Influence of Composition and Prior Deformation on Phase Transformation Temperatures and Hardness in Direct Quenching Using Physical Simulation

Abstract

For conventional reheated and quenched (RQ) steels, the level of alloying necessary to achieve a given hardenability and hardness can be estimated from calculated Jominy curves or ideal critical diameters (DI) given in ASTM A255-10(2014). However, for thermomechanically rolled direct quenched (DQ) steels, little data are available. In this study, the accuracy of the ASTM approach was estimated by designing an experiment to study the main effects of seven alloying elements (C, Mn, Cr, Ni, Mo, Nb, and V) at two levels with eight boron steels based on an eight-run resolution III partial factorial designed experiment. Continuous cooling transformation (CCT) diagrams covering cooling rates of 1.5°C/s–48°C/s were determined using Gleeble simulations with or without controlled deformation below Tnr. The effects of deformation below Tnr and the alloying elements were clearly revealed. In general, low temperature straining of the austenite led to higher levels of hardness than quenching from unstrained austenite. The start of the bainite transformation Ar3 (bainite) was modelled as a function of chemical composition and cooling rate. The data in the CCT diagrams were used to derive equivalent ideal critical diameters (DIB) for strained and unstrained prior austenite. The hardenability index DIB calculated from experimental Jominy curves generally agreed well with those of CCT data for unstrained austenite. In order to apply the ASTM A255 approach to the calculation of DIB, boron factors (BF) for steels with alloy factors (AFs) greater than 26 were estimated by extrapolating the ASTM data. However, this approach did not give satisfactory predictions for either strained or unstrained austenite. Preliminary analysis indicated that Cr, Mo, and V might be less effective at increasing hardenability than implied from their AFs. New formulae were given to allow estimations of the hardness to be expected in connection with direct quenching.

Optimization of CCT Equations Using Calculated Grain Boundary Soluble Compositions for the Simulation of Austenite Decomposition of Steels

“…A comparison of the predictions made using the newly developed CCT equations based on the grain boundary composition and those developed previously based on nominal bulk composition has also been made, particularly with respect to the CCT measurements of high-carbon steels given in the literature. The results of the current study can also be complemented by the effect of thermomechanical processing [15][16][17][18] taking place in many steel processing operations. [19] On the other hand, the current method using IDS to include the effect of deformation on phase transformations and precipitation is still quite approximate and needs to be developed further.…”

Strengthening Mechanisms in Low Carbon Lath Martensite as Influenced by Austenite Conditioning

“…Thus, a wide range of dislocation substructures in the austenite before the quench can be produced. While the focus of this study is on lath martensite microstructures, TMP can influence the hardenability during direct quenching and other microstructures with variations in strength and toughness can be produced depending on the cooling rate [5].…”

An Appraisal of Direct Quenching for the Development and Processing of Novel Super-High Strength Steels

“…Where direct quenching is concerned, the data are scarce and often proprietary. The use of the ASTM standard as such has been found to be inappropriate in the case of direct quenching [5]. Fresh attempts were made to develop new hardenability models through non-linear regression analysis by dynamically varying both the boron factor and multiplying factors of most elements in the alloy factor [6].…”

“…In the ASTM standard A255-10, distance hardness dividing factors for both non-boron and boron steels have been provided for a large number of ideal critical diameters in separate tables[4]. A preliminary analysis suggested that if the structure of the approach to calculating DI B is considered correct, then the terms a Mo and a Cr are too large[5]. Alternatively, the boron factor is perhaps not described well as a function of AF, particularly beyond AF > 26.Because of inherent difficulties with the complex form of DI B (Eqs.…”

“…1 and 2, then the solution sought would contain too many variable coefficients to be determined by using say 9 or 10 different compositions and corresponding measured values of DI B : the variable parameters alone consume 7 degrees of freedom and the solution sought essentially contains numerous interaction terms, such as a MnMo Mn*Mo. Notwithstanding these difficulties of non-linear regression analysis, a few promising solutions were found to give good convergence with reasonable predictions for all the steels, both for strained and unstrained matrices[6]. Predicted DI B(1.11) data for different steels both in strained and unstrained conditions are plotted against the experimental DI B(1.11) values (solid diamonds and square…”