die steel
Appropriate requirements for hardness, focusing on
Yuhong
It's hard,
conductivity for heat
Wear resistance. Therefore, the carbon content is low,
alloying agent
increase
hardenability
, improve wear resistance and red hardness.
Hot working die steel
1. Working conditions of hot working dies
The hot working mold includes
hammer forging
Die, hot extrusion die and
Compression mould
Three categories. As mentioned above, the main feature of the working conditions of hot working dies is contact with hot metal, which is the main difference from the working conditions of cold working dies. Therefore, it will bring the following two problems:
(l) Heating the surface metal of the mold cavity. Usually, when the hammer forging die works, the surface temperature of the die cavity can reach above 300 ~ 400℃, and the hot extrusion die can reach above 500 ~ 800℃. The cavity temperature of die casting mold is related to the type of die casting material and pouring temperature. Such as die casting
black metal
The cavity temperature can reach above 65438 0000℃. Such a high service temperature will make the mold cavity
surface hardness
And is easy to accumulate in use. Therefore, the basic performance requirement of hot working die steel is high thermoplastic deformation resistance, including high temperature hardness and high temperature strength, which actually reflects the high thermoplastic deformation resistance of steel.
Tempering stability
. Therefore, hot die steel can be found.
Alloy method
The first way, that is, adding alloying elements such as Cr, W and Si, can improve the tempering stability of steel.
(2) Metal generation on the surface of mold cavity
heat fatigue
(crack). The working characteristics of hot die are intermittent. Every time hot metal is formed, the surface of the mold cavity should be cooled by water, oil, air and other media. Therefore, the working state of the hot die is repeated heating and cooling, thus repeatedly producing the surface metal of the die cavity.
Expand while it is hot
That is, repeatedly subjected to tensile and compressive stress. As a result, cracks appear on the surface of the mold cavity, which is called thermal fatigue. Therefore, the second basic performance requirement for hot-working die steel is high thermal fatigue resistance. Generally speaking, the main factors affecting the thermal fatigue resistance of steel are:
① Thermal conductivity of steel. The high thermal conductivity of steel can reduce the heating degree of metal on the die surface, thus reducing the thermal fatigue tendency of steel. It is generally believed that the thermal conductivity of steel is related to carbon content, and high carbon content and low thermal conductivity are not suitable for hot-working die steel.
high-carbon steel
. Usually used in production.
medium carbon steel
Low carbon content (C 0.3% 5 ~ 0.6%) will lead to the decrease of hardness and strength of steel, which is also unfavorable.
② steel
critical point
Influence. Generally speaking, the higher the critical point (Acl) of steel, the lower the thermal fatigue tendency of steel. Therefore, the critical point of steel is generally improved by adding alloying elements Cr, W and Si. So as to improve the thermal fatigue resistance of steel.
2. Hot working steel is commonly used in molds.
(1) steel for hammer forging die. Generally speaking, steel for hammer forging dies has two outstanding problems. One is to bear impact load when working, so it requires higher mechanical properties, especially higher plastic deformation resistance and toughness; Second, the cross section of the hammer forging die is large (< < 400mm), so the hardenability of steel is required to ensure the uniform structure and performance of the whole die.
Commonly used hammer forging construction steel are
5CrNiMo
、
5CrMnMo，
5CrNiW，
5CrNiTi and 5CrMnMoSiV, etc. Different materials should be selected for different types of hammer-eye molds. For oversize or large hammer forging dies, it is best to choose 5CrNiMo, 5CrNiTi, 5CrNiW or 5CrMnMoSi. Small and medium-sized hammer forging dies are usually selected.
5CrMnMO
Steel.
(2) Hot extrusion die steel. The working characteristic of hot extrusion die is slow loading speed, so the heating temperature of die cavity is high, usually reaching 500-800℃. The performance requirements of this kind of steel should be based on high high temperature strength (that is, high tempering stability) and high thermal fatigue resistance. The requirements for ak and hardenability can be appropriately reduced. Generally, the size of hot extrusion die is small, often less than.
70~90
Hmm.
Commonly used hot extrusion dies are hot working die steels such as 4CrW2Si, 3Cr2W8V and 5% Cr. Their chemical compositions are shown in Table 4. 16.
Including 4CrW2Si. It can be used as both cold working die steel and hot working die steel. Due to different uses, different heat treatment methods can be used. The cold die adopts low quenching temperature (870-900℃) and low or medium temperature tempering. Hot die adopts high quenching temperature (generally 950- 1000℃) and high temperature tempering treatment.
(3) Steel for die casting. Generally speaking, the service performance requirements of steel for die casting die are similar to those of steel for hot extrusion die, that is, high tempering stability and high thermal fatigue resistance are the main requirements. Therefore, the steel grades usually used are basically the same as those used in hot extrusion dies. As usual, 4CrW2Si. , 3Cr2W8V and other steel products. But there are some differences, such as 40Cr, 30CrMnSi, 40CrMo, etc. Suitable for low melting point zinc alloy die casting die. The die casting dies for aluminum and magnesium alloys can be 4CrW2Si and 4Cr5MoSiV.
Equal pairs of copper alloy die-casting dies mostly use 3Cr2W8V steel.
In recent years, with the application of ferrous metal die casting technology, aluminum alloy and nickel alloy with high melting point are mostly used, or Cr-Al-SI ternary co-infiltration is carried out on 3Cr2W8V steel to manufacture ferrous metal die casting dies. Recently, high-strength copper alloy is being tested as a die-casting die material for ferrous metals at home and abroad.