Brief introduction of sandblasting process applied to titanium alloy surface
1, the scope of adaptation
This standard provides cleaning and descaling methods for titanium and titanium alloy surfaces, applicable to the production and use of titanium and titanium alloys and manufacturers to remove general pollution, oxides and dirt, as well as foreign substances in the form of surface pollution in the process of heat treatment.
When titanium and titanium alloys are machined, cast, or molded, these methods are not mandatory for the removal of existing dirt, but rather provide a guide.
This standard applies to cleaning prior to chemical etching, joining, plating, assembly, and removal of foreign matter in any state that affects corrosion resistance, stability, and quality of the final product.
When there is a layer of oxidative contamination or alpha on the surface, pickling must be carried out after cleaning.
2. program
Alkali or emulsion immersion cleaning agents and alkaline electrolytic systems are recommended for oils, oils and lubricants used in machining, forging and manufacturing operations of titanium and titanium alloys. In electrolysis, the workpiece may be either anode or cathode. these grime is reasonably good before heat treatment or acid treatment as specified in 4.2. When using electrolysis, voltage should be controlled to prevent spark discharge, resulting in pits on the surface of products.
3, impact cleaning
3.1 Mechanical descaling methods, such as sand blasting, shot blasting and steam spraying. The state and cleaning method described in Chapter 4 can be used for hot working scales and lubricants on QICHU titanium surface.
3.2 The sand used for sandblasting shall be high quality, cleaned and iron-free silica sand. If carbon steel or low alloy steel products have been sandblasted with this equipment, the sand used for these products should not be used for cleaning the titanium surface, but should be provided with separate cleaning sand.
3.3 If the entire surface is sandblasted, the exposed surface will be roughened by coarse or shot sand. To protect surface accuracy, local cleaning with appropriate pickling procedures should be given priority.
3.4 When using steel sand or sand containing iron for sandblasting to clean the titanium surface, pickling should be used after sandblasting to remove the steel particles embedded in the titanium surface.
3.5 Any grinding or shot peening may cause residual compressive stress and local deformation on the surface of the material or titanium structure, which should be treated by chemical milling or contour machining.
3.6 In most cases, sandblasting does not mean complete exclusion of pickling. Grinding does not remove layers of contamination caused by interstitial elements such as carbon, oxygen and nitrogen. When these elements are present in excess, pickling control as shown in 4.3 is more suitable for removal.
4. Pickling and descaling
4.1 In addition to the recommended rapid sandblasting or grinding treatment of titanium surface, pickling shall be carried out in accordance with 4.3.2 to ensure complete removal of metal iron, oxide, scale and other surface contamination. If the product is chemically milling to remove oxidized contamination on the surface and the product profile is not conducive to overall sandblasting, salt bath treatment can be used to avoid localized surface corrosion.
4.2 Scales and lubricant residues formed during rolling, casting, forging, or assembly of titanium products are normally treated by one of the following industrial methods prior to final pickling in accordance with 4.3.2 to obtain a completely uncontaminated surface.
4.2.1 All corrosion base solutions shall be mixed with tap water as recommended by the manufacturer.
The molten base salt bath shall be conducted at 399 ~ 454℃(750 ~ 850℉) according to the specified procedure.
4.2.3 The molten base salt bath shall be conducted at 204℃(400℉) according to the specified procedure.
4.2.4 Oxide and color produced by heating below 593 ° C (1100 ° F), usually by pickling, acid ratio (volume) : 10-20% (150-300 g/L) nitric acid (70%)+ 1-2% (12-24 g/L) hydrofluoric acid (60%)+ water, 120 ° C (49 ° F)
Forged and hot-worked titanium alloys are usually mixed with graphite or glass lubricants to form hot scales that can be completely dissolved in a bath of molten base salts at 454℃(850℉). For subsequent treatment, parts shall be pickled in accordance with 4.3.2.
4.2.6 Mixed hot scales containing graphite and molybdenum disulfide lubricant residues produced from solution treated and heat treatable α+β and β alloys at temperatures above 593 ° C (1100 ° F) are recommended to be treated in a base salt bath fused at 204 ° C (400 ° F). Subsequent parts shall be pickled in accordance with 4.3.2.
4.2.7 Suitable grinding methods such as wheel or belt grinding, flake wheel for cutting, and grinding or shot peening can be used if the surface structure is scales that can be easily removed.
4.3 After mechanical grinding or chemical treatment, the material can be further treated in the following way to completely clean the surface.
4.3.1 After salt bath treatment and water cleaning, titanium and titanium alloys can be immersed in sulfuric acid solution to remove deteriorated scales. The acid used is well maintained at 66 ° C (150 ° F) with a concentration of 10-40% sulfuric acid (95% weight) by volume. The final surface bleaching of the finished product can be completed by short immersion in acid as specified in 4.3.2.
4.3.2 Materials that are mechanically grinded according to 3.1 or chemically treated according to 4.2.1, 4.2.2 and 4.2.3 can be cleaned and finished by immersing them in acid solution. The ratio of acid solution is: 10 ~ 30% (volume)(150 ~ 450g/L) nitric acid (70%)+1 ~ 3%(volume)(12 ~ 36g/L) hydrofluoric acid (60%) at 49℃(120℉), keeping the ratio of nitric acid to hydrofluoric acid at 10:1.
5 Precautions for Operation
5.1 When cleaning titanium in high temperature oxidized salt, an electric current will be generated when the workpiece is in contact with the iron based material. Titanium and these iron-based materials form a positive electrode, or anode, forming a circuit breaker voltage of about 0.60V. The release from the rack to the workpiece results in surface overheating and possible fire. This effect can be reduced by maintaining a salt bath temperature not higher than 455 ° C (850 ° F) and by using titanium fixtures or aluminum insulation between the workpiece and fixture.
5.2 Forgings or hot rolled materials with thick scales can be mechanically grinded to remove the surface contamination before salt bath treatment.
5.3 In the process of hot forming or annealing of titanium alloy, the silicon-based protective layer can reduce the formation of scale skin. All coatings shall be QICHU prior to acid treatment.
5.4 Hot-formed or heat-treated titanium, during fabrication or assembly, forms a mixed surface contamination, including graphite or molybdenum disulfide with titanium oxide, which shall be cleaned by molten salt bath. The processing temperature should be 204 ° C (400 ° F) to avoid thermal deformation.
5.5 When treated in salt bath or lye, the titanium oxides on the metal surface will react to form sodium titanate. They dissolve in acids containing sulfur and nitrogen-hydrogen fluoride. Sulfuric acid had no corrosion effect on titanium or titanium alloys, and corrosion was inhibited by the addition of 0.25 ~ 1.0 % copper or ferric sulfate.
5.6 Most pickling after salt solution is carried out in acids containing sulfur and nitrogen-hydrogen fluoride. The material is cycled through salt solution, water rinsing, and sulfuric acid until all scales are removed. The final bright finish is completed after a short cycle immersion in nitric acid and hydrofluoric acid bath.
5.7 In nitric acid and hydrofluoric acid baths, the ratio of nitric acid to hydrofluoric acid is more important than the concentration of either acid. When the ratio is maintained at 10:1, hydrogen absorption during pickling is quite small.
5.8 In the manufacturing process of titanium rolled products, it is inevitable to have oxygen-rich layers on surfaces exposed to high temperature or oxidation atmosphere. When removing the oxygen-rich and α layers by pickling with strong nitric acid and hydrofluoric acid solutions, it is important that all residual oxides and scales be completely removed to prevent preferential corrosion of the finished product.
5.9 Excess hydrogen can be removed by vacuum annealing of cleaner parts.
6 test
6.1 Materials cleaned in accordance with this standard shall be free from visible paint, oil, grease, glass, graphite, lubricants, dirt, abrasives, iron, or other forms of contamination during visual inspection.
6.2 The absorption of hydrogen during cleaning should be controlled to a relatively small extent and within the permissible limits as described in the procedure. Regular monitoring of the cleaning system shall be carried out by treating samples with known hydrogen content and completing the system by chemical analysis methods. When the hydrogen content exceeds the original product analysis result by 20ppm, the acid solution should be replaced or the acid composition should be adjusted to reduce the degree of hydrogen absorption.
6.3 Product cleanliness can also be accomplished by using a sample sample. Test sheets are quite good to use approximately 0.001 to 0.002 in.(0.025 to 0.05 mm) and corrode from both sides. After chemical corrosion, the surface of the sample should be smooth and bright, without bumps and pollution caused by residual scales.
