In conventional heat treatments or rolling schedules, the microstructure can be properly identified by its mean attributes. These parameters can properly predict the ductile-brittle (DB) transition temperatures measured by Charpy tests. However, if the austenite grain size distribution prior to transformation remains heterogeneous, after transformation, wider distributions of grain sizes will be obtained. In this context, the classical approaches do not properly predict the DB regime. This study analyzes the behavior of several ferrite-pearlite microstructures with different local heterogeneities. Grain size distributions, EBSD analysis identifying high angle misorientation boundaries and cleavage facet measurements were performed. These parameters have been incorporated in previous empirical expressions in order to quantify the contribution of these heterogeneities to the DB transition temperature.
Heterogeneity and Microstructural Features Intervening in the Ductile-Brittle Transition of Ferrite-Pearlite Steels
1. Heterogeneity and Microstructural Features
Intervening in the Ductile-Brittle Transition of
Ferrite-Pearlite Steels
October 29, 2013 – Montreal, Quebec Canada
R. Zubialde, P. Uranga, B. López and J.M. Rodriguez-Ibabe
puranga@ceit.es
(CEIT and TECNUN, Univ. Navarra)
San Sebastian, Basque Country, Spain
2. Introduction
• Mechanical strength is properly described by mean
grain sizes in ferrite-pearlite structures.
• Toughness prediction is not straightforward with
average grain sizes.
• Classical equations include dα and %pearlite to
predict the ductile-brittle (DB) transition temperatures.
• However, if austenite distribution is not properly
controlled
– Austenite heterogeneity → heterogeneous ferrite
distributions.
– Weakest link behavior: Coarsest grains will trigger brittle
fracture.
3. Objectives
– Analysis of the behavior of several ferritepearlite microstructures with different local
heterogeneity.
• Grain size distributions, EBSD analysis
identifying low/high angle misorientation
boundaries and cleavage facet measurements.
– Incorporation in previous empirical
expressions to quantify the contribution of the
heterogeneity to the ductile-brittle (DB)
transition temperature.
5. Material and Heat Treatments
• CMn steel
C
0.1
Heat treatment #
1
2
3
4
Mn
0.48
Si
Al
N
0.006 0.041 48 ppm
Thermal cycle
As-wrought microstructure
910ºC for 30 minutes and air cooling at 1.5ºC/s
980ºC for 30 minutes and furnace cooling at 0.1ºC/s
1000ºC for 30 minutes and furnace cooling at 0.1ºC/s
16. Fracture Initiation Ductile-Brittle Transition
#1: Test @ 27ºC
#4: Test @ -7ºC
• Energy absorbed by plastic deformation until brittle fracture happens.
• Brittle fracture initiation areas isolated by a ductile region.
• Crack energy lower than the matrix/matrix interface energy.
• First facet size 2-3 times bigger than average grain size.
29. Final Remarks
• Toughness of ferrite-pearlite microstructures:
– importance of microstructural heterogeneity.
– contribution of the largest grains in the
toughness of the material is one of the key
factors controlling brittle behavior.
– a modified equation has been proposed to
accurately predict ductile-brittle transition
temperature.
• Strategy extension to microalloyed steels with
complex microstructures
31. Heterogeneity and Microstructural Features
Intervening in the Ductile-Brittle Transition of
Ferrite-Pearlite Steels
October 29, 2013 – Montreal, Quebec Canada
R. Zubialde, P. Uranga, B. López and J.M. Rodriguez-Ibabe
puranga@ceit.es
(CEIT and TECNUN, Univ. Navarra)
San Sebastian, Basque Country, Spain