POST HEAT TREATMENT
3.1 Abrasive Blast Cleaning:
This method is widely used for removing all classes of scales and rust from forging, casting, and heat-treated parts. Depending on the finish requirement, blasting may be the sole means of scale removal or it may be used to remove the major portion of scale, with pickling employed to remove the remainder. In this process, the parts are generally cleaned by the use of abrasive particles such as sand, steel grit, steel shots, impelled against the surface to be cleaned.
Some cleaning is performed by high velocity air blast, with the blast directed by hand. In many cases, an air less blast machine that cleans by impact is also used. The abrasive is fed from an overhead storage hopper to the centre of the radially rotating wheel, whereupon the metallic shot or grit is hurled in a controlled steam upon the work to be cleaned. All traces of sand, scales, oxides, and other material are removed right down to the virgin metal, providing an excellent surface for bonding final finishes.
The airless blast machine is used for cleaning engine blocks, crankshafts, castings of different shapes and size, railroad cars, car wheels, oil and gas pipes, steel strips and many other purposes.
3.2 Acid Pickling:
The most common method of removing unwanted pigmented compounds, which are mostly oxides of metal, is by acid pickling. Either diluted sulphuric acid, hydrochloric acid or phosphoric acid is sprayed on the part, or the part is dipped in to a tank, agitated, and then washed and rinsed thoroughly. Alkaline cleaning of the part should be used first to remove all dirt and oils in order to obtain an even removal of the oxides during the pickling process. Sometimes it is necessary to add pickling inhibitors such as detergents, liquid glycol, ether etc to decrease the action of the acid upon the metal particularly aluminium or other non-ferrous metals.
Both hot and cold baths are used. A bath made out of 10% Sulphuric acid with water is the most common solution. Heating the bath to about 700C-850C can accelerate pickling operation. A bath, contains 25% to 100% hydrochloric acid with water is best for steels and iron.
3.3 Alkaline cleaning:
The most common type of cleaning is with alkali. It is efficient and economical in removing oil and grease. This method cannot be used for removing rust and scale effectively. Some of the alkaline agents are sodium metasilicate, caustic soda, sodium carbonate, sodium chromate, and sodium phosphate. In this process, a bath is prepared from cleaning agents. These materials are added to some type of soap to aid in emulsification. The mixture produces an alkali, which serves as the cleaning agent. The parts to be cleaned are suspended in to the hot bath. Bath is heated by an electric circuit. After cleaning, parts are thoroughly rinsed to clean off completely from any leaning agent remaining on the surface. The process is used on all metals except zinc, lead, tin, brass and aluminium. On assemblies comprised of dissimilar metals, the presence of alkaline solution in crevices may result in galvanic corrosion, and even a trace of alkali will contaminate paint and phosphate coating.
3.4 Distortion and straightening:
During quenching, large temperature differences exists between surface and interior and between light and heavy sections. This can cause severe shape distortion due to the thermal and mechanical stresses developed by the martensitic transformation. This problem is most severe if the hardenability of the steel is so low that a fast cooling rate is required to achieve full hardness. If lower hardenable steels requiring liquid quenching are used, fixturing and pressure die quenching can help to minimize distortion. Long symmetrical parts should be fixtured and should be quenched in the vertical position with vertical agitation of the quenching medium.
If the part is showing distortion even after the quenching process. This can be removed to an extent by straightening. Straightening can be done in several ways.
1. As soon as the quenching is over the piece should be clamed in a fixture for certain time.
2. Keeping the piece on two parallel blocks under fly press and slowly removing the distortion after removal.
The distortion to a certain extends operation to be carried out.
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