Influence of composition and prior deformation on phase transformation temperatures and hardness in direct quenching using physical simulation

• 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. 

Nyckelord

CCT diagram

Dilatation curves

Direct quenching

Hardenability

Ideal critical diameter

Jominy curves

Martensite

Phase transformation

Regression modelling

Alloy steel

Alloying

Alloying elements

Austenite

Bainite

Boron

Cooling

Deformation

Hardness

Manganese

Phase transitions

Quenching

Temperature

Critical diameter

Direct-quenching

Bainitic transformations

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