Embed Size (px)
Citation preview
Fine Wood Work Association
Western Australia
Presentation by Mike Wiggin
August 2013 meeting
1
A BRIEF LOOK AT TOOL STEELS FOR WOODWORKERS
Fine Wood Work Association
Western Australia
TOOL STEELS
Sources
The content in this article has been checked with two or more
sources wherever possible. The main source material for this
presentation comes from:-
American Iron and Steel Institute (AISI).
Simply Tool Steel www.simplytoolsteel.com – they make the
machines that make the steel as well as making the steel.
Powder-Metallurgy Tool Steel: an overview.
Hillskog T., MetalForming Magazine, 2003. p. 48-51.
Popular Woodworking Magazine.
Ron Hock http://www.hocktools.com/toolsteel.htm – makes and
sells after market plane blades.
Wikipedia – you pays your money and you takes your chances.
MDME website on steels - TAFE NSW
www.ejsong.com/mdme/memmods/MEM30007A/steel/steel.html
Lee Valley Tools
UK Centre for Materials Education - FE Resources
www.materials.ac.uk/resources/FE/ferrousmetallurgy.ppt
Seeing Further.
Bill Bryson ed. HarperPress 2011 ISBN 9780007302574
2
Fine Wood Work Association
Western Australia
TOOL STEELS
Iron
Pure iron is a very soft metal with a marked propensity to oxidise in
air (aka rust).
It’s also rare, most pure iron exists only in metallurgical labs.
What we call iron is in fact an alloy of iron, carbon, other metals
and a range of impurities.
Any combination of iron and more than 2% carbon is considered
to be cast iron (or pig iron).
Iron with less than 2% carbon is considered to be steel.
Microstructure of Steel
The crystalline structure of steel has five main constituents:
FERRITE - pure iron crystals at STP (standard temperature and
pressure).
AUSTENITE - structure of iron crystals at temperatures above
912°C.
CEMENTITE - iron carbide crystals, responsible for hardness and
reduced ductility in steel.
PEARLITE - alternating layers of ferrite and cementite crystals.
MARTENSITE - very hard needle-like structure of iron and carbon
formed by very rapid cooling of austenite. Needs to be modified
by tempering before acceptable properties reached.
3
Fine Wood Work Association
Western Australia
TOOL STEELS
Wrought Iron
‘Wrought’ is an old word for worked, so wrought iron means “worked
iron”.
Wrought iron was made in small furnaces in batches by forcing
air over the molten metal to burn off impurities.
It contains around 0.05% carbon, some impurities (mainly
sulphur and phosphorous) and slag inclusions.
Quality control and properties varied widely between foundries
and between batches from a single foundry.
Used for weapons, armour, cooking pots, etc since around
2000BC.
Can’t be welded or made into large structures and has been
pretty much completely replaced these days by mild steel.
Slag inclusions have a big influence on workability of wrought
iron.
Pig Iron
Pig iron is the product of the initial smelting of iron ores in a blast
furnace.
Called pig iron because it used to be poured into sand moulds
and the result looked (vaguely) like piglets suckling on a sow’s
teats.
Carbon content around 4%.
Extremely brittle and therefore of very limited use.
4
Fine Wood Work Association
Western Australia
TOOL STEELS
Cast Iron
Cast Iron is the product of heat treating pig iron to remove excess
carbon and other impurities before pouring the molten metal into
moulds.
It contains 2 to 4% carbon and 1 to 3% silicon plus other metals
depending on requirements.
Cast iron properties depend on the alloyants.
Modern cast iron production comes from blast furnaces and
mainly goes directly into steel refineries as a high temperature
liquid.
Can be heat treated to produce things like ductile iron, etc.
Grey cast iron showing the graphite flakes
in a pearlite matrix.
5
Two-dimensional view of pearlite consisting of alternating layers of
cementite and ferrite.
Fine Wood Work Association
Western Australia
TOOL STEELS
Steel
The Oxford Dictionary defines steel as a hard, strong grey or bluish-
grey alloy of iron with carbon and usually other elements, used as a
structural and fabricating material.
Steel is an alloy of iron and up to 2% carbon to which one or
more of a wide range of metals have been added to modify the
hardness, toughness and treatability of the resulting alloy.
Steel can be tempered and (normally) retains its magnetism. Its
malleability decreases and hardness (wear resistance) increases
with increasing carbon content.
6
Fine Wood Work Association
Western Australia
TOOL STEELS
Turning Iron into Steel
Two main methods of producing steel from pig iron are the
BESSEMER PROCESS (1850s) and BASIC OXYGEN STEELMAKING
(1950s).
Air (Bessemer process) or oxygen (Basic Oxygen process) is
forced through molten iron.
Silicon is converted to silica (SiO2) which is light enough to float
on the molten metal – known as slag.
Dissolved carbon is converted to carbon dioxide which is
insoluble in molten iron.
The resulting product is molten steel which is removed from the
furnace for further treatment.
Carbon and alloying metal content is dependent on quality
control and assorted other black arts, e.g.
adding manganese helps remove sulphur and dissolved
oxygen (and makes the steel stronger);
adding nickel and chromium improves hardness.
7
Fine Wood Work Association
Western Australia
TOOL STEELS
Mechanical Properties of Steel
Mechanical properties of steel are dependent on the crystal structure
of the steel, principally on how defects move and interact
All solid steel is crystalline and the alloyants modify the crystal
lattice of the iron molecules.
Steel bends because dislocations in the lattice can shift and
move within the crystals.
When the dislocations combine and interact, movement is
restricted and the metal becomes brittle (fatigue cracking and
failure).
Treatment to reduce crystal size means dislocations at crystal
boundaries are more common and the steel is therefore harder –
but more brittle.
8
Dislocation in
a Crystal Lattice
Fine Wood Work Association
Western Australia
TOOL STEELS
Mechanical Properties of Steel (cont’d)
9
0.1% carbon steel
(dead mild steel)
– the light areas are
ferrite (pure iron), dark
areas are pearlite.
0.2% carbon steel
- note the increased
amount of pearlite
compared with the 0.1%
‘dead mild’ steel.
Increasing the carbon content decreases the amount of ferrite and
increases the proportion of pearlite in the structure.
Fine Wood Work Association
Western Australia
TOOL STEELS
Steel Types
Steel hardness is mainly determined by carbon content. Increasing
the carbon percentage increases the hardness and reduces the
malleability. Steel is broken down in to four classes, based on
carbon content:
MILD AND LOW CARBON STEEL
Low carbon steel contains approximately 0.05–0.15% carbon
and mild steel contains 0.16–0.29% carbon; making it malleable
and ductile, but it can’t be hardened by heat treatment.
MEDIUM CARBON STEEL
Approximately 0.30–0.59% carbon content. Balances ductility
and strength and has good wear resistance; used for large parts,
forging and automotive components.
HIGH CARBON STEEL
Approximately 0.6–0.99% carbon content. Very strong, used for
springs and high-strength wires.
ULTRA-HIGH CARBON STEEL
Approximately 1.0–2.0% carbon content. Steels that can be
tempered to great hardness. Used for special purposes like
knives, axles or punches. Most steels with more than 1.2%
carbon content are made using powder metallurgy.
NB: All these steels can be extensively modified by adding other
materials.
10
Fine Wood Work Association
Western Australia
TOOL STEELS
Effects of Common Additives to Steel
Common additives, and the effects they have on the properties of a
steel are as follows.
CARBON
forms a variety of iron carbides,
increases wear resistance,
is responsible for the basic matrix hardness.
TUNGSTEN AND MOLYBDENUM
improve red hardness,
retention of hardness and high temperature strength of the
matrix,
form special carbides of great hardness.
VANADIUM
forms special carbides of supreme hardness,
increases high temperature wear resistance,
aids retention of hardness and high temperature strength of the
matrix.
CHROMIUM
promotes deep hardening, produces readily soluble carbides.
COBALT
improves red hardness and retention of hardness of the matrix.
ALUMINIUM
improves retention of hardness and red hardness.
11
Fine Wood Work Association
Western Australia
TOOL STEELS
Modifying the Properties of Steel
The properties of the steel can also be changed by several other
methods.
12
COLD WORKING
Bending or hammering into its
final shape at a relatively cool
temperature, (e.g. cold forging
a piece of steel into shape by a
heavy press).
cold rolling to produce a
thinner but harder sheet.
cold drawing for a thinner
but stronger rod.
CASE HARDENING
Steel is heated to about 900°C then plunged into oil or water.
Carbon from the oil can diffuse a small distance into the steel,
making the surface very hard.
promotes both toughness and hardness.
the surface cools quickly, but the inside cools slowly, making
an extremely hard surface and a durable, shock resistant
inner layer.
Fine Wood Work Association
Western Australia
TOOL STEELS
Modifying the Properties of Steel (cont’d)
13
HEAT TREATMENT
The steel is heated red-hot, then cooled quickly.
the iron carbide molecules are decomposed by the heat, but
do not have time to reform.
since the free carbon atoms are stuck, it makes the steel
much harder and stronger than before
ANNEALING
Heating the steel to 700–
800°C for several hours
and then gradual cooling.
decreases hardness
and increases
malleability.
Fine Wood Work Association
Western Australia
TOOL STEELS
Testing of Steels
Mechanical properties of steels can be evaluated using a variety of
tests, principally the Rockwell or Vickers hardness tests. The data
on iron is so consistent that it is often used to calibrate
measurements or to compare tests.
Mechanical properties of iron are significantly affected by the
sample’s purity.
Pure iron is actually softer than aluminium
The purest industrially produced iron (99.99%) has a hardness
of 20–30 Brinell.
Increasing the carbon content of the iron will initially cause a
significant corresponding increase in the iron’s hardness and
tensile strength.
14
Hardness of 65 Rockwell C Scale
(HRC) or greater is achieved with a
0.6% carbon content, although this
produces a metal with a low tensile
strength.
A Rockwell Hardness
testing machine
Fine Wood Work Association
Western Australia
TOOL STEELS
Rockwell Hardness Testing
Testing involves the application of a minor load followed by a major
load, noting the depth of penetration, i.e. the hardness value,
directly from a dial, with a harder material giving a higher number.
In order to get a reliable reading the thickness of the test-piece
should be at least 10 times the depth of the indentation. Readings
should be taken on a flat surface, perpendicular to the load (convex
surfaces give lower readings).
15
Various Rockwell Scales
Scale Abbreviation Load Indenter Use
A HRA 60 kgf 120° diamond cone† Tungsten carbide
B HRB 100 kgf 1/16 inch diameter (1.588 mm) steel sphere
Aluminium, brass and soft steels
C HRC 150 kgf 120° diamond cone Harder steels
D HRD 100 kgf 120° diamond cone
E HRE 100 kgf 1/8 inch diameter (3.175 mm) steel sphere
F HRF 60 kgf 1/16 inch diameter (1.588 mm) steel sphere
G HRG 150 kgf 1/16 inch diameter (1.588 mm) steel sphere
†Also called a brale indenter
Very hard steel (eg chisels, quality knife blades): HRC 55–66 for
hardened High Speed Carbon and Tool Steels such as M2, W2, O1
and D2.
Axes: about HRC 45-55
Brass: HRB 55 to HRB 93
Fine Wood Work Association
Western Australia
TOOL STEELS
Physical Performance of Steels
Steel is produced to provide a wide range of physical characteristics
for differing users.
16
Machinists want
to cut metals,
including steels,
so cutting tools
have to have the
right mix of
hardness and
toughness.
Extruders want the form to
remain true to very tight
tolerances while hot metal is
forced through the forms.
Woodworkers want cutting
edges that can be hand
sharpened to an edge that holds
its polished faces while being
pushed at variable speeds.
through wood of widely differing
(to us) abrasiveness, by people
with widely differing strengths,
hammers and skill levels.
Fine Wood Work Association
Western Australia
TOOL STEELS
Common Steel Designations
The AISI-SAE (American Iron and Steel Institute – Society of
Automotive Engineers) grading system for steel is the most common
scale used.
Individual alloys within a grade are given a number, for
example: A2, O1, M2 etc.
Cold-working grades: The use of Oil quenching (O-grade) and Air
hardening (A-grade) helps reduce distortion in these steels
Water-hardening grades: W-grade tool steel gets its name from
its defining property of having to be water quenched
Other grades: designated by a variety of letters and numbers
depending on the principal alloying metal (and the country of
origin?)
17
AISI-SAE Tool Steel Grades
Defining Property AISI-SAE grade Significant Characteristics
Water-hardening W
Cold-working
O Oil-hardening
A Air-hardening; medium alloy
D High carbon; high chromium
Shock resisting S
High speed T Tungsten base
M Molybdenum base
Hot-working H
H1–H19: chromium base
H20–H39: tungsten base
H40–H59: molybdenum base
Plastic mold P
Special purpose L Low alloy
F Carbon tungsten
Fine Wood Work Association
Western Australia
TOOL STEELS
W Grade Steels
W-grade steels are essentially high carbon steel.
Low cost compared to other tool steels.
Hardenability is low unless quenched in water. These steels
can attain high hardness (above HRC 60) and are rather
brittle compared to other tool steels.
Toughness is increased by alloying with manganese, silicon and
molybdenum.
Up to 0.20% vanadium is used to retain fine grain sizes during
heat treating.
Typical applications for various carbon compositions are:
0.60–0.75% carbon: machine parts, chisels, setscrews;
properties include medium hardness with good toughness
and shock resistance.
0.76–0.90% carbon: forging dies, hammers, and sledges.
0.91–1.10% carbon: general purpose tooling applications
that require a good balance of wear resistance and
toughness, such as drills, cutters, and shear blades.
1.11–1.30% carbon: small drills, lathe tools, razor blades,
and other light-duty applications where extreme hardness is
required without great toughness.
18
Fine Wood Work Association
Western Australia
TOOL STEELS
Cold Working Steels
Used on larger parts or parts that require minimal distortion
during hardening.
Oil quenching and Air hardening helps reduce distortion
compared to higher stress caused by quicker water quenching.
More alloying elements are used in these steels, as compared to
water-hardening grades. These alloys increase the steels’
hardenability and thus require a less severe quenching process.
These steels are also less likely to crack and are often used to
make knife blades.
19
Oil Hardening Grades
Most common is O1 steel.
Very good cold working steel and also makes very good knives
and chisels. It can be hardened to 57-61 HRC.
Typical O1 composition:
0.90% Carbon
1.0–1.4% Manganese
0.50% Chromium
0.50% Nickel
0.50% Tungsten
Fine Wood Work Association
Western Australia
TOOL STEELS
Air Hardened Steels
Most common (for us wood workers) is A2 Steel (HRC ~65).
Characterised by low distortion during heat treatment because of
their high chromium content.
Their machinability is good for tool steels and they have a
balance of wear resistance and toughness, hence they are harder
to sharpen than O1, but retain their edge longer.
A2 to A10 steels produced for a variety of purposes (I have no
idea how the numbering came about – order of development?).
Typical A2 composition:
1.0% Carbon
1.0% Manganese
5.0% Chromium
0.3% Nickel
1.0% Molybdenum
0.15–0.50% Vanadium
20
Fine Wood Work Association
Western Australia
TOOL STEELS
21
D Grade Steels
D2 Steel is most common (HRC ~57).
D grade steels contain between 10% and 18% chromium. These
steels retain their hardness up to a temperature of 425°C
(797°F).
D2 is very wear resistant, but not as tough as lower alloyed
steels.
Mechanical properties of D2 are very sensitive to heat treatment.
It is widely used for shear blades, planer blades and industrial
cutting tools, sometimes used for knives.
Typical D2 composition:
1.5% Carbon
10.0–13.0% Chromium
0.45% Manganese
0.030% Phosphorus
0.030% Silicon
1.0% Vanadium
0.7% Molybdenum
0.30% Sulphur
Fine Wood Work Association
Western Australia
TOOL STEELS
22
High Speed Steels
High speed steels (HSS) are a subset of tool steels commonly used
in tool bits and cutting tools.
Often used in power saw blades and drill bits because it can
withstand higher temperatures without losing its temper
(hardness). This allows HSS to cut faster than high carbon steel,
hence the name.
HSS grades generally display hardness above HRC60 and a high
abrasion resistance compared to common carbon and tool steels.
High speed steels belong to the Fe-C-X multi-component alloy
system where X represents chromium, tungsten, molybdenum,
vanadium and/or cobalt.
More than 0.60% carbon and X component is usually in
excess of 7%.
Approx 10% tungsten and molybdenum in total maximises
hardness and toughness of high speed steels at the high
temperatures generated when cutting metals.
Fine Wood Work Association
Western Australia
TOOL STEELS
23
M2 grade HSS
M2 is a HSS in the tungsten-molybdenum series.
Carbides in it are small and evenly distributed, thus it has high
wear resistance.
After heat treatment, its hardness is around 64 HRC, but its
bending strength is very high and it has exceptional toughness.
It is normally used to manufacture a variety of tools, such as drill
bits, taps and reamers.
Fine Wood Work Association
Western Australia
TOOL STEELS
24
Powder Metallurgy Steels
Powder metallurgy (PM) tool steels and HSS manufacture.
1. Produce a steel powder (~50 micron) by nitrogen gas
atomization of a prealloyed melt.
2. Encapsulation of the produced spherical powder in metal
containers.
3. Consolidation of the packed powder by hot isostatic pressing
(HIP) at 1150°C and at a very high pressure which compresses
the powder into a fully dense billet.
4. In most cases, the billet then is rolled or forged to various bar
sizes.
Fine Wood Work Association
Western Australia
TOOL STEELS
25
Powder Metallurgy Steels (cont’d)
Performance of the PM steel is, to a large extent, determined by the
steel alloy used to form the powder. PM steels are now third
generation products (i.e. the result of considerable R & D work).
PM creates a refined carbide structure compared to conventional
high alloy grades such as D2 or D3. More uniform microstructure
improves cracking and fatigue resistance while maintaining or
improving wear resistance.
PM process allows more variability in alloys to increase alloying
content and select carbide forming elements other than
chromium, most commonly vanadium. Thus, steelmakers can
increase wear resistance while maintaining a similar or better
cracking resistance.
The small, uniform carbide structure that makes PM steels easier
to grind also delivers ground surfaces with smoother edges when
compared with D2 or D3.
Tool steel providers have developed new super HSS alloy for the
cutting tool market that can achieve hardness of 70 HRC or
slightly higher.
Veritas is selling PM-V11 blades and chisels – presumably this is
an 11% Vanadium (or Veritas trial number11?) – steel with a
hardness of HRC 62.5.
Fine Wood Work Association
Western Australia
TOOL STEELS
26
Japanese Chisel Steels
Steel Alloyant Percentages
O1 A2 White
Steel 1 White
Steel 2 Blue Steel
1 Blue Steel
2
Carbon 0.95 0.95-1.05 1.25-1.35 1.05-1.15 1.25-1.35 1.05-1.15
Manganese 1.2 1.0 0.02-0.03 0.02-0.03 0.02-0.03 0.02-0.03
Silicon 0.4 0.3 0.1-0.2 0.1-0.2 0.1-0.2 0.1-0.2
Chromium 0.5 4.75-5.5 0.3-0.5 0.2-0.5
Tungsten 0.5 1.5-2.0 1.0-1.5
Molybdenum 0.9-1.4
Vanadium 0.2 0.15-0.5
Phosphorus 0.3 0.3 0.025 0.025 0.025 0.025
Sulphur 0.03 0.03 0.004 0.004 0.004 0.004
Japanese chisels can take an extremely sharp edge that lasts a long
time, due mainly to the treatment and type of steel used for the
cutting edge. Today, the most common steels used are “white steel”
and “blue steel”, named from the colour of their packaging paper.
The steel is hardened to a higher degree than most Western chisels,
with 64 HRC being not uncommon. This results in the edge being
less likely to deform under impact, such as when chopping.
Making the chisels uses a forge-welding process, where multiple
layers are repeatedly hammered out, folded over, and forged,
causing the carbides in the steel to become very small and evenly
distributed. This results in an extremely sharp and long-lasting edge.
Laminated Japanese chisels are fabricated with a very hard cutting
layer of steel along the underside of the chisel, forge-welded to a
layer of softer steel or wrought iron along the top and in the tang.
Fine Wood Work Association
Western Australia
TOOL STEELS
27
Summary
Softer steels can be more easily sharpened to a fine edge, but
harder steels keep their edge longer.
Any steel – and in particular any tool steel – is a trade-off
between toughness (wear resistance) and hardness (brittleness)
based on the additives and treatment of the steel.
In any trade-off regime, there will be a range of prices for a
range of products… and in general, you’ll get what you pay for.
There is always an element of ‘sharpening time’ vs ‘time between
sharpening’ (just how wear resistant do you want your cutting
edge to be?) that should to be considered in deciding on what
steel to buy.
There will always be new steels being developed, which means
no choice will be correct forever.
Just about any current tool steel will be better than the tool
steels of half a century or more ago because of improvements in
production Quality Control and a better understanding of what
makes a good alloy for a given use.
Additional Information
There is a good Slow Motion Video of a set of Plane Blade and Cap
Iron tests at http://giantcypress.net/post/23159548132/this-is-the-
full-version-of-the-video-created-by?8de140f8
This shows how much influence the cap iron has on the performance
of a plane blade.
