Plant Maintenance - Welding and Strength of Materials
The third digit on a welding rod refers to the suitable welding position, using #1-4. What is the significance of each #?
- 1- all positions - 2- flat horizontal fillet - 3- flat only - 4- down hand only
1PSI = _____kPa
- 6.895
What is submerged arc welding?
- a machine electric welding process, where the welding is shielded by a blanket of granular, fusible material (usually called flux) on the work -ac or dc
Nickel
- a tough silvery element w/ approx. same density as copper -excellent resistance to corrosion and oxidation -improves toughness -prevents brittleness at low temps
What are the four primary methods of metal transfer in GMAW welding?
- globular -short-circuiting -spray -pulsed spray
reverse polarity preferred for overhead welding?
- less heat is produced at the work and more heat at the electrode. This would be preferred when doing overhead welding, where the fi ller metal from the electrode must be quickly solidified to prevent it falling away from the work.
How is copper primarily used in a power plant?
- primary for electrical equipment -as an alloy, used for heat exchanger tubes, valves and fittings
What is the arc welding power source that can convert AC into DC?
-3.) the rectifier
What is the typical composition of lead based white metal?
-5% tin, 14.5% antimony, 0.3-0.6% arsenic and approx. 80% lead
What is the typical composition of tin based white metal? What is it most commonly used for?
-85% tin, 7% antimony, 7% copper, and a max of 0.35% lead -turbine bearings
What is the formula for determining the Brinell Hardness Number?
-BHM=load(kg)/area of the mark(mm^2)
True OR False? Brazing is a form of fusion welding?
-False cuz fusion welding is when metal pieces to be welded are brought to molten state and fused together but brazing heats not fuse.
What are the readings made with the diamond cone of a Rockwell Tester called?
-Rockwell C readings
physical properties of aluminum.
-Very malleable and ductile -A good conductor of electricity -An excellent conductor of heat -Highly resistant to corrosion -Low density as it is only one third as heavy as iron or steel - DISadvantage is tht in pure aluminum has low tensile strength
Tough and ductile materials have ______________ of elasticity. Hard and brittle materials have ________________ elasticity.
-a certain amount -very little
Define metal arc welding.
-a fusion welding process in which the pieces are joined by means of an electric arc -3000*C to 8300*C
Briefly describe a dye penetrant inspection.
-a low viscosity fluid containing red dye is applied to the surface being tested -it is allowed to sit for 5-15min to soak in(travels easily by capillary action) -after penetration time, excess penetrant is removed from surface -surface dried, then developer compound applied with paint spray gun developer compound draws out any penetrant indicating any defects
What is a liquid dye penetrant inspection.
-a method of inspection involving dye which gives a visual indication of cracks, leaks, and other defects which extend to the surface
What is welding?
-a process which consists of joining two or more pieces of metal by the application of heat and sometimes pressure
8.) What is toughness?
-a property that determines whether or not a material will break under a sudden impact or hard blow -also referred to as impact strength
7.) What is malleability?
-a property which allows a material to be hammered or rolled into other shapes and sizes -the malleability of most materials will increase when the material is heated
What are the main advantages of alloy steels?
-ability to respond to heat treatment -improved corrosion resistance -improved properties at high and low temperatures -combination of high strength with good ductility
How are plain carbon steels grouped?
-according to their carbon content
What are steels?
-alloys of iron and carbon with less then 2% carbon
What is white metal? lead & tin What is it chiefly used for?
-also known as babbit -alloys made primarily of lead and tin -chiefly used for bearing material because they are easily melted and cast in bearing shell
Bronze
-an alloy of copper and tin -MAY contain zinc to ensure non porous castings and lead for improve machining qualities -has resistance to corrosion equal to copper
Briefly describe the braze welding process.
-base metal is heated w/ oxyacetylene torch above 450*C -bronze filler rod applied to hot metal -filler melts and flows smoothly over entire weld area
What is the product of additions of up to 1% phosphorus to a bronze?
-bearing bronze which is hard but not abrasive
Why are DC machines usually prefered to AC machines for quality work?
-because the current from a DC machine flows in only direction which results in good control of the welding process
two most commonly used copper alloys
-brasses and bronzes
Braze welding is used extensively in the repair of ________ and ________________.
-cast iron -malleable iron parts
Pig iron must be refined into what two broad classes to be of any use?
-cast iron -steel
white cast iron
-cast iron that has most of it's carbon combined chemically with the iron -produced by rapidly cooling the molten material in a mold -white cast iron is very hard and brittle so it is used for machinery parts subjected to excessive wear (such as crusher jaws and grinding mill balls and liners)
grey cast iron
-cast iron that has most of it's carbon mechanically mixed in(in the form of graphite) -produced by slowly cooling the molten material which allows the carbon to disassociate and form graphite within the iron. - it is softer than white so it is easily machined and it is used commonly for machinery bases and supports as it has good compressive strength
Gas metal arc welding (GMAW) is a type of welding process in which a _________ and __________ wire electrode is used.
-continuous -consumable - and shielding gas are fed through a welding gun
Brass
-copper and zinc(up to 40%) -frequently small amount of lead, tin, nickel, aluminum, and manganese
What type of current is used for arc welding?
-either AC or DC
What is special high alloy steel frequently produced in?
-electric furnaces where the heat is furnished by electrical arcs
Regarding welding rods such as E6010 or E7024; -What does the E mean? -What is meant by the first two digits? -What is meant by the third digit? -What is meant by the fourth digit?
-electric welding -tensile strength in PSI (times 1000, ex; 60 is 60 000PSI which means a pull of 60 000 pounds on a material 1 square inch cross sectional area would break the material) -the welding position -special characteristics
Briefly describe a fluorescent penetrant inspection
-employs a penetrating fluid which is fluorescent under a black light -fluid is applied to surface being tested -after a short time the excess fluid is wiped off -a black light is beamed over the area and any fault or defects will glow
What two groups of metals are power engineers concerned with the applications and properties of?
-ferrous and nonferrous
What are the two types of dye that may be used in a liquid dye penetrant inspection?
-fluorescent penetrant -dye penetrant
Which properties of carbon steels increase with increased carbon content?
-hardness and strength
High and very high carbon steels respond well to _____________. Most of these materials may, in the _____________ state, be ______________________.
-heat treatments -annealed -readily machined
What properties make copper desirable as an engineering material?
-high electrical conductivity -high heat conductivity -high corrosion resistance -ductility -toughness
copper
-improves atmospheric corrosion qualities
lead
-improves machinability
Molybdenum
-increases hardness and endurance limits -decreases tendency towards creep (slow permanent stretching of a material under stress at high temp) -increases resistance to corrosion
manganese
-increases strength and hardness -promotes high impact strength -increases resistance to wear by abrasion
Briefly describe a visual inspection of a weld.
-involves looking for visble defects before, during and after welding -can be readily applied at all stages of production -is the best method to avoid errors and detect faults while they can still be easily rectified
List four types of ferrous materials that are used in the engineering of many components.
-iron -cast iron -wrought iron -steel
How is iron produced?
-iron ore is added to a blast furnace with coke (fuel) and limestone which combine with the impurities to form slag -slag floats on the surface of the molten metal and is skimmed off -the molten metal is then cast into molds to form pig iron
Why is elasticity considered to be one of the most important properties from an engineering point of view?
-it helps determine the behaviour of a material under a load
What is malleable cast iron and how is it produced?
-it is annealed (heating and cooling at controlled rate) white cast iron -annealing increases toughness and ductility -used for some farm implements, automobile parts, pipe fittings, and tools
What is the advantage of lead based babbitt?
-it's cheaper. tin is more expensive then lead
What knowledge is absolutely essential?
-knowledge of types, performance and preservation
For lower speeds
-lead based white metal
cast iron
-melting pig iron and some scrap iron together in a cupola furnace -resulting molten iron contains 2-4% carbon
What are nonferrous metals? What are some examples of nonferrous metals?
-metals which contain no iron or at most only a trace -copper, lead, aluminum, zinc, nickel, tin and magnesium
List the 8 common elements added to alloy steels.
-nickel -chromium -molybdenum -vanadium -copper -lead -manganese -tungsten
What type of electrode is used to produce the weld in a gas tungsten arc welding (GTAW) aka TIG (tungsten inert gas)
-nonconsumable tungsten
What are the two categories of steels?
-plain carbon steel (carbon and steel only) -alloy steel (carbon steels with additional elements added)
Describe the process of making wrought iron.
-produced by a process known as puddling. -starts with pig iron melted in furnace -carbon and other IMPURITIES oxidize and leave -remaining iron formed into ball using puddling bar -ball removed from furnace, then squeezed and rolled to remove slag -ductility and resistance to corrosion - was used for boiler tubes and piping but it has now been replaced with steel
tungsten
-produces fine grain structure -retains hardness and strength at high temps
Vanadium
-produces fine grain structure during heat treatment -promotes hardening ability -improves ductility
What are ferrous metals?
-pure iron and alloys of iron
Chromium
-resists oxidation from hot gases -maintains high strength at elevated temps -increases hardness and abrasion resistance -in amounts above 4%, corrosion resistance greatly improved -minimum of 12% is stainless steel
Briefly describe the basic steel making process.
-scrap iron, pig iron from the blast furnace and limestone are melted together in the basic oxygen furnace -as this mix is melted, the carbon content is reduced to the desired level via oxidization -if alloy steel is desired, the required alloying materials are added at this point -slag floats to the surface -at the correct temperature and composition the steel is tapped from the furnace
What is the ultimate criteria in choosing metals?
-serviceability and safety
What are two common tests to determine a materials toughness?
-the Izod test - Hammer sht -the Charpy test - swing sht
6.) What is elasticity?
-the ability of the material to return to its original shape after any force acting on it has been removed
1.) What is hardness?
-the ability to resist wear, abrasion, cutting and indentation
What is the majority of steel in the world produced in?
-the basic oxygen furnace
Common brazements are about 1/3 as strong as the materials they join. What can be done to create high-strength brazes?
-the brazement can be annealed (cooled at a controlled rate)
What are properties of a metal?
-the characteristics by which it can be accurately identified, or by which its range of usefulness can be determined
In alloy steels, some of the alloying elements combine with the carbon to form compounds; other elements do not form compounds, but remain in solution in the ferrite. "In solutions means _________________________________.
-the elements do not combine with the other elements, but are held suspended as crystals in the basic ferrite.
In straight polarity welding what side is the electrode holder connected to?
-the negative terminal
4.) What is plasticity?
-the opposite of brittleness -material said to exhibit plasticity if it is very soft and easily deformed
In reverse polarity welding, what side is the electrode holder attached to?
-the positive terminal
3.) What is ductility?
-the property of a material that enables it to be drawn to a considerable extent before rupture and, at the same time, to sustain appreciable load -sometimes considered to be the ability of a material to be permanently deformed without breaking -mild steel
2.) What is Brittleness?
-the property of a metal which permits no permanent deformation before breaking - cast iron
What must be known before a suitable welding inspection method can be chosen?
-the service conditions
What determines the rate of electrode consumption?
-the strength of the current
What effect do impurities like phosphorus or sulphur have on carbon steels?
-they lower ductility, malleability, and welding qualities
For higher speeds
-tin based white
What is the purpose of the electrode covering?
-to form a gaseous shield around the weld which excludes oxygen and nitrogen -also aids in stabilizing the arc -to a very large degree determines the welding characteristics of the electrode
What is GTAW most often used for?
-to weld thin sections of stainless and for light metals such as aluminum, magnesium, and copper alloys - it grants operator greater control over the weld, thus allowing stronger and better quality but it is significantly slow asf compared to other.
What is the most extensively used form of weld inspection? Why?
-visual -because it is easy to apply, is quick, is relatively inexpensive, and gives very important information with regard to the welds and general conformity of the weldment to specification requirements.
List four nondestructive weld tests.
-visual -liquid dye penetrant -radiographic -ultrasonic
What are some examples of plastic materials?
-wax, lead, and Babbitt
What is the most satisfactory source of DC power for arc welding?
1.)portable motor-generator set
How is AC supplied for arc welding?
2.) from single-phase transformers
Aluminum is produced by electrolysis from bauxite ore
and is low density only 1/3 of iron or steel
alloys of aluminum
copper, silicon, manganese, zinc, magnesium and chromium
straight polarity
for shallow wide weld deposit cuz it melts and produce faster
Ferrous materials
less than 2% = steel more than 2% = cast iron
