Ni-Cr-Mo ALLOY FOR WELDED TUBE HAVING SUPERIOR WORKABILITY AND CORROSION RESISTANCE

TECHNICAL FIELD

The present invention relates to Ni—Cr—Mo alloy, and in particular, relates to Ni—Cr—Mo alloy in which workability and corrosion resistance can be maintained even after welding.

BACKGROUND ART

Since Ni—Cr—Mo alloy is a material which is extremely superior in corrosion resistance, it is widely used under hard environment such as in chemical plants, natural gas fields, or petroleum fields. Furthermore, it is also used as a cladding tube of a heater, and this is also used under hard environment in which it may be easily corroded. In order to use it under such fields, it is necessary be welded or processed variously. There may be often a case in which welded parts are processed. Therefore, as similar to a parent material part, a welded part is also required to have workability and corrosion resistance. However, since the welded part may become a solidified structure, corrosion resistance may be reduced, cracking may occur, and workability may be deteriorated due to deterioration of ductility in the welded part. Improvement for them is required.

With respect to such Ni—Cr—Mo alloy, a method for production in which segregation of Mo is reduced in the purpose for improvement of corrosion resistance is shown in Patent Document 1, and a technique in which carbide effecting on corrosion resistance is controlled is shown in Patent Document 2. However, in these documents, there is no description about welding properties or workability of welded part.

On the other hand, with respect to the workability and corrosion resistance of welded part, documents about ferrite stainless steel is known (for example, see Patent Documents 3 and 4); however, since alloy type is different, these techniques as they are cannot be employed to Ni—Cr—Mo alloy.

Patent Documents are as follows:

- Patent Document 1: International Publication No. 2019/107456
- Patent Document 2: Japanese Unexamined Patent Application Publication No. 2019-52349
- Patent Document 3: Japanese Unexamined Patent Application Publication No. 2002-275590
- Patent Document 4: Japanese Unexamined Patent Application Publication No. 2008-231542

SUMMARY OF INVENTION

In view of the above techniques, an object of the present invention is to provide Ni—Cr—Mo alloy having superior workability and corrosion resistance of welded part.

The inventors have researched in order to solve the above subjects. As a result, in order to improve workability at welded part, it became obvious that it be necessary to improve ductility of welded part itself and amounts of Cu, Co, C and N be controlled to improve ductility of material itself. Furthermore, it became obvious that it be effective to add Nb, Ti and V at an appropriate amount in order to fine solidified structure of welded part. Furthermore, in welded part, it became obvious that generation of cracking can be reduced by restraining carbides, carbonitrides, and eutectic precipitates which lowers melting point. In particular, with respect to Cu and Sn, it became obvious that ductility of welded part can be improved by controlling amount of addition and maintaining at appropriate amount. Furthermore, in view of corrosion resistance, it became obvious it be effective that element such as Sn or W is added, and that carbides being cause of deterioration of corrosion resistance and oxides being origin of corrosion are reduced.

As mentioned above, the present invention is completed in view of experiment, that is, the present invention is a Ni—Cr—Mo alloy consisting of: in mass %, C: 0.002 to 0.020%, Si: 0.02 to 1.00%, Mn: 0.02 to 1.00%, P: not more than 0.030%, S: not more than 0.005%, Cr: 18.0 to 24.0%, Mo: 7.5 to 9.0%, Cu: 0.01 to 0.20%, Al: 0.005 to 0.400%, Ti: 0.1 to 1.0%, Fe: 3.0 to 6.0%, Nb: 2.5 to 4.0%, Co: 0.01 to 0.50%, V: 0.05 to 0.50%, N: 0.002 to 0.020%, Sn: 0.003 to 0.030%, W: 0.05 to 0.50%, Nb+Ti+V: 2.5 to 4.5%, Cu+10Sn: not more than 0.40, and Ni as a remainder and inevitable impurities.

In the present invention, it is desirable that the Ni—Cr—Mo-alloy contains O: not more than 0.005%, Mg: 0.001 to 0.010%, and Ca: 0.0001 to 0.0100%.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing relationship of Co amount and elongation ratio in the preliminary experiment in the present invention.

FIG. 2 is a graph showing relationship of Cu amount and elongation ratio in the preliminary experiment in the present invention.

FIG. 3 is a plotted diagram showing relationship of Sn amount and Cu amount, and existence of defects in the bending test in the preliminary experiment in the present invention.

FIG. 4 is a conceptual diagram showing collecting test pieces of the preliminary experiment in the present invention.

EMBODIMENTS OF INVENTION

The inventors have performed the following Experiments 1 to 3 in order to solve the above subjects, so that the present invention was completed. The studies are explained as follows.

<Experiment 1>
Evaluation of Ductility of Welded Part

In a laboratory, Ni—Cr—Mo alloy having various component of which Ni-21% Cr-8% Mo-4.5% Fe was basic composition, and C, N, Mn, Cu, Ti, Nb, Co, V, Sn, and W were added to this, was melted in a high-frequency induction furnace, and casted in a mold, so as to obtain alloy ingot. This was made as forged material having thickness of 8 mm by hot forging, annealed at 1100° C., washed by acid, and cold rolled so as to obtain cold rolled plate having thickness of 3 mm. Furthermore, this was annealed at 1100° C., and a test piece consisting of only parent material part and a test piece consisting of base material part and welded part were prepared. The test piece had a size of thickness 3 mm, width 30 mm, and length 100 mm, and was collected in a condition in which tensile direction and rolled direction were in parallel. Workability after welding was evaluated by an elongation ratio defined as follows. It should be noted that the elongation ratio indicates how much ductility of welded part is maintained with respect to the parent material.

Elongation ratio=(Elongation % of test piece including welded part)/(Elongation % of test piece consisting of only parent material part)

The welded part was prepared by a non-filler plasma welding. The welding condition was as follows: current: 100 A, voltage: 30 V, rate: 500 mm/min, center gas and back gas: 100% Ar gas, shield gas: 93% Ar+7% H₂gas, and beveling shape: I type. Furthermore, the welded part was made flat by bead cut. The test piece was collected in a condition in which welding bead was perpendicular to tensile direction and the welded part locates center of parallel part of the test piece.

Table 1 shows the above test results. As a result of researching effect by various components, elongation was extremely low in a case of sample Nos. 8, 9 and 10 in which one of Ti, Nb, and V was not added and in a case of sample No. 11 in which all of Ti, Nb and V were added and that total Nb+Ti+V amount was low. Furthermore, elongation was extremely low also in a case of sample Nos. 12 and 13 in which C amount and N amount were high.

TABLE 1

Chemical component mass % remainder Ni
Tenside test

Nb +

Elongation
CPT

Cr
Mo
Fe
C
N
Mn
Cu
Ti
Nb
Co
V
Ti + V
Sn
W
ratio
(° C.)

1
22.1
8.1
4.5
0.005
0.011
0.37
0.06
0.68
2.9
0.08
0.08
3.7
0.014
—
0.72
80

2
22.0
8.1
4.5
0.009
0.018
—
0.31
0.67
2.8
0.32
0.05
3.5
0.021
0.06
0.89
85

3
22.2
8.0
4.5
0.017
0.012
0.42
0.12
0.71
3.2
0.15
0.10
4.0
0.000
0.09
0.79
80

4
22.1
8.1
4.5
0.009
0.009
0.41
0.23
0.16
3.5
0.23
0.35
4.0
0.011
0.22
0.85
85

5
22.0
8.1
4.5
0.013
0.009
3.41
0.16
0.13
3.9
0.18
0.13
4.2
0.031
0.62
0.83
95

6
22.0
8.1
4.5
0.011
0.007
—
—
0.71
2.6
0.02
0.32
3.6
0.081
0.82
0.58
90

7
22.3
8.0
4.5
0.012
0.009
0.61
0.21
0.78
2.3
—
0.16
3.2
0.012
0.25
0.55
85

8
22.3
8.0
4.5
0.012
0.009
0.61
0.21
—
2.3
0.05
0.16
2.5
0.012
0.25
0.68
85

9
22.1
8.1
4.5
0.009
0.009
0.41
0.11
0.71
—
0.12
0.35
1.1
0.011
0.22
0.56
85

10
22.0
8.1
4.5
0.009
0.018
—
0.31
0.67
2.3
0.26
—
3.0
0.021
0.06
0.69
85

11
22.2
8.1
4.4
0.005
0.015
0.36
0.01
0.12
2.3
—
0.08
2.5
0.004
0.22
0.59
85

12
22.1
8.1
4.5
0.018
0.025
0.91
0.01
0.59
3.2
0.68
0.13
3.9
0.026
0.23
0.52
80

13
22.2
8.1
4.5
0.027
0.013
0.48
0.11
0.61
3.1
0.19
0.26
4.0
0.036
0.15
0.58
80

FIG. 1 shows relationship of Co amount and elongation ratio in sample Nos. 1 to 7, and FIG. 2 shows relationship of Cu amount and elongation ratio in sample Nos. 1 to 6. Improvement in elongation was confirmed in a case in which Cu and/or Co was added compared to a case of no addition of them (Nos. 6 and 7), and elongation of welded part was improved more accompanied by amount of addition of Cu and Co.

In order to improve ductility of welded part, it became obvious that controlling of Cu, Co, C and N amount was effective to maintain ductility of material itself. Furthermore, there was a case in which carbides, carbonitride, and eutectic precipitates lowering melting point became origin of cracking in welded part thereby deteriorating workability. Therefore, cracking at welded part was evaluated as follows.

<Experiment 2>
Evaluation of Cracking at Welded Part

In a laboratory, Ni—Cr—Mo alloy having various component of which Ni-21% Cr-8% Mo-4.5% Fe-3.5% Nb-0.010% C-0.010% N was basic composition, and Mn, Cu, Ti, Nb, Co, V, Sn, and W were added to this, was melted in a high-frequency induction furnace, and casted in a mold, so as to obtain alloy ingot. This was made as coil material having thickness of 8 mm by hot rolling, annealed at 1100° C., washed by acid, and cold rolled so as to obtain thickness of 0.7 mm. Furthermore, this was annealed at 1100° ° C. and slitted so as to obtain narrow width coil (31.4 mm). This was formed and welded in a continuous line. The welded part was made by a non-filler plasma welding. The welding condition was as follows: current: 100 A, voltage: 10 V, rate: 1000 mm/min, center gas and back gas: 100% Ar gas, and shield gas: 93% Ar+7% H₂gas. In this way, a pipe having outer diameter 10 mm was prepared and bending test was performed.

That is, a test piece having length of 500 mm along tubulating direction was collected. The bending test was performed based on press bending method using iron steel cylinder of 135R (mm). It should be noted that bead was arranged at lower side of the pipe, and the cylinder was pressed from upper side.

Defect (cracking) of bended part was confirmed using optical microscope magnifying 20 to 400 times, whether or not defect existed. It should be noted that defect was regarded to exist if cracking was more than 0.1 mm.

Table 2 shows the above test results. In addition, FIG. 3 is plotted diagram of the bending test result at 135R, and horizontal axis indicates Sn amount and vertical axis indicates Cu amount. A case no defect occurred was shown by “o”, and a case defect was observed was shown by “x”. In the diagram, cracking was not observed in a region in which Cu+10xSn being not more than 0.40; however, cracking was observed in a case in which it exceeds 0.40 and Cu and Sn were high. Therefore, it is necessary that Cu amount and Sn amount be controlled and Cu+10xSn be not more than 0.40.

TABLE 2

Chemical component mass % remainder Ni

Nb +

Cu +

Cr
Mo
Fe
Nb
C
N
Mn
Cu
Ti
Co
V
Ti + V
Sn
W
10 Sn
Cracking

1
21.1
8.0
4.5
3.3
0.010
0.010
0.82
0.16
0.31
0.13
0.42
4.0
0.009
0.08
0.25
None

2
20.9
7.9
4.5
3.5
0.009
0.010
0.65
0.13
0.36
0.46
0.31
4.2
0.019
0.36
0.32
None

3
21.0
8.0
4.4
3.4
0.010
0.011
0.34
0.05
0.91
0.31
0.11
4.4
0.019
0.34
0.24
None

4
20.8
8.0
4.4
3.5
0.011
0.010
0.64
0.09
0.61
0.21
0.29
4.4
0.029
0.13
0.38
None

5
21.1
8.1
4.5
3.4
0.012
0.010
0.97
0.16
0.44
0.16
0.23
4.1
0.021
0.22
0.37
None

6
21.2
8.1
4.5
3.5
0.010
0.009
0.31
0.12
0.68
0.17
0.35
4.5
0.026
0.13
0.38
Occurred

7
21.0
8.2
4.5
3.3
0.010
0.010
0.76
0.21
0.54
0.07
0.21
4.1
0.013
0.06
0.34
Occurred

8
21.0
7.9
4.5
3.5
0.010
0.011
0.21
0.16
0.43
0.13
0.33
4.3
0.026
0.65
0.42
Occurred

9
21.0
8.0
4.6
3.5
0.010
0.012
0.13
0.05
0.26
0.27
0.65
4.4
0.033
0.13
0.38
Occurred

10
21.1
8.0
4.5
3.4
0.010
0.010
0.16
0.14
0.33
0.32
0.81
4.5
0.028
0.41
0.42
Occurred

<Experiment 3>
Evaluation of Corrosion Resistance at Welded Part

Using samples similar to Experiment 1, corrosion test was performed as an evaluation of corrosion resistance at welded part. The welded part was made flat by bead cut, and ground by emery paper of #120 to finish. This test piece was immersed in 600 ml solution consisting of 6% FeCl₃and 1% HCl for 120 hours. Test was performed at 80, 85, 90 and 95° C. so as to measure critical pitting corrosion temperature (CPT). Pitting corrosion was regarded to exist in a case in which pitting corrosion was not less than 25 μm. The results are shown in Table 1.

Generation of pitting corrosion was observed even at 80° C. of test temperature in Nos. 1 and 3 in which W and Sn were not added and in Nos. 12 and 13 in which C amount and N amount were high. On the other hand, CPT was 95° C. in No. 5 in which amount of addition of W and Sn was high, and CPT was 90° C. in No. 6. Thus, improvement in corrosion resistance was confirmed. In view of these results, it is effective for corrosion resistance at welded part to add element such as Sn or W, and to reduce carbides being cause of deterioration of corrosion resistance and oxides being origin of corrosion.

Next, reason for limiting composition of Ni—Cr—Mo alloy component of the present invention is explained. It should be noted that “%” means “mass %” in every case.

C: 0.002 to 0.020%

C is an element affecting workability and corrosion resistance. In Ni—Cr—Mo alloy, C combines Nb, Ti and V to form carbides of them. Excessive carbides in welded part reduce ductility and become origin of cracking during welding. Furthermore, in heat-affected part by heat treatment process or welding, C easily combines Cr and Mo being effective for maintaining corrosion resistance to form carbides of M6C (M is mainly Mo, Ni, Cr or Si), or M23C6 (M is mainly Cr, Mo or Fe). Because a Cr and Mo lacking layer is easily formed around these carbides and necessary corrosion resistance is reduced, content is set to be not more than 0.020%. On the other hand, carbides in welded part fines solidification structure to improve ductility, it is necessary to contain not less than 0.002%.

In view of the above, C content is set to be 0.002 to 0.020%. It is desirably 0.003 to 0.015%, and most desirably 0.003 to 0.010%.

Si: 0.02 to 1.00%

Because Si is not only an effective element for deoxidation but also improves flowability of melt during welding, it is necessary to add not less than 0.02%. However, because a convex bead cannot be maintained at welded part if flowability of melt is excessively high, content should be not more than 1.00%. In addition, Si is an element helping formation of M6C and M23C6 and reducing grain boundary corrosion resistance. Therefore, Si content is set to be 0.02 to 1.00%. It is desirably 0.03 to 0.80%, and more desirably 0.05 to 0.50%.

Mn: 0.02 to 1.00%

Because Mn segregates at grain boundary and fixes P and S occurring welding cracking so as to restrain welding cracking, it is necessary not less than 0.02%. However, because Mn is an element helping formation of MnS and reducing pitting corrosion resistance, it is necessary to add not more than 1.00%. Therefore, Mn content is set to be 0.02 to 1.00%. It is desirably 0.03 to 0.80%, and more desirably 0.05 to 0.50%.

P: Not More than 0.030%

P is an element which segregates at grain boundary and deteriorates hot workability and corrosion resistance. In addition, P generates eutectic crystal of low melting point with Ni and thereby increasing sensitivity of welding cracking. Therefore, it is desirable to reduce P. Therefore, P content is set to be not more than 0.030%. It is desirably not more than 0.028%, and more desirably not more than 0.020%.

S: Not More than 0.005%

S is also an element which segregates at grain boundary and deteriorates hot workability as similar as P, in addition, forms MnS thereby reducing corrosion resistance, therefore, it is desirable to reduce as much as possible. Furthermore, S improves flowability of melt during welding; however, if flowability of melt is high excessively, a convex bead shape cannot be maintained at welded part. Therefore, S content is set to be not more than 0.005%. It is desirably not more than 0.002%, and more desirably not more than 0.0015%.

Cr: 18.0 to 24.0%

Cr is a very important element since passivation film is formed on surface of alloy and corrosion resistance is maintained. However, since excessive Cr addition helps precipitating M23C6, corrosion resistance may be reduced. Therefore, Cr content is set to be 18.0 to 24.0%. It is desirably 20.0 to 24.0%, and more desirably 21.0 to 23.0%.

Mo: 7.5 to 9.0%

Mo is also an important element since passivation film is formed and corrosion resistance is maintained as similar as Cr. However, excessive Mo addition helps precipitation of M6C thereby deteriorating corrosion resistance. Furthermore, excessive Mo addition increases strength, on the other hand, reduces ductility. Therefore, Mo content is set to be 7.5 to 9.0%. It is desirably 8.0 to 9.0%, and more desirably 8.0 to 8.5%.

Cu: 0.01 to 0.20%

Because Cu is an important element which improves ductility of parent material part and welded part, it is necessary at least 0.01%. However, excessive addition deteriorates hot workability and occurs welding cracking. In addition, Cu improves flowability of melt during welding; however, if flowability of melt is high excessively, convex beads shape cannot be maintained at welding part. Therefore, Cu content is set to be 0.01 to 0.20%. It is desirably 0.02 to 0.15%, and more desirably 0.02 to 0.10%.

Al: 0.005 to 0.400%

Because Al is an effective element for deoxidation, it is necessary at least 0.005%. By containing Al not less than 0.005%, O can be reduced to not more than 0.005%. However, excessive addition deteriorates hot workability. In addition, cluster of alumina is formed and causes liner defect on surface of alloy plate. Therefore, Al content is set to be 0.005 to 0.400%. It is desirably 0.020 to 0.300%, and more desirably 0.050 to 0.300%.

Ti: 0.1 to 1.0%

Ti fines solidification structure of welded part and improves ductility by combining C and N and forming carbide (TiC) and nitride (TiN), and in addition, restrains formation of M6C and M23C6 causing deterioration of corrosion resistance. On the other hand, excessive addition causes formation of much carbide (TiC), nitride (TiN) and oxide (TiO₂) and deterioration of hot workability and ductility. Therefore, Ti content is set to be 0.1 to 1.0%. It is desirably 0.1 to 0.8%, and more desirably 0.1 to 0.5%.

Fe: 3.0 to 6.0%

Fe is added to reduce production cost, and at the same time, has effect to reduce O amount in alloy. However, since excessive addition causes deterioration of corrosion resistance, Fe content is set to be 3.0 to 6.0%. It is desirably 3.0 to 5.0%, and more desirably 3.0 to 4.5%.

Nb: 2.5 to 4.5%

Nb fines solidification structure of welded part and improves ductility by combining C and N and forming carbide (NbC) and nitride (NbN) as similar as Ti. Furthermore, Nb restrains formation of M6C and M23C causing deterioration of corrosion resistance. On the other hand, although Nb solid-solves to increase strength, reduces ductility. Furthermore, excessive addition of Nb causes deterioration of hot workability by decreasing ductility exhibiting temperature. Therefore, Nb content is set to be 2.5 to 4.5%. It is desirably 2.8 to 4.0%, and more desirably 2.8 to 3.8%.

Co: 0.01 to 0.50%

Because Co is an important element improving ductility of parent material part and welded part, it is necessary to add at least 0.01%. However, excessive addition deteriorates hot workability and causes welding cracking. Therefore, Co content is set to be 0.01 to 0.50%. It is desirably 0.01 to 0.30%, and more desirably 0.01 to 0.20%.

V: 0.05 to 0.50%

V fines solidification structure of welded part and improves ductility by combining C and N and forming carbide (VC) and nitride as similar as Nb and Ti. Furthermore, V restrains formation of M6C and M23C causing deterioration of corrosion resistance. On the other hand, although V solid-solves to increase strength, reduces ductility. V content is set to be 0.05 to 0.50%. It is desirably 0.10 to 0.50%. It is more desirably 0.10 to 0.30%.

N: 0.002 to 0.020%

N combines Nb, Ti and V so as to form nitrides or carbonitrides. Appropriate amount of nitrides and carbonitrides fine solidification structure of welded part and improve ductility. On the other hand, excessive nitrides and carbonitrides in welded part deteriorates ductility and forms origin of cracking during welding. Furthermore, as N amount increases, number of blow holes at welded part increases. Therefore, N content is set to be 0.002% to 0.020%. It is desirably 0.002 to 0.016%. It is more desirably 0.002 to 0.010%.

Sn: 0.003 to 0.030%

Sn is an element which improves corrosion resistance by addition at small amount. On the other hand, Sn causes cracking at welded part by forming compounds having low melting point. Therefore, Sn content is set to be 0.003 to 0.030%. It is desirably 0.004 to 0.020%, and more desirably 0.006 to 0.010%.

W: 0.05 to 0.50%

W has an effect improving corrosion resistance similar to Mo; however, excessive addition causes formation of carbides and thereby deteriorating corrosion resistance. Therefore, W content is set to be 0.05 to 0.50%. It is desirably 0.10 to 0.40%. It is more desirably 0.10 to 0.30%.

Nb+Ti+V: 2.5 to 4.5%

Nb, Ti and V forms carbides, nitrides, and carbonitrides by combining C and/or N. Appropriate amount of nitrides and carbonitrides fines solidification structure of welded part thereby improving ductility of welded part. On the other hand, excessive nitrides and carbonitrides in welded part reduce ductility and cause origin of cracking during welding. Nb+Ti+V is set to be 2.5 to 4.5%. It is desirably 2.8 to 4.5%, and more desirably 3.0 to 4.0%.

Cu+10Sn: Not More than 0.40%

In a case in which Cu and Sn are added, if amount of addition of Sn with respect to Cu is large, compound having low melting temperature is formed, thereby causing cracking at welded part. Therefore, it is set to be not more than 0.40. It is desirably not more than 0.35, and more desirably not more than 0.30.

In the alloy of the present invention, it is desirable that concentration of O, Mg and Ca are controlled as follows.

O: Not More than 0.005%

O forms oxides, and deteriorates welding properties and hot workability. In addition, it causes forming blow hole during welding. Furthermore, it improves flowability of melt during welding; however, if flowability is excessively high, convex bead shape cannot be maintained at welded part. Therefore it is desirable to reduce O. Furthermore, O causes forming cluster of Al₂O₃and Ti oxides, thereby deteriorating hot workability and forming linear defects. Therefore, O content is set to be not more than 0.005%. It is desirably not more than 0.004%, and more desirably not more than 0.003%.

Mg: 0.001 to 0.010%

Mg segregates at grain boundary and fixes P and S occurring welding clacking, thereby restraining welding cracking as similar as Mn. On the other hand, if Mg is contained not less than a certain amount, inclusions aggregate on welding bead, they deteriorate workability and become origin of corrosion, and thus, corrosion resistance is deteriorated. Furthermore, Mg forms inclusions of MgO, MgO forms cluster, and causes surface defects in final products. Therefore, Mg content is set to be 0.001 to 0.010%. It is desirably 0.002 to 0.008%. It is more desirably 0.002 to 0.005%.

Ca: 0.0010 to 0.0100%

Ca segregates at grain boundary and fixes P and S occurring welding cracking thereby restraining welding cracking as similar as Mn. On the other hand, if Ca is contained not less than a certain amount, inclusions aggregate on welding bead, they deteriorate workability and become origin of corrosion, and thus, corrosion resistance is deteriorated. Furthermore, Ca forms inclusions of CaO, CaO forms cluster, and causes surface defects in final products. Therefore, Ca content is set to be 0.0010 to 0.0100%. It is desirably 0.0020 to 0.0070%. It is more desirably 0.0020 to 0.0050%.

Vickers Hardness at Parent Material Part, Welded Part, and Heat-Affected Part after Welding: Each not More than 280 HV

In welded part and heat-affected part after welding, there is a possibility increasing hardness due to formation of carbides and carbonitrides and due to change of structure. However, if hardness is increased, workability is deteriorated. Therefore, Vickers hardness at parent material part, welded part, and heat-affected part after welding is set to be each not more than 280 HV. It is desirably not more than 270 HV, and more desirably not more than 260 HV. Although it is not limited in particular, from the viewpoint of maintaining strength, it is desirable to be not less than 180 HV.

In Ni—Cr—Mo alloy of the present invention, the remainder is Ni and inevitable impurities. Here, the inevitable impurity means component which enters into alloy by various reasons during industrially producing Ni base alloy, and which is allowed to be contained at a range not affecting adversely to action and effect of the present invention.

Furthermore, in order to improve workability, it is desirable to perform heat treatment to the entire part which is processed and a part including welded part. In a case of alloy of the present invention, a heat treatment at 1000° C. for about 1 minute in the atmosphere is enough. That is, a heat treatment at high temperature, for example up to 1160° C., and a long term, for example about 1 hour at the longest, can be avoided. In a case of such heat treatment at high temperature and long term, oxidation scale may be generated in the atmosphere and it may become necessary to remove it by acid washing or mechanical polishing; however, this can be avoided in the present invention.

Next, the method for producing Ni—Cr—Mo alloy of the present invention is explained. Although the method for producing Ni—Cr—Mo alloy of the invention is not limited in particular; however, it is desirable to produce by the following method. First, raw material such as scrap, Ni, Cr, Mo and the like, are melted by an electric furnace, decarburization is performed by oxygen blowing by AOD (Argon Oxygen Decarburization) and/or VOD (Vacuum Oxygen Decarburization). Then, Cr reduction is performed by putting Al and lime stone into it. Furthermore, lime stone and fluorite are put into it so as to form CaO—SiO₂—Al₂O₃—MgO—F type slag on melted alloy, so that deoxidation and desulfurization are performed. The melted metal obtained is casted by a continuous casting machine so as to produce a slab, and then, hot rolling is performed, cold rolling is performed if necessary, so as to obtain thick plate and thin plate such as hot rolled steel plate and cold rolled steel plate.

EXAMPLES

The present invention is further explained with reference to the following Examples. It should be noted that the present invention is not limited within these Examples without departing from the spirit of the invention. First, raw material such as scrap, Ni, Cr, Mo and the like, were melted by an electric furnace, decarburization was performed by oxygen blowing by AOD and VOD. Then, Cr reduction was performed by putting Al and lime stone into it. Furthermore, lime stone and fluorite were put into it so as to form CaO—SiO₂—Al₂O₃—MgO—F type slag on melted alloy, so that deoxidation and desulfurization are performed. The melted metal refined in this way was casted by a continuous casting machine so as to produce a slab, and then, the slab was hot rolled by a Steckel mil, and subsequently cold rolled so as to obtain cold rolled plate having plate thickness of 3 mm. Table 3 shows chemical compositions of alloys produced and Table 4 shows measuring conditions and evaluation results.

Evaluation of Ductility at Welded Part

After the cold rolled plate having thickness of 3 mm was annealed at 1100° C., ductility at the welded part was evaluated. FIG. 4 shows a conceptual diagram in which a test piece was collected. The cold rolled plate 1 includes a welded bead 2. From the welded cold rolled plate 1, a test piece 3 only including parent material part and a test piece 4 including parent material part and welded part were prepared, and tensile test was performed. The welded part was prepared by non-filler plasma welding. The welding conditions were current: 100 A, voltage: 30 V, speed: 500 mm/min, center gas and back gas: 100% Ar gas, shield gas: 93% Ar+7% H₂gas, and beveling shape: I type. In addition, the welded part was made flat by bead cut. The test piece 4 was collected so that the welded bead was perpendicular to the tensile direction and the welded part located center of parallel part of the test piece. It should be noted that the test pieces 3 and 4 had a size of plate thickness 3 mm, width 30 mm, and length 100 mm, and were collected so that tensile direction was parallel to rolling direction. Workability after welding was evaluated by an elongation ratio defined as follows. a case the elongation ratio was not less than 0.8, a case the ratio was from not less than 0.7 to less than 0.8, a case the ratio was from not less than 0.6 to less than 0.7, and a case the ratio was less than 0.6, were defined as A, B, C and D, respectively.

Elongation ratio=(elongation % of test piece including welded part)/(elongation % of test piece of only parent material)

Evaluation of Cracking of Welded Part

The cold rolled plate having thickness of 3 mm was annealed at 1100° C., washed with acid, and then, rolled to have thickness of 0.7 mm by cold rolling. After that, pipe was produced by forming and welding in a continuous line. As an evaluation of cracking at welded part, a pipe having diameter of 10 mm was produced using 0.7 mm material so as to perform bending test. Welding conditions were current: 100 A, voltage: 10 V, and velocity 1000 mm/min. 100% Ar gas was used as center gas and back gas, 93% Ar+7% H₂gas was used as shield gas. Furthermore, a test piece having length of 500 mm was collected along a producing direction of pipe. Using a cylinder made of steel, bending test at 135R, 115R, and 95R was performed based on press bending method. It should be noted that bead was located at lower side of the pipe, and the cylinder was pressed from upper side.

As a confirmation of defect (cracking) at bending part, whether defect existed or not was confirmed by magnifying 20 to 400 times using optical microscope. It should be noted that a cracking more than 0.1 mm was regarded as defect occurred. A case cracking did not occur even at 95R, a case cracking occurred at 95R and did not occur at 115R, a case cracking occurred at 115R and did not occur at 135R, and a case cracking occurred at 135R were evaluated as A, B, C, and D, respectively.

Evaluation of Corrosion Resistance at Welded Part

As an evaluation of corrosion resistance at welded part, corrosion test was performed. 3 mm material was used as a test piece. Furthermore, welded part was made flat by bead cut, and then, polished by emery paper of #120. This test piece was immersed in 600 ml solution made of 6% FeCl₃and 1% HCl for 120 hours. The tests were performed at 80, 85, 90, and 95° ° C., and critical pitting temperature (CPT) was measured. Pitting corrosion of not less than 25 μm was regarded as pitting corrosion occurred. A case CPT was 95° C., a case CPT was 90° C., a case CPT was 85° C., and a case CPT was 80° C. were evaluated as A, B, C, and D, respectively.

Evaluation of Hardness at Welded Part

As an evaluation of hardness at welded part, Vickers hardness at parent material, welded part, and heat-affected part were measured. 3 mm material was used as a test piece, and its cross section was polished by emery paper of #120. Measurement was performed at three points of each of the parent material, welded part, and head-affected part in a condition of 1 kgf load during the measurement, and average hardness among each three points was evaluated.

TABLE 3

Chemical component mass % remainder Ni

C
Si
Mn
P
S
Cr
Mo
Cu
Al
Ti
Fe

Exam-
1
0.009
0.31
0.12
0.009
0.0013
2.28
8.4
0.03
0.078
0.31
3.1

ples
2
0.007
0.47
0.13
0.006
0.0014
23.8
8.2
0.09
0.095
0.48
3.6

3
0.008
0.23
0.21
0.008
0.0011
21.8
8.4
0.08
0.064
0.29
3.1

4
0.009
0.45
0.23
0.011
0.0013
22.8
8.1
0.10
0.213
0.18
4.2

5
0.008
0.48
0.31
0.008
0.0021
22.9
8.4
0.06
0.210
0.32
4.4

6
0.008
0.32
0.41
0.013
0.0017
21.1
8.8
0.09
0.116
0.21
3.6

7
0.009
0.22
0.22
0.006
0.0033
21.3
8.8
0.06
0.135
0.15
3.5

8
0.008
0.21
0.07
0.007
0.0013
21.8
8.5
0.03
0.184
0.11
3.8

9
0.010
0.33
0.13
0.014
0.0012
22.1
8.3
0.04
0.142
0.11
3.4

10
0.009
0.32
0.18
0.013
0.0011
22.6
8.0
0.06
0.133
0.26
4.2

11
0.006
0.09
0.12
0.009
0.0014
22.7
8.1
0.03
0.123
0.34
4.1

12
0.014
0.17
0.46
0.015
0.0023
22.9
7.8
0.09
0.211
0.41
4.4

13
0.010
0.31
0.48
0.027
0.0019
22.1
8.1
0.09
0.361
0.16
3.6

14
0.007
0.33
0.47
0.008
0.0014
22.5
7.7
0.05
0.079
0.13
5.6

15
0.010
0.26
0.49
0.013
0.0013
22.4
8.2
0.06
0.087
0.16
3.2

16
0.018
0.29
0.36
0.023
0.0023
19.8
8.1
0.02
0.011
0.65
3.7

17
0.011
0.04
0.78
0.011
0.0014
22.3
8.8
0.18
0.113
0.92
3.4

18
0.009
0.24
0.33
0.009
0.0037
18.4
8.8
0.13
0.215
0.22
3.2

19
0.005
0.91
0.31
0.025
0.0031
22.9
8.4
0.08
0.210
0.81
3.3

20
0.002
0.48
0.18
0.011
0.0027
21.8
7.8
0.01
0.213
0.74
3.6

Com-
21
0.018
0.48
0.13
0.027
0.0040
22.3
7.8
0.08
(0.002)
0.34
(2.8)

para-
22
(0.028)
0.38
0.31
0.028
0.0039
23.8
8.8
0.18
0.021
0.78
3.3

tive
23
0.010
(1.05)
0.26
(0.038)
0.0028
19.3
8.6
0.16
0.312
(1.10)
3.6

Exam-
24
0.012
0.58
0.11
0.016
(0.0056)
22.7
8.5
0.03
0.265
(0.03)
3.4

ples
25
0.017
0.32
0.56
0.022
0.0017
22.9
8.9
0.19
(0.410)
0.58
3.8

26
0.018
0.44
0.17
0.025
0.0021
21.6
7.8
0.07
0.012
0.34
(6.3)

27
0.019
0.78
0.47
0.009
0.0018
22.1
8.6
(−)
0.277
0.47
3.1

28
0.015
0.51
0.21
0.017
0.0015
22.8
8.4
(0.29)
0.284
0.48
4.2

29
0.007
0.27
0.36
0.026
0.0024
20.9
8.1
0.13
0.241
0.31
3.6

30
0.018
0.31
0.38
0.014
0.0037
21.8
8.4
0.13
0.185
0.67
3.1

34
0.012
0.66
0.54
0.019
0.0032
21.6
8.1
0.14
0.274
0.32
3.5

32
0.016
0.53
0.13
0.008
0.0014
22.4
8.3
0.17
0.233
0.28
3.8

33
0.009
0.15
0.57
0.021
0.0013
21.6
8.4
0.19
0.254
0.16
3.2

34
0.013
0.13
0.39
0.011
0.0011
21.1
8.8
0.18
0.134
0.25
4.2

35
0.011
0.21
0.41
0.014
0.0023
20.7
8.1
0.09
0.173
(0.08)
3.2

36
0.017
0.22
0.18
0.013
0.0016
22.1
8.0
0.17
0.147
0.36
3.3

37
0.014
0.74
0.44
0.014
0.0018
22.2
8.1
0.19
0.156
0.36
3.6

38
(0.001)
(0.01)
0.32
(0.11)
0.0029
22.2
8.1
0.11
0.153
0.22
3.3

Chemical component mass % remainder Ni

Nb +
Cu +

Nb
Co
V
N
Sn
W
Ti + V
10 Sn
O
Mg
Ca

Exam-
1
2.9
0.11
0.22
0.009
0.008
0.23
3.4
0.11
0.003
0.003
0.0045

ples
2
2.8
0.21
0.19
0.009
0.007
0.18
3.5
0.16
0.002
0.003
0.0061

3
3.3
0.13
0.28
0.005
0.009
0.28
3.9
0.17
0.002
0.004
0.0011

4
3.5
0 16
0.11
0.009
0.008
0.21
3.8
0.16
0.003
0.004
0.0034

5
3.3
0.16
0.15
0.007
0.009
0.28
3.8
0.15
0.002
0.003
0.0033

6
3.3
0.18
0.11
0.004
0.008
0.13
3.6
0.17
0.002
0.004
0.0045

7
3.1
0.09
0.20
0.008
0.005
0.16
3.6
0.11
0.002
0.003
0.0048

8
2.9
0.11
0.13
0.009
0.008
0.14
3.1
0.11
0.003
0.003
0.0036

9
2.9
0.05
0.12
0.007
0.009
0.24
3.1
0.13
0.002
0.004
0.0024

10
2.8
0.06
0.24
0.008
0.007
0.17
3.3
0.13
0.003
0.004
0.0009

11
3.4
0.03
0.22
0.009
0.008
0.18
4.0
0.11
0.002
0.007
0.0026

12
2.8
0.06
0.19
0.007
0.005
0.22
3.4
0.14
0.003
0.003
0.0130

13
3.1
0.08
0.13
0.015
0.006
0.23
3.4
0.15
0.003
0.004
0.0021

14
3.6
0.03
0.08
0.015
0.015
0.19
3.8
0.20
0.002
0.002
0.0019

15
3.7
0.23
0.06
0.009
0.006
0.17
4.0
0.12
0.002
0.008
0.0071

16
3.6
0.07
0.21
0.013
0.009
0.07
4.4
0.11
0.006
0.006
0.0069

17
3.1
0.42
0.13
0.002
0.021
0.21
4.2
0.39
0.003
0.002
0.0033

18
3.2
0.14
0.17
0.015
0.003
0.45
3.8
0.16
0.005
0.005
0.0047

19
3.3
0.08
0.15
0.019
0.026
0.28
4.3
0.32
0.002
0.003
0.0015

20
3.2
0.02
0.41
0.011
0.005
0.21
4.4
0.06
0.004
0.012
0.0009

Com-
21
3.7
(−)
0.16
0.016
0.007
(0.03)
4.2
0.07
0.007
—
—

para-
22
3.4
0.42
0.31
0.008
0.011
0.31
4.5
0.29
0.003
0.004
0.0026

tive
23
3.8
0 34
0.46
0.009
0.008
0.38
(5.4)
0.22
0.002
0.003
0.0029

Exam-
24
(2.3)
0.16
(0.04)
0.018
0.015
0.17
(2.4)
0.18
0.002
0.004
0.0071

ples
25
3.6
(0.85)
0.31
0.019
0.011
0.47
4.5
0.30
0.001
0.004
0.0026

26
3.4
0.18
0.16
0.016
(0.001)
0.13
3.9
0.08
0.005
0.005
0.0031

27
3.4
0.02
0.21
0.007
0.004
0.15
4.1
0.04
0.003
0.015
0.0151

28
3.6
0.15
0.15
0.009
0.005
0.14
4.2
0.34
0.005
0.004
0.0035

29
(4.4)
0.17
0.22
0.014
0.004
0.26
(4.9)
0.17
0.004
0.005
0.0067

30
2.8
0.21
(0.76)
0.009
0.008
0.22
4.2
0.21
0.003
0.006
0.0049

34
(1.5)
0.16
(−)
0.012
0.003
0.15
(1.8)
0.17
0.003
0.004
0.0041

32
3.6
0.23
0.38
(0.027)
0.007
0.28
4.3
0.24
0.003
0.006
0.0061

33
3.4
0.11
0.24
0.020
(0.035)
0.17
3.8
(0.54)
0.004
0.005
0.0036

34
3.2
0.08
0.26
0.015
0.028
0.11
3.7
(0.46)
0.005
0.006
0.0022

35
2.9
0.05
0.36
0.011
0.011
(0.02)
3.3
0.20
0.004
0.004
0.0013

36
3.6
0.13
0.33
0.016
0.008
(0.66)
4.3
0.25
0.003
0.003
0.0074

37
(4.1)
0.06
0.31
0.013
0.027
0.21
(4.8)
(0.46)
0.004
0.004
0.0027

38
3.6
0.08
0.25
(0.001)
0.009
0.25
4.1
0.20
0.008
0.003
0.0022

TABLE 4

Tenside test

Hardness HV

Elongation

Cracking

Base
Heat-affected
Welded

ratio
Decision
136R
116R
95R
Decision
CPT
Decision
material
part
part

Examples
1
0.81
A
None
None
None
A
95° C.
A
218
221
230

2
0.88
A
None
None
None
A
95° C.
A
239
240
245

3
0.82
A
None
None
None
A
95° C.
A
238
251
285

4
0.89
A
None
None
None
A
95° C.
A
260
259
254

5
0.88
A
None
None
None
A
95° C.
A
255
258
260

6
0.83
A
None
None
None
A
95° C.
A
260
263
271

7
0.84
A
None
None
None
A
95° C.
A
255
258
259

8
0.85
A
None
None
None
A
95° C.
A
238
240
245

9
0.83
A
None
None
None
A
95° C.
A
234
233
246

10
0.85
A
None
None
None
A
95° C.
A
218
228
248

11
0.74
B
None
None
None
A
95° C.
A
260
261
278

12
0.78
B
None
None
None
A
85° C.
C
231
230
239

13
0.81
A
None
None
Occurred
B
95° C.
A
214
221
233

14
0.77
B
None
None
None
A
95° C.
A
253
256
269

15
0.88
A
None
None
None
A
95° C.
A
248
250
266

16
0.74
B
None
Occurred
Occurred
C
90° C.
B
251
258
271

17
0.88
A
None
Occurred
Occurred
C
95° C.
A
255
260
288

18
0.87
A
None
None
None
A
85° C.
C
248
250
251

19
0.73
B
None
Occurred
Occurred
C
95° C.
A
255
256
280

20
0.69
C
None
None
Occurred
B
90° C.
B
258
280
261

Com-
21
0.58
D
None
Occurred
Occurred
C
90° C.
B
240
261
273

parative
22
0.67
C
None
Occurred
Occurred
C
85° C.
C
278
281
298

Examples
23
0.92
A
Occurred
Occurred
Occurred
D
85° C.
C
280
289
320

24
0.59
D
None
Occurred
Occurred
C
95° C.
A
170
179
178

25
0.88
A
Occurred
Ocourred
Occurred
D
90° C.
B
278
271
264

26
0.78
B
None
None
None
A
80° C.
D
283
281
273

27
0.51
D
None
Occurred
Occurred
C
95° C.
A
260
263
274

28
0.68
C
Occurred
Occurred
Occurred
D
95° C.
A
255
285
273

29
0.64
D
Occurred
Occurred
Occurred
D
90° C.
B
260
265
281

30
0.52
C
Occurred
Occurred
Occurred
D
95° C.
A
244
249
254

31
0.59
D
None
Occurred
Occurred
C
80° C.
B
178
198
216

32
0.55
D
Occurred
Occurred
Occurred
D
95° C.
C
240
241
247

33
0.71
B
Occurred
Occurred
Occurred
D
95° C.
A
245
244
256

34
0.74
B
Occurred
Occurred
Occurred
D
95° C.
A
245
251
264

35
0.79
B
None
Occurred
Occurred
C
80° C.
D
221
226
231

36
0.74
B
None
None
Occurred
B
80° C.
D
265
275
286

37
0.70
B
Occurred
Occurred
Occurred
D
90° C.
B
241
246
251

38
0.57
D
None
None
None
A
85° C.
C
220
221
218

Examples shown in Tables 3 and 4 are explained as follows. Nos. 1 to 20 are Examples of the invention since they have only one C decision at the most, therefore, they are within acceptable range. Since they satisfy the range of the present invention, they have superior workability and corrosion resistance at welded part.

Nos. 21 to 38 are Comparative Examples since they have D, or have not less than two C even if they do not have D, therefore, they are not within the acceptable range. The Comparative Examples Nos. 21 to 38 are explained as follows.

No. 21 was out of the acceptable range since ductility was D because Co was not added.

No. 22 was out of the acceptable range since ductility, cracking and corrosion resistance were C and hardness at welded part was high because of high C amount.

No. 23 was out of the acceptable range since cracking was C and hardness at welded part was above the range because Nb+Ti+V was above the content range.

No. 24 was out of the acceptable range since ductility was D because Nb was below the content range and Nb+Ti+V was below the content range.

No. 25 was out of the acceptable range since cracking was D because Co was above the content range.

No. 26 was out of the acceptable range since corrosion resistance was D because Sn was below the content range.

No. 27 was out of the acceptable range since ductility was D because Cu was not added.

No. 28 was out of the acceptable range since cracking was D because Cu was above the content range.

No. 29 was out of the acceptable range since Nb was above the content range, ductility and cracking were D, and hardness at welded part was above the acceptable range.

No. 30 was out of the acceptable range since V was above the range and ductility and cracking were D.

No. 31 was out of the acceptable range since ductility was D because V was not added.

No. 32 was out of the acceptable range since N was above the content range and ductility and cracking were D.

Nos. 33 and 34 were out of the acceptable range since cracking was D because Cu+10Sn was above the content range.

No. 35 was out of the acceptable range since W is below the content range and corrosion resistance was D.

No. 36 was out of the acceptable range since W was above the content range and corrosion resistance was D.

No. 37 was out of the acceptable range since cracking was D because Cu+10Sn was above the content range.

No. 38 was out of the acceptable range since ductility was D because C and N were below the content range.

EXPLANATION OF REFERENCE NUMERALS

- 1: Cold rolled plate (annealed), 2: welding bead, 3: test piece including only parent material part, 4: test piece including parent material part and welded part.

Ni-Cr-Mo ALLOY FOR WELDED TUBE HAVING SUPERIOR WORKABILITY AND CORROSION RESISTANCE

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