Effect of overaging on mechanical properties and stress corrosion cracking of HY-180M steel
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ALLOYHY-180 M
is a tough, high strength steel relying upon secondary precipitation hardening of a lath martensite for development of mechanical properties. It has a nominal composition of 10Ni-2Cr1Mo- 14Co-0.16C, and is a modification of H Y - 180 alloy. It was developed by Little and Machmeier ~to meet the structural requirements of airframes. They found that o p t i m u m strength and toughness could be achieved by a double austenitizing treatment, with interim water quenching, followed by a final aging treatment at 510 ~ for 5 h. This o p t i m u m aging treatment gave a yield strength of 1550 M P a and a fracture toughness K m value of 160 M P a . mlJ2. * The treat-
* Recently, Machmeier et al published additional information on HY-180 M, which they referred to as AF 1410 steel.2~ merit for o p t i m u m strength and toughness was subsequently confirmed by Snide? The properties of HY-180 M initially made it a possible contender for hydrofoil applications in sea water environments, where high strength/weight ratio was a factor, but the stress corrosion cracking (SCC) behavior was not known. Recently, Bala and T r o m a n s 3,4 studied SCC of the o p t i m u m aged alloy in aqueous 315 pct NaC1 solutions. They showed 3 that at 22 ~ SCC could be prevented by an applied electrochemical potential of E = - 0 . 5 2 VSHE,which was close to the thermodynamically reversible potential of iron. However, at a more noble applied potential of E = - 0.28 VSHE,3'4 representative of freely corroding conditions in aerated solutions, the SCC was poor, K1scc being - - 5 M P a ~m 1/2 for forged bar materiaP and ~ 12 M P a - m lj2 for plate stock? At a cathodic protection potential of E SATHISH RAO BALA, formerly Research Associate with the Department of Metallurgical Engineering, University of British Columbia, is now with Dominion Bridge Co. Ltd., Lachine, P.Q. DESMOND TROMANS is Associate Professor, Department of Metallurgical Engineering, University of British Columbia, Vancouver, B.C. V6T 1W5, Canada. Manuscript submitted August 9, 1979. METALLURGICAL TRANSACTIONS A
= - - 0 . 8 V S H E , 3'4 employed in practice to simultaneously protect submerged aluminum and steel c o m p o n e n t s in metallic contact, the SCC was slightly improved, K~scc being ~40 M P a . m l/2 for plate stock 4 and 50 to 60 M P a 9 m l/2 for forged b a r ) ,4 The purpose o f the present study was to examine the effect of overaging (i.e. aging temperatures ~ 510 ~ on mechanical properties and SCC behavior in 3.5 pct NaC1 at 22 ~ in order to determine whether or not significant improvements in SCC resistance could be obtained without overwhelming losses in strength and fracture toughness. One guide in this direction was the reported L2 observation that higher aging temperatures caused reversion to austenite, some of which was retained upon cooling to r o o m temperature. Ritchie et a l 5 have shown that retained austenite in high strength steels has a beneficial effect on SCC resistance in both water and 3.5 pct NaC1 solutions, principally by lowering the c
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