Effect of Nickel on Microstructure and Hardness and Toughness of Medium Carbon Low Alloy Steel

Nickel is an important alloying element in medium carbon low alloy steels and can play a critical role in influencing their structure, hardness and toughness. Nickel is added to increase the strength of these steels and to reduce their tendency to form fine pearlite which is more prone to cracking. Many microstructural changes occur during heat treatment of steel containing nickel and as a result, results in significantly improved mechanical properties.
    
The addition of nickel to medium carbon low alloy steel has been found to result in an increase in hardness and strength. This is because it increases the proportion of pearlite in the microstructure. When the steel is heat treated, nickel forms Bainitic or Martensitic microstructures. These microstructures have fine grain sizes and are more uniform than the pearlite microstructure which would form without nickel. The fine grain size increases the number of intragranular dislocations which results in an increase in the tensile strength of the steel.
    
In addition to increasing the strength of the steel, nickel can also increase its toughness. This occurs when nickel forms a eutectoid transformation which increases the matrix toughness. This happens when the nickel is present in amounts which form a eutectoid transformation of the iron carbide. The transformation results in a larger volume fraction of martensite, which has increased toughness compared to pearlite.
    
Nickel is also known to improve the wear and fatigue resistance of the steel. Nickel is generally added to low alloy steels in amounts of between 0.1 - 0.8 %. At concentrations higher than 0.8% the cost of nickel becomes the overriding factor in its selection. When the concentration is below 0.2%, the effects on the microstructure are relatively small.
    
When nickel is added to steel, a variety of microstructural changes occur. The addition of nickel leads to an increased volume fraction of austenite when heated above 910°C which results in a higher degree of hardenability. It also decreases the eutectoid transformation temperature resulting in an increase in the amount of bainite and low temperature martensite formed in the steel during cooling. The addition of nickel also leads to an increase in the grain size in the steel, resulting in a more uniform, fine grain structure. This fine grain structure is more resistant to cracking, resulting in improved toughness.
    
The influence of nickel on the microstructure and mechanical properties of medium carbon low alloy steel is clearly evident. The addition of nickel increases the strength and hardness and also improves the wear and fatigue resistance of the steel. It also leads to an increase in the toughness due to the formation of a eutectoid transformation. The overall result is improved properties which make medium carbon low alloy steels an ideal choice for many applications.

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