Rare earth metals in stainless steelmaking

Abstract

A considerable amount of research has been done between 1950 and 1960 in the US and Canada using various types of Rare Earths to improve certain properties of Cr-Ni austenitic stainless steels. After encouraging research work, almost all stainless steel producers started field application of rare earths primarily to improve yield by eliminating poor 'Hot Workability' of certain chemically 'unbalanced' alloys. Only a very few papers were published in those years basically because of contradicting results. Several patents have been granted in the U.S. and the Special Alloy Manufacturers anticipated a very bright future for the future use of these elements. Rare Earths are plentiful and chemists were working with them during the past 150 years. Their separation was expensive and scarce. With the wide use of Thorium in nuclear technology, large quantities of Rare Earths became available as a by-product after World War II. Despite the relative abundancy, the use of Rare Earths is more and more limited in the Steel Industry and was practically withdrawn by 1960 from the manufacture of stainless steels in North America. Surprisingly, authors from other countries claimed recently that the addition of Rare Earths to stainless steels can improve 'Hot Workability' and permit the reduction of Ni content from 10.6% to 9.6% resulting in significant cost saving. It appears proper to review the history of making stainless steels with the addition of Rare Earths, particularly in light of recent work on properties of Rare Earths. It is unfortunate that most of the scientific work on phase diagrams, thermodynamic properties of Rare Earth alloys were not known at the time of Stainless Steel Industry experimented with their use, as most of the work of this nature was published after 1960. The information available today is still far from being complete but it is certainly confirmative for the discontinued use of Rare Earths for stainless steels. The present paper will discuss : (i)The Chemistry of Rare Earths (ii) Historical Review of Experimental work in Connection with Stainless Steels (iii) Physical Properties of Rare Earth metals (iv) Alloying Behaviour of Rare Earth metals (v) Rare Earths-Stainless Steel experiments done by ATLAS (vi) Interpretation of Results in light of thermodynamic data available. It can be concluded that further specific work is needed to investigate some properties, the improvement of which could justify the expensive use of rare Earths additions. (Mr. Leslie Nemethy, Manager, Atlas India Project, Atlas Steels Company, Ontario, Canada)