Effect of Mo and Ni on Continuous Cooling Transformation of Low Alloy Wear Resistant Steel

Low alloy wear resistant steels of the type used in engineering components and tools have various alloying elements added to them such as molybdenum (Mo) and nickel (Ni). These alloying elements have a major effect on the continuous cooling transformation of these alloys, which is an important factor in affecting their mechanical strength and wear resistance.
    
Molybdenum (Mo) is an important alloying element in low alloy wear resistant steels and can improve the wear resistance and hardenability of the alloys. Mo tends to stabilize the austenitic phase, making it difficult for bainite to form. This increases the temperature range of the upper bainite-austenite transformation and favors the formation of martensite. Without Mo, the transformation temperatures can decrease significantly. The presence of Mo changes the morphology of the retained austenite. Highly Mo-alloyed steels are inhomogeneous and can contain Widmanstatten and lath-like structures of austenite and ferrite which can decrease the strength of the bainite, yet result in higher wear resistance.
    
Nickel (Ni) is another major element in low alloy wear resistant steels that also affects the continuous cooling transformation of these alloys. The presence of Ni in these alloys greatly accelerates the austenite-to-bainite transformation, while also decreasing the amount of retained austenite. This is due to the fact that Ni increases the nuclear diffusion rate which results in more rapid transformation of austenite to bainite. Additionally, Ni can influence the morphology of the austenite, resulting in a fine and homogeneous structure which helps to maintain a greater amount of bainite in the alloy. The amount of bainite formed also directly affects the wear resistance of the alloy.
    
In addition to these effects, Mo and Ni also affect the relative amounts of ferrite and pearlite that form during the continuous cooling transformation of low alloy wear resistant steels. The presence of Mo increases the amount of ferrite present, while Ni tends to decrease it. This is due to the fact that Mo tends to suppress the formation of pearlite, while Ni favors its formation. This effect can be seen in the diagrams below.
    
In conclusion, molybdenum (Mo) and nickel (Ni) are two important alloying elements in low alloy wear resistant steels, and they have a significant effect on the continuous cooling transformation of these alloys. Mo increases the temperature range of the bainite-austenite transformation and favors the formation of martensite, while Ni increases the nuclear diffusion rate and favors the formation of bainite. The presence of Mo increases the amount of ferrite present, while Ni decreases it. All of these effects help to improve the wear resistance and toughness of these alloys.

---