Effect of Filter Functional Coating on Detrimental Nonmetallic Inclusions in 42CrMo4 Steel and Its Resulting Mechanical Properties

Abstract

Carbon‐bonded ceramic foam filters with different functional coatings are immersed in a 42CrMo4 steel melt within a steel casting simulator. The solidified steel is analyzed with respect to the size distribution and the chemical composition of the remaining nonmetallic inclusions (NMI). Cyclic loading and quasi‐static tests are performed to determine the fatigue limit, the strength, deformability, and toughness of the steel after filter immersion. The immersion of filter with calcium hexaluminate (CA6) coating significantly reduces the population of small (4–20 μm) NMIs. This leads to an increased deformability and, thus, ability for energy dissipation during deformation. However, the maximum size of NMIs is increased from 100 to 150 μm, which results in fatigue limit reduction, despite the decrease in NMIs total density. The majority of inclusions are found to be pure alumina. Large (up to 150 μm) plate‐like alumina inclusions introduce most of the detrimental effects on cyclic strength, whereas significant effect on quasi‐static strength is not found.

On the formation of ridges and burnished debris along internal fatigue crack propagation in 42CrMo4 steel

“…Due to the surface protection by nitriding, all of the investigated fatigue cracks were originated from the interior with a clearly recognizable fisheye (Figure 2A,D). Corresponding stress versus cycles to failure ( S‐N f ) curves are not in the focus of present work and can be found elsewhere 32 . Most of the fatigue cracks were initiated by a plate‐shaped Al 2 O 3 particle, surrounded with FGA in case of N f ≥ 10 6 .…”

“…Following the above‐mentioned assumption, the SA should be somehow related with the plastic zone size and martensitic structure. The size of the cyclic plastic zone around a propagating fatigue crack can be estimated by a radius r c , which starts from the crack tip and ends at the local stress level equal to the yield strength σ YS 21 ,p. 306 where σ YS = 950 MPa is the yield strength of the analysed 42CrMo4 steel 32 . This value matches almost perfectly with the average grain size of 3.63 μm, which corresponds to the average size of a martensite block (Figure 1C).…”

“…Around 100 cylindrical specimens for USFT (Figure 1A) were manufactured from several cast 42CrMo4 ingots after hot isostatic pressing. Specimens undergone specific treatment: (1) austenitization at 840°C for 20 min in vacuum; (2) quenching to 20°C in helium; (3) tempering at 560°C on tempered martensite (Figure 1B) with the core hardness of 310 HV0.1 32 . In addition, plasma nitriding (1 hr at 420°C and 2 hr at 570°C) for the surface hardness of 650 HV0.1 was implemented to block surface crack initiation 33 .…”

Ultrasonic fatigue testing of cast steel G42CrMo4 at elevated temperatures

“…In the present study, an average value of ΔK th = 7.0 ± 2.2 MPa√m for all failed specimens was determined using the √area parameter of Murakami 18 and the polygon method to determine the defects' area. Stanzl-Tschegg et al 1,12 and Seleznev et al 50 also reported on lower surface roughness at the inner FIE, the so-called 'smooth area' for RT fatigue experiments. In the VHCF regime, the surface roughness inside the FiE is connected to the crack growth rate.…”

“…Other authors evaluated values of SIF for FGA formation from 4 up to 5 MPa√m. 12,[48][49][50][51] Li et al 52 reported a SIF threshold value of 6.5 MPa√m for the formation of an FGA. In the present study, an average value of ΔK th = 7.0 ± 2.2 MPa√m for all failed specimens was determined using the √area parameter of Murakami 18 and the polygon method to determine the defects' area.…”