Hardenable Steels

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