Chapter 7 Heat Treatment of Carbon Steel

 

HEAT TREATMENT OF CARBON STEEL

Zeraul Jun D. Luarez Eliseo A. Grasparil

 

PLAIN CARBON STEELS AND ALLOY STEELS ARE AMONG THE RELATIVELY FEW ENGINEERING MATERIALS WHICH CAN BE USEFULLY HEAT TREATED IN ORDER TO PROPERTIES. VARY THEIR MECHANICAL

 

THE VARIOUS HEAT-TREATMENT PROCESSES APPROPRIATE TO PLAIN CARBON STEELS ARE: •

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 Annealing.  Normalising.  Hardening.  Tempering.

 

IN ALL THE ABOVE PROCESSES THE STEEL IS HEATED SLOWLY TO THE  APPROPRI  APPROP RIATE ATE TEMPERATURE FOR ITS CARBON CONTENT AND THEN COOLED.

 

ANNEALING  All anne anneali aling ng proce processes sses are

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concerned with rendering steel soft and ductile so that it can be cold worked and/or machined.

 

THERE ARE THREE BASIC  ANNE  AN NEAL ALIN ING G PR PROC OCESS ESSES ES:: •

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 Stress-relief annealing at subcritical temperatures.  Spheroidised annealing at subcritical temperatures.  Full annealing for forgings and castings.

 

STRESS-RELIEF ANNEALING ('PROCESS ANNEALING‘) •

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it is often used for softening cold worked low carbon (0.4 % carbon content) steel or mild steel. Process involved heating steel to a temperature just below be low the lower critcal temp. (723 degrees Celcius) of steel. Cold worked steel has high hardness hardn ess and low ductility generally carried out in either batch-type or continuous furnaces, usually with an inert atmosphere of burnt coal gas, though cast-iron annealing "pots" are still used, their lids being luted on with clay.

 

SPHEROIDISED ANNEALING •

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produced by annealing the steel at a temperature between 650 and 700 C, just below the lower critical temperature. During this treatment cementite forms as spheroidal partisles in a ferrite matrix, putting the steel into a soft, but very tough, condition.

 

FULL ANNEALING •

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It is the treatment given to produce the softest possible condition in a hypoeutectoid steel. It involves heating the steel to a temperature within the range 30 – 50 C above the upper critical temperatures and then allowing the steel to cool slowly within the furnace. an expensive treatment and when it is not absolutely essential for the steel to be in a very soft condition c ondition

 

This operation need a very specific controlling on the heat temperature of annealing because if any fault is occurs, it will make some undesired phases in the steel

 

OVER HEATING •

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heating for too long a period in the austenitic range, will obviously cause grain growth of the newly formed austenite crystals, leading to a structure almost as bad as the original Widmanstatten Widmanstatten structure. In fact, castings are sometimes air-cooled to about 650° C and then cooled more slowly to room temperature, by returning to a furnace to prevent stresses due to rapid cooling from being set up.

 

BURNING (EXCESSIVE OVER HEATING) •

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Excessive overheating will probably cause oxidation, or "burning", of the surface, and the penetration by oxide films of the crystal boundaries following Recarburization of the surface.  To prevent "burning", castings are often annealed in cast-iron boxes into which they are packed with lime, sand, cast-iron turnings or carbonaceous material, material, according to the carbon content of the castings.

 

UNDER-QUNEALING •

 As the t he lower l ower crit critical ical tem tempera perature ture (723 C) is is reached on heating, the patches of pearlite change to austenite, but these crystals of austenite are very small, since each grain of pearlite gives rise to a number of new austenite crystals.

 

NORMALISING •

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Is accelerated by taking the work from the furnace and allowing it to cool in free air. The fine grain structure resulting from the more rapid cooling associated with normalising improved strengthitsand toughness togives the steel but reduces ductility and malleability.

 

PURPOSE OF NORMALISING •

To improve machinability

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To modify and refine last dendritic structures

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To refine grains

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To make steels suitable for the further heat

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treatment To relieve the internal stress.

 

 AD VANT  ADVA NTAG AGES ES OF NO NORM RMALI ALISI SING NG OV OVER ER  ANEA  AN EALIN LING G PR PROC OCES ESS S •

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It is faster process as compared to anealing Quenching media is air.

 

HARDENING •

When a piece of steel, containing sufficient carbon, is cooled rapidly from above its upper critical temperature it becomes considerably harder than it would be if allowed to cool slowly.

 

THE DEGREE OF HARDNESS PRODUCED CAN VARY, AND IS DEPENDENT UPON SUCH FACTORS AS THE INITIAL QUENCHING TEMPERATURE; •

the size of the work

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the constitution

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properties and temperature of the quenching medium the degree of agitation and final temperature of the quenching medium.

 

THE FOLLOWING LIST OF MEDIA IS ARRANGED IN ORDER OF QUENCHING SPEEDS •

5 % Caustic soda

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5 – 20 % Brine

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Cold water 

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Warm water 

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Mineral oil.

 

TEMPERING •

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to relieve the stresses and reduce the brittleness. any increase in toughness is accompanied by some decrease in hardness

 

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Tempering temperatures below 200 °C only relieve the hardening stresses, but above 220 C the hard, brittle br ittle martensite starts to transform into a fine pearlitic structure called s condary troostit (or just 'troostit ').

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Tempering above 400 C causes any cementite particles present to"ball-up" giving a structure called sorbit .

 

THE END