Chapter7 alloy steels 2 - Teaching Learning Site 1613 : Materials Science 2007 Heat Treatment...

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Transcript of Chapter7 alloy steels 2 - Teaching Learning Site 1613 : Materials Science 2007 Heat Treatment...

  • Alloy SteelsAlloy Steels

    77

  • SME 1613 : Materials Science 20073

    Low Alloy High Alloy

    low carbon

  • SME 1613 : Materials Science 2007

    Introduction

    Alloy steels differ from carbon steels in that they have compositions that extend beyond the limits set for carbon steels.

    As a general guide, alloy steel will have:-

    Manganese content >1.65%

    Silicon content >0.5%

    Copper content >0.6%

    Type of alloy steels:-

    1. Tool steels

    2. High speed steels

    3. High strength low alloy steels (HSLA)

    4. Maraging steels

  • Used in manufacturing processes as well as for machining metals.

    Able to withstand high specific loads

    Stable at elevated temperatures (200oC)

    Composition:

    high carbon: 0.6-1.4 %wt C

    and others elements : chromium (Cr) ,vanadium (V), tungsten (W), and

    molybdenum (Mo) to form carbide

    Six types of tool steels

    1. High speed,

    2. Hot work,

    3. Cold work,

    4. Shock resisting,

    5. Special purpose and

    6. Water hardening

    Tool Steels

  • SME 1613 : Materials Science 2007

    Effects of Alloying Elements on Tool Steel Properties

    Carbon (C): increases hardness slightly and wear resistance.

    Manganese (Mn): small amounts reduce brittleness and improve forgeability. Larger amounts of manganese improve hardenability, permit oil quenching, and reduce quenching deformation.

    Silicon (Si): Improves strength, toughness, and shock resistance.

    Tungsten (W): Improves hardness - used in high-speed tool steel.

    Vanadium (V): Refines carbide structure and improves forgeability, also improving hardness and wear resistance.

    Molybdenum (Mo): Improves deep hardening, toughness, and in larger amounts, "hot hardness". Used in high speed tool steel because it's cheaper than tungsten.

    Chromium (Cr): Improves hardenability, wear resistance and toughness.

    Nickel (Ni): Improves toughness and wear resistance to a lesser degree.

  • SME 1613 : Materials Science 2007

    Heat Treatment

    Heated to austenite temperature prior to quenching

    Tempered to form martensite

    Never allow cooling to room temperature after quenching as this will lead to cracking.

    The tool steel must be transferred to the tempering furnace whilst still warm - about 50-80C. During multiple tempering operations tool steels may be allowed to cool to room temperature between tempers.

    Residual stresses need to be removed prior to heat treatment and it is recommended that a stress relief heat treatment at 500-550C be carried out allowing the tooling to cool to room temperature after stress relief

  • SME 1613 : Materials Science 2007

    The normalising: cycle involves heating slowly and carefully to the

    normalising temperature for that particular steel, holding at

    temperature sufficient to allow homogenisation to occur and then air

    cooling to room temperature.

    The full annealing: process involves heating the steel slowly and

    uniformly to a temperature above the upper critical transformation

    point and holding until complete austenitisation and homogenisation

    occurs. Cooling after heating is carefully controlled at a particular

    rate as recommended by the steel manufacturer for the grade of tool

    steel involved.

    Cooling at this specified cooling rate is continued down to 550C

    when the steel may be removed from the furnace and air cooled to

    room temperature.

    Normalizing & Annealing

  • SME 1613 : Materials Science 2007

    Properties of tool steels & Applications

    wear resistant

    capable of holding sharp cutting edge

    very hard

    Applications

    wood working tools

    concrete drills

    cutting tools

    drawing dies

    embossing dies and etc

  • SME 1613 : Materials Science 2007

  • SME 1613 : Materials Science 2007

    TYPICAL APPLICATIONS

    Woodworking tools1.1C, 0.15Cr, 0.2Ni, 0.1Mo. 015W and 0.1 V

    W1

    Cutlery and drawing dies

    1.5C, 12Cr, 0.3Ni, 0.95Mo and 1.1 V (No tungsten)

    D2

    Punches and dies1.0C, 5.5Cr, 0.3Ni, 1.15Mo and 0.35 V (No tungsten)

    A2

    Drills, saws, lathe, and planer tools

    0.85C, 3.75Cr, 0.3Ni, 8.7Mo 1.75W and 1.2 V

    M1

    ApplicationsCompositions AISI NUMBER

  • SME 1613 : Materials Science 2007

    High Speed Steel (HSS)

    Capable of cutting metal at a much higher rate (high speed) than

    carbon tool steel

    Used to cut other metals/alloys

    continues to cut and retain its hardness even when the point of the tool

    is heated to a low red temperature.

    Composition

    Tungsten is the major alloying element

    Also combined with molybdenum, vanadium and cobalt in varying

    amounts.

  • SME 1613 : Materials Science 2007

    High Speed Steel (HSS)

    Mechanical Properties

    1. High attainable hardness

    Minimum attainable hardness of High-Rupturing Capacity (HRC)

    Typical cutting tools may be HRC 64/68, depending on grade & application.

    High carbon, along with elements to promote strong secondary hardness (W, Mo), are common to all high speed steels for this purpose

    2. High hardness at elevated temperatures (600oC)

    This involves both red hardness (the ability to stay hard at elevated temperature during cutting) and temper resistance (the ability to resist permanent softening over time due to high temperature exposure).

    The W and/or Mo contents of high speed steels promote these properties, and cobalt enhances it further when needed.

  • SME 1613 : Materials Science 2007

    High Speed Steel (HSS)

    3. High wear resistance

    To promote edge retention during cutting.

    Constant abrasion wears away tool surfaces.

    The high volumes of wear-resistant carbides in high speed steel

    microstructures aids in resisting this abrasion.

    4. Sufficient impact toughness

    To handle interrupted cutting applications, to avoid chipping during

    cutting, and to avoid breakage in fragile tools.

    High speed steels are notably tougher than carbide or ceramic

    materials.

    CPM (Crucible Powder Metallurgy)-produced high speed steels offer

    the ultimate in impact resistance for cutting tools.

  • SME 1613 : Materials Science 2007

    Influence of alloying elements on the properties

  • SME 1613 : Materials Science 2007

    Microstructure of HSS

    Fine grain structure at 1000x showing uniform dispersal of

    carbides. Sample was austenitized at 2240F and triple

    tempered at 1025F to a hardness of HRC 70.

  • SME 1613 : Materials Science 2007

    Turning, planning tools of all types, broaches and hobs,

    taps, twist drills, economical high speed steel grade,

    drills, reamers, milling tools, wood working tools, cold

    work tools, tool bits

    High Speed Steel: Applications

  • SME 1613 : Materials Science 2007

  • SME 1613 : Materials Science 2007