The tempering characteristics of three different hot and warm forging die steels (FX, 2714, and WF) were systematically studied over a range of temperatures (316–677°C) and a range of times (1–300h). ...The softening rate for each steel was determined by the change in room temperature hardness. In this study, the hardness data are quantitatively related to the tempering parameter via regression analysis. The tempering parameter (also known as the Hollomon–Jaffe parameter or the Larson–Miller parameter) accounts for the effects of both tempering time and tempering temperature. Room temperature hardness is a measure of the microstructural change that occurs during the tempering process. Results from this study show a bilinear softening as a function of the tempering parameter. For hot and warm forging application the second portion of the curve is more applicable, since these die steels are tempered to some extent before initial use. The slope of the curve can be used as a measure of softening, which is one of the contributing factors on how well the die steel will perform in actual forging operations. These results indicate that WF has the highest resistance to softening during use, FX is somewhat less resistance to softening, and 2714 is the least resistant of the three die steels studied.
Dual‐phase (DP) steels offer an outstanding combination of strength and formability as a result of their microstructure, in which a hard martensitic or bainitic phase is dispersed in a soft ferritic ...matrix. DP steels have a high ultimate tensile strength combined with a low initial yielding stress. The yield strength of DP steels is further increased by the paint baking, also called bake hardening (BH), process. In the current research, the effect of BH parameters, namely pre‐strain, baking temperature, and holding time, on yield strength increment of DP steels with different martensite volume fractions is investigated. DP steels with 20 and 30% martensite volume fraction are pre‐strained up to 5%, then they are baked at 100, 170, and 300 °C for various times. The results show that maximum yield strength is obtained as optimum values of BH parameters are applied. It is also found that at high values of baking temperature and holding time, martensite tempering effects prevail and yield strength increment decreases. The results also indicate that the increment of pre‐strain enhances the Lüders band and decreases the yield strength increment.
The effect of bake‐hardening (BH) parameters, namely pre‐strain, baking temperature, and holding time, on yield strength increment of DP steels with different martensite volume fractions is investigated. The results show that maximum yield strength is obtained as optimum values of BH parameters are applied. The increment of pre‐strain enhances the Lüders band and decreases the yield strength increment.
Short time tempering has been applied to steels to improve toughness keeping the hardness at the same level. For AISI D2 tool steel, typical thermal cycles are quench and temper. The austenitizing ...temperature is around 1040 ºC and the subsequent double tempering at temperatures close to 500 ºC, for 2 hours each, in order to get the maximum secondary hardness. The aim of the present work is to verify the effects of short time tempering in the AISI D2. For such purpose the Hollomon-Jaffe tempering parameter was used to determine the equivalent time at 600 and 700 °C, being 500 °C for 2 hours the standard temperature and time. The thermal cycles were carried out at a dilatometer. All the specimens were austenitized at 1040 °C followed by double tempering at: 500 and 700 °C for 2 hours each; and 500 °C, 600 °C, and 700 °C for 10 s and 1 min each. The dilatometric curves were analysed. Scanning electron microscopy, X-ray diffraction and Vickers hardness were carried out to characterize the microstructure and mechanical properties. The short time tempering conducted at high temperatures led to the formation and growth of tempering carbides and to matrix recrystallization, which were responsible for greatly reducing the hardness of the material. The short time tempering up to the intermediate temperature of 600 ºC, for 1 minute or 10 seconds, produced results like the conventional tempering in microstructural and hardness terms, denoting that the Hollomon-Jaffe tempering parameter predictability close to the secondary hardness peak.
Das Presshärten ermöglicht als Kombinationsprozess aus Wärmebehandlung und Formgebung die Herstellung von hochfesten Karosseriestrukturbauteilen aus Stahl im Serienmaßstab. Der konventionelle ...Austenitisierungsprozess im Ofen bestimmt dabei maßgeblich die Prozessgesamtdauer. In dieser Arbeit wird ein alternativer Ansatz zur herkömmlichen, auf Zeit‐Temperatur‐Austenitisierungsschaubildern oder Erfahrungswerten beruhenden, Ermittlung der nötigen Austenitisierungszeit untersucht. Die Berechnung der optimalen Austenitisierungszeit basiert dabei auf experimentell gewonnenen Werkstoffkennwerten. Die Grundlage zur phänomenologischen Beschreibung des Austenitisierungsvorgangs bildet der durch den Hollomon‐Jaffe Parameter beschriebene Zusammenhang zwischen Temperatur und Zeit für den Reaktionsfortschritt thermisch aktivierter Prozesse. Die nötigen Werkstoffkennwerte werden mittels Austenitisierungsversuchen in einem Dilatometer anhand von Kleinproben bestimmt. Die Anwendbarkeit dieses Parameters zur Berechnung der optimalen Austenitisierungsdauer für isotherme und prozessnahe Temperaturführung wird für die Stähle 22MnB5 und X15Cr13 exemplarisch beschrieben und nachgewiesen.
Translation
The press hardening technology combines heat treatment and forming in a serial production process to produce car body components made of high‐strength steels. The conventional austenitization process in the furnace determines the overall process time essentially. In this work, an alternative approach to the traditional assessment of the austenitization time based on time temperature austenitization diagrams or empirical values is investigated. The calculation of the optimal austenitization duration is based on experimental material values. The basis for the phenomenological description of the austenitization process is the relationship between temperature and time for the reaction progress of thermally activated processes described by the Hollomon‐Jaffe parameter. The necessary material characteristics are determined by means of austenitization tests in a dilatometer using small samples. The applicability of this parameter for calculating the optimum austenitization duration for isothermal and process‐oriented temperature control is described and demonstrated exemplary for the steels 22MnB5 and X15Cr13.
The Hollomon-Jaffe parameter is usually used to stablish a equivalence between time and temperature in a tempering treatment, but not to predict the harness of the alloy after the treatment. In this ...paper this last possibility has been studied. A group of cast iron samples was annealed and cooled at different rates in order to obtain samples with three different hardness values. These samples were tempered using different times and temperatures. The Hollomon-Jaffe parameter was calculated for each case and a relationship based on a logistic function between that parameter and the final hardness was stablished. This relationship was found to depend on the initial hardness and the lowest hardness achievable.
Abstract
Das Presshärten ermöglicht als Kombinationsprozess aus Wärmebehandlung und Formgebung die Herstellung von hochfesten Karosseriestrukturbauteilen aus Stahl im Serienmaßstab. Der ...konventionelle Austenitisierungsprozess im Ofen bestimmt dabei maßgeblich die Prozessgesamtdauer. In dieser Arbeit wird ein alternativer Ansatz zur herkömmlichen, auf Zeit‐Temperatur‐Austenitisierungsschaubildern oder Erfahrungswerten beruhenden, Ermittlung der nötigen Austenitisierungszeit untersucht. Die Berechnung der optimalen Austenitisierungszeit basiert dabei auf experimentell gewonnenen Werkstoffkennwerten. Die Grundlage zur phänomenologischen Beschreibung des Austenitisierungsvorgangs bildet der durch den Hollomon‐Jaffe Parameter beschriebene Zusammenhang zwischen Temperatur und Zeit für den Reaktionsfortschritt thermisch aktivierter Prozesse. Die nötigen Werkstoffkennwerte werden mittels Austenitisierungsversuchen in einem Dilatometer anhand von Kleinproben bestimmt. Die Anwendbarkeit dieses Parameters zur Berechnung der optimalen Austenitisierungsdauer für isotherme und prozessnahe Temperaturführung wird für die Stähle 22MnB5 und X15Cr13 exemplarisch beschrieben und nachgewiesen.
Translation abstract
The press hardening technology combines heat treatment and forming in a serial production process to produce car body components made of high‐strength steels. The conventional austenitization process in the furnace determines the overall process time essentially. In this work, an alternative approach to the traditional assessment of the austenitization time based on time temperature austenitization diagrams or empirical values is investigated. The calculation of the optimal austenitization duration is based on experimental material values. The basis for the phenomenological description of the austenitization process is the relationship between temperature and time for the reaction progress of thermally activated processes described by the Hollomon‐Jaffe parameter. The necessary material characteristics are determined by means of austenitization tests in a dilatometer using small samples. The applicability of this parameter for calculating the optimum austenitization duration for isothermal and process‐oriented temperature control is described and demonstrated exemplary for the steels 22MnB5 and X15Cr13.
Die Härtemessung bei erhöhten Temperaturen erlaubt eine erste Einschätzung des Werkstoffs hinsichtlich seiner Warmfestigkeit. Der Vorteil dieser Prüfmethode ist, dass mit einer einzigen kleinen Probe ...in kurzer Zeit die Temperaturabhängigkeit der Härte über einen großen Temperaturbereich gemessen werden kann. Insbesondere eignet sich die Warmhärteprüfung für Vergleichszwecke zwischen unterschiedlichen Wärmebehandlungen. In dieser Arbeit wird der Einfluss der Anlasstemperatur und ‐dauer auf die Warmhärte von P92 untersucht. Dabei ergab sich ein mathematischer Zusammenhang zwischen der Härte bei unterschiedlichen Temperaturen und des Hollomon‐Jaffe‐Parameters, mit welchem die Anlasszeit‐ und Anlasstemperatur in gewissen Grenzen ausgetauscht werden kann.
Influence of tempering on the hot hardness of P92 sheet metal
The measurement of hardness at elevated temperatures allows a first estimation of the material concerning its high temperature strength. The advantage of this testing method is to determine the hardness as function of the temperature in a wide temperature range in a short time on a small specimen. Especially the hot hardness testing is suited to compare the influence of different heat treatments. In this work the influence of tempering time and temperature on the hot hardness of P92 were investigated. The results show a mathematical relation between the hardness at different temperatures and the Hollomon‐Jaffe‐Parameter, which allows in a certain range a replace of tempering time and tempering temperature.
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