Abstract
The process of brazing of different steel grades by pure liquid copper foil was studied to reveal the critical conditions when cracks do or do not appear in the braze upon cooling without any external load. Steel grades C45 (S 0.030 wt.%, no Mn and no Cr), S103 (Mn 0.25 wt.% and S 0.020 wt.% with no Cr), CK60 (0.75 wt.% Mn, 0.07 wt.% S and 0.15 wt.% Cr) and EN 1.4034 (Cr 12 wt.%, Mn 1.0 wt.% and S 0.035 wt.%) are studied under identical conditions using copper foils of 70-microns-thick. The samples were held above the melting point of copper at 1100 °C under high vacuum for different time durations (between 180 and 3600 s) and then cooled spontaneously. The joints were found cracked during cooling after a certain critical holding time. This critical holding time for cracking was found to decrease with increasing the Mn content and the S content of steel. It is observed that cracking is due to the precipitation of a critical amount of MnS phase upon cooling. The MnS/Cu interface is the weakest interface in the joint (with adhesion ensured only by van-der-Waals bonds), which is broken/separated upon cooling due to difference in heat expansion coefficients of the sulfide and copper phases. Higher is the Mn and S content, shorter is the probable time required for crack to appear in the joint. The braze integrity diagram is constructed as function of solubility product of MnS in steel and holding time showing a stable crack-free technological region and an unstable technological region with high probability of crack formation.
Highlights
Brazing has been a field of industrial significance specially for heat exchanger and structural joint fabrication (Ref 1-3)
Our study focuses on the microstructure of the steel/Cu joint as function of liquid time of the braze and the effect of steel composition
The joint thickness obtained for all the steel/Cu/steel sandwiches can be written by the following approximated equation: 70 dj ffi 1 þ z Á tL
Summary
Brazing has been a field of industrial significance specially for heat exchanger and structural joint fabrication (Ref 1-3). Majlinger and Szabo (Ref 10-12) investigated the effect of copper and copperbased alloy fillers on austenitic steels. They have observed the grain boundary dissolution of the substrate by the copper braze followed by intergranular cracking. Kozlova et al (Ref 13, 14) used Cu-Ag eutectic alloy as braze fillers for stainless steel brazing They reported that a combination of deoxidation of steel surface and dissolutive wetting helps achieve contact angles around 10°30°. Ghovanlou et al (Ref 31) studied the braze joint reliability of pure copper filler with low carbon steels. During holding time at the brazing temperature, partial dissolution and diffusion of the steel components into liquid copper are inevitable. The measured liquid time was found to depend linearly on measured holding time (see Fig. 3), and can be approximately written as: tL ffi th þ 68 ðin secondsÞ ðEq 1Þ
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