BEHAVIOUR OF MATERIALS SUBJECT TO DEEP SUBZERO TEMPERATURES PRELIMINARY FINDINGS AND IDEAS

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BEHAVIOUR OF MATERIALS SUBJECT TO DEEP SUBZERO
TEMPERATURES PRELIMINARY FINDINGS AND IDEAS
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GENESIS

• Sub Zero Processing for Tool Steels up to 40oC
• Improve Dimensional Stability
• Eliminate Retained Austenite

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NEED FOR SUB ZERO PROCESSING

• Instabilities Developed During Manufacturing
  Process
• Unstable / Quasi Stable / Meta Stable Phases
  Developed during Hardening
• Eliminate Hardness Drop during Tempering

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MATERIAL CHARACTERISTICS

• ALL ALLOY AND TOOL STEELS
• PH HARDENING STEELS
• SUPERALLOYS
• COPPER COPPER ALLOYS
• ALUMINIUM AND ALUMINIUM ALLOYS
• PLASTICS AND POLYMERS
• PRECIOUS METALS
• CARBIDES AND SINTERED MATERIALS

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DEEP SUB ZERO TECHNOLOGY

• Operation at low temperatures (-195OC)
• Long Residence times for homogenization
• Slow cooling and heating rates

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THEORY

• Deep Sub Zero Processing based on Third Law of
  Thermodynamics
• Stress Relieving at Atomic Level
• Elimination of Point Line Defects
• Low Temperature Provides adequate DG for
  transformation

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BENEFITS IN TOOL AND ALLOY STEELS
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MANUFACTURING PROCESS FOR TOOL STEELS

• Rough Machining
• Stress Relieving
• Finish Machining
• Hardening
• Deep Sub Zero Processing
• EDM, Grinding Polishing

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HARDENING TEMPERING
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THEORY

• Annealed steel C Alloy elements as Carbides
• Ni Co in solid solution
• Heat to dissolve Carbides
• Avoid grain coarsening embrittlement

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THEORY

• Ferrite -- Austenite
• Lattice parameters change
• Carbon alloy elements occupy lattice points
• Austenite -- Martensite
• No time for C to reposition
• High microstress -- high hardness

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RETAINED AUSTENITE

• Incomplete Austenite -- Martensite Transformation
• Increase with
• Increased Alloy content
• Higher Hardening Temperature
• Longer Soaking Times

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TEMPERING

• Reheating to relieve inherent stresses
• Prevent Cracking
• Transform Retained Austenite -- Martensite
• Immediately after hardening
• Microstructure consists of
• Tempered Martensite / Martensite
• Carbides / Retained Austenite
• Tempering Dependent on Tool Steel

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PRACTICE

• Stress Relieving
• Hardening / Quenching
• Tempering

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STRESS RELIEVING

• After Rough Machining
• Y. S. low so unable to resists stresses
• Stress Relieved, lower distortion
• Right Sequence
• Rough Machining
• Stress Relieving
• Finish Machining

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HARDENING

• Vacuum / Salt Bath / Muffle Furnace / Fluidized
  Bed
• Protection against Decarburisation
• 1-2 Preheat steps
• Holding for temperature equalization

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QUENCHING

• Oil
• Salt Bath
• Air
• Cooling rate depending on Hardenability
• Avoid Bainite Formation

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TEMPERING

• Immediately after job cooled to 50 0C
• Temperature depends on
• Hardness required
• Toughness required
• Dimensional stability

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DEEP SUB ZERO PROCESSING
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WHY DEEP SUB ZERO PROCESSING

• Minimize Retained Austenite
• Improves Toughness with no change in Tensile
  Properties
• Hardness Profile Correction

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R. A. CONVERSION

• Conversion of Retained Austenite in situ to
  Temper Martensite.
• Stress relieving provides driving force for
  transformation.
• Hardness Correction across cross section

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THEORY

• Athermal Conversion of Austenite to Martensite
• Volume Change accompanies Phase Transformation
• Structural Integrity of Steel prevents complete
  transformation
• Tempering precipitates Carbides leading to Stress
  Relief further Transformation
• Prolonged Tempering leads to complete relieving
  of stresses but drop in hardness

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CARBIDE PRECIPITATION

• Eta Carbides precipitated
• Sub micron size particles characterized by shape
• Concrete Effect
• Toughness Increases, Catastrophic Shattering
  decreases significantly

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THEORY

• Alloy Element Solid Solubility decreases with
  decreasing temperature
• No preferred Precipitation Site
• Precipitation over entire matrix
• Very high Specific Surface
• Approx 10 of Total Carbide Volume

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THEORY CONCRETE EFFECT

• Eta Carbide Particles act as Binder between
  Matrix Primary Carbides
• Establish Coherency in Steel Microstructure
• Reduce Stress Concentration around Primary
  Carbides
• Minimise Chipping Tendency by promoting Abrasive
  wear

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THEORY CATASTROPHIC SHATTERING

• Minimise Stress Concentration leading to delay in
  Crack Initiation
• Secondary Carbide Particles provide barrier for
  Crack Propagation
• Crack Growth rate decreases leading to
  minimisation of Catastrophic shattering

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PH HARDENING STEELS
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MANUFACTURING PROCESS FOR PH HARDENING STEELS

• Rough Machining
• Stress Relieving
• Finish Machining
• Solution annealing
• Deep Sub Zero Processing
• Finish Tempering
• EDM, Grinding Polishing

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THEORY

• Steels hardened by precipitation of dispersed
  phase due to instability in parent phase
• Solutionising for homogenisation of phases
• Deep Sub zero processing for uniform
  precipitation of nuclei
• Finish tempering for growth of precipitate phase

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THEORY

• Lower tempering temperatures possible
• Alloys exhibit uniform distribution of phases and
  much higher physical characteristics
• Dimensionally stable and fully transformed phase
  distribution in finished component

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NON FERROUS MATERIALS, SUPER ALLOYS AND SINTERED
MATERIALS
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• Solid Solution Strengthening
• Inter atomic distances (Bragg distances) reduced
• Atomic packing density increased
• More Uniform Dispersion of nucleation sites in PH
  alloys
• Increased Mechanical Properties due to more
  ordered structure.
• Effects more dramatic in Single Phase Alloys as
  compared with Multi Phase Alloys

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• Preferably carried out in finished components as
  ductility malleability decreases significantly
• Pure metals show better results as compared to
  alloys
• Even applicable in Noble Metals such as Au, Ag,
  Pt, etc.
• Sintered carbides and other Powder metallurgical
  components exhibit varying degrees of improvement
  based on metallic nature of the product.

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OTHER IMPROVEMENTS

• Long Range order established
• Corrosion Resistance increased
• Thermal, Electrical Conductivity Hot Hardness
  improved
• Dimensional stability, structural compactness
  improved
• Grain Shape and Size refined.
• Vacancies erased, Dislocations and Stacking
  Faults eliminated by forming fresh grain
  boundaries.

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CONCLUSIONS

• EXPOSURE OF MATERIALS TO DEEP SUB ZERO
  TEMPERATURES CAUSES PERMANENT IRREVERSIBLE
  CHANGED IN THE MICROSTRUCTURE OF THE METALS
• PHENOMENON BEING EXPLORED AND SCIENTIFIC BASIS
  FOR THE IMPROVEMENTS BEING ANALYSED

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CONCLUSIONS

• BENEFITS QUANTIFIABLE IN THE CASE OF ALLOY
  STEELS.
• PRODUCT LIFE INCREASES (50 - 700)
• UNIFORM WEAR PATTERN ESTABLISHED
• AMOUNT OF REGRINDING/REDRESSING MINIMIZED
• HARDNESS INCREASE DETECTED IN NON FERROUS
  MATERIALS

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CONCLUSIONS

• DIFFERENT PROCEDURES FOR DIFFERENT MATERIALS AS
  WELL AS AT DIFFERENT STAGES IN THE MANUFACTURING
  PROCESS
• OPTIMUM PROCEDURES STILL UNDER STUDY FOR VARIOUS
  MATERIALS
• REAL AND TANGIBLE BENEFITS SEEN THOUGH EXACT
  REASONS FOR THE SAME ARE UNDER INVESTIGATION