Validation of a New Quality Assessment Procedure for Ductile Irons Production Based on Strain Hardening Analysis

A mathematical procedure based on the analysis of tensile flow curves has been proposed to assess the microstructure quality of several ductile irons (DIs). The procedure consists of a first diagram for the assessment of the ideal microstructure of DIs, that is, the matrix where mobile dislocations move, and a second diagram for the assessment of the casting integrity because of potential metallurgical discontinuities and defects in DIs. Both diagrams are based on the dislocation-density-related constitutive Voce equation that is used for modeling the tensile plastic behavior of DIs. The procedure stands on the fundamental assumption that the strain hardening behavior of DIs is not affected by the nature and the density of the potential metallurgical discontinuities and defects, which are expected to affect only the elongations to fracture. However, this fundamental assumption is not obvious, and so its validity was evaluated through tensile testing Isothermed Ductile Irons (IDIs) 800, showing a wide scatter of elongations to rupture. The analysis of the strain hardening behaviors supported by strain energy density calculations of IDIs tensile tests proved that the fundamental assumption was valid and the quality assessment procedure could be applied to IDIs. A modified Voce equation was also introduced to improve the fitting of the experimental tensile flow curves and the strain energy density calculations.

Tensile Flow Behavior of 9Cr–2WVTa Reduced-Activation Ferritic/Martensitic Steel

Tensile flow behavior of 9Cr–2WVTa ferritic/martensitic (RAFM) steel in normalized-tempered condition has been studied based on Voce equation over the temperature range of 25–600 °C. Yield strength (YS) and ultimate tensile strength (UTS) decrease with increase in temperature. However, the elongation decreases with increase in temperature up to 400 °C and then increases beyond 400 °C. True stress–true plastic strain curves at all temperatures are adequately described by the Voce equation. While saturation stress (σs) decreases with temperature, the rate at which the stress approaches the saturation value (nV) increases with temperature. The variation of the stress increment up to saturation stress (σun) with temperature shows a plateau in the temperature range of 200–400 °C. Moreover, the product of σun and nV (σun·nV) is inversely proportional to the elongation. The relation of elongation to σun·nV can be described by a power law with the exponent of −1.63.

Flow Curve Modelling of an Austenitic Stainless Steel at High Temperatures Starting from the One-Parameter Model of Strain Hardening

The flow curves of an austenitic stainless steel deformed at temperatures 700-1000°C with strain rates 10-5-10-2s-1were modelled with the Voce equation. The parameters needed to draw the Voce equation, are the saturation stressσVthat defines the height of the flow curve, the critical strainεCthat defines the velocity to achieveσV, and the stressσo, namely the back-extrapolated flow stress to zero strain. A modified strain hardening analysis based on the one-parameter model was used to analyze the strain hardening rate dσ/dεvs. the flow stressσin order to obtainσVandεC. The modified approach was based on the assumption that the dislocation multiplication component of strain hardening was temperature and strain rate dependent through the thermal activation termsof flow stress. A parameters’ proportional toswas obtained from the strain hardening analysis and a relationship betweens’ and temperature and strain rate was found. Relationships betweenσV,σo,εCands’ were finally established and at this stage the Voce equation could reproduce the experimental flow curves at any imposed deformation conditions of temperature and strain rate.

PREDICTION OF A MODIFIED PTW MODEL FOR VARIOUS TAYLOR IMPACT TESTS OF TANTALUM

The strain hardening part of the Preston-Tonks-Wallace (PTW) model, developed for the description of the plastic constitutive behavior of materials at wide ranges of strain, strain rate, and temperature, has been modified by employing the Voce equation. The prediction capability of the modified PTW (MPTW) has been investigated with reference to Taylor impact test results in the literature, and comparison has been made with the models of Johnson-Cook (JC), Steiberg-Guinan (SG), Zerilli-Armstrong (ZA), and PTW. Of the compared existing models, no model was appropriate for describing the results of various Taylor impact tests. However, the modified PTW is shown to predict fairly accurate results in terms of the length, diameter, and shape of the deformed specimen tested at different temperatures and impact velocity.

Representation of the High-Temperature Tensile Behavior of Reannealed Type 304 Stainless Steel by the Voce Equation

Tensile tests were performed on a reference heat of type 304 stainless steel (heat 9T2796) in the laboratory reannealed condition. Testing temperatures ranged from 25 to 760 deg C (77 to 1400 deg F) and strain rates were varied from 1.5 × 10−6 to 8.3 × 10−2/s. Several models were developed to represent the tensile curves, and each model was restricted to a specific range of applicability. For the inelastic strains below 0.001 a Ludwik-type formulation was developed; it was independent of strain rate but applicable for temperatures up to 760 deg C. For the strain range 0.001 to 0.05 a second Ludwik-type formulation was developed with minimal strain rate dependence, and the model was restricted to temperatures not exceeding 593 deg C (1100 deg F). For inelastic strains above 0.001 an alternative model was based on the Voce equation. The Voce model was applicable for all temperatures and strain rates and for strains to the uniform strain at the ultimate strength. The ability of the Voce model to represent tensile behavior at strain rates well below those that are practical in the laboratory experimentation was checked by comparing predicted ultimate tensile strength data against creep strength data for minimum creep rates corresponding to the tensile strain rates. The agreement was good. The ability of the Voce model to predict uniform strain behavior was also examined. In the creep range the Voce model overestimated uniform strains for temperatures below 649 deg C (1200 deg F) and underestimated them for temperatures above 649 deg C (1200 deg F).