Diffusion Bonding of Sialon to AISI 420 Martensitic Stainless Steel with Cut Off Heat upon Cooling

In the past years, not many works on joining sialon to martensitic stainless steel have been carried out. The effect of the cut off heat upon cooling on the joint has yet to be discussed. In this paper, this effect is studied in terms of the microstructure and hardness of the reaction layer. Diffusion bonding was utilized to join sialon and AISI 420 martensitic stainless steel. The joining processes were conducted at 1150°C and 1200°C for one hour under a uniaxial pressure of 17 MPa in a vacuum (i.e. 1.0x10-5Torr). The heat was cut off upon cooling and the samples were left in the furnace for about 20 hours to cool down. Thicker reaction layer was formed in 1200°C sample because interdiffusion and reaction of elements occurred more rapidly at this temperature. The cut off heat had caused the sialons to crack and it was very severe in 1150°C sample due to the formation of a very thin interface layer. This layer did not have the sufficient strength to bind sialon and steel together because one of the sialons was completely detached from the joint. Diffusion layer and parent steel were segregated due to the difference in properties between the regions and it was more noticeable in 1200°C sample. Iron silicides and aluminium oxide might be formed in the interface layer. Microstructure of the parent steel transformed from globular coarse carbide in ferrite matrix to large grain size with carbide’s precipitation along the grain boundaries. More precipitates were formed in 1200°C sample and they were concentrated near the segregation line of the diffusion layer and the parent steel. The weakest part of the joint was at the border of sialon and interface layer since their hardness were extremely different.