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Lecture bainite, bainitic alloys and bulk nanocrystalline steel

This document discusses the development of bulk nanocrystalline steel with exceptionally high strength. It describes how bainite formation through isothermal or continuous cooling transformation can produce a nanocrystalline microstructure in steel. A specific alloy composition is presented that achieves a nanocrystalline microstructure with 20-40nm thick ferrite plates after transforming at 200C for 10 days. This results in an ultrahigh strength of 2.5GPa but maintains good ductility. The technique allows large component manufacturing and is cost effective.

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MM-501 Phase Transformation in Solids Fall Semester - 2016 Engr. Muhammad Ali Siddiqui Assistant Professor, m.siddiqui@neduet.edu.pk Department of Metallurgical Engineering NED University of Engineering & Technology, Pakistan
Bainite Formation
Upper and Lower Bainite (Microstructure) 3
Upper bainite 4Upper bainite in Fe–0.095C–1.63Si–2Mn–2Cr wt% steel transformed isothermally at 400◦C.
5 Lower bainite
By using Atomic Force microscope or Scanning Tunneling Microscope in order to study at higher Magnification. 6 Surface Relief Shape Change:Surface Relief Shape Change: Intense dislocation debris at a bainite/austenite interface. TEM
• Bainite grows at relatively high temperatures compare to Martensite. • The large strains associated with the shape change cannot be sustained by the austenite, the strength of which decreases as the temperature rises. • These strains are relaxed by the plastic deformation of the adjacent austenite. • The local increase in dislocation density caused by the yielding of the austenite blocks the further movement of the glissile transformation interface. • This localized plastic deformation therefore stops the growth of the ferrite plate so that each sub-unit only achieves a limited size which is much less than the size of an austenite grain.
8 Optical micrograph: Microstructure of lower bainite. Fe–0.8C wt% steel transformed at 300◦C, showing sheaves of lower bainite. • Black line is not a single plate it is actually a cluster of plate (thousand of plates). • Each plate is stopped by plastic accommodation.. • This produce fine structure than martensite.
9
Introduction to Bainitic Alloys
Carbides in Lower Bainitic Ferrite plates Since long range diffusion is not allowed at lower temp, so only iron carbides (like ε, η, κ, or cementite) precipitates.
Harmful Effect of Cementite θ : how can be avoided? Brittle Fracture = initiates cleavage cracks. Ductile Fracture = initiate nucleation of voids. As consequence of carbides there is reduction in Toughness.
Bainitic Alloys: (Carbide Free Alloys) 1 Fe - 0.4C 2Si 3Mn 2 Fe - 0.2C 2Si 3Mn 3 Fe - 0.4C 2Si 4Ni Wt% ?? Suppress the θ Precipitation * Mn/Ni for hardenability. Stop other transformation product * Al can do the same job as Si, but presently don’t have any prove. [2004] Role of Si, Mn & Ni =?
Microstructure of Bainitic Alloy • Ferrite + Carbon enriched films of Austenite. • No carbide particles in that material • Both strength and toughness are depend upon the scale of bainitic ferrite & films of C-enriched-γ γ
Advantage as a Results 1. Can achieve very fine structure just by phase transformation. Fine Plates of Bainite 0.2μm thick and 10μm in length. 2. Got a mixture of ferrite & films of austenite. 3. Each ferrite plate is only about 10μm long b/c of plasticity associated with shape deformation; stop it from growing, once it reaches about that length. So actually finer than martensite. 0.2 μm 10 μm
3. Tougher than all structure; strength is due to fine structure. (it is considered as an ideal microstructure; grain refinement is only mechanism for increase both strength and toughness). 4. Due to austenite in the microstructure; “H” embrittlement problem would be solved. (diffusion rate of hydrogen in austenite is slow)
Now have a look on toughness Notice that the impact transition temp is more than 100o C, so that completely unacceptable for any engineering material that below 100o C one can get fracture by cleavage. So, something is very wrong in our science?
• As soon as we apply stress over here the austenite is transformed into untampered martensite which is extremely hard and brittle. • Why do we have these large region of austenite left in our material; we have transformned isothermally? Bainite Sheaf Untransformed high carbon Austenite Microstructure of that Alloy
α γ γα T Ae3Ae1 o Carbon Concentration TemperatureFreeEnergy T 1 T 1
How many ways can one increase volume fraction of bainitic alloys?
1. Reduce the average carbon concentration; shift to the Y-axis (means lowering the “carbon”) 2. Addition / modify of substitutional solute Mn etc. shift/move the To curve to higher carbon. 3. Lower the transformation temperature but this is limited to Ms temperature.
Changed Original
Fe-0.2C -3Mn-2Si Fe-0.4C -4Ni-2Si
Product of these alloy Fig: Section of railway line What is the normal structure of Sections?
Microstructure of Pearlite
Tunnel b/w Britain and France ;under the sea
Talk about World first Bulk Nanostructured steel ever created Bulk Nanocrystalline Steel
• Imagine, a steel 1.Exceptionally strong, = GPa 2.Be made in large chunks = bulk crystalline 3.Easy to manufacture 4.Low cost which is affordable = cheap How ?
Problem: to design a bulk nanocrystalline steel which is very strong, tough, cheap ….
Before describing this novel material, it is important to review the meaning of strength,
• Put an apple on 1 m2 = 1 pa • 100 MPa = I00 million apples on 1 m2 • 1GPa = billion apples on 1 m2 • 1TPa = 1000 billion apples on 1 m2 Understanding unit
Brenner, 1956 10 GPa Theoretical Strength• Brenner achieved tensile strength = greater than 13 GPa in an iron whisker about 1.5 mm in length. • Theoretically = possible to achieve a tensile strength of 21 or 22 GPa in ideal crystals of iron.
• The strength of a crystal increases sharply as it is made smaller because the probability of avoiding defects increases. • Note these are the crystals only. • Strength collapses as we make bigger in size because of defects increases. • Now remember Aim ~ 22 Gpa, if we eliminating the defects in the materials.
1. Strengthening by Deformation • It has been possible for some time to obtain commercially, steel wire which has an ultimate tensile strength of 5.5 GPa and yet is very ductile in fracture. • made by Kobe Steel Japan. Scifer, Scientific Iron
• See strength 5.5 GPa and ductility (tie knot) • We can not make a knot with Carbon fiber which has 3.3GPa strength & virtually zero ductility. • Scifer, as the wire is known is made by drawing a dual- phase microstructure of martensite and ferrite in Fe–0.2C– 0.8Si–1Mn (wt-%) steel. • So can we make a cable bridge from this = ?
1 Denier: weight in grams, of 9 km of fibre or yarn. 50-10 Denier Scifer is 9 Denier So we can use it for cutting semi conductors not for making bridge cables
Figure: Comparison of size-sensitivity of single-crystals whiskers of iron and Scifer
2. Strengthening of Carbon Nanotubes Carbon nanotube to catalyze to grow
Morinobu Endo, 2004
Claimed strength of carbon nanotube is 130 GPa Edwards, Acta Astronautica, 2000 Claimed modulus is 1.2 TPa (1000 GPa) 6X greater than Steel Terrones et al., Phil. Trans. Roy. Soc., 2004
Space-elevator concept (originally due to Arthur C. Clark), requiring a cable 120 000 km in length.2 Cable would be launched in both directions from geosynchronous orbit at a height of 36 000 km People starting research to built an Space elevator (Russian Concept) What is wrong with this ?
as soon as make it big the strength collapses due to increase in the defects as we scale up [as we know that about Fe in 1956. (22 GPa) ] Equilibrium number of defects (1020 ) Strength of a nanotube rope 2 mm long is less than 2000 MPa.
Limit of Nanotube
•Strength produced by deformation limits shape: wires, sheets... •Strength in small particles relies on perfection. Doomed as size increases. Summary So far; we are unsuccessful to produce Bulk Nanocrystalline Steel
3. Thermomechanical processing • Smallest size possible in polycrystalline substance? • Back in 1960 (Micro alloying = dramatic change in grainsize improves the quality of steel) • 10 billion tons of steel are in service today by micro alloying only. (HSLA steels)
Yokota & Bhadeshia, 2004
Limit of Thermomechanical Treatment
Thermomechanical processing limited by recalescence Summary Need to store the heat Reduce rate Transform at low temperature Heating up the steel by itself
Courtesy of Tsuji, Ito, Saito, Minamino, Scripta Mater. 47 (2002) 893. Howe, Materials Science and Technology 16 (2000) 1264. Another problem = ?
Fine crystals by transformation 1. Introduce work-hardening capacity--- How … 2. Need to store the heat 3. Reduce rate 4. Transform at low temperature Requirement for Scale up:
Design criteria for Bulk Nanocrystalline Steel 1. It should ideally be possible to manufacture components which are large in all dimensions, not simply in the form of wires or thin sheets.
2. There are commercially available steels in which the distance between interfaces is of the order of 250–100 nm. The novelty is in approaching a structural scale in polycrystalline metals that is an order of magnitude smaller. 3. The material concerned must be cheap to produce. A good standard for an affordable material is that its cost must be similar to that of bottled water when considering weight or volume.
• The following conditions are required to achieve this: 1.
2. 3.
4.
• All of these conditions can in principle be met by the phase transformation of austenite into bainite, partly because the reaction is particularly amenable to control by either isothermal or continuous cooling heat treatment. • Furthermore, the transformation is displacive, i.e., it leads to a shape deformation which is macroscopically an invariant plane strain with a large shear component, as illustrated in figure.
There is in principle no lower limit to the temperature at which bainite can be generated. How the bainite-start BS and martensite-start MS temperatures vary as a function of the carbon concentration?
0 200 400 600 800 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Carbon / wt% Temperature/K Fe-2Si-3Mn-C wt% BS MS Temperature?
1.E+00 1.E+04 1.E+08 0 0.5 1 1.5 Carbon / wt% Time/s Fe-2Si-3Mn-C wt% 1 month 1 year Take 100 year to produce bainite at room temperature
• On the other hand, the rate at which bainite forms slows down drastically as the transformation temperature is reduced, as shown by the calculations in the right plot of Fig. • It may take hundreds or thousands of years to generate bainite at room temperature. • For practical purposes, a transformation time of tens of days is reasonable.
C Si Mn Mo Cr V P 0.98 1.46 1.89 0.26 1.26 0.09 < 0.002 wt% Low transformation temperature Bainitic hardenability Reasonable transformation time Elimination of cementite Austenite grain size control Avoidance of temper embrittlement
Temperature Time 1200 oC 2 days 1000 o C 15 min Isothermal transformation 125 o C-325 o C hours-monthsslow cooling Air cooling Quench AustenitisationHomogenisation
X-ray diffraction results 0 20 40 60 80 100 200 250 300 325 Temperature/o C Percentageofphase bainitic ferrite retained austenite
200 Å γ γ α α α Caballero, Mateo, Bhadeshia Transformation took 10 days at 200 o C C Nano tube same X
Low temperature transformation: 0.25 T/Tm Fine microstructure: 20-40 nm thick plates Harder than most martensites (710 HV) Carbide-free Designed using theory alone
Effect of Elongation due to increase in volume fraction of austenite Strain is uniform
“more serious battlefield threats”
ballistic mass efficiency consider unit area of armour
200 Å γ γ α α α Very strong 2.5GPa, 710HV Huge uniform ductility No deformation No rapid cooling No residual stresses Cheap Uniform in very large sections
Chatterjee & Bhadeshia, 2004Fe-1.75C-Si-Mn wt% 2104
Further Reading
Cobalt (1.5 wt%) and aluminium (1 wt%) increase the stability of ferrite relative to austenite Refine austenite grain size Faster Transformation C Si Mn Mo Cr V P 0.98 1.46 1.89 0.26 1.26 0.09 < 0.002
Original 5h 3/4d 63 550 Co 4h 11h 77 640 Co + Al 1h 8h 76 640 200o C 250o C 300o C Steel Beginning End % Bainite HV Original 4d 9d 69 618 Co 2d 5d 79 690 Co+ Al 16h 3d 78 690 Original 2.5h 1/2d 55 420 Co 1h 5h 66 490 Co + Al 0.5h 4h 66 490
original
Co
Co+Al
References • H.K.D.H Bhadeshia (Online Lectures)
Thanks