Steels Flashcards
What are Hypereutectoid steels
Hypereutectoid steels describe steels with compositions of carbon greater the 0.76 % which is to the right of the Eutectoid point.
Why are Hypereutectoid steels difficult to machine
Hypereutectoid steels are difficult to machine due to brittle network of pearlite
what is Spherization annealing? Why is it applied?
heating, soaking and cooling (very slowly). Breaks up the hypereutectoid steel – cementite & Pearlite
In water hardened tool steels, what is the role of Chromium (Cr) and Vanadium?
Presence of Cr Inhibits pearlite formation and therefore increases hardness & hardenability.
Vanadium is often added to inhibit grain growth when austenitzing, this works by forming V4C3 which then pins the grain boundaries, increasing hardness, improving wear resistance
What contributes to an alloys increased shock resistance?
Most important property is Toughness (hardness is secondary). It is therefore necessary to have lower carbon contents than other tool steels to improve impact strength.
heat extraction rate transfer air, water or oil quenching
Air: 2.5-25 W/m2K < Oil: 20 – 500 W/m2K < Water: 100 – 15,000 W/m2K
S7 Shock Resisting Tool Steels hardenability
S7 has increased hardenability due to the 1.8% Mo, 6.5% Cr and oil quenching
What is the benefit of high Cr and C cold worked tool steels?
Oxidation resistance due to Cr
What is secondary hardening and how does it improve the performance of hot-worked tool steels?
It is the introduction of Mo and W into the grain structure to produce M-carbides which reduce the softening occurring in an alloy at high temperatures. Without these elements’ diffusion of the Fe-carbides occurs readily softening the alloy at high temps.
What contributes to high speed tool steels hardness
W and/or Mo provide carbide formation and “red hardness”
V provides increased abrasion resistance, reduced oxidation and increased hardness.
Co provides high temperature hardening.
Why are duplex Steels heated above 1000*C
Duplex stainless steels are heated to 1000 - 1150˚C only the α and γ phases are present. Generally fairly rapid cooling is required to prevent other phases forming as the steel cools.
Below ~ 1000 ˚ C duplex stainless steels are not stable and various carbides precipitate at grain boundaries
Intergranular corrosion and its occurrence in duplex steels
Heating 482-760°C (welding). Cr form with C at grain boundaries (𝐶𝑟23𝐶6). Creates Cr depleted zones, decreasing corrosion from protective passive film.
What are the two phases within Duplex stainless steel
they have a two-phase microstructure consisting of ferritic and austenitic stainless steel.
Main improvements in microalloyed steel
- Refinement of the ferrite grain size, by formation of a fine sub grain structure.
- Strain induced precipitation of carbides and nitrides.
what are microalloyed steels
low carbon (~ 0.05 – 0.10 wt% C) plain carbon steels with a very small amount (< 1%) of strong carbide and nitride forming alloying additions, eg. Nb (Niobium), Ti and V.
strength enhancement of microalloyed steel
Hot rolling - Grain refinement, Sub grain formation (means grains are very small & orientation between them is very small 0.5-1˚). Precipitation hardening.
What is the practicality of different inset shapes in Cemented Carbides?
There is a trade off between strength, power requirements and vibration tendency and versatility and accessibility. Increasing one decreases the other.
What is CVD & PVD? What are the influences of residual stresses in the formation of coatings by these two processes and how do they influence the fracture resistance? (Cemented carbides)
Chemical Vapour deposition and Physical Vapour Deposition. CVD produces coatings with tensile stresses due to the mismatch between the thermal expansion coefficients of the coating and substrates, lowering the fracture resistance. PVD produces compressive stresses due to the same reason, however will show improved fracture resistance. 1
What does the basic grain structure of cemented carbide consist of? What is the role of the individual phases within
WC (α) Angular, Co (β) (Binder- Glue)
What are the three modes of failure of a tool and how do they occur?
Fracture failure: Cutting force becomes excessive and/or dynamic, leading to brittle fracture. Temperature failure: Cutting temperature is too high for the tool material. Gradual wear: Gradual wearing of the cutting tool.
Where does gradual wear occur in the tool bit and why?
Crater wear – occurs on top rake face
Flank wear – occurs on flank (side of tool). Abrasion - dominant cause of flank wear.
Diffusion – Loss of hardening atoms at tool-chip boundary (contributes to crater wear).
Plastic deformation – contributes to flank wear
