Abstract
Within the alloying range of 20 to 28 % Cr, up to 5 % Mo, 4 % Ni and 1 % Cu about thirty experimental heats of low interstitial ferritic steel grades have been investigated in view of their mechanical properties as well as corrosion resistance. To prevent intergranular corrosion the sum of carbon and nitrogen contents must be restricted to less than 100 ppm or, with addition of a stabilizing element such as titanium, columbium and zirconium, to less than 500 ppm. Increasing contents of chromium and especially molybdenum enhance remarkably the corrosion resistance in hot chloride containing media. Nickel additions are beneficial on the resistance against crevice corrosion; copper extends the resistance in reducing acids e.g. sulfuric acid.
On the other hand the contents of alloying elements are limited by occurence of deleterious structure decomposition phenomena. In the temperature range of 950 - 550 °C chromium rich sigma and/or molybdenum rich chiphase can be precipitated; below 550 °C the socalled 475°-embrittlement can occur, both resulting in a breakdown of mechanical properties and corrosion resistance. Stabilization by titanium enhances these structure reactions, whereas columbium has no influence. Considering all these points a columbium stabilized steel with 28 % Cr, 2,5 % Mo and 3,5 % Ni shows the optimum combination of good mechanical properties with excellent corrosion behaviour also in as-welded condition
