HEAT TREATMENT TECHNOLOGY - MODULE-1 - Solid State Phase Transformation - Diffusion - Practice Problem

Oct 26, 2026

Upload by Mimi

Heat Treatment Technology module 1 focuses on solid-state phase transformation with emphasis on diffusion as a transport phenomenon. The module explains atomic and molecular diffusion driven by concentration gradients, introduces flux relation and the role of diffusivity D, and distinguishes steady-state from nonsteady-state diffusion using the governing concentration variations over space and time. It concludes with a practice problem on carburizing in steel to estimate time for a target carbon concentration at increased depth, yielding a numerical answer.

LanguageEnglish

CategoryTechnology

FormatPDF

Pages28

Reading Time1 h 11 min

AccessFree to download

Module-1: Solid State Phase Transformation

1.1 Diffusion

Steady State and Nonsteady Diffusion

Practice Problem

HEAT TREATMENT TECHNOLOGY COMPILED BY : ARABINDA NAYAK LECTURER IN METALLURGY GOVERNMENT POLYTECHNIC MAYURBHANJ TIKARPADA HEAT TREATMENT TECHNOLOGY MODULE-1 1.0 SOLID STATE PHASE TRANSFORMATION: 1.1 DIFFUSION At any temperature different from absolute zero all atoms, irrespective of their state of aggregation (gaseous, liquid or solid), are constantly in motion. Since the movement of particles is associated with collisions, the path of a single particle is a zigzag one. However, an aggregation of “diffusing” particles hasan observable drift from places of higher to places of lower concentration . For this reason diffusion is known as a transport phenomenon In each diffusion reaction (heat flow, for example, is also a diffusion process), the flux (of matter, heat, electricity, etc. ) follows the general relation Flux = (conductivity) x (driving force) In the case of atomic or molecular diffusion, the “conductivity” is referred to as the diffusivity or the diffusion constant, and is represented by the symbol D. We realize from the above considerations that this diffusion constant (D) reflects the mobility of the diffusing species in the given environment and accordingly assumes larger values in gases, smaller ones in liquids, and extremely small ones in solids. The “driving force” for many types of diffusion is the existence of a concentration gradient. The term ΔC/Δx → dC/dx “gradient” describes the variation of a given property as a function of distance in the x-direction. Ifa material exhibits a linear variation of concentration with distance in the distance x-direction, we speak of a constant Concentration gradient ΔC /Δx concentration x-direction. The gradient itself is the rate of change of the concentration with distance (dc/dx), which is the same as the slope of a graph of concentration vs. position (Δc/Δx) Steady State and Nonsteady Diffusion Diffusion processes may be divided into two types: (a) steady state and (b) nonsteady state. Steady state diffusion takes place at a constant rate-that is, once the process starts the number of atoms (or moles) crossing a given interface (the flux) is constant with time. This means that throughout the system dc/dx = constant and dc/dt = 0. Nonsteady state diffusion is a time dependent process in which the rate of diffusion is a function of time. Thus dc/dx varies with time and dc/dt ≠ 0. Both types of diffusion are described quantitatively by Fick’s laws of diffusion. The first law concerns both steady state and nonsteady state diffusion, while the second law deals only with nonsteady state diffusion. PRACTICE PROBLEM For a steel alloy it has been determined that a carburizing heat treatment of 10-h duration will raise the carbon concentration to 0.45 wt% at a point 2.5 mm from the surface. Estimate the time necessary to achieve the same concentration at a 5.0-mm position for an identical steel and at the same carburizing temperature Ans: 40 hr

FAQs

Why is diffusion considered a transport phenomenon in heat treatment technology?

Because atoms and molecules move continuously and collectively drift from higher to lower concentration regions. This drift transports matter (and related quantities like heat) through the material.

What does the diffusion constant D represent?

D is the diffusivity (diffusion constant) that reflects the mobility of the diffusing species in the given environment. It is larger in gases, smaller in liquids, and extremely small in solids.

How do steady-state and nonsteady-state diffusion differ?

Steady-state diffusion occurs at a constant rate, so the flux is constant with time and dc/dt = 0 while dc/dx remains constant. Nonsteady-state diffusion depends on time, so dc/dx varies with time and dc/dt ≠ 0.

Oct 26, 2026

Upload by Mimi