Giáo trình thí nghiệm vật liệu học phần 2

Lesson MICROSTRUCTURE ANALYSIS OF GRAPHITE - CAST IRON 3.1 PURPOSE - Able to observe the microstructure of graphite-cast iron by using a metallurgical microscope - Able to classify graphite-cast irons through the microstructure 3.2 THEORETICAL BACKGROUND 3.2.1 Definition Cast iron is generally defined as an alloy of Iron with 2.14% to 6.67% carbon, more commonly - 5% C Cast iron has low melting temperature, brittle, and so easiest to cast Graphite is an allotropic form of carbon that occurs as a mineral in some rocks and can be made from coke Graphite has simple layered hexagonal type a = 2.5Å; c/a = 2.74 The covalent bond strength in each layer is very strong, however, the force between layers is very weak, because the distance between layers is too far so graphite is very soft and easy to separate (Figure 2.6) Graphite cast iron is carbon-iron alloys in which a part or all of its carbon exists under the free state, due to the concentration of carbon atoms 3.2.2 Graphite forming conditions Through the equilibrium diagram for iron and carbon, only steel and white cast iron exist without any presence of graphite The formation of graphite depends on many factors such as temperature, cooling rate, chemical composition Graphite is an allotropic form of carbon that occurs as a mineral in some rocks and can be made from coke Graphite is more stable than cementite Cementite is a metastable phase; Graphite formation is promoted by being added Si > 1% and slowly cooling with the form of flakes Graphite Cementite decomposes to ferrite and graphite: Fe3C → Fe (α) + C (graphite) 74 The addition of Si in the alloy Fe and carbon will inhibit the formation of Fe3C Carbon will tend to react with Si if the amount is sufficient Therefore, the addition of Si is only 1-4% which aims to decompose Fe3C to promote graphite formation The slower cooling rate leads to a higher graphitization rate rather than s rapid cooling rate If the graphitization process takes place, it will form graphite in cast iron 3.2.4 Classification in graphite cast iron Basing on the shape of graphite, it is classified into three groups: - Grey cast iron: laminate graphite (Figure 3.1) - Malleable iron: graphite has the shape of dark rosettes (Figure 3.2) - Nodular iron: spheroidal graphite iron is cast iron in which the graphite is present as tiny balls or spheroids (Figure 3.3) Ferrite matrix Graphite Figure 3.1 Laminate graphite (gray cast iron- ferrite matrix) (4% Nital) Graphite Figure 3.2 Dark rosettes (malleable cast iron - ferrite matrix) (4% Nital) 75 Graphite Figure 3.3 Spherical form (Nodular cast iron - ferrite matrix) (4% Nital) 3.2.5 Gray cast iron It is a kind of graphite cast iron in which graphite has a plate shape Gray iron has low hardness and is brittle due to the flake-like graphite, weak and brittle under tension, stronger under compression, excellent vibrational damping capacity, good wear resistance Depending on the level of graphitization, there are three types of gray iron: - Ferritic gray cast iron: The microstructure is the graphite plate on the ferrite matrix (Figure 3.1) - Ferrite + pearlite gray cast iron: The microstructure is the graphite plate on the ferrite and pearlite matrix (Figure 3.4) - Pearlitic gray cast iron: The microstructure is the graphite plate on the pearlite matrix (Figure 3.5) Figure 3.4 Ferrite + pearlite gray cast iron (4% Nital) 76 Figure 3.5 Pearlitic gray cast iron (4% Nital) 3.2.6 Malleable cast iron A cast iron which has graphite cluster shape is formed by heating white cast iron at high temperature ± 800°C for a prolonged time, ± 30 hours Cementite will be decomposed to graphite precipitates, clusters or rosettes Malleable iron has higher durability than grey cast iron and more ductile than white cast iron The heating process of white cast iron to become malleable cast iron: Figure 3.6 Process of heating white cast iron → Malleable cast iron Malleable iron has higher durability than grey cast iron Basing on the level of graphitization by time, there are three groups: 77 - Ferritic malleable cast iron (Figure 3.2) - Ferrite + pearlite malleable cast iron (Figure 3.7) - Pearlitic malleable cast iron (Figure 3.8) Graphite Figure 3.7 Ferrite + pearlite malleable cast iron (4% Nital) Pearlite Graphite Figure 3.8 Pearlite malleable cast iron (4% Nital) 3.2.7 Nodular cast iron Nodular cast iron which has nodular graphite shape is formed by converting flakes graphite to nodular graphite through the addition of a small amount of Mg or Ce Spheroidal (nodules) graphite will be precipitates rather than flakes The ductility increased by a factor of 20, strength is doubled, approaching mechanical properties of steel Depending on the graphitization, there are three types: - Ferritic nodular cast iron (Figure 3.3) - Ferrite + pearlite nodular cast iron (Figure 3.9) 78 - Pearlitic nodular cast iron (Figure 3.10) Graphite Figure 3.9 Ferrite + pearlite nodular cast iron (4% Nital) Graphite Figure 3.10 Pearlite nodular cast iron (4% Nital) 3.3 EXPERIMENTAL CONTENT Observe samples by using a microscope and collect the results - Classify the types of cast iron - Determine the percentage of phases - Redraw the microstructure of each sample Types of samples: - Nodular iron: samples - Malleable iron: samples - Gray iron: samples 79 3.4 REPORT - Summary of theoretical contents - Draw/take picture of the microstructure of the samples and write as follows: Redraw the picture 80 Sample number: Sample name: Magnification: 80 Lesson HARDNESS TESTING AND QUENCHING STEEL PROCESS 4.1 PURPOSE - Measuring the hardness of a metal by the Rockwell method - Researching on the steel quenching process - Quenching environments steel at different temperatures and cooling 4.2 THEORETICAL BACKGROUND 4.2.1 Hardness 4.2.1.1 Definition Hardness can be defined as resistance to local plastic deformation of the materials, with applying the compression load through indenter onto the surface of the materials There are kinds of indenter shapes namely: steel ball, diamond cone and diamond pyramid, Figure 4.1 Basically, there are three methods of hardness testing, as follows: - Scratch - Indentation - Rebound Figure 4.1 Measurement of Brinell hardness testing method The most common hardness testing is the indentation method, Figure 4.2, which has three kinds of hardness testing methods 81 a) Ball b) Cone c) Pyramid Figure 4.2 Indenter shapes 4.2.1.2 Hardness testing methods Brinell hardness testing method (HB) The Brinell test method, as defined in ASTM E10 and ISO 6506, is mostly used for heavy casted materials such as cast iron, aluminum or steel The compression load is being applied through a steel ball indenter onto the surface of the material; after a few seconds, the load is removed and the dent will appear in the form of a certain inner circle with a certain depth, Figure 4.3 The indenter is a hardened steel ball (HBS) or a tungsten carbide ball (HBW) with the diameter: D = 2.5; 5; 10 (mm) The corresponding weight is P = 1875; 7500; 30000 (N); P can be measured in kilograms force (Kgf) The relationship between P and D: P/D2 = 300 Indenter Material δ Figure 4.3 Indenter and sample 82 Indenter Anvil Hanger with load Hand wheel Figure 4.4 Brinell hardness tester Measurement methods: Applying the compression load through the steel ball onto the surface of the sample for 10 to 30 seconds Remove the load from the surface of the sample and dent will appear on the surface of the sample Brinell measurements are calculated by the formula: HB = x 0,1 (N/mm2) In which: P is function load (N), S is the surface area of the dent (mm ) If D is the diameter of the ball, the depth of dent is h We have: S = Dh However, measuring the diameter d of the dent is much easier than measuring the depth h, so acreage of the dent can be calculated by the formula: S= HB  D( D  D  d ) P  0,1 P  0,1 (N/mm2)   D Dh (D  D  d ) 83 Dial Indenter Anvil Knob for changing loads Rotary wheel Bar Figure 4.6 Rockwell hardness tester Table 4.2． Scales of the Rockwell Hardness Method Scale Indenter Weight P (N) K Application Measure ment limit HRA Diamond – cone α = 120˚ 600 100 Very hard materials (alloy steel, hard alloy, WC, TiC, ) 70÷85 HRC Diamond – cone α = 120˚ 1500 100 Hard materials (chilled steel, martensite) 20÷67 HRB Balls d =1/16”= 1,588 mm 1000 130 Soft materials, thick materials, thin materials 25÷100 Vickers hardness The Vickers test method, as defined in E 384 for microhardness materials Vickers hardness number, HV, a number related to the applied load and the surface area of a permanent impression made by square based pyramid diamond indenter Vickers’ method of measuring has the same principle as the Brinell method but replacing the steel ball with the pyramid diamond tip, Figure 86 4.1c, the angle between the two sides is 136˚ Usable weight P = (50÷ 1500) N, depends on sample thickness The Vickers hardness is determined by: HV = x0,1 In which: P is the weight (N), S is the surface area of the indentation (mm2) Figure 4.7 Vickers testing method For convenience, we can count S through diagonal d and α = 136˚ x0,1= 1,854x x0,1 (N/mm2) HV= x0,1= Table 4.3 Conversion table for metals with the relationship of hardness number Brinell (DBrinell =10mm) - Rockwell - Vickers Brinell dmm HB 2.00 Rockwell HRB HRC Vickers HRA HV Brinell Rockwell Vickers dmm HB HRB HRC HRA HV 946 3.70 269 - 28 65 272 2.05 898 3.75 262 - 27 64 261 2.10 857 3.80 255 - 26 64 255 2.15 817 3.85 248 - 25 63 250 2.20 782 - 72 89 1220 3.90 241 100 24 63 240 2.25 744 - 69 87 1114 3.95 235 99 23 62 235 2.30 712 - 67 85 1021 4.00 229 98 22 62 226 2.35 683 - 65 84 940 4.05 223 97 21 61 221 2.40 652 - 63 83 867 4.10 217 97 20 61 217 87 Brinell Rockwell Vickers Brinell Rockwell Vickers dmm HB HRB HRC HRA HV dmm HB HRB HRC HRA HV 2.45 627 - 61 82 803 4.15 212 96 19 60 213 2.50 600 - 59 81 746 4.20 207 95 18 60 209 2.55 578 - 58 80 694 4.25 201 94 - 59 201 2.60 555 - 56 79 649 4.30 197 93 - 58 197 2.65 532 - 54 78 606 4.35 192 92 - 58 190 2.70 512 - 52 77 587 4.40 187 91 - 57 186 2.75 495 - 51 76 551 4.45 183 89 - 56 183 2.80 477 - 49 76 534 4.50 179 88 - 56 177 2.85 460 - 48 75 502 4.55 174 87 - 55 174 2.90 444 - 47 74 474 4.60 170 86 - 55 171 2.95 429 - 45 73 460 4.65 167 85 - 54 165 3.00 415 - 44 73 435 4.70 163 84 - 53 162 3.05 405 - 43 72 423 4.75 159 83 - 53 159 3.10 388 - 41 71 401 4.80 156 82 - 52 154 3.15 375 - 40 71 390 4.85 152 81 - 52 152 3.20 363 - 39 70 380 4.90 149 80 - 51 149 3.25 352 - 38 69 361 4.95 146 78 - 50 147 3.30 341 - 37 69 344 5.00 143 76 - 50 144 3.35 331 - 36 68 335 5.05 140 76 - - - 3.40 321 - 35 68 320 5.10 137 75 - - - 3.45 311 - 34 67 312 5.15 134 74 - - - 3.50 302 - 33 67 305 5.20 131 72 - - - 3.55 293 - 31 66 291 5.25 128 71 - - - 3.60 285 - 30 66 285 5.30 126 69 - - - 3.65 277 - 29 65 287 5.35 123 69 - - - Vickers method is often used to measure the hardness of thin objects, micro phases, and be able to measure very soft or hard materials 88 4.2.1.2 Advantages of hardness measurement methods - Hardness is the resistance to local plastic deformation and the strength is the resistance to plastic deformation It is possible to calculate the strength of the metal through hardness calculation, especially steel - Hardness measurement is relatively simple, less time consuming (less than minute / nose) - Be able to measure thick or thin parts - Be able to identify the working ability of the part 4.2.1.3 The hardness of common details - Hardness suitable for cutting: (160 ÷ 180) HB - Springs part, hot molds: (40 ÷ 45) HRC - Small, slow-speed gears (machine tools): (52 ÷ 58) HRC - All gears with high load, high speed; all cutting tools; cold stamping molds; rolling bearings; friction discs; and other similar parts need greater hardness (60 ÷ 62) HRC 4.2.2 Quenching 4.2.2.1 Definition Figure 4.8 Schematic diagram of the heat treatment process Quenching is a part of the heat treatment method by heating the specimen the temperature of γ state, holding until several time t, and cooling down with the cooling rate faster than the critical cooling rate by dipping into the quenching medium The schematic diagram for quenching process is depicted in Figure 4.8 89 The critical cooling rate is the minimum cooling rate that the Austenite will transform into 100% martensite, the microstructure of martensite is shown in Figure 4.9b Different steel grades have different critical cooling rate, depending on the chemical composition a) AISI 1045 steel (Annealed) b) AISI 1045 steel (after quenching) Figure 4.9 Microstructure of 1045 steel before and after quenching (4% Nital) 4.2.2.2 Some quenching media - Water: + Hot water (40÷60)°C + Normal water (25 ÷30)°C + Cold water (5÷ 15)°C - NaOH or NaCl solution - Oils - Gas/air (least severe) - Molten salt - Emulsion: oil + water - Liquid nitrogen Cooling rate: Vcooling = Vcritical+ (30 ÷ 50)0C/s 4.2.2.3 Temperature The temperature directly affects the mechanical properties of the steel after quenching With carbon steel, it can be based on the Fe-C diagram and %C to determine heating temperature 90 Dial Figure 4.10 Heat treatment furnace For hypo-eutectoid steel (%C ≤ 0.8%) Figure 4.11 Phase diagram of Fe-C Determine a temperature higher than AC3, which means heating the steel to a fully austenite state This method is called austenitization quenching t° = AC3 + (30 ÷ 50)0C 91 In the range of 0.1 ÷ 0.8%C line AC3 of steel reduce slowly Slow heating AC3 = A3 For hyper- eutectoid and eutectoid steel (0.8% < %C ≤ 2.14%) The austenitization temperature for hyper-eutectoid steel is higher than AC1, but lower than Accm, means to heat up to the state of intermediate phase austenite + cementite t° = AC1 + (30 ÷ 50)0C Slow heating AC1 = A1, about (760 ÷ 780°C)°C, it does not depend on carbon content We can identify AC3 and AC1 either by relying on the carbon-iron state diagram or by using a heat treatment handbook Table 4.4． Time and temperature quenching of specimens Shape Cylinder Square Plate Time (minutes) Heating For 1mm diameter For mm thickness temperatures 600°C 700°C 1.5 2.2 800°C 1.0 1.5 900°C 0.8 1.2 1.6 1000°C 0.4 0.6 0.8 Holding time τ: Depends on many factors: - Temperature range - Part size - Machine part shape - How to arrange the part Experience: calculate the minimum thickness in the largest section, or check the following table: 92 Note: Because these samples are small, they are cooled down fastly after opening the furnace Therefore, we must rapidly; the time between opening the door and pull the sample into water/oil must be less than three seconds 4.3 EXPERIMENTAL CONTENT 4.3.1 Receive samples Sample TCVN C45/AISI 1045/ alloy steel 4.3.2 Numbering A: Group numbers AB B: Sample numbers 4.3.3 Hardness testing of HRB/HB - Measure three times - Calculate the average hardness number and write on Table 4.5 Table 4.5． Hardness of samples Sample Steel Alloy C45 C45 C45 C45 C45 Average (before quenching) Average (after quenching) 4.3.4 Quenching sample steel Table 4.6 Temperature and quenching media Sample Steels Temperature Cooling (˚C) environment Cooling rate (oC/s) Alloy 780 Oi 150 C45 730 Water 600 C45 780 Water 600 C45 830 Water 600 C45 830 Oil 150 C45 830 Still air 30 Holding time ~1 minute/ mm 93 Note: Small samples must be pulled out rapidly and the time between opening the door and dropping samples into water/oil must be less than three seconds because these samples are small and are quickly cool down when opening the door 4.3.5 Hardness measurement after quenching Using Rockwell method with C or B scale, calculate the average hardness number and write on Table 4.5 4.4 REPORT Summarize the theoretical background Record the hardness of the steel after measuring Draw 02 diagrams the relationship between temperature and cooling rate compared to the hardness of the sample Conclusion 94 Lesson TEMPERING QUENCHED – STEEL 5.1 PURPOSE - Understand how to temper carbon steel, choose a suitable temperature and time - Determine the effect of tempering temperature on the hardness 5.2 THEORETICAL BACKGROUND a) Before quenching Martensite (dark) residual austenite (bright) b) After quenching Figure 5.1 Microstructure of high-carbon steel (4% Nital) 95 Tempering quenched-steel is a method of heat treatment of saturated martensite and residual austenite into more stable microstructure, in accordance with the requirements Tempering also reduces or eliminates the residual stress, as well as increased flexibility for the mechanical part When quenching-steel carbon, the conversion of martensite to ferrite and pearlite, saturated carbon is released from the network forming of carbide ε, then the carbide ε is gradually converted to Fe3C, and austenite residuals are transferred into tempered-Martensite For high-carbon and alloy steels after hardening, they could have a large amount of residual Austenite Depending on the size of the microstructure of the cementite and ferrite while tempering, we can have troostite or sorbite These process depends on the temperature and heating time Depending on temperature, can divides into: 5.2.1 Low tempering (150°C ÷ 250°C) The microstructure received is tempered martensite; the hardness is almost unchanged; the stress is slightly reduced; the hardness resistance and abrasion resistance are still high Applications: For products with high hardness or abrasion resistance such as metal cutting tools, cold dies, roller bearings, carburizing parts,… Figure 5.2 Microstructure of tempered martensite (4% Nital) 5.2.2 Medium tempering (300°C ÷ 450°C) Microstructure received is troosite The hardness is rather high; the stress decreases; the toughness increases; the elastic modulus reaches the maximum value 96 Applications: For mechanical parts that require high elasticity, high stiffness such as hot stamping molds, forging, springs 5.2.3 High tempering (500°C ÷ 650°C) Microstructure received was sorbite, which has the optimal impact strength The hardness decreases quite a lot, it is necessary to have surface-treatment to achieve high hardness in some necessary positions Applications: For parts that need durability, high yield strength, high impact strength such as axles, gears… Figure 5.3 Microstructure of troosite (4% Nital) Figure 5.4 Microstructure of sorbite (4% Nital) 5.3 EXPERIMENTAL CONTENT Determine the effect of tempering temperature and tempering time on the hardness Each group receives some AISI 1045 steel samples, quenching at 830°C/water, tempering Measure the average hardness 97 Table 5.1 Tempering carbon steel Temperature (°C) Time (min) 250 45 450 45 650 45 650 20 650 80 Hardness (HRC) 5.4 REPORT Summarize the theory Draw the diagram representing the dependence of hardness on temperature and time Comment on the results Determine the effect of temperature and time on hardness 98 TÀI LIỆU THAM KHẢO [1] Đặng Vũ Ngoạn –Thí nghiệm vật liệu học xử lý– NXB ĐHQG TP HCM, 2008 [2] V.M Zuyev, A Laboratory Manual for Trainess in Heat Treatment, Mir Publishers, 1985 [3] Lê Công Dưỡng (chủ biên) - Vật liệu học đại cương – NXB KHKT – Hà Nội, 2002 [4] Nghiêm Hùng - Kim loại học nhiệt luyện – NXB Giáo Dục - Hà Nội, 1993 [5] Trần Thế San – Giáo trình Vật liệu đại cương – NXB ĐHQG TP HCM, 2013 [6] Krauss, G., (1995): Steels: Heat Treatment and Processing Principles, ASM International, Ohio [7] ASM Handbook, (1990): Vol 4, Heat Treating, ASM International Committee [8] Thelning, K.E., (1981): Steel and Its Heat Treatment, Butterworth, London [9] Wilson, R., (1975): Metallurgy and Heat Treatment of Tool Steels, McGraw Hill, London 99 ... 63 25 0 2. 20 7 82 - 72 89 122 0 3.90 24 1 100 24 63 24 0 2. 25 744 - 69 87 1114 3.95 23 5 99 23 62 235 2. 30 7 12 - 67 85 1 021 4.00 22 9 98 22 62 226 2. 35 683 - 65 84 940 4.05 22 3 97 21 61 22 1 2. 40 6 52. .. HB 2. 00 Rockwell HRB HRC Vickers HRA HV Brinell Rockwell Vickers dmm HB HRB HRC HRA HV 946 3.70 26 9 - 28 65 27 2 2. 05 898 3.75 26 2 - 27 64 26 1 2. 10 857 3.80 25 5 - 26 64 25 5 2. 15 817 3.85 24 8 - 25 ... 95 18 60 20 9 2. 55 578 - 58 80 694 4 .25 20 1 94 - 59 20 1 2. 60 555 - 56 79 649 4.30 197 93 - 58 197 2. 65 5 32 - 54 78 606 4.35 1 92 92 - 58 190 2. 70 5 12 - 52 77 587 4.40 187 91 - 57 186 2. 75 495 -