JPH0285339A - Structural member including welded part, method for manufacturing structural member including welded part, and low C-Cr-Mo steel for structural member including welded part - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a structural member including a welded portion such as a turbine diaphragm, a method for manufacturing the same, and a low C-〇r-Mo steel for the same.

[Conventional technology]

FIG. 2 is a plan view showing the structure of the turbine diaphragm. As shown in FIG. 3, which is a cross-sectional view taken along the line ■-■ in FIG. It also includes an inner ring 3, an outer ring 2, and support rings 6, 7.8, which are assembled by welding.

First, nozzle blade 1. is fixed to the two support spacers 4, 5, then the inner ring 3. is attached to each spacer 4, 5. The outer ring 2 is joined, and then the support 111i! 6.7.8 are joined.

The welding method used is electron beam welding for welding the spacers 4, 5 of the nozzle blade 1 and the inner and outer rings 2, 3. 9(a), 9
(b), 10(a), and 10(b) show electron beam welding parts. On the other hand, the support rings 6, 7.8 are arc welded to the inner and outer rings 2, 3 and both spacers 4.5. 11(a), 11(
b), 12(a), 12(b), 12(c) show arc welding parts. By the way, nozzle blade support spacers 4, 5.
The inner and outer rings 2, 3 and the support rings 6, 7.8 are usually made of Cr-Mo steel as shown in Table 1, which has excellent corrosion resistance, since they are in contact with wet steam at temperatures below 250°C.

However, since these Cr-Mo steels contain a particularly large amount of C, which is a self-hardening element, they have the disadvantage of being susceptible to weld cracking. To prevent this, when welding, preheat to 250'C or more, maintain the temperature between passes, and after welding, 400'CX
High-temperature and long-term heat treatment, such as 30-minute dehydrogenation treatment, was required. Such heat management not only imposes a severe labor burden on welding workers, but also consumes energy and complicates the work.Therefore, there was a demand for the development of Cr-Mo steel with excellent weldability. .

These issues are not limited to the turbine diaphragm.
The same problem occurred in welded structures for thermal power generation, chemical equipment, etc., as well as feed water heaters for nuclear power generation using Cr-Mo steel.

In order to improve weldability, weld hardening index, C equivalent (C
+Si/24+Mn/6+Ni/40+Cr15+Mo
/4+V/14) or weld crack susceptibility index IPcM value (C+Si/30+Mn/20+Cu/20+Ni/6
As is clear from 0+Cr/20+Ao/15+V/10+5B), the reduction in the amount of C is Ao/15+V/10+
5B For example, JP-A-61-56309 or JP-A-61-
As described in Publication No. 56310, the amount of C was 0.0.
Materials have been developed that have a reduction of 2% to 0.14%.

[Problem to be solved by the invention]

However, when the conventional materials mentioned above are electron beam welded, the solidification structure of the weld metal nail head is coarse and there is segregation of impurities, so welding structures with a high degree of restriction such as a bottle diaphragm using an extremely thick plate cannot be used. There was a problem in that cracks occurred in the weld metal during the stress relief annealing process performed after welding for the purpose of removing residual stress and improving toughness. If such cracks are left behind, power generation will be hindered.

An object of the present invention is to provide a structural member including a welded part that does not cause cracking during the stress relief annealing process of electron beam welding, a method for manufacturing the same, and a low C-Cr-Mo steel for the same. .

[Means to solve the problem]

Cr-M with low carbon to improve weldability.

When steel was electron beam welded, there was a problem with cracking during the stress relief annealing process, but we initially thought that this might be due to the low carbon content, and first investigated the effect of C on stress relief annealing cracking. It was investigated. However, no knowledge was obtained that the difference in carbon content makes it easier to break.

Furthermore, as a result of basic research, we found that electron beam welded metals involve solidification segregation compared to conventional cracking in the heat-affected zone, so even if the base metal has a composition that does not easily cause cracking, it may exhibit high cracking susceptibility. I understand.

That is, in low-carbon Cr-Mo steel, ■ Elements such as Sb and Sn, which are trace impurity elements, are known to be elements that increase cracking susceptibility during stress relief annealing. However, low carbon Cr-M
o steel has high cracking susceptibility even if these are reduced as much as possible. It is desirable that these values be as low as possible, but at the same time it is necessary to regulate the amount of S. It has been found that during electron beam welding, solidification and segregation cause about six times as much segregation at dendrite interfaces as in the base metal, promoting the precipitation and aggregation of carbides that weaken grain boundaries.

It has also been found that refinement of crystal grains by adding Ti and fixation of solid solution S by adding Ca or Mg are effective.

■AΩ is included at a maximum of 0.08% because coexistence with N makes the crystal grains finer and contributes to improving toughness, but AQ has been found to segregate at grain boundaries and weaken them. In fact, it is necessary to strictly select the raw materials to be dissolved so as to contain as little AQ as possible.

■In electron beam welding of low carbon Cr-Mo steel, the solidification structure of the nail head of the weld metal is high carbon Cr-Mof14.
It was found that the size of the austenite crystal grains was much coarser than that of the austenite crystal grains.

Based on the above, the present invention provides a structural member including a welded portion subjected to stress relief annealing after electron beam welding of steel materials, in which at least one of the steel materials to be welded is
Weight ratio: C 0.05-0.1%, Si 0.8% or less, Mn 0.4-0.8%, Cr 0.94-2.6
2%, Mo 0.40-1.15%, B 0.0005
~0.0015%, Ti 0.01-0.5%, S 0
．． The steel material is characterized in that it contains 0.002% or less, with the remainder consisting of Fa and unavoidable impurities.

In the structural member, at least one of the steel materials to be welded further contains Ca and/or Mg of 0°00.1 to 0.0°.
It is desirable that the steel material contains 1%.

In the structural member, it is desirable that at least one of the steel materials to be welded is a steel material further containing 0.002% or less of AQ.

In the structural member, at least one of the steel materials to be welded further contains Ca and/or Mg of 0°001 to 0.1
%, AQ is preferably 0.002% or less.

An example of a structural member including the welded portion is a nozzle blade;
A support spacer fixed to both ends of the nozzle blade, an inner ring and an outer ring each electron beam welded to the surface support spacer,
A turbine diaphragm includes a support ring that is arc welded to the inner ring and its support spacer, and the outer ring and its support spacer.

Further, examples of other structural members include a feed water heater, a condenser, a heat exchanger, etc. of nuclear power generation equipment. Each of these structural members also includes a welded portion similar to the turbine diaphragm, and the present invention can be applied thereto.

In addition, the manufacturing method according to the present invention provides a method for manufacturing a structural member including a welded portion, which includes a step of electron beam welding steel materials and a step of stress-relieving annealing treatment of the welded portion. Apply an auxiliary plate to the auxiliary plate, perform electron beam welding from above the auxiliary plate, form a nail head portion formed by the electron beam welding within the auxiliary plate, remove the auxiliary plate, and remove the nail head portion. It is characterized by including the step of.

Another manufacturing method according to the present invention is a method for manufacturing a structural member including a welded portion, which includes a step of electron beam welding steel materials and a step of stress-relieving annealing treatment of the welded portion. The method is characterized by including a step of overlay welding after removing the nail head portion.

Another manufacturing method according to the present invention is a method for manufacturing a structural member including a welded part, which includes a step of electron beam welding the steel material and a step of stress-relieving annealing treatment of the welded part, in which the welded part on the surface of the steel material is deleted. The method is characterized in that it includes the steps of forming a groove, performing electron beam welding within the groove, and then performing overlay welding within the groove.

[Effect]

The reasons for limiting the above component ranges are as follows.

C content is an important requirement in this invention. From the viewpoint of weldability, the lower the C content, the better, but in order to ensure strength at room temperature, it is required to be 0.05% or more. On the other hand, if it exceeds 0.10%, weldability deteriorates, so the important ratio is set to 0.05 to 0.10%.

Si is an element that has a deoxidizing effect and increases the strength at room temperature, but if it exceeds O and SO%, it reduces weldability.

Mn has deoxidizing and desulfurizing effects, and at least 0.4% is required to provide strength. On the other hand, if it exceeds 1%, weld hardening problems will occur, so the content should be 0.8% or less.

Both Cr and MO are important elements that increase high temperature strength and corrosion resistance. From that point of view, Cr is 0.94% to 2.
62%, and Mo is 0.40 to 1.15%. In turbine diaphragms for nuclear power generation equipment, the Cr content is 0.94 to 1.56% or 1.88 to 2.62% depending on the part.
, Mo content 0.4-0.7% or 0.85-1.1
It is best to limit its use to 5%. Cr is an element that improves hardenability and contributes to the formation of a high-strength structure, and MO is a precipitation-hardening element that precipitates fine carbides during tempering, contributing to an increase in strength. do.

B is an element that improves hardenability and is effective for strengthening, and is particularly used to improve the strength level. High strength due to the action of B makes it possible to reduce the C content, but 15p
If it exceeds pm+, the toughness and weldability will deteriorate, so the upper limit is set at 0.0015%. The appropriate range of B is particularly preferably 0.0005 to 0.0010%.

Next, A12yTl, Ca, etc. will be described.

These are important requirements that influence the stress relief annealing cracking susceptibility of the low-carbon Cr-Mo steel weld metal of the present invention.

First, the addition of Ti is effective in suppressing stress removal annealing cracking in B-added steel.Tx forms oxides like Al, so it has a deoxidizing effect, and also forms nitrides. .

At present, the effective reason for adding Ti is not clear, but 0.01 to 0.5% was an appropriate range.

Ca or Mg has a strong affinity with Mn, Fe, and S, and especially S promotes stress relief annealing cracking susceptibility.
The addition of Ca or Mg was found to be effective in suppressing the segregation of oxides into grain boundaries.

The appropriate amount is as follows.

Ca and Mg are o.001% or less, CaS or M
The amount is insufficient to fix S as gS, and cracking cannot be prevented. Further, addition of a large amount exceeding 0.1% of each is not preferable because oxides such as CaS or MgO may increase in the steel, resulting in deterioration of toughness.

On the other hand, S in steel materials is generally fixed as a sulfide as described above, so that grain boundary segregation of S in a solid solution state can be suppressed, but the smaller the amount, the better. The amount needs to be 0.002% or less.

Next, Al1 has a deoxidizing effect, and is known to partially dissolve in austenite and bind nitrogen, etc. to make crystal grains finer. I discovered the surprising fact that it is an element.

Therefore, the lower the AQ, the better, but since there are restrictions on steel manufacturing, the upper limit was set at 0.002%.

The reasons for limiting the composition of the low C-Cr-Mo steel for structural members such as turbine diaphragms of the present invention have been explained above.
The steel of the present invention is produced as a steel material by Wj rolling with the above-mentioned composition adjustment, followed by one normal rolling process, normalizing, tempering or annealing.

On the other hand, as mentioned above, cracks in electron beam welded metal of low C-Cr-Mo steel are caused by solidification segregation, but the nail head part where the residual stress is maximum is the most likely to crack.
In order to prevent stress relief annealing cracking, a method of reducing the residual stress as much as possible can be considered, but it is also effective to remove nail heads with a lot of solidification segregation.

〔Example〕

An embodiment of the present invention is shown below in Table 29, Table 3, and Figures 1 and 4.
This will be explained with reference to FIGS. 9 to 9.

The chemical composition of the sample material is shown in Table 2. No. in Table 2.

Steels No. 5 to No. 8 are inventive steels belonging to the composition range of the present invention, as noted above. Steels No. 1 to No. 4 are comparison steels. Incidentally, steels No. 3 and No. 4 are commercially available steels with high C, which greatly exceed the upper limit of 0.10% of the present invention.

After melt rolling, these steels were annealed by heating at 930° C. for 1 hour, and then tempered. The tempering treatment was performed at 620℃ for materials equivalent to SCMVa, and
V4 equivalent material was maintained at 690°C for 2 hours.

Using these test materials, it was first examined whether the strength as Cr-Mot* was satisfied. The test piece is a round bar test piece with a diameter of 8 mm, a parallel portion of 35 mm, and a gauge length of 28 mm.

In addition, the weldability test used a diagonal Y-shaped cracking test specified in JIS Z3158, and the preheating temperature to prevent welding cracking was investigated. The test piece had a length of 200 mm, a width of 150 mm, and a plate thickness of 25 mm, with a diagonal Y group having a length of 80 mm in which a test bead was welded at the center in the longitudinal direction.

Test welding used commercially available co-metallic welding materials, varying the preheating temperature, welding current 165A, and welding voltage 25V.

The welding speed was 15 as/win.

Inspection of cracks in the welded area was carried out by examining the cut surface of the weld length into six equal parts after 72 hours of test welding.

Next, we will describe a method for evaluating the stress relief annealing cracking susceptibility of electron beam welded metal nail heads. There are various test methods for evaluating stress relief annealing cracking susceptibility, but the present invention shows high cracking susceptibility at a limited location, the electron beam welded metal nail head, and therefore it is suitable for welding conditions for turbine diaphragms. I decided to make a welded joint along the lines. A circumferential notch was cut into a round bar made of all weld metal taken from the nail head, and the cracking critical stress obtained from a stress relaxation test simulating the actual stress relief annealing process was evaluated.

4 and 5 are a perspective view and a front view of the main parts showing the shape of a welded joint, and FIG. 6 shows the shape of a round bar test piece and the direction of loading. In the figure, 13 and 14 are members to be welded, 15 is an electron beam welded part, 16 is a round bar test piece, and P is a load direction. The welding joint has an accelerating voltage of 90kv and a welding beam current of 3
It was made under the conditions of 20 mA, a welding speed of 15 cn/win, and a downward attitude with the focus position set at -80 mm from the surface of the workpiece.

The above test results are summarized in Table 3. As a result, the following was found regarding the steel of the present invention.

(1) Mechanical Properties Although the lower limit of C content was reduced to 0.05% to improve weldability, it was not observed that the strength at room temperature was below the standard value.

(2) Weldability Weld cracking susceptibility varies depending on the C content.

C content of high C materials No. 3, No. 4, and 4 materials is 0.14~
At 0.16%, the cracking prevention temperature is 175 to 200°C, whereas in the steel of the present invention, by limiting the C content to 0.10% or less, the preheating temperature can be lowered to 100°C or less.

(3) Stress-relief annealing cracking susceptibility The relationship between stress-relief annealing cracking critical stress and C content (wt%) is summarized in Figure 1.

In the figure, the target value is the cracking limit stress based on conventional results. In other words, obtaining a welded portion that is equal to or higher than this target value of 19 kgf/m'' is a measure of the soundness of the actual machine.

As is clear from the figure, the comparison steel was 5 to 8 k lower than the target value.
gf/m2 showed a low crack initiation stress. As mentioned above, the reason for this is that excessive impurities such as S segregated at the solidification interface are included, which promotes the precipitation and aggregation of carbides at grain boundaries during the stress relief annealing process. It was due to the results.

Therefore, in electron beam welding, it is necessary to suppress the amount of impurities more strictly than the measures to prevent cracking in the heat-affected zone.

It was also found that by adding Ca, Mg, etc., segregation of S to grain boundaries can be suppressed, resulting in improved cracking resistance.

The steels of the present invention, No. 5 to No. 008 shown in Table 2, have improved cracking resistance due to low S and low Afl and the addition of Ti, Ca or Mg, but as shown in Table 3, It is clear that the critical stress for cracking is improved by 50% or more compared to the comparative steel.

On the other hand, since impurities are segregated in the final solidified nail head as described above, a method of removing this nail head is also considered to be effective in preventing stress relief annealing cracking.

FIGS. 7(a) to 7(d) show the lamination state by overlay welding after grinding the nail head portion 17. Reference numeral 618 indicates the overlay welding portion. Here, the build-up welding is performed by submerged arc welding, with a welding current of 600 A and a welding voltage of 32 V after preheating to 100°C.
, carried out at a welding speed of 28 cm/win.

Figures 8(a) to (d) show joint shapes fabricated to confirm that stress relief annealing cracking can be prevented by simply removing the nail head, rather than by sensual welding as described above. . This is a method in which the auxiliary plate 19 is applied to the part to be electron beam welded, the nail head 17 is formed on the auxiliary plate 19, and the auxiliary plate 19 is removed after welding. The material of the auxiliary plate 19 is the same as that of the materials to be welded 13 and 14.

FIGS. 9(a) to 9(d) show still another manufacturing method.

A groove 20 is formed in advance, and the nail head 1 is attached to this groove 20.
This is a method of overlay welding to bury 7.

The results of stress relief annealing cracking susceptibility in these welds are shown in FIG. In both cases, it was found that the stress relief annealing cracking resistance was improved compared to the nail head portion. Particularly when using sensual welding, a remarkable effect can be seen.

This is because the structure of the weld metal is improved by the heat received by the subsequent weld bead.

〔Effect of the invention〕

According to the present invention, the preheating temperature for welding is set to 100°C.
By being able to reduce the temperature to a considerably low temperature, it is possible to not only improve the welding work environment but also to obtain a structural member including a welded part without the risk of stress relief cracking at the edge of the electron beam welded part. can. In particular, if the present invention is applied to a turbine diaphragm for nuclear power generation equipment, a highly reliable product can be obtained.

Further, according to the manufacturing method of the present invention, it is possible to easily obtain a structural member with less risk of occurrence of stress relief edge annealing cracks.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between the crack initiation limit stress and the amount of carbon in the stress relief edge annealing crack test, Figure 2 is a plan view of the turbine diaphragm, Figure 3 is a sectional view taken along the line ■-■ in Figure 2, and Figure 4 The figure is a perspective view showing the shape of a welded joint, Figure 5 is an enlarged front view of the main part of Figure 4, Figure 6 is a side view of a round bar test piece, and Figures 7 (a) to (
d), Figures 8(a) to (d) and Figures 9(a) to (d)
are process diagrams showing different manufacturing methods according to the present invention,
FIG. 10 shows a relation diagram between stress relief edge annealing crack generation limit stress and sampling position. 1... Nozzle blade, 2... Outer ring, 3... Inner ring, 4, 5
・Support spacer, 6, 7.8...Support wheel, 15...
Electron beam welding section, 17... Nail head, 18...
Overlay welding part, 19... Auxiliary plate. Diagram

Claims (1)

[Claims] 1. In a structural member including a welded part that has been subjected to stress relief annealing after electron beam welding of steel materials, at least one of the steel materials to be welded has a C content of 0.05 by weight. ~0.
1%, Si 0.8% or less, Mn 0.4-0.8%,
Cr: 0.94-2.62%, Mo: 0.40-1.1
5%, B 0.0005 to 0.0015%, and Ti 0.0005% to 0.0015%.
01-0.5%, S is 0.002% or less, the balance is Fe
and a structural member including a welded part, characterized in that it is a steel material containing unavoidable impurities. 2. In claim 1, at least one of the steel materials to be welded further contains 0.001 to 0.1 of Ca and/or Mg.
Structural members including welded parts that are steel materials containing %. 3. A structural member according to claim 1, including a welded portion, wherein at least one of the steel materials to be welded is a steel material further containing 0.002% or less of Al. 4. In claim 1, at least one of the steel materials to be welded further contains 0.001 to 0.1 of Ca and/or Mg.
%, a structural member including a welded part that is a steel material containing 0.002% or less of Al. 5. In any one of claims 1 to 4, the structural member including the welded portion includes a nozzle blade, a support spacer fixed to both ends of the nozzle blade, and an inner ring and an outer ring each of which is electron beam welded to both support spacers. , the inner ring and its supporting spacer,
and a structural member including a weld, which is a turbine diaphragm consisting of an outer ring and a support ring arc welded to its support spacer. 6. In a method for manufacturing a structural member including a welded part, which includes a process of electron beam welding steel materials and a process of stress-relieving annealing treatment of the welded part, an auxiliary plate is applied to the part to be welded with the electron beam, and the auxiliary plate is The method includes the steps of performing electron beam welding from above the plate, forming a nail head portion formed by the electron beam welding in the auxiliary plate, and removing the auxiliary plate to remove the nail head portion. A method for manufacturing a structural member including a welded part. 7. In a method for manufacturing a structural member including a welded part, which includes a process of electron beam welding steel materials and a process of stress-relieving annealing the welded part, after removing the nail head part formed by electron beam welding, A method for manufacturing a structural member including a welded part, the method comprising a step of overlay welding. 8. In a method for manufacturing a structural member including a welded part, which includes a process of electron beam welding the steel material and a process of stress-relieving annealing treatment of the welded part, the welded part on the surface of the steel material is deleted to form a groove, A method for manufacturing a structural member including a welded part, the method comprising the steps of electron beam welding within the groove and then overlay welding within the groove. 9. Low C-Cr for structural members including welded parts, characterized by being made of the steel material composition according to any one of claims 1 to 4.
-Mo steel.