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Annealing & pickling process


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Annealing & pickling process

  1. 1. 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.
  2. 2. Steps for all HT (anneals): 1. Heating 2. Holding or “soaking” 3. Cooling Time and temperature are important at all 3 steps
  3. 3. (Stress-relief)
  4. 4. 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
  5. 5. 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 A3,1line, holding there (about 20h.or more) and then slowly cooling
  6. 6. Normalizing Allows steels to cool more rapidly, in air Produced structure – fine pearlite Faster cooling provides higher strength than at full annealing
  7. 7. Process Annealing – 3 stages Recovery (stress-relief anneals) Recrystallization (process anneals) Grain Growth
  8. 8. 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.
  9. 9. Stresses may result from: Plastic deformation (cold work, machining) Non-uniform heating (ex. welding) Phase transformation (quenching)
  10. 10. Stress-relief: Is held at fairly low temperature Is held for a fairly short time So that recrystallization does not occur
  11. 11. 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
  12. 12. TS and elongation
  13. 13. 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
  14. 14. 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
  15. 15. How hot is hot? Most metals have a recrystallization temperature equal to about 40% of the melting point K,4.0  mr TT =
  16. 16. 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
  17. 17. 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
  18. 18. Size of grains vs. temperature G R A I N S I Z E Temperature, deg.C 200 600400
  19. 19. Microscope images show: Cold rolled steel 90% reduction recrystallized after 2 830°C Grain growth after 2min @ 930°C.
  20. 20. Grain-Growth is not recommended mainly because: Energy consumption Need of expensive equipment Large grain metals get surface distortion under tensile forces
  21. 21. 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
  22. 22. 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.
  23. 23. Quenching media Involves the principles of heat transfer There are 9 possible choices (air, furnace, tap water, oil, brine etc.)
  24. 24. Pickling
  25. 25. 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!
  26. 26. Pickling chemistry H+ HF CrF3 Cr(NO3)3Fe(NO3)3 H+ HF FeF2 + NO3 - CrF2 + FeF2 + FeF3 NO3 - H+ CrF2 + FeF3 Fe(NO3)3 Cr(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
  27. 27. 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 -
  28. 28. 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 Concentration 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!
  29. 29. 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!
  30. 30. 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.
  31. 31. 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!
  32. 32. 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!
  33. 33. 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!
  34. 34. 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!
  35. 35. 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:
  36. 36. Thank-You