is least compatible with the carbon steel component of the tube 20cr 38ni 1 6mo has a similar coefficient of linear expansion as carbon steel but the coefficient of linear expansion for 30cr 60ni and 22cr 63ni 9mo are even more similar to carbon steel rosen u s pat no 5 324 595 teaches that a composite tube containing 20cr 38ni 1 6mo as an outer component is an improvement for use as floor tubes in a recovery boiler based on a selection process that considered both thermal fatigue resistance and differences in thermal expansion coefficients as the key criteria the results for this alloy were much better than for 18cr 8ni in other examples of prior art alloys even more resistant to thermal fatigue than 20cr 38ni 1 6mo are also used as outer components of composite tubes in recovery boilers particularly those containing about 22cr 63ni 9mo alloys within the composition range that is the subject of this invention also have much better resistance to thermal fatigue than 18cr 8ni and nearly the same as 20cr 38ni 1 6mo advantages of the present invention were discovered when tests specific to the stress corrosion cracking and corrosion mechanisms in a kraft recovery boiler were employed for example corrosion of many alloys including those of groups containing 20cr 38ni 1 6mo and 22cr 63ni 9mo has occurred at the bends of tubes that make air port openings where combustion air is injected into the cavity of the boiler as seen in fig5 an alloy of the present invention is substantially more.