1. CGThermal CGThermal LLC
TWINSBURG * OHIO * USA
INTRODUCTION TO STEEL & STAINLESS STEEL PICKLING AND TITANIUM CHEMICAL MILLING
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2. CGThermal Pickling Basics
Before cold forming, hot-rolled steel is always descaled, most commonly in an operation known as pickling.
Scale consists of thin layers of iron oxide crystals
Scale itself is chemically very similar to rust, being made up of iron and oxygen bonded together in various molecular arrangements.
Like rust, scale develops at the steel’s surface where the iron is exposed to air, except that it forms at higher temperatures such as those used during hot rolling.
The scale layer grows deeper into the steel over time at a rate that increases rapidly as the temperature rises; for instance, slabs exit the reheat furnace at the Hot Mill with a layer of scale up to 1/8” thick.
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3. CGThermal Pickling Basics Con’t
The heart of the Pickling Line, its acid baths, consists of four tanks in a row containing hydrochloric acid in concentrations ranging from 2% to 10% at temperatures held between 180 and 200 degrees Fahrenheit.
Normally hot hydrochloric or sulfuric acid is used, but for some alloy steels a different acid, such as a combination of HF and nitric acid is needed.
Because of its high reactance to treatable steels, acid concentrations and solution temperatures must be kept under control to assure desired pickling rates
Hydrochloric acid is more expensive than sulfuric acid, but it pickles much faster while minimizing base metal loss.
Carbon steels, with an alloy content less than or equal to 6%, are often pickled in hydrochloric or sulfuric acid.
Steels with an alloy content greater than 6% must be pickled in two steps and other acids are used, such as nitric and hydrofluoric acid.
Most copper alloys are pickled in dilute sulfuric acid.
Brass is pickled in concentrated sulfuric and nitric acid mixed with sodium chloride and soot.
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4. Titanium Chemical Milling Basics
Chemical milling or industrial etching is the subtractive manufacturing process of using baths of temperature-regulated etching chemicals to remove material to create an object with the desired shape.
The process essentially involves bathing the cutting areas in a corrosive chemical known as an etchant, which reacts with the material in the area to be cut and causes the solid material to be dissolved
Typical applications of chemical milling include Inconel jet engine mixer components, titanium exhaust system cowlings, forgings (titanium, steel, or hi-temp), engine combustion cases, and thin wall tubing.
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5. Titanium Chemical Milling Basics Con’t
It is used in the aerospace industry to remove shallow layers of material from large aircraft components, missile skin panels, and extruded parts for airframes.
There is a very positive effect on the overall quality of the final product as well as production cost savings to control the bath temperature as precisely as possible.
Typically a HF/HNO3 solution is used. HF is expensive and many Companies have incorporated an Acid Purification Unit (APU) to remove solids from the milling solution and extend the life of the acid. Better control on the bath temperature increases the APU performance greatly.
Ti milling is an exothermic reaction, creating heat that can have a detrimental effect on quality and production rates.
The most popular industry practice is to use plastic immersion coils to maintain proper temperature.
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6. CGThermal Continuous Push-Pull Pickling Line (CPL)
The SMS Siemag-developed and patented turbulence pickling technology for removing the scale from steel strip surfaces by means of hydrochloric acid.
Shallow tanks, using high pressure spray nozzle on strip surface for cleaning action.
Experience shows either (4) or (5) tanks used in each line.
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7. Continuous Annealing and Pickling Line for stainless steel strip (CAPL)
Designed either for hot or for cold strip, and for hot and cold strip.
In both cases, the hot strip leaving the annealing and pickling line is coiled, then rolled in a rolling mill and recoiled, and finally annealed and pickled a second time as cold strip.
Typically coils are cleaner but there is a presence of oils from rolling operation. Oils increase potential for fouling.
300 series (3xx) typically could require tank heating (at start up) and steam heat to maintain temperature.
Certain 400 series (4xx) cause an exothermic reaction in the bath and require constant cooling.
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8. Hot Annealing Pickling Lines for stainless steel strip (HAPL)
Processing of stainless steel from a tandem or Steckel hot rolling mill. Thermal processing to achieve uniform metallurgical structure and subsequent surface treatment (shot blasting and pickling) prior to cold rolling.
Utilizes HF/Nitric acid combination.
Experience shows either (4) or (5) tanks used in each line.
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9. Stainless Steel , pickling Titanium, Chemical Milling
Process Overview Summary
In the below chart pickling for batch processing of steel, as present in batch galvanizing operations, is not taken in to account. In these operations typically sulfuric acid is used and heated with zirconium or graphite immersion heaters, with zirconium being the more popular.
Steel
Stainless Steel , pickling
Titanium, Chemical Milling
Bath Chemistry
2-10% HCL
5-10% HF, 15-20% HNO3
2-5% HF, 6-10% HNO3
Bath Temperature
Deg F
140 Deg F
Operation
Continuous
Continuous and batch
Batch
Control Requirements
Heating
Heating and/or cooling
Cooling
Heating Medium
Steam
Cooling Medium
Water
Heat Exchanger Technologies
Phenolic impregnated cylindrical block
TFE "Spaghetti" Immersion
Poly Coils
Tantalum Shell/Tube
TFE "Impregnated" cylindrical block
Very common to have no temp control
Tantalum Immersion coils (more rare)
Direct Steam Sparging (even more rare)
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10. CGThermal
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11. CGThermal
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12. CGThermal Umax® Ceramic Alpha Sintered
Umax Tubing – Alpha Sintered Silicon Carbide
Alpha
No Fillers
No Free Silicon
No trace Contaminations
Sintered
Over 98% Theoretical Density
50% Harder than WC
HF, BR2, HNO3, Mixed acids, H2SO4 and High fouling applications
Replaces graphite, glass, reactive metal and nickel alloy material of construction.
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13. Properties Comparison of Common CPI Materials
Umax Advanced Ceramic
Impervious Graphite
Tantalum
304SS
Borosilicate Glass
Specific Gravity
3.1
1.9
16.6
8.0
2.2
Flexural (psi)
60,000
6,380
50,750
75,000
1,000
Compressive (psi)
560,000
11,310 NA
150,000
Mod. Elast.
(x 106 psi) 59
2.3 27 28 98
CTE
(10-6 in/in f)
1.04
5.8
9.3
1.8
Conductivity
(btu/ft-hr F)
72.6 58 32
9.1
0.67
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14. Ceramic
Umax Tubing – Alpha Sintered Silicon Carbide
UNCONDITIONAL 2 YEAR GUARANTEE AGAINST CORROSION AND EROSION!
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15. CGThermal 14mm, ½”, ¾” OD tubes available Standard o-ring sizes
Single pass or Multi-pass process designs
HD and LD designs
HD – Heavy Duty psig
LD – Light Duty. 75 psig
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16. CGThermal
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17. TYPICAL PROCESS HEAD DESIGNS
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18. Tube to Tubesheet Sealing
Tubes can expand independently of tubesheet.
Easy tube replacement if required.
Back-up rings create captured sealing mechanism.
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19. ASSEMBLY PROCESS FOR UMAX® ADVANCED CERAMIC EXCHANGERS
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20. ASSEMBLY PROCESS FOR UMAX® ADVANCED CERAMIC EXCHANGERS
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Ceramic. Graphite . Heat Exchangers. Process Equipment

21. ASSEMBLY PROCESS FOR UMAX® ADVANCED CERAMIC EXCHANGERS
CGThermal
Ceramic. Graphite . Heat Exchangers. Process Equipment