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Carburizing heat treatment process of alloy steel

1 Carburizing technology of mold

After carburizing, quenching and low-temperature tempering, the mold can obtain high surface hardness, wear resistance, contact fatigue strength and bending fatigue strength, while the core still maintains good toughness. Therefore, the carburizing process is mainly used in molds that are subjected to severe wear and large impact loads at the same time. For cold work molds, hot work molds and plastic molds, carburization can play a role in increasing the life of the mold. Gas carburizing and ion carburizing are commonly used for molds.

Both medium and high carbon low alloy mold steels and high alloy steels can be carburized or carbonitrided. For example, after carburization of W6Mo5Cr4V2 and 6Cr4W3Mo2VNb (65Nb) steel molds, the service life is significantly improved.

01.Mold gas carburizing technology

It is a process that uses liquid or gaseous hydrocarbons as carburizing agents for carburizing, among which drip gas carburizing is the most widely used. Gas carburizing is mostly used for medium and small molds that can withstand large impact loads, high strength and hardness of 58 to 62HRC.

02.Mold ion carburizing technology

Put the mold into the vacuum furnace and start to evacuate. When the vacuum degree in the furnace is 13~1.3Pa, fill the furnace with methane gas and the internal pressure is 1300~2600Pa. Glow discharge is stimulated between the anode and cathode and infiltration is carried out. Carbon, the voltage at this time is 500~800V. Ion carburizing has a deeper carburizing layer and higher surface hardness than vacuum carburizing and atmospheric pressure carburizing.

03.CD carburizing technology for molds

That is, the carbide dispersion carburizing method is to heat mold steel containing a large amount of strong carbide forming elements (such as Cr, Ti, Mo, V) in a carburizing atmosphere. While carbon atoms diffuse from the surface to the interior, the carburized layer A large number of dispersed alloy carbides are deposited in the mold, which are finely and evenly distributed. After quenching and tempering, the mold surface can obtain high hardness, wear resistance and anti-seize properties.

04.Application of carburizing technology

Carburizing is mainly used for small molds and cold-formed plastic molds that can withstand large impact loads and require high strength and good crack resistance.

2 Carbonitriding technology of mold

Compared with single carburizing, carbonitriding has many advantages, mainly in improving the surface hardness, wear resistance and fatigue strength of the mold. Moreover, since the carbonitriding temperature is lower than the carburizing temperature, it can generally be Direct quenching and reducing mold distortion. Medium temperature gas carbonitriding is commonly used for molds. The depth of carbonitriding layer can reach 0.6~0.7mm. Due to the increased carbon and nitrogen content, the strength of the infiltration layer is increased, the compressive strength of the base material is increased, the friction factor is reduced, and the toughness and thermal conductivity of the core are maintained, so that the mold has good overall performance.

3.Nitriding technology of mold

Compared with carburizing, mold nitriding can achieve higher fatigue strength, wear resistance, hot hardness, seizure resistance, lower notch sensitivity, and good corrosion resistance than carburizing. In addition, the nitriding temperature is low (500~570℃), no heat treatment is required after nitriding, and the mold distortion is very small. Commonly used nitrided steels for molds include 38CrMoAlA, Cr12, Cr12MoV, 3Cr2W8V, 5CrNiMo, 4Cr5MoSiV, etc. Molds are usually tempered before nitriding. Nitriding is suitable for molds with higher precision requirements. Since the nitriding layer is very thin, nitriding is only suitable for lightly loaded molds. Gas nitriding, ion nitriding, etc. are commonly used for molds. Alloy steel molds tempered at high temperatures of 500 to 650°C can be surface nitrided or nitrocarburized at a temperature lower than the tempering temperature or while tempering.

01.Mold gas nitriding technology

Gas nitriding is a chemical heat treatment of nitriding in a gas medium. After the mold is gas nitrided, the surface hardness can reach 950~1200HV. Nitriding can make the mold have high thermal hardness and high fatigue strength, improve the anti-seizure ability of the mold, and reduce the surface roughness of the mold. Molds commonly use gas isothermal nitriding (one-stage nitriding), gas two-stage nitriding, etc.

02.Mold ion nitriding technology

Ion nitriding is a chemical heat treatment of nitriding using the glow discharge generated between the mold (cathode) and the anode under a certain vacuum, so it is also called glow ion nitriding. Advantages of ion nitriding: fast infiltration speed, 3 to 4 times that of gas nitriding; nitriding layer has certain toughness; mold distortion after nitriding is small; surface is silvery white, good quality; strong adaptability to materials, carbon steel , cast iron, alloy steel, etc. can be ion nitrided.

Ion nitriding can improve the wear resistance, corrosion resistance, thermal fatigue resistance and adhesion resistance of the mold. It is widely used in hot forging dies, hot extrusion dies, die-casting dies, cold extrusion dies, etc. that are subject to small impact loads. Blanking dies, etc., can significantly improve the life of the mold.

4.Nitrocarburizing technology of mold

Nitrocarburizing is a chemical heat treatment based on nitriding at a temperature of 500 to 600°C. Nitrocarburizing has the characteristics of low processing temperature, short time and small distortion. The nitrocarburizing mold has certain heat resistance, wear resistance, corrosion resistance, anti-seizure performance and low friction factor, and can be used for hardening requirements. Cold work molds and hot work molds with thin layers and small loads. Nitrocarburizing has good application effects on cold heading dies, cold extrusion dies, punching dies and drawing dies. After nitrocarburizing die-casting molds and hot extrusion molds, the thermal fatigue performance and lifespan can be significantly improved.

Commonly used gas nitrocarburizing process: co-penetrating temperature is 570~600℃, holding time is 3~5h, co-penetrating media include: ammonia + ethanol, urea, formamide, etc.