ABSTRACT

Modern melting and refining techniques have given stainless and nickel alloy users the capability to control alloy content to precise narrow ranges. This ability in conjunction with market needs for heat resistant stainless steels that could offer properties competitive with more costly nickel alloys led to the development of alloys such as $30815 in the 1970s, and $35315 in the 1980s. More recently these techniques were also applied to develop N06025, as a means to enhance a nickel alloy so that it could offer improved properties versus traditional alloys such as alloy 600 and alloy 601. Microalloy additions of rare earth metals such as cerium and yttrium enhance scaling resistance and creep strength. Elevated levels of nitrogen and/or carbon enhance mechanical properties at elevated temperatures.

Results of laboratory oxidation testing and creep rupture property comparisons are provided along with several case histories.

INTRODUCTION

Development work was started on alloy 253MA °) (UNS $30815) at the former Avesta Stainless (Now Outokumpu Stainless) in 1971. The aim at that time was to create an alloy with at least the properties of the established heat resistant stainless steel 310S (UNS $31008), but with a lesser alloying content.

(1) 253MA and 353MA are registered trademarks of Outokumpu Stainless The chemistry of $30815 is compared to 310 stainless in Table 1. $30815, like 310 stainless, has a fully austenitic structure. Dramatic improvements in high temperature properties result from microalloying with cerium (oxidation, creep strength), silicon (oxidation), slightly higher carbon (creep strength), and significantly higher nitrogen (creep strength)

Alloy 353 MA development by Avesta took an alloy with similar nickel content (30-35%) as an RA330® alloy (2) (UNS N08330) or an alloy 800H (UNS N08010) which through microalloying could offer properties rivaling nickel alloys such as alloys 600 (UNS N06600) and 601 (UNS N06601). Table 1 shows a comparison of chemistries between $35315 and the other grades listed. Like alloy $30815, dramatic improvements in high temperature properties result from microalloying with cerium (oxidation, creep strength), silicon (oxidation), slightly higher carbon (creep strength), and significantly higher nitrogen (creep strength).

Alloy 602CA (3) is a nickel based alloy that has been developed by ThyssenKrupp VDM of Germany. The goal in the development of this alloy was to offer a material that extends the operating range of established nickel grades such as alloy 600 or 601 without the use of elements such as tungsten, molybdenum, or cobalt. These dements are commonly added to nickel superalloys, such as alloys X and 617 to enhance creep rupture properties, however, they also greatly increase the cost of the alloy and negatively impact oxidation resistance at more extreme temperatures. The yttrium addition in N06025 serves a similar purpose as the cerium addition in the "MA'" alloys. Increased levels of aluminum also enhance oxidation resistance. An elevated carbon content coupled with additions of zirconium and titanium further improves creep rupture properties.

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