tool steels Flashcards
main countermeasure against tool wear
increase tool hardness (e.g. by martensitic hardening)
how to increase the resistance of a tool edge used for stamping and cutting
Small and well-dispersed carbides with a fraction of around 20 % of the volume
(need higher toughness due to notch effects)
how to increase the
resistance of wear from gravel
protected by coarse carbides or borides with a fraction of around 50 volume % (hardness is the main requirement)
needed characteristic of hard phases
✓ The hard phases (HP) have to be harder than the abrasive material and at least as large as the width of the
groove to act as a obstacles to the scratching abrasive
✓ The hard phases should also have a high fracture toughness. If KIC,HP > KIC,AB the
mineral grain will tend to fracture rather
than the hard phase
✓ the matrix has the task of supporting the
hard phases and incorporating them in the microstructure. It should have not only a
high hardness but also a high yield point and a certain degree of ductility
what is hardfacing/cladding
Hardfacing/cladding is the deposition of a surface layer of a material with superior properties than the substrate by «welding».
locally improve surfaces requiring abrasion-corrosion resistance.
✓ can be designed for «preventive» purposes on new parts or for «remedial» purposes
✓ Hardfacing is a welding process that applies a high-wear surface to add protection, typically hardmetals
containing carbides (WC).
✓ Cladding typically uses overlay material that is similar to the base material but in many cases with improved
properties
thermal spry coating
metallic, ceramic, composite materials in the form of powder, wire, rod are fed into a torch or gun, they are heated and sprayed against the surface to be coated
(✓ Dilution with substrate is now almost negligible since the substrate in principle does not melt
✓ Metallurgical compatibility between the substrate and the deposit is not a concern anymore )
hardness ranges to preserve shape of tools depending on application
✓ pure polymer processing 30 – 35 HRC;
✓ metal processing (hot) 40 – 50 HRC
✓ metal processing (cold) 55 – 65 HRC
Service hardness is reached by hardening and tempering:
describe how to harden
✓ dissolved carbon increases the hardening capacity
✓ dissolved alloying elements increase the hardness penetration (hardenability)
✓ formation of alloy carbides during tempering results in secondary hardening
tool steels claassification
- cold-work tool steels (CWS) with a high hardening capacity and optional carbides to improve wear protection
- hot-work tool steels (HWS) with increased creep resistance through precipitation hardening
- high-speed tool steels (HSS) containing carbides in a creep-resistant, hardened matrix
cold work tool steels
✓ steels of group 1 with ≈ 0.5% C do not reach their full martensite hardness. They are hardened at
temperatures above Ac3 and are substantially carbide-free.
✓ Hard steels of group 2, with higher C content contain small undissolved secondary carbides because their hardening
temperature usually lies just above Ac1
✓ Wear-resistant chromium steels of group 3 contain coarser eutectic carbides with a higher hardness.
✓ High-speed steels, which were originally developed for machining applications, are frequently used for cold-work
tools as well. They contain carbides of W, Mo and V (M6C, M2C, MC) that are embedded in a martensitic metal matrix.
✓ Thermal stability of carbides is important for service applications and to set the proper thermal treatment.
hot work tool steels
Hot-work tools are used at workpiece temperatures between ≈400 and 1200°C
✓ The surface temperature of the tools approaches that of the workpiece as the contact time increases and
the relative cooling time decreases
✓ Creep-resistant and high-strength Q&T steels are used depending on the temperature conditions
✓ At very high temperatures, creep-resistant and oxidation resistant austenitic steels and nickel-base alloys
can also be used
