Steels Flashcards
What characteristics do water-hardened tool steels have?
High tensile strength & hardness, low ductility and toughness
What percentage carbon and alloying elements do water-hardened tool steels have?
0.85 - 0.95%C, Vanadium, Chromium
What is the typical heat treatment process of water-hardened steels?
Preheating an annealed tool, typically at 670°C
Austenitizing (soaking at high heat)
Quenching (create hard brittle (martensite) condition
Tempering or Annealing (draw to desired hardness)
What are hypereutectoid steels? Why are they difficult to machine?
Hypereutectoid steels are steels with carbon content >0.76wt%C
Due to a brittle network of pearlite
What is spheroidization annealing? What does it do? What are the aims?
Heating, soaking and cooling (very slowly)
Breaks up hypereutectoid steel: the pearlite and cementite network
- minimise hardness
- maximise ductility, machineablility and softness
What is the purpose of chromium in water hardened tool steels? What are the benefits?
Cr inhibits pearlite formation
Increases hardness and hardenability (the ability of the material to form martensite)
What is the purpose of vanadium in water hardened tool steels? What are the benefits
Vanadium inhibits grain growth when austenitizing. Works by forming V4C3 and pinning grain boundaries.
Increases hardness and improves wear resistance.
What are applications of water hardened tool steels?
- Hammers
- Forging Dies, Cutting Dies
- Blanking and boring tools
- Chisels, scissors, knife blades
- Watch making tools
- Engraving tools
What are some characteristics of shock-resisting tool steels?
- High impact resistance
- Excellent toughness
- Good wear resistance
- High hardenability and softening resistance
Toughness (most important)
Hardness (second most)
What is the definition of toughness?
The ability of a material to withstand a suddenly applied load.
The energy absorbed by a material before failure.
What is the carbon content and main alloys of shock-resisting tool steels?
0.5%C, Chromium, Molybdenum.
What is the typical heat treatment process for S7 shock-resistant tool steel?
- Annealing (heat slow and uniform to 815-845°C, furnace cool to 510°C at a rate of 8-15°C/hr)
- Stress relieving (warm to 650°C, furnace cool)
- Hardening (pre-heat 650 - 700°C, austenitize: heat 925°C - 960°C)
- Quench (by oil)
- Temper (150-200°C, holding 30 mins/25mm)
What does tempering do? How does it work?
Increases ductility but reduces strength.
Works by inducing recrystallisation, making crystals more equiaxed.
What are some applications of shock-resisting tool steels?
- Chisels
- Punches
- Shear blades
- Driver bits
What contributes to the shock resistance of shock-resistant tool steels?
- Toughness then hardness are most important factors
- A lower carbon content than other tool steels is used to improve impact strength.
What contributes to S7 (shock-resistant) increased hardenability?
1.8wt% Mo and 6.5wt% Cr
Oil quenching
What are general characteristics of cold-worked tool steels?
- High abrasion and wear resistance
- High toughness
- High impact resistance
What are the 3-grades of cold-worked tool steels?
- Oil hardening steels [O]
- Air hardening steels [A]
- High carbon, high chromium steels [D]
What are some characteristics of oil-hardening steels?
- Hardened by oil quenching
- Contain high carbon with manganese, chromium and molybdenum
- High as-quenched hardness
- Low red hardness
- High hardenability, resist cracking on quenching
What is a popular cold-worked oil- hardening steel?
O1 (0.5wt%Cr, 0.5wt% W)
Can’t be used for hot working, restarts tempering
What are applications of air-hardening steels?
Applications where high toughness and fair wear resistance are required:
- Blanking
- Forming dies
- Knives
- Coining dies
What elements compose air-hardening steels? What are important alloys?
1.0-2.0wt%C, Cr, Mn, Mo, V, Ni
A2 and A4 are important alloys
What are properties of high carbon, high chromium steels? What are the applications?
Properties:
- Excellent wear resistance
- Non-deforming properties
- Good oxidation resistance (Cr)
Applications:
Blanking and piercing dies, coining dies.
What are some characteristics of hot-worked tool steels
- 0.3-0.5wt%C
- High temperature (200-800°C) metal forming operations
- Resistance to deformation at hot working temperatures
- Resistance to mechanical and thermal shock (low wt%C)
- Wear and erosion resistant
What are the 3 groups of hot-worked tool steels?
Chromium based [H11 - H19]
Tungsten based [H20 - H26]
Molybdenum based [H41 - H43]
What is secondary hardening?
It is the introduction of Mo and W into the grain structure to produce M-carbides which reduce the softening in an alloy at high temperatures.
Without these elements, diffusion of Fe-carbides occur readily causing softening.
What is happening during secondary hardening?
- Cementite (Fe3C) forms as carbon diffuses out of the martensite.
- The microstructure of the matrix coursens (strength and hardness fall)
- If the cementite can be replaced by a more stable carbide e.g. Mo or W, softening is greatly reduced
-IF sufficient amounts of these alloying elements are present from 500-600°C, secondary hardening occurs.
What are some characteristics of high speed tool steels?
- Resistance to softening in red heat (high red hardness)
- Wear resistance
- High hardness
- Edge retention
What are the two groups of high speed steels (HSS)?
- Molybdenum high speed steel [M]
- Tungsten high speed steel [T]
What are the purpose of each alloy in high speed steels (HSS)?
- W and Mo provide carbide formation and secondary hardness.
- V provides increased abrasion resistance, reduced oxidation, increased hardness.
- Co provides high temperature hardening.
List in order of heat extraction rate air, water or oil quenching?
- Air: 2.5-25 W/m2K
- Oil: 20-500 W/m2K
- Water: 100-15,000 W/m2K
What is the benefit of high Cr and C in cold worked steels?
Oxidation resistance due to Cr
What is the application of HSS
Mill cutters operating at high speeds
What contributes to HSS materials hardness?
W and or Mo provide carbide formation and red hardness/secondary hardness
- V provides abrasion resistance, reduced oxidation and increased hardness.
- Co provides high temperature hardening.
In Taylor’s tool life equation vT^n=C, what are the units of C?
m/min
What is sintering?
Sintering is the process of forming a solid mass of material through heat and pressure without melting the material. Usually done with powdered metal.
What are cermets?
Combinations of TiC, TiN, TiCN, with nickel or molybdenum as binders
What are applications of cermets?
High speed finishing of steels, stainless steels and cast irons.
- higher speeds and lower feeds than steel cutting cemented carbide grades
- better finish achieved, often eliminating need for grinding
How is sintered polycrystalline diamond (SPD) fabricated?
By sintering fine grained diamond crystals under high temperatures and pressures into desired shape with little or no binder
What are applications of sintered polycrystalline diamond (SPD)?
High speed machining of non-ferrous metals and abrasive non-metals such as fiberglass reinforced polymer, graphite and wood
What is the hardest material known next to diamond?
Cubic boron nitride (cBN)
What are applications of cubic Boron Nitride?
Machining steel and nickel-based alloys
What are applications of cemented carbide?
Used in lathes,
milling machines of cast iron, non ferrous alloy and non metallic materials
What are general properties of cemented carbides?
- High compressive strength
- Low to moderate tensile strength (brittle)
- High hardness (90 - 95 HRA)
- Good hot hardness
- Good wear resistance
- High thermal conductivity
- High elastic modulus 600GPA
- Toughness lower than HSS
What are coated carbides?
Cemented carbide insert coated with one or more layers of TiC, TiN and or Al2O3 or other hard materials
What is chemical vapour disposition? (CVD)
CVD coated carbides make up 70% of all coated WC-Co tools.
They offer high thermal stability, high adhesion (wear resistance) and ability for multi layer coating.
