High-alloyed corrosion resistant steels for the chemical process industry, power engineering and marine technology-past and future

摘要

Today's most common high-alloyed corrosion resistant steels are in their majority characterised by very low contents of carbon and sulphur and, in many cases, by substantial amounts of nitrogen as an alloying constituent. Their broad use ai the chemical process industry, power generation and marine technology has become possible when new metallurgical processes for steel making had been introduced in the 1960s. The time before had seen mainly stabilised grades, being highly alloyed with copper in many cases, which have disappeared to a large extent in our days. The superferritic grades (ferritic steels with ≥ 25% chromium) had been the materials of great expectations in the 1970s, but have found a very limited application only in the chemical industry since then, e.g. for the handling of hot concentrated sulphuric acid, due to the high risks of low ductility cracking of these materials at greater wall thickness. These risks can be managed better if the highly alloyed ferritic phase is present in a finely dispersed compound with an austenitic phase where the ferritic part is adding its advantages, higher strength and resistance to stress corrosion cracking, to the duplex compound. This can result in low weight and corresponding cost saving. The application of the corrosion resistant duplex grades will expand further as much as users will better learn the special requirements of manufacturing of these materials and to take advantage of their unique properties. However, the most important alloy developments since the 1960s have been seen in the field of the austenitic stainless steels being highly alloyed with chromium, molybdenum and nitrogen, Especially the austenitic 6% Mo grades as e.g. X1NiCrMoCuN25-20-7 - alloy926 (1.4529) have found many applications in chemical process industry, power generation and marine technology. Higher alloyed grades as e.g. X1NiCrMoCu32-28-7 - alloy 31(1.4562) are excelling in extraordinary resistance to corrosion by acids and pitting attack. In addition today's upper limits of alloying austenitic corrosion resistant grades have been explored with grade X1CrNiMoCu33-32-1 alloy 33 (1.4591) for chromium additions up to about 33% and with grade X1NiCrSi24-9-7 - alloy 700 Si (1.4390) for additions of silicon up to about 7%, providing a high corrosion resistance mainly in oxidising acids. When considering the prospects of further development of the corrosion resistant duplex grades the ferritic phase within these materials is both offering chances and setting limits. The high-alloyed austenitic corrosion resistant steels have a potential being unexplored so far in the alloy range where molybdenum and nitrogen are becoming more prominent compared to the chromium content.