Annealing & pickling process

This document contains Annealing & Pickling process

2. 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.

3. Steps for all HT (anneals):
1.Heating
2.Holding or “soaking”
3.Cooling
Time and temperature are important
at all 3 steps

4. (Stress-relief)

5. Full Annealing
Heat the steel to a temperature within the
austenite (FCC, γ) phase region to dissolve
the carbon. (50 deg.F above A
3
-A
cm
line)
The temperature is kept at the bottom of this
range to minimize growth of the austenitic
grains. Then, after cooling ferrite (a) and
cementite structures will be fine as well

6. Spheroidizing – improving
machinability
Used on steels with carbon contents above
0.5%
Applied when more softness is needed
Cementite transforms into globes, or
spheroids
These spheroids act as chip-breakers – easy
machining
Performed by heating to just below A
3,1
line,
holding there (about 20h.or more) and then
slowly cooling

7. Normalizing
Allows steels to cool
more rapidly, in air
Produced structure –
fine pearlite
Faster cooling
provides higher
strength than at full
annealing

8. Process Annealing – 3 stages
Recovery (stress-relief anneals)
Recrystallization (process anneals)
Grain Growth

9. Stress-relief Annealing
Heats the steel to just below the eutectoid
transformation temperature (A
1
) to remove the
effects of prior cold work and grain
deformation.
This allows further forging or rolling
operations.

10. Stresses may result from:
Plastic deformation (cold work, machining)
Non-uniform heating (ex. welding)
Phase transformation (quenching)

11. Stress-relief:
Is held at fairly low temperature
Is held for a fairly short time
So that recrystallization does not occur

12. 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

13. TS and elongation

14. 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

15. 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

16. How hot is hot?
Most metals have a recrystallization
temperature equal to about 40% of the
melting point
K,4.0

mr TT=

18. Minor factors for recrystallization
Pure metal
If an alloy – host atom – solvent
foreign atom – solute
Solute atoms inhibit dislocations motion, higher
temperature is needed
Insoluble impurities (oxides and gases) become
nucleation sites and refine grains
Smaller initial grain size will recrystallize easier –
at less temperature and time

19. 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

20. Size of grains vs. temperature

G
R
A
I
N
S
I
Z
E
Temperature, deg.C
200 600400

21. Microscope images show:
Cold rolled steel
90% reduction
recrystallized after
2 min.at 830°C
Grain growth after
2min @ 930°C.

22. Grain-Growth is not recommended
mainly because:
Energy consumption
Need of expensive equipment
Large grain metals get surface distortion
under tensile forces

23. 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

24. 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.

25. Quenching media
Involves the principles of heat transfer
There are 9 possible choices (air, furnace,
tap water, oil, brine etc.)

26. Pickling

27. What is pickling?
Removal of annealing oxides and mill scales.
Removal of Chromium depleted zone
STAINLESS STEEL
HNO
3
/ HF
CHROMIUM-DEPLETED ZONE
SCALE
Mechanism of pickling, HNO
3
/ HF
Pickling is the final step in making stainless steel corrosion resistant!

28. Pickling chemistry
H
+
HF
CrF
3
Cr(NO
3
)
3
Fe(NO
3
)
3
H
+
HF
FeF
2
+
NO
3
-
CrF
2
+
FeF
2
+
FeF
3
NO
3
-
H
+
CrF
2
+ FeF
3
Fe(NO
3
)
3
Cr(NO
3
)
3
CrF
3
NO
3
-
HF
Chemical composition in the pickling bath
CHEMICAL REACTIONS
Dissolving of metals:
Fe + 4H
+
+ NO
3
-
« Fe
3+
+ NO + 2H
2
O
Cr + 4H
+
+ NO
3
-
« Cr
3+
+ NO + 2H
2
O
3Ni + 8H
+
+ 2NO
3
-
« 3Ni
2+
+2NO + 4H
2
O
Complex reactions:
3HF + Fe
3+
® FeF
3
+ 3H
+
2HF + Fe
3+
® FeF
2
+
+ 2H
+
3HF + Cr
3+
® CrF
3
+ 3H
+
2HF + Cr
3+
® CrF
2
+
+ 2H
+
HF + Ni
2+
® NiF
+
+ H
+
The relation between HNO
3
and HF is important for the pickling process!
Pickle tank

29. Mixed acid pickling
Role of HNO
3
:
-H
+
Generator
-Powerful Oxidising Agent
-Brightener for the pickled product
Role of HF:
-Complexing Agent for Fe
3+
, Cr
3+
, Ni
2+
-H
+
supplier

Environmental impacts from:
-Emission of NO
X
gases
-Presence of NO
3
-
and NO
2
-

30. Uncontrolled pickling
Acid
conc.
Metal
conc. Time
DUMP SPENT BATH
Over pickled surface
Waste of material
High chemical consumption
Rapid destruction of pickle
solution
Bad surface quality
Production rejects
Acid must be wasted
Sludge formation
C
o
n
c
e
n
t
r
a
t
i
o
n
Uniform surface quality
Less production rejects
Best 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!

31. 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 surface
Waste of material
High chemical consumption
Rapid destruction of pickle
solution
Bad surface quality
Production rejects
Acid must be wasted
Sludge formation
Uniform surface quality
Less production rejects
Best use of chemicals
involved
ZONE 1 ZONE 2 ZONE 3
Uncontrolled pickling results in bad quality in the material produced!

32. Uncontrolled pickling (cont.)
Over pickled surface
Waste of material
High chemical consumption
Rapid destruction of pickle
solution
Bad surface quality
Production rejects
Acid must be wasted
Sludge formation
Uniform surface quality
Less production rejects
Best 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.

33. 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!

34. 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!

35. 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!

36. 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
•NO
x
gases formed
For a more economical and environmental sound pickling process the
by-products need to be taken care of!

37. 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
- NO
x
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:

38. Thank-You