Pertemuan Ke 9 (Sifat Mekanik Besi Karbon)

  MK TRANSFORMASI FASA

  Pertemuan ke 9

Nurun Nayiroh, M.Si

  Hardness Brinell, Rockwell Yield Strength Tensile Strength Ductility % Elongation Effect of Carbon Content

  Mechanical Properties: Influence of Carbon Content Pearlite (med) Pearlite (med) ferrite (soft) Cementite

  (hard) < 0.76 wt% C 0.76 wt% C Hypo eutectoid

  Hyper eutectoid Gambar 1. Pearlit, cementit, dan ferit

  Sementit bersifat lebih keras dan lebih rapuh dari perlit karena itu dengan menaikan fraksi Fe C pada baja sementara elemen

  3

  lain konstan maka material akan lebih keras dan lebih kuat, grafik sifat mekanik pearlit bisa dilihat pada Gambar 1. Ketebalan lapisan pearlit sementit juga mempengaruhi sifat mekanik sifat bahan. Pearlit halus lebih keras dan kuat dibandingkan dengan pearlit kasar. Hal ini dperlihatkan secara grafik pada Gambar 2. Alasan kenapa pearlit mempunyai sifat ini dikarenakan fase sementit lebih kuat dan kaku dibandingkan ferit yang lebih lunak sehingga bisa menahan deformasi dan dengan kata lain sementit sebagai penguat ferit. Pearlite halus akan lebih kuat di bandingkan dengan pearlit kasar karena butiran pearlit halus lebih banyak sehingga luas batas fase perunit volume akan lebih besar sehingga mempunyai kemampuan yang lebih besar menahan gerakan dislokasi.

  Gambar 2 Sifat Mekanik Pearlite

  Spheroitide mempunyai kekuatan dan kekerasan dibawah pearlit. Fenomena ini bisa diterangkan dengan metode penguatan oleh sementit dan hambatan gerakan dislokasi.

  Luas permukaan batas butir spherodit pesatuan volume lebih sedikit dari pearlit sehingga kekuatannya dan kekerasannya lebih rendah.

  Karena baja banite mempunyai struktur kristal yang lebih halus maka banite lebih kuat dan keras dari pearlit.

  Dari bagian bentuk struktur mikro paduan baja, martensit adalah yang paling kuat dan keras namun paling rapuh. Kekerasannya tergantung kandungan karbon. Pengaruh kandungan karbon terhadap kekerasan martensit bisa dilihat pada Gambar 3.

  Kekuatan dan kekerasan martensite tidak dikaitkan dengan struktur mikro tetapi lebih dikaitkan dengan efektifitas atom karbon yang larut dalam bentuk interstisi yang akan menghalangi gerakan dislokasi dan juga karena sistem slip yang lebih sedikit untuk kristal BTC.

  Gambar 3. Kekerasan sebagai fungsi konsentrasi karbon untuk baja martensit dan fine peralite

  Martensit bersifat keras sehingga sebagian besar tidak bisa dipakai aplikasi. Disamping itu tegangan internal karena proses quencning juga memberikan efek perlemahan. Ketangguhan dan keuletan martensit bisa ditingkatkan dan tegangan internal bisa dibuang dengan cara perlakuan panas yang disebut tempering. Tempering dilakukan dengan memanaskan baja martensit sampai temperatur di bawah eutectoid pada periode waktu tertentu. Biasanya tempering dilakukan pada temperatur antara 250 650 0C. Tegangan internal akan hilang pada suhu ± 200 C. Proses tempering akan membentuk “tempered maetensite”.

  martensite (bct, satu fase) → → → → tempered martensite (α + Fe3c)

  Foto struktur mikro tempered martensite sama dengan spheroidit hanya partikel sementit lebih banyak dan lebih kecil. Tempered martensit mempunyai sifat sekeras dan sekuat matensit namun ketangguhan dan keuletan lebih baik. Hubungan antara tegangan tarik, kekuatan luluh dan keuletan terhadap temperatur temper pada baja paduan bisa dilihat pada Gambar 4. Tempering reduces internal stresses caused by quenching. The small particles are cementite; the matrix is α%ferrite

  Gambar 4. Tensile dan yield strength dan ductility versus tempering temperature untuk paduan baja yang diquenching dg minyak

  Hardness versus tempering time for a water%quenched eutectoid plain carbon steel (1080) that has been rapidly quenched to form martensite.

  Rockwell C and Brinell Hardness Pada proses tempering beberapa baja bisa mengalami penurunan ketangguhan, hal ini disebut perapuhan temper. Fenomena ini terjadi bila baja ditemper pada suhu diatas 575 C dan diikuti pendinginan lambat sampai temperatur ruangan, atau jika tempering dilakukan pada o suhu antara 375 – 575 C. Perapuhaan ini disebabkan oleh kandungan elemen lain dalam jumlah yang cukup signifikan seperti mangan, nikel, crom dan phospor, arsen, timah putih. Perapuhan temper bisa dicegah dengan :

  1. Pengontrolan komposisi o o

  2. Tempering diatas 575 C atau dibawah 375 C diikuti dengan quenching pada temperatur ruang. Ketangguhan baja yang telah mengalami perapuhan bisa o diperbaiki dengan pemanasan sampai kira kira 600

  C, dan kemudian secara cepat didinginkan sampai temperatur o dibawah 300 C. Kekuatan dan kekerasan beberapa paduan logam dapat dikembangkan oleh pembentukan partikel kecil terdispersi secara ekstrim dan uniform ( precipitates ) pada fasa kedua dengan matrika fasa utama. Alloys that can be precipitation hardened or age hardened :

  Copper beryllium (Cu Be) Copper tin (Cu Sn) Magnesium aluminum (Mg Al) Aluminum copper (Al Cu) High strength aluminum alloys

Phase Diagram for Precipitation Hardened Alloy

  Criteria: Maximum solubility of 1 component in the other ( M ); Solubility limit that rapidly decreases with decrease in temperature ( M→N ). Process:

  Solution Heat Treatment – first

  heat treatment where all solute atoms are dissolved to form a single phase solid solution. Heat to T and dissolve B phase. Rapidly quench to T

  solid solution supersaturated with B atoms; alloy is soft, weak no ppts ).

  Precipitation Heat Treatment

  The supersaturated α solid solution is usually heated to an intermediate temperature T

  2

  within the α+β region ( diffusion rates increase). The β precipitates ( PPT ) begin to form as finely dispersed particles. This process is referred to as aging . After aging at T , the alloy is

  2 cooled to room temperature.

  Strength and hardness of the alloy depend on the ppt temperature (T ) and the aging

  2 time at this temperature.

  Heat treatable aluminum alloys gain strength from subjecting the material to a sequence of processing steps called solution heat treatment, quenching , and aging .

  The primary goal is to create sub micron sized particles in the aluminum matrix, called precipitates that in turn influence the material properties. While simple in concept, the process variations required (depending on alloy, product form, desired final property combinations, etc.) make it sufficiently complex that heat treating has become a professional specialty. The first step in the heat treatment process is solution heat treatment. The objective of this process step is to place the elements into solution that will eventually be called upon for precipitation hardening.

  Developing solution heat treatment times and temperatures has typically involved extensive trial and error, partially due to the lack of accurate process models.

  The supersaturated solid solution is unstable and if, left alone, the excess θ will precipitate out of the α phase. This process is called aging .

  Types of aging: Natural aging process occurs at room temperature Artificial aging If solution heat treated, requires heating to speed up the precipitation

  After solution heat treatment the material is ductile, since no precipitation has occurred. Therefore, it may be worked easily. After a time the solute material precipitates and hardening develops. As the composition reaches its saturated normal state, the material reaches its maximum hardness.

  The precipitates, however, continue to grow. The fine precipitates disappear . They have grown larger , and as a result the tensile strength of the material decreases.

  This is called overaging .

  Precipitation Heat Treatment

  PPT behavior is represented in the diagram: With increasing time, the hardness increases, reaching a maximum ( peak ), then decreasing in strength. The reduction in strength and hardness after long periods is

  overaging (continued particle growth).

  Small solute enriched regions in a solid solution where the lattice is identical or somewhat perturbed from that of the solid solution are called Guinier Preston zones.

  Guinier Preston (GP) zones Tiny clusters

  of atoms that precipitate from the matrix in the early stages of the age hardening process.

  The hardness and tensile strength vary during aging and overaging.

• 2014 Al Alloy:

  TS peak with precipitation time. • %EL reaches minimum • • Increasing T accelerates with precipitation time. process.

  )

  30

  a ) P le M p

  400

  ( m

  20

  th a g s n 300 n e i tr

  10

  149°C (2 s

  200

  204°C ile s n %

  100

  te 1min 1h 1day 1mo 1yr 1min 1h 1day 1mo 1yr

  precipitation heat treat time precipitation heat treat time

  Effects of Temperature

  Characteristics of a 2014

  aluminum alloy (0.9 wt% Si, 4.4

  wt% Cu, 0.8 wt% Mn, 0.5 wt% Mg) at 4 different aging temperatures.

  Alloys that experience significant precipitation hardening at room temp, after short periods must be quenched to and stored under refrigerated conditions. Several aluminum alloys that are used for rivets exhibit this behavior. They are driven

  while still soft , then allowed to age harden at the normal room temperature.

  Several stages in the formation of the equilibrium PPT (θ) phase. (a) supersaturated α solid solution; (b) transition (θ”) PPT phase; (c) equilibrium θ phase within the α matrix phase.

• Ex: Al%Cu system
• Procedure: %% Pt B : quench to room temp.

  2 A • Particles impede dislocation motion.

  ! " # $ % & ' ( ' ) ( ' * + , & - . , %- ' % / σ y

  Pt C (precipitate θ) ^{At room temperature the stable state of an aluminum copper alloy is an aluminum rich solid solution (α) and an intermetallic phase with a tetragonal crystal structure having} _{nominal composition CuAl 2 (θ).}

  Pt B C

  Pt A (solution heat treat) B

  %% Pt A : solution heat treat (get α solid solution)

  Time

  Temp.

  α phase.

  (retain α solid solution) %% Pt C : reheat to nucleate small θ particles within

  CuAl

  10

  

composition range

available for precipitation hardening

  (Al) (°C)

  600 700

  300 400 500

  θ θ+

  α α+θ

  50 wt% Cu α+

  40

  30

  20

  ~ Aging either at room or moderately elevated temperature after the quenching process is used to produce the desired final product property combinations. The underlying metallurgical phenomenon in the aging process is precipitation hardening . Due to the small size of the precipitate particles, early understanding was hampered by the lack of sufficiently powerful microscopes to actually see them. With the availability of the transmission electron microscope (TEM) with nanometer scale resolution, researchers were able to actually image many precipitate phases and build on this knowledge to develop improved aluminum alloy products.

  Aluminum is light weight, but engineers want to improve the strength for high performance

  applications in automobiles and aerospace.

  To improve strength, they use precipitation

  hardening.

  Age%hardening heat treatment phase diagram Quenching is the second step in the process.

  Its purpose is to retain the dissolved alloying elements in solution for subsequent precipitation hardening.

  Generally the more rapid the quench the better, from a properties standpoint, but this must be balanced against the concerns of part

  distortion and residual stress if the quench is non uniform.

  

Changes in Microstructure due to quenching