MSE-226 Engineering Materials Lecture-7 ALLOY STEELS Tool Steels
TYPES of FERROUS ALLOYS FERROUS ALLOYS Plain Carbon Steels Alloy Steels Cast Irons - Low carbon Steel - Medium carbon steel - High carbon steel - Low alloy steels - High alloy steels - Stainless steels - Grey irons - White irons - Malleable irons - Nodular irons
ALLOY STEELS Carbon steels are regarded as steels containing not more than 1.65% Mn, 0.6% Si and 0.6% Cu, all other steels are regarded as alloy steels. Purpose of alloying 1) Increase hardenability 2) Improve mechanical properties at either high or low temperatures 3) Improve toughness at any minimum hardness or strength 4) Increase wear resistance 5) Increase corrosion resistance 6) Improve magnetic properties
ALLOY STEELS ALLOYING ELEMENTS Group 1 Group 2 Elements dissolve in ferrite Elements which combine with carbon to form simple and complex carbides ALLOYING ELEMENT Nickel Silicon Aluminum Copper Manganese Chromium Tungsten Molybdenum Vanadium Titanium GROUP1 Dissolved in ferrite Ni Si Al Cu Mn Cr W Mo V Ti GROUP2 Combined in carbide Mn Cr W Mo V Ti
ALLOYING ELEMENTS: Group 1 Group 1: Elements dissolve in ferrite Ni, Al, Si, Cu and Co are all found largely dissolved in ferrite. In addition, in the absence of carbon Group2 elements will also be found dissolved in ferrite. The effectiveness of these alloying elements in strengthening iron The hardening effect of the dissolved elements is small and they have very little effect on strengthening the steel.
ALLOYING ELEMENTS: Group 2 2) Carbide forming alloying elements; (Mn, Cr, W, Mo, V, Ti) Carbides found in steel are hard and brittle, their effect on the room temperature tensile properties is similar regardless of the specific composition. Following carbides may occur; * Cr ; Cr 7 C 3, Cr 23 C 6 * Mo ; Mo 2 C * V ; V 4 C 3, VC * W ; WC W 2 C * Ti ; TiC The presence of elements that form carbides influences; 1) Hardening temperature 2) Soaking time 3) Hardenability
ALLOY STEELS : TOOL STEELS Tool steel : High quality special steels used for cutting or forming purposes both in hot and cold condition. The carbon content is between 0.1-1.6% and they also contain alloying elements like Cr, Mo and V. Tool steels offer better durability, strength, corrosion resistance and thermal stability. They are used in applications such as blanking, die forging, forming, extrusion and plastic forming
CLASSIFICATION OF TOOL STEELS WATER HARDENING STEELS
CLASSIFICATION OF TOOL STEELS Tool Steel Type Prefix Specific Types COLD WORK W = Water Hardening O = Oil Hardening A = Medium alloy Air Hardening D = High Carbon, High Chromium W1, W2, W5 O1, O2, O6, O7 A2, A4, A6, A7, A8, A9, A10, A11 D2, D3, D4, D5, D7 SHOCK RESISTING S S1, S2, S4, S5, S6, S7 HOT WORK H H10-H19 Chromium types H20-H39 Tungsten types H40-H59 Molybdenum types HIGH SPEED M T MOLD STEELS P P6, P20, P21 SPECIAL PURPOSE L and F series L2, L6 Molybdenum types (M1, M2, M3-1, M3-2, M4, M6, M7, M10, M33, M34, M36, M41, M42, M46, M50 Tungsten types (T1, T4, T5, T6, T8,T15)
SELECTION OF TOOL STEELS Most tool steels are used in the following applications: Cutting, Shearing, Forming, Drawing, Extrusion, Rolling During selection of tool steels in any applications service requirements for application should be carefully examined.
SELECTION OF TOOL STEELS 1-CUTTING: Lathe Drills Tap Service requirements: Tool must have high hardness, good heat and wear resistance
SELECTION OF TOOL STEELS Shear blades Automotive parts made by blanking dies Service requirements: Tools have high wear resistance and fair toughness
SELECTION OF TOOL STEELS 2-FORMING Forming is carried out at high or low temperature and done by forging press Forging press Solid metal is forced into tool impression either hot or cold by using hot forging or coldheading die Piston rod impression die made up of tool steel Service requirements: Tools must have high strength, high toughness, and may require high red hardness (resistance to heat softening)
SELECTION OF TOOL STEELS 3-ROLLING Service requirements: Rolling dies must be hard enough to withstand the forces in forming and must have sufficient wear resistance and toughness to adjust the stress developed.
SELECTION OF TOOL STEELS 3-DRAWING Wire drawing Deep drawing Ao die die Ad tensile force Dies made up of tool steel Dies used for wire drawing Parts produced by deep drawing: cups, pans, cylinders and irregular shaped products Service requirements: Drawing dies require high strength and high wear resistance
SELECTION OF TOOL STEELS 4-EXTRUSION Done at high or low temperature by forcing material into a die Commonly extruded materials include metals, polymers, ceramics and concrete Die (tool steel) ram Work piece Service requirements: Hot extrusion Cold extrusion dies require toughness to withsatnd outward pressures and wear resistance. Hot extrusion dies must additionally posses high red-hardness.
IMPORTANT SELECTION FACTORS FOR TOOL STEELS Hardness Toughness Wear resistance Red Hardness 1-Depth of Hardening: Hardenability increases with alloy content. Shallow hardening steels; Group W, carburising grades of Group P, Group F 2-Toughness: Energy absorbed by a material up to fracture point. High toughness : S and H groups Low toughness: cold work tool steels
IMPORTANT SELECTION FACTORS FOR TOOL STEELS 3-Wear resistance: Resistance to abrasion or resistance to loss of dimensional changes In general a correlation exists between the hard, undissolved carbide particles and wear resistance. 4-Red-hardness (Hot hardness): Defined as resistance of the steel to the softening effect of heat. Steels that have high red-hardness contain W, Cr and Mo due to formation of stable carbides.
WATER HARDENING TOOL STEELS These are essentially plain carbon steels. Some high carbon grades contain small amount of Cr, V to improve hardenability and wear resistance Group1: 0.6-0.75%C- High toughness. e.g. Hammers, concrete breakers, rivet sets Group2: 0.75-0.95%C High toughness, hardness. e.g. Punches, dies, shear blades Group3: 0.95-1.40%C Increased wear resistance. e.g. Drills, turning tools Concrete breaker Shear blades Punches Turning tools
COLD WORK TOOL STEELS Most important group of tool steels (used in majority of tool applications) Types: O-type, A-type, D-type Group-O (oil hardening): - Contain Mn (~1%), and smaller amounts of Cr and W. - Relatively inexpensive - adequate wear resistance,, fair toughness and red-hardness Group-A (air hardening): - Medium alloy type (~1%C, up to 3% Mn, up to 5% Cr, ~1% Mo) - good wear resistance, fair toughness and red-hardness Group-D - High carbon-high alloy types( Up to 2.25%C and 12% Cr + Mo,V,Co) - excellent wear resistance and nondeforming properties
COLD WORK TOOL STEELS Applications: blanking and piercing dies, drawing dies for wires, bars, tubes, taps, forming tools, thread rolling die Blanking die and cut metal by blanking die Die for wire drawing thread rolling is processes for forming screw threads
HOT WORK TOOL STEELS In many applications tool is subjected to excessive heat (hot forging, extruding, die casting and plastic moulding) Tool steels which have developed for such high temp. applications are called Hot work tool steels and they have high red hardness For high red hardness Cr, Mo and W is used Sum of these alloying elements must be at least 5%. Basic types; 1- Chromium types 2- Tungsten types 3- Molybdenum types
HOT WORK TOOL STEELS 1) Hot-work Chromium Base (H11-H19) Composition : Min. 3.25% Cr, Rest: V, W, Mo (carbide former elements) They are resistant to heat softening. They also have good weldability Used in extrusion dies, die casting dies, forging dies 2) Hot-work Tungsten Base (H21-H26) Composition : Min. 9%W and min. 2-12% Cr They are resistant to heat softening. But they are more susceptable to brittleness Used in extrusion dies, madrels 3) Hot-work Molybdenum Base (H41-H43) Composition : 8% Mo, 4% Cr and small amounts of W and V More resistance to heat checking than tungsten grades
HOT WORK TOOL STEELS Hot forging die for connecting rod Casting die for stirring wheel As a summary, Hot-work tool steels; have good toughness because of their low carbon content good to excellent red-hardness fair wear resistance and machinability
HIGH SPEED TOOL STEELS (HSS) The most highly alloyed tool steels and contain large amounts of W or Mo along with Cr, V, Co. Carbon content varies between 0.7-1% C, some contains 1.5%C They have excellent red hardness, good wear resistance, poor machinability Two types of HSS: Mo base (group M), Tungsten base (group T) Applications: Cutting tools; such as milling cutters, drills, saws, taps The presence of hard carbides makes the tool wear-resistant
PRODUCTION OF TOOL STEELS Tool and die steels are produced in Electric arc furnaces (EAF) in small amounts and then cast into Ingots and Billets. Largest amount: 500 kgs Most common : 200-250 kgs (HSS) During casting at eutectic point; eutectic carbides (M6C, M4C3, M7C3, M23C6) form Alloy carbides
PRODUCTION OF TOOL STEELS After production tool steels are hot forged to obtain homogenous distribution of alloy carbides HOT FORGING To break down carbides There are two advantages in forged microstructures; 1) Uniformity 2) Solutionizing becomes easier
HEAT TREATMENT OF TOOL STEELS 1) Heating: During heating much damage may be done to the steel on heating as cooling heat slowly or preheat at a lower temperature (to prevent large temperature gradients) Overheating should be prevented to overcome grain growth problem. Also, quenching from excessive temperatures may result in cracking 2) Atmosphere: Surface should be protected against scaling and decarburization. Inert atmospheres may be used. 3) Quenching media: Water, brine, oil and air Carbon and low-alloy steels are quenched in brine and water. High alloy tool steels are quenched in oil, air or molten salts Sometimes to prevent cracking and distortion interrupted quenching is applied. Steel is quenched in a liquid bath of salt, then cooled in air
HEAT TREATMENT OF TOOL STEELS 4) Tempering: They should be tempered immediately just after quenching and before they have cooled to room temperature to minimize the danger of cracking due to strains introduced by cracking. Generally, double tempering is applied to high speed tools
HEAT TREATMENT STEPS OF TOOL STEELS Heating: The parts must be heated slowly or preheated at a lower temperature (to prevent large temperature gradients) When the steel is heated for hardening, the basic idea is to dissolve the carbides to such a degree that the matrix acquires an alloying content that gives the hardening effect without becoming coarse grained and brittle. Temp. ( o C) 1300 Austenitization Austenitization period is extremely important. So to limit or to prevent austenite grain coarsening small percentages of undissolved alloy carbides should be left in microstructure III.Preheating: If the component is large T= 1150-1200 o C II.Preheating: Partial dissolution of alloy carbides( Least stable carbide, Cr, will dissolve), T= 1050-1100 o C Air cooling Critical temp. Above this temperature crystal contraction (bcc to fcc) I.Preheating: Just below critical temp.; Aim: To equalize the temp. İnside and outside of the component Oil hardening tool steels time HSS
HEAT TREATMENT STEPS OF TOOL STEELS 1) Complex carbides don t dissolve even at high temperatures. This serves to lower the carbon and alloy content of austenite. Higher temperatures and soaking times are required for dissolution of alloy carbides. 2) Undissolved carbides also reduce the grain growth. Both these effects reduce the hardenability of steel
HEAT TREATMENT STEPS OF TOOL STEELS COOLING: Tool steels may be hardened by quenching in oil or cooling in air. 1300 o C HSS During cooling; 1) Contraction due to cooling 2) 4% expansion due to martensite formation Oil hardened tool steels QUENCH CRACKS occur (solution: use step quenching) STEP QUENCHING To equalize the temperature inside and outside of the component, parts are quenched to T>M s and wait for long time then cooled in air. As quenched tool or die 1)Undissolved alloy carbides (If this is HSS: M 6 C; If D1 or D2 : Cr-carbides) 2) Martensite 3) Retained Austenite (quenching medium T (R.T) > M f )
HEAT TREATMENT STEPS OF TOOL STEELS TEMPERING:Since high wear resistance is required temper the steel at around 550 o C HRc As-quenched hardness Secondary hardening (alloy carbides) ~550 o C Tempering temp. At 550 o C; 1) Some alloy carbides precipitate 2) Martensite becomes tempered martensite 3) Additionally, there is retained austenite in the structure
HEAT TREATMENT STEPS OF TOOL STEELS COOLING AFTER TEMPERING: Subsequent to tempering cooling to room temperature results in transformation of some austenite to martensite T( o C) 550 o C 2 hrs of tempering pearlite bainite CONDITIONING Log t Example: 12% retained- 84% Martensite 4% Undissolved carbides Tempering + Quench 1.5% retained- 10.5% fresh martensite 84% Temp. Martensite 4% Undissolved carbides I. Tempering II. Tempering (for tempering 10.5% fresh martensite) III. Tempering (sometimes necessary) MULTIPLE TEMPERING