Ni-Cr-Mo-W alloy

Abstract

To provide Ni-Cr-Mo-W alloys with excellent corrosion resistance. [Solution] A Ni-Cr-Mo-W alloy having a composition in mass%, consisting of Cr: 13.0~25.0%, Mo: 10.0~18.0%, W: 1.00~5.00%, C: 0.001~0.030%, Si: 0.01~0.10%, Mn: 0.10~1.50%, Al: 0.001~0.170%, Fe: 2.00~8.00%, Nb: 0.001~0.100%, Ag: 0.001~0.005%, P: 0.030% or less, S: 0.0050% or less, Cu: 1.00% or less, Co: 3.00% or less, V: 0.50% or less, with the remainder being Ni and unavoidable impurities.

Description

Technical Field

[0001] The present invention relates to a Ni-Cr-Mo-W alloy excellent in corrosion resistance and workability.

Background Art

[0002] Ni-Cr-Mo alloys containing Ni (nickel) as the main component and Cr (chromium) and Mo (molybdenum) as the main alloying elements exhibit excellent corrosion resistance in various corrosive environments such as chloride-containing acidic environments, reducing acids, oxidizing acids, and even mixed acid environments. Therefore, they are widely used in chemical process equipment, natural gas field and oil field facilities, heat exchangers exposed to seawater environments, flue gas desulfurization equipment, etc. <U+

[0003] For example, in Patent Document 1, a Ni-Cr-Mo alloy is disclosed which contains, in mass%, 18.0 to 24. <U+ % of Cr and 8.0 to 10.0% of Mo in Ni, and a predetermined amount of C, Si, Mn, P, S, Al, Ti, Fe, Nb, W, V is added, and it is said to be excellent in corrosion resistance and also in workability. Here, by controlling NbC carbide and (Ti,Nb)N nitride to a predetermined level, it is possible to control the machinability at room temperature, particularly the 0.2% proof stress, which is the stress at which plastic deformation starts, within a range suitable for machining, such as 270 to 400 MPa.

[0004] Even in such Ni-based alloys excellent in corrosion resistance, precipitates such as carbides, intermetallic compounds, and Laves phases may be formed at grain boundaries. If the formation of these precipitates causes a decrease in the concentrations of Cr and Mo in the vicinity of grain boundaries, it can cause intergranular corrosion and intergranular stress corrosion cracking. Furthermore, since it is not only used as plates, tubes, and bars, but is also often used as welded structural members, weldability should also be considered from the viewpoint of ensuring the reliability of welded joints.

[0005] Patent Document 2 discloses a Ni-Cr-Mo alloy that can maintain workability and corrosion resistance even after welding, in which the alloy contains, by mass%, 18.0-24.0% Cr and 7.5-9.0% Mo in Ni, along with predetermined amounts of C, Si, Mn, P, S, Cu, Al, Ti, Fe, Co, V, Nb, N, Sn, and W. It states that controlling the amounts of Cu, Co, C, and N is effective in improving the ductility of the material itself, and that adding appropriate amounts of Nb, Ti, and V is effective in refining the solidification structure of the weld. It also states that crack occurrence is reduced in the weld by suppressing carbides, carbonitrides, and eutectic precipitates that lower the melting point.

[0006] Furthermore, Patent Document 3 discloses a Ni-Cr-Mo alloy that may contain W in an amount of 10% by mass or less, in which the Ni content is 40-60% by mass, and within this alloy, only 20-30% Cr, 4.0-10% Mo, and 2.0-5.0% Cu are added, along with predetermined amounts of C, Si, Mn, P, S, and Al. By increasing the Mo content, a stable passivation film can be obtained and corrosion resistance can be improved, but considering the decrease in weldability and workability due to the formation of the σ phase, components such as Cr and Cu are adjusted along with the amount of Mo. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2021-161528 [Patent Document 2] Patent No. 7009666 [Patent Document 3] International Publication No. 2009 / 119630 [Overview of the project] [Problems that the invention aims to solve]

[0008] Even in Ni-Cr-Mo alloys containing a predetermined amount of W, a decrease in the concentration of Cr and Mo near grain boundaries can cause intergranular corrosion and intergranular stress corrosion cracking, and it has been observed that these alloys are strongly affected by the solidification segregation of Cr and Mo during the solidification of the molten metal. On the other hand, since high temperatures and long periods of time are required to resolve precipitates and eliminate solidification segregation through heat treatment, new methods were needed from the perspective of operational and environmental burden.

[0009] This invention has been made in view of the above circumstances, and its objective is to provide a Ni-Cr-Mo-W alloy with excellent corrosion resistance. [Means for solving the problem]

[0010] The Ni-Cr-Mo-W alloy according to the present invention is characterized by having a component composition consisting of, by mass%, Cr: 13.0-25.0%, Mo: 10.0-18.0%, W: 1.00-5.00%, C: 0.001-0.030%, Si: 0.01-0.10%, Mn: 0.10-1.50%, Al: 0.001-0.170%, Fe: 2.00-8.00%, Nb: 0.001-0.100%, Ag: 0.001-0.005%, P: 0.030% or less, S: 0.0050% or less, Cu: 1.00% or less, Co: 3.00% or less, V: 0.50% or less, with the remainder being Ni and unavoidable impurities.

[0011] These characteristics make it possible to create an alloy with improved local corrosion resistance and excellent corrosion resistance.

[0012] In the above-described characteristics, the component composition includes Ce: 0.00001 to 0.010, and when [M] is the mass % of element M, 0.002 ≤ [Ag] + [Ce] ≤ 0.010 This characteristic may be highlighted. Such a characteristic ensures a reliable improvement in local corrosion resistance, resulting in an alloy with excellent corrosion resistance.

[0013] In the invention described above, the component composition may be characterized by containing Te: 0.00001 to 0.002. With this characteristic, local corrosion resistance can be reliably improved, and an alloy with excellent corrosion resistance can be obtained.

[0014] In the above-described invention, the critical crevice corrosion initiation temperature may be 85°C or higher when the ASTM G48 Method D test solution is an aqueous solution containing 7% by mass of sulfuric acid, 4% by mass of hydrogen chloride, 1% by mass of ferric chloride, and 1% by mass of copper(II) chloride, and the test period is 30 hours, and the critical pitting corrosion initiation temperature may be 110°C or higher when the ASTM G48 Method C test solution is an aqueous solution containing 7% by mass of sulfuric acid, 4% by mass of hydrogen chloride, 1% by mass of ferric chloride, and 1% by mass of copper(II) chloride, and the test period is 30 hours. According to these features, an alloy with excellent corrosion resistance can be obtained. [Brief explanation of the drawing]

[0015] [Figure 1] This diagram illustrates the mechanism of localized corrosion in conventional Ni-Cr-Mo-W alloys. [Figure 2] This figure illustrates the mechanism for improving local corrosion resistance in a Ni-Cr-Mo-W alloy according to an embodiment of the present invention. [Figure 3] This is a list of the chemical components of the alloys used in the manufacturing tests. [Figure 4] This is a table showing the results of the manufacturing tests. [Modes for carrying out the invention]

[0016] The embodiments of the Ni-Cr-Mo-W alloy according to the present invention will be described in detail below.

[0017] First, based on FIG. 1, in the Ni-Cr-Mo-W alloy system, the mechanism that reduces the resistance to local corrosion such as intergranular corrosion and intergranular stress corrosion cracking due to the decrease in the concentrations of Cr and Mo near the grain boundaries will be described. Note that this mechanism is associated with the normal solidification of the molten alloy during the process of obtaining ingots such as slabs or ingots by continuous casting or ingot casting before hot working such as hot rolling.

[0018] As shown in FIG. (a), during the solidification process of the molten alloy, Cr and Mo that improve corrosion resistance tend to move from the solid phase 1 to the liquid phase 2 side (see the small arrows in the figure), and while the concentration in the solid phase 1 decreases and it becomes concentrated on the liquid phase 2 side, the solid phase 1 grows (see the white arrows in the figure). That is, the concentrations of Cr and Mo increase in accordance with the growth of the solid phase 1. Then, as shown in FIG. (b), in the region of the grain boundary 2' that remained as the liquid phase 2 until the end, with respect to the crystal grains 1' that solidified first as the solid phase 1, the concentrations of Cr and Mo become high. That is, Cr and Mo undergo solidification segregation, and the concentration near the grain boundary rises excessively. Further, as shown in FIG. (c), in the region of the grain boundary 2', a large number of precipitates 3 such as carbides, nitrides, and intermetallic compounds containing Cr and Mo are generated. As a result of a large number of precipitates 3 being generated, the concentrations of Cr and Mo decrease around the precipitates 3 in the region of the grain boundary 2'. This Cr and Mo depletion phase having a concentration gap 4 can cause intergranular corrosion and intergranular stress corrosion cracking. Note that the graphs shown at the bottom of FIGS. (a) to (c) are images of the concentration profiles of Cr and Mo along the line segment AB.

[0019] Therefore, the inventors of the present application earnestly studied whether it is possible to suppress the decrease in the concentrations of Cr and Mo near the grain boundaries. As a result, they came up with a Ni-Cr-Mo-W alloy having the following component composition.

[0020] In this embodiment, the Ni-Cr-Mo-W-based alloy is a Ni-based alloy having a component composition in mass% of Cr: 13.0 to 25.0%, Mo: 10.0 to 18.0%, W: 1.00 to 5.00%, C: 0.001 to 0.030%, Si: 0.01 to 0.10%, Mn: 0.10 to 1.50%, Al: 0.001 to 0.170%, Fe: 2.00 to 8.00%, Nb: 0.001 to 0.100%, Ag: 0.001 to 0.005%, P: 0.030% or less, S: 0.0050% or less, Cu: 1.00% or less, Co: 3.00% or less, and V: 0.50% or less. Such a component composition containing a trace amount of Ag can suppress the decrease in local corrosion resistance and can be made into a Ni-Cr-Mo-W-based alloy excellent in corrosion resistance.

[0021] That is, as shown in Fig. 2(a), in the case of the Ni-Cr-Mo-W-based alloy having the above-described component composition, especially by containing Ag, during solidification in the process of obtaining the ingot before hot working as described above, Ag preferentially moves from the solid phase 1 to the liquid phase 2 side, and it is considered that the movement of Cr and Mo to the liquid phase 2 side is suppressed. As a result, as shown in Fig. 2(b), the difference in the concentrations of Cr and Mo in the grain boundary 2' with respect to the concentrations of Cr and Mo in the crystal grain 1' immediately after solidification becomes smaller than that in the conventional Ni-Cr-Mo-W-based alloy (see Fig. 1). That is, it is considered that the grain boundary segregation of Cr and Mo is suppressed. Further, as shown in Fig. 2(c), it is considered that during cooling while suppressing the generation of precipitates 3 such as carbides, nitrides, and intermetallic compounds containing Cr and Mo in the region of the grain boundary 2'. As a result, the precipitates 3 generated in the grain boundary 2' are few, and the degree of concentration decrease of Cr and Mo around the precipitates 3 and the range in which the concentration decrease occurs are also slight. That is, it is considered that the concentration decrease of Cr and Mo in the vicinity of this grain boundary 2' can be suppressed, and intergranular corrosion and intergranular stress corrosion cracking can be suppressed.

[0022] (Reasons for limiting each element) The content range of each element in the above-described Ni-Cr-Mo-W-based alloy is determined in consideration of corrosion resistance, mechanical properties, workability, weldability, economy, etc. comprehensively, and the reasons are shown below.

[0023] C:0.001~0.030% by mass Carbon (C) is an element that contributes to improving high-temperature strength through solid solution strengthening and carbide formation. However, if present in excess, it causes large amounts of carbides to precipitate at grain boundaries, leading to Cr deficiency in the surrounding area and inducing intergranular corrosion and intergranular stress corrosion cracking. Taking these factors into consideration, the amount of C is in the range of 0.001 to 0.030% by mass, preferably in the range of 0.002 to 0.015%, and more preferably in the range of 0.003 to 0.012%.

[0024] Si:0.01~0.10% by mass Si acts as a deoxidizing agent, contributing to the acquisition of a clean alloy ingot by removing oxygen during melting. Furthermore, up to a certain amount, Si contributes to improved mechanical strength through solid solution strengthening. However, excessive Si content can lead to the formation of a brittle phase or a decrease in corrosion resistance. Considering these factors, the amount of Si is typically within the range of 0.01% to 0.10% by mass, preferably 0.02% to 0.08%, and more preferably 0.03% to 0.06%.

[0025] Mn:0.10~1.50% by mass Mn acts as a deoxidizing agent, reducing the sulfur concentration in the matrix by combining with sulfur in the molten metal to form sulfides such as MnS. It also functions as a solid solution strengthening element. However, if present in excess, it can coarseen sulfides such as MnS, potentially causing intergranular corrosion and a decrease in fatigue strength. Considering these factors, the amount of Mn is set to be in the range of 0.10 to 1.50% by mass, preferably in the range of 0.15 to 1.00%, and more preferably in the range of 0.20 to 0.60%.

[0026] P: 0.030% by mass or less P is an impurity element that tends to segregate at grain boundaries, leading to grain boundary embrittlement and reduced corrosion resistance. Therefore, it is desirable to reduce its content as much as possible. Accordingly, the amount of P is set to 0.030% or less by mass, preferably 0.015% or less, and more preferably 0.013% or less.

[0027] S: 0.0050% by mass or less S is an impurity element that tends to segregate at grain boundaries as sulfides such as MnS, and can be the starting point for stress corrosion cracking and intergranular corrosion. Therefore, it is desirable to reduce its content as much as possible. Accordingly, the amount of S is set to 0.0050% or less by mass, preferably 0.0010% or less, and more preferably 0.0005% or less.

[0028] Cr:13.0~25.0% by mass Cr is one of the most important elements for forming a stable Cr2O3-based passivation film in oxidizing environments and imparting corrosion resistance to alloys. However, if present in excess, it can easily form brittle phases such as the σ phase, potentially leading to a decrease in toughness and workability. Considering these factors, the amount of Cr is set to be in the range of 13.0 to 25.0% by mass, preferably in the range of 14.0 to 23.0%, and more preferably in the range of 15.0 to 22.0%.

[0029] Mo:10.0~18.0% by mass Mo significantly contributes to suppressing pitting and crevice corrosion in the presence of chloride ions, and also improves corrosion resistance in reducing environments. However, excessive content promotes the formation of intermetallic compounds and σ phases, leading to decreased ductility, deterioration of weldability, and increased costs. Considering these factors, the amount of Mo is set to within the range of 10.0 to 18.00% by mass, preferably within the range of 12.00 to 17.00%, and more preferably within the range of 13.00 to 16.00%.

[0030] W:1.00~5.00% by mass Like Mo, W contributes to suppressing local corrosion and synergistically enhances the local corrosion suppression effect of Mo, further shifting the pitting potential in a chloride environment towards the noble side. It also improves high-temperature strength. However, excessive content leads to increased specific gravity, difficulty in processing, and increased costs. Considering these factors, the amount of W is set to within the range of 1.00 to 5.00% by mass, preferably within the range of 2.50 to 4.50%, and more preferably within the range of 3.00 to 4.00%.

[0031] Cu: 1.00% by mass or less While copper (Cu) can improve corrosion resistance in some reducing acid environments, excessive amounts can promote localized corrosion through the sealing of corrosion products and the formation of local galvanic cells. It may also increase the susceptibility to cracking at welds. Therefore, the amount of Cu is set to 1.00% or less by mass, preferably 0.50% or less, and more preferably 0.20% or less.

[0032] Co: 3.00% by mass or less Co contributes to solid solution strengthening in Ni-based alloys and may have a positive effect on high-temperature strength and creep strength. However, excessive coagulation can lead to increased costs and may impair corrosion resistance under certain environmental conditions. Therefore, the amount of co is set to 3.00% or less by mass, preferably 1.00% or less, and more preferably 0.50% or less.

[0033] Al:0.001~0.170% by mass A1 acts as a deoxidizing agent and also contributes to improved oxidation resistance by forming an Al2O3-based oxide film in high-temperature environments. However, if present in excess, it can lead to hardening and embrittlement due to γ' precipitation, increasing susceptibility to weld cracking. Considering these factors, the amount of Al is set to be in the range of 0.001 to 0.170% by mass, preferably in the range of 0.002 to 0.130%, and more preferably in the range of 0.003 to 0.100%.

[0034] Fe:2.00~8.00% by mass Fe contributes to reducing raw material costs and, in appropriate amounts, also contributes to improving mechanical strength through solid solution strengthening. However, if included in excess, it tends to reduce the corrosion resistance of the Ni-based alloy and may cause the formation of ferrite or brittle phases in the microstructure. Taking these factors into consideration, the amount of Fe is set to be in the range of 2.00 to 8.00% by mass, preferably in the range of 3.00 to 7.50%, and more preferably in the range of 5.00 to 7.00%.

[0035] V: 0.50% by mass or less V acts as a carbonitride-forming element and can improve high-temperature strength through precipitation strengthening, but if present in excess, it can lead to embrittlement due to the formation of coarse carbides. Therefore, the amount of V is set to 0.50% or less by mass, preferably 0.30% or less, and more preferably 0.10% or less.

[0036] Nb:0.001~0.100% by mass Nb forms carbides and carbonitrides, contributing to grain boundary stabilization and improved high-temperature strength. However, excessive Nb content can lead to the formation of coarse NbC and other materials, resulting in reduced toughness and poor workability. Considering these factors, the Nb content is set to within the range of 0.001 to 0.100% by mass, preferably within the range of 0.002 to 0.080%, and more preferably within the range of 0.005 to 0.050%.

[0037] Ag:0.001~0.005% by mass Even in trace amounts, Ag has the effect of improving local corrosion resistance. However, if included in excess, it may have adverse effects such as increased costs and excessive solidification segregation. Taking these factors into consideration, the amount of Ag is set to be in the range of 0.001 to 0.005% by mass, preferably in the range of 0.0015 to 0.0045%, and more preferably in the range of 0.0020 to 0.0040%.

[0038] (Addition of Ce) In addition to the above-mentioned component composition, it is also preferable to add Ce. Ce is a rare earth element that has deoxidizing and desulfurizing properties, and can improve corrosion resistance and fatigue strength by contributing to the refinement and dispersion of oxide inclusions. Furthermore, it is thought that by being incorporated into the passive film, it increases the stability of the film. In addition, together with Ag, it preferentially segregates at grain boundaries, suppressing grain boundary segregation of Cr and Mo, and can improve resistance to localized corrosion. However, if included in excess, it may lead to the formation of coarse oxides and oxide clusters, potentially creating starting points for localized corrosion. Considering these factors, Ce may be included in a range of 0.00001 to 0.010% by mass.

[0039] (Relationship between Ag and Ce) When Ce is included, the following relationship should be satisfied, where [M] is the mass percentage of element M. 0.002 ≤ [Ag] + [Ce] ≤ 0.010 This relationship is a composite inequality relating to the local corrosion resistance of Ag and Ce as described above. When the value of the middle side of this composite inequality ([Ag] + [Ce]) is less than 0.002, the effect of suppressing grain boundary segregation of Mo and Cr by the preferential segregation of Ag and Ce at grain boundaries is not sufficiently exhibited. On the other hand, when the value of the middle side exceeds 0.010, it leads to excessive segregation of Ag and the formation of coarse inclusions, increasing the risk of local corrosion. This relationship defines the overall "effective amount" of Ag and Ce, thereby improving local corrosion resistance. The value of the middle side of this relationship is preferably in the range of 0.0030 to 0.0080, and more preferably in the range of 0.0030 to 0.0060.

[0040] (Addition of Te) In addition to the component composition described above, it is also preferable to include Te. Te, when present in trace amounts in the Ni-based alloy, contributes to modifying the solidification structure and controlling grain boundary behavior, thereby improving corrosion resistance and workability in a balanced manner. On the other hand, excessive content may cause segregation and embrittlement. Taking these factors into consideration, Te may be included in a mass% range of 0.00001 to 0.002%, preferably in the range of 0.00005 to 0.0015%, and more preferably in the range of 0.0001 to 0.001%.

[0041] (Manufacturing Test) Next, we will describe the manufacturing tests conducted to specifically confirm the effects of the above-mentioned component composition.

[0042] First, cold-rolled sheets were produced from Ni-Cr-Mo-W alloys with the component compositions shown in Examples 1-20 and Comparative Examples 1-7 in Figure 3. In detail, raw materials such as scrap, Ni, Cr, and Mo were melted in an electric furnace and decarburized by oxygen blowing with AOD and VOD. Then, Al and limestone were added to reduce Cr, and further limestone and fluorite were added to form a CaO-SiO2-A12O3-MgO-F slag on the molten alloy to perform deoxidation and desulfurization. The molten alloy refined in this way was cast in a continuous casting machine to obtain slabs. Subsequently, the slabs were hot-rolled in a Steckel mill and then cold-rolled to produce cold-rolled sheets with a thickness of 3 mm. The chemical composition shown in the figure is the result of quantitative analysis of the obtained cold-rolled sheets using an X-ray fluorescence analyzer.

[0043] Next, we will describe the evaluation methods for crevice corrosion resistance, pitting corrosion resistance, and hot workability of the obtained cold-rolled sheets, as well as their overall evaluation.

[0044] Furthermore, for determining crevice corrosion resistance and pitting corrosion resistance, crevice corrosion tests according to ASTM G48 Method D and pitting corrosion tests according to ASTM G48 Method C can be used. These tests were conducted under extended high-temperature conditions (100°C), but no significant crevice corrosion or pitting corrosion was observed in any of Examples 1-20 or Comparative Examples 1-7, making it difficult to adequately distinguish differences in corrosion resistance between the materials in the examples and comparative examples. In other words, at least Examples 1-20 were suggested to have corrosion resistance equivalent to or better than conventional Ni-Cr-Mo-W alloys. To grasp clearer differences in corrosion resistance, tests and evaluations were conducted under more stringent conditions using the aqueous solutions described later as test solutions.

[0045] Crevice corrosion resistance evaluation: Test specimens were prepared from cold-rolled sheets, and the critical crevice corrosion temperature (CCT) was measured and evaluated in accordance with ASTM G48 Method D. The CCT was measured by the following test. A multi-clevis fixture in accordance with ASTM G48 Method D was used for the test, and the test specimens, which had been wet-polished beforehand with SiC#80 abrasive paper, were mounted with a tightening torque of 0.28 N·m. Next, to provide a more severe corrosive environment than the Green Death Solution, the test specimens were immersed in an aqueous solution containing 7% by mass of sulfuric acid, 4% by mass of hydrogen chloride, 1% by mass of ferric chloride, and 1% by mass of copper(II) chloride. The test temperature was changed in 5°C increments from -5°C to 95°C, and the specimens were held for a predetermined holding time at each test temperature. Although the holding time is 24 hours in the same ASTM standard, it was set to 30 hours in order to more precisely evaluate the difference in corrosion resistance. After the test, the test specimens were cleaned and dried, and the surface, including the crevice, was observed to measure the maximum crevice corrosion depth. If the maximum crevice corrosion depth was 25 μm or more, it was determined that crevice corrosion had occurred, and the lowest test temperature at which crevice corrosion occurred was defined as the CCT. A higher CCT indicated superior resistance to crevice corrosion. A CCT of 95°C or higher was marked "◎" for excellent performance, "○" for good performance if it was between 90°C and below 95°C, "△" for acceptable performance if it was between 85°C and below 90°C, and "×" for poor performance if it was below 85°C.

[0046] Pitting corrosion resistance evaluation: For pitting corrosion resistance, test specimens were prepared by cutting cold-rolled sheets to predetermined dimensions and pre-polishing them wet with SiC#80 abrasive paper. In accordance with ASTM G48 Method C, and to provide a more severe corrosive environment than the Green Death Solution, an aqueous solution containing 7% by mass of sulfuric acid, 4% by mass of hydrogen chloride, 1% by mass of ferric chloride, and 1% by mass of copper(II) chloride was used as the test solution. The critical pitting temperature (CPT) was measured and evaluated. The CPT was measured by the following test. The test temperature was varied in 5°C increments from 5°C to 125°C, and the test specimens were immersed in the above-mentioned test solution and held for a predetermined holding time at each test temperature. Although the holding time is 24 hours in the same ASTM standard, it was set to 30 hours to more precisely evaluate the difference in corrosion resistance. After the test, the test specimens were washed and dried, and the surface of the test specimens was observed to measure the maximum pitting depth. Pitting corrosion that occurred on the end faces of the test specimens was excluded from the measurement. Pitting corrosion was determined to have occurred when the maximum pitting depth was 25 μm or more, and the lowest test temperature at which pitting corrosion occurred was defined as the CPT (Critical Point Test). A higher CPT was considered to indicate superior pitting corrosion resistance. Specifically, a CPT of 120°C or higher was marked as "◎" for excellent performance, "〇" for good performance between 120°C and 115°C, "△" for acceptable performance between 115°C and 110°C, and "×" for poor performance below 110°C.

[0047] Hot workability evaluation: During the hot rolling process, which is an intermediate step in the manufacturing of cold-rolled sheets, the surface of the rolled sheet was observed along its entire length, and 10m 2 If there were 0 visible surface defects caused by hot working, the result was marked as "Excellent" (◎); if there was 1 defect, it was marked as "Good" (〇); if there were 2 defects, it was marked as "Acceptable" (△); and if there were 3 or more defects, it was marked as "Poor" (×). Surface defects included cracks, breaks, laps, deep slag inclusions, and other defects judged to be caused by insufficient hot plastic deformation capacity or structural non-uniformity.

[0048] comprehensive evaluation: First, if any of the crevice corrosion resistance, pitting corrosion resistance, or hot workability evaluations were rated as poor (×), the overall evaluation was recorded as poor (×). For all other evaluations, points were assigned to each of the crevice corrosion resistance, pitting corrosion resistance, and hot workability evaluations: 3 points for excellent (◎), 2 points for good (〇), and 1 point for acceptable (△). The total score was then ranked to determine the overall evaluation. If the total score was 9 points, the overall evaluation was recorded as excellent (◎); if it was 6-8 points, it was recorded as good (○); and if it was 3-5 points, it was recorded as acceptable (△).

[0049] As shown in Figure 3, Examples 1 to 20 all satisfied the above-described range of component composition, exhibited excellent crevice corrosion resistance and pitting corrosion resistance, and had few surface defects due to hot workability, resulting in good overall evaluations of "acceptable" (△) or higher. In particular, Examples 18 to 20 contained a predetermined amount of Ce while satisfying the above-described composite inequality, and / or contained a predetermined amount of Te, exhibiting excellent crevice corrosion resistance, pitting corrosion resistance, and hot workability, and received good overall evaluations.

[0050] On the other hand, in Comparative Examples 1 to 7, the content of some elements all deviated from the range described above, and the evaluation results further supported the reasons for limiting each element in this embodiment, as detailed below.

[0051] Comparative Example 1 had a high Ag content of 0.0058%. As a result, it is thought that the solidification segregation and localized galvanic cell formation acted as initiation points for corrosion, leading to a deterioration in corrosion resistance. Furthermore, it is believed that the segregation increased the non-uniformity of the microstructure, resulting in an increase in the number of surface defects generated during hot rolling and a decrease in hot workability.

[0052] In Comparative Example 2, the Ag content was low at 0.0004%, and it is thought that the segregation-suppressing effect of Ag on Cr and Mo did not manifest, resulting in a significant decrease in CCT and CPT. In other words, the crevice corrosion resistance and pitting corrosion resistance were significantly reduced. From this, it was confirmed that the inclusion of Ag can improve local corrosion resistance.

[0053] In Comparative Example 3, the Mo content was high at 18.10%, and it is thought that intermetallic compounds (e.g., Laves phase) were formed due to the excess Mo, resulting in an increase in surface defects due to reduced hot workability.

[0054] In Comparative Example 4, the Mo content was low at 9.86%, which is thought to have resulted in insufficient local corrosion resistance, leading to poor resistance to crevice corrosion and pitting corrosion. Both CCT and CPT were low, and crevice corrosion and pitting corrosion occurred even at relatively low temperatures. In other words, it was confirmed that the inclusion of a predetermined amount of Mo can provide high resistance to pit corrosion and crevice corrosion.

[0055] In Comparative Example 5, the Cr content was low at 12.84%, resulting in insufficient formation and maintenance of a passive film in an oxidizing environment. This is thought to have prevented sufficient local corrosion resistance, leading to low resistance to crevice corrosion and pitting corrosion. CCT and CPT were significantly lower compared to Examples 1-20, confirming the importance of including a predetermined amount of Cr.

[0056] In Comparative Example 6, the C content was high at 0.032%, which is thought to have caused coarse carbides to precipitate at the grain boundaries, resulting in a significant decrease in corrosion resistance and hot workability. It is believed that a chromium-deficient layer was formed due to the precipitation of carbides at the grain boundaries, and the susceptibility to intergranular corrosion and intergranular stress corrosion cracking also increased.

[0057] Comparative Example 7 had a high Si content of 0.11%, and a decrease in corrosion resistance was observed with increasing Si content.

[0058] These results indicate that the examples in which the component composition was controlled to within the specified range of each element's content were superior to the comparative examples in overall evaluation of crevice corrosion resistance, pitting corrosion resistance, and hot workability.

[0059] As described above, the Ni-Cr-Mo-W alloy in this embodiment exhibits extremely excellent resistance to localized corrosion (pit corrosion, crevice corrosion) in acidic environments containing chlorides or mixed acid environments. In particular, it is believed that controlling the Ag content within the above-mentioned range results in excellent resistance to localized corrosion. Although the detailed reasons are not entirely clear, as mentioned above, it is presumed that Ag preferentially segregates at the grain boundaries, suppressing excessive segregation of Mo and Cr at the grain boundaries, thereby suppressing the precipitation of carbides, nitrides, intermetallic compounds, etc., and inhibiting the formation of Cr and Mo deficient layers near the grain boundaries.

[0060] Furthermore, the excellent resistance to localized corrosion was demonstrated by setting the ASTM G48 Method D test solution to an aqueous solution containing 7% by mass of sulfuric acid, 4% by mass of hydrogen chloride, 1% by mass of ferric chloride, and 1% by mass of copper(II) chloride, with a test duration of 30 hours, resulting in a critical crevice corrosion initiation temperature of 85°C or higher, and setting the ASTM G48 Method C test solution to an aqueous solution containing 7% by mass of sulfuric acid, 4% by mass of hydrogen chloride, 1% by mass of ferric chloride, and 1% by mass of copper(II) chloride, with a test duration of 30 hours, resulting in a critical pitting corrosion initiation temperature of 110°C or higher.

[0061] Furthermore, the addition of small amounts of Ce and the control of Ag and Ce content to satisfy the above-mentioned composite inequality are also preferable, as this is thought to suppress the occurrence and progression of localized corrosion and maintain stable corrosion resistance over a long period of time. Although the detailed mechanism is not entirely clear, it is presumed that Ce, like Ag, contributes to suppressing the segregation of Cr and Mo at grain boundaries. In addition, Ce may contribute to the fine dispersion of oxide inclusions and to the stabilization of the passive film by being incorporated into it.

[0062] Furthermore, as mentioned above, it is preferable to include a small amount of Te in the component composition, as this contributes to modifying the solidification structure and controlling grain boundary behavior, thereby reliably improving local corrosion resistance and resulting in an alloy with excellent corrosion resistance.

[0063] As described above, the Ni-Cr-Mo-W alloy according to this embodiment exhibits excellent corrosion resistance in harsh corrosive environments such as acidic solutions containing chlorides, mixed acid solutions, and high-temperature, high-pressure water environments. Therefore, it is useful in equipment handling corrosive liquids, such as chemical industrial equipment, petrochemical plants, pharmaceutical plants, fertilizer plants, the paper and pulp industry, and various flue gas desulfurization systems.

[0064] Although representative embodiments of the present invention and modifications thereof have been described above, the present invention is not necessarily limited thereto, and those skilled in the art will be able to find various alternative embodiments and modifications without departing from the spirit of the present invention or the scope of the attached claims.

Claims

1. In mass percent, Cr: 13.0-25.0%, Mo: 10.0-18.0%, W: 1.00-5.00%, C: 0.001-0.030%, Si: 0.01 to 0.10%, Mn: 0.10 to 1.50%, Al: 0.001-0.170%, Fe: 2.00-8.00%, Nb: 0.001 to 0.100%, Ag: 0.001-0.005%, P: 0.030% or less, S: 0.0050% or less, Cu: 1.00% or less, Co: 3.00% or less, V: 0.50% or less, A Ni-Cr-Mo-W alloy characterized by having a component composition consisting of the remainder being Ni and unavoidable impurities.

2. In the above component composition, Ce: 0.00001 to 0.010 is included. When [M] is the mass percentage of element M, 0.002≦[Ag]+[Ce]≦0.010 The Ni-Cr-Mo-W alloy according to claim 1, characterized in that it is the Ni-Cr-Mo-W alloy according to claim 1.

3. In the above component composition, Te is included, Te: 0.00001~0.002% The Ni-Cr-Mo-W alloy according to claim 1 or 2, characterized in that it is the Ni-Cr-Mo-W alloy according to claim 1 or 2.

4. The Ni-Cr-Mo-W alloy according to claim 1, characterized in that when the ASTM G48 Method D test solution is an aqueous solution containing 7% by mass of sulfuric acid, 4% by mass of hydrogen chloride, 1% by mass of ferric chloride, and 1% by mass of copper(II) chloride, and the test time is 30 hours while varying the test temperature T1, the lowest test temperature T1 at which the maximum crevice corrosion depth measured by observing the surface including the crevice is 25 μm or more is 85°C or higher, and when the ASTM G48 Method C test solution is an aqueous solution containing 7% by mass of sulfuric acid, 4% by mass of hydrogen chloride, 1% by mass of ferric chloride, and 1% by mass of copper(II) chloride, and the test time is 30 hours while varying the test temperature T2, the lowest test temperature T2 at which the maximum pitting corrosion depth measured by observing the surface is 25 μm or more is 110°C or higher.

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