Heat Treatment of Steel PDF

Annealing Purpose: Produces soft, ductile steel with a fine grain structure and no internal stresses. Effect: Results in steel having its lowest strength; opposite of hardening. Process: o Heat steel just above its upper critical temperature. o Soak a...

Annealing Purpose: Produces soft, ductile steel with a fine grain structure and no internal stresses. Effect: Results in steel having its lowest strength; opposite of hardening. Process: o Heat steel just above its upper critical temperature. o Soak at that temperature (~1 hour per inch of material thickness). o Cool very slowly to achieve maximum softness. Cooling Methods: o Allow the furnace and steel to cool together to 900°F or lower before removing. o Alternatively, bury the heated steel in insulating materials like ashes or sand. Advantages of Annealing Increases machinability and formability Increases ductility Reduces internal stresses Enhances metal uniformity Increases toughness Disadvantages of Annealing Time consuming Expensive May alter properties of the material in cases where it is not favorable Case Hardening Overview: Case hardening enhances the surface hardness of metal parts while maintaining a tough, durable core, ideal for components needing wear resistance and internal toughness. Low carbon and low alloy steels are best suited for this process, as high carbon steels risk brittleness. The process involves chemically altering the surface with carbon or nitrogen compounds, followed by heat treatment. Types of Case Hardening: 1. Carburizing a. Adds carbon to the surface of low-carbon steel. b. Results in a high-carbon, hard outer layer and a low-carbon, tough interior. c. Methods: i. Pack Carburizing: Steel parts are packed with carbon-rich material, heated to ~1700°F, causing carbon penetration depending on the soak duration. ii. Gas Carburizing: Carbon-rich gas produces the same effect as pack carburizing. iii. Liquid Carburizing: Steel is immersed in a molten salt bath with carbon-rich chemicals. 2. Nitriding Nitriding is a surface hardening process in which nitrogen is introduced into the steel surface at lower temperatures. This process forms hard nitrides, enhancing surface hardness and corrosion resistance, while the core properties remain largely unaffected. Method o Steel is exposed to a nitrogen-containing atmosphere. o Nitrogen diffuses into the steel surface. o Nitrogen reacts with alloying elements to form hard nitrides. Surface Effects: o Significant increase in surface hardness. o Improved wear and corrosion resistance. Core Effects: o Core properties remain almost unchanged. o Nitrides are confined to the surface layer. Applications: Case hardening is commonly used in industries requiring durable components, with carburizing and nitriding as preferred methods, depending on material and desired properties. Normalizing of Steel Normalizing is a heat treatment process for steel that alleviates internal stresses caused by heat treating, welding, casting, forming, or machining. These stresses, if unmanaged, can lead to structural failure. The process improves the physical properties of steel, making it suitable for aircraft applications, particularly in its normalized state rather than annealed state. Key Points: 1. Purpose in Aircraft Work: a. Commonly applied to welded parts to address strains caused by welding. b. Refines grain structure to harmonize the differences between the cast structure of welds and the wrought structure of the surrounding material. 2. Process: a. Steel is heated above the upper critical point and allowed to cool in still air. b. This air-cooling method results in a harder and stronger material compared to annealing, which uses furnace cooling. 3. Applications: a. Essential for treating welded aircraft components after fabrication. b. Recommended normalizing temperatures vary based on the type of aircraft steel. This process enhances durability and consistency, critical for demanding applications like aviation. Summary of Tempering Tempering is a heat treatment process that follows hardening to reduce brittleness and achieve specific physical properties in steel. It also softens the material and is conducted at temperatures below the steel's low critical point, differentiating it from annealing, normalizing, and hardening. Key Points: Purpose: o Reduces brittleness imparted by hardening. o Produces desired hardness and strength by selecting a specific tempering temperature. o Softens the steel to improve workability. Process: o Begins when reheated hardened steel reaches 212°F and continues as the temperature approaches the low critical point. o Tempering temperatures and resulting tensile strengths can be predetermined. o Minimum tempering time is 1 hour, with an additional hour for each inch of thickness beyond 1 inch. Characteristics: o Ultimate tensile strength of tempered steels used in aircraft ranges from 125,000 to 200,000 psi. o Cooling rate after tempering has no impact on the steel structure, so air cooling is typically used. Tempering is essential for refining steel's mechanical properties while ensuring durability and reduced brittleness.  NDI 1 Ultrasonic Inspection Ultrasonic inspection is a valuable method for detecting internal delamination, voids, or inconsistencies in composite components that are not visible or detectable using visual or tap tests. Principle: o Uses high-frequency sound waves (several MHz) introduced into a part. o Sound waves travel through the part and are monitored for changes indicating flaws. o Similar to light waves, ultrasonic waves can be absorbed or reflected when encountering interruptions. Process: o A transducer will send sound waves through the material o If no defects are found, the sound waves will pass through the material, but if there are the sound waves hit a defect, they will bounce of it indicating its presence. o A receiving transducer captures the disrupted waves, converting them into visual displays on devices like oscilloscopes or chart recorders. Calibration & Standards: o Reference standards are crucial for calibrating equipment and ensuring accurate comparisons. o These standards must account for variations in composite components due to in-service exposure or prior repairs. Applications & Challenges: o Works well in repetitive manufacturing environments but is more complex in repair scenarios due to the diversity and complexity of composite components. Advantage is portable Disadvantage Cast iron and other coarse-grained materials are difficult to inspect due to low sound transmission and high signal noise Ultrasonic inspection is an essential nondestructive testing method, adaptable to various scenarios with proper calibration and understanding of composite behaviors. 2 Radiography Radiography, commonly known as X-ray inspection, is a nondestructive testing (NDI) method that enables the internal examination of parts or assemblies by capturing X-ray absorption on a film. Key Points: Principle: o X-rays pass through the material, and variations in absorption are recorded on an X-ray- sensitive film. o Developed film provides a visualization of the internal structure by showing changes in density. Applications: o Ideal for detecting defects parallel to the X-ray beam's centerline, such as: ▪ Corner delamination. ▪ Crushed or blown cores. ▪ Water in core cells. ▪ Voids in foam adhesive joints. ▪ Positions of internal details. o Less effective for defects normal to the ray direction, such as planar delaminations. Limitations: o Most composites are nearly transparent to X-rays, requiring low-energy rays for effective imaging. o Safety concerns make it impractical for use around aircraft. Safety Measures: o Operators must use lead shields and maintain a safe distance to avoid exposure to X-rays or scattered radiation. Radiography is a powerful tool for internal inspection but requires careful handling due to safety risks and is less suited for specific defect orientations. Summary of Heat Treatment of Magnesium Alloys Magnesium alloy castings, widely used in aircraft construction, are highly responsive to heat treatment. This process enhances their mechanical properties and includes two primary types: 1. Solution Heat Treatment: Purpose: Improves tensile strength, ductility, and shock resistance. Designation: o -T4: Solution heat-treated. o -T6: Solution heat-treated and artificially aged for full property development. Process: o Heated between 730°F and 780°F, depending on the alloy type (as per MIL-H-6857). o Soaking time ranges from 10 to 18 hours, longer for parts over 2 inches thick. o Air quenching is used after treatment. Safety: o Never heat magnesium alloys in salt baths due to explosion risks. o Furnaces must have safety cutoffs to prevent overheating and potential ignition. o Some alloys require sulfur dioxide gas for fire prevention. 2. Precipitation (Aging) Heat Treatment: Purpose: Increases hardness and yield strength, stabilizes alloys, and relieves stress to prevent dimensional changes during machining. Process: o Conducted at 325°F to 500°F. o Soaking time ranges from 4 to 18 hours. Effects: o Improves corrosion resistance closer to the “as-cast” state. o Slight loss of ductility in exchange for increased hardness and yield strength. Summary: The controlled heat treatment of magnesium alloys enhances their strength, stability, and corrosion resistance. Careful temperature control and safety measures are critical to avoid fire hazards. Rivets Alloy Types & Characteristics: o Alloy 1100: ▪ Used "as fabricated" without heat treatment. ▪ Ideal for low-strength applications in aluminum sheets. o Alloy 5056: ▪ Also used "as fabricated." ▪ Specifically designed for riveting magnesium alloy sheets. o Alloy 2117: ▪ Offers moderate strength. ▪ Requires only one heat treatment by the manufacturer and can be anodized post- treatment. ▪ Most commonly used alloy in aircraft construction, ready for use at any time. o Alloy 2017: ▪ High-strength alloy requiring reheat treatment before use. ▪ Becomes too hard to drive about one hour after quenching. o Alloy 2024: ▪ Provides high strength, similar to Alloy 2017. ▪ Hardens faster, within 10 minutes of quenching, giving superior shear strength. Storage & Reheat Treatment for 2017 and 2024 Rivets: o Rivets can be stored in a refrigerator below 32°F post-quenching to remain soft for several days. o Reheat treatment is necessary if not used within the specified timeframe after refrigeration. Heat Treatment Processes: o Alloy 2017: ▪ Heat to 930–950°F for ~30 minutes, then quench in cold water. ▪ Achieves maximum strength approximately nine days after installation. o Alloy 2024: ▪ Heat to 910–930°F, followed by cold water quenching. ▪ Attains maximum shear strength one day post-installation. Driving Timeframe: o 2017 Rivets: Must be driven within one hour of heat treatment or refrigeration removal. o 2024 Rivets: Should be driven within 10–20 minutes of heat treatment or refrigeration removal. o 2117 This is a moderately high-strength alloy that is used for rivet aluminium sheets. It is once heat treatment by the manufacturer and requires no further heat treatment. It must be anodized after being heat treated. it is essentially one of the best Alloys to be used for riveting. 2017 This is a high-strength aluminium alloy rivet, it is heat treated by the manufacturer however due to its ageing characteristics it must be reheat treated before use. It hardens within 1 hour of quenching. Before reheat treating it must be anodised to avoid intergranular oxidation. To undergo solution heat treatment. It is done in tubular containers using a salt bath or wire baskets In the air furnace. 2017 is heated to approximately 930°F to 950°F and left to soak for 30 minutes, then is immediately quenched in cold water. it takes about 9 days to fully harden and should be used within 1 hour. however, if refrigerated below 32°C it will remain soft and usable for several days. 2024 Similar to 2017 it is a high-strength alloy rivet which initially is heat treated by the manufacturer because of its ageing characteristics. It hardens within 10 to 20 minutes. It must be reheat- treated before use. It must also be anodized before reheat treating to avoid intergranular oxidation. If kept below 32°F it will remain soft and usable for several days. The heat treatment also happens in a tubular container with a salt bath or an air furnace. it is heated to 910°F to 930° F and is immediately quenched in cold water. It takes about 1 day to harden but must be used within 10 to 20 minutes 2024 develops a greater sheer strength than 2017.