Hardenable Steels
Post on 19-Jan-2016
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Hardenable Steels
Carbon0.15-0.8%
Alloy> 1%
Resistance Welding
Lesson ObjectivesWhen you finish this lesson you will understand:•
Learning Activities1. View Slides; 2. Read Notes, 3. Listen to lecture4. Do on-line workbook
Keywords
Alloying Element Effect on Spot WeldingAluminum,Silicon,Calcium
Purifiers in Steelmaking
Carbon Increased Hardness/Strength Increased Hardenability
Post Heat Requiredabove 0.15%
Nickel Increased Hardenability Increased Toughness Mild Corrosion Resistance
Higher ForceRequired
Molybdenum
Increased Hardenability Increased Hot Strength/Creep
Higher Forcesrequired
Chromium Increased Corrosion Resistance Increased Hardenability
Little Effect
Vanadium Grain Refinement Slightly HigherForces
Sulfur Improved Machinability Hot ShortnessManganese Increased Tensile Strength >2% Need Special
ProceduresSilicon Solid Solution Strengthening
Some Corrosion Improvement >1% Electrical Steels (Brittle)
>1% Need SpecialProcedures
Copper Corrosion Resistance Strengthening
Probably None
Aluminum/Nitrogen
Grain Refinement
Tungsten High Speed Tool Steels Hot Hardness
High Force Post Heat
Cobalt Reduces Hardenability (unusual)
% Carbon
Har
dnes
s
TIME
TEM
PE
RA
TUR
E
Martensite
Ferrite +Pearlite
Tempering Temperature - 1 hr.
Hard
ness
0.35% C
0.8% C
1.2% C MetallurgicalConsiderations
Finer GrainCobalt
Mn, Ni, Mo, C, Cr
Factors to be Considered When Welding Hardenable Steels
• Surface Cleaning• Weld Force• Weld Current• Weld Time• Upslope• Downslope• Indentation• Preheat• Post Weld Temper
Surface Cleaning
Scale usually more tenacious (especially Cr, Cu)Surface Cleaning must be more complete
• Pickle (preferred)• Grind / Polish• Shot Blast
Weld Force
Alloy Grades have:• Higher Strength Than Plain Carbon Steels• High Hot Hardness Than Plain Carbon Steels
Therefore:• An increase in weld force (up to 10% greater) is needed over plain carbon steel at the same gage
Weld Current
• Base Metal Resistance is Higher Than Plain Carbon Steel Because of Higher Alloy Content
Therefore:• Lower Currents are needed (approximately 90%of that required for Plain Carbon steel)
Weld Time
For Lower Alloy Grades:A longer weld time by as much as 2 to 3 times helps.
TIME
TEM
PE
RA
TUR
E
Martensite
Ferrite +Pearlite
Short TimeHigher CurrentRapid Cooling
Long Time Lower CurrentSlower Cooling
For Higher Alloy Grades:Nose is too far back and longer weld time is not effective
Long Time Lower CurrentSlower Cooling
Indentation
Weld Cycles
Cur
rent
Upslope
• Helps Set Electrodes• Helps Bring Parts Together• Some slight enlargement of HAZ but no great effect on Cooling Rate
TIME
TEM
PE
RA
TUR
E
Martensite
Ferrite +Pearlite
Weld Cycles
Curr
ent
Downslope
With Downslope
WithoutDownslope
Time in Cycles
Curr
ent
Tem
pera
ture
Temper Time
Har
dnes
s
Weld Time
Quench Time
TemperTime
Temper Heat
Short Quench Time
Time in Cycles
Cur
rent
Tem
pera
ture
% Carbon
Min
Que
nch
Tim
e
Time
Tem
pIncrease CarbonMoves Range Down
10101020
10351045
Thicker Material
Minimum Quench Time
Minimum Temper Heat
% Carbon
Min
Tem
per
Hea
tNiCrMo (86xx)CrMo(4xxx)More Resistant To Tempering
Plain Carbon
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