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Materials for making Press Toolings

Types and characteristics of tool steels

The steel material used to fabricate press tool is also called tool steel as this steel is used to cut, shear or form materials. They may be either carbon or alloy steels. Steels in the lower carbon content (0.7-0.9%) are used for tools subject to shock, and higher carbon content tool steels (1.1-1.3%) are used for making tools with keen cutting edges. 

Tool steels must be hard, tough and resistant to wear and shock. The depth of the hardened layer within the tool steel differs based on applications, e.g. a hardened tooling surface is important for tensile and compressive forming while tooling subject to heavy shocks and loading would need the entire steel piece to be hardened. Any deformation after heat treatment must be minimal.

Carbon Tool steel

Carbon tool steel can be described as the basic tool steel material. It is divided into 7 classes according to the carbon content with class 1 having the highest carbon content. Carbon tool steel has the following distinctive features:

• Hardness - In general, carbon tool steel only requires water cooling within the  tempering process. Although water cooling is effective, it results in non-uniform hardness on the steel surface due to the presence of air bubbles during water cooling. The excellent cooling effect also causes a difference in hardness between the steel surface and the interior. As the heat treatment process for carbon steel involves relatively low hardening and tempering temperatures, exposure to strong heat during the die operation would inevitably subject the carbon steel to repeated heat treatment resulting in lower hardness. 
  
• Resistance to wear - High resistance to wear
  
• Deformation after heat treatment - Rapid cooling with water leads to substantial deformation and even cracks after heat treatment.
  
• Toughness - Hardened and tempered carbon steel is hard but brittle. As a thick piece of carbon steel is normally used as material for the press die, the rolled structure is relatively tough. Hardened carbon steel is brittle because the carbon within the steel does not react with iron to form iron carbides, but remain as free carbon in between the crystal structure.

S-Type Alloy Tool Steel (Special Tool Steel Type 3)

SKS-3 is widely used as a tooling material in Japan. SKS-3 is SK-4 (carbon tool steel) with 0.5% to 1.0% of chrome and tungsten added to make the structure denser so as to minimize deformation from heat treatment, and stop heat from affecting the hardness and improve resistance to wear respectively.

• Hardness - Hardening and tempering by oil cooling gives HRC of 60 or above. Due to oil cooling, special tool steel (oil hardened steel) possesses less variation in hardness compared to carbon tool steel.
  
• Resistance to wear - The addition of chrome makes the structure denser. The added tungsten greatly enhances the resistance to wear, which is better than that of carbon tool steel.
  
• Deformation after heat treatment - While the added chrome reduces deformation arising from heat treatment, it is noted that only 0.5-1.0% of chrome has been added. As such, SKS-3 also deforms considerably but less than SK material.
  
• Toughness - Special Tool Steel is tougher than carbon tool steel due to the former`s denser structure.

High Speed Tool Steel

High speed tool steel is used to make tools which are used for deeper cuts at higher cutting speeds than regular tool steels above. High speed tool steel has at least 4% chrome and tungsten. It also contains vanadium for better heat treatment and molybdenum and cobalt for enhanced toughness.

• Hardness - The tungsten carbide added to the martensitic structure makes it the hardest tool steel. Both chrome and tungsten carbide combine to minimize the variation in hardness. Red hardness is also a term to refer to hardness at high temperatures.
  
• Resistance to wear - It has the highest resistance to wear among the tool steel due to its tungsten carbide content.
  
• Deformation after heat treatment - As high speed steel has a high hardening temperature, its structure is not generally vulnerable to deformation, but deformation by gravity occurs frequently.
  
• Toughness - It has high toughness due to addition of molybdenum and cobalt.

Powder High-Speed Steel

It is commonly accepted that material formed from powder has high hardness but low toughness. However, research and development have been done to give it hardness and toughness higher then the conventional SKH material.

Cemented Carbide/ Tungsten Carbide

Tungsten carbide is the hardest human-made metal. It is shaped by molding tungsten, carbon and cobalt powders under heat and pressure during sintering. The metals fuse together without melting.

Tungsten carbide is capable of maintaining high hardness even at high temperatures. Its hardness and formability depends on its cobalt content. Cemented carbide used for making dies has 12-20% cobalt. When selecting which carbide to use, check the requirements of the work, and then choose the carbide materials with the appropriate cobalt content. Tungsten carbide far excels in hardness, resistance to wear, resistance to heat when compared against other tool steels. However, it has the following weaknesses:

• Low resistance to vibration- Particles of tungsten enveloped in cobalt tend to fall off from the material.
  
• Low toughness - Cemented carbide has no crystalline structure and is liable to break if a bending force is applied.
  
• Low resistance against chemicals - Care must be taken when abrasive agent or coolant is used during blanking as this steel tends to react with sulphur.

Tungsten carbide tools are normally coated by titanium nitrate (TiN) or titanium carbide (TiC) about 2 microns thick to increase their tool life by about 3-8 times longer. The coating on the tools will resist the formation of built-up edge, thus allowing the tools to run longer at cooler temperatures, hold tolerances better and provide better surface finish in the workpiece.