Sunghak Lee

Effects of alloying elements on microstructure and fracture properties of cast high speed steel rolls: Part I: Microstructural analysis

Abstract A study was made of the effects of alloying elements on microstructural factors of six high speed steel (HSS) rolls manufactured by centrifugal casting method. Particular emphasis was placed on the role of hard carbides located along solidification cell boundary and the type of the martensite matrix. Microstructural observation, X-ray diffraction analysis, and hardness measurement were conducted on the rolls to identify carbides. Various types of carbides were formed depending on the contents of strong carbide forming elements. In the rolls containing the high Cr content, MC carbides inside cells and M 7 C 3 carbides along cell boundaries were primarily formed, while in the rolls containing the high W and Mo contents, MC carbides inside the cells and fibrous M 2 C carbides in the intercellular regions were dominantly formed. The most important microstructural factor affecting overall roll hardness was the intercellular carbides and their distribution. The effects of alloying elements were analyzed on the basis of the liquidus surface diagram, suggesting that the proper contents of carbon, tungsten, molybdenum, chromium, and vanadium were 1.9–2.0, 3–4, 3–4, 5–7, and 5–6%, respectively.

Adiabatic shear band formation during dynamic torsional deformation of an HY-100 steel

Abstract Dynamic torsion experiments have been conducted on thin-walled tubular specimens of a tempered martensitic HY-100 steel, causing adiabatic shear bands to form. The strain rates imposed were ∼ 10 3 s −1 and local temperature increases up to 600°C within the shear bands were measured. The shear band microstructure was examined by transmission electron microscopy (TEM), revealing two distinct microstructures. In some regions, highly elongated narrow subgrains extended in the shear direction, while in other regions, fine equiaxed cells were characteristic. The proportions of the two microstructures varied for different specimens, and the observations were interpreted to indicate that a process of dynamic recovery accompanying large deformation and a high temperature rise occurred within the shear band. Although thermal effects were apparent, there was no evidence to support a phase transformation to austenite followed by martensite formation. On the basis of present findings, it appears that the thermodynamic stability of the original microstructure can influence the tendency toward shear localization under dynamic loading conditions.

Effect of carbide distribution on the fracture toughness in the transition temperature region of an SA 508 steel

Abstract An investigation was conducted into the effect of carbide distribution on fracture toughness in the ductile–brittle transition temperature region of an SA 508 steel used for nuclear reactor pressure vessels. Tensile properties and elastic–plastic cleavage fracture toughness were measured in the transition temperature region, and the fracture toughness data were interpreted by using a simple fracture model containing carbide size distribution. This modeling study indicated that the critical nearest-neighbor distance between coarse carbides was an important microstructural factor affecting elastic–plastic fracture toughness, since it satisfied a linear relationship with the critical distance between a crack tip to a cleavage initiation site. These findings suggested that reducing the total number of carbides, particularly the number of M3C carbides larger than the critical size, and homogeneously distributing fine M2C carbides, were useful ways to improve fracture toughness in the transition temperature region.

EFFECT OF TRIGGERING ON THE ENERGY ABSORPTION CAPACITY OF AXIALLY COMPRESSED ALUMINUM TUBES

Abstract The energy absorption performance of extruded aluminum tubing for space frames was evaluated using computer-simulated compressive tests and quasi-static compressive deformation tests. An experimental deformation test and its simulation were conducted for seven extruded tube specimens on which various types of triggering dents were introduced, and the test data were investigated via observation of deformation mode, maximum repulsive force, and absorbed energy. The results indicated that the computer simulation results correlated well with the compressive deformation mode, indicating that the simulation was useful for the evaluation of absorbed energy. When triggering dents were introduced at the folding sites pre-estimated by the computer simulation, energy absorption could be improved, and the half-dented specimens absorbed more effectively than the full-dented specimens. On the other hand, when triggering dents of the same interval were introduced without consideration of the peak location of the folding wave, inhomogeneous deformation, together with overall bending, occurred, and deteriorated energy absorption because energies in bending were not as effectively absorbed as on folding.

Effects of volume fraction and stability of retained austenite on formability in a 0.1C–1.5Si–1.5Mn–0.5Cu TRIP-aided cold-rolled steel sheet

The effects of volume fraction and stability of retained austenite on formability of a 0.1C–1.5Si–1.5Mn–0.5Cu (hereafter all in wt.%) TRIP-aided cold-rolled steel sheet was investigated after various heat treatments (intercritical annealing and isothermal treatment). Tensile tests and limiting dome height (LDH) tests were conducted on the heat-treated sheet specimens, and the changes of retained austenite volume fraction as a function of tensile strain were measured using an X-ray diffractometer. The results showed a plausible relationship between formability and retained austenite parameters such as stability and initial volume fraction. The formability was improved with increasing volume fraction of retained austenite. However, when the volume fraction of retained austenite was same, the better formability was obtained in the specimens with the higher stability of retained austenite. This indicated that the strain-induced transformation of retained austenite to martensite could be stably progressed, thereby leading to the improvement of formability. Thus, the heat-treatment conditions should be established in consideration of the maximum volume fraction and high stability of retained austenite, and the optimal conditions were found to be intercritical annealing in the temperature range at which the austenite volume fraction was about 50%, followed by isothermal treatment at Ms temperature.

Correlation of fatigue properties and microstructure in investment cast Ti-6Al-4V welds

Abstract The effect of microstructural characteristics on high-cycle fatigue properties and fatigue crack propagation behavior of welded regions of an investment cast Ti-6Al-4V were investigated. High-cycle fatigue and fatigue crack propagation tests were conducted on the welded regions, which were processed by two different welding methods: tungsten inert gas (TIG) and electron beam (EB) welding. Test data were analyzed in relation to microstructure, tensile properties, and fatigue fracture mode. The base metal was composed of an alpha plate colony structure transformed to a basket-weave structure with thin α platelets after welding and annealing. High-cycle fatigue results indicated that fatigue strength of the EB weld was lower than that of the base metal or the TIG weld because of the existence of large micropores formed during welding, although it had the highest yield strength. In the case of the fatigue crack propagation, the EB weld composed of thinner α platelets had a faster crack propagation rate than the base metal or the TIG weld. The effective microstructural feature determining the fatigue crack propagation rate was found to be the width of α platelets because it was well matched with the reversed cyclic plastic zone size calculated in the threshold Δ K regime.

Transformation strengthening by thermomechanical treatments in C-Mn-Ni-Nb steels

The purpose of this study is to clarify the correlation between microstructural factors and mechanical properties of ultrafine steels processed by thermomechanical controlled treatments. Three steels deformed at high strain rates in a pilot plant rolling mill showed very fine ferritic microstructure, whose grains became more equiaxed and finer with increasing fraction of alloying elements, and had good tensile and fracture properties, although they contained only about 0.01 pct carbon. Especially in the Ni-added steel, tensile properties were greatly improved because of the high dislocation density and the fineness of the ferritic substructure, readily satisfying the requirements for commercial-grade high-strength, high-toughness steels. The formation of ultrafine equiaxed grains in the steels might be explained by a possible strain-induced dynamic transformation mechanism associated with the austenite → ferrite transformation caused by heavy deformation in the austenite range.

Correlation of microstructure and fracture properties in weld heat-affected zones of thermomechanically controlled processed steels

This article presents a correlation study between the microstructural parameters and fracture properties in the weld heat-affected zones (HAZs) of high-strength low alloy (HSLA) steels,i.e., a normalized steel and four thermomechanically controlled processed (TMCP) steels. The influence of the local brittle zone (LBZ) on toughness was investigated by means of simulated HAZ tests as well as welded joint tests. The intercritically reheated coarse-grained HAZ ex-hibited the lowest impact energy over the testing temperature range, indicating that this region was the LBZ. By comparing the volume fraction of martensite islands with impact energy val-ues, this LBZ was attributed mainly to the significant increase in the amount of martensite. Niobium was also found to have a deleterious effect on the HAZ fracture toughness because of martensite hardening. This suggests that the formation of martensite islands must be controlled by proper design of chemical compositions to reduce the carbon equivalent and by using the proper welding conditions to limit cooling rates in order to optimize the fracture toughness of welded joints of TMCP steels.

Effects of alloying elements on microstructure and fracture properties of cast high speed steel rolls: Part II. Fracture behavior

This study is concerned with the effects of the microstructural factors such as hard carbides located along solidification cell boundary and the type of the martensite matrix on fracture property of six high speed steel (HSS) rolls manufactured by centrifugal casting method. In situ fracture observation, fracture toughness measurement, and fractographic observation were conducted on these rolls to clarify the microfracture mechanism. The in situ test results indicated that hard carbides located along cell boundary provided easily intercellular fracture sites under low stress intensity factor levels. In the rolls containing a small amount of intercellular carbides and lath-type tempered martensite matrix, fracture occurred along a tortuous transcellular path since carbides were well spaced, thereby leading to the improvement of fracture toughness. In order to obtain better microstructure, hardness, and fracture toughness of the HSS rolls, it is suggested that the optimization of alloying elements is required to achieve the more homogeneous distribution of hard intercellular carbides and the lath-type tempered martensite matrix.

Effect of direct quenching on microstructure and mechanical properties of copper-bearing high-strength alloy steels

Abstract This study aims to investigate the effects of direct quenching on microstructural modification and mechanical properties of copper-bearing high-strength alloy steels. Two direct quenched and tempered (DQ&T) steel plates were rolled at different finish rolling temperatures, and their microstructures and mechanical properties were compared with those of a reheat quenched and tempered (RQ&T) steel plate. The as-quenched microstructure of the DQ plates consisted of refined lath martensite with high density of dislocations, which acted as preferred precipitation sites for NbC or e -Cu particles during tempering. These fine precipitates were not coarsened much up to the tempering temperature of about 650°C, and thus played a role in improving the tempering resistance. Especially in the DQ&T plate quenched at 760°C and tempered at 660°C, yield strength reached 1050 MPa, and Charpy impact energy at −18°C showed 140 J, indicating the potent effect of the DQ&T process. These findings indicated that the copper addition and the application of the DQ&T process to low-carbon alloy steels contributed to the production of steel plates with excellent strength and toughness.