Thermal Processing Effects on Mechanical Integrity and Fatigue Performance of CK45 Carbon Steel

CK45 medium-carbon steel is extensively employed in automotive, agricultural, and industrial applications owing to its favorable combination of strength, machinability, and cost-effectiveness. Nevertheless, components manufactured from CK45 steel are frequently exposed to cyclic loading environments where mechanical integrity and fatigue reliability are of paramount importance. In the present study, the influence of thermal processing on the microstructure, mechanical properties, and fatigue performance of CK45 carbon steel was systematically investigated using four representative conditions: as-received (S1), normalized (S2), quenched (S3), and quenched–tempered (S4). Optical microscopy, FESEM, and AFM analyses were performed to evaluate the microstructural evolution and surface characteristics, while Rockwell hardness, tensile, Charpy impact, and rotating bending fatigue tests were employed to assess the mechanical behavior. The results demonstrated that normalization refined the ferrite–pearlite structure and improved the overall performance, increasing the hardness from 18.6 ± 0.5 HRC to 22.8 ± 0.7 HRC, the yield strength from 425 ± 8 MPa to 485 ± 10 MPa, the ultimate tensile strength from 680 ± 12 MPa to 745 ± 15 MPa, and the fatigue limit from 290 ± 10 MPa to 340 ± 12 MPa. Quenching produced a predominantly martensitic microstructure, resulting in the highest hardness (55.4 ± 0.9 HRC), yield strength (1015 ± 18 MPa), and ultimate tensile strength (1185 ± 20 MPa), but significantly reduced the elongation (8.2 ± 0.4%) and impact toughness (18 ± 2 KJ) because of increased brittleness and residual stresses. Subsequent tempering transformed the brittle martensitic structure into tempered martensite with finely dispersed carbides, leading to a superior balance of properties, including a hardness of 44.1 ± 0.8 HRC, yield strength of 860 ± 15 MPa, ultimate tensile strength of 985 ± 18 MPa, elongation of 14.6 ± 0.5%, impact energy of 68 ± 3 KJ, fatigue limit of 395 ± 13 MPa, and fatigue life of 9.4 × 10⁵ cycles at an applied stress of 500 MPa. Furthermore, AFM analysis revealed that the quenched specimen exhibited the highest surface roughness (Ra = 72.4 nm; Rrms = 91.8 nm), whereas the quenched–tempered condition showed the lowest roughness values (Ra = 28.9 nm; Rrms = 37.2 nm). The findings confirm that thermal processing is an effective strategy for tailoring the performance of CK45 steel. Among the investigated conditions, the quenched–tempered treatment emerged as the optimum route, providing the best compromise between strength, toughness, and fatigue reliability for engineering components operating under repeated loading conditions.