CONTROL OF PHASE TRANSFORMATIONS THROUGH THERMAL PROCESSING MODIFICATIONS OF HIGH-STRENGTH LOW-ALLOY STEELS

Abstract

HY-80 steel was developed by the U.S. Navy and its industrial partners following World War II to address metallurgical problems associated with catastrophic low-temperature brittle fractures of the incumbent hull material at the time. It represented a large step forward in the performance of hull steel alloys at that time, with significant increases in strength and toughness, particularly at low temperatures, while remaining weldable and unsusceptible to stress-corrosion cracking. It is also a versatile alloy system, being able to be produced in both wrought and cast forms, with some limitations. Shipbuilding requires creating large castings with thick sections to withstand the harsh operational conditions, which is challenging due to issues with elemental segregation during solidification and slow heating and cooling rates in thick sections during heattreatments. One purpose of the work described in this thesis was to improve the mechanical properties of HY-80 castings by heat-treatment process modifications by incorporating an intercritical heat treatment with the targeted purposes of stabilizing austenite and limiting grain growth.

Performing intercritical heat treatments on steel requires near complete knowledge of the transformation processes taking place in the alloy during the heat treatment, which are affected by every prior step in the production of a steel ingot, including ingot design, solidification, and other heat treatments. To that end, an experimental investigation on cast HY-80 material was undertaken to measure secondary-dendrite-arm spacing (SDAS) and used in conjunction with Scheil solidifications simulations to create homogenization simulations to achieve more uniform properties. Intercritical heat treatments were carried out on a martensitic HY-80 microstructure created by quenching from the single-phase austenite phase at high temperatures.

To better understand the martensite transformation, a martensite-start temperature, Ms, model for lath, plate, and ϵ martensite was developed by the CALculation of PHAse Diagrams method (CALPHAD) using an open-source steels thermodynamic database, which was also developed for this work. In addition, a novel martensite-type prediction Gaussian process classification (GPC) machine-learning model was developed to increase the accuracy of the Ms model in the numerous instances where the type of martensite is unreported in the literature. The GPC and ϵ martensite models were published for the first time, while the lath- and plate-martensite were updated and published using an open-source steels database vice a closed, proprietary database for the first time. This was necessary because the closed-source databases and commercial software provide limited opportunity for learning the fundamentals of their property models or quantifying their uncertainty.

The transformation of lath martensite to austenite in the intercritical temperature regime of HY-80 was studied using metallography and differential scanning calorimetry (DSC), with experimental results being used to calibrate the CALPHAD equilibrium predictions to be more relevant for industrial heat treatments, which was also published for the first time. Quantitative optical metallography with tint etching of the martensite/ferrite phase was coupled with machine learning to measure the phase fractions after long heat treatments. This method can be applied to a variety of materials to increase the industrial applicability of CALPHAD predictions to each material.

The DSC and metallographic results were used to select a short, intercritical heat treatment for HY-80 by replacing an austenitization step with an intercritical step at a temperature near the Ac3 to enrich interlath austenite films, primarily with carbon and nickel, to improve its mechanical properties. Kinetic simulations using Thermo-Calc’s DICTRA and open-source phase-field software were developed and are compared. The phase-field models should be more physically accurate due to the incorporation of gradient-energy effects that help drive microstructures toward equilibrium. Both simulations show that interlath austenite films becomes enriched in austenite-stabilizing elements at the austenite/martensite boundary during short intercritical heat treatments, increasing its local stability against decomposition. This is a novel approach tograin-boundary toughening of lath martensite via local austenite stability of interlath films.

Fully heat treated HY-80 material was characterized by high-energy x-ray diffraction using a monochromatic synchrotron source, which indicated an increase in the volume fraction and a change in the composition of retained austenite from the intercritical heat treatment as compared to the baseline quench-and-tempered microstructure. The mechanical properties were evaluated using quasistatic tensile and instrumented Charpy V-notch impact energy testing at two temperatures. The intercritical heat treatment improved the low-temperature toughness of HY-80 by shifting the ductile-to-brittle transition temperature downwards while maintaining acceptable tensile strength and ductility. An apparent ductility reduction was investigated by examination of the fracture surfaces in a scanning electron microscope, which indicated that microporosity from the casting process was the likely cause. Additional work is needed to optimize the heat treatment and assess its feasibility for thick sections; however, the initial results show promise for at least a subset of HY-80 castings.