South Ural State University, Chelyabinsk, Russia:

Yu. N. Goykhenberg, Cand. Eng., Professor, Senior Researcher, Dept. of Materials Science and Physical Chemistry of Materials, e-mail: goikhenbergyn@susu.ru

Scientific and Technical Center KONAR Ltd., Chelyabinsk, Russia:
D. S. Polukhin, Executive Director, e-mail: Polukhin.dmitriy@konar.ru

Possible causes of defects during deposition and subsequent heat treatment (HT) of nickelphosphorus (Ni – P) coatings containing dispersed silicon carbides SiC on steel sheets with a thickness of 65–180 mm are investigated. The phase composition, microstructure, and microhardness of 40–70 μm thick coatings chemically deposited on workpieces made of low- and medium-carbon low-alloy steels in various structural states and subjected to crystallization annealing of various durations were studied. According to X-ray diffraction studies, after deposition coatings are in an amorphous state and have low hardness and plasticity. Heat treatment in the temperature range of 400–450 ºC is accompanied by an increase in coating adhesion, microhardness to the required values (1000 HV) according to specifications, and a drop in elongation to zero. This is due to the crystallization with the formation of carbides SiC and Si₅C₃, nickel with a face-centered cubic (fcc) lattice, and nickel phosphide Ni₃P. The conducted studies and the revealed regularities made it possible to choose 09G2S thick steel sheet for some articles of oil and gas equipment, to establish the technological parameters for deposition of composite Ni–P coatings and the final HT modes that do not lead to the appearance of defects in the form of surface cracks, micropores and other discontinuities. The more homogeneous and stable the substrate structure, the less defects will appear in the coating. Another reason for rejection may be due to the transition from a less dense amorphous initial state to a denser crystalline state upon heating, accompanied by a reduction in volume in combination with a drop in the relative elongation of the coating to zero.

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