재료과학 II 재료공학과공학 5 판. 저자 : Donald R. Askeland and Pradeep P. Phule. [International Student Edition] [ 강충길외공역 / 사이텍미디어 ] Thomson

Transcription

1 [International Student Edition] 재료공학과공학 5 판 THE SCIENCE AND ENGINEERING OF MATERIALS [4 th Ed.] 재료과학 II 저자 : Donald R. Askeland and Pradeep P. Phule [ 강충길외공역 / 사이텍미디어 ] Thomson

2 [ 무기재료 금속재료 철재료 ] Objectives of Chapter 12 Understand how the ideas of solid solution strengthening, strain hardening, and dispersion strengthening are applied to the ferrous alloys Discuss how to use the eutectoid reaction to control the structure and properties of steels through heat treatment and alloying. Examine two special classes of ferrous alloys: stainless steels and cast irons. Key words: 철합금, 공석반응, 철강, 열처리, 스테인리스, 주철, 구조, 특성 철강에포함된 5 대원소 : 탄소, 규소, 망간, 황, 인 [Fe + C, Si, Mn, P, S] ** 철재료 (ferrous alloy): Fe 가 major 를이루는금속재료

3 [ 다음을읽고, 본문에들어가자!!] 이유 : 호기심 ( 의문점 ) 유발

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7 Figure 12.1 ( 고로 ) ( 전기로 ) ( 전로 ) (a) In a blast furnace, iron ore is reduced using coke (carbon) and air to produce liquid pig iron. The highcarbon content in the pig iron is reduce by introducing oxygen into the basic oxygen furnace to produce liquid steel. An electric arc furnace can be used to produce liquid steel by melting scrap. (b) Schematic of a blast furnace operation. ** 질문 : 제선 (Pig Iron) vs. 제강차이는? 강의생산과정 : (a) 용선제조후또는고철을용융하여용강제조, (b) 고로작업도 **Pig Iron 의 5 대불순물 : C(~4wt.%), Si, Mn, P, S

8 Section 12.1 Designations and Classification of Steels Designations - The AISI (American Iron and Steel Institute) and SAE (Society of Automotive Engineers) provide designation systems for steels that use a four- or five-digit number. [Table 12-1 연습 ] Classifications - Steels can be classified based on their composition or the way they have been processed. ** 질문 : Composition vs. component 차이는? 강의명칭과구분 : AISI-SAE 4 or 5 digit number

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10 Example 12.1 Design of a Method to Determine AISI Number An unalloyed steel tool used for machining aluminum automobile wheels has been found to work well, but the purchase records have been lost and you do not know the steel s composition. The microstructure of the steel is tempered martensite, and assume that you cannot estimate the composition of the steel from the structure. Design a treatment that may help determine the steel s carbon content. 2 가지방법소개함.

11 Example 12.1 SOLUTION: The first way is to heat the steel to a temperature just below the A 1 temperature and hold for a long time. The steel overtempers and large Fe 3 C spheres form in a ferrite matrix. We then estimate the amount of ferrite and cementite and calculate the carbon content using the lever law. If we measure 16% Fe 3 C using this method, the carbon content is: ( x ) % Fe3 C = 100 = 16 or = 1.086% ( ) x A better approach, however, is to heat the steel above the A cm to produce all austenite. If the steel then cools slowly, it transforms to pearlite and a primary microconstituent. If, when we do this, we estimate that the structure contains 95% pearlite and 5% primary Fe 3 C, then: x % Pearlite = 100 = 95 or = x 1.065%

12 [ 신기한현상 : 부피변화 ] [ 참고 ] Figure 12.2 (a) The eutectoid portion of the Fe-Fe 3 C phase diagram. (b) An expanded version of the Fe-C diagram. 중요!!! 12 장의핵심!!!

13 ** 저온에서밀도가낮은상 [BCC] 이안정!!! [74%] [A2 변태 ] [68%]

14 1. Carbon steels: ~ 2% C ( Si <0.6%, Cu <0.6%, Mn < 1.65%) Decarburized steels: < 0.005%C Ultra-low carbon steels: < 0.03%C Low-carbon steels: < 0.04~0.15%C [Car bodies 등 ] Mild steels: 0.15~0.3% C [Building, piping 등 ] / 연강 Medium-carbon steels: 0.3~0.6%C [Machinery 등... ] / 중탄소강 High-carbon steels: >0.6%C [Railroad car wheel 등... ] 2. Cast irons: Fe-C alloys 2~4%C 3. Alloy steels: more significant levels of alloying elements (Si >0.6%, Cu >0.6%, Mn > 1.65%) hardenability!!! Total alloying elements < 5%, and Total carbon < 1%. 4. Specialty Steel: tool steel, IF steel, DP steel, TRIP steel,..

15 Ferrite + Cementite Ferrite + Cementite (more rounded than in pearlite) Ferrite + Cementite (very fine & nearly round) Figure 12.3 Electron micrographs of (a) pearlite, (b) bainite, and (c) tempered martensite, illustrating the differences in cementite size and shape among three microconstituents ( 7500).

16 Section 12.2 Simple Heat Treatments Process Annealing Eliminating Cold Work: A lowtemperature heat treatment used to eliminate all or part of the effect of cold working in steels. Annealing and Normalizing Dispersion Strengthening: Annealing - A heat treatment used to produce a soft, coarse pearlite in steel by austenitizing, then furnace cooling. Normalizing - A simple heat treatment obtained by austenitizing and air cooling to produce a fine pearlitic structure. Spheroidizing Improving Machinability: Spheroidite - A microconstituent containing coarse spheroidal cementite particles in a matrix of ferrite, permitting excellent machining characteristics in high-carbon steels.

17 Prevents the formation of brittle, continuous film of Fe 3 C at the GB Figure 12.4 Schematic summary of the simple heat treatments for (a) hypoeutectoid steels and (b) hypereutectoid steels.

18 Figure 12.5 The effect of carbon and heat treatment on the properties of plain-carbon steels.

19 Figure 12.6 The microstructure of spheroidite, with Fe 3 C particles dispersed in a ferrite matrix ( 850).

20 Example 12.2 Determination of Heat Treating Temperatures Recommend temperatures for the process annealing, annealing, normalizing, and spheroidizing of 1020, 1077, and steels.

21 Figure 12.2 (a) The eutectoid portion of the Fe-Fe 3 C phase diagram. (b) An expanded version of the Fe-C diagram, adapted from several sources.

22 Example 12.2 SOLUTION From Figure 12.2, we find the critical A1, A 3, or A cm, temperatures for each steel. We can then specify the heat treatment based on these temperatures. ** 질문 : quenching, annealing, normalizing, tempering 차이는?

23 Section 12.3 Isothermal Heat Treatments Austempering - The isothermal heat treatment by which austenite transforms to bainite. Isothermal annealing - Heat treatment of a steel by austenitizing, cooling rapidly to a temperature between the A 1 and the nose of the TTT curve, and holding until the austenite transforms to pearlite.

24 More uniform property of pearlite than that by annealing and normalizing Figure 12.7 The austempering and isothermal anneal heat treatments in a 1080 steel.

25 Figure 12.8 The TTT diagrams for (a) 1050 and (b) steel.

26 Ferrite + Austenite + Pearlite Figure Austenite Producing complicated structures by interrupting the isothermal heat treatment of a 1050 steel. ** 질문 : TTT 와 CCT curve 차이

27 Figure Dark feathers of bainite surrounded by light martensite, obtained by interrupting the isothermal transformation process ( 1,500).

28 Section 12.4 [ 조질처리 : QT] Quench and Temper Heat Treatments Retained austenite - Austenite that is unable to transform into martensite during quenching because of the volume expansion associated with the reaction. Tempered martensite - The microconstituent of ferrite and cementite formed when martensite is tempered. Quench cracks - Cracks that form at the surface of a steel during quenching due to tensile residual stresses that are produced because of the volume change that accompanies the austenite-to-martensite transformation. Marquenching - Quenching austenite to a temperature just above the M S and holding until the temperature is equalized throughout the steel before further cooling to produce martensite. ** 열처리시, 변화할수있는 phase 는?

29 Figure The effect of tempering temperature on the mechanical properties of a 1050 steel.

30 Figure Retained austenite (white) trapped between martensite needles (black) ( 1000).

31 Figure Increasing carbon reduces the M s and M f temperatures in plaincarbon steels.

32 Figure Formation of quench cracks caused by residual stresses produced during quenching. The figure illustrates the development of stresses as the austenite transforms to martensite during cooling.

33 Figure The marquenching heat treatment designed to reduce residual stresses and quench cracking.

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35 CCT : Longer times are required for transformations to begin and no bainite region is observed. Figure The CCT diagram (solid lines) for a 1080 steel compared with the TTT diagram (dashed lines).

36 Figure The CCT diagram for a low-alloy, 0.2% C Steel.

37 Section 12.5 Effect of Alloying Elements Hardenability - Alloy steels have high hardenability - even cooling in air may produce all martensite. Effect on the Phase Stability - When alloying elements are added to steel, the binary Fe-Fe 3 C stability is affected and the phase diagram is altered. Shape of the TTT Diagram - Ausforming is a thermomechanical heat treatment in which austenite is plastically deformed below the A1 temperature, then permitted to transform to bainite or martensite. Tempering - Alloying elements reduce the rate of tempering compared with that of a plain-carbon steel. 핵심사항 : 경화능의정의와경화능에미치는합금과 C 의영향?

38 Figure (a) TTT and (b) CCT curves for a 4340 steel. All common alloying elements in steel shift the TTT and CCT diagrams to longer times, permitting us to obtain all martensite even in thick sections at slow cooling rates. ** 질문 : Alloying element 가 hardenability 에미치는영향에대해설명하시오.

39 Figure The effect of 6% manganese on the stability ranges of the phases in the eutectoid portion of the Fe-Fe 3 C phase diagram.

40 Figure When alloying elements introduce a bay region into the TTT diagram, the steel can be ausformed.

41 Alloy steels Figure The effect of alloying elements on the phases formed during the tempering of steels. The air-hardenable steel shows a secondary hardening peak.

42 Section 12.6 Application of Hardenability Jominy test - The test used to evaluate hardenability. An austenitized steel bar is quenched at one end only, thus producing a range of cooling rates along the bar. Hardenability curves - Graphs showing the effect of the cooling rate on the hardness of as-quenched steel. Jominy distance - The distance from the quenched end of a Jominy bar. The Jominy distance is related to the cooling rate. Jominy test 를정의하시오!

43 Figure The set-up for the Jominy test used for determining the hardenability of a steel.

44 Figure The hardenability curves for several steels. An alloy steel with a high hardenability (e.g., 4340) maintains a rather flat hardenability curve; A plain-carbon steel (e.g., 1050 & 1080) has a curve that drops off quickly.

45 [2 inch length]

46 Figure The Grossman chart used to determine the hardenability at the center of a steel bar for different quenchants.

47 Section 12.7 Specialty Steels [ 특수강 ] / 용어의정의!! Tool steels - A group of high-carbon steels that provide combinations of high hardness, toughness, or resistance to elevated temperatures. Secondary hardening peak - Unusually high hardness in a steel tempered at a high temperature caused by the precipitation of alloy carbides. Dual-phase steels - Special steels treated to produce martensite dispersed in a ferrite matrix. Maraging steels - A special class of alloy steels that obtain high strengths by a combination of the martensitic and age-hardening reactions.

48 TRIP (transformation induced plasticity) steels - better ductility and formability. It contains of a continuous ferrite matrix and a dispersion of a harder second phase (martensite/bainite). In addition, the microstructure consists of retained austenite, which transforms to martensite during plasitc defomation. IF (interstitial-free) steels containing Nb and Ti to react with C and S to form precipitates of carbides and sulfides. Thus, no carbon remains in the ferrite. Very formable and therefore attractive for the automobile industry.

49 Figure Microstructure of a dual-phase steel, showing islands of light martensite in a ferrite matrix ( 2500).

50 Section 12.8 Surface Treatments [ 표면처리방법과특징연구 ] Selectively Heating the Surface - Rapidly heat the surface of a medium-carbon steel above the A 3 temperature and then quench the steel. Case depth - The depth below the surface of a steel at which hardening occurs by surface hardening and carburizing processes. Carburizing - A group of surface-hardening techniques by which carbon diffuses into steel. Cyaniding - Hardening the surface of steel with carbon and nitrogen obtained from a bath of liquid cyanide solution. Carbonitriding - Hardening the surface of steel with carbon and nitrogen obtained from a special gas atmosphere (carbon monoxide and ammonia).

51 ** Case depth 를정의하시오. Figure (a) Surface hardening by localized heating. (b) Only the surface heats above the A 1 temperature and is quenched to martensite.

52 Figure Carburizing of a low-carbon steel to produce a highcarbon, wear-resistant surface. The surface of the steel is normally above the A3 temperature.

53 Example 12.7 Design of Surface-Hardening Treatments for a Drive Train Design the materials and heat treatments for an automobile axle and drive gear (Figure 12.28). Good fatigue / wear resistance and inexpensive A 1010 steel to avoid wear : Ferrite matrix gas carburizing A 1050 steel for more severe loading conditions : Ferrite + pearlite matrix surface hardening Figure Sketch of axle and gear assembly (for example 12.7).

54 Section 12.9 Weldability of Steel Figure The development of the heat-affected zone in a weld: (a) the structure at the maximum temperature, (b) the structure after cooling in a steel of low hardenability, and (c) the structure after cooling in a steel of high hardenability. Preheating the material Minimizing H incorporation ** HAZ 를정의하시오.

55 Section Stainless Steels Stainless steels - A group of ferrous alloys that contain at least 11% Cr, providing extraordinary corrosion resistance. Categories of stainless steels: 5가지 Ferritic Stainless Steels Martensitic Stainless Steels Austenitic Stainless Steels Precipitation-Hardening (PH) Stainless Steels Duplex Stainless Steels ** Stainless steel [STS] 에서자성체와비자성체를구분하시오.

56 Figure (a) The effect of 17% chromium on the ironcarbon phase diagram. At low-carbon contents, ferrite is stable at all temperatures. (b) A section of the ironchromium-nickel-carbon phase diagram at a constant 18% Cr-8% Ni. At low-carbon contents, austenite is stable at room temperatures.

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58 Figure (a) Martensitic stainless steel containing large primary carbides and small carbides formed during tempering ( 350). (b) Austenitic stainless steel ( 500).

59 Section Cast Irons [ 주철 ] Cast iron - Ferrous alloys containing sufficient carbon so that the eutectic reaction occurs during solidification. [ 대략, C 함량 2.1 wt.% 가경계 ] Eutectic and Eutectoid reaction in Cast Irons Types of cast irons: 5가지 Gray cast iron[ 회주철 ]: L γ + Graphite[ 박편, 판상 ], 낮은강도 / 연성가짐 White cast iron[ 백주철 ]: L γ + Fe 3 C, 단단하고깨지기쉽다. Malleable cast iron[ 가단주철 ]: 백주철의장시간열처리로 Fe 3 C를둥근 Graphite화 Ductile or nodular, cast iron[ 연 / 구상흑연주철 ]: 응고중 Mg첨가로스페로이다이징 Compacted graphite cast iron[ 콤팩트흑연주철 ]: 흑연은편상과구상의중간형태 구상이고서로연결된흑연형상, Vermicular graphite라고부르기도함.

60 흑연덩어리 응고시박편흑연 회주철 / 일반화됨 응고시세멘타이트 백주철 열처리로둥근흑연 가단주철 강도 / 연성 / 인성 Mg( 구상화 ) 강도와연성 응고시구상흑연 연주철 응고시괴상흑연 콤팩트흑연주철 Mg, Ti 연결된산호모양 Figure Schematic drawings of the five types of cast iron: (a) gray iron, (b) white iron, (c) malleable iron, (d) ductile iron, and (e) compacted graphite iron.

61 Graphite 100 Figure The iron-carbon phase diagram showing the relationship between the stable iron-graphite equilibria (solid lines) and the metastable iron-cementite reactions (dashed lines).

62 노냉 수냉 공냉 Figure The transformation diagram for austenite in a cast iron.

63 스케치 현미경사진 Figure (a) Sketch and (b) photomicrograph of the flake graphite in gray cast iron (x 100). / 회주철에서흑연박편.

64 Figure The effect of the cooling rate or casting size on the tensile properties of two gray cast irons. ** Class 20: 20,000 psi 인장강도

65 [** 백주철의열처리로가단주철제조 ] Martensite +Graphite Figure The heat treatments for ferritic and pearlitic malleable irons.

66 Figure (a) White cast iron prior to heat treatment ( 100). (b) Ferritic malleable iron with graphite nodules and small MnS inclusions in a ferrite matrix ( 200). (c) Pearlitic malleable iron drawn to produce a tempered martensite matrix ( 500). (d) Annealed ductile iron with a ferrite matrix ( 250). (e) As-cast ductile iron with a matrix of ferrite (white) and pearlite ( 250). (f) Normalized ductile iron with a pearlite matrix ( 250).

67 From white CI

68 단원정리를위한숙제 1. 재료 (materials) 는유기재료와무기재료로구분할수있다. 구분기준을말하고, 각각에대해서구체적으로세분하여구분하고각각에대한예를 1 개이상들어보세오. 2. Fe-C diagram 에서 Fe-Fe 3 C 준평형상태도를상세하게도시하고설명하시오. 3. AISI-SAE steel 의표시방법에대하여예를들어설명하시오. 4. Carbon steel, Cast iron, Alloy steel 을구분설명하시오. 5. TTT 와 CCT curve 를정의하고사용용도를설명하시오. 6. Hardenability 를정의하고, 첨가원소의영향에대해서논하시오. 7. 표면처리 (surface treatment) 의종류와원리에대해서설명하시오. 8. Cast iron 과 stainless steel 를정의하고특징에대해서간단히논하시오. 9. Ferrous alloys 를정의하고중요성에대해논하시오. [ 제 4th 영문판과 5 판한글판의연습문제번호대조표 ]: 12-2(13-1), 12-7(13-6), 12-15(13-14), 12-22(quench cracking 의발생원인은?), 12-25(13-21), 12-38(13-32), 12-44(13-38)