Creep & Spheroidisasi

Albet Ahmad

Creep & spheroidisasiRLA memakai klasifikasi Wedel & Neubaeur serta Toft & Mardsen unutk prediksi sisa umur.Pendekatan Wedel dan Neubauer (Creep Cavitations) melihat ada tidaknya creep.

Pendekatan Toft & Mardsen (Spheroidization of pearlite & Carbide particles) melihat ada tidaknya spheroidisasi

Apa itu creep?• Materials are often placed in service at elevated temperatures and

exposed to static mechanical stresses (e.g., turbine rotors in jet engines and steam generators that experience centrifugal stresses, and high-pressure steam lines). Deformation under such circumstances is termed creep.... It is observed in all materials types; for metals it becomes important only for temperatures greater than about 0,4Tm ( Tm = absolute melting temperature). Callister: Materials Science and Engineering 7th edition, page 238

• creep is not too important with most ferrous metals unless the operating temperature is above 800 oF (426 oC). Kenneth G. Engineering materials properties and selection 5th editon. Page 41

• the dislocation creep mechanism is operative at intermediate and high stress and at temperatures above 0.4 of the melting point and is the only mechanism of significancefor most enginnering materials and applications. R. Viswanathan, Damage Mechanism and life assesment of high temperature components, Page 62

• Stress produced at elevated temperature produces a condition of continuous strain called creep. By definition, creep is deformation as a function of time at constant load or stress. Creep, after a period of time, may terminate in stress-rupture fracture, also known as creep rupture. ASM Metals Handbook Vol 11: Failure Analyse & Prevention

• Creep is time-dependent strain, which can cause failures when a component operating at high temperatures under steady load is so deformed or distorted that it can no longer perform its intended function. terminate in stress-rupture fracture, also known as creep rupture. ASM Metals Handbook Vol 11: Failure Analyse & Prevention page 621

Apa itu creep?

• If a steel alloy having either pearlitic or bainitic microstructures is heated to, and left at, a temperature below the eutectoid for a sufficiently long period of time—for example, at about 700 o C( 1300 F)) for between 18 and 24 h—yet another microstructure will form. It is called spheroidite (Figure 10.19). Callister : Materials Science and Engineering 7th edition, page 330.

Apa itu spheroidisasi?

Photomicrograph of a steel having a spheroidite microstructure. The small particles are cementite; the continuous αphase is ferrite. (Copyright 1971 by United States Steel Corporation.)

Alloys containing pearlitic microstructures have greater strength and hardnessthan do those with spheroidite. Calister page 341

A photomicrograph of a pearlitic steel that has partially transformed to spheroidite. 2000 . Calister Materials Science and Engineering 7th edition, page 311.

0 % C (500X)

1,4 % C (1000X)

0.8 % C

0.38 % C (635X)

Apa itu spheroidisasi?

Struktur mikro baja karbon: ferrit (terang) dan pearlite (gelap)

Photomicrographs of (a) pearlite

“Damage mechanisms and life assessment of high-temperature components” by Viswanathan