Research On The Correlation Between Electrochemical Reactivation Characteristics And Plasticity Of Typical Ni-Cr Alloy

Due to the high content of Cr,the corrosion-resistant alloy has better corrosion resistance than ordinary carbon steel.In order to ensure sufficient strength and hardness,corrosion-resistant alloys usually contain much other alloying elements,so precipitates are precipitated easily during the heat treatment.Due to the higher energy at the grain boundary,most of the precipitated phases are concentrated at the grain boundary.The strength and hardness of the alloy can be improved effectively result from the dispersed small-sized precipitates.However,when the precipitated phase is accumulated and grew at the grain boundary,the alloy becomes embrittled.Because of the large difference of the composition and structure between the precipitated phase and the matrix,the structure of passivation film near the precipitated phase is also weaker than that of the matrix.Therefore,the precipitated phase and its adjacent areas are prone to preferential corrosion in corrosive media.In short,the mechanical properties and corrosion resistance of the corrosion-resistant alloy can be affected by the precipitated phase at the same time.In this article,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X-ray diffraction analysis（XRD）,double-loop electrochemical reactivation（DL-EPR）and other analytical methods were used in Cr13 super martensitic stainless steel and 625 alloy surfacing layer to analyze the relationship between toughness and electrochemical reactivation performance after heat treatment.The results showed that:The impact toughness of super martensitic stainless steel is significantly reduced when samples temper before quenching and tempering.The crystal grains are strengthened because of the precipitation of M₇C₃ carbides in the crystals,and the grain boundary is weakened because of the precipitation of a large amount of M₂₃C₆ carbide at the martensite lath boundary,and the two together lead to embrittlement.The impact toughness of super martensitic stainless steel can be improved and restored when the sample is solution treated at 1323K before quenching and tempering.The sample with the lowest impact toughness has the highest electrochemical reactivation parameter Ra value.With the increase of Ra value,the impact toughness of Cr13 super martensitic stainless steel is increased first and then decreased rapidly.Ra is positively correlated with the coarsening of Cr-rich carbides in super martensitic stainless steel,while the degree of precipitate coarsening is negatively correlated with impact toughness.Therefore,the reduction of impact toughness of super martensitic stainless steel can be predicted by the electrochemical reactivation test.In the temperature range of 873～993K,as the aging temperature increases,the elongation of surfacing 625 alloy is decreased rapidly and then slightly increased.When aging at 973K,elongation of sample reaches the lowest.During aging,γ″phase and M₂₃C₆ carbide are precipitated along the dendrite.With the increase of the age temperature,the size of the precipitated phase increases sharply.When aging at 973K,the size of the precipitate reaches the maximum.The aggregation and growth of the precipitated phases along dendrite is the main reason for the decrease in elongation.The result of electrochemical reactivation test is consistent with the size of precipitated phase.As the aging temperature increases,the Ra value first increases and then decreases,and reaches the highest at 973K.As the Ra value increases,the elongation of the sample increases first and then decreases,which indicated that Ra is positively correlated with the growth of the second phase in the 625 alloy.At the same time,there is a negative correlation between the size of the precipitated phase and the elongation.Therefore,the reduction of elongation of surfacing 625 alloy can be predicted by the electrochemical reactivation test.