JPH0718324A - Corrosion resistant tough cast iron pipe and its production - Google Patents

Abstract

PURPOSE:To produce the cast iron pipe having excellent toughness and corrosion resistance in combination and to provide the process for production of this cast iron pipe. CONSTITUTION:This corrosion resistant tough cast iron pipe is formed by coating the surface of a pipe body subjected to ferritization annealing with an Fe-Sn alloy layer formed by diffusion of Sn. On the other hand, this process for production of the cast iron pipe comprises subjecting the pipe body after casting to a blasting treatment and subjecting the pipe body to thermal spraying of Sn, then subjecting the cast iron pipe coated with the thermally sprayed layer of the Sn to the ferritization annealing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cast iron pipe having excellent toughness and corrosion resistance, and a suitable method for producing the same.

[0002]

2. Description of the Related Art Conventionally, cast iron pipes of various materials have been manufactured by centrifugal casting, and if necessary, in order to improve toughness, ferritic annealing is performed after casting. This heat treatment transforms the cementite in the matrix of the cast iron pipe in the as-cast state into ferrite, and usually 900 to 1
It is carried out by holding at a high temperature of about 000 ° C. for several tens of minutes.

In order to impart not only toughness but also good corrosion resistance to a cast iron pipe, in recent years, the surface of the cast iron pipe after ferritic annealing is subjected to Zn thermal spraying to form a Zn thermal sprayed layer on the surface of the pipe.

[0004]

However, since the Zn sprayed layer is porous, it is inferior in corrosion resistance, and since it is only mechanically attached to the surface of the pipe, it is easily peeled off, and it is localized from the peeled portion. There is a problem that corrosion progresses.
The present invention has been made in view of the above problems, and an object of the present invention is to provide a cast iron pipe having excellent toughness and corrosion resistance, and a manufacturing method thereof.

[0005]

In the corrosion resistant tough cast iron pipe of the present invention, the surface of the ferritic annealed pipe body is coated with an Fe-Sn alloy layer formed by diffusion of Sn. On the other hand, in the method for producing a cast iron pipe according to the present invention, the pipe body after casting is subjected to blasting treatment, Sn thermal spraying, and then the Sn treatment.
A cast iron pipe coated with a thermal sprayed layer is annealed by ferrite. S
The n-sprayed layer is usually formed on the outer surface of the tube, but if the inner diameter is large, it can be applied to the inner surface of the tube. The layer thickness of the sprayed layer is usually 100 to 150 μm.
About m is sufficient.

[0006]

The surface of the cast iron pipe main body in the as-cast state is treated by shot blasting or sand blasting, so that the Sn sprayed layer is directly formed on the background of the pipe main body without interposing an oxide film or the like. When the tube body coated with the Sn sprayed layer is subjected to ferritic annealing, the cementite in the matrix transforms into ferrite, the toughness is improved, and Sn is rapidly diffused and permeated from the Sn sprayed layer to the tube surface, A dense Fe-Sn alloy layer having excellent corrosion resistance is formed on the surface portion. This alloy layer is metallurgically formed integrally with the tube surface,
There is no risk of peeling.

Although Sn can be expected to diffuse in the Sn plating layer instead of the Sn sprayed layer, large-scale equipment is required as compared with spraying, and the running cost is increased. In this respect, thermal spraying can be easily performed with simple equipment, and a coating having a uniform thickness can be easily formed. Although Zn can be sprayed instead of Sn as in the conventional case, Zn has a low boiling point and is not preferable because it vaporizes during high temperature heating in ferritic annealing.

[0008]

Example: Shot blasting was performed on the outer peripheral surface of an as-cast ductile cast iron pipe body having an inner diameter of 100 mm x wall thickness of 7.5 mm x length of 4 m to remove the oxide layer, and then 155 g of Sn.
/ M ² thermal spraying (average thermal sprayed layer thickness 75 μm), and this cast iron pipe was annealed while rolling in a horizontal converter according to the heat cycle of FIG. 1. In the figure, the section at 980 ° C. for 20 minutes is the ferritic annealing section. After annealing, a sample was taken from this cast iron pipe, the metal structure in the thick section was microscopically observed, and the elemental analysis was performed by EPMA analysis. As a result, it was confirmed that a Fe—Sn alloy layer having a layer thickness of about 80 to 120 μm was formed on the tube surface.

Next, a 75 × 150 mm corrosion test piece was sampled from the cast iron pipe to prepare a sample in which only the outer surface of the pipe was exposed and the other part was embedded in a synthetic resin, and this was used for salt water corrosion. The test was conducted. This test is carried out by keeping the material for 30 days in a test chamber in which a 3% concentration saline solution is constantly sprayed and measuring the corrosion weight loss.
As a result, the corrosion weight loss was 0.0015 g. Since this value is within the measurement error range, it is found that the Fe-Sn alloy layer is hardly corroded, and the Fe-Sn alloy layer has extremely excellent corrosion resistance.

[0010]

As described above, according to the corrosion resistant tough cast iron pipe of the present invention, since the surface of the pipe body annealed for ferriticity is coated with the Fe-Sn alloy layer formed by diffusion of Sn, the toughness is improved. And a dense Fe-Sn alloy layer is formed metallurgically on the surface of the tube, there is no fear of peeling, and the corrosion resistance is extremely excellent. Further, according to the manufacturing method of the present invention, the corrosion-resistant tough cast iron pipe can be easily obtained by simply using a simple thermal spraying device without newly providing special equipment.

[Brief description of drawings]

FIG. 1 is a heat treatment diagram of a cast iron pipe in an example.

Claims (2)

[Claims] 1. A corrosion-resistant high-strength cast iron pipe characterized in that the surface of a ferritic-annealed pipe body is coated with a Fe—Sn alloy layer formed by diffusion of Sn. 2. A method for manufacturing a cast iron pipe in which flight-annealing is performed on a cast pipe body, wherein the cast pipe body is subjected to blast treatment, Sn spraying, and then the Sn sprayed layer is coated. A method for producing a corrosion-resistant tough cast iron pipe, characterized by subjecting the pipe to ferritic annealing.