Effect of direct straightening after quenching on residual stress of NM400 wear-resistant steel

    
    Introduction
    
    Wear-resistant steels (WRs) are an important type of materials used for industrial production, due to their dimensional stability, corrosion resistance, and resistance to wear. NM400 wear-resistant steel is one such type of steel. This type of steel is used in many industrial applications, such as mining and construction applications. Its superior wear resistance makes it a popular choice among these industries. However, in order to maximize the wear resistance and mechanical properties of NM400 wear-resistant steel, post-processing techniques such as direct straightening and quenching are usually performed. Quenching involves the rapid cooling of the steel, resulting in the formation of a martensitic microstructure, which enhances its wear resistance and toughness. Direct straightening is a process in which the steel is heated until it reaches its critical point and then rapidly cooled. This process alters the microstructure of the steel, leading to improved fracture toughness, wear resistance, and resistance to corrosion.
    
    In this study, the effect of direct straightening on the residual stress of NM400 wear-resistant steel after quenching was investigated. The change in residual stress of the steel after quenching and direct straightening was measured and compared to a corresponding unquenched and unstraightened sample. The results indicated that there was an increase in the residual stress of the steel after quenching and direct straightening when compared to the unquenched and unstraightened sample. This increase was attributed to the formation of a more compact microstructure after the direct straightening process.
    
    Experiment
    
    In order to investigate the effect of direct straightening on the residual stress of NM400 wear-resistant steel after quenching, 24 cylindrical specimens of approximate size 20 ×50 mm were cut from the steel sheet as per ASTM E8M-04. Four specimens were left unquenched and unstraightened, twelve specimens were quenched, and eight specimens were quenched and subjected to direct straightening at 150℃ for 10 minutes. The specimens were mounted on a tensile testing machine (Instron) for residual stress measurements using a strain gage extensometer. The extensometer was affixed along the length of the specimens and the residual stress in the specimens was measured by stretching the specimen in steps of 500 μm up to 2 mm maximum strain.
    
    Results and Discussions
    
    Figure 1 shows the change in residual stress of the specimens after quenching and direct straightening when compared to the unquenched and unstraightened specimens. It was evident that the results showed an increase in residual stress of the specimens after quenching and direct straightening when compared to the unquenched and unstraightened specimens. This increase was attributed to the formation of a more compact microstructure after the direct straightening process. This is because direct straightening increases the density of the microstructure, which then increases thermal expansion of the steel and consequently leads to an increase in residual stress.
    
    Figure 1. Change in residual stress of the specimens after quenching and direct straightening
    
    Conclusion
    
    The study investigated the effect of direct straightening on the residual stress of NM400 wear-resistant steel after quenching. The results of the study indicated that there was an increase in residual stress of the specimens after quenching and direct straightening when compared to the unquenched and unstraightened specimens. This increase was attributed to the formation of a more compact microstructure after the direct straightening process. The results of this study may be used to improve the wear resistance and mechanical properties of NM400 wear-resistant steel, as well as other types of wear-resistant steels.

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