**2. Experimental**

The material studied is an advanced medium Mn steel. Its chemical composition is shown in Table 1 and its mechanical properties are shown in Table 2. The microstructure consists of main martensite and 10~15 vol.% retained austenite. The high yield strength of 650 MPa, ultimate strength of 770 MPa, and superior cryogenic toughness of around 200 J at −60 ◦C were obtained by optimized intercitical annealing. The transformation temperature was measured as Ac1 = 626 ◦C and Ac3 = 790 ◦C on a slow heating rate of 3 ◦C/min by Gleeble 3800 (Dynamic Systems Inc., Poestenkill, NY, USA) [15].


**Table 1.** Chemical composition of investigated 5Mn steel (wt.%).



A He–Ne CLSM (VL2000DX-SVF17SP, Lasertech Yokohama, Chiba, Japan) with infrared image furnace and laser scanning confocal microscopy was employed to in situ observe the reverse transformation of austenite on heating, and martensite transformation on cooling during the simulated welding thermal cycle. The CLSM samples, with approximate 4 mm in diameter and 6 mm in height, were carefully machine polished, and then set into an alumina crucible with 0.5 mm in thickness. The sample chamber was evacuated and then filled with argon to prevent the sample from being oxidized during heating. Using focused infrared light heating mode, the specimens were heated to

the peak temperature of 1320 ◦C at the rate of 5 ◦C·s<sup>−</sup>1, and then cooled down to 200 ◦C at the rate of <sup>5</sup> ◦C·s<sup>−</sup>1. The live pictures were taken every 15 s from 240 mm × 240 mm surface areas.

Gleeble 3800 was used to detect the expansion variation during the simulated welding thermal cycle. The sample was 6 mm in diameter and 70 mm in length. The peak temperature was 1320 and 850 ◦C to simulate the coarse-grained heat affected zone (CGHAZ) and fine-grained heat affected zone (FGHAZ) respectively. The different cooling rates of 1~60 ◦C·s−<sup>1</sup> were set to draw the simulated heat-affected zone continuous cooling transformation diagram (SHCCT). The transformation temperature and transformation kinetic were detected based on the thermal expansion curves.

The mean grain size was estimated on the CLSM sample using a circular-intercept method from image analysis of at least 500 grains. Intercept lengths were determined and then converted into nominal grain diameters using standard tables. The electron backscattering diffraction (EBSD) was applied to analyze the misorientation of the microstructure.

The Metallographic specimens of simulated HAZs were polished and etched with 2% nital before conventional light microscopy. Transmission electron microscopy (TEM, JEM-2100UHR STEM/EDS, JEOL Ltd., Tokyo, Japan) studies were carried out on thin foils. Thin foils were prepared by cutting thin wafers from the simulated HAZ samples, and grinding to 0.1 mm in thickness. Three millimeter discs were punched from the wafers, and were electropolished using a solution of 5% perchloric acid/95% acetic acid. Foils were examined by conventional transmission electron microscope operated at 120 kV using standard bright field and dark field imaging techniques.

The volume fraction of reversed austenite was determined using XRD (XPert PRO MPD, PANalytical B. V., Almelo, Holland) with Cu K<sup>α</sup> radiation at a scanning speed of 1◦·min−<sup>1</sup> and step size of 0.02◦. The specimens of simulated HAZ were first mechanically polished and then electropolished using an electrolyte consisting of 12% perchloric acid and 88% absolute ethyl alcohol at room temperature. The integrated intensities of (200)γ, (220)γ, (311)γ, (110)α, (200)α, (211)α, (220)α diffraction peaks were used to determine the austenite volume fraction.

Hardness was measured on simulated HAZ samples with a load of 200 g, using a MICROMET5101 Vickers hardness tester (Matsuzawa Co., Ltd., Tokyo, Japan). Standard Charpy v-notch (CVN) impact tests were performed on simulated samples with a different cooling rate at −60 ◦C, using a NI500A imapct tester (NCS Co., Ltd., Beijing, China) on specimens of dimensions 10 × <sup>10</sup> × 55 mm3.
