--- title: "Notes on powder metallurgy and heat treatment" subject: "Machine Design II" chapter: 3 key: "semester_7/machine_design_ii:powder-metallurgy-heat-treatment-notes" source: https://www.bctengineer.com/q/semester_7/machine_design_ii/powder-metallurgy-heat-treatment-notes site: BCT Engineering --- # Notes on powder metallurgy and heat treatment Machine Design II · Chapter 3: Interaction of Materials, Processing and Design - Asked in: year not given, 3+3 marks - Video: [Powder Metallurgy Process](https://www.youtube.com/watch?v=MgukjCT9o80) — The Engineering Science - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/machine_design_ii/powder-metallurgy-heat-treatment-notes - Whole chapter: https://www.bctengineer.com/md/semester_7/machine_design_ii/chapter-3 ## Question Write short notes on: (a) powder metallurgy and its design limitations; (b) heat treatment processes for steel (annealing, normalising, hardening and tempering) and their effect on design. ## Short answer Powder metallurgy compacts and sinters metal powders into near-net shapes, but dies require ejectability, uniform sections, and porosity reduces strength. Annealing gives softness, normalising refines grain, hardening yields martensite, and tempering adds toughness; design must avoid sharp corners and section changes to prevent quench cracking. ## Answer #### (a) Powder metallurgy (P/M) Metal powder is **blended, compacted** in a die under high pressure (hundreds of MPa) and **sintered** below the melting point (about 70 to 90% of the melting temperature in a protective atmosphere). Optional sizing, impregnation with oil (self-lubricating bearings) or infiltration follows. - Advantages: near-net shape, little scrap, good for gears, bushings, filters, cutting tool tips, and alloys that cannot be cast (tungsten carbide). - Design limitations: part must be ejectable from the die (no undercuts or cross holes), uniform section, limited size and length-to-diameter ratio, porosity lowers strength and ductility, sharp corners and thin walls crack, die cost high so it suits large quantity. #### (b) Heat treatment of steel | Process | Operation | Result | |---|---|---| | Annealing | Heat above critical temperature, soak, cool slowly in furnace | Soft, ductile, stress-free; improves machinability | | Normalising | Heat above upper critical, cool in still air | Fine uniform grain, moderate strength | | Hardening | Heat to austenite, quench in water/oil | Hard, brittle martensite | | Tempering | Reheat hardened steel to 150 to 650 °C | Reduces brittleness and internal stress, gives toughness | **Effect on design:** quenching causes distortion and cracking, so avoid sharp corners, sudden thickness changes and blind holes; keep sections uniform; allow grinding allowance after hardening; choose a steel with enough hardenability for the section size; design stresses must use the strength of the heat-treated condition. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the sequence of the powder metallurgy process? A. Melt, pour, solidify B. Heat, hammer, quench C. Sinter, blend, compact D. Blend the powder, compact it in a die, then sinter below the melting point Answer: D. Powders are blended, compacted at high pressure and sintered below the melting point (about 70 to 90 percent of it). (Key point: P/M process) 2. Which feature is a design limitation of a powder metallurgy part? A. Undercuts and cross holes, because the part must eject from the die B. Near-net shape C. Little scrap D. Use of hard alloys such as tungsten carbide Answer: A. The compact must be ejected from the die, so undercuts and cross holes are not possible. The others are advantages. (Key point: P/M design limitation) 3. How does normalising differ from annealing? A. Normalising uses a furnace cool; annealing uses air B. Both quench in water C. Normalising cools in still air and gives fine uniform grain with moderate strength; annealing cools slowly in the furnace for a soft, ductile steel D. Normalising never heats above the critical temperature Answer: C. Cooling rate is the difference: furnace cooling for annealing, still air for normalising. (Key point: Annealing vs normalising) 4. Why is hardened steel tempered, and what design care does quenching need? A. To increase brittleness; no care is needed B. To make it harder; use sharp corners C. To melt the martensite; use blind holes D. To reduce brittleness and internal stress; avoid sharp corners and sudden thickness changes because quenching causes distortion and cracks Answer: D. Tempering toughens hard, brittle martensite. Uniform sections and no sharp corners limit distortion and cracking. 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