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The Role of Chemical Elements in High-speed Tool Steel

1. C:

Mechanism: K formation, solid solution strengthening.

Carbon has a great influence on the hardness of high-speed steel and is the main strengthening element;

C% increased, HRC and hot hardness increased after quenching and tempering.

Disadvantages:

The increase of carbon content increases the total amount of carbides in the steel, and the inhomogeneity of carbides increases →→forging and rolling

Amount of retained austenite after quenching↑→→multiple tempering

Solidus temperature ↓, make quenching temperature ↓

For tungsten series high-speed steel, C%↑ makes the bending strength and toughness of steel↓, and has little effect on tungsten-molybdenum series

2. W:

Existing form:

M6C is the main composition of eutectic carbides, and it also precipitates from austenite as secondary carbides. .

Function: mainly to improve thermal hardness;

It can prevent the growth of austenite grains and improve the toughness of steel;

Improve the tempering stability of martensite;

W2C is precipitated during tempering at 560°C, resulting in dispersion strengthening.

3 .Mo:

Same as W-, mainly to improve thermal hardness;

Tungsten and molybdenum are of the same family and can be substituted for each other, and 1% Mo can replace 1.5% W;

The molybdenum-containing eutectic carbide changes from a coarse fishbone shape to a fine bird’s nest shape, reducing the precipitation of carbides;

After forging, the molybdenum-containing M6C particles are smaller.

★Comparison of performance of tungsten series, molybdenum series and tungsten-molybdenum series:

The hot hardness of molybdenum series high-speed steel Mo18Cr4V is inferior to that of W series; during heat treatment, the possibility of decarburization of molybdenum series is high;

The bending strength and impact toughness of molybdenum series high-speed steel are much higher than those of W series;

Tungsten and molybdenum series high-speed steel W6Mo5Cr4V2 has the advantages of tungsten series and molybdenum series, and overcomes the shortcomings of both.

4. Cr:

Existence form: M23C6 mainly exists, also dissolved in M6C and MC: mainly to improve the hardenability of steel;

It can improve the corrosion resistance and oxidation resistance of steel, and improve the cutting ability.

5. V:

Existence form: mainly exists in VC, but also dissolves in other types of carbides

Function: mainly improve the wear resistance of steel and play an important role in improving the hot hardness of steel;

VC is partially dissolved in A, and M is stable against tempering after quenching; VC is precipitated during tempering to produce secondary hardening.

6. Co:

Function:Mainly improve the hot hardness of steel;

Improve the tempering stability of martensite;

Cobalt is a non-carbide forming element in steel. It dissolves in A during quenching and heating, and dissolves in M after quenching, which slows down the precipitation and aggregation growth of alloy K, and strengthens the secondary hardening effect (increases the starting melting temperature of grain boundaries).

Disadvantages: reduce toughness, increase the tendency to remove C. There are three levels: 5%, 8%, and 12%.

The role of microalloying elements:

Nitrogen:Increase the stability of M6C, reduce aggregation tendency; refine A grains, increase the melting temperature of grain boundaries, improve quenching and alloying.

The dissolved amount of the element increases the quenching and tempering hardness and hot hardness, and at the same time increases the flexural strength and deflection, and improves the toughness.

Rare earth elements: reduce the segregation of sulfur at grain boundaries and improve thermoplasticity.