new method for heat treatment of spheroidal graphite cast iron pipes.

That is, the present invention reduces the Si content of the spheroidal graphite cast iron pipe to 23
~2.8%, and as-cast with a large amount of ferrite (
After casting the tube by centrifugal force casting or other casting methods with a precondition of having a ferrite content of 50% or more, the tube is removed from the mold at a high temperature (5) as shown in Figure 1. , -The tube cooled to a temperature just below the Arl transformation point (13700 to 750°C) is cooled to a temperature just above the Ac1 transformation point (0850°C
After the pearlite is transformed to zero austenite, the temperature is increased to about 10° to 700 to 650°C (D).
C (10°C±2.5°C)/long or less, followed by air cooling.

In the present invention, among the chemical components of the spheroidal graphite cast iron pipe to be cast, the Si content is particularly limited to a range of 2.3 to 28%.

This is from the standpoint of ensuring the above number of parts and the amount of ferrite.
If it is less than 2.3%, the rate of ferritization in terms of parts is poor, and therefore ferritization cannot be sufficiently achieved even by carrying out the heat treatment of the present invention.
% or more, Si strengthens the ferrite (solid solution strengthening),
This is because the impact value begins to decrease.

One of the characteristics of the heat treatment method of the present invention as described above is that the amount of pearlite in the tube body is smaller than that in the conventional method, so there is no need for graphitization.
The holding time at the maximum holding temperature of 850° C. during annealing can be significantly shortened compared to that in the conventional method.

According to the heat treatment method of the present invention as described above, the spheroidal graphite cast iron pipe is supported even in the following actual results.
It is possible to obtain the desired structural state and mechanical properties with almost no difference from conventional heat treatment methods.

<Example> Tables 1 and 2 below compare the heat treatment examples according to the present invention and the conventional heat treatment examples, and show the structure, mechanical properties, and dimensional accuracy of the heat-treated spheroidal graphite cast iron pipes. Display test results.

However, the chemical composition of the tube subjected to the test was T-C: 3.
4±0.2%, Mn: 0.5% or less, P: 0.05% or less, S: 0.012% or less (Si is in the table).

The test tubes were cast with multiple inoculations to promote ferrite formation.

The results shown in the table above (according to the results), the structure of the spheroidal graphite cast iron pipe obtained by heat treatment according to the present invention is lower than that obtained by conventional heat treatment. There is no inferiority in terms of mechanical properties; rather, after heat treatment (
It is clear that there is little variation in the tubular body dimension l, which is excellent in ensuring dimensional accuracy.

The heat treatment method of the present invention is characterized in that a spheroidal graphite cast iron tube that has been as-cast to produce ferrite is pulled out of the mold at a high temperature, and the temperature begins to rise from 700 to 750°C. However, apart from this heat treatment method, a tube drawn at a temperature higher than the Ar1 transformation point temperature is treated with Ar1
Without cooling to below the transformation point temperature, after holding the temperature at Ar1 transformation point temperature or above for a period of time, slowly cooling it to 650 to 700 °C,
It can also be proposed to apply a heat treatment method in which air cooling is performed thereafter.

However, in the case of this constant temperature heat treatment method, in order to secure the same mechanical properties as this heat treatment method, it is necessary to
It is necessary to hold it for about 20 minutes, and this heat treatment method usually takes 1
Compared to the case where a heating time of about 0 minutes is sufficient, the problem arises that the annealing holding time is extended.

The reason for this is that in the constant temperature heat treatment method, the transformation of austenite riferite is followed, whereas in this heat treatment method, the transformation is austenite → ferrite + pearlite → ferrite +
Following the transformation from austenite to ferrite, A
This is presumably because repeated transformations at one transformation point contribute to recrystallization, promotion of diffusion of impurity elements, and removal of casting stress and internal stress.

As described in detail above, according to the heat treatment method of the present invention, during the cooling process of spheroidal graphite cast iron pipes after casting, the self-retained heat is effectively utilized, and heating is performed under specific conditions.
[Thus, even when compared with the conventional heat treatment method in which the cast iron pipe is cooled once after casting, then heated and annealed and slowly cooled, the ferrite rate and In addition to having no inferiority in mechanical properties, the thermal energy required for heat treatment can be greatly reduced, and at the same time, the time required for heat treatment is also significantly shortened, making it economical and efficient. It has the advantage of being able to obtain a product with a small size and excellent dimensional accuracy.

In fact, the processing time in this heat treatment method is approximately 10 minutes for heating up and 15 to 20 minutes for slow cooling, which is shorter than conventional methods, and the amount of thermal energy supplied is also equivalent to the amount of kerosene consumed. 15, #/T, which is much less than the conventional 40-501/T.

Further, the heating time in the conventional method is about 30 minutes, while in the present heat treatment method, it is about 10 minutes, so the length of the heating zone of the heat treatment furnace can be reduced to one-third.

Comparing an example of the structure of a heat treatment furnace used for conventional heat treatment and that used for this heat treatment method, the former has a heating zone of 10 m,
The total length is 33m with 10m of soaking zone and 13m of slow cooling zone.
The latter has a heating zone of 3.3 m and an annealing zone of 6.7 m, totaling 10 m, which makes the entire furnace equipment more compact and has the advantage of reducing the area occupied within the factory premises.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a temperature curve of the heat treatment method according to the present invention. Figures 2 and 3 are micrographs showing the structure of spheroidal graphite cast iron pipes. Figure 2 shows the structure of the spheroidal graphite cast iron pipe. Figure 2 shows the structure obtained by the conventional heat treatment method, and Figure 3 shows the structure obtained by the heat treatment method of the present invention. (not shown), each shows an annealed structure.

Claims (1)

[Claims] After casting a spheroidal graphite cast iron tube with a Si content of 2.3 to 2.8%, the tube body is taken out of the mold at a high temperature and charged into a heat treatment furnace at a temperature of 700 to 750°C. 85
After heating to around 0℃, it is heated to 700-650℃.
A method for heat treatment of spheroidal graphite cast iron pipes, characterized by slow cooling at a cooling rate of ℃/vtm or less.