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The critical failure factor and damage mechanism of 316 stainless steel during fretting corrosion under the molten lead-bismuth eutectic (LBE) are specially investigated. Representative F τ - D -cycle curves are shown to reflect the changing process and characteristics of different cycles for each parameter during fretting corrosion tests. The fretting operation regime is the mixed fretting regime (MFR) with minor stroke. The increase in stroke and temperature convert the fretting operation regime from MFR to the gross sliding regime (GSR). According to the dissipated energy and wear volume among all the samples, the results demonstrate more information as follows: the change of dissipated energy and wear volume is positively correlated with stroke, cycle number and temperature. The most serious damage occurred at 450 ℃− 80 µm-2 × 106 cycles. Multivariate linear regression model was established for 18 groups of data, and it was concluded that temperature was the key failure factor within the parameter range of this experiment. The first effect of temperature increase is to increase the rate of diffusion of Ni element into LBE, and the inevitable dissolution corrosion will occur. The problem is that a large number of vacancy defects are generated. The second effect of temperature increase is that the austenite substrate is transformed into ferrite, which cannot resist damage caused by fretting, due to the dissolution of large amounts of Ni. • The degree of wear damage is positively correlated with the stroke, cycle number and temperature. • The contribution of the temperature to the material damage is about 2 times of the stroke and cycle number. • The damage is the most serious at 450 ℃− 80 µm − 2 × 106 cycles. • The dissolution of Ni will transform the substrate from austenite to ferrite which cannot resist damage caused by fretting. [ABSTRACT FROM AUTHOR]

• The Co-based alloy/WC/CaF 2 composite coating is fabricated by laser cladding to improve the fretting wear resistance. • The surface microhardness of the composite coating can reach to 600 HV. • The fretting wear resistance are significantly improved, which can lead to the decrease of the COF and the wear loss. • This work guide for the protection of heat transfer tube in nuclear power plants. The coating fabricated by laser cladding on the surface has been an effective way to strengthen and enhance the wear resistance of alloys. However, the traditional Co-based alloy coatings rarely pay attention to the addition of self-lubrication constituents to form the composite coating. In addition, the effects of the Co-based composite coating on fretting wear resistance were few addressed. In this study, a Co-based alloy/WC/CaF 2 composite coating was fabricated by laser cladding on the surface of 316 stainless steel to improve the surface hardness and self-lubrication performance. The microstructure, phase constituents and fretting wear resistance at both room temperature (RT) and 500 °C were investigated. Results confirm that the composite coating is consisting of α-Co, Cr 23 C 6 and WC phases. And the F element is effectively dissolved into the α-Co phase. The fabricated composite coating is dense and uniform. The microhardness of the composite coating can reach to 600 HV 0.2 due to the presence of WC and Cr 23 C 6 phase. In addition, the grain refinement is another reason resulting in the higher microhardness. Fretting tests demonstrate that the fretting wear resistance at both RT and 500 °C are significantly improved by the composite coating, which can lead to the decrease of the COF and the wear loss. The lower COF can be ascribed to the dissolved F element in α-Co phase, especially at high fretting temperature. Additionally, the composite coating can effectively prevent the oxidation during fretting both at RT and 500 °C. This work provides an excellent method for the metallic materials to improve fretting wear resistance at both RT and high temperature. [ABSTRACT FROM AUTHOR]

The synergistic effect of lead-bismuth eutectic (LBE) corrosion and fretting on the damage of a 316 stainless steel was thoroughly investigated. The interaction between the LBE corrosion and fretting plays a key role in materials damage. The dissolution of Ni element leads to a large number of vacancies, lattice defects and built-in internal strain, making the material itself more prone to damage during the wear. Additionally, the shear strain induced by fretting can break the oxide layer formed during friction, leading to the exposure of new materials. This study can provide essential information on damage of heat transfer tubes. [Display omitted] • The dissolution of Ni results in the transformation of the substrate from austenite to ferrite, accelerating the fretting. • Shear strain breaks the oxide layer leading to the exposure of new materials to LBE, accelerating the corrosion. • The synergistic effect between LBE corrosion and fretting accelerates the damage of materials. • This work can rich the data of the investigation on the damage of the heat-transfer tubes in nuclear power plants. [ABSTRACT FROM AUTHOR]

Heat transfer tubes in nuclear power plants are easily damaged by fretting wear. The fretting behaviour of the heat transfer tube related to the lead-bismuth eutectic (LBE) alloys needs to be investigated. In this work, the effect of pre-immersion corrosion in LBE of AISI 316 stainless steel on fretting behaviour is explored in detail. Following short-duration corrosion (specimens denoted C0 and PC1), no significant change occurred during corrosion, and after the fretting test, the wear scars were shallow, with adhesive wear being the dominant wear mechanism. Following long-duration corrosion (specimens denoted PC2 and PC3), the dissolution of Ni and the formation of an oxide layer were the main corrosion mechanisms. After the fretting tests, the wear scars were deep, demonstrating a degraded anti-wear property, and fretting fatigue wear was dominant. The degradation of the fretting wear resistance in specimens PC2 and PC3 can be attributed to the absence of the protection of NiO in the tribolayer, the large grain size, and the weak plastic deformation accommodation ability. The underlying mechanism is the dissolution of Ni induced by pre-immersion corrosion in the LBE. The present work provides useful information on the fretting behaviour of heat-transfer tubes in nuclear power plants related to the LBE environment. • The dissolution of Ni element and the formation of oxide layer are main corrosion mechanisms in LBE. • The pre-immersion corrosion behaviour results in a larger wear loss in the fretting behaviour. • The effect of pre-immersion corrosion on fretting wear mechanisms are illustrated systematically. • This work guide for the protection of heat transfer tube in nuclear power plants. [ABSTRACT FROM AUTHOR]