MECHANICAL PROPERTIES OF NICKEL BASE WIDE GAP BRAZED JOINT
WEI DAKE
WEI DAKE
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Abstract
The effects of gap filler content, gap width and post-braze heat treatments on the microstructure and room temperature tensile properties of nickel base wide gap brazed joints have been investigated using IN625 as the base metal and Nicrobraz LC and Nicrogap 16 as the filler metal and gap filler, respectively. Three different gap widths were studied, i.e., 0.4, 0.8 and 1.2mm. The gaps were first prepacked with extra braze mixes containing different amounts of gap filler powder with braze mixes deposited on top of the gaps. Then, they were brazed in vacuum at 1175°C. Three post-braze heat treatment temperatures were investigated, i.e., 1000, 1050 and 1100°C, with holding times of up to 72 hours.
The results show that with increasing gap filler content in the braze mix, the amounts and sizes of the various brittle phases in the brazes, i.e. discrete chromium boride particles, a nickel-rich eutectic and a chromium-rich eutectic phase, decrease while the tensile strength and ductility of the brazed specimens increase accordingly, the latter being measured in terms of joint elongation-to-fracture and work-to-fracture of the specimens. The vield strength is less sensitive to the amount of gap filler in the braze mixes used.
Post-braze heat treatments produce a similar effect in that the higher the heat treatment temperature and the longer the holding time, the lesser' are the amounts of brittle phases in the resultant brazes and the more superior are the tensile fracture properties.
Gap width produces little effects on the braze microstructure, provided that sufficient amounts of gap filler are used in the braze mix, i.e. >40¾. By and large, for a given gap filler content, the UTS decreases while the Joint elongation-to-fracture increases steadily with increasing gap width. The effects of gap width on the tensile yield strength and the work-to-fracture of the brazed specimens are less pronounced.
The present work shows that both the tensile strength and the ductility of nickel base brazed joints can be appreciably improved through the addition of sufficient amounts of gap filler in the braze mixes and/or through an appropriate post-braze heat treatment. This, with the proper selection of gap width, offers a wide range of choices for one to engineer the properties of nickel base brazed joints to suit a particular application.
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Date
1993
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