Powder Interlayer Bonding in Dissimilar Steels

Abstract

In the current project, mild steel was bonded to stainless steel using two different techniques: diffusion bonding (DB) and powder interlayer bonding (PIB). The two approaches were compared and contrasted through various methods, including Energy Dispersive Spectroscopy (EDS), hardness testing, and microstructural evaluations, specifically focusing on grain size. The PIB process was conducted in both a single step (6.5 MPa/975°C for 60 minutes) and multiple steps (10 MPa/975°C for 5 minutes and 6.5 MPa/975°C for 55 minutes). On the other hand, DB samples were bonded in a single step under 6.5 MPa and 975°C for 60 minutes. The microstructure of both the stainless steel and mild steel was revealed using Kalling’s etchant since oxalic acid and Nital proved ineffective. Grain sizes were determined using ImageJ software. Before bonding, the grain sizes for the base materials were measured at 20.97 microns for mild steel and 20.86 microns for stainless steel, respectively. After bonding, the grain sizes were as follows: for single-step PIB (bond 1), mild steel had a grain size of 22.21 microns, and stainless steel had a grain size of 49.45 microns. For double-step PIB (bond 3), mild steel measured 33.49 microns, while stainless steel had a grain size of 57.50 microns. In the case of DB (bond 2), mild steel exhibited a grain size of 20.66 microns, and stainless steel had a grain size of 56.17 microns. In bonds 1 and 3, the EDS results indicated that the interlayer region contained higher amounts of Ni compared to stainless steel, which also contained substantial amounts of Ni and Cr. Specifically, in bond 1, the EDS results showed a higher concentration of Mo in the interlayer region compared to bond 3, suggesting possible diffusion of these elements from the base materials. In contrast, for bond 2, the bondline contained low levels of Ni, as the DB sample lacked an interlayer. The hardness results for the base materials before bonding were consistent with those reported in the literature: mild steel measured 114.72 Hv, and stainless steel measured 225.58 Hv. However, for the bonded materials, both mild steel and stainless steel exhibited reduced hardness, which can likely be attributed to the high temperatures experienced during the bonding process. The hardness of the bondline fell somewhere between the values for stainless steel and mild steel. SEM results revealed the presence of some level of porosities in the bondline for bonds 1 and 3. Overall, these findings provide valuable insights into the effects of different bonding techniques on the microstructure and properties of the bonded materials.