Giáo trình Thí nghiệm Vật liệu học: Phần 2

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Phần 2 cuốn giáo trình Thí nghiệm Vật liệu học cung cấp cho người đọc các kiến thức: Microstructure analysis of graphite - cast iron, hardness testing and quenching steel process, tempering quenched – steel. Mời các bạn cùng tham khảo nội dung chi tiết.

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Lesson 3 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 3 - 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: 3 samples - Malleable iron: 3 samples - Gray iron: 3 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 4 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 steel at different temperatures and cooling environments. 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 1. 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 2 (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: D( D  D 2  d 2 ) S= 2 P  0,1 P  0,1 HB   (N/mm2) Dh D (D  D 2  d 2 ) 2 83
For steel and cast iron usually use P = 30000 N, D = 10mm. To determine HB hardness, we need to measure the diameter of the dent and use the above formula to calculate (but can use the sheet available in the table - Table 4.3) The requirements for conducting Brinell hardness testing: - The sample thickness is not less than 10 times the depth of the dent. - The surface of the test specimen must be clean, flat, and free of scratches. - Width, length of the sample and distance between 2 dents must be greater than 2D. - The duration of applying load will affect the measurement results. Normally this time can be checked according to table 4.1. Usage range: Brinell's method can measures soft and hard materials (from 16 HB to 627 HB) such as non-ferrous metals (copper, nickel,…), non-ferrous alloys, steel, graphite cast iron. Brinell's method cannot measure thin materials (δ < 10h). Table 4.1. The requirements for conducting Brinell hardness testing Materials Hardness Minimum P/D2 D P(N) Duration HB thickness (mm) of effect (mm) (s) >6 300 10 30000 10 140 - 450 3–6 300 5 7500 10 Ferrous 6 300 10 30000 30 < 140 3–6 300 5 7500 30 6 100 10 10000 30 Copper 31,8 – 3–6 100 5 2500 30 alloys 130 6 25 10 2500 60 Aluminum 8 - 35 3–6 25 5 625 60 alloy
2. Rockwell hardness measurement method (HRA; HRB; HRC) The Rockwell test method, as defined in ISO 6508 and ASTM E- 18, is the most commonly used method to determine a material's hardness and is suitable for almost all metals and to some degree for plastics. It measures the permanent depth of indentation produced by a force/load on an indenter. The Rockwell test is generally easier to perform, and quicker than other types of hardness testing methods. The main advantage of Rockwell hardness testing is its speed of testing and its ability to display hardness values directly after penetrating the material. Test principles: - Rockwell hardness number based on an inversed relationship to the measurement of the additional depth, in which an indenter is forced by a heavy (major) load beyond the depth resulting from a previously applied (minor) load. - This method has two types of indenter: diamond cone (Figure 4.2 b), have an angle of the tip is 120˚, or hardened steel/tungsten carbide ball, have diameter d =1/16” = 1,588 mm. - The regulation of this method: Indenter moves down 0,002 mm, the hardness will decrease by 1 unit. - The distance between 2 dents or between the dent and the edge must not be less than 1,5 mm for diamond indenter, or 4 mm for ball indenter. Figure 4.5. Measurement Rockwell hardness test method - Rockwell hardness measurements are determined by: h HR  K  0,002 Where: K is constant with each nose stabbed h is the depth of dent (mm) 0,002mm is the value of 01 line of the dial 85
Dial Indenter Knob for Anvil changing loads Rotary wheel Bar Figure 4.6. Rockwell hardness tester Table 4.2． Scales of the Rockwell Hardness Method Weight Measure Scale Indenter K Application ment P (N) limit Very hard materials Diamond – cone HRA 600 100 (alloy steel, hard 70÷85 α = 120˚ alloy, WC, TiC, ...) Diamond – cone Hard materials HRC 1500 100 (chilled steel, 20÷67 α = 120˚ martensite) Balls Soft materials, thick HRB d =1/16”= 1,588 1000 130 materials, thin 25÷100 mm. materials 3. 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˚. HV= x0,1= x0,1= 1,854x x0,1 (N/mm2) Table 4.3. Conversion table for metals with the relationship of hardness number Brinell (DBrinell =10mm) - Rockwell - Vickers Brinell Rockwell Vickers Brinell Rockwell Vickers dmm HB HRB HRC HRA HV dmm HB HRB HRC HRA HV 2.00 946 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 1 minute / 1 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 1. 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. 2. 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 1 mm thickness temperatures 600°C 2 3 4 700°C 1.5 2.2 3 800°C 1.0 1.5 2 900°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 do 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 Average (before Average (after quenching) quenching) 1 Alloy 2 C45 3 C45 4 C45 5 C45 6 C45 4.3.4. Quenching sample steel Table 4.6. Temperature and quenching media Sample Steels Temperature Cooling Cooling rate Holding (˚C) environment (oC/s) time 1 Alloy 780 Oi 150 2 C45 730 Water 600 3 C45 780 Water 600 ~1 minute/ 4 C45 830 Water 600 1 mm 5 C45 830 Oil 150 6 C45 830 Still air 30 93