Cracking of Copper Brazed Steel Joints Due to Precipitation of MnS upon Cooling
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Cracking of Copper Brazed Steel Joints Due to Precipitation of MnS upon Cooling Dheeraj Varanasi, Daniel Koncz-Horvath, Anna Sycheva, Peter Baumli, and George Kaptay Submitted: 17 August 2020 / Revised: 13 October 2020 / Accepted: 13 October 2020 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. Keywords
brazing, copper, cracking, manganese sulfide, steels, wetting
1. Introduction Brazing has been a field of industrial significance specially for heat exchanger and structural joint fabrication (Ref 1-3). Though copper and its various alloys are widely used braze materials for joining steels, there are questions on the reliability of the joint in practical applications due to their occasional failure. Brazing temperature, holding time at this temperature, and all the processes that take place during brazing play a crucial role in determining the microstructural evolution of the joint. Cracking during and after brazing is one of the most important problems of the brazing industry. Copper brazing of steels was of scientific interest for quite some time now. The earlier studies focused on behavior of molten copper on the steel surfaces (Ref 4-6). Copper in its purest form wets very well steels owing to metal/metal interactions, in absence of an oxide layer. Varanasi et al. (Ref 7, 8) investigated the behavior of copper as braze filler for medium carbon and low Cr steels and found excellent Dheeraj Varanasi, Daniel Koncz-Horvath, and Peter Baumli, Institute of Physical Metallurgy, Metal Forming and Nanotechnology, University
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