Papers by Author: John G. Speer

Abstract: Austenitic stainless steels are commonly used for hydrogen storage and transportation. These alloys have a high nickel (Ni) content, which increases alloy cost. In this study, high manganese (Mn) austenitic alloys were evaluated as potential lower cost alternatives. Two heats of high Mn alloys with different stacking fault energies (SFE) of ~29 mJ·m^-2 and 49 mJ·m^-2 were acquired. Additionally, a new vanadium (V)-microalloyed high Mn alloy was designed to achieve a SFE of 47 mJ·m^-2 to minimize planar slip deformation mechanisms. Post-processing via cold working in conjunction with aging was also performed on the V-microalloyed high Mn steel. Hydrogen embrittlement sensitivity was investigated using circumferential notch tensile specimens cathodically charged with hydrogen in a 0.05M NaOH electrolytic solution. The alloys were compared to a cold-worked 316L stainless steel, which exhibited no strength loss due to hydrogen. The high Mn alloys with SFE of ~29 mJ·m² and 49 mJ·m^-2 had notch strength losses of 11 and 6 pct, respectively. The V-microalloyed high Mn steel in the as-hot-rolled condition had a notch strength loss of 17 pct. The V-microalloyed high Mn steel in the cold worked and aged condition indicated no notch strength loss in hydrogen, which was comparable to the performance of the 316L stainless steel.

Abstract: Double soaking (DS) has been proposed as an alternate processing route for medium manganese steels. DS consists of soaking in the intercritical annealing region to stimulate manganese enrichment of austenite by depletion of ferrite followed by secondary soaking at a higher temperature and cooling to room temperature to obtain a martensite/austenite microstructure. DS is different from more traditional medium manganese heat treating which usually involves a single soaking step in the intercritical region to generate a ferrite/austenite microstructure. DS has been shown effective at generating attractive tensile properties notably tensile strength levels in excess of typically observed levels in medium manganese steels. A review of properties and microstructural evolution obtained by DS of medium manganese steels is presented here.

Abstract: Extensive efforts have been undertaken worldwide to develop new high strength steels with substantial fractions of retained austenite, for lightweight automobile manufacturing and other applications requiring improved combinations of strength and formability. These “3rd Generation” Advanced High Strength Steels (AHSS) are being implemented, and spot-welding has been found to present new challenges for these steels when Zn-based corrosion resistant coatings are involved, wherein zinc liquid metal embrittlement (LME) can occur. Some recent work is highlighted here that was designed to examine the separate effects of prior microstructure and alloy composition on LME sensitivity. LME behavior was assessed by comparing hot-ductility of steels with and without a Zn coating tested under conditions simulating spot-weld thermal cycles. Effects of prior microstructure on LME susceptibility were assessed with a single AHSS alloy composition, using annealing modifications to produce martensitic, Q&P, TBF and dual-phase substrates. The dual-phase steel exhibited less sensitivity to LME, perhaps because the Zn penetration and cracking are unable to follow (prior) austenite boundaries in this microstructure. With respect to alloy composition, carbon and manganese variations did not lead to noticeable effects on LME sensitivity, while silicon clearly leads to increased LME sensitivity. Addition of 1.3 wt. pct. aluminum to a 0.5 wt. pct silicon-containing AHSS steel further increased LME sensitivity at some test temperatures. The effects of alloying are interpreted in terms of the propensity to form an intermetallic reaction layer that consumes liquid and physically separates the substrate and liquid zinc.

Abstract: The effects of Mo and V on impact toughness in martensitic steels tempered at low temperatures were investigated using three low-alloy medium-C steels. Previous examination of these alloys had identified differences in impact toughness without a clear cause. In this work, the Base alloy with a reduced Mo addition experienced a significant loss in hardenability leading to the formation of small fractions of bainite during quenching even at relatively high quench rates. The use of different quench media to simulate cooling rates throughout a heavy section demonstrated that the variation in previously reported Charpy V-notch impact absorbed energies was readily explained by some regions cooling fast enough to avoid bainite while others formed some small fraction of upper bainite leading to increased cleavage fracture and decreased impact toughness. Small amounts of bainite transformation were not detected by dilatometry or tensile properties. These results emphasize the importance of effective through-hardening and careful microstructure evaluation in alloys that are meant to maintain good toughness and strength in thicker sections.

Abstract: Resistance spot welding is a critical joining technique in automobile assembly. The load carrying properties of spot welds are generally accepted to correlate with weld diameter, which increases with increasing weld current or duration. The formation of a softened layer, or weld halo, surrounding the fusion zone in a spot-welded third generation (Gen3) advanced high strength steel (AHSS) was recently reported in the literature. To optimize weld performance by schedule design, it is necessary to understand the halo formation characteristics and potential impacts. Accordingly, welding of a Gen3 AHSS was performed using weld times between 130 – 1300 ms. Microhardness mapping characterized weld microhardness and the evolution of the halo during welding. Electron probe microanalysis and timeof-flight secondary ion mass spectrometry enabled measurement of solute distributions through the weld halo, while scanning electron microscopy was used for microstructural characterization. The solidified structure was examined using light-optical microscopy, and with the microhardness and compositional data, used to infer the mechanism by which the halo forms during welding. It was found that the halo develops due to solute rejection from a cellular solidification front that advances towards the center of the fusion zone while weld current is applied. Extended weld times increase the size of the weld halo and the solute content of the inner fusion zone. The decrease in weld halo microhardness and the increase in inner fusion zone microhardness is largely explained by the changes in local carbon content associated with halo formation.

Abstract: Advanced High Strength Steel (AHSS) developments have largely focused on automotive applications using metallurgical approaches to develop retained austenite-containing microstructures in a variety of new steels, using the transformation-induced plasticity (TRIP) effect to achieve better combinations of strength and ductility. These efforts have been extended in recent studies to explore the potential to improve wear resistance, using metastable retained austenite to enhance wear resistance for earth-moving and other applications. This paper provides selected highlights of the authors’ efforts to develop wear resistant steels using AHSS processing approaches. Some attractive product/process development opportunities are identified, and it appears that martensite-austenite microstructures produced using “quenching and partitioning” exhibit increased wear resistance.

Abstract: Three AISI 1045 steels: a base steel, one modified with vanadium (V), and one modified with V and niobium (Nb) were studied to evaluate microstructural conditioning prior to induction hardening. Simulated bar rolling histories were evaluated using fixed-end hot torsion tests with a Gleeble® 3500. The effects of chemical composition and thermomechanical treatment on final microstructures were examined through analysis of laboratory simulations of steel bar rolling and induction hardening processes in order to provide additional insights into the morphological evolution of austenite of microalloyed steels. Analysis of prior austenite grain size (PAGS) is complemented with analysis of austenite recrystallization and pancaking during rolling. The potential for utilizing TMP, in conjunction with microalloy additions, to enhance bar steel microstructures and subsequent performance is assessed by evaluating the induction hardening response of each steel systematically processed with different preconditioning treatments.

Abstract: Extensive efforts are underway worldwide to develop new steels with substantial fractions of retained austenite, for lightweight automobile manufacturing and other applications requiring improved combinations of strength and formability. It is likely that microalloying can provide product enhancements in these emerging products, such as Q&P, TBF, medium-Mn TRIP, etc. and this paper examines the expected behavior of niobium using inferences based on published AHSS literature and principles of Nb microalloying. Some benefits of Nb in terms of microstructure refinement and precipitation strengthening have been reported. The potential influences of Nb are complex due to the sensitivity of Nb dissolution and precipitation to chemical composition and processing; differences in the expected role of Nb are pointed out with respect to different product forms produced via hot-rolling or annealing after cold-rolling, and microstructures with or without substantial quantities of primary ferrite. Some issues that warrant further examination are identified, as a deep understanding of Nb microalloying and other fundamental behaviors will be needed to optimize the performance of these next-generation steels.

Abstract: The potential to utilize controlled thermal processing to minimize banding in a DP780 steel with 2 wt pct Mn was evaluated on samples processed on a Gleeble® 3500 thermomechanical processing simulator. All processing histories were selected to result in final dual-phase steel microstructures simulating microstructures achievable during annealing of initially cold rolled sheet. Strip samples were processed to evaluate the effects of heating rate, annealing time, annealing temperature, and cooling rate. The degree of banding in the final microstructures was evaluated with standard light optical microscopic techniques. Results are presented to illustrate that the extent of banding depended on control of both heating and cooling rates, and a specific processing history based on a two-stage heating rate can be used to minimize visible banding in selected final heat treated products.

Abstract: Low carbon bainitic steels are important in applications such as linepipe, and the details of the bainite microstructure control strength and toughness. The transformation of austenite to bainitic ferrite has been widely researched over the years, although recent use of electron backscatter diffraction techniques has provided opportunity to advance the characterization of various crystallographic aspects. In recent work, microstructures were characterized in a base steel containing 0.04 C and 1.7 Mn (wt. pct.) and two additional steels having modest carbon and manganese variations to influence the transformation behavior, with an interest in the MA (martensite-austenite) constituent and characteristics of the bainite developed at different transformation temperatures. Effects of austenite conditioning were also examined, as these steels contained an addition of 0.04 wt. pct. Nb. Microstructural details including crystallographic characteristics assessed using EBSD are presented, along with comments related to the implications of the results.