Residual stresses Flashcards
What causes residual stresses?
Contraction from cooling and volume expansion from martensitic tranf
Volume expansion from martensitic transf increases with in ________ in CC, _________ Ms temp, and _____ the Ms-Mf interval
increase in CC; lower the Ms temp; higher the Ms-Mf interval
What does the decrease in volume due to cooling depend on?
austenitizing temp, temp of quenching medium, and thermal expanion coefficients of austenite and martensite
What do the extent of residual stresses depend on?
CC of steel, Ms temp, Ms-Mf, austenitizing temp, rate of cooling, quenching medium and component geometry
What happens if residual stresses exceed yield stress?
Plastic deformation
Residual stresses from thermal contraction
shell: compressive stresses
core: tensile stresses
Residual stresses from formation of martensite and thermal contraction
case 1: expansion > thermal contraction > shell: tensile stress > core: compressive stresses case 2: expansion < thermal contraction > shell: compressive stresses > core: tensile stresses > NOT common, but maybe rapid cooling of thick components can achieve this if CC isn't high case 3: expansion = thermal contraction > no residual stresses
Residual stresses in induction hardened steel
> shell: compressive stresses
core: tensile stresses
only case depth dc reaches austenitization temp
Residual stresses associated with other phase transformations in steel
> austenite –> bainite or pearlite involves less volume expansion of steel compared to martensite
no significant residual stresses occur as result
Effect of Ms temp on magnitude of residual stresses in quenched hardened steel
> higher the temp at which martensite forms, lower the volume expansion
high Ms temp, most phase transformation occurs above room temperature, smaller volume expansion, lower residual stresses, lower tendency to crack
Effect of Alloying elements on residual stresses in quench hardened steel
> Cr, Mn and Mo increase hardenability, decrease Ms temp, increases residual tensile stresses at surface; however, higher hardenability gives lower cooling rates for martensitic transformtion and less residual stress
Bo increases hardenability of steel WITHOUT decreasing Ms temp, gives lower residual stresses
Effect of CC on residual stresses in quench hardened steel
> higher CC, lower Ms, higher volume change, more residual sterss, more brittle and more cracking
Delayed cracking of quenched steel components
> excessive tensile residual stresses cause quench cracks that may delay is formation
the delay is caused by the transformation of retained austenite to martensite over time
to avoid this, temper the steel immediately after quenching
