Vacuum Carburizing Process of 20CrMnTi Steel

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20CrMnTi steel is a carburizing plastic mold steel with good performance in carburizing and vacuum heat treatment. It has a fine-grained structure below 960 °C. When heated for a long time at a commonly used carburizing temperature, the grains have no tendency to grow. There is very little retained austenite, so it has high strength and wear resistance, good cutting performance, and the main properties are similar to 20CrMnTi and 20CrNi steel.

Tensile strength σb≥1080MPa, yield strength σs≥885MPa, elongation δ5≥10%, area shrinkage φ≥45%, impact energy Aku≥55J, impact toughness value αku≥69J/cm2, hardness≤207HB, sample The blank size is 15mm.

Chemical composition (mass fraction, %) C 0.17～0.23, Si 0.17～0.37, Mn 0.80～1.10, P 0.030, S 0.013, Cr 1.00～1.30, Ti 0.04～0.10, Cu 0.09.

Critical point temperature (approximate value) Ms=374℃.

Quenching and tempering specification Austenitizing temperature 880℃, time 25min, grain size 7～8.

The process route of steel is blanking→forging die blank→vacuum annealing→mechanical roughing→cold extrusion forming→recrystallization annealing→mechanical finishing→vacuum carburizing→vacuum quenching, vacuum tempering→grinding and polishing→assembly.

Isothermal normalizing is adopted for the gear blank of 20CrMnTi steel containing Mo, which can eliminate the granular bainite structure and reduce the hardness of the blank after normalizing, which can fully meet the machining requirements. It can be seen from the results of the isothermal normalizing test that this process has the characteristics of wide process range and stable process. Compared with 20CrMnTi steel, the isothermal normalizing temperature is 550-650℃ and the holding time is more than 30min, both pearlite and iron can be obtained. The body has a mixed structure, and the hardness is between 160 and 170HBS. Apparently this process contributes to the wider use of Mo-containing 20CrMnTi steel.

In conventional normalizing production, not 100% granular bainite appears, which brings great difficulties to machining. According to the production statistics of automobile reverse gear, planetary gear and side gear, about 40% of them have this problem.

Typical application example

①For small precision cavity inserts, carburizing can be used to increase surface hardness and improve wear resistance.

② Moulds with larger wear, larger loads and larger production batches.

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