Metalworking Processes: Hot Working Techniques

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.