ABSTRACT

A laboratory study was performed to investigate the aged mechanical properties of standard chemistry static cast 20Cr32Ni1Nb alloy versus a modified chemistry version of the alloy. The modified chemistry consisted of minimizing the niobium-to-carbon ratio and the silicon content and increasing the manganese content. Mechanical testing at room temperature and elevated temperature, high temperature creep rupture testing, and microstructural analyses were performed on material in the ascast condition as well as material aged from 10 to 10,000 hours at a constant temperature of 1472oF (800oC). Room temperature mechanical testing indicated that the aged tensile strength and aged ductility of the modified chemistry alloy was consistently superior to the standard chemistry alloy. Microstructural analysis indicated the tensile strength and ductility degradation of the standard chemistry material was directly correlated to the precipitation of a nickel-niobium-silicon rich phase. The chemistry modification mitigated precipitation of the detrimental nickel-niobium-silicon rich phase.

INTRODUCTION

Hydrogen reformer outlet manifold components are routinely fabricated from statically or centrifugally cast 20Cr32Ni1Nb material. These components require good aged ductility in order to resist failure by cracking. In previous studies of aged components of 20Cr32Ni1Nb, microstructural analysis found precipitation of a nickel-niobium-silicon rich phase in addition to the expected chromium and niobium carbides1. Mechanical testing found notable decreases in the room temperature ultimate tensile strength and elongation of the aged components. The aged mechanical property decreases were attributed to the precipitation of the nickel-niobium-silicon (Ni-Nb-Si) rich phase. The microstructural analysis results indicated that the aged mechanical properties of the 20Cr32Ni1Nb material could be improved via chemistry modifications intended to mitigate precipitation of the Ni-Nb-Si rich phase. Using regression analysis of aged outlet manifold components, Shibasaki et al developed an algebraic expression known as the P factor to predict aged ductility as a function of alloy composition2. The P factor is defined as

P = 7 x wt% C + 5 x wt% Si ? 3 x wt% Mn + 8 x wt% Nb.

In that study, unacceptable long-term aged ductilities, below 10% tensile elongation, were indicated for P factor values exceeding 9. Shibasaki et al also pointed out the link between the formation of Ni-Nb-Si rich phase and the inferior ductility.

Others have indicated that the aged mechanical properties of niobium containing castings are controlled by the niobium-to-carbon (Nb/C) ratio3. From stoichiometry, a Nb/C ratio of 7.7 ensures that there is an atom of niobium for every carbon atom while minimizing the amount of excess niobium in solid solution, assuming no segregation has occurred.

Typically, the Nb/C ratio is specified to be a minimum of 7.7, and often much higher, to ensure that all carbon is tied up as niobium carbides. The underlying principle for maintaining the high Nb/C ratio is that, since all the carbon will be tied up as NbC, deleterious precipitation of M23C6 secondary chromium carbides during subsequent aging will be minimized and that the high thermodynamic stability of the NbC will ensure high temperature strength is maintained. The latter understanding does not consider the possible formation of deleterious Nb-rich silicides, during prolonged aging, which may have a considerable effect on the aged ductility or tensile strength

In the present study, static cast bars were obtained from a set of heats of 20Cr32Ni1Nb alloy having a standard chemistry and another set of heats with a modified c

This content is only available via PDF.
You can access this article if you purchase or spend a download.