JPH05200583A - Welding structure excellent in hic resistance and ssc resistance - Google Patents

Abstract

PURPOSE:To provide a welding structure showing a good SSC resistance even in excess of Hv248 by adequately controlling the component of a welding metal. CONSTITUTION:In reference to a base material part, in accordance with the conventional technique, the shape of the inclusion is controlled and the component is set so as to satisfy Hv248. Consequently, the welding structure secures HIC resistance and SSC resistance, under a welding metal of C; 0.03-0.14%, Si; 0.1-0.6%, Mn; 0.70-1.8%, P<=0.02%, S<=0.02%, O; 0.015-0.05%, Al; 0.01-0.055%, Cr<=0.5%, Mo<=0.5%, Ni<=1.0%, Cu<=1.0%, and also if B; 0.0006-0.005%, Ti; 0.005-0.05%, by making Pwm; 0.25-0.43%; and if B<=0.0005%, then by making Pwm; 0.25-0.55%; provided Pwm=C+Si/24+Mn/5+Cr/7, +Mo/4+Ni/10+Cu/14.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welded structure such as a spherical tank exposed to a hydrogen sulfide atmosphere, and more particularly to a welded structure having excellent HIC resistance and SSC resistance.

[0002]

2. Description of the Related Art Welded structures such as LPG and spherical tanks for gas storage that are exposed to hydrogen sulfide atmosphere may undergo hydrogen-induced cracking (HIC) or sulfide stress corrosion cracking (SSC) during use. When these occur, the reliability of the structure is greatly affected, and various strict requirements are made. As specific examples of steel materials and steel materials HAZ, non-metallic inclusions such as sulfide (MnS) are reduced and their morphology is controlled to prevent HIC, and NACE proposes to prevent SSC. The maximum hardness of the Rockwell C hardness is 22 or less (corresponding to Vickers hardness of 248 or less, hereinafter referred to as Hv248 or less), and Ni is 1
There is a demand to keep it below%. In order to satisfy these characteristics, for steel materials, a method of securing strength and other characteristics of the steel material is currently adopted by a method such as a controlled rolling control cooling process in which the additive element is kept low.

On the other hand, also in the case of weld metal, in order to secure the reliability of the structure, the same characteristics as those of the base material and the weld heat affected zone are required. Unlike the steel material, the weld metal does not undergo a rolling process, so that non-metallic inclusions are present in a spherical shape and are extremely fine compared to the steel material, so that HIC does not occur. Similar to the base metal, SSC may occur under certain conditions, and its risk increases as hardenability increases. However, the characteristics of the weld metal are almost determined by the cooling process and the components of the weld metal that are determined by the welding conditions. Therefore, in the same composition as the base metal, it is often impossible to obtain equivalent characteristics. Therefore,
Conventionally, in order to obtain the same properties as the base metal, the alloying element of the weld metal is set higher than that of the base metal to solve this problem. This method is indispensable for ensuring the strength and toughness of the weld metal, but the addition of alloying elements results in an increase in the hardenability of the weld metal and is not preferable from the viewpoint of SSC resistance. This tendency becomes more remarkable especially when the weld metal is hardened due to the heat effect of the subsequent welding.

According to the prior art, Japanese Patent Application Laid-Open No. 63-258.
As disclosed in Japanese Patent Publication No. 8, it is usual to apply the requirement that the maximum hardness proposed by NACE is Hv 248 or less to weld metal as well, and secure characteristics such as toughness while securing SSC resistance. It was However, the application range of the method of securing strength and toughness while maintaining low hardness of Hv 248 or less is naturally limited in view of satisfying the originally contradictory characteristics. In particular, when the strength of the base metal is improved and the same strength is required for the weld metal, it is extremely difficult to achieve both low hardness and high strength. This problem becomes more difficult when repair welding is performed while the structure is in use because it is more likely that the weld metal will have more additive elements than the base metal and will be hardened under the influence of the heat. ..

[0005]

In order to overcome such a problem, not only when the weld metal satisfies Hv248 or less, but also when the weld metal does not satisfy Hv248 or less, low hardness is achieved. It is necessary to have the same characteristics as the SSC resistance of the base material and the base material HAZ (satisfying Hv 248 or less). In other words, the maximum hardness that does not cause SSC (hereinafter simply referred to as the limit hardness) is Hv248.
The invention of weld metal that exceeds the above is required. This means that when repair welding is required while using the welded structure,
Depending on the welding conditions, Hv24 for weld metal
Considering that the condition of 8 or less may not be satisfied, its significance is great. That is, the present invention is Hv248
It is an object of the present invention to provide a welded structure characterized by having a weld metal exhibiting good SSC resistance even when the following conditions are not satisfied.

[0006]

The present inventors have paid attention to the above-mentioned circumstances, and because the weld metal has a large amount of oxygen as compared with the base metal, the SSC sensitivity is high. Being convinced that it must be different from that of the base metal, I have mainly studied the effect of the components in the weld metal on the SSC susceptibility. The present invention has been completed as a result of such research, and its constitution is that the base material portion is C by weight% and is C;
0.02-0.06%, Si; 0.6% or less, Mn;
1.0-1.6%, P; 0.020% or less, S; 0.0
01% or less, Al; 0.001-0.060%, Nb;
0.005-0.04%, Ti; 0.005-0.03
0%, Ca; 0.001-0.006%, N; 0.00
6% or less, if necessary, Mo; 0.05 to 0.5%,
Ni: 0.05-0.5%, Cu: 0.05-0.5
%, V: 0.01 to 0.10% in the range of 1 type or 2 types or more, the balance is iron and unavoidable impurities, and the weld metal has an index Pwm of 0.25; ~ 0.
43% and C: 0.03 to 0.14%, S
i; 0.1 to 0.6%, Mn; 0.70 to 1.8%,
P; 0.02% or less, S: 0.02% or less, Ti;
005-0.05%, B; 0.0006-0.005
%, O; 0.015 to 0.05%, Al; 0.01 to
0.055%, and if necessary, Cr; 0.5% or less, Mo; 0.5% or less, Ni; 1.0% or less, Cu;
1.0% or less of one kind or two kinds or more, the balance consisting of iron and unavoidable impurities, or Pwm;
0.25-0.55%, C; 0.03-0.14%, S
i; 0.1 to 0.6%, Mn; 0.70 to 1.8%,
P: 0.02% or less, S: 0.02% or less, B; 0.0
Less than 006%, O; 0.015-0.08%, Al;
0.01 to 0.055%, and if necessary, Cr;
0.5% or less, Mo; 0.5% or less, Ni; 1.0% or less, Cu; 1.0% or less, and one or more kinds, and the balance consists of iron and unavoidable impurities. The gist exists in the welded structure characterized by. Pwm = C + Si / 24 + Mn / 5 + Cr / 7 + Mo / 4 + Ni / 10 + Cu / 14 (1) The present invention will be described in detail below.

[0007]

[Function] First, the base metal portion will be described. The HIC resistance and SSC resistance of the base material should be achieved by reducing the sulfide content in the base material and controlling its morphology, and ensuring the maximum hardness of the base material and HAZ of Hv 248 or less, which has been conventionally adopted. And The component range therefor will be described below.
C and Mn are essential components for ensuring the strength and toughness of the base material. However, excessive addition raises the hardenability too much, so the range is set to 0.02-0.06, respectively.
%, 1.0 to 1.6%. If the amount of Si added is too large, the HAZ toughness deteriorates, so the upper limit was made 0.6%. Although P and S are impurities in the present invention, they deteriorate the toughness of the base material and HAZ, and S forms sulfides, so the upper limits are 0.020% and 0.001%, respectively.
And

Al is 0.001 to 0.060% in terms of the amount required for deoxidation and the amount that does not cause deterioration of toughness.
And Nb is 0.0 to improve strength due to precipitation effect.
05% is necessary, but the upper limit was made 0.04% to suppress the HAZ hardness. Ti is a base material and HA as TiN
It is essential because it is effective for making Z fine. But T
Since excessive addition of both i and N deteriorates the toughness of the base metal and HAZ, the range is set to 0.005 to 0.030%, 0.0
It was set to 06% or less. Ca is necessary for controlling the morphology of sulfide and improving toughness.
If a large amount of O and CaS is generated, on the contrary, the toughness is deteriorated, so the range is set to 0.001 to 0.006%. The above is the basic components, but the base material portion of the structure of the present invention may contain one or more of the following additional components, if necessary.

Mo improves the strength and toughness of the base material,
If the addition amount is too large, toughness and weldability will be deteriorated.
It was set to 0.5% or less. The lower limit is set to 0.05% in the sense that a substantial effect is obtained. For the lower limit,
The same applies to Ni and Cu. The upper limit of Ni is HA
From the viewpoint of not adversely affecting the Z toughness, it was set to 0.5%. The upper limit of Cu is set to 0.5% from the viewpoint of preventing Cu cracks during the production of the base material. V, like Nb, contributes to the precipitation effect, but does not work as much as Nb, so the range was made 0.01 to 0.10%. By subjecting the steel having the above composition range to controlled rolling controlled cooling or quenching and tempering as required, the strength is increased to 50 kgf /
It becomes possible to secure a high HIC resistance and a low hardness (Hv 248 or less), that is, a good SSC resistance, with the thickness being at least ² mm ² .

Next, the weld metal will be described. Regarding weld metal, it is said that HIC does not occur as described above. For SSC, H for base metal and HAZ
Good SSC resistance by satisfying v248 or less
However, it is necessary to set the component range for the weld metal to exhibit good SSC resistance even if the condition of Hv 248 or less is not satisfied.

First, C, Si, Mn, Cr, Mo, Ni,
Regarding Cu, these components have the function of lowering the maximum hardness (critical hardness) at which SSC does not occur in view of the SSC resistance of the weld metal. However, since the degree of influence is different, the index Pw calculated by equation (1)
m was introduced. By setting Pwm ≦ 0.55%, good SSC resistance can be obtained. vice versa,
If Pwm> 0.55%, Hv advocated by NACE
Even if 248 or less is satisfied, SSC may not be prevented. This is the reason why the upper limit of Pwm is set to 0.55%.

However, the upper limit of Pwm also changes depending on other elements. That is, the upper limit of Pwm decreases due to B described later. However, since B is added to the weld metal together with Ti and is very effective in improving the toughness, the present inventors considered it unreasonable not to add B to the weld metal. Therefore, 0.43% was found as the upper limit at which good SSC resistance can be obtained even if B is added.

On the other hand, if Pwm ≦ 0.43% is satisfied, B
It is possible to obtain good SSC resistance even if Hv 248 or less is not satisfied with or without addition of Pw.
In the range of m <0.25%, it becomes difficult to secure the strength equivalent to that of the base metal, and the maximum hardness becomes Hv248 or less even at the low heat input amount that is considered during normal welding work. This is the same as securing SSC resistance by the conventional technique. Therefore, assuming that the range of Pwm <0.25% is out of the range of the present invention, the lower limit of Pwm is 0.25%.
And

Next, each component of the weld metal will be described with respect to the reason for limiting the range. When C is added excessively,
The hardenability is excessively increased, solidification cracking is caused during welding, and the SSC resistance may be deteriorated, so the upper limit was made 0.14%. The lower limit of 0.03% is the minimum value that secures strength and hardenability. Mn is C
Similarly, it is essential to secure strength and toughness.
The lower limit of Mn, 0.70%, is the minimum value required to secure these. On the other hand, if Mn exceeds 1.8%, excessive hardenability occurs and the limit hardness becomes Hv 248 or less, so this value was made the upper limit.

Si is a deoxidizing element and has a great influence on the amount of O described later. In addition, Si greatly affects workability during welding work. From the viewpoint of ensuring SSC resistance, the smaller the amount of Si, the better (the larger the amount of O, the better).
However, the lower limit is set to 0.1% from the viewpoint of ensuring practically sufficient workability and preventing an excessive increase in O 2. The upper limit is 0.6 from the viewpoint of excessive increase in hardenability, deterioration of toughness, and deterioration of SSC resistance due to reduction of O content.
%.

P and S are impurities in the present invention. However, P and S are easily segregated at the grain boundaries in the weld metal, and there is a possibility that SR embrittlement and toughness deterioration may occur due to this point, so the upper limits were made 0.020% respectively.

B reduces the critical hardness by adding it. Therefore, the function of B is not large, 0.00
If it is less than 06%, the upper limit of Pwm shown in the formula (1) may be 0.55%, but if it is added more than that, the upper limit of Pwm must be 0.43%.
This means that by adding B, C, Si, Mn, Cr, M
This means narrowing the selection range of o, Ni, and Cu, which is not preferable, but the addition of B together with Ti can drastically improve the toughness of the weld metal. Moreover, the decrease in the critical hardness due to the addition of B is 0.
If it is 001% or more, the effect of B on the critical hardness is very small. From the above, the present inventors considered that addition of B was practically effective even if the range of Pwm was narrowed. The lower limit of 0.0006% of B addition is the minimum value for obtaining a substantial effect. However, if B is added excessively, it segregates at the austenite grain boundaries, suppresses the growth of pro-eutectoid ferrite, and causes excessive hardenability, so the upper limit of B addition was made 0.005%.

O is the only component among the weld metal components handled in the present invention, which can increase the critical hardness, that is, the SSC resistance by increasing the addition amount. However, most of O exists as non-metallic inclusions in the weld metal and acts as nuclei for ferrite formation,
If added excessively, the hardenability becomes too low, and there is a possibility that the predetermined toughness cannot be secured. Furthermore, O
When B is present in the weld metal, it combines with B to form B ₂ O ₃ , which may reduce the effective B. Therefore, when B is added, the upper limit of O is set to 0.05% so as not to impair the effect. In the case where B was not added, the upper limit for ensuring the predetermined toughness was 0.08%.
The lower limit of 0.015% is the minimum value for obtaining good SSC resistance.

Al forms a nonmetallic inclusion with O. A
If the amount of 1 is insufficient, sufficient deoxidation is not performed, and excessive O in the weld metal reacts with C, Si, Mn, etc., so that the prescribed hardenability may not be ensured. The lower limit of 0.01% of Al is the minimum value for sufficient deoxidation. On the other hand, excessive addition of Al may cause excessive deoxidation and the amount of O in the weld metal may not be in an appropriate range. Al
The upper limit of 0. is the maximum value that secures an appropriate amount of O.
It was set to 055%.

The above is the basic components of the weld metal. In addition to these, if necessary, Cr, Mo, N
One or more of i, Cu, and Ti can be added. Cr and Mo are elements to be added in order to utilize the excellent properties of the present invention. However, Cr, Mo
Has a function of improving hardenability like C and Mn, and is more expensive than C and Mn, so the upper limit of each is 0.5.
%.

Cu is the coefficient of Cu in Pwm, Mn, C
As can be understood by comparing with r, Mo, etc., S resistance
The effect on SC characteristics is small. Therefore, the selection range can be made wider than that of Cr and Mo. However, there is no influence at all, and excessive addition deteriorates the toughness of the weld metal and further the weld metal after SR is performed, so the upper limit was made 1.0%.

Like Ni, Ni has a relatively small coefficient in Pwm. However, excessive addition raises the hardenability of the weld metal too much and causes deterioration of toughness, so the upper limit is 1.0%.
I decided to.

Ti should be added together with B and used to improve the toughness of the weld metal. Lower limit of Ti, 0.005
% Is the minimum amount required to obtain a substantial effect. T
i becomes TiN, TiO, etc., which acts as a nucleus for forming ferrite and has an effect of forming a fine structure. On the other hand, however, precipitates such as TiC are formed, and if they are excessively present, the toughness of the weld metal is deteriorated. Therefore, the upper limit is 0.0
It was set to 5%.

Although the component ranges of the base metal and the weld metal have been described above, the component range of the weld metal is not limited by the welding method. Further, as a means for limiting the composition of the weld metal within a predetermined range, there is a limitation of the composition of the welding wire or the composition of the welding flux, but the weld metal composition after welding is within the predetermined range. If so, it does not depend on these means.

[0025]

Example: A base material having the components shown in Table 1 was JIS
According to Z3101 of No. 3, by performing the maximum hardness test of the welding heat affected zone and measuring the maximum hardness of the base metal and HAZ,
These SSC resistance was evaluated. According to the present invention, the maximum weld heat-affected zone hardness does not exceed Hv248,
It can be seen that good SSC resistance is exhibited. Further, the HIC cracking ratio (CAR) under NACE environment measured by ultrasonic flaw detection is also shown. Compared to the comparative example, the present invention is HIC
Does not occur and shows good HIC resistance. Also, 4
In the case of the present invention, no crack was observed even when the SSC test of point bending was carried out by applying a bending stress corresponding to the actual yield stress.

In order to produce various welded joints, two beads were placed as shown in FIG. 1 and SSC test pieces 3 were taken from the portion shown in FIG. In FIG. 1, the second bead 2 is for exerting a thermal influence on the first bead 1, and by changing the heat input amount of the second bead,
You can control the maximum hardness of the bead. The welding method used for the first bead is submerged arc welding (SA
W), manual welding (SMAW), gas shield welding (GMA)
W) are three types, and these three types of welding methods are general and typical welding methods, except for TIG welding in stainless steel and self-shield welding in which it is difficult to secure the amount of oxygen in the weld metal. Most of the welded structures used in the sour environment are constructed by these three welding methods.

The second bead is for controlling the maximum hardness of the first bead and the base material heat affected zone.
When the SSC is generated from the second bead, the SSC test of the first bead or the heat affected zone of the base material is not performed. Therefore, for the second bead, the SSC was not caused even with the heat input amount of 5 kJ / cm. A combination of wire and shielding gas was used.

Tables 2A, 2B and 2C show the components of the welding material used for each welding method. Table 2 shows the welding conditions and welding materials used for the first bead. Table 3A, B
Is a summary of the SSC test results. The chemical composition of the weld metal refers to the composition of the first bead. The critical hardness is equivalent to the actual yield stress obtained by producing welded joints in which the maximum hardness of the first bead was changed by the second bead and sampling SSC test pieces from each joint as shown in FIG. Bending stress was applied in a NACE environment and determined as the maximum hardness that does not generate SSC. According to the present invention, SSC does not occur in the base material and the heat-affected zone of the base material, and the critical hardness of the weld metal is all above Hv248, which is good even if the condition of Hv248 or less proposed by NACE is not satisfied. It can be seen that excellent SSC resistance can be obtained.

On the other hand, in the comparative example, YC3, YC4, YD
2, YE2, YE3, YF2, YG2, MA3, SA,
SSC or HIC occurs in the base material or the base material HAZ before reaching the limit hardness of the weld metal, such as SF, and the limit hardness of the weld metal cannot be determined, or MD, ME, SL, S
Limit hardness of weld metal is Hv24 like U, SV, SY
Some have fallen below eight.

[0030]

[Table 1]

[0031]

[Table 2A]

[0032]

[Table 2B]

[0033]

[Table 2C]

[0034]

[Table 2D]

[0035]

[Table 2E]

[0036]

[Table 3A]

[0037]

[Table 3B]

[0038]

EFFECTS OF THE INVENTION The present invention provides a welded structure having a weld metal exhibiting good SSC resistance even if it does not always satisfy the condition widely believed in the art, Sv prevention standard, Hv 248 or less. can do. This means that the range of choices of weld metal components will be expanded,
Expected usefulness is extremely high, such as improving the reliability of the welded structure and improving the efficiency of repair welding work, such as ensuring SSC resistance and other properties.

[Brief description of drawings]

FIG. 1 is a perspective view showing a welded joint subjected to a welding test,

FIG. 2 is a front view showing a sampling position of a test piece used for an SSC test collected from a welded joint.

[Explanation of symbols]

 1 1st bead 2 2nd bead 3 SSC test piece 4 Base metal part

Claims (8)

[Claims] 1. A base material part, in% by weight, has a C content of 0.02 to 0.02.
0.06%, Si; 0.6% or less, Mn; 1.0 to 1.
6%, P; 0.020% or less, S; 0.001% or less,
Al: 0.001-0.060%, Nb: 0.005-
0.04%, Ti; 0.005-0.030%, Ca;
0.001 to 0.006%, N; 0.006% or less, the balance being steel composed of iron and unavoidable impurities, and the weld metal has an index Pwm of 0.
25 to 0.43%, C: 0.03 to 0.14
%, Si; 0.1 to 0.6%, Mn; 0.70 to 1.8
%, P; 0.02% or less, S; 0.02% or less, Ti;
0.005-0.05%, B; 0.0006-0.00
5%, O; 0.015 to 0.05%, Al; 0.01 to
A welded structure comprising 0.055% and the balance being iron and inevitable impurities. 2. The welded structure according to claim 1,
If the weld metal is further weight%, Cr; 0.5% or less, M
o; 0.5% or less, Ni; 1.0% or less, Cu; 1.0
%, 1 type or 2 types or more are further contained, The welded structure characterized by the above-mentioned. 3. A steel material equivalent to the base material according to claim 1 is used as a base material, and the weld metal is% by weight, and the following index Pwm is:
Pwm: 0.25 to 0.55% satisfied, C: 0.03
~ 0.14%, Si; 0.1-0.6%, Mn; 0.7
0 to 1.8%, P; 0.02% or less, S; 0.02% or less, B; less than 0.0006%, O; 0.015 to 0.0
8%, Al; 0.01 to 0.055%, the balance consisting of iron and unavoidable impurities, a welded structure. 4. The welded structure according to claim 3, wherein
If the weld metal is further weight%, Cr; 0.5% or less, M
o; 0.5% or less, Ni; 1.0% or less, Cu; 1.0
%, 1 type or 2 types or more are further contained, The welded structure characterized by the above-mentioned. 5. The welded structure according to claim 1, wherein
The base material is, by weight, Mo; 0.05 to 0.5%, Ni;
0.05-0.5%, Cu; 0.05-0.5%, V;
A welded structure further containing one or more members in the range of 0.01 to 0.10%. 6. The welded structure according to claim 2,
The base material is, by weight, Mo; 0.05 to 0.5%, Ni;
0.05-0.5%, Cu; 0.05-0.5%, V;
A welded structure further containing one or more members in the range of 0.01 to 0.10%. 7. The welded structure according to claim 3,
The base material is, by weight, Mo; 0.05 to 0.5%, Ni;
0.05-0.5%, Cu; 0.05-0.5%, V;
A welded structure further containing one or more members in the range of 0.01 to 0.10%. 8. The welded structure according to claim 4,
The base material is, by weight, Mo; 0.05 to 0.5%, Ni;
0.05-0.5%, Cu; 0.05-0.5%, V;
A welded structure further containing one or more members in the range of 0.01 to 0.10%. Pwm = C + Si / 24 + Mn / 5 + Cr / 7 + Mo / 4 + Ni / 10 + Cu / 14 (1)