Introducing vacuum nitriding and vacuum carburizing processes

---

Vacuum nitriding furnace

Nitriding treatment refers to a chemical heat treatment process that infiltrates active nitrogen atoms into the steel surface to form a nitride hardened layer under the premise that the critical point Ac1 of steel is lower than that of steel and the matrix does not undergo phase change. These nitrides have high hardness, thermal stability and high dispersion, so the nitrided steel parts can obtain high surface hardness, wear resistance, fatigue strength, seizure resistance, atmospheric and superheated steam corrosion resistance, tempering softening resistance, and reduce notch sensitivity. For example, the surface hardness of typical nitriding steel 38CrMoA1 can reach 1100~1200HV (equivalent to Rockwell hardness 68~72HRC), the mold life is increased by 1~2 times, or even higher, the fatigue strength of smooth samples is increased by 20%~40%, and the fatigue strength of notched samples is increased by 0.5~1.2 times. The high hardness can be maintained for a long time at 500℃ or for a short time at 600℃.

Generally speaking, all steel materials can be nitrided, but because iron nitrides (such as Fe₄N and Fe₂N) are brittle and unstable, they tend to aggregate and coarsen at slightly higher temperatures, and cannot fully exert the good performance after nitriding. Only low-carbon and medium-carbon alloy structural steels, tool steels, and die steels containing elements such as Cr, Mo, V, Ti, and Al are suitable for nitriding treatment and can achieve satisfactory results. This is because nitrided parts often work under complex working conditions such as friction and variable loads, and both the surface and core performance requirements are very high. Cr, Mo, V, Ti, and Al elements can form stable nitrides with nitrogen, reduce the brittleness of the steel nitriding layer, and obtain a tough surface hardening layer, and can also improve the core structure of the steel, improve the overall strength and toughness of the steel, and enable these steels to fully exert the excellent performance obtained by nitriding treatment.

Advantages of vacuum nitriding process

Vacuum nitriding is a chemical process that uses pulses to penetrate active gas N atoms into the metal surface under low vacuum (several kilopascals to tens of kilopascals) and low temperature (520~550℃, 650℃ for corrosion resistance treatment) to obtain higher surface hardness or corrosion resistance. It is a process that combines vacuum heat treatment with gas nitriding (nitriding gas is NH3). In addition to inheriting the common advantages of vacuum heat treatment, vacuum nitriding has a shorter process time and a stronger bond between the nitrided layer and the matrix than gas nitriding.

Vacuum carburizing furnace

Carburizing treatment refers to a chemical treatment process in which carbon atoms are penetrated into the surface layer of steel in the complete austenite phase region of steel (generally 900~950℃) to make the carbon content of the surface layer of steel reach the level of high carbon steel. The depth of the carburized layer generally ranges from 0.8~1.2mm, and can reach 2mm or deeper during deep carburizing. After vacuum quenching and low temperature tempering, the microstructure of the surface with high carbon content (mass fraction of C is 0.85%~1.05%) is high hardness martensite residual austenite and a small amount of carbide, with a hardness of 58~63HRC, and the core structure with low carbon content is low carbon martensite or non-martensite structure with good toughness (the proportion of ferrite should be strictly limited), with a hardness of 30~42HRC. The steel suitable for carburizing is low carbon steel, including low carbon carbon steel and low carbon alloy steel, with a carbon content of 0.15%~0.25%. For carburized workpieces under heavy load, the carbon content can be increased to 0.25%~0.30%. In this way, the workpiece surface can have high hardness and high wear resistance through carburizing, and the core of the workpiece can still maintain the toughness of low carbon steel, so that the workpiece can withstand large impact loads, and carburizing produces compressive stress on the workpiece surface, which greatly improves the fatigue strength of the workpiece. The most commonly used carbon carburizing steels are 15 steel and 20 steel. Due to their low hardenability, they are only suitable for small parts with low core strength requirements, small forces, and wear, such as bushings and chains.

Application of vacuum carburizing

Low alloy carburizing steels, such as 20Cr, 20Cr₂MnVB, 20Mn₂TiB, etc., have higher permeability and core strength than carbon carburizing steels, and can be used to manufacture more important carburized parts in general machinery, such as gears and piston pins in automobiles and tractors. Medium alloy carburizing steels, such as 20Cr₂Ni₄, 18Cr₂Ni₄W, 15Si₃MoWV, etc., have higher hardenability, strength and toughness, and are mainly used to manufacture parts with larger cross-sections, heavier loads, and complex forces, such as gears and shafts of aircraft engines. Alloying elements such as manganese, chromium, nickel, molybdenum, tungsten, vanadium, and boron can refine grains, affect the saturated carbon concentration, thickness and hardness of the carburized layer, and improve the carburizing permeability of steel.

Advantages of vacuum carburizing process

Although gas carburizing is still the mainstream process of carburizing treatment, and vacuum carburizing itself has many technical problems to be solved, the advantages of vacuum carburizing, such as no oxidation in the grain boundary, bright surface, smaller deformation, energy saving and environmental protection, make it represent the development direction of carburizing treatment. Acetylene (C₂H₂) as a carburizing medium has largely solved the carbon black and tar pollution problems caused by propane, bringing hope for the large-scale commercial application of vacuum carburizing.

Vacuum carburizing is a chemical process in which active gas C atoms are infiltrated into the metal surface in a pulsed manner under low vacuum (500-1500Pa, up to 2000Pa) and high temperature (950~1050℃) conditions to obtain a higher surface hardness. Unlike vacuum nitriding, the high surface hardness of vacuum carburizing requires vacuum quenching to achieve.