Study on phase transition of NM400 wear-resistant steel during continuous cooling

    
    NM400 wear-resistant steel is a kind of high-strength and wear-resistant steel with excellent properties. It has excellent wear resistance, impact resistance, fatigue resistance, cold and hot forming performance, excellent welding performance, and good heat treatment performance. In order to ensure the good wear-resistant performance of NM400 wear-resistant steel, it is necessary to control its continuous cooling rate, so as to enter the proper phase of the continuous cooling process. In this article, the phase transition of NM400 wear-resistant steel during continuous cooling will be studied in detail.
    
    1 Overview
    
    Phase transformation is one of the most important factors affecting the properties of metals and alloys. The phase state of steels is determined by their chemical composition, temperature and time. The transition of a substance from one phase to another is called a phase transition. During the continuous cooling process, there are several changes in the physical properties of NM400 wear-resistant steel. Such changes depend on the cooling rate and the corresponding grain size of the microstructure. Generally speaking, the solid-to-liquid transition occurs at low cooling rates and large grain sizes, while the solid-to-solid transition occurs at higher cooling rates and smaller grain sizes.
    
    2 Phase Transitions of NM400 Wear-resistant Steel
    
    NM400 wear-resistant steel is a kind of low alloy steel which consists of less than 0.25%, 0.95-1.65%, 0.40%, 0.025% and < 0.020% of carbon, silicon, manganese, phosphorus and sulfur, respectively. The balance is made up of iron. During continuous cooling, the NM400 wear-resistant steel goes through several phase transitions.
    
    2.1 Solid-Solid Transition (SS)
    
    The solid-solid transition of NM400 wear-resistant steel occurs at temperatures below 750oC and cooling rates below 10˚C/s. During this transition, the austenite grains start to transform into ferrite at a slow cooling rate. As the transformation proceeds, some of the grain boundaries start to shrink, and some of the grains recrystallize. This process leads to a homogenous microstructure and causes the material to become more resistant to wear.
    
    2.2 Solid-Liquid Transition (SL)
    
    The solid-liquid transition of NM400 wear-resistant steel occurs at temperatures above 900oC and cooling rates between 20-30˚C/s. During this transition, the austenite grains start to dissolve at high temperatures and cooling rates. As the dissolution proceeds, the austenite grains transform into liquid phase and the microstructure becomes inhomogeneous. This transition reduces the wear resistance of the material.
    
    2.3 Solid-Solid-Liquid Transition (SSL)
    
    The solid-solid-liquid transition of NM400 wear-resistant steel occurs at temperatures between 750-900oC and cooling rates between 10-20˚C/s. During this transition, the austenite grains first transform into ferrite at a slow cooling rate. Then, the ferrite grains start to dissolve at high cooling rates. As the dissolution proceeds, the ferrite grains transform into liquid phase, while some of the austenite grains remain in the solid phase. This transition leads to an inhomogeneous microstructure and increases the wear resistance of the material.
    
    3 Conclusion
    
    In conclusion, the phase transition of NM400 wear-resistant steel takes place following a continuous cooling process. The type of phase transition depends on the temperature and cooling rate as well as the type of microstructure. Generally speaking, the solid-solid transition occurs at low cooling rates and large grain sizes, while the solid-liquid and solid-solid-liquid transitions occur at high temperatures and cooling rates with small grain sizes. This knowledge can help us to choose an appropriate cooling rate to ensure the good wear-resistant performance of the material.

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