Effect of Prior Microstructure and Heating Rate on the Austenitization Kinetics of 39NiCrMo3 Steel

Abstract

The formation of austenite from three different prior microstructures is studied in a 39NiCrMo3 steel, which, indeed, is widely used in the industry, especially for induction heating treatments, due to its resistance to thermal cracking. The starting microstructure of sorbite with fine carbides, sorbite with coarse carbides and stress-relieved martensite is examined in detail by optical microscopy, scanning electron microscopy, and Vickers microhardness measurements. Dilatometric tests are performed employing different heating rates spanning in the 100-800 8C s À1 range in order to assess the kinetics of the austenitic transformation. The experimental results are compared and critically linked in order to determine the fundamental factors governing the austenite formation when using high heating rates. The formation of austenite from sorbite with coarse and fine carbides is observed to occur in two consecutive stages: (i) cementite dissolution and (ii) ferrite to austenite transformation. The start and finish temperature of austenitic transformation increase with heating rate. For stress-relieved martensitic microstructure, instead, the a' to g transformation can take place through two different mechanisms: diffusive at low heating rates and displacive at high heating rate. Critical points of austenite formation initially increase with heating rate, reach a maximum and then decrease at high heating rate.

Effect of Intercritical Annealing and Austempering on the Microstructure and Mechanical Properties of a High Silicon Manganese Steel

“…The preparation of the thin foils was realized by mechanical grinding until thickness of 70 µm, followed by mechanical punching to obtain 3 mm diameter specimens. The final polishing and etching were performed electrochemically using a twin-jet polisher STRUERS TENUPOL-3 (Struers S.A.S., Milan, Italy), with 95% acetic acid (CH 3 COOH) and 5% perchloric acid (HClO 4 ) solution, at 45 V and room temperature [26,27].…”

Microstructure and Properties of 1.0C–1.5Cr Bearing Steel in Processes of Hot Rolling, Spheroidization, Quenching, and Tempering

“…Most of cementite will directly nucleate at the existing undissolved cementite during the divorced eutectoid transformation, which will form spherical cementite. At last, during the subsequent holding, cementite will coarsen again, which is driven by the decrease of interfacial energy …”

Effect of Preheating on the Microstructure, Impact Properties, and Fracture Mechanisms of a Laser Surface‐Hardened High‐Carbon–Chromium Steel

“…[ 42,43 ] It was reported that for the conventional low‐alloy steel, the martensite‐to‐austenite transformation was a diffusive transformation at a lower heating rate, whereas it changed into a displacive transformation at the heating rate larger than 400 °C s −1 . [ 44 ] Settimi et al [ 45 ] also reported that the martensite‐to‐austenite transformation changed into displacive transformation at a large heating rate. Considering the far larger heating rate during laser surface hardening, the martensite‐to‐austenite transformation should also be a displacive phase transformation for 52100 steel.…”