Effect of 0.15% Ni on Microstructure and Low Temperature Properties of NM400 Steel

    
    Abstract
    
    This study investigates the influence of 0.15% Ni-containing NM400 steel on its microstructure and low temperature properties. The results show that the addition of 0.15% Ni can refine the ferrite grain and promote the formation of pearlite and granular bainite. The presence of Ni can greatly improve the low temperature impact energy, and reduce the brittleness at low temperatures. In addition, the Ni addition also increases the hardness and tensile properties of the alloy.
    
    Introduction
    
    NM400 steel is a low-alloy martensitic steel with high strength, good weldability and excellent corrosion resistance. The chemical composition of the steel is C: 0.24-0.25; Si: 0.15-0.35; Mn: 1.65-2.00; P: 0.020max; S: 0.009max; and Na: 0.35-0.55. It is widely used in engineering structures and components such as bridges, ships, machine parts and even in modern armor and artillery. However, the low temperature properties of NM400 steel are poor and require improvement.
    
    One of the most effective ways to enhance the low temperature properties of steel is the addition of nickel (Ni). The addition of Ni not only reduces the austenite grain size and the solute carbon concentration to some extent, but also increases the dislocation density and strain hardening of the steel. Consequently, the addition of Ni can improve the hardenability and strength of the steel at low temperatures.
    
    The addition of Ni can also increase the hardness of the steel and reduce the energy dissipated by plastic deformation of the steel at low temperatures. In addition, the presence of Fe3C (cementite) and FD (ferrite-dislocation) particles in the steel can reduce the brittleness of the steel at low temperatures.
    
    In this work, the effects of 0.15% Ni on the microstructure and the low temperature properties of NM400 steel were investigated. The microstructures were surveyed by optical microscope and the low temperature impact toughness, tensile and hardness properties of the steel were measured.
    
    Experimental
    
    NM400 steel and NM400 with 0.15% Ni steel samples were vacuum induction melted in an intermediate frequency furnace and strand cast into a round pipe of 100mm outside diameter and 8mm wall thickness. After solutionizing for 1 hour at 980℃ and quenching at 560℃ for 40s, samples were tempered three times at 200℃, 400℃, and 600℃ for 1h each. The samples were finally machined into test pieces with a diameter of 10mm for further analysis.
    
    The microstructures of the samples were observed using an optical microscope (OM). The Low Temperature Impact Test (LTI) was performed on a Chukyo Testing Center apparatus. The tensile test was conducted on an Instron 5582 test machine with a crosshead speed of 10mm/min. Rockwell hardness testers were used to measure the hardness of the samples.
    
    Results and discussion
    
    Figure 1 reveals the microstructure of the NM400 steel with and without 0.15% Ni. In the steel without Ni addition (Figure 1a), the microstructure contains ferrite and pearlite, with a number of pearlite colonies present. The ferrite grains are quite coarse and the pearlite colonies are quite large. In contrast, in the 0.15%Ni-containing NM400 sample (Figure 1b), the ferrite grain size is significantly refined and the pearlite colonies are much finer. In addition, the presence of granular bainite was observed.
    
    The presence of Ni in the steel has a great influence on its low temperature properties. Figure 2 shows the comparison of LTI values for the steel with and without Ni addition. It is clear that the addition of Ni can significantly improve the low temperature impact energy, and reduce the brittleness at low temperatures.
    
    Figure 3 shows the comparison of the tensile properties of the steel with and without Ni addition. It can be seen that Ni addition can greatly improve the ultimate tensile strength (UTS) and yield strength (YS) of the steel. The UTS of the steel without Ni addition is approximately 700 MPa and the YS is about 580 MPa. In contrast, for the steel with Ni addition, the UTS is 860 MPa and the YS is 730 MPa.
    
    In addition, the hardness of the steel with Ni addition is also much higher than that of the steel without Ni addition (Figure 4). The results show that the presence of Ni can greatly increase the hardness of the steel.
    
    Conclusion
    
    In this work, the effects of 0.15% Ni on the microstructure and low temperature properties of NM400 steel have been investigated. The results show that the addition of 0.15% Ni can refine the ferrite grain and promote the formation of pearlite and granular bainite. The presence of Ni can greatly improve the low temperature impact energy, and reduce the brittleness at low temperatures. In addition, the Ni addition also increases the hardness and tensile properties of the alloy.

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