Cryogenic Treatment/Tempering Unit 89

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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