A BRIEF LOOK AT TOOL STEELS FOR WOODWORKERS

Transcription

1 A BRIEF LOOK AT FOR WOODWORKERS Presentation by Mike Wiggin August 2013 meeting 1

2 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 they make the machines that make the steel as well as making the steel. Powder-Metallurgy Tool Steel: an overview. Hillskog T., MetalForming Magazine, p Popular Woodworking Magazine. Ron Hock makes and sells after market plane blades. Wikipedia you pays your money and you takes your chances. MDME website on steels - TAFE NSW Lee Valley Tools UK Centre for Materials Education - FE Resources Seeing Further. Bill Bryson ed. HarperPress 2011 ISBN

3 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

4 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

5 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. Two-dimensional view of pearlite consisting of alternating layers of cementite and ferrite. 5

6 Steel The Oxford Dictionary defines steel as a hard, strong grey or bluishgrey 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

7 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 (SiO 2 ) 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

8 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. Dislocation in a Crystal Lattice 8

9 Mechanical Properties of Steel (cont d) Increasing the carbon content decreases the amount of ferrite and increases the proportion of pearlite in the structure. 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. 9

10 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 % carbon and mild steel contains % carbon; making it malleable and ductile, but it can t be hardened by heat treatment. MEDIUM CARBON STEEL Approximately % carbon content. Balances ductility and strength and has good wear resistance; used for large parts, forging and automotive components. HIGH CARBON STEEL Approximately % carbon content. Very strong, used for springs and high-strength wires. ULTRA-HIGH CARBON STEEL Approximately % 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

11 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. 11 ALUMINIUM improves retention of hardness and red hardness.

12 Modifying the Properties of Steel The properties of the steel can also be changed by several other methods. 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. 12

13 Modifying the Properties of Steel (cont d) ANNEALING Heating the steel to C for several hours and then gradual cooling. decreases hardness and increases malleability. 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 13

14 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 Brinell. Increasing the carbon content of the iron will initially cause a significant corresponding increase in the iron s hardness and tensile strength. 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. 14 A Rockwell Hardness testing machine

15 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). 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 for hardened High Speed Carbon and Tool Steels such as M2, W2, O1 and D2. 15 Axes: about HRC Brass: HRB 55 to HRB 93

16 Physical Performance of Steels Steel is produced to provide a wide range of physical characteristics for differing users. 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. 16

17 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?) AISI-SAE Tool Steel Grades Defining Property AISI-SAE grade Significant Characteristics Water-hardening Cold-working Shock resisting High speed Hot-working Plastic mold Special purpose W O A D S T M H P L F Oil-hardening Air-hardening; medium alloy High carbon; high chromium Tungsten base Molybdenum base H1 H19: chromium base H20 H39: tungsten base H40 H59: molybdenum base Low alloy Carbon tungsten 17

18 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: % carbon: machine parts, chisels, setscrews; properties include medium hardness with good toughness and shock resistance % carbon: forging dies, hammers, and sledges % carbon: general purpose tooling applications that require a good balance of wear resistance and toughness, such as drills, cutters, and shear blades % carbon: small drills, lathe tools, razor blades, and other light-duty applications where extreme hardness is required without great toughness. 18

19 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. 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 HRC. Typical O1 composition: 0.90% Carbon % Manganese 0.50% Chromium 0.50% Nickel 0.50% Tungsten 19

20 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 % Vanadium 20

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 % Chromium 0.45% Manganese 0.030% Phosphorus 0.030% Silicon 1.0% Vanadium 0.7% Molybdenum 0.30% Sulphur 21

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. 22

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. 23

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. 24

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

26 Japanese Chisel Steels 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. Steel Alloyant Percentages O1 A2 White Steel 1 White Steel 2 Blue Steel 1 Blue Steel 2 Carbon Manganese Silicon Chromium Tungsten Molybdenum Vanadium Phosphorus Sulphur

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 This shows how much influence the cap iron has on the performance of a plane blade. 27