A Spinodally Hardened Cu-30Ni-2.8 Cr Alloy for Marine Use IN-732
Authors: Badia F.A. , Richardson R.R. , Hanson P.S. 
Published: October 2008

The metallurgical and corrosion characteristics of a spinodally hardened copper-30%nickel-2.8% chromium alloy (IN-732) are described. Tensile, fatigue and impact properties determined at low, ambient and elevated temperatures are included. Annealed yield strengths 12f 50,000 psi are achieved in a wide range of section sizes up - to 700 °F. The alloy is readily weldable with weldment yield strengths of 50,000 psi or higher in material thick enough to require multi-pass welds.

The sea water corrosion behavior of IN-732 can be considered essentially equivalent to conventional70-30 copper-nickel (CA 715) for seawater velocities of 6 to 25 ft/sec. At low velocities, IN-732 experiences a higher average weight loss although the corrosion manifests itself as a broad uniform type of attack. Sea water stress corrosion testing with U-bend specimens indicates no susceptibility to cracking in the annealed, cold rolled or as-welded conditions.

IN-732 is hardened by a structural change known as spinodal decomposition. In simplest terms, during, cooling from the annealing temperature, the high temperature a solid solution rapidly decomposes into two new phases possessing the same crystal structure but differing slightly in chemical composition. The hardening is attributed to coherency strains in the new lattice.

INTRODUCTION

The beneficial effect of chromium additions on the strength of copper nickel alloys has been recently described (1). The unique hardening features observed in certain compositions have been ascribed to spinodal decomposition occurring in these CU-Ni-Cr alloys during continuous cooling from the annealing temperature (1,2). The utilization of spinodal decomposition for hardening has been predicted by both Cahn (3) and Huston et al.(4), the latter dealing with spinodal decomposition during continuous cooling.

Alloy development work in the CU-Ni Cr system was prompted by the continuing need for a stronger corrosion resistant copper-nickel alloy, one in which high strengths could be achieved without resorting to heat treatment(5). The work of Badia et al.(1) suggested that a promising composition for detailed study was a copper-30% nickel alloy containing a nominal 2.8%Cr addition. This work has led to the development of a new high strength copper-nickel alloy, designated by The International Nickel Company as IN-732. By virtue of its high mechanical properties, good weldability and sea water corrosion resistance, IN-732 should find application as a new engineering material in marine environments.

COMPOSITION

The nominal composition of IN-732 is given below:(available in full paper)

Manganese, titanium and zirconium are added for deoxidation and scavenging purposes. Ii nominal zirconium residual insures an adequate hot working temperature range and good weldability. Carbon and nitrogen should be controlled to low levels since they can preferentially tie up chromium leading to lower strengths.

TENSILE PROPERTIES

Annealed Tensile Properties

The unique hardening that can be obtained through spinodal decomposition is illustrated by the annealed strengths obtained in IN-732 of varying section size. A remarkable consistency and high level of tensile properties are obtained in annealed material ranging from 1/16 inch to 2 inch plate (Table I). As seen in Table II, yield strengths vary little with final annealed grain size as final annealing temperature is varied from 1600-1800 °F.