Effects of Ferrite and Sensitization on Intergranular and Stress-Corrosion Behavior of Cast Stainless Steels

Abstract

Cast stainless steels of the CF grades (19Cr, 9Ni), which find widespread application in the chemical, petrochemical, and power generating industry, were subjected to intergranular corrosion tests (Huey) and stress-corrosion tests (vapor phase of aqueous 875-ppm sodium chloride (NaCl) solution at 477 K [400°F]) to evaluate the effect of ferrite in the range of 0 to 40 ferrite numbers, in the solution-annealed and in the sensitized condition. The intergranular corrosion tests demonstrated beneficial effects of ferrite for all the alloys tested in the solution-annealed condition and for the molybdenum-free cast steel grades in the sensitized condition. Ferrite increased the sensitization effects on intergranular corrosion for the molybdenum-bearing grades. Highest corrosion rates in Huey tests, based upon weight loss measurements, do not necessarily mean deepest intergranular attack. Intergranular attack was found to terminate at ferrite-austenite grain boundaries. The intergranular corrosion test results would indicate that ferrite numbers exceeding 10 offer greater operating safety for cast, corrosion resistant grades, CF-3, CF-3M, CF-8, and CF-8M in the solution-annealed condition and also, in the event of sensitization, for cast corrosion-resistant grades CF-3 and CF-8. Significantly increased resistance to stress-corrosion cracking with increasing ferrite content was observed for cast steels in the solution-annealed and in the sensitized condition. These stress-corrosion cracking tests would indicate that ferrite numbers of 10 or more offer greater operating safety for cast corrosion-resistant CF grades in the solution-annealed as well as in the sensitized condition.