JPH05228614A - Method for removing shrinkage cavity on inner surface of centrifugal cast steel tube - Google Patents

Abstract

PURPOSE:To remove a shrinkage cavity in the inner surface of a cast steel tube by remelting the inner surface of the steel pipe with high energy heat source. CONSTITUTION:A centrifugal cast steel tube 1 is horizontally supported to a rotary receiving tables 5 and connected to a rotary driving device and rotated at low speed. A torch 2 for welding with plasma power is connected to a longitudinal feeding device 3 and provided in the tube so as to make a nozzle hole face downward, and is movable by the setting distance in the axial direction of the tube. The torch 2 is connected to a weaving device separately to the longitudinal feeding device 3, and the weaving is executed in the tube axial direction of the tube in a prescribed weaving width. And the torch 2 is moved to the axial direction by the longitudinal feeding device 3 corresponding to the weaving width at each one turning of the tube 1, thereby the weaving is again started from the moved position. By remelting the inner surface of the tube at high temp. of the plasma jet, the shrinkage cavity is eliminated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing a sink mark formed on the inner surface of a steel pipe when the steel pipe is formed by centrifugal casting.

[0002]

[Prior Art and Problems] Conventionally, by the centrifugal casting method,
Seamless pipes such as high alloy corrosion resistant steel and heat resistant steel are manufactured from carbon steel. In this centrifugal casting method, there are few restrictions on materials and thick pipes can be easily manufactured. It is also possible to manufacture a metallurgically joined bimetal tube by alternately layering and casting different alloys.

According to the centrifugal casting method, the solidification of the molten metal charged into the centrifugal casting mold progresses from the outer surface to the inner surface, during which a centrifugal force of about 70 to 200 G is applied to the molten metal to replenish the semi-solidified portion. To obtain a sound casting pipe.

The problem of the centrifugal casting method is that even if centrifugal force is applied to the innermost surface portion, the molten metal is not completely replenished, so that the inner surface portion has a sink mark, and the wall thickness of the pipe is large. As the thickness increases, the shrinkage layer on the inner surface of this tube tends to become thicker.

Generally, these unhealthy parts are removed by machining and used for piping pipes, etc., but the cutting margin of the machining reduces the yield of the material, resulting in high cost, and the inner layer is made of expensive high alloy. The effect is particularly great for bimetal pipes made of steel.

The present invention clarifies a method for treating the inner surface of a centrifugal cast steel pipe capable of improving the material yield by remelting and removing the sinkhole layer.

[0007]

According to the present invention, a steel pipe is formed by centrifugal casting, the inner surface of the steel pipe is remelted by a high energy heat source such as a plasma jet, a laser or an electron beam, and the steel pipe is subjected to centrifugal casting. Eliminates the burrows that occur on the inside.

[0008]

[Operation and effect] When the inner surface of the tube including the shrinkage cavity is remelted by a high-energy heat source such as a plasma jet, a laser, an electron beam, etc., the shrinkage cavity disappears. Since the melted portion is minute with respect to the entire tube, heat is taken by the surrounding unmelted portion and rapidly solidifies with a large thermal gradient. For this reason, the remelted portion becomes directional solidification that solidifies from a portion close to the inner surface of the mold, and no sink mark occurs.

The problem that the cutting allowance deteriorates the yield of the material as in the case of cutting and removing the burrs can be solved.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a situation in which the present invention is carried out by plasma powder welding. The centrifugal cast steel pipe (1) is horizontally supported by the rotation pedestal (5) and is connected to a rotation drive device (not shown) to rotate at a low speed.

The plasma powder welding torch (2) is connected to the vertical feeding device (3) and is disposed inside the tube with the nozzle port facing downward, and is movable by a set distance in the axial direction of the tube.

The torch (2) is connected to a weaving device (not shown) which is separate from the vertical feeding device (3), performs a weaving with a predetermined width in the tube axis direction, and the tube (1) makes one revolution. The torch (2) is moved in the axial direction by the vertical feeding device (3) corresponding to the width of the weaving each time, and the weaving is restarted from the moving position.

Powder hopper of plasma powder welding apparatus
In (4), powder of the same material as the inner surface of the tube is put in to stabilize the arc.

Specific data will be shown below. Base material (centrifugal cast steel 2-layer pipe) Inner layer components C 0.02%, Si 0.48%, Mn 0.32%, P 0.013%,
S 0.003% (however by weight) Outer diameter 183 mm Inner diameter 133 mm Inner layer thickness 10 mm Outer layer thickness 40 mm Plasma powder welding current 150 to 500 A Voltage 30 to 50 V Powder supply amount 5 to 10 g / min Plasma gas Ar Shield gas Ar Carrier gas Ar Torch weaving width 5 to 20 mm Welding speed (peripheral speed of inner surface of pipe) 90 mm / min

In order to prevent the plasma gas or the gas generated from the base material from being caught in the remelted layer, it is important to maintain the molten pool state for a long time. Therefore, the higher the current, the more The slower the relative movement between the torch and the base metal, the better.

The current was 500 A at maximum, depending on the capacity of the welding power source and the nozzle hole diameter. On the contrary, the lower limit of the current changes depending on the welding speed, but 100A or more is required to maintain a stable arc. Therefore, the current is 150 ~
500A is desirable.

The lower the voltage, the deeper the penetration. This is because the arc length becomes shorter when the voltage is lower, and therefore the force of the plasma jet emitted from the plasma torch is stronger.

However, if the voltage is too low, the distance from the molten pool becomes small, and there is a risk that the spatter will adhere to the torch due to the disorder of the molten pool. Usually, 30 V or more is necessary.

The diameter of the molten pool formed by the plasma gas under the above conditions is 15 to 20 mm, and the penetration depth is 2-1.
It was 0 mm.

However, when the inner surface of the tube is remelted by the high temperature of the plasma jet, the hollows disappear. Since the melted portion is minute with respect to the entire tube, heat is taken by the surrounding unmelted portion and rapidly solidifies with a large thermal gradient. For this reason, the remelted portion becomes directional solidification that solidifies from a portion close to the inner surface of the mold, and no sink mark occurs.

The present invention is not limited to the configuration of the above embodiment, and the same effect as above can be obtained by remelting the inner surface of the centrifugal cast steel pipe with a high energy heat source such as laser or electron beam. For example, various modifications are possible within the scope of the claims.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of an example.

[Explanation of symbols]

 (1) Cast steel pipe (2) Torch

Claims (1)

[Claims] 1. A steel pipe (1) is formed by centrifugal casting, the inner surface of the steel pipe is remelted by a high-energy heat source such as a plasma jet, a laser, an electron beam, etc., and is produced on the inner surface of the steel pipe during centrifugal casting. How to make a nest disappear.