On the heterogeneous microstructure development in the welded joint of 12MnNiVR pressure vessel steel subjected to high heat input electrogas welding

Microstructure features of 12 MnNiVR pressure vessel steel welded joint deposited by the high heat input electrogas welding have been systematically investigated. It is revealed that the welded joint is featured by a heterogeneous juxtaposition. The coarse grained heat-affected zone(CGHAZ) primarily consists of lath bainites and minor granular bainites. The fine grained heat-affected zone(FGHAZ) is dominated by polygonal ferrites, pearlites, and fine cementite particles. Moreover, electron backscatter diffraction results further demonstrate that the CGHAZ is populated by coarse prior austenite grains(PAGs) with high frequency(61.3%) of low angle grain boundaries(LAGBs). On the other hand, the FGHAZ is filled with fine PAGs with a lower frequency(19.6%) of LAGBs. Such microstructural differences may likely contribute to differed mechanical properties for samples tested at designed positions.

Brazing temperature-dependent interfacial reaction layer features between CBN and Cu-Sn-Ti active filler metal

CBN/Cu-Sn-Ti(CBN: cubic boron nitride) composites are prepared by active brazing sintering at 1123 K,1173 K, 1223 K and 1273 K, respectively. The effects of brazing temperature on the wettability, interfacial characteristics, and elemental distribution variations are fully investigated. When the brazing temperature is below 1223 K, completely uncoated and/or partially coated CBN particles with sharp edges can still be observed, and the reaction layer, mainly composed of Ti N and Ti B2, appears to be thin and uneven.When the brazing temperature is 1223 K, all CBN particles are completely coated, suggesting that adequate wetting has taken place. Besides, as Ti diffuses thoroughly and enriches the interface, the reaction layer, filled primarily by Ti N, Ti B2 and Ti B, becomes thicker(about 1.30 μm), more uniform, stable and continuous. Further increasing the temperature to 1273 K is unnecessary or even harmful as the reaction layer thickness undergoes negligible change yet some tiny micro-cracks appear on the interface, which may likely deteriorate the grinding capability of the final brazing products.