**2. Materials and Experiments**

Two steels obtained from a steel company, i.e., type 430 (8 mm hot-rolled plate) and 410S (6 mm hot-rolled plate) stainless steel were used in this study. Type 410S stainless steel was selected as a comparison for the investigation of continuous cooling transformation kinetics. Their chemical compositions are listed in Table 1.


**Table 1.** Chemical composition of experimental steels (wt %).

Similar constituent phases appear on both phase diagrams in Figure 1 which include ferrite, austenite, chromium nitride, and carbide with body-centered cubic (BCC), face-centered cubic (FCC), hexagonal close packed (HCP), and M23C6 crystal structure, respectively. The enlarged lower left region of the phase diagram is shown in the inset. Chromium nitride and carbide would precipitate at ~850 ◦C and below. Formation of cementite is thermodynamically unfavorable. The most noticeable difference in the phase diagram was the austenite single phase region between 901 and 1037 ◦C in type 410S stainless steel while the maximum volume fraction of austenite in the dual phase region is 44.7% in type 430 stainless steel.

**Figure 1.** Phase diagram of type 430 (**a**) and 410S (**b**) stainless steels.

The heat treatment experiments in this work were conducted on a DIL 805A/D dilatometer (TA Instruments, New Castle, DE, USA). The sample size was Φ 4 mm × 10 mm. After machining, samples were all homogenized in a sealed quartz tube at 1200 ◦C for 120 min. Figure 2 shows the employed heat treatment procedure. The sample was firstly held at 1200 ◦C for 5 min. Then, in the cyclic heat

treatment where austenite–ferrite transformation kinetics were studied, a 30 min isothermal holding at 950 ◦C was carried out to create a "ferrite + austenite" dual-phase microstructure. Subsequently, one cycle of heating and cooling between 950 and 1150 ◦C was applied to the sample before quenching to room temperature. The corresponding rate of temperature change (RTC) was 10, 100, and 200 ◦C/min. In the continuous cooling experiment, which was targeted for the investigation on the formation of interphase Cr-rich precipitates, the sample was cooled at 30 ◦C/min from 1200 ◦C. Interrupted quenching was respectively conducted at 850 and 200 ◦C to examine the resulted microstructure.

**Figure 2.** Schematic illustration of heat treatment procedure for (**a**) cyclic and (**b**) continuous cooling experiments.

Microstructure examination was made by optical microscopy (OM) and scanning electron microscopy (SEM, GeminiSEM 300, ZEISS, Oberkochen, Germany) with energy dispersive spectroscopy (EDS, Ultim Max, Oxford Instruments, Abingdon, UK). The heat-treated samples have gone through the standard metallographic preparation procedure, including grinding, polishing, and etching with 2% Nital solution.
