Heat Treatment of Steels

DEEPAK KUMAR NAYAK 07318VAIBHAV SRIVASTAVA 07319ANURAG BHARDWAJ 07320MAYANK SHARMA 07321ABHINAV KAPILA 07323RAJAT MAHAJAN 07324ANKIT SOOD 07325VIPAN KUMAR 07203

PRESENTED BY

Topics to be discussed

• Heat treatments Annealing Normalizing Tempering

• Martensitic transformations• Critical temperature on heating and cooling

AnnealingAnnealing is a heat treatment wherein a material is altered, causing changes in its properties suchas strength and hardness. In it a previously cold worked metal is softened by allowing it to crystalline.

In the cases of copper, steel, silver, and brass this process is performed by substantially heating thematerial (generally until glowing) for a while and allowing it to cool slowly.

In this fashion the metal is softened and prepared for further work such as shaping, stamping, or forming

Purpose

• The purpose of annealing has following aims:

• To soften the steel and to improve machinability.

• To relieve internal stresses induced by some previous treatment (rolling, forging, uneven cooling).

• To remove coarseness of grain.

Stages

There are 2 stages in the annealing process :

• The recovery phase• The recrystallization

The Recovery Phase

• In this step softening of the metal through removal of crystal defects takes place and the internal stresses which are caused by these defects.

• Recovery phase covers all annealing phenomena that occur before the appearance of new strain-free grains.

The recrystallization

New strain-free grains nucleate and grow to replace those deformed by internal stresses.

If annealing is allowed to continue once recrystallization has been completed, grain growth will occur, in which the microstructure startsto coarsen and may cause the metal to have less than satisfactory mechanical properties

FULL ANNEALINGFull annealing is the process of slowly raising the temperature about 500C(900F) above the Austenitic temperature line A3 or line ACM in the case of Hypoeutectoid steels (steels with < 0.77% Carbon) and 500C (900F) into the Austenite- Cementite region in the case of Hypereutectoid steels (steels with > 0.77% Carbon). It is held at this temperature for sufficient time for all the material to transform into Austenite or Austenite- Cementite as the case may be. It is then slowly cooled at the rate of about 200C/hr (360F/hr) in a furnace to about 500C (900F) into the Ferrite- Cementite range. At this point, it can be cooled in room temperature air with natural convection.

The grain structure has coarse Pearlite with ferrite or Cementite (depending on whether hypo or hyper eutectoid). The steel becomes soft and ductile.

PROCESS ANNEALING

It is used to treat work-hardened parts made out of low-Carbon steels (< 0.25% Carbon). This allows the parts to be soft enough to undergo further cold working without fracturing. Process annealing is done by raising the temperature to just below the Ferrite-Austenite region, line A1on the diagram.

This temperature is about 7270C (13410F) so heating it to about 7000C (12920F) should suffice. This is held long enough to allow recrystallization of the ferrite phase, and then cooled in still air. Since the material stays in the same phase through out the process, the only change that occurs is the size, shape and distribution of the grain structure. This process is cheaper than either full annealing or normalizing since the material is not heated to a very high temperature or cooled in a furnace.

Spheroidization

It is an annealing process used for high carbon steels (Carbon > 0.6%) that will be machined or cold formed subsequently. This is done by one of the following ways:

1. Heat the part to a temperature just below the Ferrite-Austenite line, line A1 or below the Austenite- Cementite line, essentially below the 727 0C (1340 0F) line. Hold the temperature for a prolonged time and follow by fairly slow cooling.

2. Cycle multiple times between temperatures slightly above and slightly below the 7270C (13400F) line, say for example between 700 and 7500C (1292 – 13820F), and slow cool.

3. For tool and alloy steels heat to 750 to 8000C (1382-14720F) and hold for several hours followed by slow cooling.

All these methods result in a structure in which all the Cementite is in the form of small globules (spheroids) dispersed throughout the ferrite matrix. This structure allows for improved machining in continuous cutting operations such as lathes and screw machines. Spheroidization also improves resistance to abrasion

Normalizing

DEFINITION

A heat treatment process that has the object of relieving internal stresses, refining the grain size and improving the mechanical properties. The steel is heated to 800-900oC according to analysis, held at temperature to allow a full soak and cooled in still air to room temp.

HOW IT DIFFERS FROM ANNEALING

It differs from annealing in that :-1.The metal is heated to a higher temperature and then removed from the furnace for air cooling.

2.Thin pieces cool faster and are harder after normalizing than thick ones. In annealing (furnace cooling), the hardness of the two are about the same.

3.Normalized steels are harder and stronger than annealed steels. In the normalized condition, steel is much tougher than in any other structural condition

PURPOSE OF NORMALIZINGTo produce a harder and stronger steel than full annealingTo improve the machinabilityTo modify and refine the grain structureTo obtain a relatively good ductility without reducing the hardness and strengthTo homogenize the structure in order to improve the response to hardening operations.Normalizing is applied to castings and forgings

• Because of increased cooling rates as compared to furnace cooling affects the transformation of austenite. Since there is less time for the formation of the proeutectoid constituent, consequently there will be less proeutectoid ferrite in normalized hypoeutectoid steels and less proeutectoid cementite in hypereutectoid steels.

COMPARISON BETWEEN NORMALIZED AND ANNEALED STEELS

Ductility of annealed and normalized steels.

• Annealing and normalizing do not present a significant difference on the ductility of low carbon steels.

• As the carbon content increases,

-annealing maintains the % elongation around 20%.

-the ductility of the normalized high carbon steels drop to 1 to 2 % level.

Tensile strength of normalized and annealed steels.

• Tensile strength of the normalized steels are higher than the annealed steels.

• Normalizing and annealing do not show a significant difference on the tensile strength of the low carbon steels.

• Normalized high carbon steels has much higher tensile strength than those that are annealed.

Hardness of normalized and annealed steels.

• Low and medium carbon steels can maintain similar hardness levels when normalized or annealed.

• When high carbon steels are normalized they maintain higher levels of hardness than those that are annealed.

Yield point of annealed and normalized steels• Yield point of the normalized

steels are higher than the annealed steel.

• Normalizing and annealing do not show a significant difference on yield point of the low carbon steels.

• Normalized high carbon steels present much higher yield point than those that are annealed.

Martensitic Transformation

Under slow cooling rate, the carbon atom diffuses out of austenite structure. The Iron atoms then move to become B.C.C.

The gamma to alpha transformation takes place by a process of nucleation and growth and is time- dependent.

With a still further increase in cooling rate, insufficient time is allowed for the carbon to diffuse out of the solution, although some movement of iron atoms takes place, the structure cannot become B.C.C. while the carbon is trapped inside the solution.

The resultant solution is called Martensite, is a super saturated solid solution of carbon trapped in a body centered tetragonal structure.

Purpose of hardening

The basic purpose of hardening is to produce a fully martensitic structure, and the minimum cooling rate (0F per second )that will avoid the formation of any of the softer products of transformations is known as the critical cooling rate.

The critical cooling rate, determined by chemical composition and austenitic grain size, is an important property of a steel since it indicates how fast a steel must be cooled in order to form only martensite

Hardening Mechanism

• Two dimensions of the unit cell are equal, but the third is slightly expanded because of the trapped carbon.

• The axial ratio c/a increases, with carbon content to a maximum of 1.08%.

• The highly distorted lattice structure is the prime reason for the High hardness of martensite.

• Highly localized stresses cause plastic deformation of the matrix.

Mechanism of hardening

Microstructure of martensite

• After drastic cooling (quenching), martensite appears microscopically as a white needlelike structure described as pile of straw.

• In most steels, the martensitic structure appears vague and unresolvable .

Martensitic transformation characteristics

• 1. Various microstructures occur depending on Carbon content of

steel ~0.2 wt% C well-defined laths of martensite ~0.6 wt% C plates of martensite form, mixed with laths ~1.2 wt% C well-defined plates of martensite

• 2. Martensitic transformation is diffusion less (no time for atoms to intermix)

• 3. No compositional change to parent phase (relative position of carbon atoms with respect to iron atoms identical to austenite parent)

.6% c .2% c

1.2% c

Continued….

• 4. Crystal structure changes from BCC to body centered tetragonal as carbon content increases (solid solubility difference of C in FCC austenite and BCC ferrite)

• 5. Martensitic transformation begins at definite temperature called Ms being dependent on chemical composition only.

• 6.The most significant property of martensite is its potential of being very hard. The hardness of martensite increases rapidly at first reaching upto about 0.4% carbon. Its also a result of severe lattice distortions by its formation, since the amount of carbon present is many times more than can be held in solid solutions

Martensitic transformation temperature

Tempering

Definition

Tempering is a heat treatment process accomplished by heating steel to a temperature below the eutectoid temperature for a specified period of time.

According to ASME Metals Handbook, Reheating hardened steel to some temperature below

the eutectoid temperature to decrease hardness and/or increase toughness is called tempering.

Generally, it is studied for martensite structure of steel.

Eutectoid Reaction

MartensiteOf all the microstructures that can be produced for a given steel alloy, martensite is the• Hardest• Strongest• Most Brittle

Applications: crankshafts, spanners, high-tension bolts.

Martensite needs to be tempered to obtain better ductility. This happens when ferrite is allowed to precipitate from the supercooled Martensite.

Structure

Martensite has Body Centered Tetragonal Structure(BCT).

The circles represent iron atoms and crosses represent carbon atoms.

Martensite is formed by the rapid cooling of FCC Austenite to ambient temperature.

Normally, tempering is carried out at temperatures between 250 °and 650 °C (480 °and 1200 °F) even though internal stresses may be relieved at temperatures as low as 200 °C (390 °F).

Significant Points-

1. The microstructure of tempered martensite consists of extremely small and uniformly dispersed cementite particles embedded within a continuous ferrite matrix.

2. Tempered martensite may be nearly as hard and strong as martensite, but with substantially enhanced ductility and toughness.

3. The hard cementite phase reinforces the ferrite matrix along the boundaries, and these boundaries also act as barriers to dislocation motion during plastic deformation.

4. The continuous ferrite phase is also very ductile and relatively tough, which accounts for the improvement of these two properties for tempered martensite.

Martensite (BCT, single phase) Tempered martensite (α + Fe3C phases)

Tensile and yield strengths and ductility (%RA) versus tempering temperature for an oil- quenched alloy steel.

The changes during the tempering of martensite can be categorized into stages.

• During the first stage, excess carbon in solid solution segregates to defects or forms clusters within the solid solution. It then precipitates, either as cementite in low-carbon steels, or as transition iron-carbides in high-carbon alloys. The carbon concentration that remains in solid solution may be quite large if the precipitate is a transition carbide.

• Further annealing leads to stage 2, in which almost all of the excess carbon is precipitated, and the carbides all convert into more stable cementite. Any retained austenite may decompose during this stage.

• Continued tempering then leads to the coarsening of carbides, extensive recovery of the dislocation structure, and finally to the recrystallisation of the ferrite plates into equiaxed grains.

Temper Embrittlement

Tempering is frequently necessary to reduce the hardness of martensite and increase toughness,

But

The heat-treatment can lead to embrittlement when the steel contains impurities such as phosphorus, antimony, tin and sulphur. This is because these impurities tend to segregate to the prior austenite grain boundaries and reduce cohesion across the boundary plane, resulting in intergranular failure.

Tempering at temperatures around 650o promotes the segregation of impurity elements such as phosphorous to the prior austenite grain boundaries, leading to intergranular failure along these boundaries.

Critical temp. on heating and cooling

Definition- The temperatures at which the transformations in

the solid state takes place are called critical temperatures. there are two types of transformation : On heating On cooling

• At a temperature just above AC1 , the structures of steel being considered will be composed of grains of pro eutectoid ferrite and the grains of austenite.

• As the temperature is raise above AC1 the ferrite will be transformed to austenite gradually until, at the a temperature, the transformation will be completed.

• in hypo eutectoid steels the transformation of steel begins at the AC3,1 temperature by the transformation of pearlite to austenite. Heating the steel above to temperature above AC3,1 will bring about the solution of pro eutectoid cementite in the austenite. At the ACM temperature, the cementite is completely dissolved in the austenite.

Heating transformation

Cooling transformations

• Steel containing .3% carbon is cooled slowly from above the AC3 temperature, pro eutectoid ferrite will begin to precipitate from the austenite at Ar3 point.

• Upon slow cooling a hypereutectoid steel, proeutectoid cementite will begin to precipitate at the ACM temperature.

• Also, the transformation of austenite of eutectoid composition involves the simultaneous transformation of gamma to alpha iron and the precipitation of cementite due to the low solubility of carbon in alpha iron.

References

• Introduction to physical Metallurgy- by Sidney H Avener

• Physical Metallurgy for Engineers- by Donald S Clark and Wilbur R Varney

• Material Science and Engineering- by William D Callister

Images taken from web resources.