Abstract
Application of a systems approach to computational materials design led to the theoretical design of a transformation toughened ultratough high-strength plate steel for blast-resistant naval hull applications. A first prototype alloy has achieved property goals motivated by projected naval hull applications requiring extreme fracture toughness (C v > 85 ft-lbs or 115 J corresponding to K Id> 200 ksi.in1/2 or 220 MPa.m 1/2) at strength levels of 150-180 ksi (1,030-1,240 MPa) yield strength in weldable, formable plate steels. A continuous casting process was simulated by slab casting the prototype alloy as a 1.75″ (4.45 cm) plate. Consistent with predictions, compositional banding in the plate was limited to an amplitude of 6-7.5 wt% Ni and 3.5-5 wt% Cu. Examination of the oxide scale showed no evidence of hot shortness in the alloy during hot working. Isothermal transformation kinetics measurements demonstrated achievement of 50% bainite in 4 min at 360 °C. Hardness and tensile tests confirmed predicted precipitation strengthening behavior in quench and tempered material. Multi-step tempering conditions were employed to achieve the optimal austenite stability resulting in significant increase of impact toughness to 130 ft-lb (176 J) at a strength level of 160 ksi (1,100 MPa). Comparison with the baseline toughness-strength combination determined by isochronal tempering studies indicates a transformation toughening increment of 65% in Charpy energy. Predicted Cu particle number densities and the heterogeneous nucleation of optimal stability high Ni 5 nm austenite on nanometer-scale copper precipitates in the multi-step tempered samples was confirmed using three-dimensional atom probe microanalysis. Charpy impact tests and fractography demonstrate ductile fracture with C v > 80 ft-lbs (108 J) down to -40 °C, with a substantial toughness peak at 25 °C consistent with designed transformation toughening behavior. The properties demonstrated in this first prototype represent a substantial advance over existing naval hull steels. Achieving these improvements in a single design and prototyping iteration is a significant advance in computational materials design capability.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 201-233 |
| Number of pages | 33 |
| Journal | Journal of Computer-Aided Materials Design |
| Volume | 14 |
| Issue number | 2 |
| DOIs | |
| State | Published - Jul 2007 |
Funding
This research was carried out under the financial support by the Office of Naval Research (ONR) under the ONR Grand Challenge in Naval Materials by Design grant number N00014-01-1-0953, conducted as a part of the multi-institutional Steel Research Group program. The authors would like to acknowledge Dr. Dieter Isheim for his time and his help during the operation of the atom probe and analysis of the data and to Imago for early use of their LEAP instrument. The authors are thankful to Jamie Heisserer for her assistance during the collection of the dilatometry data. The prototype was cast by Special Metals Corporation in New Hartford, New York and rolled by Huntington Alloys in Huntington, West Virginia.
Keywords
- Austenite dispersion
- Copper precipitation
- Dilatometry
- Fracture toughness
- Impact toughness
- Materials design
- Multi-step tempering
- Optimal stability
- Tensile
- Three-dimensional atom probe (3DAP)
- Yield strength
ASJC Scopus subject areas
- General Materials Science
- Computer Science Applications
- Computational Theory and Mathematics