1.0577, S355J2 (St52-2) and the Chinese Equivalent Q345D: Technical Guide on Hot-Rolled Challenges, Carbonitriding, and Oil-Free Blackening

In the global industrial supply chain, material equivalence and surface engineering are the twin pillars that determine the success of high-precision components. For engineers sourcing from China or designing for international markets, the correlation between the European grade 1.0577 (S355J2 or St52-2) and the Chinese National Standard Q345D is one of the most critical benchmarks. At Tuofa CNC Machining China, we specialize in navigating the complexities of these materials, particularly when they are supplied in hot-rolled forms and subjected to advanced thermal treatments like carbonitriding followed by oil-free blackening. This article explores the metallurgical properties of these steels, the challenges of warping in hot-rolled materials, and the technical requirements for achieving a high-performance, clean surface finish.

Understanding the material bridge begins with the mechanical specifications. 1.0577 (S355J2) is a high-strength, non-alloy structural steel known for its minimum yield strength of 355 MPa and its exceptional toughness at low temperatures, specifically tested at -20°C. Its Chinese equivalent, Q345D, shares these exact performance characteristics, with the "Q" representing yield strength and "D" indicating the same -20°C impact testing requirement. These steels are favored for heavy-duty applications—such as gear housings, structural brackets, and transmission parts—because they offer a high strength-to-weight ratio and excellent weldability.

When these materials are supplied as hot-rolled plates or bars, they bring unique advantages and significant challenges to the CNC machining process. Hot-rolled steel is produced at temperatures above the recrystallization point, which makes it cost-effective and easier to produce in large dimensions. However, the hot-rolling process leaves behind internal residual stresses. During the subtractive process of CNC machining, as layers of metal are removed, these internal stresses are released, which can lead to significant warping or "warpage." This phenomenon is particularly problematic for long, thin components or parts with asymmetrical geometries. At Tuofa CNC Machining, we mitigate this by employing stress-relief annealing or strategic machining sequences that balance material removal to maintain the dimensional integrity of the 1.0577 or Q345D substrate.

The true transformation of these steels occurs during the surface treatment phase. For components that require high surface hardness, wear resistance, and fatigue strength, carbonitriding is an ideal choice. Carbonitriding is a thermochemical process that involves the simultaneous diffusion of both carbon and nitrogen into the surface of the steel. This is typically performed in a gas atmosphere at temperatures slightly lower than standard carburizing, usually between 700°C and 850°C. For a low-carbon structural steel like Q345D, carbonitriding creates a hard, wear-resistant "case" while the core remains tough and ductile. The addition of nitrogen is crucial; it lowers the critical cooling rate, allowing for a more uniform hardness and improving the material's resistance to tempering and softening under high-temperature friction.

However, the heat-treatment process itself is a secondary source of warping. The rapid heating and subsequent quenching required to lock in the carbonitrided case can cause S355J2 parts to distort. This is a critical stage where "warpage" from the hot-rolled material's history meets the thermal stresses of the furnace. To solve this, Tuofa utilizes precision quenching techniques and fixture hardening to minimize distortion, ensuring that even after the surface is hardened to 55-62 HRC, the part remains within the specified geometric tolerances.

Following the carbonitriding process, the component requires a finish that provides both corrosion resistance and a professional aesthetic. Black oxide, or "blackening," is often the preferred choice for industrial tools and mechanical parts. Traditionally, blackening involves a series of chemical baths that convert the surface of the steel into magnetite (Fe3O4). However, many modern applications in the electronics, medical, and clean-room sectors require an "oil-free" blackening finish. Standard blackening usually relies on a final oil dip to provide corrosion protection, but for "dry" environments, this is unacceptable as oil can migrate and contaminate sensitive assemblies.

Achieving an oil-free blackening finish on carbonitrided 1.0577 or Q345D requires a high degree of process control. Since the carbonitrided surface is rich in nitrogen and carbon, it reacts differently to blackening salts than raw steel. The resulting black oxide layer must be dense and chemically stable enough to resist oxidation without the help of a supplemental oil film. At Tuofa CNC Machining, we utilize a specialized post-treatment "dry" sealant or a high-temperature blackening process that creates a more robust magnetite layer. This ensures that the parts have a sleek, matte-black appearance that is clean to the touch and safe for use in environments where lubrication or oil residue would be a hazard.

The combination of hot-rolled Q345D, carbonitriding, and oil-free blackening is frequently used in the manufacturing of precision hand tools, specialized fasteners, and high-load sliding components. The hot-rolled base provides the necessary bulk strength and cost efficiency. Carbonitriding provides the diamond-like surface hardness needed to resist abrasive wear and crushing forces. Finally, the oil-free blackening provides a non-reflective, professional finish that prevents rust during storage and transit without the mess of industrial oils.

In the CNC machining shop, the workflow for such parts is a delicate dance of precision and foresight. Because carbonitriding adds a "case" thickness—usually between 0.1mm and 0.5mm—and blackening is a conversion process with almost zero dimensional change, the machinist must focus on the pre-heat-treatment dimensions. At Tuofa, we machine the 1.0577 steel to a high precision, accounting for the slight thermal expansion that occurs during carbonitriding. If the part is made from hot-rolled material, we often include a "roughing" stage followed by a natural or thermal aging period to allow the material to stabilize before the final precision cuts are made. This extra step is what separates standard machining from high-precision engineering.

In conclusion, 1.0577, S355J2, and Q345D are versatile and powerful alloys that form the backbone of modern structural engineering. While hot-rolled materials present a risk of warping during the machining and hardening phases, these challenges can be overcome with expert process management and stress-relief techniques. By applying carbonitriding, we give these structural steels the surface performance of high-alloy tool steels. And by perfecting the oil-free blackening process, we ensure that the final product is as clean and professional as it is durable.

At Tuofa CNC Machining China, we take pride in our ability to manage these complex manufacturing chains. We don't just see a piece of steel; we see the potential for a high-performance component that can withstand the test of time and stress. Whether you are seeking a reliable equivalent for St52-2 or need a specialized carbonitrided finish on Q345D, our team is equipped to deliver parts that exceed international standards for quality and precision.