annealing & pickling process

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Heat Treatment

In the process of forming steel into shape and producing the desired microstructure to achieve the required mechanical properties, it may be reheated and cooled several times.

Steps for all HT (anneals):

1. Heating

2. Holding or “soaking”

3. Cooling

Time and temperature are important

at all 3 steps

(Stress-relief)

Full Annealing

Heat the steel to a temperature within the austenite (FCC, γ) phase region to dissolve the carbon. (50 deg.F above A3-Acm line)

The temperature is kept at the bottom of this range to minimize growth of the austenitic grains. Then, after cooling ferrite (α) and cementite structures will be fine as well

Spheroidizing – improving machinability

Used on steels with carbon contents above 0.5%Applied when more softness is neededCementite transforms into globes, or spheroidsThese spheroids act as chip-breakers – easy machiningPerformed by heating to just below A3,1 line, holding there (about 20h.or more) and then slowly cooling

Normalizing

Allows steels to cool more rapidly, in air

Produced structure – fine pearlite

Faster cooling provides higher strength than at full annealing

Process Annealing – 3 stages

Recovery (stress-relief anneals)

Recrystallization (process anneals)

Grain Growth

Stress-relief Annealing

Heats the steel to just below the eutectoid transformation temperature (A1) to remove the effects of prior cold work and grain deformation.

This allows further forging or rolling operations.

Stresses may result from:

Plastic deformation (cold work, machining)

Non-uniform heating (ex. welding)

Phase transformation (quenching)

Stress-relief:

Is held at fairly low temperature

Is held for a fairly short time

So that recrystallization does not occur

Recovery (Stress-relief)

If you only add a small amount of thermal energy (heat it up at little) the dislocations rearrange themselves into networks to relieve residual stresses

Ductility is improved

Strength does not change

TS and elongation

Recrystallization

Add more heat and wait some more time, and new grains start to grow at the grain boundaries.

The new grains have not been strain hardened

The recrystallized metal is ductile and has low strength

How much time to wait?

Incubation period – time needed to accumulate stored energy from the lattice strain and heat energy

Then lattice starts to recrystallize

At first fast (lots of nucleation sites)

Slower at the end

How hot is hot?

Most metals have a recrystallization temperature equal to about 40% of the melting point

K,4.0 mr TT =

Minor factors for recrystallization

Pure metalIf an alloy – host atom – solvent

foreign atom – soluteSolute atoms inhibit dislocations motion, higher temperature is neededInsoluble impurities (oxides and gases) become nucleation sites and refine grainsSmaller initial grain size will recrystallize easier – at less temperature and time

Grain Growth

If you keep the metal hot too long, or heat it up too much, the grains become large

Usually not good

Low strength

Size of grains vs. temperature

GRAIN

SIZE

Temperature, deg.C200 600400

Microscope images show:

Cold rolled steel90% reduction

recrystallized after 2 min.at 830°C

Grain growth after 2min @ 930°C.

Grain-Growth is not recommended mainly because:

Energy consumption

Need of expensive equipment

Large grain metals get surface distortion under tensile forces

Solution Annealing

Solution annealing is the heat treatment most frequently specified for stainless steels.

The main objective is to dissolve the phases that have precipitated during the thermomechanical processing of the material, especially the chromium-rich carbides

Bright Annealing

Stainless steels can be bright annealed in a pure hydrogen or dissociated ammonia atmosphere.The dew point should be kept below 508 °C (608F) The sheet should be dry and clean before entering the furnace. If the dew point is not kept sufficiently low, some thin green/blue oxide film may be formed, which will be difficult to remove.

Quenching media

Involves the principles of heat transfer

There are 9 possible choices (air, furnace, tap water, oil, brine etc.)

Pickling

What is pickling?

Removal of annealing oxides and mill scales.

Removal of Chromium depleted zone

STAINLESS STEEL

HNO3 / HF

CHROMIUM-DEPLETED ZONE

SCALE

Mechanism of pickling, HNO3 / HF

Pickling is the final step in making stainless steel corrosion resistant!

Pickling chemistry

H+ HF

CrF3

Cr(NO3)3Fe(NO3)3

H+

HF

FeF2+

NO3-

CrF2+ FeF2

+FeF3

NO3-

H+

CrF2+ FeF3

Fe(NO3)3Cr(NO3)3

CrF3

NO3-

HF

Chemical composition in the pickling bath

CHEMICAL REACTIONS

Dissolving of metals:

Fe + 4H+ + NO3- ↔ Fe3+ + NO + 2H2O

Cr + 4H+ + NO3- ↔ Cr3+ + NO + 2H2O

3Ni + 8H+ + 2NO3- ↔ 3Ni2+ +2NO + 4H2O

Complex reactions:

3HF + Fe3+ → FeF3 + 3H+

2HF + Fe3+ → FeF2+ + 2H+

3HF + Cr3+ → CrF3 + 3H+

2HF + Cr3+ → CrF2+ + 2H+

HF + Ni2+ → NiF+ + H+

The relation between HNO3 and HF is important for the pickling process!

Pickle tank

Mixed acid pickling

Role of HNO3:- H+ Generator- Powerful Oxidising Agent- Brightener for the pickled product

Role of HF:- Complexing Agent for Fe3+, Cr3+, Ni2+

- H+ supplier

Environmental impacts from:- Emission of NOX gases

- Presence of NO3- and NO2

-

Uncontrolled pickling

Acid conc.

Metal conc. Time

DUMP SPENT BATH

Over pickled surfaceWaste of materialHigh chemical consumptionRapid destruction of pickle solution

Bad surface qualityProduction rejectsAcid must be wastedSludge formation

Co

nce

ntr

atio

n

Uniform surface quality Less production rejectsBest use of chemicals involved

ZONE 1 ZONE 2 ZONE 3

Change of bath composition over time

Uncontrolled pickling results in variations in the pickling process!

Uncontrolled pickling (cont.)

Oxide remains on the steel surface

Total removal of surface oxide and chromium depleated layer

The mixed acid has attacked both grains and grain-boundaries resulting in a dull surface appearance

Under pickled surfaceGood surfaceOver pickled surface

Over pickled surfaceWaste of materialHigh chemical consumptionRapid destruction of pickle solution

Bad surface qualityProduction rejectsAcid must be wastedSludge formation

Uniform surface quality Less production rejectsBest use of chemicals involved

ZONE 1 ZONE 2 ZONE 3

Uncontrolled pickling results in bad quality in the material produced!

Uncontrolled pickling (cont.)

Over pickled surfaceWaste of materialHigh chemical consumptionRapid destruction of pickle solution

Bad surface qualityProduction rejectsAcid must be wastedSludge formation

Uniform surface quality Less production rejectsBest use of chemicals involved

ZONE 1 ZONE 2 ZONE 3

Pickle solution conditions

Uncontrolled pickling results in uneconomical and environmentally unsound use of chemicals!

Concentration of acid in the pickle solution is too low. Pickle solutions must be wasted. Pickle tank must be manually cleaned from sludge.

Concentration of acid in the pickle solution is too high.

Concentration of acid in the pickle solution good for pickling.

Controlled pickling = Efficient Pickling

Controlled pickling leads to efficient pickling with the following benefits:

•Uniform pickling conditions

•High productivity

•Reduced cost for acid chemicals

•Reduced cost for waste treatment

•Improved environment conditions

Controlled pickling is a must for competitive production!

First step to efficient pickling

Analysis of free acid components is very important for the pickling process!

Pickling efficiency is directly related to Free acid:- Free acid is acid that has not yet reacted with the metals- Total acid is composed of both free acid and acid already reacted and spent

We need to measure the concentration of free acid in order to control the pickling

Knowledge of the free acid in the pickle bath composition is the first step to efficient pickling!

Second step to efficient pickling

Definitions of parameters that effect the pickling process:

Free acid concentration

Acid re-circulation (agitation)

Process temperature

Exposure time of material to acid

Four parameters effect the pickling efficiency!

Third step to efficient pickling By-products formed in an optimised pickling process:

• Dissolved metals salts- Limits the pickle bath life time

- Precipitates and forms sludge

- Frequent bath replacement and sludge leads to high environmental impact and cost for disposal

• Oxide scales (Specially Hot material)- Remains in the process as sludge

- Limits the pickle bath life time

• NOx gases formed

For a more economical and environmental sound pickling process the by-products need to be taken care of!

Conclusion

1. Analyse the pickle bath condition

- Analyser for measuring the free acid components

2. Control the pickling process

- Efficient acid re-circulation system

- Fresh acid addition system

3. Minimize pickling by-product

- Acid retardation to remove dissolved metal salts

- Mechanical filtration to remove oxide scales and other solids

- NOx suppression

The following steps are required in order to achieve an efficient pickling with high productivity,

optimal use of chemicals involved and with a minimum impact on the environment:

Thank-You