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

2. Comparative Carbide Structure Microstructure of T15 high speed steel Cast and wrought P/M processed, HIP After “Tool Steels”, ASM International

3. 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

4. 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

7. Alloy Apparent Density Hall Flow Compressibility Green Strength Sieve Carbon Oxygen g/cc sec. psi Wt% Wt% ppm M2 2.20 – 2.40 <40 >6.50 @100kpsi 0.75MgSt >3000 @87kpsi DWL <2 >150µm >20 <45µm 0.75-0.90 <850 M3/2 2.25 – 2.45 <45 >6.00 @87kpsi DWL >2000 @87 kpsi DWL <2 >150µm >20 <45µm 0.90-1.10 <1000 T15 2.20 – 2.35 <45 >5.80 @87 kpsi DWL >2000 @87kpsi DWL <2 >150µm >20 <45µm 1.50 – 1.65 <1000 Typical Specifications

8. 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

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

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

12. 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.

13. 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

14. Mixed Powder Microstructures 100 μm 100 μm As sintered Oil quenched and tempered 4600 type material containing 20% HSS

15. HSS – Valve Seats High duty automotive use Fe-HSS mixes used to match required duty Some heavy duty Applications use 100% HSS

16. 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