PSI - Issue 68

2

S. Ghosh et al. / Procedia Structural Integrity 68 (2025) 1329–1336 S. Ghosh et al. / Structural Integrity Procedia 00 (2025) 000–000

1330

1. Introduction The novel concept of quenching and partitioning (Q&P) is a potential processing route for improving the balance of elongation to fracture and tensile strength for advanced high-strength steels (Speer et. al. (2004), Li et. al. (2010), Moor et. al. (2008)). In the Q&P processing, the steel is austenitised, quenched to a temperature in the M s - M f range, and held at a suitable temperature range for a particular time to allow partitioning of carbon from martensite to austenite, thereby stabilizing it partly or fully down to room temperature. Unlike in the case of tempering, the formation of iron carbides and decomposition of austenite are intentionally suppressed using Si, Al, or P alloying (Speer et. al. (2005), Clarke et al. (2008), Miettunen et al. (2021)). A martensitic matrix has the potential to provide the required strength, while a fraction of RA distributed finely between the martensitic laths is expected to provide improved work hardening and uniform elongation without a loss of impact toughness. Based on the concepts of Q&P process, the direct-quenching and partitioning (DQP) processing route has been developed at the University of Oulu, Finland, with the specific aim of producing these innovative steels with yield strengths > 1100 MPa combined with good uniform and total elongations, and adequate low temperature impact toughness (Ghosh et al. (2022)). To start with, relatively inexpensive compositions based on 0.2 - 0.3 wt.% C containing Mn, Cr, and up to 1.5% Si+Al, were designed for the development of these high-strength steels with good impact toughness and deformation capacity as assessed by the uniform elongation. The DQP process developed and patented during this development comprised sequential deformation in the recrystallization and no-recrystallization regimes in the austenitic range followed by direct quenching to a quench stop temperature (T Q ) in the range 200 300°C. This was followed by holding for a pre-decided time or very slow cooling to room temperature. The final microstructure consisted of fine lath-martensite with 5-15% RA films. Recently the process has been extended to 0.4% C steels, but the low M s temperature poses a challenge in respect of carbon partitioning that requires holding at somewhat higher temperatures, unlike in the case of 0.2 and 0.3% C steels. Not only the tensile properties of the DQP steels are significantly improved as compared to their direct quenched variants, but also their weldability and wear resistance are promising. High- and ultrahigh-strength martensitic steels with small fractions of finely divided austenite are particularly interesting for applications in mobile structures and machines, where performance requirements are high and the need for weight-saving is critical. Although extensive research has been conducted on the characterization of the microstructures and mechanical properties of Q&P/DQP steels, their behaviour under fatigue loading, particularly fatigue crack growth, remains least understood. Given that potential applications of martensitic-austenitic steels are in the automotive sector, especially in load-bearing structures subject to cyclic loading, further research in this area is crucial. Therefore, in this study, we examine the effects of alloying, particularly Si, on the stability of RA and corresponding fatigue crack propagation characteristics in medium-carbon DQP steels. 2. Experimental Methods 2.1. Materials, thermomechanical processing, and characterisation Two medium-carbon steels (0.4 wt.% C) with varying Si (1.51 wt.% and 0.68 wt.%) contents were obtained from OCAS NV, Zelzate, Belgium, as 70 kg vacuum-cast ingots. The steels ¢ chemical compositions, analysed via spark optical emission spectroscopy and carbon combustion analysis, are presented in Table 1.

Table 1. Chemical compositions of the test materials.

Steel code

Fe

C

Si

Cr 1.0 1.0

Ni

Mn 2.05 2.04

Al

H-Si M-Si

Bal. Bal.

0.4 0.4

1.51 0.68

0.49 0.49

0.02 0.02

Steel blocks, approximately 60 × 80 × 130 mm in size, were cut from steel castings and fully austenitized by soaking at 1200 °C for 2 hours in a furnace. These blocks were then hot-rolled using a 1 MN Carl Wezel laboratory