Effect of Pressure and DCT on Microstructure and Strength of Diffusion Bonds of PLCS-HSLAS

In the present study, eight pairs of samples were used in four groups for diffusion bonds of plain low carbon steel (PLCS) to high strength low alloy steel (HSLAS) so that, diffusion bonding was performed on two groups without pressure and two groups with the pressure of 2 MPa. Then, the groups alternatively underwent deep cryogenic treatment at -196°C (DCT) for 10 h; finally, all the groups were tempered at 150°C for 1 h. The results show under the non-pressure conditions, DCT had a positive effect on the bond strength because population density of carbide increased from 1400000 to 2300000 mm^-2. While under the pressure conditions, the DCT led to 59% decrease in the bond strength because discontinuity on the bond metal increased from 7 to 15 percent. In addition, in the absence of DCT, applying pressure during diffusion bonding caused the strength of the bond to increase 25.8%, the reason of which could be attributed to the increase in the bond length from 160 to 220 μm, or the larger bond metal. It is found that discontinuity in bond metal, population density of carbides and bond's length are three main factors to affect the bond strength. The role of discontinuity in bond metal is negative while the others are positive. Finally, to increase bond strength just one of these two operations (pressure during diffusion bonding or DCT after diffusion bonding) must be used.