Several new technologies in the development of heat treatment

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Since its establishment, China North Industries Group Corporation has seen its heat treatment processes evolve from outdated methods to more advanced ones, shifting from manual operations to mechanized, semi-automated, and partially automated processes. The process parameters and product quality have also improved, moving from being incompletely controllable to fully controllable. The introduction of digital simulation technology is also part of the plans, and it is expected to be implemented in the near future. Along with the current development of the industry, our company’s heat treatment technology has also undergone tremendous changes, yet shortcomings still exist. “During the 12th Five-Year Plan period and beyond, our company’s heat treatment technology must be continuously improved and developed to reach advanced levels both domestically and internationally. 1 Surface strengthening technologies　1.1 Surface cladding technology Surface cladding technology utilizes lasers, electromagnetic induction, or plasma arcs to heat self-fluxing alloy powders on the surface of a workpiece to a semi-molten state. This results in the formation of a semi-metallurgical bonding layer with the surface layer of the workpiece, thereby creating a surface cladding layer that exhibits good wear resistance, oxidation resistance, heat resistance, low-stress resistance, and impact resistance. Commonly used deposit layers include nickel-based, cobalt-based, copper-based ones, as well as those containing WC-type powders, with a hardness of 35–67 HRC. When depositing on a CNC machine, a deposit layer with uniform thickness can be obtained, and grinding is generally not required for surfaces with non-mating dimensions. 1.2 Laser-induced amorphous treatment: The metal surface is heated to a molten state using a laser, and then rapidly cooled (1×106°C/s) below a certain temperature, causing the metal to assume an amorphous structure. Amorphous materials have no grain boundaries; during crystallization, there is no segregation of components, and they achieve a high hardness, exceeding 1000 HV ； At the same time, it also has high corrosion resistance and wear resistance. This technology is widely used for laser-induced amorphous treatment of automotive cam shafts and diesel engine cast steel sleeves, among other applications. 1.3 Laser shock peening technology. Laser shock peening technology, also known as laser peening, involves the use of a laser with high power density (on the order of GW/cm²) and short pulses (10–30 ns). When this laser strikes the black paint and aluminum foil coating on the surface of a workpiece, it generates a high-pressure plasma and high-pressure shock waves. These, in turn, cause plastic deformation on the metal surface, ultimately resulting in residual compressive stress. This significantly improves the fatigue resistance of the metal surface. The corrosion-fatigue life of 7075-T6 aluminum alloy components after laser shock treatment can be increased by two orders of magnitude, while the strength of Ti6A14V compressor blades increases from 206 MPa to 412 MPa following laser shock treatment. 1.4 Surface mechanical strengthening techniques involve using methods such as mechanical rolling and shot blasting on the surface of materials or workpieces to create a layer of residual compressive stress. These techniques are commonly used for surface shot blasting and rolling strengthening of shafts, shot blasting strengthening of springs and gears, rolling strengthening of threads and spline teeth, torsional + surface shot blasting strengthening of torque shafts, as well as work hardening strengthening of austenitic steels. 1.5 Digital Technology for Surface Heat Treatment Strengthening: As software for heat treatment processes and computer technology are increasingly applied in engineering contexts, digital technologies for surface heat treatment strengthening are being used more and more widely. These include electromagnetic induction quenching and tempering, laser surface quenching and tempering, electron beam surface quenching and tempering, laser shock surface strengthening, as well as surface heat treatments such as carburizing, nitriding, and carbonitriding. This technology not only enables the optimization of heat treatment process parameters through simulation, but also allows for digital control of the process and quality characteristics. 2 New Process Technologies 2.1 Application of Low-Vacuum Variable-Pressure Nitriding Technology Generally, nitriding is carried out under constant furnace gas and furnace pressure conditions, which makes it difficult to nitride small blind holes. The variable pressure gas exchange method of vacuum pumping – negative pressure – pressure holding – gas injection – pressure holding – pumping is employed, which enhances the activity of nitrogen on the workpiece surface as well as its adsorption capacity for nitrogen, thereby accelerating the penetration rate of the nitrided layer. Coupled with the use of a rare earth catalyst for acceleration of nitriding, the nitridation time is further reduced. 2.2 New process of ion nitriding + oxidation composite treatment (an improvement on the soft nitriding process) This process first involves ion nitriding to create a hardened layer, followed by one round of ion oxidation treatment in an ion furnace. This approach not only solves the problem of environmental contamination caused by cyanides in the salt bath used in QPQ treatment but also enhances the wear and corrosion resistance of the parts. The process of gas nitridation combined with conventional chemical oxidation can be tried to improve the wear and corrosion resistance of the nitrided layer. Studies have shown that oil cooling the vast majority of parts after nitriding has no effect on deformation, while effectively improving the wear and corrosion resistance of the nitrided layer. 2.3 Liquid-equivalent ion electrolytic carburizing and nitriding technologies: Similar to conventional electrolytic techniques, the metal to be treated is used as the cathode, while graphite or stainless steel serves as the anode. Organic compounds containing C and N, such as methanolamine and urea, are added to the NaCl electrolyte. A sufficiently high direct current is applied so that positive ions accumulate at the cathode, resulting in the formation of carbon-rich layers with a nanoscale size of 50–70 nm. This allows the hardness of 20# steel to reach 1200 HV and that of H13 steel to reach 870 HV, with the entire process taking only about 25 minutes. 2.4 Plasma-enhanced electrochemical surface ceramization technique: By connecting colored alloy parts made of Al, Ti, Mg, and Zr to the anode of an electrolyte and applying a high voltage, a micro-plasma arc discharge is generated on the surface of the parts. This discharge is sufficient to melt and sinter substances such as the oxide film formed on the part’s surface as well as the electrolyte in the interfacial layer, thereby creating a layer of Al2O3 ceramic oxide with excellent properties and high hardness. The fiber hardness ranges from 1,000 to 3,000 HV, which is comparable to that of cemented carbide, and it boasts excellent wear resistance, corrosion resistance, and high-temperature resistance. 2.5 Application of the reversion treatment process (RRA process): For the solution-treated Al-Zn-Mg-Cu-based 7055-T77 aluminum alloy, during aging, as the aging temperature rises, GP zones and alloy phases remelt. Within a short period after reaching the reversion temperature, its strength decreases while its corrosion resistance increases; thus, it can undergo rapid correction and deformation processing ； At the peak aging stage of T6, the density of GP regions within the crystal is higher, resulting in greater strength. The correction of Al-Mg-Cu series cast aluminum products in our company can also be carried out using regression analysis; this method has become the recommended technique for further enhancing the strength of aluminum alloys through heat treatment. 3 Gear heat treatment deformation control technology 1) Internal stresses exist in gears during cutting processes; although stress-relief annealing can release these stresses and allow the distorted lattice structures within the metal to return to their normal state ； It is worth noting, however, that the response process can also cause deformation. For gears that are prone to deformation before carburizing and quenching, performing multiple stress-relief finishing steps before quenching results in less deformation compared to quenching them directly after a single stress-relief process. 2) The control measures taken to address the deformation of gears during quenching are to ensure uniform heating and uniform cooling. The heating furnace is equipped with a stirring fan; the heating process should be slow. When the thickness of the structure is uneven, measures such as cooling the thicker areas first or adding thickening sleeves to the thinner areas should be taken. If gas quenching can be used for the cooling process, oil quenching will not be employed ； If nitrate quenching can be used, oil quenching will not be employed ； If quenching with a water-based quenchant is possible, oil cooling is not used. 4 Technologies to Improve Mold Life Currently, the areas where heat treatment technologies for molds are developing rapidly at home and abroad are vacuum heat treatment, surface strengthening, and material pre-hardening techniques. Vacuum heat treatment is mainly used to address the quenching deformation of molds ； Surface strengthening involves various methods such as nitriding, boriding plus quenching, electron beam surface hardening, laser surface hardening, and surface ion implantation (in which the mold and substances such as C, N, Ti, and B to be implanted are placed in a vacuum chamber; under the influence of a high-voltage electric field, these substances are transformed into high-speed ions that are injected into the surface of the mold, thereby forming highly dispersed wear-resistant layers such as TiC and TiN) ； Material pre-hardening technology refers to the process of subjecting molds to mechanical rolling and shot peening after quenching. 5 Energy-saving and clean technologies for heat treatment of large forgings 5.1 Improvements to heating furnaces and loading methods The forgings produced by a major forging factory in China weigh between ten tons and hundreds of tons; the depth of these heat treatment furnaces is greater than 20 meters, and the temperature variation within the furnace is maintained at ±5°C. The stress level in large rotors can reach up to 10 MPa. To overcome the limitations imposed by the plant height and uneven furnace temperature, the furnace adopts an open-and-close shaft furnace, as shown in Figure 1. The improvements to the loading method are as follows: 1) The doors on the side of the furnace are foldable, and the workpieces are lifted in from the side ； 2) The workpiece is supported by a base instead of being hung, with the flange located above the furnace top ； 3) The lifting device is not heated; a specialized cold lifting device is used when removing the workpiece from the furnace, followed by quenching. This method eliminates the need to heat large lifting devices, reduces the mass of the fixtures used to hold the workpiece, effectively solves the problem of heating large forgings during heat treatment, and offers significant energy savings. 5.2 Changes in the cooling methods for large forgings Typically, the cooling media used for large forgings include water and oil. To address the problems of oil quenching causing contamination in high-alloy steels and cracking that occurs during water quenching, spray or mist quenching is employed; the spraying speed of water or mist can be controlled as needed, which is a key advantage that enables precise control over the cooling rate during the quenching of large forgings. 5.3 Hydrogen diffusion annealing: Among the annealing processes for large forgings, hydrogen diffusion annealing is the one that takes the longest time. In previous non-vacuum methods, the hydrogen content in the molten steel was greater than 2×10-6, and the time required for hydrogen diffusion below the critical point was generally several hundred to several thousand hours. After vacuum degassing, the hydrogen content in the molten steel is kept at around 1×10-6. Most large carbon steel forgings do not require hydrogen expansion treatment; ordinary annealing is sufficient to address the issue. 6 Amorphous materials: High-strength, tough, corrosion-resistant, wear-resistant, and soft-magnetic amorphous alloys (commonly known as metal glasses) have an atomic structure similar to that of glass, being generally disordered. To enhance their strength and hardness and to achieve high toughness and soft magnetism, elements such as P, C, B, Zr, and Ti are added to improve their properties, resulting in higher strength, hardness, and fracture toughness compared to metals. When the molten steel is rapidly cooled to the freezing point (near the melting point of glass) at a rate of 1×105°C/s, it remains in a liquid state. As the temperature drops, the molten steel gradually solidifies, taking on an amorphous structure. It can be seen that only by achieving such a high cooling rate, and in liquid nitrogen at < -100°C, can a bulk amorphous structure be obtained. By utilizing the principles of amorphous metal production, ultra-high strength nanocrystalline amorphous aluminum alloy materials have been developed and are now used in the aerospace industry. Their tensile strength can reach 1000–1500 MPa, which is far higher than that of super-hard aluminum alloy materials and comparable to that of titanium alloy materials. Moreover, their recovery time is longer, allowing for the shaping and repair of components. 7 Carbon fibers and reinforced carbon-based composites: Carbon-carbon composites are composites in which carbon fibers serve as reinforcing agents while carbon acts as the matrix, and extensive application research on them has been conducted both domestically and internationally. The gun barrel is made of carbon-carbon composite material with a ceramic lining formed by winding, which reduces its weight by 30% to 50%. For armor protection, lightweight composite armor is used, with an armor-piercing capacity of up to 700 mm, which significantly reduces the weight of the turret. 8 Conclusion The degree of adoption of plasma oxidation-free heat treatment is a key indicator to measure the level of advancement in heat treatment in a company or region. Using vacuum, induction, laser, multi-purpose furnace heat treatment, as well as gas quenching and pressure quenching are effective methods to reduce heat treatment distortion in workpieces and promote environmental sustainability. The adoption of digital heat treatment technology is an emerging trend, enabling precise control over heat treatment parameters and quality. Utilizing isothermal annealing of the residual heat in cast and forged parts, quenching, as well as the recovery of residual heat, along with deformation reduction and post-correction tempering, are effective ways to save energy in heat treatment. The widespread use of high-strength or ultra-high-strength functional materials is an important means to achieve component lightweighting. The widespread use of surface quenching, surface coating, surface alloying, as well as surface ion implantation and plating techniques provides technical support for improving the wear and corrosion resistance of workpieces.