Alloying Elements in Tool Steels for Molds

Mold steel is a type of tool steel used to manufacture various forming tools, including cold work, hot work, and plastic mold steels.

What elements are used to manufacture mold steel?

Major Alloying Elements: Carbon (C), Silicon (Si), Manganese (Mn), Phosphorus (P), Sulfur (S), Nickel (Ni), Chromium (Cr), Molybdenum (Mo), Aluminum (Al), Tungsten (W), Vanadium (V), Cobalt (Co).
Minor Alloying Elements: Titanium (Ti), Niobium (Nb), Copper (Cu), Aluminum (Al).
Non-metallic Elements: Nitrogen (N), Boron (B).

Effects of Various Alloying Elements on Mold Steel

1.Carbon (C)
Main alloying element, increases hardenability and wear resistance. Forms carbides with other elements during heat treatment, which improves hardness. Excess carbon can cause carbide coarsening, negatively impacting toughness, machinability, and weldability.

2.Silicon (Si)
Advantages: Improves hardenability and wear resistance, enhances elasticity.
Disadvantages: Reduces electrical conductivity, toughness, thermal conductivity, and polishing ability.

3.Manganese (Mn)
Advantages: Helps remove oxygen during smelting and combines with sulfur to improve machinability, increases yield strength and tensile strength.

4.Phosphorus (P)
Disadvantages: Can cause segregation during solidification and lead to brittleness and poor toughness.

Non-negative effects: In Austenitic stainless steel, phosphorus can improve yield strength and assist in the precipitation-hardening process with nickel and chromium.

5.Sulfur (S)
Sulfur in steel forms iron sulfide (FeS), which can lead to segregation during solidification of steel ingots, particularly affecting copper alloyed steels. This segregation causes a network of sulfides that encircle grain boundaries during hot forging, resulting in a weakened structure. Sulfur combines with manganese to form manganese sulfide (MnS), which is considered an impurity, also reducing toughness and poor weldability, increasing the likelihood of cracks. Additional issues caused by sulfur include poor polishability, resulting in suboptimal mirror finishes; uneven etching, leading to poor texturing and surface flaws like hairline patterns on the mold surface; negative impact on coatings, such as hard chrome plating and electroless nickel plating, reducing the effectiveness of the surface treatments.

6.Chromium (Cr)
Chromium enhances the hardenability of steel, especially through oil or air cooling, which helps form a martensitic structure. However, if the martensite content becomes too high, it can negatively affect impact strength (toughness). Chromium also combines with carbon to form chromium carbide (M7C3), improving wear resistance and increasing toughness, while offering resistance to hydrogen embrittlement. In steels containing more than 13% chromium (Gr) (like stainless steels), it imparts corrosion resistance. However, excessive chromium content can reduce thermal conductivity, electrical conductivity, polishability, and affect the effectiveness of electrical discharge machining (EDM) and chemical etching.

7.Nickel (Ni)
Nickel does not form carbides, making it a single-phase alloy element that improves toughness, corrosion resistance, polishability at high temperatures, specifically above 600°C, where it provides resistance to oxidation and maintains strength and ductility. However, nickel-containing steels typically exhibit poor machinability, leading to challenges like tool sticking and inefficient chip removal during cutting. Nickel also contributes to low thermal expansion and poor thermal conductivity.

8.Molybdenum (Mo)
Molybdenum typically participates in solid-solution strengthening with other alloy elements, forming alloy carbides (M6C), which enhance the base hardness and improve hardenability. In hot-work steels, molybdenum boosts the resistance to tempering softening, corrosion resistance, and provides high-temperature heat and erosion resistance. It also improves resistance to temper embrittlement and increases both yield strength and tensile strength. In HSS (e.g., M-35, M-42, M-45, M-50, M-52), molybdenum significantly enhances cutting performance and high-temperature strength, making these alloys suitable for high-stress, high-heat applications.

9.Vanadium (V)
Vanadium is typically added during secondary refining and plays a key role in inhibiting grain growth during the solidification of steel ingots. It strengthens carbide formation and, during subsequent heat treatment, enhances the solubility of vanadium carbides, preventing grain coarsening. This results in optimal hardening ability.
Vanadium carbide has a hardness of HV 2600–3200 and offers excellent resistance to adhesive and abrasive wear. It also improves the resistance to temper softening and provides high toughness at the cutting edge of tools, reducing the likelihood of chipping.

10.Tungsten (W)
Tungsten is a crucial element for forming carbides, with tungsten carbide (MC) reaching a hardness of HV 2250–3200. It enhances hardenability, red hardness, high-temperature strength, and resistance to temper softening. Tungsten is widely used in hot-work and high-speed steels and exhibits strong magnetic properties, making it useful in magnetic materials.

11.Cobalt (Co)
Cobalt does not participate in the formation of carbides, making it unique. It inhibits grain growth at high temperatures, maintains hardness and strength under heat, and resists wear at high temperatures. Cobalt also improves hardening ability, base hardness, and creep strength. Its high magnetic saturation and thermal conductivity make it ideal for high-grade magnetic materials and alloys.

12.Niobium (Nb)
Niobium enhances carbide formation, strengthens the base hardness, and offers strong resistance to chemical corrosion. It increases high-temperature strength, creep resistance, fracture toughness, and wear resistance. Recently, small amounts of niobium have been added to cold-work tool steels to improve mechanical properties.