Prototype evaluation of transformation toughened blast resistant naval hull steels: Part II

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.in 1/2 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. A. Saha et al. 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.

Strength Recovery in a High-Strength Steel During Multiple Weld Thermal Simulations

“…[1,2] BA160 is a low carbon martensitic steel additionally strengthened primarily by nanometersized Cu-rich precipitates and M 2 C precipitates (where M = Cr, Mo, and V). The yield strength of BA160 is 160 ksi (1104 MPa).…”

“…(1) In subcritical HAZ regions (SCHAZ; T P < A c1 ), no detectable transformation of ferrite to austenite occurs. (2) In the intercritical HAZ (ICHAZ), partial transformation of ferrite to austenite occurs because the peak temperature is between A c1 and A c3 . (3) In the fine-grained HAZ (FGHAZ), the samples are heated to a temperature slightly above the A c3 temperature, after complete austenitization.…”

Concurrent design of hierarchical materials and structures

“…17. High resolution microanalysis confirmed that the predicted strengthening and toughening dispersions were achieved, including the desired optimal austenite composition [87]. Continued development of variants of this steel by QuesTek has already demonstrated exceptional ballistic performance for fragment protection.…”

“…With steadily improving accuracy of design models and their supporting fundamental databases, an important milestone has been the demonstration of a successful design within a single iteration of prototyping. Described in detail elsewhere [86,87], a weldable high strength plate steel for naval blast protection applications was designed to maintain the impact toughness of the Navy's current HSLA 100 steel out to [87] significantly higher strength levels. Adopting a tight carbon limit to promote high weldability, highly efficient strengthening was designed through a combination of copper precipitation and optimized alloy carbide precipitation.…”

Computational design of advanced steels

“…Cementite particles then become the prime fracture initiators. A goal in the computational design of blast-resistant steel [1] was therefore to eliminate cementite by controlled alloy-carbide precipitation which leads to finer dispersions by virtue of the need for substitutional solutes to diffuse [1,2]. The intended matrix would then be based on a secondary-hardened bainite and martensite mixture, containing dispersed austenite, the stability of which is designed to exploit transformation plasticity.…”