1. Cryogenic Treatment/Tempering
Unit 89

2. Objectives Explain the cryogenic treatment/tempering process
Describe the typical cycle of the cryogenic process
Discuss the changes in metallurgy in a typical cryogenic treatment
List the advantages of cryogenic tempering

3. Deep Cryogenic Treatment
One-time, permanent process
Improves physical and mechanical properties of various materials (ferrous, nonferrous metals, aluminum and their alloys)
Uses subzero temperatures to stabilize, refine and close grain structures
Releases internal stresses for longer wearlife

4. Technical Data Austenite Crystalline form of steel
Face-centered cubic structure with iron atoms at corner and center
Form octahedron with carbon atoms in spaces
Quenching (high temp) austenite becomes martensite (different crystalline form of steel)
Ductile but soft

5. Crystal Structures Hard, but tempered to overcome brittleness
Soft and ductile
Face-centered cubic austenite cell with iron atoms at corners and center of each face
Martensite crystal with iron atoms in corners, but not in center of each face but in center of cell

6. Cold Treatment
Properties of many materials enhanced by cooling them to below room temperature
Parts cooled to -110 degrees F in vats of alcohol cooled by dry ice
Temperature range that refrigeration units can reach

7. Cryogenics
Developed methods to reduce temperatures to –300 degrees F in early 1900s
Greater benefits than cold treatment
Extension of heat treating
Heat treating: Cooling or quenching from high temperature to well below cryogenic practice
Gives steel hardness, toughness, wear resistance and ductility

8. Cryogenic Process
Temperatures of -300ºF (-185º C) required to create complete molecular change in most alloy steels
Makes most of retained austenite turn to martensite
Denser, refined mix, smaller and more uniform
Physically transforms microstructure stronger and more wear-resistant

9. The Procedure
Nitrogen circulated into vacuum-sealed chamber over period of several hours
Workpiece kept in chamber up to 36 hours
Depends upon shape of metal and total weight
Cross-sectional area, material, and bottom line factors determine rate and uniformity of temperature penetration
Changes take place at molecular level

10. The Procedure, cont. Molecules tightly packed together
Material brought back to room temperature
Molecules change back to normal separation, but molecules and complex carbides evenly spaced
Eliminates pockets of high density
Slow return to room temperature over 12 hours
Typical cycle of cryogenic process runs about three days

11. Stress
Stress in steel comes from cooling of uneven sections and machining
Create complex, invisible, random patterns
Parts expand from heat generated during operation (retained stress)
Uneven expansion, less fatigue life
Increased dimensional instability, increased wear
Stress boundary areas susceptible to microcracking

12. Residual Stress Relief
Residual stress cause parts to progressively warp when overheated
Uneven and located throughout structure
Deep cryogenic processing effective method for decreasing residual stress
Increase durability (wear life) of steel
parts finish machined do not move: thus less wear from abnormal tensions

13. Stress in Steel Tensile stresses Thermal stresses
Created by machining, boring and forming
Thermal stresses
Created after heat treating through quench hardening process
Differential of coefficients of expansion
Stress shear imparted due to differing rates of thermal growth

14. Depth of Cryogenic Stress Relieving
Stress relief
Entire mass is at equal temperature (core and surface) and cycled slowly through wide temperature range
Taking mass to extremely low temperatures creates very dense molecular state
Rate of temperature low enough and slow enough, thermal compression and expansion take place equally from core to surface
Result is homogenously stabilized material

15. Carbide Increase
Number of countable small carbides increase throughout heat treating steel
Increase in carbides adds to wear resistance
Make refined, flat, superhard surface on metal
Tests show various metals processed at -300ºF, wear resistance 2-5 times greater than that processes at -120ºF

16. Wear Resistance Deep cryogenic strengthening
Permanent, one-time process
Creates stronger, more durable tools
Improves dimensional stability
Minimizes retained austenite levels
Increases surface hardness
Improves wear properties
Some metals increase from 100% to over 800% using cryogenic treatment vs. cold treatment

17. Metallurgy Changes Typical cryogenic treatment
Slow-down rate from ambient temperature to near temperature of boiling point of liquid nitrogen
Cool-down cycle can be programmed to avoid thermal shock by using gaseous nitrogen

18. Kinetics of Cryogenic Treatment
Two theories
Transformation of retained austenite nearly complete
Strengthening material via precipitation of submicroscopic carbides and reduction in internal stresses in martensite ( reduces tendency to microcrack)
Cryogenic temperatures required to effect molecular change
E modulus (modulus of elasticity) of ferrous metals increased without changing maco-hardness of conversion hardness testing
All ferrous metals have same E-modulus

19. E modulus Measure of rigidity of metal
Ratio of stress to strain
Young's modulus (stretch or extensibility)
Obtained in tension or compression
Shear modulus (modulus of torsion)
Obtained in torsion or shear

20. E Modulus Bulk modulus Tangent modulus Secant modulus
Covers ratio of mean normal stress to change in volume per unit volume
Tangent modulus
Slope of stress-strain curve at specified point
Secant modulus
Slope of line from the origin to specified point on stress-strain curve

21. Material's Mechanical and Physical Properties
Reveal elastic/inelastic behavior where force
Mechanical applications
Modulus of elasticity, tensile strength, elongation, hardness and fatigue limit
Physical properties
Density, electrical conductivity, heat conductivity and thermal expansion
Elastic limits
Maximum stress material may be subjected without permanent strain remaining upon release of stress

22. Advantages of Cryogenic Tempering
Permanent, one-time process that creates stronger, more durable metals
Improves performance and durability
High-speed steel
Martensitic
Aluminum
Stainless steel
Titanium
Composites
Polymers
High-strength, high-alloy steels

23. Benefits of Cryogenic Treatment
Increases wear resistance of material
Cost of process very small
Changes entire structure, not just surface
Refinishing or regrinding does not effect
Closes and refines grain structures to create denser molecular structure
Transforms soft-retained austenite to martensite

24. Benefits of Cryogenic Treatment
Forms microfine complex carbide to strengthen larger carbide structures and add wear resistance
Increases performance and durability
May decreases residual stresses in tool steels
May increase tensile strength, toughness and release internal stresses