What is the difference between cold-working die steel and hot-working die steel? Why?

1, different carbon content: the carbon content of cold-working die steel is generally1.45% ~ 2.30%; The carbon content of hot-working die steel is 0.3% ~ 0.6%;

3. Different chromium content: the chromium content of cold-working die steel is11%~13%; The chromium content of hot working die steel varies with the properties of alloy steel.

3. The addition of other elements is not exactly the same: multi-element alloy steel, containing carbide-forming elements, such as Cr, Mo, W, V, etc. , mostly used for cold-working die steel; The alloying elements added to hot working die steel are chromium, manganese, silicon, nickel, tungsten, molybdenum, vanadium and other alloying elements.

Second, the reason:

1. The alloying elements Cr, Mn, Si and Ni in hot work die steel are used to strengthen ferrite and improve hardenability, while the alloying elements W and Mo are used to prevent temper brittleness. The alloying elements Cr, W and Si can increase the phase transition temperature, so that the mold does not undergo phase transition during alternating heating and cooling, thus improving its thermal fatigue resistance.

2. Cold working die steel is usually composed of high carbon to meet the needs of high hardness and high wear resistance. If it is necessary to increase the toughness in order to improve the impact resistance of the die, medium carbon steel can be selected or hot-working die steel can be used instead. The main purpose of adding alloying elements to cold-working die steel is to improve hardenability and wear resistance. For dies with high wear resistance, multi-element alloy steel containing carbide forming elements, such as Cr, Mo, W and V, is used. , often used.

Extended data

1. Because the hot working die has been working at high temperature and high pressure for a long time, the die material is required to have high strength, hardness and thermal stability, especially high thermal strength, thermal fatigue, toughness and wear resistance.

Second, the hot working die bears great impact when working, and the cavity is in contact with high-temperature metal, which is repeatedly heated and cooled, and the use conditions are extremely bad. In order to meet the requirements of hot-working die steel, hot-working die steel should have the following basic characteristics:

(1) High temperature strength and good toughness. Hot working dies, especially hot forging dies, bear great impact when working, and the impact frequency is very high. If molds do not have high strength and good toughness, they are easy to break.

(2) Good wear resistance. Hot-working die steel is required to bear not only friction and wear, but also high-temperature oxidation corrosion and grinding of iron oxide chips when the blank is deformed, so it is required to have high hardness and adhesion resistance.

(3) High thermal stability. Thermal stability refers to the ability of steel to maintain its mechanical properties at room temperature for a long time at high temperature. When hot working dies work, they contact hot metal, even liquid metal, so the surface temperature of dies is very high, generally 400 ~ 700℃. This requires that the hot-working die steel does not heat up at high temperature and has high thermal stability, otherwise the die will be plastically deformed, leading to collapse and failure.

(4) Excellent thermal fatigue resistance. The working characteristics of hot working die are repeated heating and cooling, and the die expands with heat and contracts with cold, resulting in great thermal stress, which is generated alternately in opposite directions. Under the action of repeated thermal stress, mesh cracks (cracks) will form on the surface of the mold, which is called thermal fatigue. The premature fracture of die caused by thermal fatigue is one of the main reasons for the failure of hot working die. Therefore, hot-working die steel must have good thermal fatigue performance.

(5) High hardenability. Hot working dies are generally larger in size, especially hot forging dies. In order to make the mechanical properties of the whole die section uniform, hot-working die steel is required to have high hardenability.

(6) Good thermal conductivity. In order to prevent the die from accumulating too much heat, which leads to the decrease of mechanical properties, it is necessary to reduce the temperature difference between the die surface and the die interior as much as possible, which requires the hot-working die steel to have good thermal conductivity.

(7) Good processability to meet the needs of processing and molding.

References:

Baidu encyclopedia-hot work mold steel