: Donald R. Askelandcontents.kocw.net/KOCW/document/2014/Pusan/joyeongrae/2.pdf · 2016-09-09 · Pusan National University Department of Materials Science& Engineering 재료과학–

Pusan National University Department of Materials Science& Engineering

재료과학– II

저자: Donald R. Askeland and Pradeep P. Phule

[International Student Edition]

Thomson

재료공학과 공학 5판

THE SCIENCE AND ENGINEERING OF MATERIALS [4th Ed.]

[강충길 외 공역 / 사이텍미디어]


Chapter 12. Ferrous Alloys

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 철강에 포함된 5대 원소: 탄소, 규소 , 망간, 황, 인 [Fe + C, Si, Mn, P, S]

[ 무기재료 금속재료 철재료]

** 철재료(ferrous alloy): Fe가 major를 이루는 금속재료



[다음을 읽고, 본문에 들어가자!!]

이유: 호기심(의문점) 유발









Figure 12.1

(a) In a blast furnace, iron ore is reduced using coke (carbon) and air to produce liquid pig iron. The high-carbon 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.

강의 생산과정: (a) 용선 제조후 또는 고철을 용융하여 용강 제조, (b) 고로 작업도

(전로)

(전기로)

(고로)

**질문: 제선(Pig Iron) vs. 제강 차이는?

**Pig Iron의 5대 불순물: C(~4wt.%), Si, Mn, P, S

발표자

프레젠테이션 노트

Basic oxygen furnace (전로)



강의 명칭과 구분: AISI-SAE 4 or 5 digit number

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.

Section 12.1 Designations and Classification of Steels

**질문: Composition vs. component 차이는?





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. 2가지 방법 소개함.

Example 12.1 Design of a Method to Determine

AISI Number



Example 12.1 SOLUTION: The first way is to heat the steel to a temperature just below the A1 temperature and hold for a long time. The steel overtempers and large Fe3C 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% Fe3C using this method, the carbon content is:

%086.1or 16100)0218.067.6(

)0218.0(CFe % 3 ==×

−

−= xx

A better approach, however, is to heat the steel above the Acm 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 Fe3C, then:

%065.1or 9510077.067.6

-6.67Pearlite % ==×

−= xx



Figure 12.2 (a) The eutectoid portion of the Fe-Fe3C phase diagram. (b) An expanded version of the Fe-C diagram.

중요!!! 12장의 핵심!!!

[참고]

[신기한 현상: 부피변화]


[74%]

[68%] [A2변태]


** 저온에서 밀도가 낮은 상[BCC]이 안정!!!



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,..



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).

Ferrite + Cementite Ferrite + Cementite (more rounded than in pearlite)

Ferrite + Cementite (very fine & nearly round)



Process Annealing — Eliminating Cold Work: A low-temperature 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.

Section 12.2 Simple Heat Treatments



Figure 12.4 Schematic summary of the simple heat treatments for (a) hypoeutectoid steels and (b) hypereutectoid steels.

Prevents the formation of brittle, continuous film of Fe3C at the GB



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



Figure 12.6 The microstructure of spheroidite, with Fe3C particles dispersed in a ferrite matrix (× 850).



Recommend temperatures for the process annealing, annealing, normalizing, and spheroidizing of 1020, 1077, and 10120 steels.

Example 12.2 Determination of Heat Treating Temperatures



Figure 12.2 (a) The eutectoid portion of the Fe-Fe3C phase diagram. (b) An expanded version of the Fe-C diagram, adapted from several sources.



Example 12.2 SOLUTION

From Figure 12.2, we find the critical A1, A3, or Acm, temperatures for each steel. We can then specify the heat treatment based on these temperatures.

**질문: quenching, annealing, normalizing, tempering 차이는?



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 A1 and the nose of the TTT curve, and holding until the austenite transforms to pearlite.

Section 12.3 Isothermal Heat Treatments



Figure 12.7 The austempering and isothermal anneal heat treatments in a 1080 steel.

More uniform property of pearlite than that by annealing and normalizing



Figure 12.8 The TTT diagrams for (a) 1050 and (b) 10110 steel.



Figure 12.9 Producing complicated structures by interrupting the isothermal heat treatment of a 1050 steel.

**질문: TTT 와 CCT curve 차이

Ferrite + Austenite + Pearlite

+ Austenite



Figure 12.10

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



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 MS and holding until the temperature is equalized throughout the steel before further cooling to produce martensite.

Section 12.4 [조질처리: QT] Quench and Temper Heat Treatments

** 열처리시, 변화할수있는 phase 는?



Figure 12.11 The effect of tempering temperature on the mechanical properties of a 1050 steel.



Figure 12.12

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



Figure 12.13 Increasing carbon reduces the Ms and Mf temperatures in plain-carbon steels.



Figure 12.14 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.



Figure 12.15 The marquenching heat treatment designed to reduce residual stresses and quench cracking.



발표자

프레젠테이션 노트

brine〔brain〕 n. 소금물, 함수(salt water);【화학】 (식)염수;[the brine]



Figure 12.16 The CCT diagram (solid lines) for a 1080 steel compared with the TTT diagram (dashed lines).

CCT : Longer times are required for transformations to begin and no bainite region is observed.



Figure 12.17 The CCT diagram for a low-alloy, 0.2% C Steel.



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-Fe3C 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.

Section 12.5 Effect of Alloying Elements

핵심사항: 경화능의 정의와 경화능에 미치는 합금과 C의 영향 ?



Figure 12.18 (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 element가 hardenability에 미치는영향에 대해 설명하시오.



Figure 12.19 The effect of 6% manganese on the stability ranges of the phases in the eutectoid portion of the Fe-Fe3C phase diagram.



Figure 12.20 When alloying elements introduce a bay region into the TTT diagram, the steel can be ausformed.



Figure 12.21 The effect of alloying elements on the phases formed during the tempering of steels. The air-hardenable steel shows a secondary hardening peak.

Alloy steels



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.

Section 12.6 Application of Hardenability

Jominy test를 정의하시오!



Figure 12.22 The set-up for the Jominy test used for determining the hardenability of a steel.



Figure 12.23 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.



[2 inch length]



Figure 12.24 The Grossman chart used to determine the hardenability at the center of a steel bar for different quenchants.



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.

Section 12.7 Specialty Steels [특수강] / 용어의 정의!!



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.



Figure 12.25 Microstructure of a dual-phase steel, showing islands of light martensite in a ferrite matrix (× 2500).



Selectively Heating the Surface - Rapidly heat the surface of a medium-carbon steel above the A3 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).

Section 12.8 Surface Treatments [표면처리 방법과 특징 연구]



Figure 12.26 (a) Surface hardening by localized heating. (b) Only the surface heats above the A1 temperature and is quenched to martensite.

** Case depth 를 정의하시오.



Figure 12.27 Carburizing of a low-carbon steel to produce a high- carbon, wear-resistant surface. The surface of the steel is normally above the A3 temperature.



Design the materials and heat treatments for an automobile axle and drive gear (Figure 12.28).

Example 12.7

Design of Surface-Hardening Treatments for a Drive Train

Figure 12.28 Sketch of axle and gear assembly (for example 12.7).

A 1010 steel to avoid wear :

Ferrite matrix ⇒ gas carburizing

A 1050 steel for more severe

loading conditions :

Ferrite + pearlite matrix ⇒

surface hardening

⇒ Good fatigue / wear resistance and

inexpensive



Section 12.9 Weldability of Steel Figure 12.29 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 를 정의하시오.



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

Section 12.10 Stainless Steels

** Stainless steel [STS]에서 자성체와 비자성체를 구분 하시오.



Figure 12.30 (a) The effect of 17% chromium on the iron-carbon phase diagram. At low-carbon contents, ferrite is stable at all temperatures. (b) A section of the iron-chromium-nickel-carbon phase diagram at a constant 18% Cr-8% Ni. At low-carbon contents, austenite is stable at room temperatures.





Figure 12.31 (a) Martensitic stainless steel containing large primary carbides and small carbides formed during tempering (× 350). (b) Austenitic stainless steel (× 500).



Cast iron - Ferrous alloys containing sufficient carbon so that the eutectic reaction occurs during solidification. [대략, C함량 2.1 wt.% 가 경계]

Eutectic and Eutectoid reaction in Cast Irons

Types of cast irons: 5가지 • Gray cast iron[회주철]: L γ + Graphite[박편, 판상], 낮은 강도/연성 가짐 • White cast iron[백주철]: L γ + Fe3C, 단단하고 깨지기 쉽다.

• Malleable cast iron[가단주철]: 백주철의 장시간열처리로 Fe3C를 둥근 Graphite화 • Ductile or nodular, cast iron[연/구상흑연 주철]: 응고중 Mg첨가로 스페로이다이징 • Compacted graphite cast iron[콤팩트흑연주철]: 흑연은 편상과 구상의 중간형태 구상이고 서로 연결된 흑연 형상, Vermicular graphite라고 부르기도함.

Section 12.11. Cast Irons [주철]



Figure 12.32 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.

응고시 박편흑연 회주철/일반화됨

응고시 세멘타이트 백주철

열처리로 둥근흑연 가단주철

응고시 구상흑연 연주철

응고시 괴상흑연 콤팩트흑연주철

Mg, Ti 연결된 산호모양

Mg(구상화) 강도와 연성

강도/연성/인성

흑연 덩어리



Figure 12.33 The iron-carbon phase diagram showing the relationship between the stable iron-graphite equilibria (solid lines) and the metastable iron-cementite reactions (dashed lines).

Graphite

100



Figure 12.34 The transformation diagram for austenite in a cast iron.

노냉

공냉 수냉



Figure 12.35 (a) Sketch and (b) photomicrograph of the flake graphite in gray cast iron (x 100). / 회주철에서 흑연 박편.

스케치 현미경사진



Figure 12.36 The effect of the cooling rate or casting size on the tensile properties of two gray cast irons.

** Class 20: 20,000 psi 인장강도



Figure 12.37 The heat treatments for ferritic and pearlitic malleable irons .

Martensite +Graphite

[**백주철의 열처리로 가단주철 제조]



Figure 12.38 (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).



From white CI



단원정리를 위한 숙제

1. 재료(materials)는 유기재료와 무기재료로 구분할 수 있다. 구분 기준을 말하고, 각각에 대해서 구체적으로 세분하여 구분하고 각각에 대한 예를 1개 이상 들어 보세오. 2. Fe-C diagram 에서 Fe-Fe3C 준평형 상태도를 상세하게 도시하고 설명하시오.

3. AISI-SAE steel의 표시방법에 대하여 예를 들어 설명하시오. 4. Carbon steel, Cast iron, Alloy steel 을 구분 설명하시오. 5. TTT와 CCT curve를 정의하고 사용용도를 설명하시오. 6. Hardenability를 정의하고, 첨가 원소의 영향에 대해서 논하시오. 7. 표면처리(surface treatment)의 종류와 원리에 대해서 설명하시오. 8. Cast iron 과 stainless steel 를 정의하고 특징에 대해서 간단히 논하시오.

9. Ferrous alloys를 정의하고 중요성에 대해 논하시오.

[제 4th 영문판과 5판 한글판의 연습문제 번호 대조표]: 12-2(13-1), 12-7(13-6), 12-15(13-14), 12-22(quench cracking 의 발생원인은?), 12-25(13-21), 12-38(13-32), 12-44(13-38)

Documents

: Donald R. Askelandcontents.kocw.net/KOCW/document/2014/Pusan/joyeongrae/2.pdf · 2016-09-09 · Pusan National University Department of Materials Science& Engineering 재료과학–