JPH0431790B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to welding ultra-high strength maraging steel, and more particularly to electron beam welding or TIG welding of 18% Ni maraging steel to obtain high joint efficiency. 18%Ni maraging steel usually has the following weight ratio: C: 0.03% or less, Si: 0.10% or less, Mn: 0.010% or less, P: 0.010% or less, S: 0.010% or less, Ni: 17.0-19.0%, Co : 7.0 to 15.0%, Mo: 3.0 to 6.5%, Ti: 0.2 to 1.5%, Al: 0.3% or less, and further contains B, Ca or Zr as necessary.
It is a precipitation-strengthened steel containing Because this steel has both high strength and good toughness through relatively simple heat treatment, it is used in solid fuel rocket motor cases, deep-sea submersibles, and uranium centrifuge rotating barrels. In these applications, the strength/density ratio is an important factor, so there is a strong demand for higher strength, and to meet this demand, 18%Ni-8-9%Co-3-5%Mo- 0.4~0.5
In addition to class ² steel with a tensile strength of 180 to 210 Kgf/mm, which consists of %Ti, even higher strength maraging steels have been developed. That is, 18%Ni-12.5%Co-3%Mo-1.2%Ti steel, 18%Ni-15~20%Co-5~6.5%Mo-1%Ti
−0.05 to 0.3% Al (Japanese Unexamined Patent Publication No. 58-25457) etc. are 245Kgf/ ^mm2 and 270Kgf/mm2, respectively.
It has a tensile strength of Kgf/ ^mm2 . The concept behind the development of these ultra-high strength maraging steels is as follows. The strength of maraging steel after aging treatment is
Mo, Co and Ti mainly contribute, and the strength increases as the content of these elements increases. On the other hand, among these elements, Mo and Ti are elements that significantly lower the temperature at which the transformation from austenite phase to martensite phase starts (hereinafter referred to as the Ms point) during cooling after solution treatment. When the element content of Hardening cannot be obtained. Furthermore, when Mo is contained in a large amount, it forms coarse intermetallic compounds at high temperatures exposed during manufacturing, impairing toughness. From the above viewpoint, by increasing the Co content, which has a small effect of lowering the Ms point, and keeping the Mo content low, an increase in strength is ensured and a decrease in toughness is suppressed. Now, in many of the above applications, steel plates manufactured in a factory are processed into the required shape and then welded, then subjected to solution treatment and aged in pairs, or directly aged after welding. use. In this case, TIG welding is usually used, but the strength of the welded part is reduced due to concentration segregation that occurs during solidification of the weld metal. In other words, the solidification phase of weld metal basically develops into a dendritic shape, similar to the solidification phase of a normal steel ingot, but the final solidification phase between the dendrites is
Since Ni, Mo, and Ti are concentrated, the Ms point of that part decreases and becomes a retained austenite phase after cooling.
This austenite phase is stable and can be heated up to 800℃ after welding.
Since it is not eliminated by solution treatment at ~900°C, the strength of the weld metal becomes lower than the strength of the steel sheet. However, in order to increase the strength of steel plates,
As mentioned above, it is necessary to increase the content of Co, Mo, and Ti, and by suppressing the addition of Mo and Ti, Ms.
Although the decrease in the point is suppressed, as the strength of the steel sheet increases, the amount of retained austenite phase in the weld metal increases, and the decrease in strength becomes more significant. Since the design of a structure is based on the strength of the lowest strength part, if the strength of the welded joint is low, even if the strength of the steel plate itself increases, this will not be reflected in the design. As a measure to solve the above-mentioned problems, a technique is disclosed in JP-A-51-122616 in which cold working is applied to the welded part after welding. This means that the strength and toughness of the welded joint can be made equivalent to that of the base metal by performing a solution treatment after welding and then an aging treatment with 60 to 85% cold working. However, cold working of welded structures is limited to very specific structures and cannot be applied to all welded structures. One method is to heat only the weld metal or the entire structure after welding to a temperature at which the concentrated segregated parts in the weld metal are sufficiently diffused and homogenized, but for this purpose, it is necessary to heat the weld metal to a temperature of 1000℃ or higher. In this case, deterioration of the base material properties due to high temperature heating is unavoidable. The present inventors, in order to overcome such difficulties,
The influence of these elements on the strength and Ms point,
As a result of a detailed study of the degree of segregation during solidification, we found that insert metal or TIG in electron beam welding
By appropriately determining the components of the filler rod in welding, we have achieved the present invention, which achieves excellent joint efficiency by adjusting the composition of the weld metal within a certain range. Hereinafter, the tests conducted by the present inventors will be explained in detail. 17~19%Ni-7~15%Co-4~6.5%Mo-0.3
Maraging steel with a composition of ~1.5% Ti was melted in an experimental furnace, thoroughly soaked, and then rolled into a 15 mm thick steel plate. For these steel plates, 850℃~900℃
In addition to measuring the Ms point after solution treatment at °C,
After aging at 480°C for 5 hours, a tensile test was conducted. In addition, for some parts, the heat input is 6000 to 8000 J/cm.
Electron beam welding under the conditions of
TIG welding was performed under the conditions of 20,000 J/cm and 15 passes, and after solution treatment and aging treatment similar to steel plates, the amount of retained austenite in the weld metal was measured.
The amount of concentrated segregation was measured using an electron probe microanalyzer (EPMA). Figure 1 shows the relationship between the strength of a steel plate and the Mo, Co, and Ti contents. Mo equivalent = (%Mo) + 1/3 x (%Co) + 3 x (%Ti) ... (1) When defined as , it can be seen that there is a relationship as follows: tensile strength = 9.7 x (Mo equivalent) + 117... (2). Figure 2 shows the relationship between the Ms point of the steel plate and the alloying element content. It is expressed as (%Mo) −18×(%Ti) (3) and is compared with the actual measured value at the Ms point, and an extremely good agreement can be seen. On the other hand, the degree of segregation of weld metal by EPMA, that is, the segregation content ratio/average content ratio, was measured as follows. Ni: 1.1, Co: 1.0, Mo: 1.5, Ti: 2.0. This degree of segregation was almost constant regardless of the heat input when looking only at electron beam welding and TIG welding. By multiplying the coefficient of each element in the above equation (3) by this segregation degree, the Ms point of the concentrated segregation part of the weld metal can be estimated as shown in the following equation (4). Ms=941−28×(%Ni) −1.7×(%Co) −71(%Mo) −38(%Ti) ……(4) However, (%Ni), (%Co), (%Mo) , and (%Ti) are the average content of each element in the weld metal. Figure 3 shows the relationship between the proportion of retained austenite phase in the electron beam weld metal structure and the estimated Ms point of the weld metal concentrated and segregated area.
When the temperature drops below 110℃, austenite begins to remain,
It can be seen that as the estimated Ms point decreases, the proportion of retained austenite phase increases. As shown above, the present invention substantially eliminates retained austenite, which causes strength reduction, by limiting the composition of the weld metal so that the Ms point of the maximum concentration segregation zone is 110°C or higher. to 0
() In addition, by limiting the Mo equivalent to more than the Mo equivalent of the base material, the two conditions of ensuring strength greater than that of the base material due to aging of the martensite single phase can be ensured. High welded joint efficiency is obtained by this method, and as mentioned above, its hardening is obvious. Furthermore, if the Ms point of the maximum concentration segregation part exceeds 200, the Mo equivalent will become small and sufficient strength as a maraging steel will not be obtained, according to the relationships in equations (1), (2), and (4) above. Limit the upper limit of Ms points to 200 or less. Furthermore, if the Mo equivalent of the weld exceeds (Mo equivalent of the base metal + 1), the strength of the weld metal increases more than necessary and becomes brittle, which is not preferable. The above findings were obtained for so-called bead-on-plate joints that do not use insert metal or filler rods, but in electron beam welding or TIG welding that uses insert metals or filler rods, the In addition to considering the amount of filler metal, it is necessary to design the composition of the insert metal or filler rod so that the weld metal composition is as limited by () and () above. The amount of base metal penetration varies depending on the welding method and welding conditions, but it is 40 to 60% in the case of electron beam welding using insert metal, and 40% to 60% in the case of electron beam welding using insert metal, and
For TIG welding, 20-40% is normal. Next, the curing of the present invention will be specifically explained with reference to Examples. Example 18% Ni maraging steel with the composition shown in Table 1
50Kg steel ingots No. 1 and No. 2 were melted in vacuum. Steel No. 1 is a 210Kgf/mm ² maraging steel, and Steel No. 2 is a 245Kgf/mm ² grade maraging steel. After homogenizing at 1200℃ for 24 hours, it was hot forged and rolled into a 15mm thick steel plate, which was then solution-treated at 900℃. On the other hand, 10 kg steel ingots A, B, C, and D of 18% Ni maraging steel having the composition shown in Table 2 were melted in vacuum to be used as filler rods or insert metals. After homogenization at 1200℃ for 24 hours, hot forging,
A steel plate of 3 mm was finished by rolling, and after solution treatment at 900°C, some were cold rolled to a thickness of 2 mm and others to a thickness of 1 mm. These cold-rolled sheets were solution-treated at 900°C and pickled, and then the 2-mm-thick steel sheets were finished into 2-mm square bars. By combining steel A with steel No. 1, and by combining steel C with steel No. 2, the limited range of the present invention is satisfied. Steel B and Steel D
are combined with steel No. 1 and steel No. 2, respectively, but steel B has a lower Mo equivalent than steel No. 1, which is the steel to be welded, and steel D has a Mo equivalent that is lower than that of the steel to be welded. However, the Ms point is lower than that of Steel No. 2, which is a weld metal, and the Ms point of the weld metal deviates from the limitation and simplicity of the present invention. The following tests were conducted using the above hot-rolled sheets, cold-rolled sheets, and square bars. (1) Tensile tests were conducted on 15 mm hot rolled sheets after aging treatment at 480°C for 5 hours. (2) As shown in Figure 4, a groove is machined perpendicular to the rolling direction of the 15 mm hot-rolled plate 1 and perpendicular to the plate surface. Welding (b) A cold-rolled plate 4 with a thickness of 1 mm was sandwiched between the grooves, electron beam welding was performed to fabricate a joint, and solution treatment was performed at 900°C. Fourth
In the figure, 3 indicates the electron beam, and the welding heat input is 8000J/
cm. The proportion of retained austenite phase was measured by microstructural observation of the weld metal of these joints, and a tensile test piece with a parallel part diameter of 10 mm and a parallel part length of 50 mm was prepared at right angles to the bead and with the weld metal at the center of the parallel part of the test piece. After processing and aging at 480°C for 5 hours, a tensile test was conducted. We also conducted a compositional analysis of the weld metal. (3) As shown in Figure 5, the V
Process the mold groove, use the same metal or the 2 mm square rod mentioned above as a filler rod, and use a backing metal 5 made of the same material as the base material to perform TIG.
The joint was manufactured by welding and solution-treated at 900℃. Welding was performed in 18 passes at a welding speed of 150 mm/min, at a current of 120 to 210 A, and a voltage of 11 to 13 V.
The same tests as in (2) above were conducted on these joints. Table 3 summarizes the test results. Combinations in which Steel No. 1 is the steel plate to be welded and Steel A is the insert metal in electron beam welding or filler rod in TIG welding, and Steel No. 2 is the steel plate to be welded and Steel C is the insert metal in electron beam welding. Alternatively, in the case of a combination as a filler rod in TIG welding, the Ms point falls within the limited range of the present invention, showing a joint efficiency of 99 to 100%. On the other hand, the combination of steel No. 1 and steel B, steel No. 2 and steel D
or in the case of electron beam welding without insert metal for Steel No. 1 and Steel No. 2, or TIG welding using a matching metal filler rod, the Mo equivalent is smaller than that of the steel plate to be welded or Ms score is 100
℃, and the joint efficiency was as low as 84-96%. As described above, it is clear that the method of the present invention greatly improves the joint efficiency of welded parts of 18% Ni maraging steel. In addition, although the above examples mainly described 18% Ni maraging steel, the method of the present invention can be similarly applied to other maraging steels.

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【table】 [Brief explanation of drawings]

Figure 1 is a graph showing the relationship between the composition and tensile strength of 18% Ni maraging steel sheets, and Figure 2 is a graph showing the relationship between the composition and tensile strength of 18% Ni maraging steel sheets.
A graph showing the relationship between the composition of Ni maraging steel sheet and the Ms point. Figure 3 shows the relationship between the estimated Ms point of the enriched segregated part of electron beam welded weld metal of 18% Ni maraging steel and the proportion of retained austenite phase in the structure. 4a and 4b are both explanatory diagrams showing the butt state in the electron beam welding method.
The figure is an explanatory diagram showing the groove shape in TIG welding. 1... Steel plate to be welded, 2... Welding part, 3... Electron beam, 4... Insert metal, 5... Backing metal.

Claims (1)

[Claims] 1. In electron beam or TIG welding in which 18% Ni maraging steel is welded and then the welded part is subjected to aging treatment and/or solution treatment, the average composition of the weld metal is determined by the following formula (1). The composition of the insert metal or filler rod should be adjusted so that the value of the Ms point, expressed by A welding method with excellent joint efficiency characterized by adjusting. (1) Formula Ms point = 941−28×(%Ni)−1.7×(%Co) −71(%Mo)−38(%Ti) (2) Formula Mo equivalent =(%Mo)+1/3×( %Co)+3×(%Ti) However, (%Ni), (%Co), (%Mo) and (%
Ti) is the average content of each element in the welded steel.