40Cr square steel is a versatile medium-carbon chromium alloy steel widely used in mechanical engineering for components like shafts, gears, bolts, and structural parts. Its composition—approximately 0.37–0.44% carbon, 0.8–1.1% chromium, and trace manganese and silicon—endows it with excellent hardenability, high tensile strength, good toughness, and wear resistance, making it a preferred choice for load-bearing applications. The processing of 40Cr square steel follows a systematic sequence to unlock its optimal properties. First, raw material preparation begins with selecting high-quality billets, typically produced via continuous casting to ensure uniform composition. These billets undergo rigorous chemical analysis using spectrometry to verify compliance with 40Cr standards. Surface defects like scaling, cracks, or inclusions are removed through grinding or shot blasting to prevent quality issues in subsequent steps. Next, forming is achieved primarily through hot rolling. Billets are heated to 1100–1200°C, a temperature that transforms the steel into an austenitic structure for easy shaping. They are then passed through a series of rolling mills with square dies, gradually reducing the cross-section to the desired square dimensions while maintaining tight tolerances. For applications requiring higher precision or smoother surfaces, cold rolling may be employed post-hot rolling, though this increases production cost. Heat treatment is a critical phase to tailor the steel’s mechanical properties. Normalization is often the first step: heating to 850–880°C, holding to homogenize the structure, then air cooling. This refines grain size, reduces internal stress, and improves machinability. For parts needing enhanced hardness and strength, quenching and tempering (QT) follow: the steel is heated to 840–860°C, quenched in oil (to avoid cracking), then tempered at 500–650°C. Tempering adjusts the balance between hardness (22–32 HRC for general use) and toughness, eliminating brittleness from quenching. Machining comes next if the square steel is fabricated into finished components. Turning, milling, drilling, and grinding are common operations. 40Cr in QT state has good machinability, though carbide tools are recommended to handle its hardness and reduce tool wear. Surface treatment is optional but essential for specific applications. Induction hardening creates a hard surface layer (50–60 HRC) for wear resistance while preserving a tough core. For corrosion protection, galvanizing, phosphating, or epoxy coating may be applied. Quality control is integrated at every stage: dimensional checks using calipers and coordinate measuring machines ensure accuracy; mechanical tests (tensile, impact, hardness) validate performance; metallographic analysis inspects microstructures; and non-destructive tests (ultrasonic, magnetic particle) detect hidden defects. The careful execution of these steps ensures 40Cr square steel meets the stringent requirements of industrial applications, delivering reliability and durability in demanding environments.