KR20190064815A - Method for manufacturing the pipe fitting having excellent low-temperature toughness in heat affected zone - Google Patents

Abstract

The present invention relates to a method for manufacturing a pipe fitting having a weld heat-affected part with low temperature toughness during high heat input welding using a high-strength heat-treated steel used in an offshore plant, a land plant, and the like. According to one aspect of the present invention, provided is the method for manufacturing a pipe fitting, wherein the process, specifically, the welding process can be optimized to form a welding part having a homogeneous texture, thereby improving productivity.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method for manufacturing a pipe fitting having a welded heat affected zone having excellent low temperature toughness,

The present invention relates to a method for manufacturing a pipe fitting having a weld heat affected zone, which is excellent in low temperature toughness during high heat treatment using high-strength heat-treated steel used for offshore plants and onshore plants.

Pipe fittings are piping materials that are used to change the direction of piping or change the pipe diameter of a power plant, shipyard, or chemical plant to branch piping from the main piping, and are processed through a die such as a pipe and an iron plate.

These pipe fittings are mainly Elbow, Tee, Reducer, Cap, and are defined as follows (see FIG. 1).

The elbow is used to change the length of the pipe and the direction of the pipe. In general, 45 ° elbow, 90 ° elbow, 180 ° elbow and 60 ° elbow are commonly used. It can be manufactured using various materials such as cast iron, stainless steel, alloy steel, carbon steel, high performance steel, nonferrous metal and plastic.

Tee, one of the industrial pipe joints, is used when separating or joining flowing fluid on the way. Especially, it is used in electric power, petroleum, natural gas, chemical industry, shipyard, heating, metallurgy, etc.

Reducer is a joint part used to connect pipes with different diameters. It is mainly classified into concentric reducers used for vertical pipes and eccentric reducers used for horizontal pipes. They are used at the pump inlet to change the flow of the fluid or prevent cavitation of the pump, and can also be used as a connection device for measuring instruments such as flowmeters and control valves.

A cap is a kind of piping material that is used to close a pipe end or a pipe hole in piping construction. It is mainly used in medical facilities, petrochemical, shipbuilding, machining, plumbing and waterproofing.

The pipe fittings are manufactured through such processes as cutting, heat treatment, welding, hot forming, heat treatment (for example, Q & T heat treatment and normalizing heat treatment) of a general high strength steel.

At this time, seamless pipes are produced without welding process, but most of them are manufactured using butt-welding.

However, butt-welding is a method of welding two surfaces that are in contact with each other at a constant angle, and is mainly used in gas metal arc welding (GMAW), flux cored arc welding (FCAW), submerged arc welding SMW) have been applied.

Such a welding process is a multi-layer welding method, which requires a long manufacturing time in manufacturing a pipe fitting, and also causes inhomogeneous materials in the welding metal occurring in the multi-layer welding, thereby preventing the welding metal and the fusing line There is a possibility that the mechanical properties are deteriorated.

Accordingly, there is an increasing demand for improving the welding process and improving the efficiency in manufacturing the pipe fitting.

Korean Patent Publication No. 2007-0022408

An aspect of the present invention is to provide a manufacturing method of a pipe fitting capable of forming a welded portion having a homogeneous structure by optimizing the process, particularly a welding process, when manufacturing a pipe fitting, and improving productivity.

An aspect of the present invention provides a method of manufacturing a pipe joint, comprising the steps of: a) preparing, cutting and rolling a steel material for a pipe fitting; b) welding the cut and rolled steel material to produce a pipe; c) Stress relief processing the pipe; d) hot-molding the SR-treated pipe; And e) quenching and tempering after the hot forming,

Wherein the welding is performed in a one-pass manner using an electro-gas arc welding. The present invention also provides a method of manufacturing a pipe fitting having a weld heat affected zone (HAZ) excellent in low temperature toughness.

According to the present invention, manufacturing productivity can be improved to 50 to 80% or more as compared with manufacturing a pipe fitting by a conventional method.

Further, by forming a weld heat affected zone having a homogeneous structure, excellent low temperature toughness can be secured.

Fig. 1 shows the kind of pipe fitting.
2 shows a process of manufacturing a pipe fitting according to an embodiment of the present invention.
FIG. 3 is a photomicrograph of a microstructure of a weld heat affected zone (HAZ) after quenching and tempering heat treatment according to an embodiment of the present invention.
FIG. 4 shows the result of measuring the low temperature impact energy of a weld heat affected zone (HAZ) after quenching and tempering heat treatment according to an embodiment of the present invention.

Normally, a pipe is first made for manufacturing a pipe fitting. The pipe can be manufactured by various processes, for example, a welding pipe through L-Seam welding (Longitudinal Seam Welding). The L-Seam welding is mainly accomplished by multi-layer welding using a welding process such as gas metal arc welding (GMAW), flux cored arc welding (FCAW), and submerged arc welding (SAW). The welded pipe produced through such a welding process is hot-formed to be transformed into the intended pipe fitting shape. The finished pipe fittings are quenched, tempered, or normalized according to the material to obtain the final material.

Thereafter, the product is subjected to non-destructive inspection painting, beveling, and calibration before shipment.

Thus, there is a problem in that the manufacturing process is very low due to the complicated process of producing the pipe fitting currently known, and the material variation of the final product frequently occurs due to the heat treatment process.

Accordingly, the present inventors have made intensive researches to improve the production efficiency of pipe fittings and at the same time to produce a final product having a uniform material.

As a result, it has been confirmed that the 1-pass welding process is applied to obtain the welded pipe used for pipe fitting, thereby realizing the improvement of the productivity and the homogenization and fineness of the material by optimizing the heat treatment process. And has reached the completion of the invention.

Hereinafter, the present invention will be described in detail.

A method of manufacturing a pipe fitting according to an aspect of the present invention will be described in detail later, but includes the steps of: a) preparing, cutting and rolling a steel material for a pipe fitting; b) welding the cut and rolled steel material to produce a pipe; c) SR treating the pipe; d) hot-molding the SR-treated pipe; And e) quenching and tempering heat treatment after the hot forming (refer to FIG. 2).

First, the material for obtaining the intended pipe fitting in the present invention, that is, the steel material for a pipe fitting, may be a steel material usually used for manufacturing a pipe fitting, and is not particularly limited.

(Excluding 0%), phosphorus: 0.03% or less, sulfur: 0.01% or less, and the like. Or less, and the balance Fe and other unavoidable impurities (for example, ASTM A960, A860, etc.). The high carbon carbon steel may further include Cr, Ni, Mo, Cu, Ti, and V in addition to the alloy composition described above, but is not limited thereto.

After the above-mentioned steel material is prepared, it can be cut and rolled to an appropriate size for welding with a pipe.

It is then preferable to weld the cut and rolled steel material and make it into a pipe.

At this time, it is preferable that the welding is performed by butt-welding. In one aspect of the present invention, it is preferable to apply an electro-gas arc welding (EGW) process.

In the present invention, it is preferable to perform 1-pass in the welding by the EGW process.

The heat input during the 1-pass welding can be defined through the thickness of the steel material, the groove angle, and the root gap, and can be expressed in detail by the following relationship. That is, the amount of the heat of welding is proportional to the thickness of the material, and can be appropriately selected according to the thickness of the tube material to be manufactured by a general engineer through the following relationship. In the present invention, it can be performed within a range of +/- 10% based on the amount of heat of welding (H.I.) derived from the following relational expression.

HI = 0.0289 x T x (G + t x tan ((? / 2)? / 180)) + 3.2246

H.I: heat input

T: thickness (material thickness)

G: Root Gap

tan: tangent

θ: groove angle (improvement angle)

When 1-pass welding is performed at the welding heat amount obtained according to the above, the average size of the old austenite grain size may be formed to be 250 to 300 탆.

As described above, relatively coarser crystal grains can be formed by 1-pass welding, but the desired properties can be obtained by enabling the grain refinement by the subsequent heat treatment step as described later.

A pipe having the weld heat affected portion described above may be subjected to SR (stress relief) treatment and hot formed to produce a desired shape.

The SR (Stress Relief) process is a process for removing the internal residual stress generated in the welding process, and can be performed according to the ASME standard for each steel type or thickness. Preferably at a temperature lower than the recrystallization temperature, that is, at a temperature range in which the mechanical properties do not change.

The hot forming can be performed by mounting the SR-treated pipe to a mold and then compressing the pipe. For example, a tube may be formed by heating at a temperature of about 900 ° C or higher and pressing to form a tube, but the present invention is not limited thereto.

The mold is preferably selected according to the shape of the pipe fitting to be manufactured.

After completion of the hot forming in the intended shape according to the above, it is preferable to quench and temper heat treatment the manufactured pipe fitting.

 The heat treatment process is for recrystallizing and refining the microstructure of the weld heat affected zone which is formed in the welding process. That is, according to the present invention, a welded heat affected zone obtained by the 1-pass EGW process has an austenite diameter of 250 to 300 μm and a coarse texture is formed. By heat-treating the hot-formed pipe joint having such a weld heat affected zone, Recrystallization and micronization.

The quenching step is preferably carried out in a temperature range of 910 to 950 DEG C for the entire austenitization. If the temperature is lower than 910 ° C, the entire austenitization may not be performed and the final structure may be uneven. If the temperature exceeds 950 ° C, coarse austenite is formed and the effective ferrite grain size after cooling may increase . Coarsening of the effective ferrite grain size is undesirable because it causes deterioration of impact toughness.

After completing the quenching process, the weld heat affected zone forms a martensitic structure, which can remove internal stresses from subsequent tempering processes and soften the structure.

In the present invention, the tempering process is performed in order to reduce the brittleness of the steel cured by the martensite produced after quenching and to improve the toughness. Through the tempering process, residual stress of the carbide generated during quenching, segregation of the impurities, Deformation and the like can be removed.

The tempering step is preferably performed at a temperature range of 590 to 670 캜. If the temperature is higher than 670 DEG C, there is a problem that the strength and hardness are lowered. On the other hand, when the temperature is lower than 590 DEG C, there is a problem that the toughness is lowered.

When the quenching and tempering process is completed after 1-pass welding according to the present invention, the welding heat affected zone has a fine average austenite size of 50 to 100 탆 and a homogeneous structure with the base metal So that the low-temperature toughness can be ensured excellently.

More specifically, the weld heat affected zone includes tempered martensite and tempered bainite phase as a pillar phase, and may include a small amount of ferrite. Preferably, the ferrite may have an area fraction of 20% or less.

FIG. 3 is a graph showing the relationship between the weld heat affected zones FL (a), FL + 1 mm (b), FL + 2 mm (c) and BM (Base Metal) after the quenched and tempered heat treatment of the hot- d)) is the result of observing the microstructure.

As shown in FIG. 3, the microstructure of the fusion line (FL) formed relatively coarsely by performing the 1-pass high-temperature heat welding (EGW) with the heat input amount of 140 kJ / cm 2 is recrystallized through the heat treatment process It can be confirmed that they are formed homogeneously and finely.

FIG. 4 shows the result of measuring the low temperature impact energy of the weld heat affected zone (FL, FL + 1 mm, FL + 2 mm) of FIG. 3, and it is possible to secure an impact energy of 180 J or more even at a very low temperature of -40 ° C. can confirm.

Claims (5)

a) preparing and cutting and rolling a steel material for a pipe fitting;
b) welding the cut and rolled steel material to produce a pipe;
c) Stress relief processing the pipe;
d) hot-molding the SR-treated pipe; And
e) quenching and tempering after the hot forming step,
Wherein the welding is carried out by one-pass using an electro-gas arc welding. 2. The method of manufacturing a pipe fitting according to claim 1, wherein the welding is performed by one-pass using an electro-gas arc welding.
The method according to claim 1,
Wherein the weld heat affected zone formed after welding has a weld heat affected zone (HAZ) having excellent low temperature toughness with austenite particle size of 250 to 300 mu m.
The method according to claim 1,
Wherein the quenching is performed in a temperature range of 910 to 950 占 폚.
The method according to claim 1,
Wherein said tempering is carried out at a temperature range of 590 to 670 캜, wherein said hot heat affected zone (HAZ) is excellent in low temperature toughness.
The method according to claim 1,
Wherein the weld heat affected zone after quenching and tempering has a weld heat affected zone (HAZ) having excellent low temperature toughness with a spherical austenite particle diameter of 50 to 100 占 퐉.

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