Determination of CCT Diagram by Dilatometry Analysis of High-Strength Low-Alloy S960MC Steel

Abstract

High-strength steels are used more than general structural steel due to their combination of properties such as high strength, good toughness and weldability. They are mainly used in the manufacture of heavy vehicles for the mining industry, cranes, transportation, etc. However, welding these grades of steel brings new challenges. Also, a simulation for welding high-strength steel is required more often. To insert a material database into the simulation program, it is necessary to conduct investigations using CCT (Continuous Cooling Transformation) diagrams, welded joints research, and more. To investigate the behavior of S960MC steel during heating and cooling, we used dilatometry analysis supported by EBSD (Electron Backscatter Diffraction) analysis. A CCT diagram was constructed. The transformation temperatures of Ac1 and Ac3 increase with increasing heating rate. The Ac1 temperature increased by 54 °C and the Ac3 temperatures by 24 °C as the heating rate increased from 0.1 °C/s to 250 °C/s. The austenite decomposition temperatures have a decreasing trend in the cooling phase with increasing cooling rate. As the cooling rate changes from 0.03 °C/s to 100 °C/s, the initial transformation temperature drops from 813 °C to 465 °C. An increase in the cooling rate means a higher proportion of bainite and martensite. At the same time, the hardness increases from 119 HV10 to 362 HV10.

Phase Composition and Temperature Effect on the Dynamic Young’s Modulus, Shear Modulus, Internal Friction, and Dilatometric Changes in AISI 4130 Steel

“…The microstructure of the same quenched alloy is predominantly martensite with characteristic lamellar morphology, but some retained austenite can also be present (lighter areas in Figure 2a). The microstructures observed are typical for annealed and quenched low-alloyed hypoeutectoid carbon steels [34][35][36].…”

“…The microstructure of annealed AISI 4130 (Figure 1) is composed of equiaxed grains of primary ferrite (α-Fe) and of the lamellar component pearlite (α-Fe + Fe3C) The microstructure of the same quenched alloy is predominantly martensite with characteristic lamellar morphology, but some retained austenite can also be present (lighter areas in Figure 2a). The microstructures observed are typical for annealed and quenched lowalloyed hypoeutectoid carbon steels [34][35][36]. Figure 3 shows the experimental XRD patterns of annealed and quenched AISI 4130.…”

Temperature Effect on the Dynamic Young’s Modulus, Shear Modulus, Internal Friction, and Dilatometric Changes in Quenched and Annealed AISI4130 Steel

“…However, different trends in the CTE with increasing temperature were observed for each sample. While the CTE for the annealed sample shows a linear behavior up to 760 °C (Figure 4a,c), which is the expected temperature for the onset of austenite formation from ferrite and cementite [40,[44][45][46][47][48][49][50], the CTE for quenched sample shows a non-linear behavior in the interval from 100 to 300 °C, where it can be expected the decomposition of martensite, as observed in similar systems [45,46], with the formation of ferrite and cementite, and the decomposition of retained austenite, at higher tempering temperatures, accompanied by slight expansion and compression of the sample, as shown in Figure 4 b,d.…”

“…2.a). The microstructures observed are typical for annealed and quenched low-alloyed hypoeutectoid carbon steels [32][33][34]. Figure 3 shows the experimental XRD patterns of annealed and quenched AISI 4130.…”

“…With respect to temperatures above 760 °C, both samples exhibit a volume contraction, which would be caused by the formation of austenite from ferrite and cementite phases [47]. Since austenite has a higher atomic packing factor than ferrite [48], this macroscopic volume contraction is attributed to lattice shrinkage.…”