What Are HSS and Tool Steels

1
What Are HSS and Tool Steels

• Ferrous based alloys used to manufacture tools
  dies and moulds that shape form and cut other
  materials, in particular, metals.
• They comprise Fe plus C and other elements that
  allow the alloys to be hardened and tempered
• In HSS correct heat treatment can generate a
  hardness greater than the quenched hardness. This
  material retains its hardness at high
  temperatures and has good toughness

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Comparative Carbide Structure
Microstructure of T15 high speed steel
Cast and wrought
P/M processed, HIP
After Tool Steels, ASM International
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Secondary Hardening Curves
M2 Data from Stora Steels Sweden
T15 Data from Jessop-Saville, England
After Metallurgy and Heat Treatment of Tool
Steels, R. Wilson
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Key Features of High Speed Steel

• High hardness
• Extreme wear resistance
• Resistance to high temperature softening
• Reasonable toughness
• Properties dominated by thermal history

5
Why Use a Powder Route

• High speed steel shapes are very difficult to
  manufacture cost effectively
• Compressible powder allows shapes to be formed
• Segregation is eliminated

6
Powder Production

• Powder is water atomised
• Atomisation centralised in Belgium
• Atomised powder is vacuum annealed
• Vacuum annealing is centralised in England
• Process provides close control of final chemistry
  and physical properties
• Reduces oxides
• Provides a compressible powder

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Typical Specifications
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Pressing

• Lubricants and pressing technologies are similar
  to those used conventionally
• The powder is generally more abrasive and high
  quality tooling is required
• Densities achieved are lower than iron powders
  and full density is generally around 8.2 g/cc

9
Sintering

• Not possible to provide generic information about
  HSS sintering
• Applications are specialised and a process route
  is developed for each application
• Users are generally quite secretive
• The thermal history of the part is the key to the
  success of a product

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Sintering

• Gas sintering
• Conventional furnaces
• 1050C upwards for around 30 minutes
• Small size change
• Vacuum sintering
• Approximately 1250C
• Produces full density material
• Densification is very rapid
• Exceptionally good temperature control and
  uniformity required

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Sintering Atmospheres

• Typically a pure hydrogen or nitrogen/hydrogen
  mix is required with a dewpoint better than -25C
• Dissociated ammonia is not generally recommended
  but should be suitable providing it is dried
  correctly
• Carbon control is required to avoid
  decarburisation

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Post Sinter Treatments

• Conventional heat treatments include many stages
• Austenitisation, Quenching, Triple tempering
• Properties achieved are a result of the thermal
  history
• Full heat treatment can be applied to sintered
  parts but this is expensive
• Control of the cooling rate after sintering can
  be used to produce useful products with limited
  post sintering heat treatment.

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Powder Mixtures

• HSS contents of 10 and above mixed with a
  structural steel powder can find applications
• Mixed materials can have the toughness and
  strength of the structural steel powder with a
  dispersed hard phase to provide wear resistance

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Mixed Powder Microstructures
100 µm
100 µm
As sintered
Oil quenched and tempered
4600 type material containing 20 HSS
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HSS Valve Seats
High duty automotive use Fe-HSS mixes used to
match required duty Some heavy
duty Applications use 100 HSS
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Other Applications

• Wear resistant products
• As pure high speed steel or as a mixture to
  provide powders for wear applications
• Useful where wear resistance is required at high
  temperatures
• Cutting tools
• Requires full density product
• Complex vacuum sintering process for super
  solidus liquid phase sintering
• Competes with WC-Co cutting tools