Quenching Microstructure and Composition Optimization of Thick Section Low Alloy Wear Resistant Steel

The development of thick section low alloy wear resistant steel (HARDOX®) is a major breakthrough in the steel industry. HARDOX® is an ultra-high strength quench and tempered martensitic steel, designed to deliver optimal wear resistance in demanding applications requiring high strength, wear resistance and excellent toughness. This alloy is an important industrial material, widely applied in many areas, such as construction, transportation (also automotive and marine), agricultural, defence and aerospace engineering. The high wear resistance of HARDOX® makes it an ideal material choice in specialised applications, such as hard rock excavation, metal forming and shot blasted components.
    
Quenching and Tempering
HARDOX® is typically produced by quenching and tempering (Q&T) processes. The quenching and tempering stages are essential for obtaining its ultra-high strength and wear resistance properties. Quenching employs a rapid cooling of the work-piece, typically by immersing in quenchant, whereas tempering follows quenching and further heats the quenched steel at a lower temperature, known as the tempering temperature or tempering time, depending on the desired final properties. The tempering temperature also affects the microstructure and composition of the steel, which influences its properties.
    
Microstructure and Composition Optimization
The microstructure and composition of the HARDOX® must be optimized to ensure its ideal wear resistance. For example, the microstructure should contain a combination of hard Bainite and Martensite, the thermochemical treatment should produce a higher amount of hematite (Fe2O3), and the manganese (Mn) content should decrease.
    
The first step to optimize the microstructure of HARDOX® is to identify the process parameters that will produce the desired microstructure. This can involve an iterative approach in which several process parameters are varied to obtain the desired effect. Such parameters may include the cooling rate, the tempering temperature and the alloy composition. The cooling rate is the rate at which the work-piece is cooled from quenching temperatures and is a key factor affecting the microstructure.
    
Another important step to optimize the microstructure is to control and adjust the alloy composition. Depending on the desired properties, increasing and decreasing the content of certain alloying elements, such as Mn and Cr, can result in a more desirable final microstructure.
    
For the optimization of the composition, the Selection and Alloying (S&A) process is essential. This process involves selecting the most suitable alloying elements and then combining them in the optimum ratios for the intended application and microstructure. For example, for improved wear resistance and higher strength, a higher manganese content may be preferred. On the other hand, for improved hardness, a higher chromium content may be preferred.
    
HARDOX® is also produced with a plastic deformation process, known as hot rolling. The hot rolling process can further affect the microstructure and composition of the steel by redistributing alloying elements, thus affecting its properties. During hot rolling, the cooling rate is a significant factor to consider, since a too-high cooling rate can disrupt the desired microstructure. The plastic deformation process also affects the grain size. Too-small grain sizes can lead to brittle material, so the grain size should be controlled and adjusted.
    
Conclusion
In conclusion, the optimization of the microstructure and composition of HARDOX®, is essential to ensure its optimal wear resistance. This optimization encompasses process parameters such as the cooling rate, tempering temperature, alloy composition and plastic deformation. The Selection and Alloying process is also important to obtain the much-coveted combination of hard Bainite and Martensite.

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