Title of Invention

A SYNERGISTIC MILLING BATH COMPOSITION USEFUL FOR CHEMICAL MACHINING OF ALLOY STEEL

Abstract

A synergistic milling bath composition useful for chemical machining of steel alloy: The present invention provides a synergistic milling bath composition for chemical machining of alloy steel such as Cr-Mo-V steel which comprises of mixing nitric acid and sulphuric acid with control additives such as urea, sodium nitrate, cetyltrimethyl ammonium bromide and 1,2,3 benzotriazole, which results in a homogeneous dissolution at a reasonable machining rate giving a very smooth surface finish without pits or voids.

Full Text

The present invention relates to a chemical milling bath composition for chemical machining of alloy steel. Chemical milling or cheimcaL machining is a process for controlled dissolution of substrate material with chemical solutions in order to obtain a desired profile, shape or for weight reduction. The chemical machining is a method of machining a work piece, which is immersed in a chemical etching bath for a particular time for the dissolution of a desired depth of material from the surface thereof. Otherwise the chemical machining is used to reduce the overall dimensions of a work piece by the complete immersion in an etching solution. The chemical machining of a particular material is controlled by the solution composition and its constant supply, temperature and time. In the chemical milling process, the metal is removed from the work surface by chemically converting it into a metallic salt. Here, the metal is gradually removed by the chemical etching solution, which is constantly changed at the work surface. The finishes are dependent on the composition of the chemical milling bath, process parameters and the initial micro-roughness and grain size. The main usage of the present invention is in machining to the desired depth of a work piece by chemical milling using a specially formulated chemical milling bath. Another particular usage of the present invention is that complicated shapes of work piece such as stabilizer spar tube can be internal, gradual and smooth taper chemical milled to reduce the wall thickness without any steps or line formation. US Patent No. 3673094 and GB 1387458 to Kreml John F of Armo Steel Corporation discloses for chemical milling of stainless steels of most grades and high nickel alloys using a solution consisting of 3 - 15% by volume H2SC>4, 3 -15% HC1, 2 - 6% HNO3, 0.1 - 5% extract of Acacia Mollisima bark with remainder water. The bath is maintained at a temperature of 80 to 90°C. The drawback of the above patent is that the above bath was used only for stainless steel material and high nickel alloys. The bath has to be heated to 90°C. The above chemical milling bath is not suitable for chemical machining of Cr-Mo-V steel. US Patent No: 4353780 to Fishter Robert et al of United Technologies Corporation claims that chemical milling of cast super alloys having high tungsten content (greater than 6%) is possible with an etchant having a composition of by volume percent of 40 - 60 concentrated HNO3, 0.6 - 0.8 concentrated HF, 30 - 70 water and 0.008 moles / litre CuS04 and 0.0016 - 0.025 moles / litre FeCl3. Preferably the molar ratio of CuS04 to FeCl3 is maintained at about 2:1. The etching is accomplished at 50 - 80°C. The drawback of this patent is that the above solution was used for chemical milling of super alloys, particularly those with high tungsten content and is not suitable for chemical machining of Cr-Mo-V steel. US Patent No: 4534823 to Fishter Robert et al of United Technologies Corporation claims that chemical milling of the commercial gamma-gamma prime nickel super alloy IN-100 was carried out by using an aqueous solution comprising of 32.5 - 85 volume percent HC1 with 40ml/l HNO3, 0.0141 M/l metal sulfate ion, 0.0326 M/l metal chloride ion, 0.0168 M/l alkali metal ion and 0.0073 M/l citric acid. The operating temperature of the bath is 55 - 60°C. The drawback of this patent is that the above process was used mainly for the chemical milling of the surface of nickel base super alloys for the article prior to conducting a fluorescent penetrant inspection. The above chemical milling bath is not suitable for chemical machining of Cr-Mo-V steel. US Patent No: 4592854 to Lindner Walter and Coggins Dolphus of Me Donnell Douglas Corporation describes the chemical etching of steels, including 4130, 4140, 4340, 4330, 300M, H-HP 9-4-30, D6AC, 1018 and other low and intermediate steel alloys using a solution consisting of 10-300 ml 75% HsPC^, 100-700 ml HNO3, 80-100 ml HF (70%), 1-50 g NaNO3, 10-100g Urea, 1-30 g sodium benzoate, 5-200 g dissolved iron or steel and balance water. The drawback of this patent is that the above solution was not useful for chemical milling of Cr-Mo-V steel. Hydrofluoric acid is very dangerous chemical to handle and use it in the process. Generally there are more number of methods and compositions available for etching steels. These prior methods and compositions generally are limited in effectiveness to a particular steel or type of steel and are ineffective on a variety of steels. There is no literature information available on chemical milling of Cr-Mo-V steel special alloy material. Therefore a new formulation of bath for chemical milling of Cr-Mo-V steel needs to be developed. The main object of the present invention is to provide a chemical milling bath composition for chemical machining of alloy steel which obviates the drawbacks of earlier methods. Another object of the present invention is to provide a chemical milling bath composition which is cost effective. Still another object of the present invention is to provide a chemical milling bath composition, which will produce a chemical milled surface as smooth, clean, uniform and bright with good etching rate, specially in alloy steel. Yet another object of the present invention is to provide an environmentally friendly chemical milling bath composition. The present invention relates to the development of a chemical milling bath composition for chemical machining of alloy steel such as Cr-Mo-V steel which comprises of mixing nitricacid and sulphuric acid with control additives such as urea, sodium nitrate, cetyltrimethyl ammonium bromide and 1,2,3 benzotriazole, which results in a homogeneous dissolution at a reasonable machining rate giving a very smooth surface finish without pits or voids. Accordingly the present invention provides a synergistic milling bath composition useful for chemical machining of steel alloy which comprises on a per liter basis: Nitric acid - 10 to 45W% ( having specific gravity of 1.41) Sulphuric acid - 8 to 45 W% (having specific gravity of 1.84) Urea - 0.08 to 0.8 W% Sodium nitrate - 0.08 to 0.8 W% Cetyl trimethyl - 0.004 to 0.04 W% ammonium bromide (CTAB) - 1,2,3 benzotriazole (BTA) - 0.004 to 0.04 W% Distilled water - Balance to make 1 litre of solution In an embodiment of the present invention the nitric acid used may be 70% LR grade, with specific gravity of 1.41. In another embodiment of the present invention the sulphuric acid used may be 98% LR grade, with specific gravity of 1.84. In still another embodiment of the present invention the urea used may be LR grade. In yet another embodiment of the present invention the sodium nitrate used may be LR or commercial grade. In a further embodiment of the present invention cetyl trimethyl ammonium bromide (CTAB) used may be AR grade. In still further embodiment of the present invention 1,2,3 Benzotriazole (BTA) used may be extra pure or synthesis reagent grade. In still yet another embodiment of the present invention the water used may be distilled water. The chemical milling bath composition of the present invention is not a mere admixture but is a synergistic mixture having properties which are not a mere aggregation of the individual properties of the ingredients and there is no chemical reactions. The chemical milling bath of the present invention is particularly useful for chemical machining of Cr-Mo-V alloy steel resulting in of very smooth surface finish without any pits or voids and also provides homogenous dissolution at a reasonable machining rate. The chemical machining using the novel chemical milling bath composition of the present invention may be carried by conventional methods. The novelty of the chemical milling bath composition of the present invention is that, it is capable of machining of Cr-Mo-V alloy steel, for which no chemical milling bath composition is known. The novelty of the present invention has been achieved by the inventive steps of mixing the individual ingredients such as herein before described, to obtain the synergistic chemical milling bath composition. The following examples are given to illustrate the invention and should not be construed to limit the scope of the present invention. Example 1 The composition of the chemical milling bath taken was as follows on a per liter basis: Nitric acid - 12.9390 W% (140 gpl) Sulphuric acid - 8.3179 W% (90 gpl) Urea - 0.0924 W% (1 gpl) Sodium nitrate - 0.0924 W% (1 gpl) Cetyl trimethyl - 0.0046 W% (0.05 gpl) ammonium bromide (CTAB) 1,2,3 Benzotriazole (BTA) - 0.0046W% (0.05 gpl) Distilled water - 78.5491 W% (849.9 gpl) Example 2 The composition of the chemical milling bath taken was as follows on a per liter basis: Nitric acid - 24.0550 W% (280 gpl) Sulphuric acid - 15.4639 W% (180 gpl) Urea - 0.0859 W% (1 gpl) Sodium nitrate - 0.1718W% (2 gpl) Cetyl trimethyl - 0.0086 W% (0.1 gpl) ammonium bromide (CTAB) 1,2,3 Benzotriazole (BTA) - 0.0086 W% (0.1 gpl) Distilled water 60.2062 W% (700.8 gpl) Example 3 The composition of the chemical milling bath taken was as follows on a per liter basis: Nitric acid - 32.6340 W% (420 gpl) Sulphuric acid - 27.9720 W% (360 gpl) Urea - 0.1554W% (2 gpl) Sodium nitrate - 0.2331 W% (3 gpl) Cetyl trimethyl - 0.0155 W% (0.2 gpl) ammonium bromide(CTAB) 1,2,3, Benzotriazole(BTA) - 0.0078 W% (0.1 gpl) Distilled water - 38.9822 W% (501.7 gpl) Example 4 The composition of the chemical milling bath taken was as follows on a per liter basis: Nitric acid - 44.9799 W% (560 gpl) Sulphuric acid - 14.4578 W% (180 gpl) Urea - 0.2410 W% (3 gpl) Sodium nitrate - 0.3213 W% (4 gpl) Cetyl trimethyl - 0.0241 W% (0.3 gpl) ammonium bromide(CTAB) 1,2,3, Benzotriazole(BTA) 0.0161 W% (0.2 gpl) Distilled water 39.9598 W% (497.5 gpl) Example 5 The composition of the chemical milling bath taken was as follows on a per liter basis: Nitric acid - 34.8548 W% (420 gpl) Sulphuric acid - 14.9378 W% (180 gpl) Urea - 0.2490 W% (3 gpl) Sodium nitrate - 0.4150 W% (5 gpl) Cetyl trimethyl - 0.0166 W% (0.2 gpl) ammonium bromide(CTAB) 1,2,3, Benzotriazole(BTA) - 0.0166 W% (0.2 gpl) Distilled water - 49.5102 W% (596.6 gpl) Example 6 The composition of the chemical milling bath taken was as follows on a per liter basis: Nitric acid - 33.7079 W% (420 gpl) Sulphuric acid - 21.6693 W% (270 gpl) Urea - 0.3210 W% (4 gpl) Sodium nitrate - 0.4815 W% (6 gpl) Cetyl trimethyl - 0.0080 W% (0.1 gpl) ammonium bromide(CTAB) 1,2,3, Benzotriazole(BTA) - 0.0160 W% (0.2 gpl) Distilled water 43.7963 W% (545.7 gpl) Example 7 The composition of the chemical milling bath taken was as follows on a per liter basis: Nitric acid - 29.5608 W% (350 gpl) Sulphuric acid - 15.2027 W% (180 gpl) Urea - 0.1689 W% (2 gpl) Sodium nitrate - 0.4223 W% (5 gpl) Cetyl trimethyl - 0.0169 W% (0.2 gpl) ammonium bromide(CTAB) 1,2,3, Benzotriazole(BTA) - 0.0169 W% (0.2 gpl) Distilled water - 54.6115 W% (646.6 gpl) Example 8 The composition of the chemical milling bath taken was as follows on a per liter basis: Nitric acid - 40.0000 W% (490 gpl) Sulphuric acid - 14.6939 W% (180 gpl) Urea - 0.0816 W% (1 gpl) Sodium nitrate - 0.4898 W% (6 gpl) Cetyl trimethyl - 0.0082 W% (0.1 gpl) ammonium bromide(CTAB) l,2,3,Benzotriazole(BTA) - 0.0082 W% (0.1 gpl) Distilled water - 44.7183 W% (547.8 gpl) Example 9 The composition of the chemical milling bath taken was as follows on a per liter basis: Nitric acid - 22.4719 W% (280 gpl) Sulphuric acid - 28.8924 W% (360 gpl) Urea - 0.1605 W% (2 gpl) Sodium nitrate - 0.3210 W% (4 gpl) Cetyl trimethyl - 0.0080 W% (0.1 gpl) ammonium bromide(CTAB) 1,2,3, Benzotriazole(BTA) - 0.0080 W% (0.1 gpl) Distilled water - 48.1382 W% (599.8 gpl) Example 10 The composition of the chemical milling bath taken was as follows on a per liter basis: Nitric acid - 21.0843 W% (280 gpl) Sulphuric acid - 40.6626 W% (540 gpl) Urea - 0.3765 W% (5 gpl) Sodium nitrate - 0.7530 W% (10 gpl) Cetyl trimethyl - 0.0150 W% (0.2 gpl) ammonium bromide(CTAB) 1,2,3, Benzotriazole(BTA) - 0.0150 W% (0.2 gpl) Distilled water - 37.0936 W% (492.6 gpl) Using the above chemical milling bath composition the special alloy steel samples at the coupon level were chemical milled and analysed. The size of the coupons used were 80 x 27 mm and the thickness of the flat sheet was 2 mm. The volume of the solution used for the experiment was 1 litre. The temperature of the bath was maintained between 28 and 35°C by cooling water. The dissolution has been carried out for 2 to 10 minutes. The effect of weight loss, milling rate, temperature rise and volume of solution have been studied. After chemical milling, the samples were characterized by image analysis and surface finish measurements. After chemical milling the surface obtained was smooth, clean, uniform and bright. The etching rate achieved was about 20 to 60 µm/cm2/min. The surface roughness of the sample before chemical milling was 2.11 - 2.60 µm. The surface finish obtained after chemical milling was 0.28 to 1.20 µm. In fact the surface finish has improved after chemical milling. The advantages of the present invention are : i. The overall surface finish has improved after chemical milling without any pits or voids. ii. The etching rate of about 10 - 60 µm/cm2/min has been achieved, iii. The decarbonisation depths on the chemical milled surface is comparable to the raw material depth of decarbonisation. iv. Sub-surface condition does not show any change in structure, v. Hydrogen absorption levels on the chemical milled surfaces are lesser than the level on the raw material, vi. The chemical milling process has not altered the basic structure and the properties of the raw material, vii. The above mentioned bath can be used for several applications where chemical milling is necessary. As an example, this can be used for machining complicated internal taper chemical milling of stabilizer spar tube. We Claim: 1. A synergistic milling bath composition useful for chemical machining of steel alloy which comprises on a per liter basis: Nitric acid _ 10 to 45 W% (having specific gravity of 1.41) Sulphuric acid - 8 to 45 W% ( having specific gravity of 1.84) Urea - 0.08 to 0.8 W% Sodium nitrate - 0.08 to 0.8 W% Cetyl trimethyl - 0.004 to 0.04 W% ammonium bromide (CTAB) - 1,2,3 benzotriazole (BTA) - 0.004 to 0.04 W% Distilled water - Balance to make 1 litre of solution 2. A synergistic milling bath composition useful for chemical machining of steel alloy substantially as herein described with reference to the examples.

Documents:

572-del-2002-abstract.pdf

572-del-2002-claims.pdf

572-del-2002-correspondence-others.pdf

572-del-2002-correspondence-po.pdf

572-del-2002-description (complete).pdf

572-del-2002-form-1.pdf

572-del-2002-form-19.pdf

572-del-2002-form-2.pdf

572-del-2002-form-3.pdf