Enhancement of Hydrogen Gas Embrittlement by Nickel or Copper Coating in Ultrahigh Strength Maraging Steels

In order to make clear the abnormal phenomenon that the notch tensile strength of ultrahigh strength maraging steels is lowered by nickel or copper coating in hydrogen gas atmosphere, the effect of the residual stress of the coating layer on the embrittlement behavior has been investigated.Specimens that had been heated at 773 K after coating showed the enhanced embrittlement by the coatings, and the residual stress before tensile testing was tensile and approximately equal to the yield stress of the coatings. When the residual stress had been shifted toward compressive stress by preloading for nickel coated specimens or by subzero cooling at 77 K for copper coated specimens, the strength of nickel coated specimens became higher than that of the uncoated ones, i.e., a positive effect of the coating was observed. The strength of copper coated specimens also increased by the shift of residual stress, although it was lower than that of the uncoated ones due to an inadequate relief of internal hydrogen that had been absorbed into the specimens during electrodeposition. The enhancement of hydrogen gas embrittlement by the coatings was explained in terms of both the hydrogen absorption at the fresh surface that was producecl by the plastic deformation from the early stage of loading and the residual stress that controlled the beginning of plastic deformation of the coatings.