CNC Machining Hardening Steel Parts
Hardening Service Means That With The Increase Of Cold Deformation Degree, The Strength And Hardness Of Metal Materials Are Improved, But The Plasticity And Toughness Are Decreased.
The Strength And Hardness Of Metal Materials Increase When Plastic Deformation Occurs Below Recrystallization Temperature, Which Hinders The Further Deformation Of Metal, While The Plasticity And Toughness Decrease. Also Known As Cold Hardening. The Reason Is That During Plastic Deformation, Grains Slip And Dislocation Entanglement Occur, Resulting In Grain Elongation, Breakage And Fibrosis, And Residual Stress In The Metal. The Degree Of Work Hardening Is Usually Expressed By The Ratio Of Microhardness Of The Surface Layer After Work And The Depth Of The Hardened Layer Before Work.
Work Hardening Can Also Occur In Nanomaterials. At This Time, The Hardening Behavior Is Considered To Be Closely Related To Dislocation Movement.
Work Hardening Brings Difficulties To The Further Processing Of Metal Parts. For Example, In The Process Of Cold Rolling, The Hardening Will Become Harder And Harder, So Intermediate Annealing Should Be Arranged In The Process Of Processing To Eliminate The Work Hardening By Heating. For Example, In The Cutting Process, The Surface Of The Workpiece Is Brittle And Hard, Thus Accelerating Tool Wear And Increasing Cutting Force.
On The Other Hand, It Can Improve The Strength, Hardness And Wear Resistance Of Metals, Especially For Pure Metals And Some Alloys Which Can Not Be Enhanced By Heat Treatment. For Example, Cold-Drawn High Strength Steel Wire And Cold Coil Spring Are Used To Improve Their Strength And Elastic Limit By Cold Working Deformation. For Example, Crawlers Of Tanks And Tractors, Jaw Plates Of Crushers And Turnouts Of Railways Are Also Made Use Of Work Hardening To Improve Their Hardness And Wear Resistance.
Role In Mechanical Engineering
1, After Cold Drawing, Rolling And Shot Peening (See Surface Strengthening), The Surface Strength Of Metal Materials, Parts And Components Can Be Significantly Improved.
2, Local Stresses In Some Parts Often Exceed The Yield Limit Of The Material, Resulting In Plastic Deformation. Work Hardening Restricts The Development Of Plastic Deformation And Improves The Safety Of Parts And Components.
3, When Metal Parts Or Components Are Stamped, Their Plastic Deformation Is Accompanied By Strengthening, Which Transfers The Deformation To The Hardened Part Around Them. The Cold Stamping Parts With Uniform Cross-Section Deformation Can Be Obtained By Repeated Alternating Action.
4, The Cutting Performance Of Low Carbon Steel Can Be Improved And The Chips Can Be Easily Separated. But Work Hardening Also Brings Difficulties To The Further Processing Of Metal Parts. For Example, Cold-Drawn Steel Wire, Because Of Work Hardening, Further Drawing Consumes a Lot Of Energy, Even Is Broken, So It Must Be Annealed In The Middle To Eliminate Work Hardening Before Drawing. For Example, In Order To Make The Surface Of The Workpiece Brittle And Hard In The Cutting Process, The Cutting Force Is Increased And The Tool Wear Is Accelerated.
Practical Significance In Production
Benefits: Work Hardening Is One Of The Ways To Strengthen Metals (Increase Strength), Especially For Pure Metals And Metals That Cannot Be Strengthened By Heat Treatment. For Example, Cold-Drawing, Rolling And Shot Peening Can Be Used To Improve The Surface Strength Of Metal Materials, Parts And Components; Or When Parts Are Subjected To Force, Local Stress Often Exceeds The Yield Limit Of Materials, Resulting In Plastic Deformation. As Work Hardening Restricts The Development Of Plastic Deformation, The Safety Of Parts And Components Can Be Improved. Degree;
Disadvantage: Work Hardening Improves Deformation Resistance And Makes It Difficult For Metal To Continue Working. For Example, Cold-Drawn Steel Wire, Because Of Work Hardening, Further Drawing Consumes a Lot Of Energy, Even Is Broken, So It Must Be Annealed In The Middle To Eliminate Work Hardening Before Drawing. For Example, In The Process Of Cutting, It Will Make The Surface Of The Workpiece Brittle And Hard, Increase Cutting Force And Accelerate Tool Wear.
Factors Affecting Work Hardening Of Surface Layer
Work Hardening Of Surface Layer Is a Process In Which The Metal On The Surface Of Workpiece Is Subjected To Cutting Force, Resulting In Strong Plastic Deformation, Which Seriously Distorts The Character Of The Metal, Breaks Up, Elongates And Fibrozes The Grains, Thus Hindering Further Deformation Of The Metal, Increasing The Surface Hardness And Reducing The Plasticity Of The Workpiece.
However, Under Certain Conditions, The Heat Of Cutting Will Cause The Hardening Of The Workpiece Surface To Recover (The Hardened Metal Returns To Its Normal State), Which Is Also Called Softening; Higher Temperature Will Also Cause Phase Transformation. Therefore, The Final Work Hardening Of The Machined Metal Surface Is The Result Of The Combination Of Hardening, Softening And Phase Transformation.
The Factors Affecting Surface Hardening Are As Follows:
(1) Cutting Force: The Greater The Cutting Force, The Greater The Plastic Deformation, The Greater The Hardening Degree And The Deeper The Hardening Layer. Therefore, Increasing The Feed Cutting Depth And Reducing The Front Angle Will Increase The Cutting Force And Make The Work Hardening Serious.
(2) Cutting Temperature: The Heat Generated During Cutting Will Soften The Surface Hardening Of The Workpiece Most, So The Higher The Cutting Temperature, The Greater The Recovery Degree Of The Surface Hardening.
(3) Deformation Speed (Cutting Speed): When The Deformation Speed Is Fast, The Contact Time Of The Workpiece Is Short And The Plastic Deformation Is Insufficient, So The Hardening Degree Will Be Reduced.
(4) When The Hardness Of The Workpiece Material Is Low And The Plasticity Is Large, The Hardening Of The Cutting Surface Is Serious.
Elimination Of Work Hardening
There Are Several Ways To Eliminate Work Hardening:
1, Recrystallization Annealing: Heat The Cold-Deformed Metal Above The Recrystallization Temperature, And Then Cool It After Holding For a Certain Time To Recrystallize The Heat Treatment Process. In Production, Recrystallization Annealing Is Used To Eliminate Work Hardening And Improve Plasticity Of Processed Products. Residual Stress Can Also Be Completely Eliminated. Recrystallization Annealing Occurs Sometimes During Cold Deformation, Which Is To Restore Plasticity For Further Processing. After The Heating Temperature Of Cold Deformed Metal Is Higher Than That Of Recovery Stage, When The Temperature Continues To Rise, The Microstructures Of The Metal Change Obviously From Broken, Elongated Or Flattened Grains To Uniform And Fine Equiaxed Grains Due To The Increase Of Atomic Activity.
This Process Is Essentially a Process Of Recrystallization And Growth Of New Grains, So It Is Called “Recrystallization”. After Recrystallization, Only The Grain Shape Has Changed, But The Lattice Type Has Not Changed, And It Is Still The Same As The Original Grain. Recrystallized Nuclei Are Usually Formed At The Grain Boundaries Or Slip Bands Of Deformed Grains And At Places Where Lattice Distortion Is Serious. After Nuclei Are Formed, They Grow Around By Atom Diffusion Until The Nuclei Grow Up To Contact Each Other And Form New Equiaxed Grains.
Through Recrystallization, The Microstructures Of Metals Have Undergone Radical Changes, So The Strength And Hardness Of The Metals Have Been Significantly Reduced, While The Plasticity And Toughness Have Been Greatly Improved, The Work Hardening Phenomenon Has Been Eliminated, And All Mechanical And Physical Properties Of Deformed Metals Have Been Restored To The State Before Cold Deformation. Therefore, Recrystallization Is Mainly Used In Industry To Reduce The Hardness And Increase The Plasticity Of Metals After Cold Deformation Or During Deformation, So As To Facilitate Further Processing. Such Heat Treatment Is Called Recrystallization Annealing.
2, Solid Solution Annealing(Commonly Used Method For Chromium-Nickel Stainless Steel): That Is, Carbide Solution Annealing, a Process Of Removing Carbide Precipitation (i.e. Carbon Escaping From Stainless Steel Solid Solution) By Heating The Finished Product To 1010 Degrees Celsius, And Then Cooling It Rapidly, Usually By Water Quenching, The Carbides Contained In It Are Returned To The Solid State Of Stainless Steel. In Bulk Solution. Solution Annealing Can Be Applied To a Series Of Alloy Steel And Stainless Steel Components. Solution Treatment Of 300 Series Stainless Steel Castings Can Produce a Homogeneous Microstructure Without Carbide Impurities. For Precipitation Hardening Alloy Castings And Forgings, Solution Annealing Can Produce Soft Microstructures.