Which factors degrade a surface?
wear and friction
corrosion
oxidation
thermal fatigue
What are the key selection criterias for selecting a surface hardening process?
surface hardness
hardening depth
What is the meaning of carburizing (also case hardening)?
Thermochemical treatment of steel consisting of surface Carbon enrichment, followed by quench hardening.
What are the two steps of carburizing?
Surface C enrichment
Quenching
What are the steps in carburizing?
What is the carburizing temperature?
900-950°C
What are the differences between direct hardening, single quench and double quenching?
Direct hardening: directly from the carburing media
Single quench: quenching after C enrichment and
following cooling to room temperature
Double quench: direct hardening (850-900°C) followed by
a second quenching from lower T (780-820°C)
Give the order of hardenability of the following steels:
Cr-Mo steels
Ni-Cr-Mo steels
Ni-Cr steels
Cr steels
C steels
What decreases the hardness at the surface in the follwoing steels?
Decrease in hardness due to retaining austenite.
What determines the steel core hardenability?
The chemical composition: Alloying elements: Cr, Mn, Ni, Mo, (B)
Until which composition increases molybdenum, chromium and
manganese the hardenability?
0.5%
What are the process parameters of carburizing?
Temperature
Time
Carburizing atmosphere
What is the carburizing reaction?
What are the temperatures and effects of a double quenching?
direct quench (850-900°C):
complete transformation into austenite to achieve fine martensite structure in the core
second quench (780-820°C):
holding until carburized layer is completely transformed into fine austenite => fine martensite at outer layer
core has no time to tranform
What are the parameters of stress reliving treatment?
Heating to 150-200°C
holding time =2h
air cooling
What are the reasons for stress reliving treatment?
transform tetragonal Martensite into cubic
reduce internal stresses
recover toughness in case hardened layer
What are the effects of introducing nitrogen into the diffucion atmosphere?
Nitrogen stabilises austenite and martensite
Nitrogen lowers the martensite start temperature
lowers carbon content of eutectoidic react.
better hardenability
better tempering behaviour
What happenes if the retained austenite transforms after hardening?
Inducing of compressive residual stresses
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