Piotr Skubisz

Warm-Forging Characteristics and Microstructural Response of Medium-Carbon High-Strength Steels for High-Duty Components

Paper presents deformation behaviour and microstructural response of selected medium-carbon high-strength steels commonly used for high-duty components deformed under high-strain-rate and warm work temperature range. The investigation of material behaviour is oriented at analysis of hot and warm workability of material and microstructure evolution resultant from deformation mechanisms, strain induced recrystallization and hardening at temperatures of lower forging regime and high strain rate deformation. The effect of these factors on microstructure after forging and subsequent direct-cooling was studied. Metallographic work aided with numerical methods of simulation of the metal flow and microstructure evolution during forging were used to correlate thermo-mechanical parameters observed with microstructure and mechanical properties after forging and cooling.

Automated Determination and On-Line Correction of Emissivity Coefficient in Controlled Cooling of Drop Forgings

The paper presents the methodology of determination of emissivity coefficient of steel in the direct cooling temperature range, which allow self-definition of this parameter in automated temperature control system used for direct cooling of drop-forged parts. Based on the actual temperature of the surface of the workpiece, the system automatically adjusts the velocity of fan-forced air to produce desired cooling rate and course of cooling curves. To enable automatic correction of cooling rate, the system must rely on exact remote temperature measurements. This is conditioned by correct definition of emissivity in pyrometer gauge system. Traditionally, pyrometers use presets of a constant value, selected as to get optimum consistency for the temperature range concerned. As the emissivity is time-dependent, the idea of the approach presented in the paper is to use time dependent characteristics, to attain the minimum discrepancy between actual and measured values during the whole cooling cycle.

Estimation of Strain-Hardness Correlation in Cold-Forged Austenitic Stainless Steel

The paper presents selected aspects of analysis cold micro-forging process of a screw made of austenitic stainless steel, concerning relation between strain and hardness. Strain hardening character of a material in consecutive forming operations was analyzed experimentally by the measurement of hardness distribution made on longitudinal axial sections of screws. The relationship between hardness and effective strain (hardness curve) was determined, which made it possible to obtain strain distributions in different regions of a material subjected to cold deformation on the basis of strain distribution numerically estimated with FEM simulation performed using QForm2D/3D commercial software. Conclusions were formulated concerning strain inhomogeneity and strain-hardening intensity with respect to the correlation between strain and hardness. It was also concluded, that nonuniformity of hardening rate in a bulk can lead to local variations in flow stress and eventually, to occurrence of the metal flow related defects, which was illustrated with a case study of cold heading of self-tapping screw of AISI 304Cu stainless steel, with large head diameter to shank diameter ratio. In order to validate the obtained results, the same method was used for analysis of hardness development in steel 19MnB4.

Effect of High Strain-Rate Thermomechanical Processing on Microstructure and Mechanical Properties of Ti-10V-2Fe-3Al Alloy

Results of beta forging of titanium alloy Ti-10V-2Fe-3Al and subsequent thermal treatment are presented, with analysis of the effect of the processing route on the final mechanical properties, correlated with microstructure of thermomechanically processed material. Investigation of response to high strain-rate hot-forging of microstructure and mechanical properties is focused on the effect of the strengthening mechanisms in the material after two common manners of deformation typical of that alloy. The effect of deformation conditions on final microstructure and mechanical properties was analyzed in three crucial stages of thermomechanical processing, e.i. after deformation, quenching and aging. In result, conclusions were formulated as for processing conditions promoting high strength and/or ductility.

Determination of Emissivity Characteristics for Controlled Cooling of Nickel-Alloy Forgings

The paper presents the methodology and the results of determination of emissivity coefficient for nickel alloy Inconel 718 in the temperature range predicted for thermomechanical processing of these alloys and its application for correction of temperature during cooling and modeling. The idea presented in the paper is an iterative use of automatic system of temperature measurement and control, facilitated with thermocouples and pyrometer gauge to provide temperature dependent characteristics of the alloy. In addition to estimation of temperature dependence of the emissivity coefficient, the results of cooling tests for simple and complex geometry of nickel alloy forgings are contained, obtained with modeling based on assumption of the determined characteristics. To verify obtained characteristic, they were used in modeling of cooling and compared to those obtained in measurements based on assumption of constant value of emissivity coefficient. Measurement errors obtained were used to indicate show the significance of application of functional dependence in comparison with constant value, as well as, magnitude of error made during temperature measurement or modeling, when neglecting the temperature dependence.

Effect of high strain rate beta processing on microstructure and mechanical properties of near-β titanium alloy Ti-10V-2Fe-3Al:

The article presents microstructures and mechanical properties of near-β Ti-10V-2Fe-3Al titanium wrought alloy after two differentiated processing cycles. The assumed processing paths involved high strain rate hot deformation in β-phase range, followed by solution treatment with variation of cooling rate and subsequent single or double aging. To estimate the effects of the assumed treatment cycles, optical microscopy and scanning electron microscopy microstructure analysis was conducted, with special attention paid to evolution of alpha precipitates in consecutive stages of processing and their role in grain refinement. The correlation between tensile properties and grain size, as well as the amount of precipitates amount was found to be connected with alpha-plates’ size and morphology. It was concluded that in case of Ti-10V-2Fe-3Al titanium alloy, proposed cycles of thermomechanical processing allow reduction of inhomogeneous recrystallization resulting in necklace substructure. On the other hand, high strain rate promotes mechanical properties improvement, as it favors fragmentation of continuous grain-boundary α precipitates.

Estimation of heat transfer coefficient of forced-air cooling and its experimental validation in controlled processing of forgings

ABSTRACT Estimation of the cooling efficiency of an accelerated air for the needs of cooling of die forgings is presented. Temperature dependence of heat transfer coefficient (HTC) was calculated for different cooling conditions varied by airflow velocity, covering the range from 18 to 48 m/s. Time–temperature measurements performed on a full-scale semi-industrial cooling line provided similarity to conditions typical of industrial conveyor, which gives the results utilitarian significance in design of controlled processing (of steel forged products). Acquired HTC values, ranging from 164.7 to 298 W/m2 · K, were validated in numerical simulation of cooling complex-shape forgings and subject to experimental verification, indicating perfect agreement with physical measurements.