Steels Questions Flashcards
In shock resisting tool steels, what contributes to the alloys increased shock resistance?
The most important property is toughness (hardness is secondary).
Lower carbon contents than other tool steels are necessary to improve impact strength.
What hardenability does S7 shock resisting steel display and what contributes to it?
S7 has increased hardenability due to 1.8wt% Mo and 6.5wt% Cr additions, and oil quenching.
What is cold worked tool steels maximum operating temperature?
200°C
For cold worked cool steels, list in order of heat extraction rate: air, water or oil quenching?
Air: 2.5-25W/m^2K < Oil: 20-500W/m^2K < Water: 100-15000W/m^2K
What is the benefit of high Cr and C in cold worked tool steels
High chromium provides oxidation resistance, high carbon makes the material harder.
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 in the alloy at high temperatures.
Without these elements diffusion of the Fe-carbides occurs readily, softening the alloy at high temperatures.
Whit is the application of high speed steels? (HSS)
High speed steels are used for mill cutters operating at high speeds.
What contributes to high-speed steels hardness?
W and Mo provide carbide formation and red hardness (secondary hardness)
V provides increased abrasion resistance, reduced oxidation and increased hardness.
Co provides high temperature hardening.
What are the three modes of failure of a tool and how do they occur?
Fracture failure: cutting forces becomes too 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 on a tool bit and why?
Crater wear: occurs on top rake face.
Diffusion causes the loss of hardening atoms at the tool chip boundary.
Flank wear: occurs on the flank (side) of the tool.
Abrasion is the dominant cause of flank wear, plastic deformation also contributes.
What does the basic grain structure of cemented carbide consist of? What is the role of the individual phases within?
The basic grain structure consists of
Tungsten carbide, WC (a), angular grains.
Cobalt (b).
Cobalt acts as a binder holding the material together.
What is the practicality of different inset shapes for tool bits?
There is a trade off between strength, power requirements, vibration tendency, and versatility and accessibility.
Increasing one decreases the other.
Circular > fine angle tip.
What effect does increasing grain size have on cemented carbides toughness and deformation resistance?
What effect does increasing Cobalt concentration have on WC grain size?
The toughness increases and deformation resistance decreases.
For a Co increase of 5-12%, the WC grain size increases from 1-5ųm, becoming more coarse.
What are micro-alloyed steels and what are their beneficial properties,
Low C (0.05-0.10wt%C) steels with very small (<1%) M-carbides and nitrides alloying additions e.g. Nb, Ti and V.
Causes refinement of the ferrite grain size by formation of a fine sub grain structure. Strain induced precipitation of carbides and nitrides (example of dispersion strengthening).
What factors contribute to the strength enhancement of microalloyed steels?
Grain refinement, sub-grain formation (grains and very small and orientation is very small 0.5-1°). Precipitation hardening.
