Diffusion redistribution of elements in transition layers of bimetals

Conclusions

• 1. The main element diffusing into the transition zone of bimetals consisting of carbon steel clad with stainless steel is carbon. Carbon diffuses into the cladding layer of chromium or chromium — nickel steel, creating an inverse concentration gradient. The diffusion of carbon occurs in the bimetal even at 425°C. The character of the carbon distribution depends on the method of producing the bimetal.

• 2. The diffusion of carbon into stainless steels Kh18N10T and 0Kh13 leads to the formation of carbides M₂₃C₆ and M₇C₃ in the form of films 500–2000 Å thick in the transition zones.

• 3. To limit the diffusion of carbon and iron into the cladding layer it is expedient to use a nickel. interlayer, which is easily accomplished in pack rolling.

• 4. In a chromium — nickel clad bimetal the nickel interlayer goes into solution during heat treatment and in its place is formed an Fe−Cr−Ni−C alloy whose composition depends on the heat treatment; the bonding strength increases in this case.

• 5. In the St.3 + Monel, bimetal the properties of the transition zone are affected mainly by the diffusion of iron into Monel, which leads to the formation of brittle intermediate layers, particularly after heat treatment. The embrittlement of this bimetal can be prevented by use of a nickel interlayer.

• 6. By plotting the results of electron probe analysis of the composition of transition layers on phase and equilibrium diagrams it is possible to predict the characteristics of these layers and the reason for changes in them.