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Steel and Other Metals Chapter 3.

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1 Steel and Other Metals Chapter 3

2 Objectives Describe steelmaking process.
List metalworking processes used to shape and improve steel. State proper use of each heat-treating process for steel. Describe internal structures of metals. Name various alloying elements and their effects.

3 Objectives List various types of ferrous metals and their applications. List various types of nonferrous metals and their applications. Describe various systems used to designate metals. Explain heating and cooling effects on weldment and how they can be controlled.

4 Metal Major Groupings Ferrous Nonferrous High iron content Includes:
Many types of steel and its alloys Cast iron Wrought iron Nonferrous Almost free of iron

5 Nonferrous Metals Common Precious Radioactive
Copper, lead, zinc, titanium, aluminum, nickel, tungsten, manganese, brass, bronze Precious Gold, platinum, silver Radioactive Uranium, radium

6 Steel Combination of iron and carbon Weldments
Percentage of carbon determines how strong and hard the steel Weldments 80% fabricated from steel 85% welded is in mild (low carbon) steel classification

7 History of Steel Assyrians (3700 B.C.) first recorded use of iron
Low carbon iron first produced in low flat hearth furnaces 1350 B.C. to 1300 A.D. all iron tools and weapons produced directly from iron ore Furnaces increased in height and charge introduced through top (shaft furnaces) Modern blast furnace

8 History of Steel Little known of first process for making steel
Tools found dating back to 1000 to 500 B.C. Prior to Bessemer process two methods used Cementation process Increased carbon content by heating iron in contact with hot carbon in absence of air Still used to limited extent Crucible process Melting wrought iron in crucibles in which carbon already added Replaced by various electric furnace processes

9 Steelmaking in the US Extends back over 300 years
Ironworks in Saugus, Massachusetts (1646–1670) First patent for steel issued in 1728 Succession of events spurred growth New uses for iron Discovery of large iron ore deposits Development of Bessemer and open hearth processes Civil War and America’s industrial growth Expansion of railroads World Wars I and II

10 Annual Steel Production

11 Changes in U.S. Steel Production
Reduction in number of blast furnaces 250 down to 36 No open hearth furnaces Increased use of recycled steel Perfection of welding process to join metals speeded up and expanded use of steel

12 Raw Materials United States well supplied with basic resources
Iron ore, limestone and coal Other countries provide other necessary materials Manganese, tin, nickel and chromium

13 Iron Ore 5% of Earth’s crust Large deposits
Northern Minnesota near Lake Superior in U.S. Principally taconite Brazil Largest and best source Sweden Purest iron ore

14 Iron Ores Magnetite (Fe3O4) Taconite Jasper Hematite (Fe2O3)
Brownish, richest, least common, 65–70% iron Hematite (Fe2O3) Red, mined in US, 70% iron Limonite (2Fe2O3 • H2O) 52–66% iron Siderite (FeCO3) 48% iron Taconite Green, 22–40% iron Jasper Iron-bearing rock Predominately magnetite or hematite

15 Iron Ore Mining Underground Open pit
Vertical shaft sunk in rock next to ore body Tunnels drilled from shaft and blasted horizontally into ore body at number of levels Open pit Mineral lying relatively near surface Earth and rock first removed Blast holes drilled, explosives shatter ore and hauled out of pit by truck, train or conveyor belt

16 Oxygen Most abundant element on earth Steel industry major consumer
One half weight of land, 21% by weight of air and 90% by weight of sea Steel industry major consumer Used to purify the material Oxidizes the carbon, silicon, manganese and other elements Speeds up process by supporting combustion of other fuels

17 Fuels Three major natural fuels
Coal Most important Oil Natural gas Used to provide heat essential in making steel mill products

18 Coal Supplies more than 80% of total heat and energy requirements
Large part used in making coke for blast furnace About 1,300 pounds of coke per each ton of pig iron Coking quality coal mined in 24 states 90% comes from West Virginia, Pennsylvania, Kentucky and Alabama

19 Oil Used as both fuel and lubricant
Heaviest grade of oil most commonly used Percentage of use 70% consumed in melting iron 20% burned in heating and annealing furnaces for special heat treatments 10% used in all other applications

20 Natural Gas Burned in furnaces and places where clean burn necessary
More heating value than all other gases employed 1,000 BTU per cubic foot Steel industry consumes over 400 billion cubic feet per year 50% used in heat-treating and annealing furnaces

21 Coke Supplies heat for smelting iron in blast furnaces
Solid residue obtained when coal heated to high temperature in absence of air Causes gases and other impurities to be released Hard, brittle substance consisting chiefly of carbon 1919 – coal chemical process of producing coke developed

22 Coke Leading fuel of steel industry
Volatile products which pass out of ovens piped to chemical plant Yields gas, tar, ammonia liquor, ammonium sulfate, and light oil Further refinement of light oil produces benzene, toulene, and other chemicals Production in United States exceeds 64 million tons per year (92% consumed as blast furnace fuel)

23 Coke Oven Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

24 Steel Scrap Oxygen furnaces (BOFs) capable of using scrap
66% of steel used is recycled Integrated producer 80% liquid metal (from blast furnace) 20% scrap Best source is old automobiles

25 Limestone Used as flux in blast furnace to separate impurities from iron ore Sedimentary rock Consists largely of calcium carbonate Color changes with presence of impurities Silica makes it harder Clay makes it softer

26 Refractory Materials Nonmetallic materials which can tolerate severe or destructive service conditions at high temperatures 2,600ºF for light duty fireclay 5,000ºF for magnesia brick Applications Linings for blast furnaces, steelmaking furnaces, soaking pits, reheating furnaces, ladles, submarine cars

27 Producing Refractory Materials
Produced from quartzite, fireclay, alumina, magnesia, iron oxide, graphites, coal, coke and tar Materials crushed, combined with binder and fed to forming machines Methods for forming refractory bricks Power pressing Extrusion Hand molding

28 Iron Blast Furnace Slag
Residue produced from interaction of molten limestone and impurities of iron Contains oxides of calcium, silicon, aluminum and magnesium (also iron oxide and sulfur) Processed for use in cement, road materials, insulating roofing material and soil conditioner

29 Carbon Nonmetallic element that can form compound with other elements
Organic compounds Three pure carbon forms Diamond (hard crystalline form) Graphite (soft form) Carbon black (amorphous form)

30 Blast Furnace First step in converting iron ore into steel
Iron freed from most impurities Furnace charged with iron ore, limestone and coke Heat melts iron, limestone form slag and two liquids separate (remove and repeat: 5–8 hrs) Liquid iron poured into molds (pigs of iron) Hard and brittle

31 Blast Furnace Schematic
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. American Iron & Steel Inst.

32 Steelmaking Processes
Cementation Crucible Bessemer furnace Invented in both Europe and United States in 1856 Open hearth furnace Invented in 1868 in the United States

33 Cementation Process Oldest method of steelmaking
Consists of heating wrought iron with carbon in a vacuum Increases carbon content of surfaces and edges Edges hardened by heating and quenching Impurities not removed Only surface is affected Later process layered soft and hard metal for strength

34 Crucible Process Revived in England during early 1740s
Process involved melting wrought iron in clay crucible to remove impurities When fluid, slag skimmed off top Metal then poured into mold to solidify into a workable mass United States used graphite crucibles (100 lb. capacity) in gas-fired furnace

35 Electric Furnace Processes: Electric Arc
French metallurgist Paul Heroult in 1899 Introduced into U.S. in 1904 Produce more than 800 tons of steel in 24 hrs. Electricity used solely for production of heat Uses three carbon electrodes (4–24 inches) for direct arc Circular furnace shape which can be titled to pour molten steel into ladle

36 Electric Arc Furnace Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

37 Electric Furnace Processes: Electric Induction Furnace
Transformer with molten metal acting as core Consists of magnesia crucible Surrounded by layer of tamped-in magnesia refractory Copper tubing coil around this connected to current source; encased in heavy box with silica brick bottom lining Charge melted down in 45 minutes Further heated for 15 minutes to tapping temperatures Alloys and deoxidizers added Furnace tilted and liquid metal runs out

38 Electric Induction Furnace
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

39 Oxygen Process Also known as Linz-Donawitz process
First established in Linz, Austria (1952) First used in United States in 1954 Method of pig iron and scrap conversion whereby oxygen is injected downward over bath of metal Chemical reaction of oxygen and fluxes refines pig iron and scrap into steel Temperature reaches 3,000ºF Refining continues for 20 to 25 minutes

40 Oxygen Process After scrap and hot
metal are charged into furnace, dust cap is put on, and oxygen blown through the lance to the surface of the molten metal in order to burn out impurities. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

41 Vacuum Furnaces and Degassing
Melting of steel and other alloys in vacuum reduces gases in metal and produces metal with minimum of impurities Gases formed in vacuum furnace pulled out by vacuum pumps Two general types of furnaces Vacuum induction melting Consumable electrode vacuum arc melting

42 Vacuum Induction Melting
First used in 1940s Charge melted in furnace within airtight, water-cooled steel chamber Advantages include: Freedom form air contamination Close control of heat Fewer air inclusions

43 Vacuum Induction Melting
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

44 Consumable Electrode Vacuum Arc Melting
Refining process for steel prepared by other methods Steel electrodes of predetermined composition are remelted by an electric arc in airtight, water-cooled crucible Principle similar to arc welding Furnace consists of water-cooled copper crucible, vacuum system for removing air from crucible during melting, and a d.c. power source for producing arc

45 Consumable Electrode Vacuum Arc Melting
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

46 Vacuum Furnaces and Degassing
Produce high quality steel and steel alloys Advantages: Production of alloys too expensive to manufacture by air-melt processes Use of reactive elements Decreased amounts of hydrogen, oxygen, and nitrogen in finished product Improved mechanical properties Close heat control Better hot and cold workability

47 Vacuum Degassing Refining operation
Purpose to reduce amounts of hydrogen, oxygen and nitrogen in steel Process carried out after molten metal removed from furnace and before poured into ingots Three processes today Steam degassing Ladle degassing Vacuum lifter degassing

48 Steam Degassing Steel is poured into a tank from which air has been removed. Collected in ingot mold or ladle. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

49 Ladle Degassing Process
A ladle of molten steel placed in tank and then air removed from tank, exposing it to vacuum. Can process smaller amounts of steel than steam degassing. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

50 Vacuum Lifter Degassing
A vacuum is created in a chamber suspended above a ladle of steel. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Metal forced upward into vacuum chamber through nozzles by means of atmospheric pressure.

51 Benefits From Degassing
Reduction of hydrogen eliminates flaking of steel Reduction of oxygen promotes internal cleanliness Oxygen reduction not as low as achieved in vacuum-melted steels Nitrogen content reduced slightly Transverse ductility of most degassed forced products nearly double that of air-cast steel

52 Continuous Casting of Steel
Process by which molten steel solidified into semifinished billet, bloom, or slab for subsequent finishing Prior method was forming ingots Improved yield, quality, productivity and cost efficiency Various shapes cast Complete operation can be achieved in 2 hours

53 Example of Continuous Casters
into tundish American Iron & Steel Inst.

54 Casting Process Sections
Tundish to feed liquid steel to mold Primary cooling zone to generate solidified outer shell Secondary cooling zone to further solidified the strand Unbending and straightening section Severing unit to cut solidified strand

55 Liquid Steel Transfer Two steps involved in transferring liquid steel from ladle to molds From ladle to tundish From tundish to molds Regulated by orifice control devices of various designs Designs: slide gates, stopper rods, or metering nozzles

56 Tundish overview Enhances oxide inclusion separation
Provides continuous flow of liquid steel to mold during ladle exchanges Maintains steady metal height above nozzles to molds Provides more stable stream patterns to molds

57 Mold Purpose to allow establishment of solid shell sufficient in strength to contain liquid core Open-ended box structure containing water-cooled inner copper lining Oscillation necessary to minimize friction and sticking of solidifying shell Achieved either hydraulically or via motor-driven cams or levers

58 Secondary Cooling Series of zones
Sprayed medium either water of air and water Three basic forms of heat transfer Radiation: to atmosphere Conduction: by direct contact Convection: by moving airflow Purpose of spray chamber Enhance and control rate of solidification Regulate strand temperature Control machine containment cooling

59 Casting and Soaking Ingots
Molten steel cast into molds directly gives us cast steel Cast steel inferior to wrought steel Molten steel poured into ingot molds or continuous casting gives inside chance to become solid while outside kept from cooling off too much Lowered into soaking pit Heat steel for rolling

60 Deoxidation Type of steel determined by control of amount of gas evolved during solidification Increasing degrees of gas evolution Killed steels Semikilled steels Capped steels Rimmed steels

61 Killed Steel Strongly deoxidized
Relatively high degree of uniformity in composition and properties Suitable for applications involving Forging Piercing Carburizing Heat treatment

62 Semikilled Steels Intermediate in deoxidation between killed and rimmed grades Composition more uniform than rimmed steels Used where neither cold-forming and surface characteristics of rimmed steel nor uniformity of killed steels essential requirements

63 Capped Steels Have thin low-carbon rim
Remainder of cross section approaches degree of semikilled steels Great increase in use of capped steels over rimmed steels in recent years

64 Rimmed Steels Surface and cold-forming characteristics of capped steels Only slightly deoxidized Low-carbon surface layer very ductile Rolling produces sound surface Used when surface is of prime importance

65 Environmental Progress in the Steel Industry
Each year, 15% of steel industry’s capital spent for environmental facilities $10 to $20 per ton of steel produced Amount of energy to produce ton of steel decreased by 45% from 1975 to 1998 Accurate and efficient microprocessor controls Two-thirds less labor producing more steel From 12 labor hours to 45 labor minutes

66 Environmental Progress in the Steel Industry
Air quality greatly improved since 1970 Discharge of air and water pollutants reduced by over 90% Made great strides in terms of recycling Over 95% of water Has worked cooperatively with federal environmental agencies

67 Metalworking Processes
Shape it and improve its characteristics Forging Rolling Destroy the cast structure “Orienting the grain” Steel stronger More ductile Greater shock resistance

68 Forging Method of reducing metal to desired shape
Usually done with steam hammer Today most done with hydraulic presses Can take cooler ingots and work to closer dimensions Drop forging Piece of roughly shaped metal placed between die-shaped faces of exact form of finished piece Metal forced to take form by drawing dies together Many automobile parts made this way

69 Rolling Steel rolled hot except for finishing passes
After rolling, ingots known by size and shape Bloom Square or oblong with minimum cross-sectional area of 36 inches Billet Square or oblong, but smaller than bloom Slab Oblong and varies in thickness from 2 to 6 inches and in width from 5 to 6 feet

70 Samples of Various Shapes Produced by Hot Rolling
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

71 Processes for Rolling Steel
One-half rolled steel products in U.S. are flat rolled Includes plates, sheet and strip Flat-rolled steel divided into two categories Hot rolled Finished at temperatures between 900 and 2,400ºF Black iron Cold rolled Finished at room temperature Coated with zinc (galvanized), tin (tin plate), tin and lead (Terne plate)

72 Tubular Steel Products
Classified according to method of manufacture Welded (flash welding steel strip) Metal pieces heated until contacting surfaces plastic state, then forced together quickly under pressure Seamless Piercing: Heated steel bar pierced by mandrel and rolled to desired diameter and wall thickness Cupping: Heated plate formed around cup-shaped dies

73 Piercing Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

74 Structural Steel Shapes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Steel may also be shaped into wire, bars, forging, extrusions, rails and structured shapes. These are just a few of the basic steel shapes with which welder fabricator works.

75 Rolling Directions X Direction: Best strength and ductility Y Direction: 30% reduction in strength 30% reduction in ductility Z Direction: Lower strength; virtually no ductility In rolling operations, grains are oriented in direction of rolling.

76 Drawing Operation of reducing cross section and increasing length of metal bar or wire Draw through series of conical, tapering holes in die plate Each hole smaller than preceding one Shapes varying in size from finest wire to very large are drawn

77 Extrusion Forming by pressing through an opening
Can obtain perfectly round rods Metal placed in closed chamber fitted with opening at one end and piston at other end Forced out through opening by hydraulic pressure Used to form brass rod

78 Cold Working Shaping of metals by working at ordinary temperatures
Methods Hammered Rolled Drawn

79 Heat Treatment Process of heating and cooling metal for purpose of improving its structural or physical properties Done to remove stresses caused by welding, casting, or heavy machining Can make it easier to work with or increase hardness for wear resistance

80 Important Variables in Any Heat Treatment Process
Carbon content Temperature of heating Time allowed for cooling Cooling medium Water, oil, or air

81 Hardening Process in which steel heated above its critical point and cooled rapidly Critical point is point at which carbon changes structure of steel Produces hardness superior to that of steel before heating and cooling Only medium, high, and very high carbon steel can be treated

82 Plain carbon steels and alloy steels are often case hardened.
Case Hardening Process that gives steel hard, wear-resistant surface while leaving interior soft and tough Processes Cyaniding Carburizing Nitriding Flame hardening Hard surfacing by welding Metal spraying Plain carbon steels and alloy steels are often case hardened.

83 Cyaniding Method of surface-hardening low-carbon steels
Carbon and nitrogen absorbed in outer layer of steel to depth of to inch Done in liquid or gas form For hard, but very thin, surface over steel

84 Carburizing Process whereby low carbon steel made to absorb carbon in its outer surface Depth to which carbon will penetrate Time heat held Temperature reached Carburizing compound used Can use carbonaceous solids, cyanidizing liquids, or hydrocarbon gases

85 Nitriding Process used only with group of low alloy steels
Contain elements such as vanadium, chromium or aluminum Will combine with nitrogen to form nitrides Nitrides act as super hard skin on surface of steel Parts heated in nitrogenous atmosphere to temperature of 900 to 1,000ºF Quenching unnecessary with little distortion or warpage

86 Flame Hardening Most recent of hardening processes
Permits localized treatment with complete control Steel must contain enough carbon for hardening to take place Article heat treated and drawn Surface exposed to oxyacetylene flame that heats to high temperature quickly Cooled quickly by water (depth of hardness controlled by temperature of water) Can be used on parts too bulky to put into furnace

87 Annealing Includes several different treatments Effects of annealing
To remove stresses To induce softness for better machining properties To alter ductility, toughness, or electrical, magnetic, or other physical properties To refine crystalline structure To produce definite microstructure Changes depend on annealing temperature, rate of cooling and carbon content

88 Difference Between Hardening and Softening of Steels
Due to rate of cooling Fast cooling hardens Slow cooling softens Both tempering and annealing reduce hardness of material

89 Tempering Process wherein hardness of steel reduced after heat treatment and relieve stress Heat hardened steel to predetermined temperature between room temperature and critical temperature Hold temperature for length of time Cooling in air or water Reduction of hardness depends on 3 factors Tempering temperature Amount of time steel is held at temperature Carbon content of steel

90 Normalizing Improves grain structure of metal and returns it to normal by removing stresses Stresses caused by uneven cooling following welding, casting, or forging Requires faster rate of cooling than used for annealing Results in harder, stronger metal than annealing

91 Metal Internal Structures
Metallurgy Science that deals with internal structure of metals Four states of matter Solids, liquids, gases, and plasmas Subatomic particles Electrons – carry negative charge Protons – carry positive charge Attraction and repelling forces effect properties of metals

92 Metal Internal Structures
Atoms are in constant state of vibration Heat energy increases atomic movement Temperature rises, atomic structure expands Rises high enough, atoms move freely and solid becomes liquid Continues to rise, vaporization occurs Liquid to gas Superheated, it ionizes and becomes plasma Gas that has become electrical conductor

93 Solid Metals Take on three-dimensional crystalline structure
Atoms align themselves into orderly layers, lines and rows Common phases of metals Body-centered cubic (BCC) Face-centered cubic (FCC) Body-centered tetragonal (BCT) Hexagonal close-paced (HCP)

94 Metals Crystalline Structures
Iron Carbon steels Chromium Molybdenum Tungsten Martensite Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Aluminum Copper Nickel Silver Austenitic Stainless Steels Zinc Cadmium Magnesium

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107 Cooling rate critical above 1333 Not so critical below 1333
Cannot go between Matensite, Bainite, or Pearlite without going through Austenite first. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

108 Solidification Starts at interface between molten weld metal and cooler unmelted heat-affected zone. Clusters of atoms form grains and grain boundaries. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

109 Grain Size Effects on Metals
Fine-grained metals Good tensile strength Good ductility Good low temperature properties Coarse-grained metals Slightly lower strength Slightly less ductility Good high temperature properties

110 Welding Effect on Grain Size
Heat input Cooling rate (preheat) Long or short arc Slow or fast travel speed Welding on high or low end of parameter ranges Process selected

111 Alloying Another method of affecting mechanical properties of metals
Changes the orderly rows, lines, and layers of the three-dimensional crystalline structure Interstitial alloying Small atoms such as carbon, nitrogen and hydrogen can occupy spaces between larger atoms Substitutional alloying Additional elements create irregularities in crystal

112 Alloying Schematic Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

113 Physical Properties of Metals
Common properties divided into three general classifications Those related to the absorption and transmission of energy Internal structure of the metal Resistance to stress

114 Properties Related to Energy
Melting point Temperature at which substance passes from solid to liquid condition Higher carbon content, lower melting point Weldability Capacity of metal substance to form strong bond of adherence while under pressure or during solidification from liquid state

115 Properties Related to Energy
Fusibility Ease with which metal may be melted Volatility Ease with which substance may be vaporized Measured by degree of temperature at which metal boils under atmospheric pressure Electrical conductivity Ability of substance to conduct electrical current

116 Properties Related to Energy
Electrical resistance Opposition to electric current as it flow through wire Measured by unit called ohm Thermal conductivity Ability of substance to carry heat Hot shortness Brittleness in metal when hot

117 Properties Related to Energy
Coefficient of thermal expansion Amount of expansion metal undergoes when it is heated and amount of contraction that occurs when cooled Linear coefficient of thermal expansion Increase in length of bar 1 inch long when its temperature raised 1ºC Overheating When temperature exceeds its critical range Heated to such a degree that properties impaired

118 Properties Related to Internal Structure
Specific gravity Unit of measurement based on weight of volume of material compared with equal volume of water Density Expressed as quantity per unit volume Dense metal is compact and does not contain discontinuities Porosity Internal structure that lacks compactness of have discontinuities that leave voids in metal

119 Typical Stresses of Metals
Compression: squeezing Shear: strain on lap joint pulled in opposite directions Bending: deflection as result of compressive force Tension: pulling in opposite directions Fatigue: result of repeated cycles of forces applied and released in all directions Torsion: twisting force in opposite direction

120 Typical Stresses Compression – The application of pressure
Tension – A pulling action Compression – The application of pressure Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

121 Typical Stresses Shear – A pulling action causing two bodies to slide on each other, parallel to their plane of contact Bending – Pressure applied to force away from a straight line Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

122 Typical Stresses Torsion – A turning or twisting action
Fatigue – Condition caused by repeated stretching, twisting, compression, while in service Torsion – A turning or twisting action Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

123 Properties Related to Stress Resistance
Plasticity Ability of material to deform without breaking Combined with strength is most important combination of properties metal can have Strength Ability of material to resist deformation Express ultimate tensile strength in pounds per square inch Ultimate tensile strength of material is its resistance to breaking

124 Properties Related to Stress Resistance
Toughness Has high tensile strength and ability to deform permanently without breaking Opposite of brittleness No direct method of measuring accurately Impact resistance Ability of material to withstand maximum load applied suddenly Often taken as indication of its toughness

125 Properties Related to Stress Resistance
Brittleness Fail without any warning as deformation, elongation, or change of shape Lacks plasticity and toughness Malleability Ability to deform permanently under compression without breaking or fracturing Must have to be forged

126 Properties Related to Stress Resistance
Hardness Ability of one material to penetrate another material without fracture Greater the hardness, greater resistance to marking Measured by pressing hardened steel ball into material Brinell hardness test – diameter of impression measured Rockwell hardness test – depth of impression measured

127 Properties Related to Stress Resistance
Elasticity Ability of material to return to original shape after load been removed Elastic limit Greatest load that may be applied after which material will return to its original condition Once reached, no longer behaves elastically Permanent deformation

128 Properties Related to Stress Resistance
Modulus of elasticity Ratio of stress to strain Measure of relative stiffness High modulus, material resist movement or distortion; low modulus, material stretches easily Resilience Energy stored in material under strain within its elastic limit that causes it to resume its original shape when load removed

129 Properties Related to Stress Resistance
Yield point Point at which definite increase in length of specimen occurs with no increase in load Expressed as pounds per square inch Ductility Ability of material to be permanently deformed by loading and yet resist fracture Amount of stretching expressed as percent of elongation

130 Properties Related to Stress Resistance
Fatigue failure Failure under repeated or alternating stress Fatigue limit: load which may be applied for indefinite number of cycles without causing failure Expressed in pounds per square inch Level of loading called endurance limit of the material Maximum load that can be applied at which no failure will occur, no matter how many cycles load is applied

131 Properties Related to Stress Resistance
Corrosion Gradual wearing away or disintegration of material by chemical process Measured by Determining loss in strength of tensile samples Determining loss in weight of materials that dissolve in corroding medium Determining gain in weight when heavy coating of rust is formed Resistance to corrosion Ability of metals to resist atmospheric corrosion and corrosion by liquids or gases

132 Effect of Common Elements on Steel: Nonmetals
Carbon Native state both as diamond (very hard) and as graphite (very soft) Part coal, petroleum, asphalt, and limestone Increased carbon content increases tensile strength of steel but decreases ductility and weldability Phosphorus Small amounts improve machinability of low and high carbon steel Considered impurity in welding

133 Effect of Common Elements on Steel: Nonmetals
Boron Occurs in nature in combination with other elements Gray, extremely hard solid with melting point in excess of 400ºF Increases hardenability of steel Silicon Main substance in sand and sandstone Added mainly as deoxidizing agent to produce soundness during steelmaking

134 Effect of Common Elements on Steel: Nonmetals
Sulfur Considered a harmful impurity in steel Makes steel brittle and causes cracking at high temps Should be kept below 0.05% Improves machinability of steel Selenium Used interchangeably with sulfur in some stainless steels to promote machinability

135 Effect of Common Elements on Steel: Metals
Manganese Very hard, grayish-white metal with reddish luster Pure state can scratch glass Addition to steel increases both tensile strength and hardness High manganese steels Very resistant to abrasion Used in equipment such as rock crushers, grinding mills, and power shovel scoops

136 Effect of Common Elements on Steel: Metals
Molybdenum Silvery white metal that increases toughness of steel Increases corrosion resistance of stainless steels Chromium Hard, brittle, grayish-white metal Highly resistant to corrosion Addition to low alloy steels increases tensile strength, hardness, and resistance to corrosion and oxidation Ductility is increased

137 Effect of Common Elements on Steel: Metals
Nickel Hard, silvery white element Used extensively for plating purposes and as alloying element in steel Increases strength, toughness, and corrosion resistance of steel Niobium Combines with carbon and improves corrosion resistance in stainless steels

138 Effect of Common Elements on Steel: Metals
Cobalt Tough, lustrous, silvery white metal Used as alloying metal in high speed steel and special alloys when high strength and hardness must be maintained at high temperatures Titanium and Zirconium Added in small amounts to certain high strength, low alloy steels to deoxidize metal, control fine grain size, and improve physical properties

139 Effect of Common Elements on Steel: Metals
Copper Soft, ductile, malleable metal that melts at 1,984ºF Has expansion rate 1-1/2 times greater than steel Thermal conductivity 10 times greater than steel Very good conductor of heat and electricity Highly corrosion resistant Added to steel to improve its resistance to corrosion Brass most common class of copper alloy (zinc) Bronzes other alloys (zinc, tin, silicon, aluminum)

140 Effect of Common Elements on Steel: Metals
Aluminum Never found in nature in pure state Derived from bauxite One of the lightest metals Good conductor of heat and electricity Highly resistant to atmospheric corrosion Ductile and malleable Used in both carbon and alloy steels Produces fine austenitic grain size

141 Effect of Common Elements on Steel: Metals
Lead Soft malleable, heavy metal Very low melting point: 620ºF Highly resistant to corrosion Additions to carbon and alloy steels improve machinability Leaded carbon steels have been used mainly for stock which is to be free machined Used extensively in plumbing industry

142 Effect of Common Elements on Steel: Metals
Tungsten Steel-gray metal more than twice as heavy as iron Melting point above 6,000ºF Improves hardness, wear resistance, and tensile strength of steel Vanadium Increases toughness of steel and gives it ability to take heavy shocks without breaking High resistance to metal fatigue and high impact resistance

143 Carbon Steels Carbon most important alloying ingredient in steel
Has direct effect on physical properties Divided into four types Low carbon Medium carbon High carbon Tool

144 Low Carbon Steels Carbon content does not exceed 0.30% and may be as low as 0.03% Referred to as mild steels and plain steels General purpose steel: 0.08–0.25% Machine steel and cold-rolled steel: 0.08–0.30% Excellent weldability May be quenched very rapidly in water or brine and do not harden to any great extent Most structures fabricated: bridges, ships, tanks, pipes

145 Medium Carbon Steels Have carbon content ranging from 0.30 to 0.60%
Stronger than low carbon steels and have higher heat-treat qualities Should be welded with shielded metal arc low hydrogen electrodes and other low hydrogen processes Best results obtained if preheated before welding and normalized after welding

146 High Carbon Steels Have carbon content ranges from 0.60 to 1.7%
More difficult to weld than low or medium carbon steels Can be heat treated for maximum hardness and wear resistance Used in springs, punches, dies, tools, military tanks, and structural steel

147 Alloy Steels Content of alloying elements exceed certain limits
Amounts of alloying elements lie within specified range for commercial alloy steels Elements added to obtain desired effect in finished product Readily welded by welding processes such as MIG/MAG, and TIG

148 High Strength, Low Alloy Steels
Group of steels with chemical compositions specially developed To give higher physical property values For materially greater corrosion resistance Generally used when need savings in weight Includes oil-hardening steel, air-hardening steel and high speed steel Readily adaptable to fabrication by shearing, plasma cutting, laser cutting, welding, riveting

149 Stainless and Heat-Resisting Steels
Possess unusual resistance to corrosion at both normal and elevated temperatures Accomplished by addition of chromium to iron Corrosion resistance increases with increasing chromium Thin layer of chromium oxide bonded to surface 11.5% chromium dividing line between low alloy steel and stainless steel Practically indefinite life Some difficulty with pitting

150 Advantages of Stainless Steels
Resist corrosion and effects of high temperatures Maintain purity of materials in contact with them Permit greater cleanliness than other steels Stainless-steel fabrications usually cost little to maintain

151 Advantages of Stainless Steels
Low strength-to-weight ratios are possible both at room and elevated temperatures Tough at low temperatures Have high weldability Highly pleasing in appearance and require minimum of finishing

152 Five Classifications of Steels
5% chromium, hardenable 500 series Martensitic 12% chromium, hardenable 400 series 17% chromium, non-hardenable 400 series Ferritic Chromium-nickel 300 series Austenitic Chromium-nickel-manganese 200 series

153 Series 400 and 500 (Martensitic)
Primarily heat resisting and retain large part of their properties at temperatures up to 1,100ºF More resistant to corrosion than alloy steels Not considered true stainless steels Satisfactory for mildly corrosive conditions Satisfactory for both hot and cold working Air hardening and must be cooled slowly or annealed after forging or welding to prevent cracking

154 Series 400 (Ferritic) Chromium content ranges from 11.5 to 27%
Carbon content low (under 0.20%) No nickel Cannot be hardened by heat treatment Hardness may be increased by cold working Low coefficient of thermal expansion Good resistance to corrosion Ductility fair Difficult to weld

155 Series 200 and 300 (Austenitic)
Chromium content ranges from 16 to 26% Nickel from 3.5 to 22% Carbon from 0.15 to 0.08% More numerous, more often used than 400 series Stable structure at low temperatures Low yield point with high ultimate tensile strength at room temperatures

156 Series 200 and 300 (Austenitic)
Provide maximum resistance to corrosion Well-suited to standard fabrication ductility required for severe deep drawing and forming High rupture and creep-strength values at high temperatures Also good oxidation resistance

157 Duplex Stainless-Steel (DSS) Alloys
Chromium content ranges from 18.0 to 29.0% Nickel from 2.5 to 8.5% Carbon from 0.03 to 0.08% Interest due to resistance to stress corrosion cracking, crevice corrosion, general corrosion and pitting Have yield strengths twice that of 300 series Used where thinner sections and weight reduction desirable

158 Tool Steels Carbon or alloy steels capable of being hardened and tempered Produced primarily for machine tools which cut and shape articles used in manufacturing Vary in chemical composition depending upon end use Many different types: carbon range from 0.80 to 1.50%

159 Tool Steels Usually melted in electric furnaces in small batches
Used in other applications when wear resistance is important Rarely welded and must be preheated to do so After-treatment also necessary Special hard-surfacing electrodes required for this work

160 Carbon Equivalency Variety of formulas for calculation One example:
Intended for use with carbon and alloy steels that contain more than 0.5% carbon, 1.5% manganese, 3.5% nickel, 1% chromium, 1% copper and 0.5% molybdenum

161 SAE/AISI Steel Numbering System
Based on chemical analysis of steel Number designations indicating percentage of predominant alloying element Table 3-7 shows classification system

162 Types of Cast Iron Iron-based material containing 91 to 94% iron
Carbon: 2.0 to 4.0% Cannot be formed by forging, rolling, drawing, bending or spinning Low ductility and lack of malleability Castings have low ductility and low tensile strength Has excellent compressive strength Four classes: gray, white, nodular, malleable

163 Gray Iron May be fusion welded or braze welded if preheating before welding cooling after are controlled Low in ductility Moderate tensile strength High compression strength High machinability

164 White Iron Produced through process of rapid cooling which causes carbon to combine with iron Hard, brittle, very difficult to machine Considered unweldable First step in making of malleable iron Has fine grain structure Silvery white appearance when fractured

165 Malleable Iron Forms when white cast iron has been heat treated by long annealing process Higher tensile strength, impact strength, ductility, and toughness than gray or white iron Fusion welding destroys properties in weld area Braze welding recommended If broken, fracture shows white rim with dark center

166 Nodular Iron Referred to as ductile iron
Amounts of magnesium and/or cerium added to iron when produced Change shape of graphite particles from flakes to spheroids Silicon contents higher than other irons Excellent machinability, shock resistance, thermal shock resistance, wear resistance, and rigidity

167 Aluminum-making in the U.S.
World’s largest producers of aluminum More than 22 million pounds of metal processed annually Refining of bauxite ore Fundamental production process of reducing alumina to aluminum by means of electricity Production done across four-fifths of country

168 Primary Products and Their Industrial Applications
Sheet: Cans, construction materials, and automobile parts Plate: Aircraft and space fuel tanks Foil: Household aluminum foil, building insulation, automotive parts Rod, bar, and wire: Electrical transmission lines and nonrust staples Extrusions: Storm windows, bridge structures

169 Environmental Progress in Aluminum Industry
Focus on reducing air emissions, water discharges, and solid waste Recycling very important Amount doubled in last decade Saves almost 95% of energy needed to extract aluminum for original ore Nearly two-thirds of aluminum beverage cans produced

170 Types of Aluminum Four-digit numbering system (see Table 3-23)
First digit indicates major alloying group Three categories of aluminum find most welding applications Commercially pure aluminum Wrought aluminum alloys Aluminum casting alloys

171 Types of Aluminum Commercially pure wrought aluminum (1100)
99% pure (little iron and silicon) Easily welded (weld strength equal to base metal) Wrought aluminum-manganese alloy 1.2% manganese, 97% aluminum Stronger than 1100 type and less ductile Welded without difficulty Welds strong

172 Types of Aluminum Aluminum-silicon-magnesium-chromium alloy
Classification number of 6151 Silicon and magnesium main alloys Welds not as strong as base metal Can be improved by heat treatment Aluminum-magnesium-chromium alloy (5052) Strong, highly resistant to corrosion Good ductility Aluminum-magnesium-silicon alloy (6053) Readily welded and can be heat treated

173 Titanium-making in the U.S.
Versatile metal Light weight, physical properties and mechanical properties Produced from heavy-mineral sands containing ilmenite and/or rutile Also titaniferous slags made by smelting of ilmenite with carbon Typically associated with iron One-third of world supply found in U.S.

174 Titanium Titanium sponge produced by Kroll process
Produced in a retort by vapor phase reduction of titanium tetrachloride with magnesium Ingot produced by melting sponge, scrap or combination of both Russia and U.S. produce bulk of world supply Vacuum arc remelt (VAR) process used to refine material Titanium electrode would be used

175 Titanium Mill products formed by rolling, forging, drawing, or extruding slabs and ingots Can be cast into variety of products Scrap and waste produced at each step of production process Large source of feedstock material with growth in cold hearth melting capacity

176 Titanium Properties Great impact properties
Durability with excellent mechanical strength Modulus of elasticity half that of stainless steel Very lightweight Coefficient of thermal expansion half that of stainless steel and copper, one third of aluminum Very corrosion resistant

177 Relative Corrosion Rates
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

178 Titanium Implants Inert to human body fluids
Natural material to use for implants Allows bone growth to adhere to implant Commonly used for reconstructive surgery applications © Scott Camazine Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

179 Titanium Applications
Largest single demand is commercial aerospace industry Chemical processing, oil and gas exploration and processing Heat exchangers Pollution control equipment Bicycles, wheelchairs, motorcycle components Eyeglass frames, writing pens, jewelry

180 Titanium Nontoxic Pyrophoric
Produces its own heat when in presence of oxidizing elements such as oxygen Small pieces with lot of surface contact area to air can ignite and burn at extremely high temperatures Form of machining or grinding chips Store in nonflammable containers submersed in water with thin layer of oil on top Extinguish with dry sand, powdered graphite, Metal-X*

181 Expansion and Contraction
All materials when loaded or stressed will deform shrink or stretch Metal expands when heated during welding Not free to move due to other welds, tackings, etc. Metal contracts when cools Combination of heating and cooling with restraint causes stresses to build up in weldment

182 Two Major Aspects of Contraction
Distortion (shrinkage) Overall motion of parts being welded from position occupied before welding to that after welding Stress Force that will cause distortion later unless relieved Residual stress Temporary distortion and stress occur while welding Remains after welded members cool

183 Physical Properties of Metal and Distortion
Result of heating and cooling and involves stiffness and yielding Heat changes physical properties of metals Yield point lowers Modulus of elasticity decreases Coefficient of thermal expansion increases Thermal conductivity decreases Specific heat increases

184 Yield Point Point at which it will stretch and elongate under load even though load is not increased Higher the yield point of weld and base metal, the greater amount of residual stress Lower the yield point, less severe residual stress

185 Coefficient of Thermal Expansion
Amount of expansion a metal undergoes when heated and the amount of contraction that occurs when it is cooled High coefficient tends to increase shrinkage of weld metal and base metal next to weld Increases possibility of distortion in weldment

186 Other Physical Properties
Thermal conductivity Measure of flow of heat through metal Low thermal conductivity retards flow of heat from weld Increases shrinkage of weld and plate next to it Modulus of elasticity Measure of relative stiffness of metal Modulus high, the material more likely to resist movement and distortion

187 Types of Distortion: Lengthwise Shrinkage
Occurs when weld is lengthwise on unclamped strip of steel Bow upwards at both ends when cools Due to contraction of weld above plate surface Minimize by small weld beads, flat deep penetrating beads or even heat on both sides of plate Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

188 Neutral Axis of Joint Center of gravity of joint
Welds kept close to neutral axis or balancing weld sequences about neutral axis minimizes lengthwise shrinkage Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

189 Types of Distortion: Crosswise Shrinkage
Also called transverse contraction Butt joint with free movement during welding allows opposite end from weld to be drawn together by contraction of weld metal Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

190 Controlling Transverse Contraction
Tack-weld at opposite ends on short seams Tack-weld at several equidistant positions on long seams Thickness of plate Type of material type of edge preparation Usually twice as long as thickness of plate and spaced 8–12 inches Clamping devices and wedges Prespacing

191 Controlling Transverse Contraction
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192 Types of Distortion: Warping
Contraction of weld deposit Uneven deposit in V-groove and U-groove butt joints Places most of weld above neutral axis Greater warping on multiple passes on joint Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

193 Counteracting Warping
Setting plates before welding so bow in opposite direction Clamping plates High internal stress Deforming occurs when residual stress exceed yield strength of metal Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

194 Types of Distortion: Angular Distortion
Fillet welds contain both longitudinal and transverse stresses Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

195 Types of Distortion: Angular Distortion
Fillet weld in T-joint will pull vertical member toward side that is welded Dotted lines indicate original position. Position after welding is indicated by solid lines. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

196 Effect on Butt Joint-Groove Welds Factors Affecting Perpendicular Shrinkage
Cross-sectional area of weld for given thickness of plate Larger cross section yields greater shrinkage Free distance spacing between roots and type of groove Total heat input: greater heat yields greater distortion Rate of heating: greater rate of heat input Weld searching like backstep procedures Peening

197 Factors that Affect Angular Distortion
Increases with number of layers Greatest in butt joints with V-grooves welds, next in U-grooves, less in double-V and double-U grooves, and least in square grooves May be controlled by peening every fill pass layer to suitable extent Practically eliminated by welding alternately on both side in multilayer welding about neutral axis in double-V and double-U groove welds Time of welding and size of electrode Rate of heating

198 Distortion Affects on Fillet Welds
Shrinkage increases with size of weld and decreases as rate of heat input increases Shrinkage proportional to length of weld, if weld intermittent Shrinkage may be decreased materially by choosing suitable sequences and procedures of welding and peening Transverse shrinkage less for lap joint than for V groove-butt joint

199 Prevention of Distortion Before Welding
Design Joints should require minimum amount of filler metal Joints arranged to balance each other Selection of process and equipment Higher welding speeds through use of powdered iron manual electrodes Reduces amount of base metal affected by heat of arc

200 Prevention of Distortion Before Welding
Prebending Plates bent in direction opposite to side being welded Shrinkage restrained curing welding by clamps More effective when welded members are allowed to cool in clamps When clamps removed, plates spring back so pulled into alignment Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

201 Prevention of Distortion Before Welding
Spacing of parts Space parts out of position before welding Arms pulled back to proper spacing by shrinkage forces of welding Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

202 Prevention of Distortion Before Welding
Jigs and fixtures Prevent warping by holding weldment in fixed position to reduce movement Widely used in production welding Strong backs Temporary stiffeners for purpose of increasing resistance to distortion Removed after welding completed and cooled

203 Distortion Control During Welding
Reduced by using sequence of welding known as wandering Making welds at different points of weldment Shrinkage set up by on weld counteracted by shrinkage set up by another Two methods Chain intermittent fillet welds Staggered intermittent fillet welds Chain Staggered Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

204 Distortion Control During Welding
Backstep method of welding Breaking up welds in short sections and depends upon welding in proper direction General progression of welding is left to right, but each bead is deposited from right to left Reduces locked-up stresses and warping Skip-stop, backstep method Direction same as backstep except short welds not made in continuous sequence

205 Backstep Method Example
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206 Skip-stop, Backstep Method
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207 Distortion Control During Welding
Balanced welding sequence Equal number of welders weld on opposite sides of structure at same time Balanced stresses Both wandering techniques and balanced welding contribute to completion of welded connections in large fabrications

208 Correction of Distortion After Welding
Shrinkage Alternate heating and cooling, frequently accompanied by hammering or mechanical working Shrink welding Variation of shrinkage in which heat applied by running beads of weld metal on convex side of buckled area (after correction, ground off) Added stiffening Pulling plate into line with strong backs and welding additional stiffeners to plate to make it retain its plane Can by used only on plate

209 Summary of Distortion Control
Metal expansion Metal with high coefficient of expansion distorts more than one with lower coefficient Distortion effects Kind of welding process has influence on distortion Use of welding positioners Allows use of larger diameter electrodes or welding procedures with higher deposition rates and faster welding speeds

210 Summary of Distortion Control
Balanced forces By prebending and presetting in direction opposite to movement caused by weld shrinkage Shrinkage pull material back into alignment Forcible restraints Restraining parts forcibly through use of clamps, fixtures, and tack welds Welder must be careful not to overrestrain parts

211 Summary of Distortion Control
Clamping parts during fabrication Clamped or welded to heavy fixture which can be stress relieved with weldment Heat distribution Distribute welding heat evenly though planned welding sequence and planned weld positions Increase speed with heat General rule about warping Decrease in speed and increase in number of passes increases warping

212 Summary of Distortion Control
Welding from both sides Distortion reduced by welding from both sides Welding from both sides at same time all but eliminates distortion Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

213 Summary of Distortion Control
Welding direction Away from point of restraint Toward point of maximum freedom Wandering sequences Skip welding and backstep welding prevents local buildup of heat thus reduces shrinkage End fixing Boxing: when fillet weld wrapped around corner of member as continuation of principal weld

214 Summary of Distortion Control
Avoid overwelding Too much welding increases distortion Too many weld passes cause additional heat input Single pass better than several passes Stringer bead produces less distortion than weave bead Use smallest leg size permissible when fillet welding Use Minimum number of passes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

215 Summary of Distortion Control
Reduce weld metal Excessive widths of groove weld increase weld shrinkage and cost Add nothing to strength Root opening, including angle should be kept to a minimum Select joints that require little weld metal Weld joints that cause most contraction first

216 Correct Edge Preparation and Good Fitup
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217 Summary of Distortion Control
Fix tack welds first Weak welds or cracked tack welds should be chipped or melted out before proceeding with weld Peening Effective Too much, causes loss of ductility and impact properties

218 Control of Residual Stress: Preheating
Necessary to control or reduce rate of expansion and contraction during welding Preheat entire structure before welding and maintaining heat during welding Care taken to make sure preheat uniform throughout structure After weld completed, structure must be allowed to cool slowly

219 Control of Residual Stress: Postheating
Most common method of stress relieving Must be done in furnace capable of uniform heating under temperature control Work must be supported When weldment reaches maximum temperature, permitted to soak One hour per one inch of thickness Reduction of temperature must be gradual and uniform

220 Control of Residual Stress: Postheating
Suggested Preheat Temperatures Carbon Equivalent (%) Temperature (Fº) Up to Optional 0.45– –400ºF Above –700ºF

221 Control of Residual Stress: Full Annealing
Superior to all other methods Very difficult to handle Must be heated to 1,600 to 1,650ºF Causes formation of very heavy scale Danger of collapse on some types of weldments

222 Control of Residual Stress: Cold Peening
Bead hammered to stretch it and counteract shrinkage due to cooling Causes plastic flow Identical to cold working steel Overpeening Cracks, loss of ductility, work hardened, new stress Root and face layers of well should not be peened

223 Control of Residual Stress: Vibratory Stress Relieving
Uses low frequency, high amplitude vibration to reduce stress levels to point where they cannot cause distortion Vibration generator clamped to workpiece Vibration level adjusted to create desired amplitude Sine waves pass through parts, relaxing microstructure Takes between 15 and 30 minutes depending on size

224 Control of Residual Stress: Cryogenic Stress Relieving
Takes various structures at very slow rate down from room temperature to 300º below 0ºF by exposing them to liquid nitrogen vapors Done at 1ºF per minute Allowed to soak at holding temperature for 24 to 36 hours Molecules in structure get closer together At end of holding period structure slowly warmed back up to room temperature Rate of 1ºF per minute

225 Control of Residual Stress: Mechanical Loading
Base metal stressed just at point of yielding by application of internal pressure to pressure vessel Works well with simple weldment Important only very small yielding takes place Hydraulic pressure used rather than air pressure Danger with air pressure of vessel rupturing

226 Control of Residual Stress: Welding Technique
Product designed to incorporate types of joints having lowest residual stress Degree of residual stress considered when choosing process Plan assembly welding sequences that permit movement of component parts during welding Avoid highly localized and intersecting welds Use electrodes that have an elongation of at least 20% in 2 inches Peening effective method of reducing stresses Root and face layer and layers more than 1/8 inch should not be peened


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