Die steel properties & requirement

Die casting is an economical process to produce complex parts in aluminium alloy, zinc, copper etc:. in modern days time die casters are demanding higher production rate, very less down time & extended die life. In addition to this the complex & large die casting parts are extending the challenge to the extreme. Surely die casting dies becomes a crucial factor to meet such challenges.
The performance of a die largely depends on following factors, namely

1. Good die design
2. Good die steel
3. Good Heat treatment of die 
4. Good workmanship of die
5. Proper maintenance of dies throughout the life cycle.
6. Process parameter setting
7. Product design

Today we will look into some of the aspects of die steel that are important to achieve good die life. 

The die is working as heat exchanger. When the hot molten metal is pressed in high temperature & high speed in die cavity the the die temperature shoots up immediately & as the solidifies cast part is ejected & die spray applied on die the temperature drops down significantly. This causes a thermal shock in die. Also when the molten metal runs through the cavity it creates a high friction with die material which causes Galling & Erosion of steel. Besides there are other severe working conditions which causes mechanical stress, thermal fatigue etc; on die steel. Therefore the die steel must be capable to withstand such severe working conditions & produce good quality castings with prolonged die life. 

Typically for die casting "Hot working tool steel" is recommended which falls in to the category of H11 & H13 grade of steel as per AISI grade. Let us have a look into the alloying elements these steel grades are made of.

H11 Grade - Carbon - 0.35 to 0.45%, Manganese - 0.2 to 0.6%, Phosphorous - 0.03% max, Sulphur - 0.03% max, Silicon - 0.8 to 1.25%, Chromium- 4.75 to 5.5%, Vanadium- 0.3 to 0.6%, & Molybdenum- 1.1 to 1.6%.

H13 Grade - Carbon - 0.32 to 0.45%, Manganese - 0.2 to 0.6%, Phosphorous - 0.03% max, Sulphur - 0.03% max, Silicon - 0.8 to 1.25%, Chromium- 4.75 to 5.5%, Vanadium- 0.8 to 1.2%, & Molybdenum- 1.1 to 1.75%.

From the above we can see that while "Mn" & "P", "S" are actually remains as a residue & other important elements are Chromium, Vanadium & Molybdenum apart from Carbon.

The hot working tool steel is characterized by 

• Good resistance of abrasion both low & elevated temperature. 
• High level of toughness & ductility
• Uniform & high level of machinability & polishability
• Good high temperature strength & resistance to thermal fatigue
• Excellent through hardening property
• Very limited distortion during hardening

Let us see how hot working tool steel get such characteristic & what are the contribution of key alloying elements.

Carbon - Carbon is essential in forming of cementite & iron-carbon martensite. The hardness of steel or more precisely hardenebility is increased by adding more carbon up to about 0.65%. Wear resistance can be increased by adding up to 1.5%. Martensite being the hardest of microstruture hence Carbon is an essential component in steel.

Chromium - This element has a tendency to increase hardness penetration. Chromium can also increase the toughness of steel, as well as wear resistance. With higher content of more than 14% this can resist stain & corrosion in steel. Steel with higher chromium is also having higher critical temperature in heat treatment.

Vanadium - This controls grain growth during heat treatment and helps increase toughness & strength of steel.

Molybdenum - This element increases the hardness penetration of steel, slows the critical quenching speed & increases high temperature tensile strength.

The typical die life of aluminum casting is varying from 50000 shots to 250000 shots in average, depending on various factors as mentioned earlier.

The die economy therefore largely depends on Raw materiel of the die. The tooling cost varies from 10 to 20 % of the component cost & out of that about 15% of the die cost is for the material. Hence choosing correct die material is very important for tool maker. 

Thee are many renowned die steel maker & one can choose the supplier depending the geographical location, availability & support provided by the die steel maker.