High-temperature metallurgy of advanced borated stainless steels
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I.
INTRODUCTION
BORATEDstainless steels are austenitic stainless steels containing significant additions (0.20 to 2.25 wt pct) of natural or enriched boron. These steels are used in the nuclear industry for neutron criticality control, spent fuel storage racks, and transportation components. Natural or enriched boron additions result in thermal neutron absorption because of the presence of the ~~ isotope, which is present as a dispersion of Cr2B-type precipitates. Although these steels can have favorable mechanical properties (e.g., yield and tensile strength minima of 205 and 515 MPa and elongation minima ranging between 6 and 40 pct), they have primarily been used as simple "strap-on" neutron shielding materials rather than structural alloys. The borated stainless steels are covered by ASTM Specification A887, which includes eight boron levels (types) and two grades per type. For each of the eight types, Specification A887 describes two grades, A and B, based on mechanical property requirements which differ primarily in ductility and impact energy minima. Typical microstructures of type 304B4 grades A and B are shown in Figure 1 and, except for the volume fraction of the boride phase, are representative of the structures observed in all grades. As shown, the microstmctures essentially consist of a dispersion of boride precipitates in an austenitic stainless steel matrix. In general, alloys processed to meet grade A property requirements must C.V. ROBINO, Senior Member of the Technical Staff, and M.J. CIESLAK, Manager, are with the Physical and Joining Metallurgy Department, Sandia National Laboratories, Albuquerque, NM 87185. Manuscript submitted June 20, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
have significantly smaller and more spherical borides. Although ASTM A887 does not stipulate processing route, grade B materials are generally processed by conventional ingot metallurgy-hot work, while grade A properties have only been achieved by proprietary powder metallurgy ingot-hot work processing, m Applications for borated stainless steels have historically been limited to shielding applications where structural requirements are minimal or nonexistent. In these cases, the alloys are typically used as bolt-on additions to a structural (usually stainless steel) framework. However, the development of grade A materials has initiated interest in uses for these alloys where full advantage of their mechanical properties can be taken. In such applications, it is envisioned that grade A steels can provide economic and technological advantages, since the alloys would address both the structural and neutron shielding requirements within the same component. Historically, there have been two major impediments to realization of this concept: (1) borated stainless steels have only recently been accepted by the ASME Boiler and Pressure Vessel Code for structural use in code construction, and (2) many conceptual designs require welded fabrication. Fusion welding subjects alloys to thermal excursions involving temperature
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