Saturday, 20 February 2016

Surface Finish After Heat treatment

          Due to heat treatment certain defects like burning, decarburisation, overheating, quenching cracks, deformation, scaling, soft spots etc., may occur. In order to remove these defects following finishing operations are carried: 

1. Acid pickling
2. Sand blasting
3. Degreasing
4. Straightening.

 1. Acid pickling:
          If the heat treatment is not performed in a controlled gas atmosphere oxide or scale layer is formed on the surface of the heat treated component. Pickling is the most common chemical procedure used to remove oxides and iron contamination. Besides removing the surface layer by controlled corrosion, pickling also selectively removes the least corrosion-resistant areas. Pickling normally involves using an acid mixture containing nitric acid (HNO3), hydrofluoric acid (HF) and, sometimes, also sulphuric acid (H2SO4). Owing to the obvious risk of pitting corrosion, chloride containing agents such as hydrochloric acid (HCL) must be avoided.
Descaling can be performed by acid pickling in the following two ways:
         Direct dissolution of a component in all acid bath:
         Mechanical circulation of acid solution:

- 8 to 12% or 20% HCL is taken for the above purpose.
- The acid bath is used at 50 to 70.
- The components removed from the pickling bath are thoroughly cleaned with steam and then neutralised in a week alkaline bath for 5 to 10 minutes. Finally they are rinsed by warm water.

2Sand blasting:
          In sand blasting process the surface to be cleaned is exposed to a jet of compressed air carrying sand composed of dry, sharp, quartz grain of 1 to 10 mm of size. This process gives fine surface finish and useful to large areas to be cleaned. 
          In another process surface cleaning is done by centrifugal action of abrasive material like iron shorts instead of sand blowing with pneumatic pressure. This method also produces cold case hardening of the surface, increasing the fatigue resistance of the articles.

          Salts are deposited on the surface of the components when treated in salt baths. These are removed by soaking the component in boiling water.
          Both slats and oil deposits can be cleaned by immersing the components in a tank containing boiling data solution and then washing in water. The operation is known as vat cleaning.

      During heat treatment certain tools and machine parts get distorted or warped. In order to bring them original shape, straightening is carried out.
     Straightening of rolled stock is done by straighteners or hydraulic straightening process.


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