The mechanical surface degradation such as abrasion and contact fatigue appears in various drive system components due to severe friction and damage accumulation. Thermochemical case hardening has been adopted to enhance durability for the components, where the surface hardness and strength of the elements increase and high compressive residual stress is induced in the surface layer [1,2]. Namely, the process results in the increase of fatigue resistance and improves usage economy of the steel parts. Nitriding is widely used as a case hardening for steel and develops a surface compound layer of ε-Fe2-3N and/or γ’-Fe4N with the thickness of a few μm at lower temperature than A1. The nitriding at higher temperature than A1 produces a diffusion layer of α’ or α’’-Fe16N2 with high content of solute nitrogen. However, applications of nitriding are limited due to its thin hardened layer, and higher wear resistance and fatigue strength are demanded [3,4]. Post-treatments of nitriding such as quench and tempering or aging are available to stabilize the microstructure and improve the mechanical properties and fatigue resistance of the surface layer. The quench-tempering treatment ensures stable surface properties and can achieve the strength and toughness of the nitriding case [5]. The quench-aging treatment may provide a hard compound layer of ε-Fe2-3N phase and α' or α''-Fe16N2 phase to improve fatigue and mechanical properties [6]. However, few studies have been done on the crack generation of the treated steels. In this study, the fatigue crack initiation and propagation in the surface hardened layer are examined for nitrided and followed by quench-tempered or -aged JIS-SCM440 (equivalent to ASTM 4110) low-alloyed steel.

A SCM440 steel (0.41C-0.19Si-0.81Mn-0.007P-0.015S-1.13Cr-0.16Mo, in mass%) continuous cast bloom was hot-rolled into a square billet of 180 mm × 180 mm and was hot-forged to a round bar of 35 mm in diameter followed by normalized at 1198 K for 7.2 ks. The cyclic tensile test specimens with 74 mm in length, 4.0 mm in necked width and 2.0mm in thickness were machined from a hot-forged bar parallel to the longitudinal direction. The specimens were nitrided in vacuum at 913 K for 5.4 ks and quenched into oil heated at 393 K and followed by aged at 553 K for 5.4 ks (NH). Some specimens were nitrided in vacuum at 1023 K for 7.2 ks and quenched into oil heated at 353 K and followed by tempered at 423 K for 3.6 ks (NQ). Cyclic tensile tests were conducted with the maximum stress below 0.2% proof stress for each material.

References

[1] Y Bai et al. Materials Science and Engineering: A, 2014, 607: 578-588.

[2] VF Terent’ev et al. Mechanics, 2007, 64.2: 12-22.

[3] G Hinojosa et al. Thin Solid Films, 1999, 349.1-2: 171-175.

[4] MJ Van Genderen et al. Metallurgical and Materials Transactions A, 1997, 28: 63-77.

[5] Y Duan et al. Surface and Coatings Technology, 2019, 360: 247-258.

[6] O Furukimi et al. Materials Transactions, 2016, 57.9: 1587-1592.