Ferrous Alloys PDF

FERROUS ALLOYS 1  TYPES OF METAL ALLOYS  Metals and alloys have many useful engineering properties and so have wide application in engineering design.  Iron and its alloys (principally steel) account for about 90 percent of the world’s production of metals mai...

FERROUS ALLOYS 1  TYPES OF METAL ALLOYS  Metals and alloys have many useful engineering properties and so have wide application in engineering design.  Iron and its alloys (principally steel) account for about 90 percent of the world’s production of metals mainly because of their combination of good strength, toughness, and ductility at a relatively low cost. 2  FERROUS ALLOYS  Iron is the main constituent in ferrous alloys.  They are produced in larger quantities than other metals.  Their widespread use is accounted for by three factors:  Iron-containing compounds exist in abundant quantities within the earth’s crust.  Ferrous alloys may be produced using relatively economical extraction, refining, alloying and fabrication techniques.  Ferrous alloys have a wide range of mechanical and physical properties.  The ferrous materials are divided into two groups according to the carbon content:  The ferrous materials with carbon content higher than 2% are called as cast irons, and those with carbon content less than 2% as steels. 3  Production of Ferrous Alloys All ferrous materials begin in a blast furnace where iron ore, limestone and coke(a form of carbon) react to form PIG IRON. Iron ores:  HEMATITE: Fe2O3 contains 70% Fe The most important iron ore.  MAGNETITE:Fe3O4 contains 72,4% Fe Coke has a dual role:  It is a fuel for the blast furnace.  It is also a reducing agent.  The coke is burned using a blast of air (sometime enriched with oxygen).The coke reduces the iron oxide into a molten Vol. iron known as pig iron. ~2000 m3  Carbon monoxide (CO) and carbon dioxide (CO2) are produced as gaseous by-products.  Limestone (CaCO3) is added as a fluxing agent to help removing impurities. The limestone decomposes and forms ~ 40 m CaO. The calcium oxide combine with impurities such as ~ 1800°C slag silica (SiO2, sulfur (S), and alumina (Al2O3 )to produce molten slag.  Slag is a by-product of the blast furnace process. ~ 1000°C ~ 10 m  The slag floats on top of the molten iron, because it is lighter. The slag are withdrawn off the top of the molten iron. 4  Reactions in the Blast Furnace  Heat generation: C+O2 CO2  Reduction of iron ore to pig iron: CO2+C 2 CO CO+Fe2O3 2 FeO+CO2 FeO+CO Fe+CO2  Purification:  Decomposition of fluxing agent: CaCO3 CaO+CO2  Forming of slag: CaO+SiO2 CaSiO3 (Molten slag: calcium silicate) 5 6   Pig iron contains about 95% iron, 4% carbon, 0.3 to 0.9% silicon, 0.5% Mn, and 0.025 to 0.05% of sulfur, phosphorus, and titanium, at about 16000C. Further refinement of the pig iron is required for both cast iron and steel  Pig iron is either used to produce cast iron or converted into steel in secondary processes.  STEELMAKİNG:  Oxygen is blown into the liquid pig iron in the converter (BOF: Basic Oxygen Furnace: steel making furnace) to eliminate the excess carbon content up to a maximum of 2% (for practical applications 1.4%).  In the converter, in addition to pig iron, scrap and limestone are added. Limestone is added to collect impurities such as P, S in slag form.  When the refining process is completed, the oxygen is shut off and the furnace is tilted to remove the slag.  Necessary alloying elements are also added.  Steel processing occurs at a very large scale. About 300 tons of pig iron can be refined into molten steel in about 30 minutes.  We can melt pig iron again in another furnace called cupola to produce cast iron. In cupola, pig irons are remelted with more coke and limestone and 7 tap it out into moulds.  SLAB, BLOOM, BİLLET  Cast steel ingots(1-300 tons) are rolled into slabs, blooms, and billets, which are latter further rolled, extruded, or drawn to different shapes and marketed as engineering materials. A bloom has a square cross section 150x150mm or larger.  A slab is rolled from a bloom and has a rectangular cross section of width 250 mm or more thickness 40 mm or more.  A billet is rolled from a bloom and is square with dimensions Semi-finished products 40 mm on a side or larger. Flat products  Finished steel products obtained upon hot rolling or hot forging of semi-finished steels (bloom, billet, slab). These cover two broad categories of products, namely long products (bars, rods) and Long products flat products (plate sheet, strip). Some of the steel products made in a rolling mill 8  STEELS  Steels are very versatile materials. Steels of low strength and super strength, soft and hard steels, ferromagnetic and paramagnetic steels, steels to resist temperature extremes, corrosion, impact, abrasion, and so on.  Steels can be classified based on their composition:  Plain carbon steels contain up to about 2% carbon. However, it is generally less than 1.0 wt%. These steels may also contain other elements, such as Mn (up to 1.65%), Si (max. 0.6%), Cu (up to 0.6%), and residual amounts of S, P (less than 0.05%).  Alloy steels contain more than 1.65%Mn, 0.60%Si, or 0.60 Cu. In addition, any steel to which any other alloying element (such as Ni, Cr, Mo, Ti, etc.) is intentionally added is considered an alloy steel.  However, the term of “alloy steel” is used for steels which contain modest amount of alloying elements and rely on heat treatment to improve the desired mechanical properties.  These steels are used for making tools (hammers, chisels, etc.) and also in making parts such as axles, shafts, and gears.  The total carbon content is up to 1% in alloy steels.  For the low alloy steels, the total alloying element content is below 5%. chisel gear hammer 9  LOW CARBON STEELS (< 0.25 wt% C)  Low-carbon steels generally contain less than about 0.25 wt % C.  They are unresponsive to heat treatments intended to form martensite. Strengthening is accomplished by cold work.  They have relatively low strengths but very high ductilities and toughnesses.  In addition, they are machinable, weldable, of all steels, are the least expensive to produce.  These steels are used for sheet material for forming applications for fenders and body panels for automobiles.  Other typical applications include structural shapes (I-beams, channel and angle iron), and sheets that are used in pipelines, building, bridges, and tin cans.  They have a yield strength of 275 MPa, tensile strengths between 415 and 550 MPa, and ductility of 25% EL(elongation). angle iron fender I-beams tin can body panels for automobiles channel 10  HIGH_STRENGTH, LOW ALLOY (HSLA) Steels  HSLA steels are developed to replace conventional low carbon steels.  They have low carbon contents plus relatively small amounts of alloying elements such as Mn, Ni,Cu, Cr, Mo (3% total of these elements)  They have better strength to weight ratios than plain carbon steels.  in addition, they are ductile, formable and machinable and weldable.  In normal atmospheres, the HSLA steels are more resistant to corrosion than the plain carbon steels, which they have replaced in many applications where structural strength is critical (e.g., bridges, towers, support columns in high-rise buildings, and pressure vessels). 11  MEDIUM CARBON STEELS (0.25-0.60 wt% C)  The medium-carbon steels contain 0.25 to 0.60% carbon.  Their mechanical properties can be improved by heat treatments.  The plain medium-carbon steels have low hardenabilities (Hardenability is a qualitative measure of the rate at which hardness drops off with distance into the interior of a specimen as a result of diminished martensite content. Hardenability is not hardness, which is the resistance to indentation).  Additions of chromium, nickel, and molybdenum improve the capacity of these alloy to be heat treated.  These steels are used in making machinery, tractors, mining equipments, railway wheels and tracks, gears, crankshafts, and other machine parts. HIGH CARBON STEELS (0.60-1.4 wt% C)  The high carbon steels normally have carbon contents between 0.6 and 1.4 wt%.  They are the hardest, strongest, and yet least ductile of the carbon steels.  They are almost always used in a hardened and tempered condition.  The tool and carbon die steels, cutters, springs are high alloys.  They usually contain chromium, vanadium, tungsten, and molybdenum. These elements combine with carbon to form very hard and wear-resistant carbide compounds (e.g., Cr23C6, V4C, and WC).  Increasing carbon content strengthens and hardens the steel, but its ductility is reduced. 12  STAINLESS STEELS  The stainless steels have excellent corrosion (rusting) resistance in many environments, especially the ambient atmosphere.  The corrosion resistance of stainless steels is due to their high Cr contents.  In order to make a “stainless steel” stainless, there must be at least 11% Cr in the steel.  Cr permits a thin protective surface layer of chromium oxide to form when the steel is exposed to oxygen. This surface oxide protects the underlying Fe-Cr alloy from corroding.  Corrosion resistance may also be enhanced by Ni and Mo additions. Ni is added for heat resistant applications (Ni≥8%).  Generally stainless steels contain very low carbon (C≤0.15%). Increasing the carbon content decreases the corrosion resistance of the alloy. Because chromium carbide forms to reduce the amount of free Cr available in the alloy. 13  STAINLESS STEELS Stainless Steels (related to their microstructure) Ferritic Stainless Martensitic Stainless Austenitic Stainless Steels (FSS) Steels (MSS) Steels (ASS)  Martensitic stainless steels are strengthened by heat treatment.(its carbon content is high) They are strong and hard. But they are the least corrosion resistant of the three groups.  For ASSs , the austenite phase field is extended to room temperature.  FSSs are composed of α-ferrite (BCC phase).  ASSs and FSSs are hardened and strengthened by cold work because they are not heat treatable.(their carbon content low)  The ASSs are the most corrosion resistant because of the high chromium contents and also the nickel additions. So, they are very common.They are 14 identified as 18-8 stainless steel.  Speciality Steels Maraging steels: They have low carbon content and high amounts of nickel (15% to 25%) and lesser proportions of cobalt, molybdenum and titanium. They are strengthened by precipitation hardening. They have very high strength as a result of martensitic transformation and following aging treatment with good toughness. They are formable, weldable, machinable. Free-machining steels: They are formulated to improve machinability. Lead, sulfur, tin, bismuth, selenium, tellurium, phosphorus are added in small amounts to increase machinability. Lead is less frequently used today because of enviromental and health concerns. These elements act to lubricate the cutting operation, reduce friction, and break up chips for easier disposal. Interstitial-free (IF) steels : These steels have extremely low carbon levels (0.005%), which result from the use of alloying elements such as niobium and titanium that combine with carbon and leave the steel virtually free of interstitial atoms. The result is excellent ductility, even greater than low carbon steel. Applications include deep drawing operations in the automotive industry. 15  DESIGNATION(CODING) OF STEELS  Steels are specified in many ways. TSE (Turkish Standards Institution), AISI (American Iron and Steel Institute), SAE (Society of Automotive Engineers), ASTM (The American Society for Testing and Materials), DIN (The German Institute for Standardization), etc. AMERICAN STANDARTS  AISI and SAE specifications: This designation systems has four-digit numbers. (If the carbon concentration is higher than 1%, the number of digits is five.)  The first two digits (or numbers) refer to the major alloying elements, and the last two numbers indicate the weight % carbon concentration multiplied by 100. Main group of the steel XXXX Carbon content % x Alloy content 100 (major alloying elements) 16  DESIGNATION OF STEELS  For plain carbon steels, the first two digits are 1 and 0. An AISI 1040 steel is a plain carbon steel with 0.4% C. An SAE 10120 steel is a plain-carbon steel containing 1.2% C.  Alloy steels are designated by other initial two-digit combinations (such as 12, 43, 86). An AISI 4340 steel is an alloy steel containing 0.4% C.  One digit in the systems of a five digit sometimes may be an alphabetical character. Then, only last two characters indicate the carbon content again. The alphabetical character refers to an additional element. Free machining steels are developed Free for fast and economic machining of parts. machining 12L10 steel Machinability is improved by addition of 0.1% C elements such as Pb, S, S-P (these are cheap), Lead addition and Te, Se and Bi (expensive, but effective). 17  DESIGNATION OF STEELS AISI/SAE Designation Systems (for plain and low-alloy steels) AISI number  1XXX carbon steels 10XX plain carbon steels 11XX free machining steels with sulfur (resulfurized) 12XX free machining steels with sufur and phosphorus (resulfurized and rephosphorized)  12LXX free machining steels with sufur and phosphorus by lead addition (Lead: Insoluble in steel. It is added for machinability) 13XX manganese  2XXX Nickel steels***  3XXX Nickel-chromium steels***  4XXX Molybdenum steels 41XX Cr-Mo steels (or shortly chromoly steels) 43XX Cr-Ni-Mo steels (≈1.75%Ni)  5XXX Chromium steels***  6XXX Chromium-Vanadium steels***  7XXX Tungsten steels***  8XXX Cr-Ni-Mo steels 86XX (0.40-0.70%Ni) 9XXX Si-Mn steels*** *** PLEASE DO NOT MEMORIZE marked designations!!! 18 AISI/SAE Designations for TOOL STEELS AISI number  W (Water hardening)  O (Oil hardening)  D (heavy Duty, air) Die steels used for cold working operations  A (Air hardening)  M, T (high speed) they are used as cutting tools in machining operations (Mo, Tungsten)  S (Shock resisting) they are used in punching, bending operations.  Applications: Cutting tools, dies, knives, razors, hacksaw blades, springs, high-strength wires etc. AISI/SAE Designations for STAINLESS STEELS AISI number  Martensitic stainless steel  410 (0.15C, 12.5Cr, 1.0Mn, 0.8Ni, 1.0Si), Applications: cutlery, surgical instrument  Austenitic stainless steel  304 (0.08C, 19Cr, 9Ni, 2.0Mn, 0.75Si), Applications: Chemical and food processing equipment, cryogenic vessels.  316 L (0.03C, 17Cr, 12Ni, 2.5Mo,2.0Mn, 0.75Si), Applications: Welding construction  304L (0.03C, …..) , 316 (0.08C…….), etc. L means LOW CARBON!!!  Ferritic stainless steel  430 (0.12 C, 17Cr, 0.75 Ni, 1Mn, 1 Si) *** PLEASE DO NOT MEMORIZE compositions!!! 19  20 Please don’t memorize compositions….  TURKISH/GERMAN STANDARTS a) Plain Carbon Steels: Plain carbon steel C35 C % content x100 C45 : Plain Carbon steel. It contains 0.45% C 21  TURKISH/GERMAN STANDARTS b) Low Alloy Steel: % of the highest C % content x 100 content element x MF Alloying element ordered with respect to amount ELEMENTS Multiplication Number (MF) Cr, Mn, Ni, Si, Al, Mo, Nb, W 4 V, Zr 10 C, P, S, N, Ce 100 B 1000 No number if concentration is less than 1% Ex: 15 Cr Mo 5 : It contains 0.15%C It contains Cr and Mo. Cr is the main alloying elements. (Its content is the highest) It contains 1.25 % Cr (Cr %= 5/4= 1.25) 22  TURKISH/GERMAN STANDARTS c) Free Cutting Steels: C % content x100 S % multiplied S by 100 Free Cutting steel Alloying element other than C and S Ex: 22 S MnPb 36: It contains 0.22 % C. It has Mn and Pb and it contains 0.36 % S. Ex: 22 S 20 : It contains 0.22 % C and 0.2 S. 23  TURKISH/GERMAN STANDARTS d) Alloy steel: If total alloying element is higher than 5%. % Alloying elements X without factors High alloy steel C % content x100 Alloying elements other than C Ex: X 50 CrMoW 9 1 1: It is a high alloy steel. It has 0.5% C Main alloying elements Cr,Mo,W. It has 9% Cr, 1% Mo, 1% W. Ex: X 5 CrNiTi 18 9 : It is a high alloy steel. It has 0.05% C. Main alloying elements Cr, Ni,Ti. It has 18% Cr, 9 % Ni, less than 1% Ti 24 25  CAST IRONS  Cast irons are a class of ferrous alloys with carbon contents above 2.14 wt%.  However, most cast irons contain between 3.0 and 4.5 wt% C and, in addition, other alloying elements, 1-3 wt% Si, Mn, etc.  Cast irons are easily melted because of their lower melting temperatures (1150- 13000C) than for steels, and amenable to casting.  They have a wide range of strength and hardness and in most cases can be machined easily.  They have relatively low impact resistance and ductility for some applications.  They are common due to their comparatively low cost.  The most common cast iron types: 26 Gray Iron Ferrite  Gray cast irons usually contain 2.5 to 4 wt% C matrix. and 1-3 wt% Si. (Depending on heat  For most of gray cast irons (CIs), carbon exist treatment, as graphite flakes. it may be pearlite.) Graphite flakes  Graphite formation is promoted by the presence of Si (graphite stabilizing element) in concentrations greater than about 1%.  Because of the graphite flakes, a fractured surface takes on a gray appearance, hence its name.  Gray iron is relatively brittle because of graphite flakes. The graphite flakes concentrate stresses and cause low strength and ductility.  However, gray iron has a number of attractive properties: high-compressive strength good machinability good resistance to wear and thermal fatigue good thermal conductivity good vibration damping. Typical applications: Diesel engine castings, cylinders, pistons, machine tool bases. 27 Ductile (or Nodular) Iron  Nodular CI contains spheroidal graphite particles. Ferrite matrix  Adding a small amount of magnesium to the gray iron before casting produces a distinctly different microstructure and set of mechanical properties.. Graphite nodules  The composition of unalloyed ductile CI is similar to that of gray CI with respect to C and Si contents.  Ductile cast iron has good castability, excellent machinability, good wear resistance.  In addition, it has a number of properties similar to those of steel such as high strength, toughness, ductility, hot workability, and hardenability. Applications: Valves, pump bodies, crank-shafts, gears, and other automotive & machine components. 28 White Iron Pearlite  It is produced by chilling (sudden cooling).  For low-Si cast irons (Si