I. What is mold steel?
Mold steel is steel used to produce molds and casting molds.
II. Classify
There are 2 ways to classify mold steel: classifying by quality and classifying by use.
1. Sort by quality
– Regular steel
Easy to process and cut, less wear and tear, easy to polish and cool. However, this type of steel has low hardness and the surface does not achieve high gloss. Low cost price. Some common steel grades such as: S45C, SCM440, S50C,...
– High quality steel
Higher hardness will give the product a perfect surface and good mechanical properties such as impact resistance, good abrasion resistance, long-term high temperature resistance, anti-rust, increased mold durability,... In addition, in this type of steel we can absorb nitrogen to harden the surface or plate with Chromium, Nickel, volume quenching,... depending on the technical requirements of each detail. High cost and processing costs. Some high quality steel grades such as: SKD11, SKD61, SKD62, 2316, 2083, NAK80,...
2. Classified by use
- Steel for plastic molds
- Steel for hot stamping dies
- Steel for cold stamping dies
- Heat pressed aluminum steel
III. Factors affecting the quality of mold steel
1. Chemical composition
Carbon: Increases the hardness of steel during tempering. Increases wear resistance, reduces ductility and weldability of steel.
Crom: One of the ingredients that increases hardness. Forms the durability and stability of steel. Improved wear resistance. When the Cr content is >12%, it has an anti-rust effect, and produces a shiny surface.
Molip: As an additive for the carbonization process of element formation, it increases wear resistance. MO > 0.5% can prevent other alloying elements from heating and breaking the steel
Vofram: Forms high hardness carbonization, increases wear resistance, reduces reaction when heat treated. Increases the durability of steel, increases stability during the steel tempering process.
Niken: Enhance toughness, improve wear resistance, along with Cr - Mo to improve heat resistance and chemical corrosion resistance.
Sulfur: Usually exists in the form of MnS
2. Production technology
Blowing oxygen furnace (BOF) and electric furnace (EAF) technology
3. Heat treatment
– Concept: Heat treatment is the process of heating steel to a specific temperature, keeping the temperature there for an appropriate time and then cooling at a specified rate to change the microstructure, thereby changing the structure. Change the mechanical properties of steel as desired.
* Heat treatment methods:
– I steel
Concept: Tempering steel is a method of heating steel above a critical temperature to create a heat-retaining austenite structure and then cooling it quickly enough for the austenite to transform into martensite or other unstable structures with high hardness.
Purpose:
+ Improve the hardness and abrasion resistance of steel.
+ This is the most important method of mold steel.
+ Required process before putting finished molds into use.
+ Depending on the steel grade, the maximum hardness achieved is different.
– Annealing steel
Concept: Annealing of steel is the process of heating steel to a certain temperature (from 200 - 10000C), keeping the heat for a long time and then cooling slowly with the furnace to achieve a stable structure.
Purpose:
+ Reduces hardness to make cutting easier.
+ Increases ductility to make it easier to stamp, roll and draw steel in a cold state
+ Reduces or eliminates internal stress after mechanical processing, casting, and welding operations.
+ Make steel grains smaller if the previous operation made large grains.
– Steel ram
Concept: Tempering is a heat treatment method of heating tempered steel below critical temperature temperatures (AC1), holding the temperature for a while and cooling. In order for martensite and residual austenite to differentiate into suitable structures suitable to specified working conditions.
Purpose:Reduce or eliminate residual stresses after quenching to the level necessary to meet the long-term working conditions of mechanical products while still maintaining mechanical properties after quenching.
4. Quality control measures
– Ultrasound using ultrasound waves
– Check chemical composition
– Check hardness
– Test tensile strength
– Check flexibility
– Check outside