JP3869576B2

JP3869576B2 - Heat-resistant steel welding method

Info

Publication number: JP3869576B2
Application number: JP6200099A
Authority: JP
Inventors: 恭佐藤; 広治田村; 浩一光畑
Original assignee: Babcock Hitachi KK
Current assignee: Mitsubishi Power Ltd
Priority date: 1999-03-09
Filing date: 1999-03-09
Publication date: 2007-01-17
Anticipated expiration: 2019-03-09
Also published as: JP2000254795A

Description

【０００１】
【発明の属する技術分野】
本発明は、耐熱鋼の溶接に係わり、特にボイラ、化学プラント等、高温あるいは高圧力の条件下で使用される高強度耐熱鋼に好適な溶接方法及び後熱処理方法に関するものである。
【０００２】
【従来の技術】
発電用ボイラや各種熱交換器等においては、多数の伝熱管群及び伝熱管を集合する管寄せと配管が高温、高圧の条件下で使用されている。近年、特に大容量の発電用ボイラにおいては発電効率向上のため蒸気条件が高温高圧化しつつあり、伝熱管材や配管材或いは鋼板材として、従来多用されてきたＣｒ含有量２．２５％以下でフェライト／パーライト組織の耐熱鋼に替わる、高温強度の高い新しい耐熱鋼が開発されている。
【０００３】
その代表的な例として以下に示すものが既に製品化され、実機で使用され始めている。
▲１▼９％Ｃｒ鋼にＭｏ，Ｎｂ及びＶを添加したもの。
▲２▼９％Ｃｒ鋼にＭｏ，Ｎｂ，Ｖ及びＷを添加したもの。
▲３▼１１％Ｃｒ鋼にＭｏ，Ｎｂ，Ｖ及びＷを添加したもの。
【０００４】
これら新しい耐熱鋼の特徴は、いずれも１０５０°Ｃ前後の焼ならし及び７８０°Ｃ前後の焼戻し処理すなわち調質処理を行うことにより、焼戻しマルテンサイト組織或いは焼戻しベイナイト組織として高温強度を高めたことにある。
【０００５】
これらの耐熱鋼を実機部材として使用するためには溶接で接合する必要がある。それぞれの耐熱鋼に合わせた共金系の溶接材料が開発されており、図４に示すように、従来鋼と同様に溶接施工し、７２０〜７５０°Ｃ前後の応力除去焼鈍を行うのが一般的である。ところで焼戻しマルテンサイト組織の鋼は、溶接の際に溶接熱影響部（ＨＡＺ）においてビッカース硬さで２０〜４０程度の軟化が生じ、大入熱の溶接では軟化領域が広くなって継手のクリープ破断強度が母材より低くなることがある。このため溶接部の構造によっては軟化領域の幅が広くならないよう、溶接施工の際に入熱を小さく抑える等細心の注意が必要であった。
【０００６】
ＨＡＺの軟化を回復させる方法としては、溶接後に上述の耐熱鋼と同じ条件で焼ならし及び焼戻し処理を行う必要がある。しかしこの場合、ＨＡＺの組織は溶接前の状態に回復するが、従来の溶接材料では溶接金属のクリープ破断強度が低下するという問題が生じた。
【０００７】
すなわち、従来の溶接材料の成分設計の考え方では、溶接金属が凝固、急冷された組織のままで７２０〜７５０°Ｃ前後の応力除去焼鈍を受けて使用することを前提としており、溶接割れの防止及び強度と組織の調整を目的としてＣを母材より低く抑え、その他の添加元素量も調整している。
【０００８】
その結果、溶接後に焼ならしを行い、さらに７８０°Ｃ前後の高温で焼戻しを行うと溶接金属の高温強度が低下する。特にサブマージアーク溶接のように大入熱溶接で１パスあたりの溶着量が多い溶接法の場合は溶接割れ防止のため、ＣとＮｂの量を低くしており、焼ならし−焼戻し処理後に溶接金属のクリープ破断強度が大きく低下するという問題があった。
【０００９】
このように一般的な従来技術による方法では、調質された高強度鋼に対しては溶接後に応力除去焼鈍した場合はＨＡＺが軟化し、また溶接後に焼ならし及び焼戻しを行った場合は溶接金属の強度が低下するため、溶接継手としての強度が低くなり、溶接施工条件や使用条件によっては設計的に安全裕度をとって肉厚を厚くする必要があった。
【００１０】
【発明が解決しようとする課題】
本発明の目的は上記した問題を解決し、容易に施工が可能で、溶接継手の強度を高めて信頼性を向上させる溶接方法及び後熱処理方法を提供することにある。
【００１１】
【課題を解決するための手段】
前記課題を解決するために、本発明は主として次のような構成を採用する。
【００１４】
Ｃｒ，Ｍｏ，Ｎｂ，Ｖ及びＷを主要添加元素として含有し、焼戻しマルテンサイト組織又は焼戻しベイナイト組織からなる耐熱鋼の溶接において、
前記耐熱鋼と同等組成のＣｒ，Ｍｏ，Ｎｂ，Ｖ及びＷに、Ｃｏを添加した溶接材料であって且つ溶接した後の６００℃の溶接金属のクリープ破断強度が前記耐熱鋼の１／０．７３以上となる溶接材料を用いて、狭開先のＴＩＧ溶接、狭開先のＭＩＧ又はＭＡＧ溶接のいずれかで溶接し、
前記狭開先のＴＩＧ溶接、狭開先のＭＩＧ又はＭＡＧ溶接のいずれかで溶接した後に、前記耐熱鋼の焼ならし温度及び焼戻し温度でそれぞれ焼ならし及び焼戻し処理を行い、さらに応力除去焼鈍の処理を行う溶接方法。
【００１５】
【発明の実施の形態】
本発明の実施形態に係る耐熱鋼の溶接方法及び後熱処理方法について、図面及び表を用いて以下説明する。
【００１６】
まず表１で、本発明の対象となる高強度耐熱鋼として代表的なものを３種類取り上げ、その基本成分系を示す。
【００１７】
【表１】

【００１８】
ここでは、９Ｃｒ−１Ｍｏ−Ｎｂ−Ｖ系で６００°Ｃのクリープ破断強度が１００ＭＰａ前後のものを「Ａ」と分類する。次に、９〜１１Ｃｒ系でＭｏ，Ｎｂ，Ｖの他に２％前後のＷを添加し、６００°Ｃのクリープ破断強度が１３０ＭＰａ前後のものを「Ｂ」と分類する。これらと類似の他の鋼種も６００°Ｃのクリープ破断強度によってＡ又はＢに分類する。６００°Ｃという温度は、これらの耐熱鋼がボイラ等でもっとも多用される温度域である。
【００１９】
次に、上記の分類に基づき表２に示すように溶接材料を選定する。
【００２０】
【表２】

【００２１】
分類Ａの耐熱鋼に対しては、当該耐熱鋼と同等程度の組成のＣｒ，Ｍｏ，Ｎｂ，ＶにＷを添加した溶接材料を適用する。分類Ｂの耐熱鋼については、当該耐熱鋼と同等程度の組成のＣｒ，Ｍｏ，Ｎｂ，Ｖ，ＷにＣｏを添加した溶接材料を適用する。これらの溶接材料に要求されるクリープ破断強度は６００°Ｃにおいて当該耐熱鋼のクリープ破断強度の約（１／０．７３）以上で、その数値を表２中に示す。その数値の根拠について以下説明する。
【００２２】
表２に示した溶接材料について溶接及び熱処理実験とクリープ破断試験を行った結果、従来の応力除去焼鈍のみを行った場合の強度σｓと焼ならし及び焼戻し後の強度σｎの間には図１に示すような相関関係があり、図中の直線の勾配が約０．７３であることが分かった。すなわち溶接後の焼ならし及び焼戻しによって、溶接金属のクリープ破断強度が０．７３倍に低下する。従って対象とする耐熱鋼自身のクリープ破断強度の（１／０．７３）以上の強度を有する溶接材料を用いればよいことが分かった。この規定に従えば、表２に示したもの以外の市販溶接材料についても、溶接及び熱処理実験やクリープ破断試験を実施しなくても、適用可否を判断することができる。
【００２３】
本発明による溶接施工及び溶接後の熱処理の流れを図２に示す。表２で選定した溶接材料を用いて溶接施工を行うが、溶接方法としては入熱を小さく抑えることができ、溶接金属の量も少ない狭開先ＴＩＧ溶接がもっとも好ましい。狭開先のＴＩＧ溶接の外に、狭開先のＭＩＧ又はＭＡＧであっても良い。溶接後に当該耐熱鋼と同じ熱処理温度で焼ならし及び焼戻し処理を行う。さらに必要に応じて応力除去焼鈍を実施してもよい。
【００２４】
次に具体的な施工例を示す。試験に用いた鋼板は板厚４０ｍｍの１１Ｃｒ２Ｗ０．４ＭｏＮｂＶ鋼で、素材圧延後に通常の調質処理（焼ならし及び焼戻し）をせず、焼き戻しのみ行ったものである。表１及び表２に従い、分類「Ｂ」の溶接材料を用いて溶接する。ここでは比較のため、分類「Ａ」の溶接材料を用いた継手も製作した。これら供試材の化学組成を表３に示す。
【００２５】
【表３】

【００２６】
溶接法は開先幅１２ｍｍの狭開先ＴＩＧ溶接とした。溶接後、図２に従い、１０５０°Ｃ×１時間の焼ならしと７８０°Ｃ×１時間の焼戻しを行い、さらに７４５°Ｃ×１時間の応力除去焼鈍を行った。熱処理後、板厚中央位置から溶接線に対して直角に継手クリープ破断試験片を採取し、試験に供した。試験片は平行部直径６ｍｍ、標点間距離３０ｍｍの丸棒型である。
【００２７】
試験結果の例を図３に示す。本発明に従った溶接材料Ｂの継手はＨＡＺで破断することなく母材部分で破断し、破断強度は耐熱鋼母材と同等であった。この結果から、ＨＡＺの軟化回復と溶接金属の強度確保が実証できた。また素材鋼板が調質されていなくても、溶接後の調質で所定の強度が得られることも確認できた。一方比較のため作成した溶接材料Ａの継手は、溶接金属の強度が母材より低くなったため溶接金属で破断し、母材強度を大幅に下回った。
【００２８】
このように本実施形態で示した溶接部では、溶接部のＨＡＺの軟化及び溶接金属の強度低下の問題が解決され、耐熱鋼母材と同等の信頼性を得ることができる。ここで、本実施形態では調質していない鋼板を素材に用いたが、従来通り調質された鋼板を素材に用いてもよいのは勿論である。即ち、当該耐熱鋼の素材として、圧延のままのもの、又は圧延後に焼なましのみを行ったものを用いても良い。
【００２９】
以上説明したように、本発明の実施形態は、次のような溶接方法及び後熱処理方法、並びに作用を奏するものを含むものである。
【００３０】
母材より合金成分が多く、高温強度の高い溶接材料を用いて望ましくは入熱量の小さい狭開先溶接法で溶接すると同時に、溶接後に焼ならし及び焼戻し処理を行ってＨＡＺの強度を回復させるものである。ここで、溶接の際に軟化したＨＡＺの組織は、焼ならし及び焼戻し処理によって母材一般部と同等に回復し、溶接金属の強度は焼ならし及び焼戻し処理で本来の強度より低下するが、母材と同等以上の強度が得られる。
【００３１】
【発明の効果】
本発明による溶接では、溶接部の強度が向上して品質信頼性が向上する。これに伴い溶接部の設計裕度を従来の非調質鋼と同等にして肉厚を薄くすることが可能で、工業的に効果が大である。
【００３２】
さらに、素材鋼管或いは鋼板の製造時には、焼ならし及び焼戻し処理が不要で素材の製造コストを低減でき、ひいてはプラントの建設コストを下げることができるので経済的な効果も大である。
【図面の簡単な説明】
【図１】本発明による実施形態で示した溶接金属のクリープ破断強度を示す図である。
【図２】本発明による実施形態で示した溶接及び熱処理の流れ図である。
【図３】本発明による実施形態で示した溶接継手のクリープ破断試験結果を示す図である。
【図４】従来技術による溶接及び熱処理の流れ図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to welding of heat-resistant steel, and more particularly to a welding method and a post heat treatment method suitable for high-strength heat-resistant steel used under high-temperature or high-pressure conditions such as boilers and chemical plants.
[0002]
[Prior art]
In power generation boilers, various heat exchangers, etc., a large number of heat transfer tube groups and headers and pipes that collect the heat transfer tubes are used under conditions of high temperature and high pressure. In recent years, especially in large-capacity power generation boilers, steam conditions are increasing at high temperatures and pressures to improve power generation efficiency, and as a heat transfer tube material, piping material, or steel plate material, the Cr content that has been widely used conventionally is 2.25% or less. New heat-resistant steels with high high-temperature strength have been developed to replace heat-resistant steels with a ferrite / pearlite structure.
[0003]
As typical examples, the following are already commercialized and are starting to be used in actual machines.
(1) 9% Cr steel with Mo, Nb and V added.
(2) Mo, Nb, V and W added to 9% Cr steel.
(3) 11% Cr steel with Mo, Nb, V and W added.
[0004]
The characteristics of these new heat-resistant steels are that the high-temperature strength has been improved as a tempered martensite structure or a tempered bainite structure by performing normalization at around 1050 ° C and tempering treatment, ie, tempering treatment at around 780 ° C. It is in.
[0005]
In order to use these heat resistant steels as actual machine members, it is necessary to join them by welding. Common metal welding materials suitable for each heat-resistant steel have been developed. As shown in FIG. 4, it is common practice to perform welding in the same way as conventional steel and perform stress-relieving annealing at around 720-750 ° C. Is. By the way, steel with a tempered martensite structure is softened by about 20 to 40 in terms of Vickers hardness in the weld heat affected zone (HAZ) during welding. The strength may be lower than the base material. For this reason, depending on the structure of the welded portion, it was necessary to pay close attention to keep the heat input small during welding so that the width of the softened region does not widen.
[0006]
As a method for recovering the softening of the HAZ, it is necessary to perform normalizing and tempering treatment under the same conditions as those of the above heat-resistant steel after welding. However, in this case, the HAZ structure is restored to the state before welding, but the conventional welding material has a problem that the creep rupture strength of the weld metal is lowered.
[0007]
That is, the conventional concept of component design of welding materials is based on the premise that the weld metal is used after undergoing stress-relief annealing at around 720 to 750 ° C. with the solidified and rapidly cooled structure. For the purpose of adjusting the strength and structure, C is kept lower than the base material, and the amount of other additive elements is adjusted.
[0008]
As a result, when normalizing is performed after welding and further tempering is performed at a high temperature of around 780 ° C., the high-temperature strength of the weld metal decreases. In particular, in the case of a welding method such as submerged arc welding with high heat input welding and a large amount of welding per pass, the amount of C and Nb is reduced to prevent weld cracking, and welding is performed after normalizing and tempering. There was a problem that the creep rupture strength of the metal was greatly reduced.
[0009]
As described above, in the conventional method according to the prior art, the HAZ is softened when the stress-relief annealing is performed after welding on the tempered high-strength steel, and the welding is performed when normalizing and tempering are performed after the welding. Since the strength of the metal is lowered, the strength as a welded joint is lowered, and it is necessary to increase the wall thickness by taking a safety margin in terms of design depending on welding conditions and usage conditions.
[0010]
[Problems to be solved by the invention]
An object of the present invention is to provide a welding method and a post heat treatment method that solve the above-described problems, can be easily constructed, and increase the strength of a welded joint to improve reliability.
[0011]
[Means for Solving the Problems]
In order to solve the above problems, the present invention mainly adopts the following configuration.
[0014]
In welding of heat-resistant steel containing Cr, Mo, Nb, V and W as main additive elements, and comprising a tempered martensite structure or a tempered bainite structure,
The creep rupture strength of the weld metal at 600 ° C. after welding, which is a welding material in which Co is added to Cr, Mo, Nb, V and W having the same composition as that of the heat resistant steel, is 1 / 0.0. Welding with either narrow groove TIG welding, narrow groove MIG or MAG welding using a welding material of 73 or more,
After welding by either narrow groove TIG welding, narrow groove MIG or MAG welding, normalization and tempering treatment are performed at the normalizing temperature and tempering temperature of the heat-resistant steel, respectively, and further stress relief annealing Welding method to perform the process.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
A welding method and post heat treatment method for heat-resistant steel according to embodiments of the present invention will be described below with reference to the drawings and tables.
[0016]
First, in Table 1, three types of typical high-strength heat-resistant steels that are the subject of the present invention are taken up and their basic component systems are shown.
[0017]
[Table 1]

[0018]
Here, a 9Cr-1Mo-Nb-V system having a creep rupture strength at 600 ° C. of around 100 MPa is classified as “A”. Next, in the 9-11Cr system, W of about 2% is added in addition to Mo, Nb, and V, and the creep rupture strength at 600 ° C. is about 130 MPa, and is classified as “B”. Other steel types similar to these are also classified as A or B according to the creep rupture strength at 600 ° C. The temperature of 600 ° C. is a temperature range in which these heat-resistant steels are most frequently used in boilers and the like.
[0019]
Next, welding materials are selected as shown in Table 2 based on the above classification.
[0020]
[Table 2]

[0021]
For a heat-resistant steel of class A, a welding material in which W is added to Cr, Mo, Nb, and V having the same composition as that of the heat-resistant steel is applied. For heat-resistant steels of class B, welding materials in which Co is added to Cr, Mo, Nb, V, W having the same composition as the heat-resistant steel are applied. The creep rupture strength required for these welding materials is about (1 / 0.73) or more of the creep rupture strength of the heat-resistant steel at 600 ° C., and the numerical values are shown in Table 2. The basis of the numerical value will be described below.
[0022]
As a result of conducting welding and heat treatment experiments and creep rupture tests on the welding materials shown in Table 2, there is a difference between the strength σs in the case of performing only conventional stress relief annealing and the strength σn after normalization and tempering. It was found that the slope of the straight line in the figure was about 0.73. That is, the creep rupture strength of the weld metal is reduced by 0.73 times by normalization and tempering after welding. Accordingly, it has been found that a welding material having a strength of (1 / 0.73) or more of the creep rupture strength of the target heat resistant steel itself may be used. If this rule is followed, it can be determined whether or not commercial welding materials other than those shown in Table 2 can be applied without carrying out welding and heat treatment experiments or creep rupture tests.
[0023]
FIG. 2 shows the flow of welding construction and heat treatment after welding according to the present invention. Welding is performed using the welding material selected in Table 2, but the narrow groove TIG welding is most preferable as the welding method because heat input can be kept small and the amount of weld metal is small. A narrow groove MIG or MAG may be used in addition to the narrow groove TIG welding. After welding, normalization and tempering are performed at the same heat treatment temperature as the heat-resistant steel. Furthermore, you may implement stress relief annealing as needed.
[0024]
Next, a specific construction example is shown. The steel plate used for the test was 11Cr2W0.4MoNbV steel with a plate thickness of 40 mm, which was subjected to only tempering without rolling the material (normalizing and tempering) after rolling the material. According to Table 1 and Table 2, welding is performed using a welding material of classification “B”. For comparison, a joint using a welding material of classification “A” was also manufactured. Table 3 shows the chemical compositions of these test materials.
[0025]
[Table 3]

[0026]
The welding method was narrow groove TIG welding with a groove width of 12 mm. After welding, in accordance with FIG. 2, normalization at 1050 ° C. × 1 hour, tempering at 780 ° C. × 1 hour, and annealing at 745 ° C. × 1 hour were further performed. After the heat treatment, a joint creep rupture test piece was taken from the center position of the plate thickness at a right angle to the weld line and used for the test. The test piece is a round bar type with a parallel part diameter of 6 mm and a distance between gauge points of 30 mm.
[0027]
An example of the test result is shown in FIG. The joint of the welding material B according to the present invention was broken at the base material portion without breaking at HAZ, and the breaking strength was equivalent to that of the heat-resistant steel base material. From this result, it was demonstrated that the HAZ was softened and the weld metal was secured. Moreover, even if the raw steel plate was not tempered, it was confirmed that a predetermined strength was obtained by tempering after welding. On the other hand, the joint of the welding material A prepared for comparison was broken by the weld metal because the strength of the weld metal was lower than that of the base metal, and was significantly lower than the base metal strength.
[0028]
As described above, in the welded portion shown in the present embodiment, the problems of softening the HAZ of the welded portion and lowering the strength of the weld metal are solved, and reliability equivalent to that of the heat-resistant steel base material can be obtained. Here, in this embodiment, the steel plate which has not been tempered was used for the raw material, but it is needless to say that the tempered steel plate may be used for the raw material. That is, as the material of the heat-resistant steel, a raw material that has been rolled, or a material that has only been annealed after rolling may be used.
[0029]
As described above, the embodiment of the present invention includes the following welding method and post-heat treatment method, and those that exhibit the effects.
[0030]
Welding with a narrow gap welding method that has a higher alloy composition than the base metal and has a high high-temperature strength, preferably with a low heat input, and at the same time normalizing and tempering after welding to restore the HAZ strength Is. Here, the structure of the HAZ softened during welding is recovered to the same level as the general base material by normalizing and tempering treatments, and the strength of the weld metal is lowered from the original strength by normalizing and tempering treatments. A strength equal to or higher than that of the base material can be obtained.
[0031]
【The invention's effect】
In the welding according to the present invention, the strength of the welded portion is improved and the quality reliability is improved. Accordingly, it is possible to make the design margin of the welded part the same as that of the conventional non-tempered steel and reduce the wall thickness, which is industrially effective.
[0032]
Furthermore, when manufacturing the raw steel pipe or steel plate, normalizing and tempering processes are not required, and the manufacturing cost of the raw material can be reduced. As a result, the construction cost of the plant can be reduced, so that an economic effect is great.
[Brief description of the drawings]
FIG. 1 is a diagram showing the creep rupture strength of a weld metal shown in an embodiment according to the present invention.
FIG. 2 is a flowchart of welding and heat treatment shown in the embodiment according to the present invention.
FIG. 3 is a diagram showing a creep rupture test result of the welded joint shown in the embodiment according to the present invention.
FIG. 4 is a flowchart of welding and heat treatment according to the prior art.

Claims

In welding of heat-resistant steel containing Cr, Mo, Nb, V and W as main additive elements, and comprising a tempered martensite structure or a tempered bainite structure,
The creep rupture strength of the weld metal at 600 ° C. after welding, which is a welding material in which Co is added to Cr, Mo, Nb, V and W having the same composition as that of the heat resistant steel, is 1 / 0.0. Using a welding material of 73 or more, welding with either narrow groove TIG welding, narrow groove MIG or MAG welding ,
After welding by either narrow groove TIG welding, narrow groove MIG or MAG welding, normalization and tempering treatment are performed at the normalizing temperature and tempering temperature of the heat-resistant steel, respectively, and further stress relief annealing The welding method characterized by performing the process.

In claim 1,
A welding method characterized by using a material of the heat-resisting steel as-rolled or a material that has been annealed after rolling.