Anisotropie embrittlement in high-hardness ESR 4340 steel forgings
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I.
INTRODUCTION
E L E C T R O S L A G remelt (ESR) processed 4340 steel tempered to high hardness levels exhibits superior ballistic properties, L2 attributed primarily to the improved shatter resistance accompanying a low sulfide inclusion content. This improved ballistic performance led to the selection of ESR 4340 steel tempered to HRC 55 hardness for ballistically-tolerant critical helicopter components. Preliminary mechanical property evaluation at AMMRC of commercial ESR 4340 steel forgings revealed a severe loss of ductility in the short transverse direction; 3 this was followed by a number of delayed failures of helicopter components in service. This study was undertaken to determine the nature and mechanism of the severe degradation of short transverse mechanical properties in the high hardness ESR 4340 steel forgings.
Tension tests were conducted on standard 0.630 cmdiameter specimens with a 3.18 cm reduced section and 2.54 cm gage length at a crosshead speed of 0.13 cm per minute. Fracture toughness was evaluated both by standard Charpy impact tests and stow-bend tests on precracked Charpy specimens; sharp-crack fracture toughness (K~c) values were estimated from maximum load measurements using a Manlabs, Inc. Physmet Slow-Bend tester. Fracture surfaces were analyzed on notched specimens fractured in vacuum in a PHI Electronics Scanning Auger Electron Microprobe operated at 3 kV. Some controlled humidity smooth bar tension tests were run on standard 0.406 cmdiameter specimens in a specially designed environmental chamber. Short transverse K~scc tests were conducted in 3.5 pct NaC1 solution using constant load SEN precracked specimens with electron-beam welded grip ends.
III. II. MATERIALS AND EXPERIMENTAL PROCEDURES Material from sixteen ESR 4340 forgings from nine commercial heats produced by three suppliers was evaluated. Also included were an ESR 300M (Si-modified 4340) heat and one vacuum-induction melted (VIM) 4340 heat for comparison. The forgings are identified in Table I, and heat analyses are given in Table II. Original ESR ingots were generally 50 cm in diameter. Particular deviations in heat compositions are noted in Table I. Unless otherwise indicated, specimens were normalized at 900 ~ for one hour and air cooled, austenitized at 840 ~ for one hour and oil quenched, and tempered at 170 ~ for one hour as per helicopter component specifications. G.B. OLSON, Principal Research Associate, is with Massachusetts Institute of Technology, Cambridge, MA 02139 and Consultant to the Army Materials and Mechanics Research Center, Watertown, MA 02172. A. A. ANCTIL, Mechanical Engineer, T. S. DeSISTO, Materials Engineer (Retired), and E.B. KULA, Metallurgist and Chief, Metals Research Division, are all with AMMRC, Watertown, MA 02172. Manuscript submitted September 22, 1982. METALLURGICALTRANSACTIONS A
MECHANICAL PROPERTIES
Table III summarizes the results of extensive mechanical testing. Properties were measured in the longitudinal (L), long transverse (LT), and short transverse (ST) directions of t
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