Chester J. Van Tyne

A Kinetics Model for Martensite Transformation in Plain Carbon and Low-Alloyed Steels

An empirical martensite kinetics model is proposed that both captures the sigmodial transformation behavior for alloy steels and remains computationally efficient. The model improves on the Koistinen and Marburger model and the van Bohemen and Sietsma model with a function that better represents the transformation rate, especially during the early stages. When compared with existing models, the proposed model exhibits better predictions of volume fraction of martensite. The proposed model also predicts various other transformation properties accurately, such as M90 temperatures and retained austenite.

Control scheme using forward slip for a multi-stand hot strip rolling mill

Forward slip is an important parameter often used in rolling-speed control models for tandem hot strip rolling mills. In a hot strip mill, on-line measurement of strip speed is inherently very difficult. Therefore, for the set-up of the finishing mill, a forward slip model is used to calculate the strip speed from roll circumferential velocity at each mill stand. Due to its complexity, most previous researches have used semi-empirical methods in determining values for the forward slip. Although these investigations may be useful in process design and control, they do not have a theoretical basis. In the present study, a better forward slip model has been developed, which provides for a better set-up and more precise control of the mill. Factors such as neutral point, friction coefficient, width spread, shape of deformation zone in the roll bite are incorporated into the model. Implementation of the new forward slip model for the control of a 7-stand hot strip tandem rolling mill shows significant improvement in roll speed set-up accuracy.

Characterization of deposited layer fabricated by direct laser melting process

Deposition dimensions are important in the final applications of products made by direct laser melting (DLM). This investigation used a 200 W fiber laser to produce single-line beads from stainless steel 316L powder using a variety of different energy distributions. To investigate the deposited layer, deposition width, height, penetration depth, and side surface roughness were measured. In order to validate the effectiveness of the two main process parameters (laser power and scan rate), multi-layered beads were fabricated by the sequential layering of single lines. It was found that with an increase in linear energy density, the wetting angle was reduced, and the average roughness was also increased with linear energy density. An equation that predicts the deposition height for a multi-layered bead is proposed and experimentally validated in this study. For deposited layer applications, the material properties of the deposited layer, such as contact angle, interfacial contact resistance, and flexural strength are estimated. The rougher deposited layers show higher contact angle and interfacial contact resistance. The flexural strength of the DLM fabricated specimen is above 250 MPa.

Effect of Roll Configuration on the Leveling Effectiveness of Tail-Up Bent Plate Using Finite-Element Analysis

The finite-element method (FEM) has been used to numerically investigate the effect of work roll configuration on the leveling effectiveness of tail-up bent plates. Leveling is a process used to minimize shape defects, including flatness imperfections and uniformity of internal stresses in shape-critical applications. Leveling plays an important role in delivering the desired plate shape and meeting the required product standards. To simulate the roller leveling effectiveness of tail-up bent plates, an initially flat plate was plastically bent prior to leveling and was passed through the leveling rolls. Leveling effectiveness was estimated by the vertical displacements of tail-up bent plates with two different roll configurations. One configuration adopts a gradually increasing roll gap, while the other configuration maintains the same roll gap in the first two sets of rolls and gradually increases the roll gap for the later rolls. For comparison purposes, the entry and exit roll gaps of the two roll configurations are set to the same roll gap. To verify the accuracy of the numerical simulations, actual leveling experiments were performed using tail-up bent plates. The results show that the roll configuration significantly influences the leveling effectiveness of the tail-up bent plates. Higher leveling effectiveness is obtained for a leveling configuration that imparts more severe deformation at the earlier leveling stages. Through the analysis, the work roll configuration is determined to be essential to increase leveling effectiveness of tail-up bent plates.

Thermal and mechanical properties of electro-slag cast steel for hot working tools

The thermal and mechanical properties of an electro-slag cast steel of a similar chemical composition with an AISI-6F2 steel are investigated and compared with a forged AISI-6F2 steel AISI-6F2 is a hot-working tool steel Electro-slag casting (ESC) is a method of producing ingots in a water-cooled metal mold by the heat generated in an electrically conductive slag when current passes through a consumable electrode The ESC method provides the possibility of producing material for the high quality hot-working tools and ingots directly into a desirable shape In the present study, the thermal and mechanical properties of yield strength, tensile strength, hardness, impact toughness, wear resistance, thermal fatigue resistance, and thermal shock resistance for electro-slag cast and fotged steel are experimentally measured for both annealed and quenched and tempered heat treatment conditions. It has been found that the electro-slag cast steel has comparable thermal and mechanical properties to the forged steel

Uniform Elongation and the Stress-Strain Flow Curve of Steels Calculated from Hardness Using Empirical Correlations

An empirical relationship between the hardness and uniform elongation of non-austenitic hypoeutectoid steels has been developed. This new hardness-elongation relationship was combined with previously developed correlations of hardness and strength (yield and ultimate tensile strength) to predict the stress-strain flow curve from a single hardness test. The current study considers both power law hardening behavior and exponential hardening behavior. Reasonable agreement was observed between the experimental and predicted flow curves of a high strength, low alloy steel. Additionally, an empirical correlation of the flow strength at instability with hardness is provided.

On the Formation of a Non-Traditional Pearlite Morphology

Continuous cooling and isothermal dilatometry was performed for a binary Fe-0.3C alloy and a ternary Fe-0.3C-1.0Mn alloy at slow (< 0.1 oC/s) cooling rates and isothermally at temperatures below the equilibrium eutectoid reaction temperature but above the bainite start temperature (625 to 715 oC). Some of the test conditions produced an unusual morphology in which fine scale ‘sub-grains’ are decorated with carbide, with additional discrete carbide particles inside the ‘sub-grains’. A detailed investigation into the network carbide formation indicates formation during austenite decomposition, as opposed to a post lamellar transformation coarsening or spheroidization reaction, but only for select temperatures, and apparently only during isothermal conditions.

Degradation Behavior of BaTiO3Dielectrics for MLCCs by an Accelerated Life Test With Voltage and Temperature Stress Factors

An accelerated life test (ALT) was designed under voltage and temperature stresses using 1005 type multilayer ceramic capacitors (MLCCs) based Ni-BaTiO3, and failure analysis was also conducted to compare the individual stress. The inverse power and Arrhenius models were applied to the voltage and temperature accelerated tests, respectively, and times to failure (TTF) of MLCCs under individual stress were measured. The stress–life relation was plotted from obtained life data, and characteristic life (B63.5) was calculated at the same condition of 130°C and 3 times rated voltage. B63.5 under the voltage stress was 15.91 min and that of the temperature stress was 17.23 min. It was determined that the voltage stress had more influence on the degradation of insulation resistance for MLCCs. As a result of an analysis of the chemical bonding state from the dielectric ceramic and inner electrodes, according to increase in the stresses, the binding energy of Ti 2p3/2 and Ni 2p3/2 peak changed, which generated oxygen vacancies. These oxygen vacancies accelerated the degradation under the high-voltage stress, caused the reduction of the BaTiO3 ceramic and oxidation of the Ni electrode, and consequently decreased the insulation resistance.

Prediction of Forming Limit Curves from Hardness for Steels

This paper presents a method for predicting the strain-based forming limit curve (FLC) for steels using hardness. The stretching side (positive minor strain component) of the FLC was calculated by using a Marciniak-Kuczyński model with a non-quadratic yield function, while the drawing side (negative minor strain component) of the FLC was predicted based on the relationship between the major and minor critical strains, in accordance with the theory of maximum sheet tension for local necking. The requisite parameter that describes the plastic flow behavior (in this case, the strain hardening exponent) was calculated, based on correlations with the measured microhardness. Additionally, the strain rate sensitivity was considered in the model by using a newly developed empirical correlation between hardness and strain rate sensitivity. This hardness-based model was used to predict FLCs that demonstrate good agreement with experimental FLCs of a high-strength low-alloy steel and a dual-phase steel. Equations are provided that enable the calculation of the FLC from given hardness values for different severities of the material inhomogeneity.

Analysis of warping during flat rolling of bimetallic slabs

The current study analyzes the warping behavior of bimetallic slabs during the flat rolling process. An analytical model that predicts the degree of warping during the rolling of bimetallic slabs is developed, based on differential bending, which occurs due to the mismatch in longitudinal elongation. The warping model focuses on the mismatch of the lengthwise elongation, due to differences in the deformation resistances of constituent materials in the bimetallic slab. With this analytical model, warping can be predicted for various processing parameters, such as initial thickness ratios and rolling reductions. To validate the model, flat rolling experiments using Al1050/Al6061 bimetallic slabs, produced by continuous clad casting, were conducted. The deformation characteristics of the bimetallic slabs during rolling were also assessed by a finite element analysis using the commercial code, FORGE™. The results from the current study show that the warping behavior of bimetallic slabs during flat rolling can be characterized with a high degree of reliability.