ABSTRACT

A comprehensive atlas of microstructures has been compiled detailing the microstructural changes as a function of exposure temperature and aging time for the HP-modified, HP-microalloyed, and 35Cr/45Ni alloy families. The microstructural characterization included optical microscopy with differential interference contrast and interference layer techniques, scanning electron microscopy, and electron probe microanalysis. The compositions of the precipitates were analyzed and the corresponding carbide and intermetallic phases identified. The concentrations of the various phases were quantified via image analysis allowing the development of diagrams characterizing the kinetics of phase transformation as a function of temperature. This level of aged microstructural information has heretofore not been available for heat resistant cast alloys.

INTRODUCTION

Furnace tubes up to and through the 1940's were typically fabricated from wrought chromium steels and/or austenitic stainless steel alloys. To increase tube life, greater carbon concentrations were required to promote precipitation hardening upon elevated temperature exposure. The use of centrifugal casting, pouring molten metal in a horizontal spinning mold, allowed founders to develop high carbon alloys as the molten metal solidified into near final shape without the need for subsequent metal working operations. Thus, refinement of centrifugal casting processes was the gateway to further alloy development and more aggressive furnace operations.

The first widely used centrifugal cast alloy for steam-methane reformer tube applications was HK40 in the 1950's timeframe. HK40 is essentially the cast equivalent to wrought 310 stainless steel nominally containing 25 wt% Cr, 20 wt% nickel, with iron as the balance. However, HK40 nominally contains 0.40 wt% carbon while wrought 310 stainless steel contains only 0.08 wt% carbon. The increased carbon content and precipitation of primary carbides resulted in HK40 having greatly improved high temperature strength as compared to wrought 310 stainless steel. In the 1960's, the cast HP alloys (nominally 25 wt% chromium, 35 wt% nickel, 0.50 wt% carbon , with iron as the balance) were developed to provide greater creep strength as compared to HK40.

The HK and HP alloys rely on precipitation of M23C6 and/or M7C3 carbides (where M is primarily chromium) for elevated temperature creep strength. The precipitated chromium carbides in the HK and HP alloys tended to coalesce as exposure temperatures approached 1800®F (982®C). Microstructural changes that occur in the HK and HP alloys with extended aging time and temperature have been well documented by Battelle Columbus Laboratories1.

User demand for higher temperature/stronger alloys fueled continued alloy development resulting in the introduction of the HP-modified alloy in the 1970's. The HP-modified alloy had the same nominal chemistry of the HP alloy along with the addition of typically 1 wt% niobium. The niobium addition results in precipitation of M23, C6, M7C3, and MC type primary carbides upon solidification. In the M23C6 and M7C3 carbides, niobium substitutes for some of the chromium with the complex niobium-chromium carbides being more stable at elevated temperatures as compared to chromium carbides.

In the 1980's, the demand for more severe design conditions and/or design lives in excess of 100,000 hours led to the introduction of the HP-micro-alloyed material. HP-micro-alloyed (or commonly designated as HPMA) material was based on the HP-modified chemistry with "micro" addi

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