CLASSIFICATION OF STEELS

Transcription

1 7 Alloy Steels

2 CLASSIFICATION OF STEELS low carbon <0.25wt%C Low Alloy med carbon wt%C high carbon wt%C High Alloy Name Additions none Example Hardenability TS EL Uses plain HSLA plain auto struc. sheet Cr,V Ni, Mo none bridges towers press. vessels heat treatable plain tool austentitic stainless Cr, Ni Cr, V, none Cr, Ni, Mo Mo Mo, W crank pistons shafts gears bolts wear hammers applic. blades wear applic. drills saws dies increasing strength, cost, decreasing ductility high T applic. turbines furnaces V. corros. resistant 3

3 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

4 Used in manufacturing processes as well as for machining metals. Able to withstand high specific loads Stable at elevated temperatures (200 o C) Composition: high carbon: %wt C Tool Steels 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

5 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.

6 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 C. 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 C be carried out allowing the tooling to cool to room temperature after stress relief

7 Normalizing & Annealing 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 550 C when the steel may be removed from the furnace and air cooled to room temperature.

8 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

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10 AISI NUMBER TYPICAL APPLICATIONS Compositions Applications M1 A2 0.85C, 3.75Cr, 0.3Ni, 8.7Mo 1.75W and 1.2 V 1.0C, 5.5Cr, 0.3Ni, 1.15Mo and 0.35 V (No tungsten) Drills, saws, lathe, and planer tools Punches and dies D2 1.5C, 12Cr, 0.3Ni, 0.95Mo and 1.1 V (No tungsten) Cutlery and drawing dies W1 1.1C, 0.15Cr, 0.2Ni, 0.1Mo. 015W and 0.1 V Woodworking tools

11 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.

12 Mechanical Properties High Speed Steel (HSS) 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 (600 o C) 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.

13 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.

14 Influence of alloying elements on the properties

15 Microstructure of HSS Fine grain structure at 1000x showing uniform dispersal of carbides. Sample was austenitized at 2240 F and triple tempered at 1025 F to a hardness of HRC 70.

16 High Speed Steel: Applications 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

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