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Heat Treatment of Steel

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1 Heat Treatment of Steel
Lecture 9

2 Heat-Treatment Heat treatment is a method used to alter the physical, and sometimes chemical properties of a material. The most common application is metallurgical It involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material It applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally 3. … heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding

3 Types of Heat-Treatment (Steel)
Annealing / Normalizing, Case hardening, Precipitation hardening, Tempering, and Quenching

4 Time-Temperature-Transformation (TTT)Curve
TTT diagram is a plot of temperature versus the logarithm of time for a steel alloy of definite composition. It is used to determine when transformations begin and end for an isothermal heat treatment of a previously austenitized alloy TTT diagram indicates when a specific transformation starts and ends and it also shows what percentage of transformation of austenite at a particular temperature is achieved. There is also Continuous Cooling Transformation (CCT) Diagram Austenite -> a solid solution of ferric carbide or carbon in iron; cools to form pearlite or martensite. In plain-carbon steel, austenite exists above the critical eutectoid temperature

5 Time-Temperature-Transformation (TTT)Curve
If austenite is cooled slowly to a temperature below LCT (Lower Critical Temperature), the structure that is formed is Pearlite The TTT diagram for AISI 1080 steel (0.79%C, 0.76%Mn) austenitised at 900°C

6 Decarburization during Heat Treatment
Decrease in content of carbon in metals is called Decarburization It is based on the oxidation at the surface of carbon that is dissolved in the metal lattice In heat treatment processes iron and carbon usually oxidize simultaneously During the oxidation of carbon, gaseous products (CO and CO2) develop In the case of a scale layer, substantial decarburization is possible only when the gaseous products can escape 4. … i.e., when the equilibrium pressures of the carbon oxides are high enough to break the scale layer or when the scale is porous.

7 Decarburization Effects
The strength of a steel depends on the presence of carbides in its structure In such a case the wear resistance is obviously decreased … therefore, loss of carbon from the surface softens and weakens the surface layers . High %age of carbon in the surface imparts compressive residual stresses that retard initiation of crack Apply special high-temperature paint on the surface

8 Annealing It is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness It the process of heating solid metal to high temperatures and cooling it slowly so that its particles arrange into a defined lattice

9 Types of Annealing Stress-Relief Annealing (or Stress-relieving)
Normalizing Isothermal Annealing Spheroidizing Annealing (or Spheroidizing )

10 1. Stress-Relief Annealing
It is an annealing process below the transformation temperature Ac1, with subsequent slow cooling, the aim of which is to reduce the internal residual stresses in a workpiece without intentionally changing its structure and mechanical properties Critical Temperature of Steel is 724°C

11 Causes of Residual Stresses
1. Thermal factors (e.g., thermal stresses caused by temperature gradients within the workpiece during heating or cooling) 2. Mechanical factors (e.g., cold-working) 3. Metallurgical factors (e.g., transformation of the microstructure)

12 How to Remove Residual Stresses?
R.S. can be reduced only by a plastic deformation in the microstructure. This requires that the yield strength of the material be lowered below the value of the residual stresses. The more the yield strength is lowered, the greater the plastic deformation and correspondingly the greater the possibility or reducing the residual stresses The yield strength and the ultimate tensile strength of the steel both decrease with increasing temperature Last -> Because of this, stress-relief annealing means a through-heating process at a correspondingly high temperature

13 Stress-Relief Annealing Process
For plain carbon and low-alloy steels the temperature to which the specimen is heated is usually between 450 and 650˚C, whereas for hot-working tool steels and high-speed steels it is between 600 and 750˚C This treatment will not cause any phase changes, but recrystallization may take place. Machining allowance sufficient to compensate for any warping resulting from stress relieving should be provided

14 Stress-Relief Annealing – R.S.
In the heat treatment of metals, quenching or rapid cooling is the cause of the greatest residual stresses To activate plastic deformations, the local residual stresses must be above the yield strength of the material. Because of this fact, steels that have a high yield strength at elevated temperatures can withstand higher levels of residual stress than those that have a low yield strength at elevated temperatures Soaking time also has an influence on the effect of stress-relief annealing … A high level of residual stress is generally to be expected with workpieces that have a large cross section, are quenched at a high cooling rate, and are made of a steel of low hardenability . … i.e., on the reduction of residual stresses Soaking time -> The hold time at maximum temperature for a heat-treatment process

15 Relation between heating temperature and Reduction in Residual Stresses
Higher temperatures and longer times of annealing may reduce residual stresses to lower levels

16 Stress Relief Annealing - Cooling
The residual stress level after stress-relief annealing will be maintained only if the cool down from the annealing temperature is controlled and slow enough that no new internal stresses arise. New stresses that may be induced during cooling depend on the (1) cooling rate, (2) on the cross-sectional size of the workpiece, and (3)on the composition of the steel 2. Composition of steel has complicated effect on residual stresses during cooling phase

17 2. Normalizing A heat treatment process consisting of austenitizing at temperatures of 30–80˚C above the AC3 transformation temperature followed by slow cooling (usually in air) The aim of which is to obtain a fine-grained, uniformly distributed, ferrite–pearlite structure Normalizing is applied mainly to unalloyed and low-alloy hypoeutectoid steels For hypereutectoid steels the austenitizing temperature is 30–80˚C above the AC1 or ACm transformation temperature Hypoeutectoid -> At L.H.S of Eutectoid point For hypereutectoid steels, normalizing is a special process

18 Normalizing – Heating and Cooling
a, Heating; b, holding at austenitizing temperature; c, air cooling; d, air or furnace cooling

19 Normalizing – Austenitizing Temperature Range
For hypereutectoid steels, whether to heat above ES line or SK line depends upon alloying materials and content. Heating above SE line is ONLY required for the alloy steels whose alloying elements form secondary carbides (e.g. Vanadium Carbide, WC, etc). For unalloyed hyper steels, heating above Ac1 (SK) line is enough

20 Effect of Normalizing on Grain Size
Normalizing refines the grain of a steel that has become coarse-grained as a result of heating to a high temperature, e.g., for forging or welding Carbon steel of 0.5% C. (a) As-rolled or forged; (b) normalized. Magnification 500 Normalizing generally gives slightly harder substrate as compared to annealing because of fine grained pearlite in normalizing as compared to coarse-grained in annealing

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