CORROSION-RESISTANT, HIGH-HARDNESS ALLOY COMPOSITION AND METHOD FOR PRODUCING SAME

Abstract

Provided is a corrosion-resistant, high-hardness alloy composition, which realizes both corrosion resistance and high hardness by using a Ni—Co—Cr—Mo-based alloy and optimizing the chemical composition, heat treatment conditions and processing conditions thereof, and a method for producing that alloy composition. The alloy composition is an alloy composition comprising 15.5% by weight to 16.5% by weight of Cr, 7.5% by weight to 15.5% by weight of Mo, 0% by weight to 30% by weight of Co, 4.5% by weight to 15% by weight of Fe and 0.5% by weight to 4.0% by weight of Cu, with the remainder consisting of Ni and unavoidably included elements, wherein the crystal phase consists only of a γ phase and the Vickers hardness at room temperature is 500 HV or more. The alloy composition is obtained by subjecting an ingot of an alloy having the aforementioned composition to homogenization treatment for 4 hours to 24 hours at 1100° C. to 1300° C., followed by subjecting to cold processing at a compression rate of 30% to 60% and then to aging treatment for 0.5 hours to 3 hours over a temperature range of 300° C. to 600° C.

Claims

1. A method for producing a corrosion-resistant, high-hardness alloy composition, comprising: subjecting an ingot of an alloy, comprising 15.5% by weight to 16.5% by weight of Cr, 7.5% by weight to 15.5% by weight of Mo, 0% by weight to 30% by weight of Co, 4.5% by weight to 15% by weight of Fe and 0.5% by weight to 4.0% by weight of Cu, with the remainder consisting of Ni and unavoidably included elements, to homogenization treatment for 4 hours to 24 hours at 1100° C. to 1300° C., followed by subjecting to cold processing at a compression rate of 30% to 60% and then to aging treatment for 0.5 hours to 3 hours over a temperature range of 300° C. to 600° C.

2. A corrosion-resistant, high-hardness alloy composition produced by the method for producing a corrosion-resistant, high-hardness alloy composition according to claim 1, comprising: 15.5% by weight to 16.5% by weight of Cr, 7.5% by weight to 15.5% by weight of Mo, 0% by weight to 30% by weight of Co, 4.5% by weight to 15% by weight of Fe and 0.5% by weight to 4.0% by weight of Cu, with the remainder consisting of Ni and unavoidably included elements; wherein, the crystal phase consists only of a γ phase and the Vickers hardness at room temperature is 500 HV or more.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 is a phase diagram of a Ni-30Co-16Cr-15Mo-6Fe-xCu (wt %) alloy relating to an embodiment of the present invention.

[0014] FIG. 2 is a phase diagram of a Ni-30Co-16Cr-6Fe-2Cu-xMo (wt %) alloy relating to an embodiment of the present invention.

[0015] FIG. 3 is a graph indicating the Vickers hardness (hardness) of a Ni-30Co-16Cr-6Fe-xMo alloy and Ni-30Co-16Cr-6Fe-2Cu-xMo (x=7% by weight to 15% by weight) alloy relating an embodiment of the present invention when subjected to homogenization treatment for 24 hours at 1250° C.

[0016] FIG. 4 is a graph indicating weight loss rates (weight loss) per unit area of a Ni-30Co-16Cr-6Fe-xMo alloy and Ni-30Co-16Cr-6Fe-2Cu-xMo (x=7% by weight to 15% by weight) relating to an embodiment of the present invention when subjected to homogenization treatment for 24 hours at 1250° C. followed by immersing for 100 hours in hydrofluoric acid (5.2 M) at 100° C.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] The reasons for limiting the composition ranges of each component of the Ni-based alloy of the present invention are as described below.

[0018] [Co: 0% by Weight to 30% by Weight]

[0019] Co is preferably added at 15% by weight to 30% by weight in terms of the added amount thereof since it demonstrates the effect of improving wear resistance properties by increasing strength. However, the Ni-based alloy of the present invention can also be provided for practical use without adding Co in the case of applications not requiring any particular consideration of wear resistance properties, and in consideration thereof, the added amount of Co is 0% by weight to 30% by weight. If the added amount exceeds 30% by weight, the μ phase precipitates easily as described in Non-Patent Document 1, thereby resulting in poor corrosion resistance. In addition, since the cost of the alloy also increases, the upper limit of the added amount of Co is set to 30% by weight.

[0020] [Cr: 15.5% by Weight to 16.5% by Weight]

[0021] Cr is added at 15.5% by weight to 16.5% by weight in order to ensure corrosion resistance of the alloy in an oxidizing atmosphere by putting Cr into a solid solution. Since a dense Cr.sub.2O.sub.3 oxide film cannot be formed in an oxidizing atmosphere if the added amount of Cr is less than 15.5% by weight, 15.5% is set for the lower limit of the added amount thereof. Since hardness and mechanical properties of the alloy decrease if the added amount exceeds 16.5%, 16.5% is set for the upper limit of the added amount thereof.

[0022] [Mo: 7.5% by Weight to 15.5% by Weight]

[0023] The amount of Mo was set to 7.5% by weight to 15.5% by weight so as to be able to form a passive film in which Mo and Cu are present in a hydrofluoric acid atmosphere in the case of having added Cu at 0.5% by weight to 4.0% by weight. Since a dense passive film cannot be formed in a non-oxidizing atmosphere (hydrofluoric acid) if the added amount of Mo is less than 7.5% by weight, 7.5% by weight was set for the lower limit. Since a Mo-rich μ phase precipitates easily, the surface composition of the alloy becomes heterogeneous and corrosion resistance to hydrofluoric acid decreases if the added amount exceeds 15.5% by weight, 15.5% by weight was set for the upper limit.

[0024] [Fe: 4.5% by Weight to 15% by Weight]

[0025] Fe is effective for improving material processability. At least 4.5% by weight or more is required to be contained particularly when Co is present. In addition, since Fe is less expensive than Ni and Co, the addition of Fe also has the effect of reducing material costs. However, the addition of Fe in excess of 17% by weight results in precipitation of a brittle a phase in the matrix phase, which has the effect of lowering alloy processability and plasticity. In this manner, since a brittle a phase precipitates if Fe is added at 17% by weight to 18% by weight or more, the amount of iron is typically preferably 4.5% by weight to 15% by weight.

[0026] [Cu: 0.5% by Weight to 4.0% by Weight]

[0027] In the case of having added Cu at 0.5% by weight to 4.0% by weight, a passive film comprised of Cu can be formed instead of Mo in a hydrofluoric acid atmosphere, thereby having the effect of reducing the amount of Mo and lowering the precipitation temperature of the μ phase. In addition, in the case of having added Cu, an effect is also demonstrated that prevents a further decrease in alloy corrosion resistance following cold processing. If Cu is added at 4.0% by weight or more, precipitation of the sigma (a) phase is promoted resulting in poor corrosion resistance. In addition, since alloy processability also becomes poor if Cu is added at 4.0% by weight or more, the amount of Cu is typically preferably 0.5% by weight to 4.0% by weight.

[0028] Unavoidably included elements are elements having high processability that enter from raw materials during production or from a crucible during casting, and consist of carbon at 0.05% or less, Mn at 0.5% or less, Al at 0.5% or less and Si at 0.5% or less.

[0029] FIG. 1 is a phase diagram of a Ni-30Co-16Cr-15Mo-6Fe alloy to which Cu was added at 0% by weight to 6% by weight as calculated based on the Ni-based alloy thermodynamic database (Ni7 Database) using ThermoCalc5 (TCWS) software available from Thermo-Calc Software (Sweden). According to FIG. 1, the precipitation temperature of the μ phase was 1370 K (about 1100° C.) or lower as a result of adding Cu at 0% by weight to 6% by weight, and was determined to lower somewhat due to the addition of Cu.

[0030] Table 1 indicates the Vickers hardness for each alloy in the table following completion of each treatment when having undergone homogenization treatment for 24 hours at 1250° C. and cold casting at a processing rate of 30% or 60% followed by aging treatment for 1 hour at 600° C. As shown in Table 1, the hardness of all of the materials clearly increases when subjected to cold processing. In addition, material hardness is able to be further increased by carrying out aging treatment after subjecting to cold processing. The hardness of alloys in which Ni was substituted with Co was much higher than the hardness of alloys not containing Co following cold processing and aging treatment. In addition, when the added amount of Co was increased from 0% by weight to 5% by weight, 10% by weight, 15% by weight or 30% by weight, although there was little change in hardness of the materials in the homogenized state, the hardness of the alloys following cold processing and aging treatment was determined to increase strongly dependent on the amount of Co.

TABLE-US-00001 TABLE 1 30% cold 60% cold Homogenization 30% cold processing + 60% cold processing + treatment state processing aging processing aging Ni16Cr15Mo6Fe4W 201 323 — 432 483 Ni5Co16Cr15Mo6Fe4W 204 331 — 442 490 Ni10Co16Cr15Mo6Fe4W 198 345 — 438 510 Ni15Co16Cr15Mo6Fe4W 200 379 — 439 525 Ni30Co16Cr15Mo6Fe 220 385 — 451 582 Ni30Co16Cr15Mo6Fe2Cu 191 374 403 476 574 Ni30Co16Cr15Mo15Fe2Cu 178 353 412 472 580 Ni30Co16Cr10Mo6Fe2Cu 157 342 385 443 562 Ni30Co16Cr10Mo6Fe 165 150 392 446 571

[0031] Table 2 indicates weight loss rates (mg/cm.sup.2) when the alloys in the table were subjected to each treatment followed by respectively immersing for 100 hours in hydrofluoric acid (5.2 M) at 100° C. As shown in Table 2, there were no effects observed on material corrosion resistance in the homogenization treatment state when the added amount of Co was increased from 0% by weight to 5% by weight, 10% by weight, 15% by weight or 30% by weight. In addition, corrosion resistance of Ni-16Cr-6Fe—Mo alloy not containing Co was determined to be superior even after cold processing. However, in the case of not adding Co, corrosion resistance of alloys to which Co had been added decreased rapidly following aging treatment for 1 hour at 600° C. In addition, corrosion resistance following cold processing clearly worsened accompanying increases in the amount of Co added. In contrast, corrosion resistance was determined to not decrease due to cold processing or aging treatment in the case of having added Cu at 2% by weight.

TABLE-US-00002 TABLE 2 30% cold 60% cold Homogenization 30% cold processing + 60% cold processing + treatment state processing aging processing aging Ni16Cr15Mo6Fe4W 6.07 3.27 — 4.10 97.21 Ni5Co16Cr15Mo6Fe4W 6.21 7.85 — 10.21 — Ni10Co16Cr15Mo6Fe4W 6.45 12.25 — 18.71 — Ni15Co16Cr15Mo6Fe4W 7.02 22.5 — 27.8 — Ni30Co16Cr15Mo6Fe 5.75 44.24 — 34.34 178.82 Ni30Co16Cr15Mo6Fe2Cu 0.90 2.05 0.84 0.62 1.52 Ni30Co16Cr15Mo15Fe2Cu 4.88 7.61 8.25 10.52 11.25 Ni30Co16Cr10Mo6Fe2Cu 0.81 1.45 — 1.22 — Ni30Co16Cr10Mo6Fe 210 260 — 170 —

[0032] Tables 3 and 4 respectively indicate Vickers hardness of a Ni-30Co-16Cr-15Mo-6Fe-2Cu (wt %) alloy that underwent aging treatment for 1 hour at 300° C. to 700° C. after having been subjected to homogenization treatment followed by the absence of cold processing, cold processing at a processing rate of 30% or cold processing at a processing rate of 60%, and weight loss rate (mg/cm.sup.2) when the alloy was immersed for 100 hours in hydrofluoric acid (5.2 M) at 100° C. following each treatment. As shown in Tables 3 and 4, cold processing and aging treatment were determined to demonstrate the effect of raising material hardness in the same manner as Tables 1 and 2. In addition, this alloy was determined to demonstrate superior corrosion resistance in comparison with a commercially available Ni-16Cr-15Mo-6Fe-4W alloy following cold processing and aging treatment.

TABLE-US-00003 TABLE 3 Initial 300° C. 400° C. 500° C. 600° C. 700° C. Homogenization 191 198 195 204 202 216 treatment 30% cold 374 375 390 407 403 378 processing 60% cold 476 521 549 555 574 541 processing

TABLE-US-00004 TABLE 4 Initial 300° C. 400° C. 500° C. 600° C. 700° C. Homogenization 0.93 1.42 3.00 2.91 2.38 0.65 treatment 30% cold 2.06 3.70 3.30 3.05 0.81 1.07 processing 60% cold 0.61 3.41 5.12 4.37 1.52 6.50 processing

[0033] FIG. 2 is a phase diagram of a Ni-30Co-16Cr-6Fe-2Cu-xMo (x=5% by weight to 20% by weight) alloy as calculated based on the Ni-based alloy thermodynamic database (Ni7 Database) using ThermoCalc5 (TCWS) software available from Thermo-Calc Software (Sweden). According to FIG. 2, the precipitation temperature of the μ phase was determined to lower rapidly when the amount of Mo decreased. For example, when the amount of Mo was decreased to 11% by weight, the precipitation temperature of the μ phase lowered to 1000° C. (1273 K) or lower, and a structure having dense crystal grains that does exhibit precipitation of the μ phase was obtained by carrying out hot casting at this temperature or higher.

[0034] FIG. 3 indicates Vickers hardness (hardness) when Ni-30Co-16Cr-6Fe-xMo alloy and a Ni-30Co-16Cr-6Fe-2Cu-xMo (x=7% by weight to 15% by weight) alloys were subjected to homogenization treatment for 24 hours at 1250° C. In addition, FIG. 4 indicates the weight loss rate (weight loss) when the alloys were immersed for 100 hours in hydrofluoric acid (5.2 M) at 100° C. following homogenization treatment. As indicated by FIGS. 3 and 4, the Vickers hardness of both types of alloys undergoes a small decrease when the amount of Mo is reduced. However, the Ni-30Co-16Cr-6Fe-xMo alloy not containing Cu demonstrated a large increase in weight loss rate following immersion caused by a decrease in the amount of Mo, and corrosion resistance worsened considerably. On the other hand, the Ni-30Co-16Cr-6Fe-2Cu-xMo alloy that contains Cu exhibited little change in weight loss rate following immersion caused by a decrease in the amount of Mo (1 mg/cm.sup.2 or less in all cases), and corrosion resistance did not worsen despite a decrease in the amount of Mo.

INDUSTRIAL APPLICABILITY

[0035] The present invention is considered to have a high degree of industrial applicability as an alloy composition for use as a member such as a screw or cylinder for resin molding of fluorine-containing resins.