ELEVATED TEMPERATURE TENSILE PROPERTIES of BORATED 304 STAINLESS STEEL: EFFECT of BORIDE DISPERSION on STRENGTH and DUCTILITY*

Conventional cast and wrought (\"Ingot Metallurgy\") borated 304 stainless steel has been used for a number of years in spent fuel storage applications where a combination of structural integrity and neutron criticality control are required Similar requirements apply for materials used in transport cask baskets, and borated stainless steel is, in fact, an attractive material for such applications. However, in the high boron contents (>1.0 wt.%) which are most useful for criticality control, the conventional cast and wrought material suffers from low ductility as well as low impact toughness. The microstructural reason for these poor properties is the relatively coarse size of the boride particles in these alloys, which act as sites for crack initiation. Recently, a \"premium\" grade of borated 304 stainless steel has been introduced (Strobel and Smith, 1988) which is made by a Powder Metallurgy (PM) process. This material has greatly improved ductility and impact properties relative to the conventional cast and wrought product In addition, an ASTM specification has been developed for borated stainless steel. This specification (ASTM A887) contains 8 different material Types with respect to boron contentwith the highest level (Type B7) having permissible range from 1.75 to 2.25 wt% boron- and each Type contains two different Grades of material based on tensile and impact properties. While the ASTM specification is properties-based and does not require a specific production process for a particular grade of material, the PM material qualifies as \"Grade A\" material while the conventional Ingot Metallurgy (IM) material generally qualifies as \"Grade B\" material. This paper presents a comparison of the tensile properties of PM \"Grade A\" material with that of the conventional 1M \"Grade B\" material for two selected Types (i.e., boron contents) as defined by the ASTM A887 specification: Types 304B5 and 304B7. Tensile properties have been generated for these materials at temperatures ranging from room temperature to 400°C (7 52°F). The data at higher temperatures are required for ASME Code Case purposes, since the use temperature of a basket under \"worst case\" cask conditions may be as high as 343°C (650°F), due to self-heating by the activated fuel elements. We will also discuss the current status of efforts aimed at obtaining an ASME Boiler and Pressure Vessel Code Case for selected grades of borated stainless steel covered by the ASTM A887 specification.