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Questions and Answers
What is a key benefit of hot working metals?
What is a key benefit of hot working metals?
Which of the following is NOT a method of metalworking?
Which of the following is NOT a method of metalworking?
What shape is referred to as a bloom in hot working?
What shape is referred to as a bloom in hot working?
Which of the following processes is used to prepare ingots for hot working?
Which of the following processes is used to prepare ingots for hot working?
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What is the main goal of heat treatment processes in metalworking?
What is the main goal of heat treatment processes in metalworking?
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Which of the following materials would commonly undergo hot working?
Which of the following materials would commonly undergo hot working?
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What is the purpose of rolling hot-rolled materials?
What is the purpose of rolling hot-rolled materials?
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What is the primary purpose of tempering steel?
What is the primary purpose of tempering steel?
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What is one critical property that metals must have for aviation applications?
What is one critical property that metals must have for aviation applications?
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Which process is most suitable for shaping small metal pieces?
Which process is most suitable for shaping small metal pieces?
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What occurs during the annealing process?
What occurs during the annealing process?
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What is one limitation of cold working?
What is one limitation of cold working?
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What defines the method of normalizing in metalworking?
What defines the method of normalizing in metalworking?
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Which process improves fatigue resistance for components under repeated stress?
Which process improves fatigue resistance for components under repeated stress?
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What is the main function of the extrusion process?
What is the main function of the extrusion process?
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What characterizes hot working processes?
What characterizes hot working processes?
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What is the primary purpose of heat treatment in metalworking?
What is the primary purpose of heat treatment in metalworking?
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Which of the following processes is NOT included in heat treatment methods for metals?
Which of the following processes is NOT included in heat treatment methods for metals?
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What can be a consequence of hardening a metal through heat treatment?
What can be a consequence of hardening a metal through heat treatment?
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Why are pure metals not suitable for hardening through heat treatment?
Why are pure metals not suitable for hardening through heat treatment?
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What is a crucial factor in the effectiveness of heat treatment processes?
What is a crucial factor in the effectiveness of heat treatment processes?
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How does the structure of alloys change with heat treatment compared to pure metals?
How does the structure of alloys change with heat treatment compared to pure metals?
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What is the impact of using furnaces beyond their rated temperatures during heat treatment?
What is the impact of using furnaces beyond their rated temperatures during heat treatment?
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Which factor can significantly affect the heat-treated parts during the heat treatment process?
Which factor can significantly affect the heat-treated parts during the heat treatment process?
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What is the maximum temperature that an electric furnace can reach?
What is the maximum temperature that an electric furnace can reach?
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Which pyrometer type provides a permanent temperature record?
Which pyrometer type provides a permanent temperature record?
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What is the role of atmosphere control in heating steel?
What is the role of atmosphere control in heating steel?
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How long is the typical soaking period for parts in a furnace?
How long is the typical soaking period for parts in a furnace?
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What is the temperature range for salt baths used in tempering or hardening?
What is the temperature range for salt baths used in tempering or hardening?
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Which thermocouple type is suitable for temperatures up to 2,200°F?
Which thermocouple type is suitable for temperatures up to 2,200°F?
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What is the primary objective when transforming pearlite to austenite?
What is the primary objective when transforming pearlite to austenite?
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What color does steel typically reach at a temperature of 1,000°F?
What color does steel typically reach at a temperature of 1,000°F?
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What is the fastest cooling medium for steel in the quenching process?
What is the fastest cooling medium for steel in the quenching process?
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Which of the following quenching media is least effective in cooling steel?
Which of the following quenching media is least effective in cooling steel?
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What temperature should water and brine quenching solutions be maintained below?
What temperature should water and brine quenching solutions be maintained below?
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What is a primary recommendation to reduce warping during the quenching process?
What is a primary recommendation to reduce warping during the quenching process?
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What should the viscosity of quenching oil be at 100°F?
What should the viscosity of quenching oil be at 100°F?
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Why is rapid transfer from the furnace to the quenching medium important?
Why is rapid transfer from the furnace to the quenching medium important?
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What effect does a vapor film on hot steel have during the quenching process?
What effect does a vapor film on hot steel have during the quenching process?
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Which quenching medium is recommended for carbon steels?
Which quenching medium is recommended for carbon steels?
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Study Notes
Metalworking Processes
- Metalworking is the process of shaping and reshaping metals to create individual parts, assemblies, or large structures.
- Three methods are hot working, cold working, and extrusion.
Hot Working
- Involves shaping metals above their recrystallization points, making the material more malleable and less likely to crack.
- Steel is typically hot worked from ingots.
- When stripped from its mold, an ingot has a solid surface but a molten interior.
- Ingots are placed in soaking pits to equalize temperature and solidify the interior.
- After soaking, ingots are rolled to intermediate sizes for easier handling.
- When rolled:
- Bloom: Square section with dimensions 6 inches x 6 inches or larger.
- Billet: Square section with dimensions less than 6 inches x 6 inches.
- Slab: Rectangular section with width greater than twice its thickness.
- Applications: These shapes are further rolled into various uniform cross-sectional shapes like sheets, bars, channels, angles, and I-beams.
- Hot-rolled materials are often finished by cold rolling or drawing for precise dimensions and smooth surfaces.
- When not rolled:
- Forging: Performed at temperatures above the critical range to shape the metal.
- Pressing: Used for large, heavy parts; slow-acting, uniformly transmits force, affecting both interior and exterior grain structure.
- Hammering: Suitable for small pieces; quick force application, affecting only a small depth.
- Smith Forging: Produces small, high-grade parts; operator controls pressure and finishing temperature, saves machining time and material.
- After hardening, tempering involves heating steel to a specified temperature and cooling it in air, oil, water, or special solutions.
- Relieves internal strain and reduces brittleness.
- Enhances hardness and toughness.
If it's too hard...
- Annealing: Involves heating to a prescribed temperature, holding, and slow cooling.
- Relieves internal stresses, softens metal, increases ductility, and refines grain structure.
- Maximum softness achieved by very slow cooling; some metals require furnace cooling, others air cooling.
- For iron base metals...
- Normalizing: Involves heating to the correct temperature, holding, and cooling in still air.
- Used to relieve stresses in metals.
Cold Working
- Takes place below the recrystallization temperature, resulting in harder, stronger metals with improved surface finishes.
- Benefits:
- Increases tensile strength and improves fatigue resistance, essential for components subjected to repeated stress.
- Limitations:
- Excessive cold working can make metals brittle, which may necessitate additional heat treatment.
Extrusion
- A process where metal is pushed through a die to produce long, continuous sections of a fixed cross-sectional shape.
- Applications: Used to create components like tubes and channels that are lightweight but durable, vital for aircraft construction.
Heat Treatment
- Essential for adjusting the mechanical properties of metals, such as hardness, toughness, and ductility.
- Involves heating and cooling metals under controlled conditions.
- Broadly includes annealing, normalizing, hardening, and tempering for steels.
- Purpose:
- Change mechanical properties to make metals more useful, serviceable, and safe.
- Can make metals harder, stronger, more impact-resistant, or softer and more ductile.
- No single heat-treating operation can produce all desired characteristics; some properties improve at the expense of others.
- Problem:
- Subjected to shock and fatigue stresses.
- Fatigue occurs from frequent reversals of loading or repeated loads, leading to cracks and eventual failure.
- Resistance to shock and fatigue is crucial for critical parts.
- Restrictions:
- Heat treatment results depend on metal structure and changes during heating and cooling.
- Pure metals cannot be hardened by heat treatment due to minimal structural change.
- Alloys respond to heat treatment as their structures change with heating and cooling.
Heating
- Objective: Transform pearlite to austenite by heating steel through the critical range.
- Requires slow heating to prevent rapid transition.
- Cold steel inserted at 300°F to 500°F below hardening temperature.
- Temperature estimation methods: commercial crayons, pellets, paints, or observing color changes.
- Steel color changes with temperature: dull red at 1,000°F, progressing to white.
- Protect steel from oxidation and decarburization using atmosphere control or covering with cast iron borings/chips.
- Vacuum furnaces used for annealing to maintain non-oxidized surfaces.
Soaking
- Maintain constant furnace temperature during soaking.
- Soaking temperatures vary by steel type and part size.
- Small parts soaked at lower range, heavy parts at upper range.
- Typical soaking period: 30 minutes to 1 hour.
Cooling
- Cooling rate through critical range determines steel's final form.
- Cooling media: still air (slow), liquids (fastest).
- Common quenching liquids: brine (strongest), water, oil (least strong).
- Oil quench for alloy steels, brine or water for carbon steels.
Quenching Media
- Quenching solutions cool steel without chemical action.
- Common media: water, inorganic salt solutions, oils.
- Cooling rates: rapid in brine, less rapid in water, slow in oil.
- Brine: 5-10% salt solution, effective in removing scale.
- Water and brine should be kept cold (below 60°F).
- Quenching oils: straight mineral oil with Saybolt viscosity of about 100 at 100°F.
- Oils have greatest cooling velocity at 100-140°F.
Quenching Process
- Film forms on hot steel surface, reducing heat abstraction.
- Agitation or pressure spray quench needed to dislodge vapor films.
- Recommendations to reduce warping:
- Avoid throwing parts into quenching bath.
- Agitate parts to destroy vapor coating.
- Immerse irregular parts with heavy end first.
Quenching Equipment
- Properly sized quenching tank with circulating pumps and coolers.
- Maintain constant temperatures during large-scale quenching.
- Rapid transfer from furnace to quenching medium is crucial.
- Use guard sheets to retain heat during transfer.
- Rinse tank to remove salt after quenching.
Furnaces
- Various types and sizes designed for specific temperature ranges.
- Using furnaces beyond rated temperatures can reduce lifespan and require repairs.
- Fuel-fired furnaces need air for combustion; usually muffler type to avoid direct flame impingement.
- Electric furnaces use wire or ribbon heating elements; additional elements at high heat loss points.
- Common maximum temperatures: electric furnaces up to 2,000°F, resistor bar furnaces up to 2,500°F.
Salt Baths
- Salt baths for tempering or hardening; temperature range 325°F to 2,450°F.
- Lead baths temperature range: 650°F to 1,700°F.
- Faster heating in lead or salt baths compared to furnaces.
Pyrometers
- Measure furnace temperature using thermocouples.
- Pyrometer components: thermocouple, extension leads, meter.
- Furnaces for tempering may have gas/electric heating and fans for hot air circulation.
- Thermocouples: copper-constantan (up to 700°F), iron-constantan (up to 1,400°F), chromel-alumel (up to 2,200°F), platinum-rhodium alloys (up to 2,800°F).
- Thermocouple life affected by maximum temperature and furnace atmosphere.
- Encased in metallic/ceramic tubes to protect from furnace gases.
- Accurate control requires placing thermocouple close to the work.
- Automatic controllers help maintain desired temperature.
- Indicating type: direct temperature reading.
- Recording type: permanent temperature record via inked stylus on calibrated paper/chart.
References
- Aircraft Systems.
- The Piping Mart.
- Aviation Metals.
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Description
Explore the fascinating world of metalworking, focusing specifically on hot working techniques. This quiz covers the methods of shaping metals above their recrystallization points and includes details about ingots and rolling processes. Test your understanding of key concepts in metalworking and the applications of different shapes.
