Heat Treatment of 25CrNi3MoAl Steel

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25CrNi3MoAl steel is a low-nickel and cobalt-free age-hardening steel. This is a new type of age-hardening steel developed by referring to the composition of similar foreign steels and improving the characteristics of my country’s smelting industry and the performance requirements of the users. The special steel grade for hardened precision plastic molds fills the gap.

1. Chemical Composition and Critical Phase Transition Point.

The chemical composition of 25CrNi3MoAl steel is shown in Table 1.

Tab 1 Chemical composition of 25CrNi3MoAl steel (mass fraction, %)

Critical phase transition point: Ac 1740℃, Ac3 780CC, Ms 290℃.

2. Mechanical Properties

Hardness: The hardness of 25CrNi3MoAl steel after solid solution and aging treatment at different temperatures is shown in Table 2 and Table 3, respectively.

Tab 2 Hardness of 25CrNi3MoAl steel after solution treatment at different temperatures

Tab 3 Hardness of 25CrNBMoAl steel after aging treatment

Mechanical properties at room temperature: The mechanical properties of 25CrNi3MoAl steel after solid solution at 880°C, tempered at 680°C, and aging at 540°C for 8 hours are shown in Table 4.

Tab 4 Room temperature mechanical properties of 25CrNi3MoAl steel

3. Heat Treatment Process

Used as a general precision plastic mold: quenching heating temperature 880℃, air cooling or water cooling quenching, quenching hardness 48~50HRC, then 680℃, 4~6h high temperature tempering, air cooling or water cooling, tempering hardness 22~23HRC, by machine Processing and forming. After aging treatment, the aging temperature is 520~540℃, heat preservation is 6~8h, air cooling, the aging hardness is 39~42HRC. After grinding, polishing or photo-etching patterns, it is assembled and used. The aging deformation rate is about -0.039%.

For high-precision plastic molds: the quenching heating temperature is 880℃, and then the high temperature tempering is 680℃, and the rest of the process is the same as above. However, after high temperature tempering, rough machining and semi-finishing of the mold are carried out, and then kept at 650°C for 1 hour to eliminate the residual internal stress after processing, and then finish machining. The subsequent aging, grinding, polishing and other processes remain the same as above. After this treatment, the aging deformation rate is only -(0.01%~0.02)%.

Used for plastic molds that do not require high impact toughness: directly rough and finish the annealed forging, and perform 520~540℃ , 6-8h aging treatment, then grinding, polishing and assembling use. After this treatment, the hardness of the mold is 40～43HRC, and the aging deformation rate is ≤0.05%.

Plastic mold used as a cold extrusion cavity process: After the mold forging is softened, the extrusion surface of the mold is processed, ground, and polished . Then the cold extrusion mold cavity and mold shape are trimmed, and finally the mold is subjected to vacuum aging treatment or surface nitriding treatment before assembling and using.

4. Features and Applications

25CrNi3MoAl steel has the following characteristics:

The steel contains low nickel content, and the price is much lower than maraging steel, and also lower than ultra-low carbon medium alloy aging steel.

Quenching and tempering hardness is 230~250HBW, conventional cutting processing and electrical processing The performance is good. The aging hardness is 38~42HRC, the temperature range of aging treatment and nitriding treatment is equivalent, and the nitriding performance is good. After nitriding, the surface hardness is above 1000HV, while the core hardness is maintained at 38~42HRC.

The mirror surface is good and the surface is rough. The degree of Ra can be 0.02~0.025μm, the surface lithography is good, and the lithography pattern is clear and uniform.

The welding repairability is good, the weld can be processed, and the hardness of the weld after aging is similar to the hardness of the substrate.

25CrNi3MoAl steel can be used to make ordinary and high-precision plastic molds. After trial, the technical and economic benefits are significant.