Wroughtable, Chromium-Bearing, Cobalt-Based Alloys with Improved Resistance to Galling and Chloride-Induced Crevice Attack

Abstract

A chromium-bearing, cobalt-based alloys amenable to wrought processing has improved resistance to both chloride-induced crevice corrosion and galling. The alloy contains up to 3.545 wt. % nickel, 0.242 to 0.298 wt. % nitrogen, and may contain 22.0 to 30.0 wt. % chromium, 3.0 to 10.0 wt. % molybdenum, up to 5.0 wt. % tungsten, up to 7 wt. % iron, 0.5 to 2.0 wt. % manganese, 0.5 to 2.0 wt. % silicon, 0.02 to 0.11 wt. % carbon, 0.005 to 0.205 wt. % aluminum, and the balance is cobalt plus impurities.

Claims

1. A chromium-bearing, cobalt-based alloys amenable to wrought processing with improved resistance to both chloride-induced crevice corrosion and galling, comprising: up to 3.545 wt. % nickel; 0.242 to 0.298 wt. % nitrogen; 22.0 to 30.0 wt. % chromium; 3.0 to 10 wt. % molybdenum; up to 5.0 wt. % tungsten; 1.71 to 7 wt. % iron; 0.05 to 2.0 wt. % manganese; 0.05 to 2.0 wt. % silicon; 0.02 to 0.11 wt. % carbon; 0.005 to 0.205 wt. % aluminum; and cobalt plus impurities as the balance.

2. The chromium-bearing, cobalt-based alloy of claim 1 comprising: 1.07 to 3.17 wt. % nickel; 27.96 to 28.12 wt. % chromium; 4.90 to 6.84 wt. % molybdenum; 2.04 to 2.26 wt. % tungsten; 2.71 to 2.92 wt. % iron; 0.77 to 0.90 wt. % manganese; 0.24 to 0.29 wt. % silicon; 0.058 to 0.067 wt. % carbon; 0.262 to 0.278 wt. % nitrogen; 0.08 to 0.13 wt. % aluminum; and cobalt plus impurities as the balance.

3. The chromium-bearing, cobalt-based alloy of claim 1 comprising: 0.695 to 3.545 wt. % nickel; 26.46 to 29.62 wt. % chromium; 4.40 to 7.34 wt. % molybdenum; 1.54 to 2.76 wt. % tungsten; 1.71 to 3.92 wt. % iron; 0.52 to 1.15 wt. % manganese; 0.04 to 0.49 wt. % silicon; 0.038 to 0.087 carbon; 0.242 to 0.298 wt. % nitrogen; 0.005 to 0.205 wt. % aluminum; and cobalt plus impurities as the balance.

4. The chromium-bearing, cobalt-based alloy of claim 1 comprising: up to 3.545 wt. % nickel; 0.242 to 0.298 wt. % nitrogen; 24.0 to 27.0 wt. % chromium; 4.5 to 5.5 wt. % molybdenum; 1.5 to 2.50 wt. % tungsten; 2.0 to 4.0 wt. % iron; 0.5 to 1.0 wt. % manganese; 0.30 to 0.50 wt. % silicon; 0.04 to 0.08 wt. % carbon; 0.005 to 0.205 wt. % aluminum; and cobalt plus impurities as the balance.

5. The chromium-bearing, cobalt-based alloy of claim 1 wherein the alloy is in a form selected from the group consisting of wrought products, castings, weldments, and powder products.

Description

DESCRIPTION OF THE DRAWING

[0017] FIG. 1 is a chart of the crevice corrosion and galling test results reported in Table 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The experimental alloys involved with this discovery were made by vacuum induction melting (VIM), followed by electro-slag re-melting (ESR), to produce ingots of material amenable to hot working. Prior to hot working (i.e. hot forging and hot rolling), ingots were homogenized at 1204° C./2200° F. Based on prior experience with this class of alloys, a hot working start temperature of 1204° C./2200° F. was used for all experimental alloys. Annealing trials indicated that a solution annealing temperature of 1121° C./2050° F. was suitable for this class of materials, followed by rapid cooling/quenching (to create a metastable FCC solid solution structure at room temperature). To enable the manufacture of crevice corrosion test samples, annealed sheets of thickness 3.2 mm/0.125 inch were produced. To enable the manufacture of galling test pins and blocks, annealed plates of thickness 25.4 mm/1 inch were produced. Two batches of Alloy 1 and two batches of Alloy 3 were produced, due to insufficient material in a single batch for both types of test.

[0019] The actual (analyzed) compositions of the experimental alloys are given in Table 1.

TABLE-US-00001 TABLE 1 Compositions of Experimental Wrought Alloys ALLOY Co Ni Cr Mo W Fe Mn Si C N Al COMMENT 1 (A) 52.76 8.98 26.68 5.07 2.10 2.77 0.93 0.29 0.062 0.114 0.15 Commercial Embodiment of U.S. Pat. No. 5,002,731 1 (B) 53.61 8.90 26.63 4.85 2.29 2.93 0.78 0.23 0.067 0.127 0.09 Commercial Embodiment of U.S. Pat. No. 5,002,731 2 60.10 3.32 26.64 5.11 2.06 2.78 0.91 0.30 0.066 0.109 0.13 3 (A) 58.07 3.17 28.12 4.90 2.04 2.71 0.90 0.29 0.067 0.262 0.12 Alloy of this Invention 3 (B) 57.01 3.08 27.96 6.84 2.26 2.88 0.77 0.24 0.058 0.278 0.08 Alloy of this Invention 4 60.16 1.07 28.10 4.52 2.24 2.92 0.80 0.25 0.061 0.270 0.13 Alloy of this Invention 5 56.63 5.37 27.85 4.55 2.19 2.85 0.78 0.26 0.060 0.233 0.10 6 56.60 3.01 29.54 4.94 2.19 2.69 0.73 0.25 0.062 0.367 0.10 Cracked during Forging 7 55.62 2.89 30.45 4.77 2.15 2.61 0.70 0.27 0.067 0.415 0.13 Cracked during Forging 8 65.47 3.08 25.01 3.78 1.37 1.05 0.42 0.05 0.023 0.095 0.08 9 50.02 3.17 31.40 5.89 3.04 4.80 1.31 0.53 0.095 0.413 0.28 Cracked during Forging

[0020] The experimental steps taken during this work were as follows:

[0021] 1. Melt and test an experimental version (ALLOY 1) of the commercial embodiment of U.S. Pat. No. 5,002,731, using the same melting, hot working, and testing procedures as intended for all the other experimental alloys. Two batches were required to make all the required samples.

[0022] 2. Melt and test a reduced (approximately 3 wt. %) nickel version (ALLOY 2), with all other elements at the ALLOY 1 level.

[0023] 3. Melt and test an increased (approximately 0.25 wt. %) nitrogen version (ALLOY 3), with nickel at approximately 3 wt. %, and all other elements at the ALLOY 1 level. Two batches were required to make all the required samples.

[0024] 4. Melt and test a further reduced (approximately 1 wt. %) nickel version (ALLOY 4), with nitrogen at approximately 0.25 wt. %, and all other elements at the ALLOY 1 level.

[0025] 5. Melt and test an intermediate (approximately 5 wt. %) nickel version (ALLOY 5), with nitrogen at approximately 0.25 wt. %, and all other elements at the ALLOY 1 level.

[0026] 6. Melt and test a further increased (approximately 0.35 wt. %) nitrogen version (ALLOY 6), with nickel at approximately 3 wt. %, and all other elements at the ALLOY 1 level.

[0027] 7. Melt and test an even further increased (approximately 0.40 wt. %) nitrogen version (ALLOY 7), with nickel at approximately 3 wt. %, and all other elements at the ALLOY 1 level.

[0028] 8. Melt and test a version (ALLOY 8) wherein all elements other than nickel (at approximately 3 wt. %) and nitrogen (at approximately 0.10 wt. %) are at the low end of the range for the commercial embodiment of U.S. Pat. No. 5,002,731.

[0029] 9. Melt and test a version (ALLOY 9) wherein all elements other than nickel (at approximately 3 wt. %) and nitrogen (at approximately 0.40 wt. %) are at the high end of the range for the commercial embodiment of U.S. Pat. No. 5,002,731.

[0030] It will be noted that the higher the nitrogen content of the experimental alloys, the higher is their chromium content. This was not deliberate, but is assumed to have resulted from higher chromium recoveries (than previously experienced) during melting of the materials. It is likely related to the use of “nitrided-chromium” charge material as a means of adding the nitrogen.

[0031] It was also the case that the actual nitrogen contents were generally higher than the aim nitrogen contents during this work. For example, the aim nitrogen content of Alloys 1 and 2 was 0.08 wt. %, whereas the actual contents were 0.114 (Alloy 1, Batch A), 0.127 (Alloy 1, Batch B), and 0.109 wt. % (Alloy 2). These variances are attributed to unanticipated, higher nitrogen recoveries during VIM/ESR melting and re-melting of the alloys.

[0032] Aluminum was added to the experimental alloys to react with, and remove, oxygen during primary melting (in the laboratory VIM furnace). Aluminum is very important in production-scale air-melting, where it is used to maintain the very high temperatures required during argon-oxygen decarburization (AOD), in addition to its function as a de-oxidizer. Manganese was added to help with the removal of sulfur during melting, at the levels suggested by U.S. Pat. No. 5,002,731. The silicon and carbon levels used in the alloys of this invention are similar to those claimed in U.S. Pat. No. 5,002,731. Such levels have provided excellent weld-ability, in the intervening years. The additional benefits of carbon at these levels, namely excellent cavitation erosion and corrosion resistance were described in U.S. Pat. No. 5,002,731. The dual benefits of chromium, molybdenum, and tungsten regarding resistance to certain forms of wear and corrosion were described in the Background section of this document; all three of these elements were kept (during this work) within the same approximate ranges as claimed in U.S. Pat. No. 5,002,731. Iron was also added to the alloys of this invention within the range claimed in U.S. Pat. No. 5,002,731, its main benefit being tolerance of iron-contaminated scrap materials during furnace charging, with significant economic benefits.

[0033] The key additions to the wrought, cobalt-based alloys described herein are nickel and nitrogen. As already mentioned, the most important and surprising discovery of this work was the powerful, positive influence upon chloride-induced crevice corrosion resistance of reducing the nickel content in the commercial embodiment of U.S. Pat. No. 5,002,731 to 3.17 wt. % and below. Furthermore, given the prior art (particularly U.S. Pat. No. 5,002,731), it was unexpected that alloys with nitrogen contents above approximately 0.12 wt. % could be processed into wrought products without difficulty, which infers that lower nickel contents might have a positive influence upon the wrought-ability of these higher nitrogen alloys.

[0034] The fact that the three alloys (6, 7, and 9) with the highest nitrogen contents (0.367, 0.415, and 0.413 wt. %, respectively) cracked during forging might mean that the solubility of nitrogen has been exceeded, leading to the presence of one or more additional phases in the high temperature, ingot microstructure. If the nitrogen contents of these alloys were reduced to levels within the range 0.262 to 0.278 wt. % of alloys 3(A), 3(B), and 4 (plus or minus the normal manufacturing allowance for nitrogen of 0.02 wt. %), these modified alloys 6, 7, and 9 would likely not crack.

[0035] Regarding the effects of reducing the nickel content upon galling resistance, these appear to be non-linear (something that current wear theory would not predict). Indeed, it was only at nickel levels of 3.17 wt. % and below, that galling resistance exceeded that of Alloy 1 (the commercial embodiment of U.S. Pat. No. 5,002,731, albeit with a slightly elevated nitrogen content, due to the aforementioned melting variance).

[0036] The melting of alloys of this type under large-scale production conditions requires not only an aim content for each element, but also practical ranges, given the variances due to elemental segregation in cast (real-time) analytical samples, variances due to secondary melting (for example ESR), and variances due to chemical analyses. “Plus or minus” allowances during melting on each of the deliberate additions to the commercial embodiment of U.S. Pat. No. 5,002,731, to accommodate these variances, are as follows: chromium±1.5 wt. %; nickel±1.25 wt. %; molybdenum±0.5 wt. %; tungsten±0.5 wt. %; iron±1 wt. %; manganese±0.25 wt. %; silicon±0.2 wt. %; aluminum±0.075 wt. %, carbon±0.02 wt. %; nitrogen±0.02 wt. %. Cobalt, as the balance, does not need such an allowance. For cobalt-based alloys with lower nickel contents than the commercial embodiment of U.S. Pat. No. 5,002,731 (for example, HAYNES 6B alloy), the plus or minus allowance for nickel is 0.375 wt. %.

[0037] Although the tests were conducted on wrought forms of the compositions, improved resistance to chloride-induced crevice corrosion and galling would be present in other product forms such as castings, weldments, and powder products (for powder metallurgy processing, additive manufacturing, thermal spraying, and welding).

Test Results

[0038] The crevice corrosion test used in this work was that described in ASTM Standard G48, Method D. It involved sheet samples of dimensions 50.8×25.4×3.2 mm/2×1×0.125 inch, with TEFLON crevice assemblies attached. Method D enables determination of the critical crevice temperature (CCT) of a material, i.e. the lowest temperature at which crevice attack is observed in a solution of 6 wt. % ferric chloride+1 wt. % hydrochloric acid, over a 72 h (uninterrupted) period. The test temperature was limited in this work to 100° C./212° F., since the ASTM Standard does not address the equipment (i.e. autoclaves) required for tests at higher temperatures.

[0039] In order to differentiate between the experimental alloys under conditions conducive to galling, a modern, LASER-based, 3-D surface measurement system was employed to study the wear scars, along with galling test hardware and procedures established in 1980. These procedures involved twisting a pin (of diameter 15.9 mm/0.625 in) against a stationary block (of thickness 12.7 mm/0.5 in) ten times through an arc of 121°, using a hand-cranked, back-and-forth movement. A load of 2722 kg/6000 lb. was applied by means of a tensile unit (in compression mode), plus a (greased) ball bearing seated on a female cone machined onto the top of the pin.

[0040] The galling tests involved self-mated samples (i.e. the pins and blocks were of the same material) and LASER-based, high-precision measurements of the root mean squared (RMS) roughness of the block scars.

[0041] All tests involved with this work were duplicated, under identical conditions. The RMS values presented in Table 2 are averages from the two galling tests. The CCT values presented in Table 2 are the lowest temperatures at which crevice attack was observed, irrespective of whether one or both samples exhibited attack at that temperature.

[0042] A higher CCT indicates higher resistance to chloride-induced crevice corrosion. A lower RMS indicates higher resistance to galling, during (self-coupled) high load/low speed, metal-to-metal sliding.

TABLE-US-00002 TABLE 2 Crevice Corrosion and Galling Test Results ALLOY CCT RMS COMMENT 1 75° C. 1.9 microns Commercial (Batch A Tested) (Batch B Tested) Embodiment of U.S. Pat. No. 5,002,731 2 85° C. 3 100° C.  1.7 microns Alloy of this Invention 4 Greater 1.4 microns Alloy of this than 100° C. Invention 5 85° C. 2.4 microns 8 Less than or 1.9 microns Equal to 75° C.

[0043] The results in Table 2 are shown in chart form in FIG. 1.

[0044] Table 3 contains the broad range and preferred range for chromium, iron, molybdenum, tungsten, silicon, manganese and carbon in the alloy disclosed in U.S. Pat. No. 5,002,731. Because the alloy of the present invention derives from the commercial embodiment of U.S. Pat. No. 5,002,731, we expect that any alloy having up to 3.17 wt. % nickel (plus the normal manufacturing allowance of 0.375 wt. %), 0.262 to 0.278 wt. % nitrogen (plus or minus the normal manufacturing allowance for nitrogen of 0.02 wt. %), and 0.08 to 0.13 wt. % aluminum (plus or minus the normal manufacturing allowance for aluminum of 0.075 wt. %), along with chromium, iron, molybdenum, tungsten, silicon, manganese and carbon in an amount within the ranges disclosed in U.S. Pat. No. 5,002,731 would have the same improved resistance to galling and chloride-induced crevice attack as the tested alloys that are disclosed here.

TABLE-US-00003 TABLE 3 Ranges for Cr, Fe, Mo, W, Si, Mn and C (Percent by Weight) Broad Range Preferred Range Chromium 22.0 to 30.0 24.0 to 27.0 Iron Up to 7 2.0 to 4.0 Molybdenum 3.0 to 10.0 4.5 to 5.5 Tungsten Up to 5.0 1.5 t0 2.5 Silicon 0.05 to 2.0 0.30 to 0.50 Manganese 0.05 to 2.0 0.50 to 1.00 Carbon 0.02 to 0.11 0.04 to 0.08

[0045] The manufacturing allowances/tolerances described above can be applied to the amounts of chromium, iron, molybdenum, tungsten, silicon, manganese, carbon and aluminum in the tested alloys of this invention to determine acceptable ranges for these elements in our alloy. Additionally, we expect that an alloy having up to 3.545 wt. % nickel and 0.242 to 0.298 wt. % nitrogen would have the same improved resistance to galling and chloride-induced crevice attack if the contents of chromium, iron, molybdenum, tungsten, silicon, manganese and carbon were identical to those claimed in U.S. Pat. No. 5,002,731.

[0046] Although we have described certain present preferred embodiments of our alloy it should be understood that the invention is not limited thereto, but may be variously embodied within the following claims.