JPS6035972B2 - Hardening method for the inner surface of steel pipes using plasma jet - Google Patents

Description

[Detailed description of the invention] The present invention relates to a method for hardening the inner surface of a steel pipe by remelting.

In order to harden the inner surface of a steel pipe or to improve defects, remelting of the inner surface of the pipe is carried out by carbon dioxide arc welding, TIC welding, or the like.

However, with the above welding method, deep penetration cannot be obtained and it is difficult to make the penetration depth uniform, resulting in inconsistent quality.
Furthermore, when using a consumable electrode type welding method, there is a problem in that if the welding rod is incorrectly selected, the desired hardness or improved results cannot be obtained. The present invention is a curing method that uses a plasma jet that can obtain deep remelting as a heat source for remelting, and by selecting the generation conditions of the plasma jet, a hardened layer can be formed on the inner surface of the tube, and defects in the tube can be improved. Regarding.

FIG. 1 shows a centrifugal steel tube 1 used for carrying out the present invention and a plasma jet torch 2 disposed within the tube so as to be movable in the vertical direction.

The Qiangang tube 1 is connected to a rotary drive device (not shown) to rotate at a low speed, and the torch 2 is connected to a vertical feed device (not shown) to move from one end of the tube to the other end with the nozzle opening facing downward. . In this example, the diameter of the nozzle opening is 3.2 to 6. He is an enemy. The torch 2 moves at a rate of one pitch for each rotation of the tube, where one pitch is the diameter length 41 of the melting part 4 where the inner surface of the tube 1 is melted by the plasma jet 3 ejected from the torch.

Here are the results of an experiment in which a plasma jet was injected from the torch 2 by changing various conditions such as plasma gas, shielding gas, plasma gas flow rate, current, voltage, etc., and the inner surface of the tube was gradually remelted from one end to the other. Shown in the table below.

The material of the tube is carbon steel, and the wall thickness is 40 ribs.

Experimental results In the results shown at the end of each example, (0) indicates that a good surface was obtained without blowholes, and (x) indicates that blowholes were generated.

As can be seen from the table, the shield gas composition is Ar containing a ratio of 5 to 10%, the plasma gas composition is pure Ar, and plasma is generated under the conditions of a plasma gas flow rate of 1.0 to 2.0 so/min and a current of 100 to 400 A. When a jet is generated, it blows onto the inner surface of the tube.
Good results were obtained, with no holes being generated, the arc being stable, and no spatter adhering to the torch. The diameter of the molten pool created by the plasma gas was 12 to 21 mm, and the penetration depth was 1 mm.

The reason for the numerical limitation will be described below. a Influence of shielding gas and plasma gas When shielding gas and plasma gas are both pure A (No.
．． 1) A blowhole occurred.

When 5% or more of H2 is added to the shielding gas, the temperature of the molten metal increases, allowing deeper penetration (No.2, No.2).
4). However, when day 2 becomes more than 20% (Ship 15), the arc becomes activated, which makes the molten pool more likely to be damaged, resulting in the occurrence of blowholes, and even after solidification, the bead surface becomes large and uneven. Not dirty. Therefore, it is preferable that the shielding gas composition is Ar containing 5 to 20% 2%, and the plasma gas composition is pure Ar.

The flow rate of plasma gas affects the penetration depth, and the flow rate is 1.0 so/
If it is less than 1 minute, deep penetration cannot be obtained.

Therefore, the plasma gas flow rate is preferably 1.0 to 2.0 cm/min. b Influence of heat input In order to prevent plasma gas or gas generated from the base material from being drawn into the remelted layer, it is important to maintain the state of the molten pool for a long time, and for this reason, the higher the current, the lower the Also, the slower the relative movement between the torch and the base material, the better.

The current is determined by the capacity of the welding power source and the nozzle hole diameter, so in this example, the nozzle hole diameter is 6 bars, and the maximum current is 40 m with a 50M power source.
It was M. Conversely, the lower limit of the current varied depending on the relative movement speed between the torch and the base metal, but was 100 A in order to maintain a stable arc.

Therefore, the current is preferably 100 to 40M.

The lower the voltage, the deeper the penetration.

This is because the lower the exposure pressure, the shorter the arc length, and therefore the stronger the force of the plasma jet emitted from the plasma torch. However, if the voltage is too low, the distance between the torch and the molten pool will become small, and the turbulence of the molten pool will cause a screen where spatter will adhere to the torch, so a voltage of 30 V or more is usually required.

In addition, as a result of investigating the relationship between the surface treatment of the tube before melting and the molten layer produced by the plasma jet, it was found that a healthy molten layer can be obtained as long as the tube is free of strong oxides such as slag from gas cutting. I was able to do it.

As described above, the present invention melts the inner surface of the tube using a plasma jet with deep penetration and a constant penetration depth.
The hardness of the solidified pipe surface after melting is constant, and even if any cavities occur, they disappear during melting, and defects in the pipe are improved, which has excellent effects.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing a state in which the torch is set. 1... Steel pipe, 2... Torch, 3...
・Plasma jet. Figure 1

Claims (1)

[Claims] 1 While rotating the steel pipe and moving the plasma jet torch placed inside the pipe in the axial direction of the steel pipe, the shield gas composition is Ar containing 5 to 20% H^2, the plasma gas composition is pure Ar, and the plasma gas composition is Flow rate 1.0-2.0l/min,
A method for hardening the inner surface of a steel pipe using a plasma jet, characterized in that a plasma jet generated at a current of 100 to 400 A is injected from the torch, and the opposing portions of the inner surface of the steel pipe facing the plasma jet are successively remelted. .