Surface chemistry and diffusion of trace and alloying elements during in vacuum thermal deoxidation of stainless steel

Abstract

Removal of the native surface oxide from steel is an important initial step during vacuum brazing. Trace and alloying elements in steel, such as Mn, Si, and Ni, can diffuse to the surface and influence the deoxidation process. The detailed surface chemical composition and grain morphology of the common stainless‐steel grade 316L is imaged and spectroscopically analyzed at several stages of in‐vacuum annealing from room temperature up to 850°C. Measurements are performed using synchrotron‐based X‐ray photoemission and low‐energy electron microscopy (XPEEM/LEEM). The initial native Cr surface oxide is amorphous and unaffected by the underlying Fe grain morphology. After annealing to ~700°C, the grain morphology is seen at the surface, persisting also after the complete oxygen removal at 850°C. The surface concentration of first Mn and then Si increases significantly when annealing to 500°C and 700°C, respectively, while Ni and Cr concentrations do not change. Mn and Si are not located only in grain boundaries or clusters but are distributed across over the surface. Both Mn and Si appear as oxides, while Cr oxide becomes metallic Cr. Annealing from 500°C up to 850°C leads to the removal of first the Mn and then Si oxides from the surface, while Cr and Fe are completely reduced to metals. Deoxidation of Cr occurs faster at the grain boundaries, and the final Cr metal surface content varies between the grains. The findings are summarized in a general qualitative model, relevant for austenite steels.

Effects of Interfacial Electron Transport on Field Electron Emission from Carbon Nanotube Paste Emitters

“…The CNT paste emitter annealed at 750 °C for 5 h had a grooved surface identified by grains and grain boundaries in the Kovar substrate area of over 10 μm. The grain boundary grooving by the vacuum annealing process is generally caused by the surface diffusion, segregation, and evaporation of metal atoms in the grain boundary, , and in the case of the Kovar alloy, it appears from about 900 °C. The grain boundary grooving shown in Figure a resulted from the diffusion of the Si elements contained from the CNT paste emitter into the substrate direction, and the grain boundary was distinguished even at the relatively low temperature of 750 °C.…”

Oxygen‐Free Production—From Vision to Application

“…To get closer to an oxygen-free process environment, metal and alloy surfaces are often thermally deoxidized in ultrahigh vacuum (UHV), especially for research applications. [17,18] Another example is the somewhat unorthodox deoxidation by an electron beam, which is only productive for the deoxidation of small substrates and structures. [19] A different option is deoxidation in thermal or nonthermal plasmas.…”

Vacuum Brazing of 18MAR300 Nickel Maraging Steel Joints Based on Additively Manufactured and Conventional Material Grades

“…Vacuum brazing of nickel maraging steels is a great challenge since the materials surface is characterized by a complex oxide layer (Fe 3 O 4 , TiO 2 , MoO 3 , and MoO 3 • xH 2 O), which is, in contrast to stainless steels, even at high vacuum not sufficiently reducible at suitable brazing process temperatures (except of Fe 3 O 4 ). [1][2][3] The process temperature is limited to 980 °C maximum since a higher temperature leads to significant grain growth, which highly reduces the toughness of the steel. [4][5][6][7] Beyond the usually outstanding low-temperature toughness, nickel maraging steels offer more unique material properties such as a ultrahigh tensile strength (up to 2400 MPa) and a particularly homogeneous hardness distribution.…”