Technology watch: Materials used in the field of tooling - General part

For more than two centuries now and the use of machine tools - the first metal-framed turning machine was invented in 1751! - the cutting speed of the tools has gradually increased from a few centimetres per second to almost 10,000 metres per minute (more than one hundred metres per second) thanks in particular to the continuous evolution of tools and machining processes.

This tool monitoring work is intended to explore several subjects concerning the evolution of technologies and processes used in the industry for tool manufacturing, and focuses here on the materials used, and prospectively on trends and innovations.

Essential Material for Tooling: Tool Steel

Tool steel is a type of alloyed steel well suited to the manufacture of tools, such as hand tools or components of industrial processing machines (boring, sawing, hammering, stamping, drilling, milling, stamping, mould making, etc.) that can be used in all conditions. Its hardness, abrasion resistance and ability to maintain its shape and mechanical performance, including its sharpness, at high temperatures, allow it to be used in the shaping of other materials by cutting, pressing, stamping or extrusion.

The performances sought lead to different levels of processing, alloys and treatments. Several types of tool steel markets can be distinguished :

  • Carbon steel for tools (non-alloy steel).
  • Alloyed tool steel.
  • High speed tool steel.

Applications and tool steel grades

The new standard NF EN ISO 4957 published in July 2018, which replaces the previous standards EN ISO 4957

(May 2000) and A35-590 (December 1992), no longer distinguishes grades according to main characteristics (wear resistance, impact resistance) or composition/structures (carbon steels, fine steels, extra-fine steels), and only retains four classes:

  • Non-alloy tool steels for cold working.
  • Alloyed tool steels for cold working.
  • Hot-work tool steels for applications where the surface temperature is above 200°C.
  • High-speed tool steels, mainly used for machining and forming. Their chemical composition provides the highest hardness and strength characteristics after hardening at high temperatures (up to about 600°C). Powder metallurgy makes it possible to produce grades with a higher content of carburising elements (vanadium, tungsten, molybdenum), which allows higher performance and longer tool life.

It should be noted that in certain special cases, it is also possible to use wear-resistant stainless steels (for shaping plastics, composite materials or corrosive glass) or even nickel-based super alloys for shaping by forging or high-temperature extrusion.

Tool steel is generally used in the heat-treated state, which gives it increased hardness.

Tool steels are characterised according to :

  • Their composition (unalloyed and alloyed).
  • The cooling agents (water, oil and air hardeners).
  • The application temperature (cold work, hot work and high speed work).

There are two main application temperature ranges, below 200°C and up to 600°C (see OTUA technical files) :

  • Cold work steels are generally used when the surface application temperature does not exceed 200°C during use,
  • While hot work steels can reach application temperatures above 200°C.
  • A special class of these hot work steels is high speed steels (HSS), in which the application temperature can reach over 600°C.

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Tool steels are available in basic unalloyed form or alloyed depending on the required performance.

 

 For example, a Vanadium alloy will increase corrosion resistance, and a Cobalt Alloy High Speed Steel (HSS)

will have a temperature resistance of up to 600°C.

Alloys are generally designed to work at high temperatures up to 600°C. These steels have in common that they have a carbon content of less than 0.60%, with the addition of chromium, molybdenum, and vanadium. The desired characteristic is the highest possible dimensional stability to resist thermal fatigue. The three most used grades are Z38CDV5, Z40CDV5 and 55NCDV7 treated. They are used, for example, to produce moulds used to make mass-produced parts in the automotive sector (crankcase, etc.), household appliances, or in companies producing screws, rivets, bolts or fatigue nuts.

The composition of the alloys must also consider the effect of additional heat treatments.

Certain grades will thus have a limited manganese content to minimise the risk of cracking during water quenching. Depending on the qualities required, other methods can be used, such as air or oil.

 

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