JP2021178994A - Fe-BASED ALLOY PIPE AND WELD JOINT - Google Patents

Abstract

To provide an Fe-based alloy pipe that can prevent cracking near a bead on the inner side of the pipe without the need for heat treatment after welding, and a weld joint.SOLUTION: An Fe-based alloy pipe has a chemical composition consisting of, in mass%, C: 0.040-0.100%, Si: 0.10-1.00%, Mn: 0.60-1.60%, P: 0.030% or less, S: 0.0001-0.0015%, Ni: 29.5-35.5%, Cu: 0.01-0.75%, Co: 0.01-1.00%, Cr: 19.0-23.0%, Mo: 0.01-1.00%, Ti: 0.10-0.60%, N: 0.0010-0.0150%, Al: 0.10-0.60%, O: 0.0004-0.0150%, with the balance being Fe and impurities, and satisfies [0.0010≤S+2O≤0.0280].SELECTED DRAWING: Figure 1

Description

The present invention relates to Fe-based alloy pipes and welded joints.

In chemical plants, power plants and energy transport equipment, Fe-based alloys, which are relatively inexpensive and have good high-temperature strength and corrosion resistance, are used. For example, Patent Documents 1 to 3 disclose Fe-based alloys having improved high-temperature strength and corrosion resistance.

Japanese Unexamined Patent Publication No. 5-98397 Japanese Unexamined Patent Publication No. 6-145913 International Publication No. 2006/003953

Some plant equipment is manufactured by assembling members by welding. An example is a heater. The heater is provided with a plurality of heater tubes, and is assembled by butt welding the tubes together at the time of manufacturing. Then, after welding, when the heater tube is used at a high temperature, cracks called stress relaxation cracks may occur in the vicinity of the bead on the inner surface side of the tube, that is, in the weld heat affected zone.

It is believed that stress relaxation cracks are caused by residual stress remaining in the weld heat affected zone. Therefore, in order to remove residual stress and suppress stress relaxation cracking, heat treatment is generally performed after welding. However, the heat treatment after welding increases the number of processes, which leads to an increase in manufacturing cost. Further, depending on the structure of the plant equipment and the place where welding is performed, it may not be possible to effectively perform the heat treatment after welding.

Based on the above, it is an object of the present invention to solve the above-mentioned problems and to provide an Fe-based alloy pipe and a welded joint capable of suppressing cracks generated in the vicinity of the inner bead of the pipe without performing heat treatment after welding. And.

The present invention has been made to solve the above problems, and the gist of the present invention is the following Fe-based alloy pipe and welded joint.

(1) The chemical composition is mass%.
C: 0.040 to 0.100%,
Si: 0.10 to 1.00%,
Mn: 0.60 to 1.60%,
P: 0.030% or less,
S: 0.0001 to 0.0015%,
Ni: 29.5 to 35.5%,
Cu: 0.01-0.75%,
Co: 0.01-1.00%,
Cr: 19.0 to 23.0%,
Mo: 0.01-1.00%,
Ti: 0.10 to 0.60%,
N: 0.0010 to 0.0150%,
Al: 0.10 to 0.60%,
O: 0.0004 to 0.0150%,
Remaining: Fe and impurities,
An Fe-based alloy tube satisfying the following formula (i).
0.0010 ≤ S + 2O ≤ 0.0280 ... (i)
However, the element symbol in the above formula represents the content (mass%) of each element contained in the Fe-based alloy, and if it is not contained, it is set to zero.

(2) In the region from the tube end of the Fe-based alloy tube to the position of 50 mm in the axial direction, the maximum wall thickness difference Dw calculated by the following equation (ii) is the following equations (iii) and (iv). The Fe-based alloy tube according to (1) above, which is satisfied.
Dw = D _MAX- D _min ... (ii)
Dw / D _MAX × 100 ≦ 20 ・ ・ ・ (iii)
Dw ≤ 1.6 ・ ・ ・ (iv)
However, each symbol in the above formula is defined as follows.
D _MAX (mm): Maximum wall thickness D _min (mm): Minimum wall thickness

(3) The chemical composition is, instead of a part of the Fe, by mass%.
The Fe-based alloy tube according to (1) or (2) above, which contains Sn: 0.030% or less.

(4) The chemical composition is, instead of a part of the Fe, by mass%.
W: 1.00% or less,
V: 0.40% or less,
Nb: 0.40% or less,
Ta: 0.40% or less,
Ca: 0.0100% or less,
Mg: 0.0100% or less,
B: 0.0100% or less, and REM: 0.0800% or less,
The Fe-based alloy tube according to any one of (1) to (3) above, which contains one or more selected from the above.

(5) A welded joint using the Fe-based alloy pipe according to any one of (1) to (4) above.

According to the present invention, it is possible to obtain an Fe-based alloy pipe and a welded joint capable of suppressing cracks generated in the vicinity of the inner bead of the pipe without performing heat treatment after welding.

FIG. 1 is a diagram showing a groove shape in an embodiment.

The present inventor investigated stress relaxation cracks in Fe-based alloy tubes and obtained the following findings (a) to (c).

(A) Cracks that occur when the pipe is used at a high temperature are likely to occur in the weld heat affected zone near the bead toe. The reason for this is due to the following mechanism. During welding, the molten metal solidifies, which makes it easier for residual stress to accumulate in the weld heat-affected zone. As a result, when the pipe is used at a high temperature, the accumulated residual stress is released and cracks occur.

(B) The above cracks are particularly likely to occur at grain boundaries. When the present inventors observed the fracture surface where cracks occurred, it became clear that S was concentrated at the grain boundaries. This is because S is an element having a high segregation energy. In addition, S segregates at the grain boundaries due to diffusion during the welding thermal cycle and use at high temperatures, reducing the binding force at the grain boundaries.

The higher the content of S, the more likely it is to occur. Therefore, it seems desirable to reduce the S content in order to suppress cracking. On the other hand, S has the effect of stably forming beads and reducing welding defects such as cracks. The reason for this is that S strengthens the inward convection in the molten pool during welding, transports heat from the arc in the depth direction, and stably forms the bead. Therefore, in order to form a stable bead while suppressing the concentration of S, it is preferable to contain O and S, which contribute to the formation of a stable bead, in a predetermined amount as in the case of S.

(C) Further, cracks are more likely to occur when the left-right step (hereinafter referred to as "irregular bead") of the butt portion of the bead becomes large. In this case, the distribution of residual stress due to welding becomes non-uniform, and local stress concentration tends to occur. Then, the residual stress at the toe of the bead and the weld heat affected zone becomes large. As a result, cracks are likely to occur.

By the way, in Fe-based alloy pipes used for heater pipes and the like, it is industrially difficult to eliminate variations in wall thickness. Therefore, in order to suppress cracking due to irregularity of the bead, it is desirable to reduce the difference in wall thickness, specifically, the difference between the maximum wall thickness and the minimum wall thickness in the pipe.

The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be described in detail.

1. 1. Chemical composition of Fe-based alloy tube The reasons for limiting each element are as follows. In the following description, "%" for the content means "mass%".

C: 0.040 to 0.100%
C has the effect of improving tissue stability and high temperature strength. Therefore, the C content is set to 0.040% or more. The C content is preferably 0.050% or more, and more preferably 0.060% or more. However, when C is excessively contained, it is combined with Cr by the welding heat cycle to form carbides at the grain boundaries of the weld heat affected zone. As a result, a Cr-deficient layer is formed in the vicinity of the grain boundaries, and the corrosion resistance is lowered. Therefore, the C content is set to 0.100% or less. The C content is preferably 0.090% or less, more preferably 0.080% or less.

Si: 0.10 to 1.00%
Si has a deoxidizing effect at the time of manufacture. It is also effective in improving corrosion resistance at high temperatures. Therefore, the Si content is set to 0.10% or more. The Si content is preferably 0.15% or more, and more preferably 0.20% or more. However, if Si is contained in an excessive amount, the structure stability of the alloy is lowered and the liquefaction cracking sensitivity of the heat-affected zone is not a little increased during welding. In addition, it may be difficult to stably form the bead on the inner surface side. Therefore, the Si content is set to 1.00% or less. The Si content is preferably 0.90% or less, more preferably 0.80% or less.

Mn: 0.60 to 1.60%
Mn has a deoxidizing effect like Si. It also has the effect of enhancing tissue stability and contributes not a little to the stable formation of beads on the inner surface side. Therefore, the Mn content is set to 0.60% or more. The Mn content is preferably 0.70% or more, more preferably 0.80% or more. However, if Mn is contained in an excessive amount, the hot workability is deteriorated. Therefore, the Mn content is set to 1.60% or less. The Mn content is preferably 1.55% or less, more preferably 1.50% or less.

P: 0.030% or less P is contained in steel as an impurity and significantly increases the liquefaction crack sensitivity of the weld heat affected zone during welding. Therefore, the P content is set to 0.030% or less. The content of P is preferably 0.028% or less, more preferably 0.025% or less. The P content is preferably reduced as much as possible, but excessive reduction increases manufacturing costs. Therefore, the P content is preferably 0.001% or more, and more preferably 0.002% or more.

S: 0.0001 to 0.0015%
S segregates at the grain boundaries of the weld heat affected zone during the weld heat cycle and use at high temperatures, weakening the grain boundaries. As a result, cracks occur at the grain boundaries near the bead on the inner surface side of the tube during use at a high temperature. Therefore, the S content is set to 0.0015% or less. The S content is preferably 0.0012% or less, more preferably 0.0010% or less. However, excessive reduction of S content significantly increases manufacturing costs. In addition, in the alloy pipe of the present invention, S, together with O, contributes to enhancing the ability to form the inner bead during welding. Therefore, the S content is preferably 0.0001% or more, more preferably 0.0002% or more, and further preferably 0.0003% or more. It is necessary that S satisfies the equation (i) with O described later.

Ni: 29.5 to 35.5%
Ni has the effect of increasing tissue stability and improving high temperature strength. It also has the effect of improving stress corrosion cracking resistance in a chloride environment. Therefore, the Ni content is set to 29.5% or more. The Ni content is preferably 29.7% or more, more preferably 30.0% or more. However, since Ni is an expensive element, if it is contained in an excessive amount, the production cost increases. Therefore, the Ni content is set to 35.5% or less. The Ni content is preferably 35.3% or less, more preferably 35.0% or less.

Cu: 0.01-0.75%
Cu enhances tissue stability, is effective in improving high-temperature strength, and has an effect of improving corrosion resistance in a chloride environment. Therefore, the Cu content is 0.01% or more. The Cu content is preferably 0.03% or more, more preferably 0.05% or more. However, if Cu is contained in an excessive amount, the hot workability is deteriorated. Therefore, the Cu content is set to 0.75% or less. The Cu content is preferably 0.70% or less, more preferably 0.60% or less, and even more preferably 0.50% or less.

Co: 0.01-1.00%
Co enhances tissue stability and is effective in improving high-temperature strength. Therefore, the Co content is 0.01% or more. The Co content is preferably 0.02% or more, more preferably 0.03% or more. However, since Co is a very expensive element, if it is contained in an excessive amount, the production cost will increase significantly. Therefore, the Co content is set to 1.00% or less. The Co content is preferably 0.90% or less, more preferably 0.80% or less.

Cr: 19.0 to 23.0%
Cr has an effect of improving corrosion resistance at high temperature and normal temperature. Therefore, the Cr content is set to 19.0% or more. The Cr content is preferably 19.2% or more, more preferably 19.5% or more. However, if Cr is contained in an excessive amount, the tissue stability is lowered and the strength is also lowered. Therefore, the Cr content is set to 23.0% or less. The Cr content is preferably 22.8% or less, more preferably 22.5% or less.

Mo: 0.01-1.00%
Mo has the effect of dissolving in a substrate to increase high-temperature strength. Therefore, the Mo content is 0.01% or more. The Mo content is preferably 0.03% or more, and more preferably 0.05% or more. However, if Mo is contained in an excessive amount, the tissue stability is lowered and the strength is also lowered. Moreover, since Mo is an expensive element, the manufacturing cost increases. Therefore, the Mo content is set to 1.00% or less. The Mo content is preferably 0.90% or less, and more preferably 0.80% or less.

Ti: 0.10 to 0.60%
Ti forms carbides and contributes to the improvement of high temperature strength. Further, Ti suppresses the formation of Cr carbides and reduces the deterioration of corrosion resistance at the grain boundaries. Therefore, the Ti content is set to 0.10% or more. The Ti content is preferably 0.20% or more, and more preferably 0.30% or more. However, if Ti is contained in an excessive amount, a large amount of carbides and carbonitrides of Ti are precipitated, and the ductility is lowered. Therefore, the Ti content is set to 0.60% or less. The Ti content is preferably 0.55% or less, more preferably 0.50% or less.

N: 0.0010 to 0.0150%
N enhances tissue stability and contributes to improvement of high temperature strength. Therefore, the N content is set to 0.0010% or more. The N content is preferably 0.0020% or more, and more preferably 0.0030% or more. However, if N is contained in an excessive amount, a nitride is precipitated and the ductility is lowered. Therefore, the N content is set to 0.0150% or less. The N content is preferably 0.0130% or less, more preferably 0.0100% or less.

Al: 0.10 to 0.60%
Al has a deoxidizing effect and also contributes to improvement of oxidation resistance at high temperatures. Therefore, the Al content is set to 0.10% or more. The Al content is preferably 0.20% or more, and more preferably 0.30% or more. However, if Al is contained in an excessive amount, Al binds to oxygen, and the cleanliness of the surface is lowered. As a result, the hot workability is lowered. In addition, it may be difficult to stably form the bead on the inner surface side. Therefore, the Al content is set to 0.60% or less. The Al content is preferably 0.58% or less, more preferably 0.55% or less.

O: 0.0004 to 0.0150%
O is an element generally contained in steel as an impurity, but in the alloy pipe of the present invention, it has an effect of enhancing the ability to form an inner bead at the time of welding together with S. Therefore, the O content is set to 0.0004% or more. The O content is preferably 0.0006% or more, and more preferably 0.0008% or more. However, if O is contained in an excessive amount, the bead on the inner surface side of the tube hangs down and tends to have a convex shape. As a result, it becomes difficult to form a stable bead. In addition, hot workability is also reduced. Therefore, the O content is set to 0.0150% or less. The O content is preferably 0.0120% or less, more preferably 0.0100% or less.

As described above, since S and O effectively contribute to the formation of beads on the inner surface side of the pipe, the Fe-based alloy according to the present invention has the following relational expression between the S content and the O content ( i) It is necessary to satisfy the equation.

0.0010 ≤ S + 2O ≤ 0.0280 ... (i)
However, the element symbol in the above formula represents the content (mass%) of each element contained in the Fe-based alloy, and if it is not contained, it is set to zero.

S and O are surface active elements and have an action of strengthening inward convection in the molten pool during welding. By transporting the welding heat in the depth direction, the bead on the inner surface side is stably formed, and this effect is obtained when the middle value of the formula (i) is less than 0.0010%. I can't. Therefore, the middle value of the formula (i) is set to 0.0010% or more. The middle value of the formula (i) is preferably 0.0012% or more, and more preferably 0.0015% or more.

On the other hand, when the middle value of the formula (i) exceeds 0.0280%, the surface tension of the molten metal becomes small and sagging occurs. As a result, the bead becomes convex and the extra height becomes high. Then, stress is easily concentrated on the toe, and the crack sensitivity of the Fe-based alloy tube is increased when used at a high temperature. Therefore, the middle value of the formula (i) is set to 0.0280% or less. The middle value of the formula (i) is preferably 0.0260% or less, and more preferably 0.0240% or less.

In the chemical composition, in addition to the above elements, Sn may be further contained in the range shown below.

Sn: 0.030% or less Sn has the effect of increasing the penetration depth and enhancing the ability to form the inner bead. Therefore, it may be contained as needed. However, if Sn is contained in an excessive amount, the hot workability is deteriorated. It also enhances the liquefaction crack sensitivity of the weld heat affected zone during welding. Therefore, the Sn content is 0.030% or less. The Sn content is preferably 0.020% or less, more preferably 0.010% or less. On the other hand, in order to obtain the above effect, the Sn content is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.003% or more.

In the chemical composition, in addition to the above elements, W, V, Nb, Ta, Ca, Mg, B, and one or more selected from REM may be further contained in the range shown below. The reason for limiting each element will be described.

W: 1.00% or less W has the effect of dissolving in the substrate and increasing the high temperature strength. Therefore, it may be contained as needed. However, excessive inclusion of W reduces tissue stability. Further, since W is an expensive element, the manufacturing cost increases. Therefore, the W content is set to 1.00% or less. The W content is preferably 0.80% or less, more preferably 0.60% or less. On the other hand, in order to obtain the above effect, the W content is preferably 0.01% or more, more preferably 0.02% or more.

V: 0.40% or less V has the effect of combining with carbon to form carbides and improving high temperature strength. It also suppresses the formation of Cr carbides. As a result, deterioration of corrosion resistance is reduced at the grain boundaries. Therefore, it may be contained as needed. However, if V is contained in an excessive amount, a large amount of carbides and carbonitrides of V are precipitated, and the ductility is lowered. Therefore, the V content is set to 0.40% or less. The V content is preferably 0.35% or less, and more preferably 0.30% or less. On the other hand, in order to obtain the above effect, the V content is preferably 0.01% or more, more preferably 0.02% or more.

Nb: 0.40% or less Nb, like V, has the effect of combining with carbon to form carbides and improving high-temperature strength. It also suppresses the formation of Cr carbides. As a result, deterioration of corrosion resistance is reduced at the grain boundaries. Therefore, it may be contained as needed. However, if Nb is contained in an excessive amount, a large amount of carbides and carbonitrides of Nb are precipitated, and the ductility is lowered. Furthermore, the sensitivity to weld cracks is also increased. Therefore, the Nb content is set to 0.40% or less. The Nb content is preferably 0.35% or less, and more preferably 0.30% or less. On the other hand, in order to obtain the above effect, the Nb content is preferably 0.01% or more, and more preferably 0.02% or more.

Ta: 0.40% or less Ta, like V and Nb, also has the effect of combining with carbon to form carbides and improving high temperature strength. Therefore, it may be contained as needed. However, if Ta is contained in an excessive amount, a large amount of carbides of Ta are precipitated and the ductility is lowered. Therefore, the Ta content is set to 0.40% or less. The Ta content is preferably 0.35% or less, more preferably 0.30% or less. On the other hand, in order to obtain the above effect, the Ta content is preferably 0.01% or more, and more preferably 0.02% or more.

Ca: 0.0100% or less Ca has the effect of improving hot workability. Therefore, it may be contained as needed. However, if Ca is contained in an excessive amount, Ca binds to oxygen and the cleanliness is significantly reduced. As a result, the hot workability is rather lowered. Therefore, the Ca content is 0.0100% or less. The Ca content is preferably 0.0080% or less, more preferably 0.0060% or less. On the other hand, in order to obtain the above effect, the Ca content is preferably 0.0010% or more, and more preferably 0.0020% or more.

Mg: 0.0100% or less Mg has the effect of improving hot workability like Ca. Therefore, it may be contained as needed. However, if Mg is contained in an excessive amount, it will be combined with oxygen and the cleanliness will be significantly reduced. As a result, the hot workability is rather lowered. Therefore, the Mg content is 0.0100% or less. The Mg content is preferably 0.0080% or less, more preferably 0.0060% or less. On the other hand, in order to obtain the above effect, the Mg content is preferably 0.0010% or more, more preferably 0.0020% or more.

B: 0.0100% or less B has the effect of segregating at the grain boundaries at high temperatures, strengthening the grain boundaries, and enhancing hot workability. Therefore, it may be contained as needed. However, if B is contained in an excessive amount, the liquefaction crack sensitivity of the weld heat-affected zone is increased during welding. Therefore, the B content is 0.0100% or less. The B content is preferably 0.0080% or less, more preferably 0.0060% or less. On the other hand, in order to obtain the above effect, the B content is preferably 0.0002% or more, more preferably 0.0005% or more.

REM: 0.0800% or less REM, like Ca and Mg, has the effect of improving hot workability during manufacturing. Therefore, it may be contained as needed. However, excessive content of REM binds to oxygen and significantly reduces cleanliness. As a result, the hot workability is rather lowered. Therefore, the REM content is 0.0800% or less. The REM content is preferably 0.0600% or less, more preferably 0.0500% or less. On the other hand, in order to obtain the above effect, the REM content is preferably 0.0005% or more, and more preferably 0.0010% or more. Here, REM refers to Sc, Y and lanthanoids, and the REM content indicates the total content of these elements.

In the chemical composition of the present invention, the balance is Fe and impurities. Here, the "impurity" is a component mixed by various factors of raw materials such as ore and scrap, and various factors in the manufacturing process when the Fe-based alloy is industrially manufactured, and is a component that does not adversely affect the present invention. Means what is acceptable.

2. 2. Thickness of Fe-based alloy tube In the Fe-based alloy tube according to the present invention, for example, a mandrel or the like is inserted into a round billet and hot-extruded to produce a hollow raw tube, which is manufactured. .. However, not limited to this method, when manufacturing a pipe, it is difficult to make the same wall thickness in all parts of the pipe due to factors such as processing accuracy, and the wall thickness tends to vary. Therefore, even in one alloy pipe, a difference in wall thickness occurs depending on the portion.

If there is a difference in wall thickness, even if pipes of the same nominal size are butt-welded to manufacture a welded joint, cracks are likely to occur when the pipe is used at a high temperature. This is due to the formation of a step at the weld toe due to the difference in wall thickness at the end of the pipe to be welded, that is, the irregularity of the bead. When the irregularity of the bead becomes large, the residual stress distribution due to welding becomes non-uniform, local stress concentration occurs, and the stress remaining at one bead toe becomes large. As a result, cracks are likely to occur when the tube is used at high temperatures.

In order to suppress irregularity of the bead, when the pipes are butted against each other, the pipes may be rotated to adjust the butting position so that the difference in wall thickness between the pipes becomes small. However, it is not easy to rotate a long pipe, and the construction efficiency is significantly reduced. Similarly, adjusting the groove shape and welding conditions may also suppress bead irregularities, but this is not desirable from the viewpoint of productivity.

Therefore, in order to reduce the difference in wall thickness between pipes and suppress irregularity of beads, it is desirable to reduce the variation in wall thickness in one pipe. Therefore, the maximum wall thickness difference Dw calculated by the following equation (ii) satisfies the following equations (iii) and (iv) in the region from the tube end of the Fe-based alloy tube to the position of 50 mm in the axial direction. It is preferable to do.

Dw = D _MAX- D _min ... (ii)
Dw / D _MAX × 100 ≦ 20 ・ ・ ・ (iii)
Dw ≤ 1.6 ・ ・ ・ (iv)
However, each symbol in the above formula is defined as follows.
D _MAX (mm): Maximum wall thickness D _min (mm): Minimum wall thickness

As described above, when the wall thickness of the pipe is measured, the measured thickest wall thickness is defined as the maximum wall thickness D _MAX, and the thinnest wall thickness is defined as the minimum wall thickness D _min . Then, the relationship between the maximum wall thickness difference Dw and the maximum wall thickness D _{MAX , which is the difference between the maximum wall thickness D MAX} and the minimum wall thickness D _min , calculated by the formula (ii), is the above formula (iii). It is preferable to satisfy. This is because when the lvalue in equation (iii) exceeds 20%, irregularities in the beads are likely to occur. The smaller the rvalue in equation (iii), the more preferable.

In addition, it is preferable that the maximum wall thickness difference Dw satisfies the equation (iv). This is because if the maximum wall thickness difference Dw exceeds 1.6 mm, it becomes difficult to butt the groove surfaces and welding work becomes difficult. The smaller the maximum wall thickness difference Dw, the more preferable.

The Fe-based alloy pipe according to the present invention can be used, for example, in a welded joint. In an Fe-based alloy pipe for a welded joint, the pipe ends of the alloy pipe cut for the purpose of size adjustment or the like may be abutted against each other and welded. In this case, the pipe end portion of the cut alloy pipe, that is, the portion abutted at the time of welding is the "pipe end portion" defined in the present invention.

If at least only the end of the pipe satisfies the equations (iii) and (iv), the desired effect in the present invention can be obtained, but (iii) and (iv) over the entire length and circumference of the alloy pipe. ) Satisfies the provisions of the present invention regardless of where the alloy tube is cut. Therefore, it is more preferable to satisfy the equations (iii) and (iv) over the entire length and circumference of the alloy tube.

Here, the wall thickness of the tube may be measured by using ultrasonic flaw detection. Measurement may be performed using a measuring instrument such as a wall thickness gauge, but the measuring method is not limited to these.

3. 3. Welded joint A welded joint of the Fe-based alloy pipe can be obtained by welding the pipe end of the Fe-based alloy pipe under predetermined conditions. The welded joint of the Fe-based alloy pipe has a welded metal formed as a joint portion by solidifying the molten metal and a base metal portion. The base metal portion includes a welding heat-affected zone that is affected by heat input due to welding. The base metal portion excluding the weld heat-affected zone inherits the chemical composition, metal structure, and other characteristics of the Fe-based alloy pipe described in items 1 and 2 above.

4. Manufacturing Method A preferred manufacturing method for the Fe-based alloy pipe and the welded joint according to the present invention will be described. The Fe-based alloy pipe and the welded joint according to the present invention can obtain the effect as long as they have the above-mentioned configuration regardless of the manufacturing method. For example, the Fe-based alloy pipe and the welded joint can be stably manufactured by the following manufacturing method. can.

4-1. Fe-based alloy tube First, the Fe-based alloy ingot, which is the material of the Fe-based alloy tube, is manufactured, or bloom is manufactured by continuous casting. The Fe-based alloy ingot is preferably produced by melting an alloy having the above-mentioned chemical composition in an electric furnace or the like, removing impurities by refining, and then casting. Subsequently, it is preferable that the obtained ingot is hot forged to form a columnar billet. Then, the obtained billet is processed to form a tube shape.

Specifically, it is preferable to hot-extrude the billet and then perform cold rolling or cold drawing. At the time of processing, softening heat treatment and intermediate pickling may be performed on the way, if necessary.

After that, it is preferable to perform a solution treatment on the tube as a heat treatment. In order for the above-mentioned maximum wall thickness difference Dw to satisfy the equations (iii) and (iv), the solution treatment is performed by heating in a temperature range of 950 ° C to 1230 ° C for 1 to 15 minutes. , It is preferable to carry out under the condition of water cooling. In addition, after the solution treatment, pickling may be performed if necessary. Further, the entire length, the entire circumference, etc. of the pipe may be machined by grinding or grinding.

4-2. Welded joint of Fe-based alloy pipe A welded joint can be obtained by using the Fe-based alloy pipe according to the present invention as a material and butt-welding the ends of the alloy pipe. The welding method is not particularly limited, but may be welded by, for example, arc welding. Further, as a condition for arc welding, for example, it is preferable to set the heat input amount in the range of 4 to 20 kJ / cm and to manufacture the welded joint of the Fe-based alloy pipe.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

An alloy having the chemical composition shown in Table 1 was melted to produce an ingot. After that, hot forging and hot rolling were performed to obtain two types of plate thicknesses of 12 mm and 6 mm for alloy types A and B, and 6 mm for other alloy types. This material was held at 1200 ° C. for 3 minutes and then subjected to a solution treatment by cooling with water. Then, in order to simulate the difference in wall thickness of the stainless steel pipe, these materials were reduced to various thicknesses shown in Table 2 by machining, and then cut into a width of 50 mm and a length of 100 mm to be used as a test material. bottom.

Subsequently, the end face of the produced test material in the rolling direction was grooved as shown in FIG. Then, the front side of the test material 1 simulating the portion having the maximum wall thickness (also referred to as "maximum wall thickness portion") and the test material 2 simulating the minimum wall thickness portion (also referred to as "minimum wall thickness portion"). The heights of the front surfaces were adjusted, butted so that there was a step on the back surface side, and the first layer was welded by automatic gas tungsten arc welding. The test materials 1 and 2 are both of the same alloy type.

At the time of welding, AWS A5.14-2009 ERNiCrMo-3 having an outer diameter of 1.2 mm was used as a filler material, and the heat input was about 9 kJ / cm. Ar was used as the shield gas and the back shield gas, and the flow rate was set to 10 L / min.

Regarding the obtained welded joint, the one in which the bead on the back surface side was formed over the entire length of the weld line was judged to have no problem in the forming ability of the bead on the inner surface side of the alloy pipe, and was judged as "passed". Among them, the one in which the width of the back surface side bead is 2 mm or more over the entire length of the weld line is "excellent", and the one in which the width is less than 2 mm but the back surface side bead of 1 mm or more is formed is "acceptable".

For welded joints for which the ability to form the inner bead was judged to be acceptable, A5.11 was placed on a commercially available steel plate (thickness 20 mm, width 150 mm, length 150 mm) equivalent to SM400B specified in JIS G 3106: 2008. -2005 EniCrMo-3 was restrained and welded around four circumferences using a specified shielded metal arc welding rod. Then, laminated welding was performed in the groove by automatic gas tungsten arc welding. For welding, AWS A5.14-2009 ERNiCrMo-3 with an outer diameter of 1.2 mm was used as a filler material, and the heat input was about 9 to 12 kJ / cm. Ar was used as the shield gas and the back shield gas, and the flow rate was set to 10 L / min.

It was confirmed in all the cases that the weld heat-affected zone was not cracked as it was welded. After that, assuming the usage state, aging heat treatment is performed at 700 ° C for 500 hours, 5 cross sections are exposed from the welded joint, mirror polished, corroded, and then microscopically examined with an optical microscope to stress the weld heat affected zone. The presence or absence of relaxation cracks was investigated. Welded joints in which cracks were not observed in all five samples were evaluated as "excellent", and welded joints in which cracks were observed in one sample were evaluated as "acceptable" and judged as "passed". Welded joints in which cracks were observed in two or more samples were judged to be "impossible".

From Table 2, the test pieces obtained by using the symbols A to H satisfying the chemical components specified in the present invention are excellent in crack resistance of the weld heat-affected zone after aging heat treatment, and also have an ability to form an inner bead. It turns out to be good.

Further, from the comparison of the test pieces A9 and A10, A16 and A17, B9 and B10 and B16 and B17, if the irregularity (step) of the bead portion on the back surface side satisfies a preferable range, the crackability and the inner surface of the welded portion after the aging heat treatment are satisfied. It can be seen that it is superior to the ability to form side beads.

On the other hand, since the S content of the test piece I1 using the reference numeral I exceeded the range of the present invention, cracks were observed in the entire cross section of the welded portion after the aging heat treatment. Further, the test piece J1 using the reference numeral J did not satisfy the formula (i) and was lower than the specified range. Therefore, the melting in the plate thickness direction was not sufficient, and the target ability to form the inner bead on the inner surface side could not be obtained.

Further, the test body K1 using the reference numeral K did not satisfy the equation (i) and was higher than the specified range. As a result, the molten metal drooped significantly, and the toe of the bead on the back surface side had an acute angle, which caused stress concentration and cracks were observed in the cross section of the welded portion after aging. As described above, it can be seen that the crack resistance of the welded portion after the aging heat treatment and the ability to form the inner bead can be compatible only when the requirements of the present invention are satisfied.

According to the present invention, it is possible to obtain an Fe-based alloy pipe and a welded joint capable of suppressing cracks generated in the vicinity of the inner bead of the pipe without performing heat treatment after welding.

Claims (5)

The chemical composition is by mass%,
C: 0.040 to 0.100%,
Si: 0.10 to 1.00%,
Mn: 0.60 to 1.60%,
P: 0.030% or less,
S: 0.0001 to 0.0015%,
Ni: 29.5 to 35.5%,
Cu: 0.01-0.75%,
Co: 0.01-1.00%,
Cr: 19.0 to 23.0%,
Mo: 0.01-1.00%,
Ti: 0.10 to 0.60%,
N: 0.0010 to 0.0150%,
Al: 0.10 to 0.60%,
O: 0.0004 to 0.0150%,
Remaining: Fe and impurities,
An Fe-based alloy tube satisfying the following formula (i).
0.0010 ≤ S + 2O ≤ 0.0280 ... (i)
However, the element symbol in the above formula represents the content (mass%) of each element contained in the Fe-based alloy, and if it is not contained, it is set to zero. In the region from the tube end of the Fe-based alloy tube to the position of 50 mm in the axial direction, the maximum wall thickness difference Dw calculated by the following equation (ii) satisfies the following equations (iii) and (iv). The Fe-based alloy tube according to claim 1.
Dw = D _MAX- D _min ... (ii)
Dw / D _MAX × 100 ≦ 20 ・ ・ ・ (iii)
Dw ≤ 1.6 ・ ・ ・ (iv)
However, each symbol in the above formula is defined as follows.
D _MAX (mm): Maximum wall thickness D _min (mm): Minimum wall thickness The chemical composition is, in mass%, instead of a portion of the Fe.
The Fe-based alloy tube according to claim 1 or 2, which contains Sn: 0.030% or less. The chemical composition is, in mass%, instead of a portion of the Fe.
W: 1.00% or less,
V: 0.40% or less,
Nb: 0.40% or less,
Ta: 0.40% or less,
Ca: 0.0100% or less,
Mg: 0.0100% or less,
B: 0.0100% or less, and REM: 0.0800% or less,
The Fe-based alloy tube according to any one of claims 1 to 3, which contains one or more selected from the above. A welded joint using the Fe-based alloy pipe according to any one of claims 1 to 4.

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