INTRODUCTION

The object of this paper is to describe the (a) metallurgical characteristics, (b) mechanical properties and (c) corrosion resistance, primarily as they relate to sea water, of two newly developed stainless steels. That first is a ferritic stainless steel referred to as 29 Cr-4 Mo (1) alloy, while the second is austenitic, designated 6X. The significance of these two steels lies in their pitting corrosion resistance which is superior to that of the currently known stainless steels and approaches that of sane Ni-base alloys. While the camion stainless steels are used in marine aerosphere and in special conditions in sea water, their pitting and crevice. corrosion resistance when immersed in sea water is not adequate. The only metals or alloys classified as virtually immune to sea water corrosion are titanium, Income 625 (2) and Hosteller c(3). The 29 er-4 M::> and 6X alloys have lONer alloy content and are therefore expected to be more economical than the three alloys mentioned above. Development of new alloys which meet certain engineering requirements previously satisfied only by materials with a higher alloy content has always been a prime objective of research efforts.

The mechanical properties and especially the toughness of the 29 Cr-4 M::> alloy receive special rephrases in this study in view of the criticality of this property in high cranium ferritic alloys.

COMPOSITION AND STRUCTURE

The nominal composition of the two alloys is shown in Table 1.

29 Cr-4 Mo is a high purity FeCrMo alloy with extremely low residual elements. The low level of residuals is essential in attaining good mechanical and corrosion properties. This requires vacuum melting processes and relatively pure raw materials. Fortunately, with the current state of development of irruption vacuum melting, the production of the 29 Cr-4 Mo is not subjected to any excessive cost condition. The structure of 29 Cr-4 Me according to Stretcher (4 is completely ferrites when marmalade above approximately 1700F. The welds are also ferrite. Exposure in the 1100 to l700F temperature range can lead to sigma and chi phase precipitation. The alloy is also susceptible to "885F" embitterment when exposed in the proper temperature range.

6X is an austenitic steel with normal residual levels which permit melting in conventional electric arc furnaces.

MECHANICAL PROPERTIES

The mechanical properties of the 29 Cr-4 Mo ferritic alloy deserve special attention. It has been known and recognized for many years that the ferritic stainless steels have outstanding corrosion and oxidation resistance. The lower alloyed ferritic steels with the chromium content in the 11 to 17 percent range gained widespread industrial acceptance. Steels with higher Cr content made, however, only limited penetration. One of the main reasons that limited their acceptance and further development was certain aspects of their mechanical proper- ties which interfered not only with the economical mill processing into various product forms but also with the fabrication into hardware involving such operations as forming and welding.

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