Sub zero treatment of steela)is used to reduce the retained austenite ...
Answer:
Sub-zero treatment of steel is a heat treatment process that is used to reduce the retained austenite in hardened steel. It involves cooling the steel to a temperature below its martensite start temperature (Ms) and holding it at that temperature for a period of time before further cooling. This process helps to transform the retained austenite into martensite, thereby improving the properties of the steel.
Below are the details regarding each option:
a) Sub-zero treatment is used to reduce the retained austenite in hardened steel:
- Retained austenite is a metastable phase that remains in the steel after quenching and tempering.
- It has lower hardness and strength compared to martensite and can reduce the overall performance of the steel.
- Sub-zero treatment helps to transform the retained austenite into martensite, thereby increasing the hardness and strength of the steel.
b) Sub-zero treatment increases the ability of steel to work in subzero atmospheres:
- While sub-zero treatment can improve the properties of steel, it does not directly enhance its ability to work in subzero atmospheres.
- Other factors such as composition, microstructure, and surface treatment play a more significant role in determining the performance of steel in subzero conditions.
c) Sub-zero treatment is used to suppress martensite transformation:
- This statement is incorrect. Sub-zero treatment actually promotes martensite transformation, helping to eliminate retained austenite.
d) Sub-zero treatment is performed after hardening operation to induce temper brittleness:
- This statement is also incorrect. Sub-zero treatment is not performed to induce temper brittleness but rather to improve the properties of the steel, such as hardness and strength.
In conclusion, the correct answer is option 'A': Sub-zero treatment is used to reduce the retained austenite in hardened steel. This process helps to transform the retained austenite into martensite, thereby improving the properties of the steel.