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Typology: Schemes and Mind Maps
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Instructors : TS. Nguyễn Anh Sơn Student : Phạm Anh Tuấn Student code : 202 17027 Class code : 75 4943
Diffusion in Solids I:
Decarburization of High Carbon Steel
I. INTRODUCTION
The objective of this experiment is to measure the diffusion coefficient of carbon in iron. In this laboratory experiment, you will measure the diffusion coefficient at various temperatures and compute the activation enthalpy (energy) of carbon diffusion. You should compare the experimental result with those from the computer simulations of diffusion (experiment # VII “ Diffusion in Solids-II : Computer Simulation) Diffusion can be defined as a mechanism of mass transport that involves the movement of one atomic species into another. The movement of atoms within a pure material is called self-diffusion. When diffusion occurs by the movement of solute atoms in a solvent lattice in which the solute and solvent atoms are approximately the same size, it is called the substitutional diffusion. The migration of interstitial solute atoms in a solvent lattice is termed as the interstitial diffusion. The interstitial diffusion of atoms in crystal lattices takes place when atoms move from one interstitial site to another neighboring interstitial site. For the interstitial mechanism to be operative the size of the diffusing atoms must be relatively small compares to the matrix atoms. For example, carbon can diffuse interstitial in BCC α iron and FCCγ iron. Diffusivity or the diffusion coefficient is a measure of the rate of diffusion in solid at a constant temperature:
D = D 0 exp( − RTQ ) The diffusivity value depends on many variables. The most important of them are:
II. EXPERIMENTAL PROCEDURE
Slices of high carbon (1080) steel, decarburized in a specially instrumented furnace. The samples decarburized for 1, 2, 2.5 hours at temperatures 880, 900 and 920oC will be supplied to you. Each student will then analyze a sample decarburized at a particular temperature and for a particular period of time. Slice the decarburized sample into four quarters using a cut-off saw and mount one of the quarters in a sample mount with one of the cut surface facing the polishing side. Then grind, polish and etch the sample. Obtain a well focused optical image of the polished surface. Display the image onto the monitor of the CCD video camera system. Use proper magnification so that the entire region of interest (from the edge where C(0) = 0 to the bulk region of the sample where C(x=8)=0,8wt%C) is displayed on the monitor screen. Capture the image using the computer equipped with an image capture card for image capturing procedure and save it.
T = 900oC; t = 2h Open and analyze the image using the Materials-Pro Analyzer application program. Find the carbon concentration at different depths from the decarburized surface for your sample by the quantitative metallographic measurement of the fractions of ferrite and pearlite.
III. RESULTS AND ANALYSIS
1. The effect of temperature to the depth of the decarburized surface T = 880oC; t = 2h; x
T = 900oC; t = 2h; x
T = 920oC; t = 2h; x
⇒ erf (^) ( 2 x √ Dt^ )
⇒ (^) 2 x √ Dt^
⇒ D = x
2 ( 0.52 × 2 )^2 × t = x
2 ( 1.04 )^2 × t
a. T = 880oC; t = 2h
With x =381.504^ μm
The diffusion coefficient is calculated by:
D = (^ 381.504^ ×^10
− (^6) )^2 ( 1.04 )^2 × 7200 =1.869 × 10 −^11 ( m^2 / s )
b. T = 900oC; t = 2h
With x =339.884^ μm
The diffusion coefficient is calculated by:
D = (^ 339.884^ ×^10
− (^6) )^2 ( (^) 1.04 )^2 × 7200 =1.483 × 10 −^11 ( m^2 / s )
c. T = 920oC; t = 2h
With x =397.687^ μm
The diffusion coefficient is calculated by:
D = (^ 397.687^ ×^10
− (^6) )^2 ( 1.04 )^2 × 7200 =2.031 × 10 −^11 ( m^2 / s )
For carbon diffusion in γ-Fe, noting (^) D = D 0 × e
− RTQ
The Arrhenius plot of the data: plot of ln^ D^ vs (^) T^1. The slope =− kQ
The diffusivity of the materials is determined by:
D = D 0 × e
− RTQ
⇒ ln D =ln D 0 + − RQ × (^) T^1
⇒ − RQ =− 2496063
We obtain the activation enthalpy as
⇒ Q = 2496063 × R = 2496063 × 8.314 ≈ 20.75 × 106 J
2. The effect of time to the depth of the decarburized surface
T = 880oC; t = 1h; x
T = 880oC; t = 2h; x
D ( m^2 / s ) ln^ D 1.869 × 10 −^11 -24. 1.483 × 10 −^11 -24. 2.031 × 10 −^11 -24.
T (^) ( oK) 1/T (^1153) 8.673 × 10 − 4 (^1173) 8.525 × 10 − 4 (^1193) 8.382 × 10 −^3
1h 2h 2,5h
Temperature T (oC)
Decarburization layer depth (
Conclusion: The decarburization layer depth should theoretically rise with increasing
heating time; but, in this instance, inconsistencies in the process resulted in the 2.5-hour
sample releasing the least carbon.
a. T = 880oC; t = 1h
With x =255.262^ μm
The diffusion coefficient is calculated by:
D = (^ 255.262 ×^10
− (^6) )^2 ( (^) 1.04 )^2 × 7200 =8.367 × 10 −^12 ( m^2 / s )
b. T = 880oC; t = 2h
With x =381.504^ μm
The diffusion coefficient is calculated by:
D = (^ 381.504^ ×^10
− (^6) )^2 ( 1.04 )^2 × 7200 =1.869 × 10 −^11 ( m^2 / s )
c. T = 880oC; t = 2.5h
With x =121.618^ μm
The diffusion coefficient is calculated by:
D = (^ 121.618^ ×^10
− (^6) )^2 ( 1.04 )^2 × 7200 =1.899 × 10 −^12 ( m^2 / s )
