Selecting interlayers for corrosion resistant bimetals

Conclusions

• 1. The effect of the interlayer on the diffusion redistribution of carbon in bimetals depends on the ratio of the activity coefficients and the effective diffusion coefficients of carbon in the components of the binetal. The interlayer is an effective barrier if the activity coefficients of carbon in it and in the components of the bimetal differ greatly. The effective diffusion coefficient of carbon in the interlayer should be smaller than in the components of the bimetal.

• 2. For operation at 425–550°C for several thousand hours the most promising interlayers are those in which the carbon activity coefficient is at least 10 times larger than the carbon activity coefficient in the base layer.

• 3. Operation at temperatures below 425°C and for short times (up to 1000 h) requires an interlayer with the minimal carbon activity coefficient (steel Kh25, for example).

• 4. The use of steels with small amounts of carbide-forming elements as interlayers is inadvisable, since carbon migration occurs in the same way as in the bimetal without an interlayer during the course of operation.

• 5. The protective properties of interlayers against redistribution of carbon in bimetals at high temperatures are also retained at lower temperatures (400–600°C).