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- 1. HARDNESS OF MATERIALS MECHANICS OF MATERIAL AND MACHINE DESIGN PRESENTED TO: PROF. SYED SAQIB PRESENTED BY: 2015-CE-78 2015-CE-64 2015-CE-51 2015-CE-72 2015-CE-73
- 2. HARDNESS: The property of a material to resist plastic deformation. The resistance may be against: • Scratching • Penetration • Cutting Figure: Hardness
- 3. HARDNESS DEPENDS UPON: • Ductility • Elastic stiffness • Plasticity • Strain • Toughness • Viscosity
- 4. SCRATCH HARDNESS(ABRASSION HARDNESS): Scratch hardness is the measure of how resistant a sample is to fracture or permanent plastic deformation due to friction from a sharp object.
- 5. PENETRATION HARDNESS(INDENTATION HARDNESS): This measures the resistance of a sample to material deformation due to a constant compression load from a sharp object.
- 6. REBOUND HARDNESS(DYNAMIC HARDNESS): This measures the height of the "bounce" of a diamond- tipped hammer dropped from a fixed height on to a material.
- 7. CUTTTING HARDNESS: This measures the resistance of the sample to material deformation due to cutting by any means.
- 8. HARDNING PROCESS • There are five hardening processes: • Hall-Petch strengthening • Work hardening • Solid solution strengthening • Precipitation hardening
- 9. Hall-Petch Strengthening • Hall-Petch hardening, also known as grain- boundary hardening • a method of strengthening materials by changing their average crystallite (grain) size • It is based on the observation that grain boundaries impede dislocation movement and that the number of dislocations within a grain Figure: Hall–Petch Strengthening is limited by the size of dislocations. Once the grain size reaches about 10 nanometres (3.9×10−7 in), grain boundaries start to slide.
- 10. HALL-PETCH STRENGTHENING • dislocation-dislocation interactions. • Dislocation creates stress fiels around them given by 𝜎 𝛼 𝐺𝑏/𝑟
- 11. WORK HARDENING • Work hardening, also known as strain hardening or cold working • It is the strengthening of a metal by plastic deformation. • This hardness occur due to dislocation movement and dislocation generation within a crystal structure of material Figure : A phenomenological uniaxial stress-strain curve showing typical work hardening plastic behavior of materials in uniaxial compression.
- 12. PRECIPITATION HARDENING • Precipitation hardening, also called age hardening • It is a heat treatment technique used to increase the yield strength and hardness of malleable materials • Precipitation hardening relies on changes in solid solubility with temperature to produce fine particles of an impurity phase, which impede the movement of dislocations, or defects in a crystal's lattice Figure: precipitation hardening
- 13. SOLID SOLUTION STRENGTHENING • Solid solution strengthening is a type of alloying that can be used to improve the strength of a pure metal. • The technique works by adding atoms of one element (the alloying element) to the crystalline lattice of another element (the base metal), forming a solid solution Figure :Solid Solution Strengthening
- 14. HARDNESS TEST • Brinell Hardness Test • Rockwell Hardness Test • Vickers Hardness Test • Knoop Hardness Test ME101: Materials Science and Technology
- 15. CLASSIFICATION OF HARDNESS TEST • Macro Hardness Test load applied by indenter>=9.8N diameter of indentation>=19 μm e.g. Brinell Hardness Test Rockwell Hardness Test
- 16. CLASSIFICATION OF HARDNESS TEST • Micro Hardness Test load applied by indenter< 9.8N diameter of indentation< 19 μm e.g. Vickers Hardness Test Knoop Hardness Test
- 17. BRINELL HARDNESS A spherical indenter (1 cm diameter) is shot with a load of 500, 1000 or 3000kg • The indenter is steel, but for harder materials it is replaced with a tungsten carbide sphere Formula : 𝐵𝐻𝑁 = 𝑙𝑜𝑎𝑑 𝑎𝑟𝑒𝑎 𝑜𝑟 𝑑𝑖𝑎𝑚𝑒𝑡𝑒𝑟 𝑜𝑓 𝑖𝑛𝑑𝑒𝑛𝑡𝑎𝑡𝑖𝑜𝑛 Figure: Brinell Hardness Figure: Brinnel Harness
- 18. LIMITATIONS OF THE BRINELL HARDNESS TEST • Sample must be ten times thicker than the indentation depth (sample usually should be at least 3/8" thick). • Test is most accurate if the indentation depth is 2.5 - 5.0 mm. Adjust load to achieve this. • It gives hardness between 50 HB to 750HB. • BHN evaluates the hardness of metallic materials
- 20. BHN PROS & CONS • Widely used and well accepted • Large ball gives good average reading with a single test. • Accurate • Easy to learn and use • Destructive • Non-portable • High initial cost ($5,000) • Error due to operator reading Brinell Microscope (10% max) ME101: Materials Science and Technology
- 21. ME101: Materials Science and Technology ROCKWELL HARDNESS TEST (HRB,HRC,ETC.)
- 22. ROCKWELL HARDNESS The shape of indentor is either steel ball or conical diamond point Formula : BHN= 𝑙𝑜𝑎𝑑 𝑑𝑒𝑝𝑡ℎ 𝑜𝑓 𝑝𝑒𝑛𝑒𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝑖𝑛𝑑𝑒𝑛𝑡𝑜𝑟 Figure: Rockwell Hardness
- 23. ROCKWELL HARDNESS METHOD • Select Scale - load and indentor depending on the scale • Press a point into material • - Diamond Point (Brale) • - 1/16" ball • - 1/8" ball • - ¼” ball
- 24. Symbol Minor(Pre-) Major(Total) Indenter Load, kg Load, kg C1 C2mm-1 Coefficients in R = C1 – C2 Δ t Normal Scales RB, 1/16 ball* RC, cone + RA, cone RD, cone RE, 1/8 ball RF, 1/16 ball RG, 1/16 ball Superficial Sales R15N, cone+ R30N, cone R45N, cone R15T, 1/16 ball R30T, 1/16 ball R45T, 1/16 ball 10 100 130 500 10 150 100 500 10 60 100 500 10 100 100 500 10 100 130 500 10 60 130 500 10 150 130 500 3 15 100 1000 3 30 100 1000 3 45 100 1000 3 15 100 1000 3 30 100 1000 3 45 100 1000 Load levels and indenter sizes for Rockwell hardness tests.
- 25. ROCKWELL TEST LIMITATIONS Sample must be ten times thicker than the indentation depth (sample usually should be at least 1/8" thick). Need 3 tests (minimum) to avoid inaccuracies due to impurities, hard spots. Test is most accurate if the Rockwell Hardness is between 0 and 100. Adjust scale to achieve this. For Steel: If HRa > 60, use HRc scale If HRa < 60, use HRb scale ME101: Materials Science and Technology
- 26. PROS & CONS (RHT) • Widely used and well accepted • Little operator subjectivity • Accurate • Fast • Destructive • Non-Portable • Initial cost ($5,000) ME101: Materials Science and Technology
- 27. VICKERS HARDNESS • The indenter is square based pyramid • Formula VH = 2PSin(θ/2)/L2 P: applied load in kg L: average diagonal length θ: angle between opposite faces of indenter=136⁰ 𝑽𝑯 = 𝑷∗𝟐 𝑫 𝟐 𝒔𝒊𝒏 𝟏𝟑𝟔/𝟐 𝑽𝑯 = 𝟏. 𝟖𝟓𝟒 𝑷 𝑫 𝟐
- 28. PROS AND CONS • It provides a continuous scale of hardness for a given load from very soft to hard metals • Measurement of diagonal length also subjects to human error • The Micro hardness methods are used to test on metals, ceramics, almost any type of material. • Small indentation compared to BHN
- 30. KNOOP HARDNESS TEST Shape of indenter: Diamond tip tool P : Applied load = 0.1 kg – 1 kg Ap : Unrecovered Proj. area of indentations, mm2 L : Length of long diagonal, mm C : A constant supplied by the manufacturer (C=0.07028 for 172° 30' between long edges and 130° 0' between short edges) ME101: Materials Science and Technology
- 31. PROS & CONS (KHN) • Accurate • Any material which is extremely soft or hard can be used to evaluate its hardness using knoop hardness . • Requires load to be normal to surface plane parallel surfaces. • Independent to ductility of material • Slow • Sensitive to surface condition • Subject to error in diagonal measurement ME101: Materials Science and Technology
- 32. ME101: Materials Science and Technology
- 33. COMPARISON OF HT METHODS ME101: Materials Science and Technology
- 34. SCLEROMETER: • Used to measure scratch hardness • Handheld instrument • Consists of a tungsten carbide tip with spring inside • Tip touched to material and spring pressed • Load increased by pressing spring until surface is scratched • Hardness measured by gauge attached TOOLS OF HARDNESS
- 35. SCLEROSCOPE: • Used to measure rebound hardness • A diamond tipped hammer is dropped from a height • Rebound height gives a measure of hardness
- 36. BRINELL HARDNESS TESTER: • Used to measure indentation hardness • A steel ball of 10mm diameter is used as indenter • Load is applied for 10-15 seconds • Formula: 𝐵𝐻𝑁 = 𝐹 𝜋 2 𝐷 𝐷 − 𝐷2 − 𝐷𝑖 2 • Gives a range of values rather than one value
- 37. VICKERS HARDNESS TESTER: • Used to measure micro-indentation hardness • Indenter is a diamond pyramid with 136o apex angle • Used to measure micro indentation • Small loads applied • Formula: 𝐻𝑉 = 1.85𝐹 𝐷2
- 38. OTHER TOOLS:
- 40. MOHS SCALE: • Developed by Friedrich Mohs, a German geologist in the 19th century. • The Mohs scale of mineral hardness is based on the ability of one natural sample of mineral to scratch another mineral visibly.
- 41. THE ROCKWELL SCALE IS A HARDNESS SCALE BASED ON INDENTATION HARDNESS OF A MATERIAL. ROCKWELL SCALE:(MACROINDENTATION)
- 43. VICKERS SCALE:
- 44. BRINELL SCALE:
- 46. • Material hardening is required for many applications: • Construction materials - High strength reduces the need for material thickness which generally saves weight and cost. • Machine cutting tools (drill bits, taps, lathe tools) need be much harder than the material they are operating on in order to be effective. • Knife blades – a high hardness blade keeps a sharp edge.
- 47. • Bearings – necessary to have a very hard surface that will withstand continued stresses. • Armor plating - High strength is extremely important both for bullet proof plates and for heavy duty containers for mining and construction. • Anti-fatigue - (Martensitic) case hardening can drastically improve the service life of mechanical components with repeated loading/unloading, such as axles and cogs.