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The Science and Engineering of Materials, 4Th Ed Donald R The Science and Engineering of Materials, 4th ed Donald R. Askeland – Pradeep P. Phulé Chapter 3 – Ferrous Alloys 1 Objectives of Chapter 3 Discuss how to use the eutectoid reaction to control the structure and properties of steels through heat treatment and alloying. Examine two special classes of ferrous alloys: stainless steels and cast irons. 2 Chapter Outline 3.1 Designations and Classification of Steels 3.2 Simple Heat Treatments 3.3 Isothermal Heat Treatments 3.4 Quench and Temper Heat Treatments 3.5 Effect of Alloying Elements 3.6 Application of Hardenability 3.7 Specialty Steels 3.8 Surface Treatments 3.9 Weldability of Steel 3.10 Stainless Steels 3.11 Cast Irons 3 (a) In a blast furnace, iron ore is reduced using coke (carbon) and air to produce liquid pig iron. The high-carbon content in the pig iron is reduced by introducing oxygen into the basic oxygen furnace to produce liquid steel. An electric arc furnace can be used to produce liquid steel by melting scrap. (b) Schematic of a blast furnace operation. 4 Classification of Steels Bsed on composition (Carbon Steels, Low alloy steels, Stainless steels, …) Bsed on steel making method (Electric arc furnace, Blast furnace, …) Bsed on process mehod (Hot rolling, Cold rolling, …) Bsed on product shape (Sheet, Strip, Bar, Plate, …) Bsed on deoxidizing method (Killed steels, Semi-killed steels, Wild steels) Bsed on Microstructure (Ferrite steels, Austenitic steels, Dual phase steels, …) Bsed on properties (Stainless steels, Heat resisting steels, Free cutting steels, …) Bsed on heat treatment (Annealed steels, Quenched and tempered steels, …) Bsed on application (Structural steels, Spring steels, High speed steels, …) Bsed on product quality (Base steels, Quality steels, Special steels, …) 5 Systems for designations of steels AISI: American Iron and Steel Institute SAE: Society of Automotive Engineers UNS: Unified Numbering System ASTM: American Society for Testing and Materials ASME: American Society of Mechanical Engineers EN: European Norms ANSI: American National Standards Institute API: American Petroleum Institute AWS: American Welding Society CSA: Canadian Standards Association DIN: Deutsches Institut für Normung (The German Institute for Standardization) JIS: Japanese Institute of Standards 6 Designations of Steels – AISI & SAE The AISI and SAE provide designation systems for steels that use a four- or five-digit number. XXXX The first number refer to the major alloying elements present. The second number designates the subgroup alloying element OR the relative percent of primary alloying element. The last two or three numbers refer to the percentage of carbon. 1xxx Carbon steels 2xxx Nickel steels 3xxx Nickel-chromium steels 4xxx Molybdenum steels 5xxx Chromium steels 6xxx Chromium-vanadium steels 7xxx Tungsten steels 8xxx Nickel-chromium-molybdenum steels 9xxx Silicon-manganese steels 7 8 Designations of Steels – AISI & SAE 10XX Plain carbon, Mn 1.00% max 11XX Resulfurized free machining 50XX Cr 0.27-0.65% Carbon steels 12XX Resulfurized/rephosphorized free machining 51XX Cr 0.80-1.05% 15XX Plain carbon, Mn 1.00-1.65% Chromium steels 50XXX Cr 0.50%, C 1.00% min Manganese steels 13XX Mn 1.75% 51XXX Cr 1.02%, C 1.00% min 23XX Ni 3.50% 52XXX Cr 1.45%, C 1.00% min Nickel steels 25XX Ni 5.00% Chromium-vanadium steels 61XX Cr 0.60-0.95%, V 0.10-0.015% 31XX Ni 1.25%, Cr 0.65-0.80% Tungsten-chromium steels 72XX W 1.75%, Cr 0.75% 32XX Ni 1.75%, Cr 1.07% 81XX Ni 0.30%, Cr 0.40%, Mo 0.12% Nickel-chromium steels 33XX Ni 3.50%, Cr 1.50-1.57% Nickel-chromium- 86XX Ni 0.55%, Cr 0.50%, Mo 0.20% 34XX Ni 3.00%, Cr 0.77% molybdenum steels 87XX Ni 0.55%, Cr 0.50%, Mo 0.25% 40XX Mo 0.20-0.25% 88XX Ni 0.55%, Cr 0.50%, Mo 0.35% Molybdenum steels 44XX Mo 0.40-0.52% Silicon-manganese steels 92XX Si 1.40-2.00%, Mn 0.65-0.85%, Cr 0-0.65% Chromium-molybdenum steels 41XX Cr 0.50-0.95%, Mo 0.12-0.30% 93XX Ni 3.25%, Cr 1.20%, Mo 0.12% Nickel-chromium- 43XX Ni 1.82%, Cr 0.50-0.80%, Mo 0.25% Nickel-chromium- 94XX Ni 0.45%, Cr 0.40%, Mo 0.12% molybdenum steels 47XX Ni 1.05%, Cr 0.45%, Mo 0.20-0.35% molybdenum steels 97XX Ni 0.55%, Cr 0.20%, Mo 0.20% Nickel-molybdenum 46XX Ni 0.85-1.82%, Mo 0.20-0.25% 98XX Ni 1.00%, Cr 0.80%, Mo 0.25% steels 48XX Ni 3.50%, Mo 0.25% 9 Alloying element Effect on the steel Increases hardness without reducing ductility. Refines grain structure and increases toughness. Chromium Simplifies heat treatment requirements. Increases strength without reducing ductility. Refines grain structure and increases toughness. Nickel Simplifies heat treatment requirements. Added as a deoxidising and desulphurising agent. Considered as alloy when above 1%. Enables Manganese oil quenching. Silicon Added as a deoxidising agent. Stabilises carbides formed by other alloying elements. Improves oil hardening and air hardening properties. Used with Chromium and Nickel to Molybdenum simplify heat treatment. Vanadium Widely used in tool steels. Steel retains its hardness at high temperatures. Tungsten Widely used in tool steels. Tool maintains its hardness even at red heat. 10 Alloying element Effect on the steel Cr is commonly added to increases corrosion resistance and oxidation resistance, to increase hardenability, or to improve high-temperature strength. As a hardening element, Chromium is frequently used with a Chromium toughening element such as nickel to produce superior mechanical properties. At higher temperatures, chromium contributes to increased strength. Chromium is strong carbide former. Ni is a ferrite strengthener. Nickel does not form carbides in steel. It remains in solution in ferrite, Nickel strengthening and toughening the ferrite phase. Nickel increases the hardenability and impact strength of steels. Mg is generally beneficial to surface quality especially in resulfurized steels. Manganese contributes to Manganese strength and hardness, but less than carbon. Increasing the manganese content decreases ductility and weldability, but less than carbon. Manganese has a significant effect on the hardenability of steel. Si is one of the principal deoxidizers used in steelmaking. Silicon is less effective than manganese in Silicon increasing as-rolled strength and hardness. In low-carbon steels, silicon is generally detrimental to surface quality. Mo increases the hardenability of steel. Molybdenum may produce secondary hardening during the tempering Molybdenum of quenched steels. It enhances the creep strength of low-alloy steels at elevated temperatures. V increases the yield strength and the tensile strength of carbon steel. The addition of small amounts of Vanadium can significantly increase the strength of steels. Vanadium is one of the primary contributors to Vanadium precipitation strengthening in micro alloyed steels. When thermo mechanical processing is properly controlled, the ferrite grain size is refined and there is a corresponding increase in toughness. The impact transition temperature also increases when vanadium is added. 11 Alloying element Effect on the steel Al is widely used as a deoxidizer. Aluminum can control austenite grain growth in reheated steels and is therefore added to control grain size. Aluminum is the most effective alloy in controlling grain growth prior Aluminum to quenching. Titanium, zirconium, and vanadium are also valuable grain growth inhibitors, but there carbides are difficult to dissolve into solution in austenite. Nb increases the yield strength and, to a lesser degree, the tensile strength of carbon steel. The addition of small amounts of Niobium can significantly increase the yield strength of steels. Niobium can also have a Niobium moderate precipitation strengthening effect. Its main contributions are to form precipitates above the transformation temperature, and to retard the recrystallization of austenite, thus promoting a fine-grain microstructure having improved strength and toughness. Ti is used to retard grain growth and thus improve toughness. Titanium is also used to achieve Titanium improvements in inclusion characteristics. Titanium causes sulfide inclusions to be globular rather than elongated thus improving toughness and ductility in transverse bending. Phosphorus P increases strength and hardness, decreases ductility and notch impact toughness of steel. S decreases ductility and notch impact toughness especially in the transverse direction. Weldability decreases with increasing sulfur content. Sulfur is found primarily in the form of sulfide inclusions. Sulfur Sulphur levels are normally controlled to low levels. The only exception is free-machining steels, where sulfur is added to improve machinability. 12 Designations of Steels – AISI & SAE Prefix letter (designate the process used to produce the steel): E = Electric furnace. EXXXX M = to designate merchant quality steel. MXXXX If a letter is inserted between the 2nd and 3rd number, B = boron has been added to increase hardenability. XXBXX L = lead has been added for improving machinability. XXLXX Suffix letter: H = when hardenability is a major requirement. XXXXH 13 Designations of Steels – AISI & SAE (Stainless steels) AISI/SAE use three digits sometimes followed by some letters to designate wrought stainless steels. The first digit specifies the alloy classification. The last two digits represent no information about the composition. 2xx Austenitic stainless steel 3xx Ferritic stainless steel 4xxx Martensitic stainless steel 301 405 302 430 Ferritic stainless steel 303 446 Austenitic stainless steel 304 410 310 420 Martensitic stainless steel 316 440 14 Designations of Steels – UNS The Unified Numbering System (UNS) for Metals and Alloys is established to correlate several internationally used alloy and metal numbering systems, which are now commonly used by trade associations, societies, producers, and users of alloys and metals. This system does not create the confusion caused due to usage of previous systems that used the same number for different metals or alloys or different identification numbers for the same metal or alloy.
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