Solidification behaviour of AISI3XX

1. Influence of intrinsic solidification behaviour on
quality of AISI 3XX during continuous casting
Presented by
Mainak Saha
M.O.S. Scholar
Department of Metallurgical and Materials Engineering
NIT Durgapur
Guide: Prof. Amit Ganguly
M.O.S. Chair Professor
NIT Durgapur
INFLUENCE OF INTRINSIC SOLIDIFICATION BEHAVIOUR ON QUALITY OF CAST AND ROLLED PRODUCTS OF STAINLESS
STEEL , “A Century of Stainless steels”, Prof. S.K. Ray, M.O.S. Chair Professor, IIT Madras

2. Why should one bother about solidification
behaviour?
• Application-specific quality requirement – an essential feature of today’s discerning customers.
• Surface and sub-surface quality of continuously cast material - largely influenced by solidification condition
inside the caster mould.
• Internal quality - affected by bulging of the solidifying strand below the mould.
• For a specific caster configuration and machine condition, steel chemistry plays an important role in controlling
its solidification characteristics.
• Quality of cast material - influenced by its solidification structure, extent of microsegregation , and the high-
temperature strength and ductility.
• These are in turn, decided by the relative proportion of δ (ferrite) and γ (austenite) during and subsequent to
solidification of the specific steel grade.

3. Nieq and Creq concept and its significance
• Unlike for plain carbon steel, the Fe-C phase diagram is not indicative of the proportion of δ and γ formed during
and subsequent to solidification of stainless steels.
• Ni and Cr being the two common alloying elements, which are respectively γ and α stabilisers.
Creq = Cr + 1.65 Mo + 1.5 Si + 2 Nb + 3 Ti
Ni eq = Ni + 0.31 Mn + 22 C + 17.5 N + Cu
• The ratio of Nieq and Creq for any stainless steel grade, largely determines the fraction of γ and δ during and
subsequent to solidification.

4. Concept of Ferrite Potential(FP)
• The relative proportion of δ or γ in the solid shell can be defined on the basis of ferrite potential ( FP )
expressed as a function of chemistry.
FP = 5.26 ( 0.74 - Ni eq / Creq )
• The local cooling rate , ( dT / dt ) during solidification influences the secondary dendrite spacing , and has a minor
influence on the relative proportion of δ and γ.

5. Grades under study
Grade Cr Ni Mn Si Mo C N S P Nieq / Creq
AISI 301 16.9 6.6 1.7 0.4 - 0.07 0.025 0.01 0.04 0.52
AISI 304 18.2 8.7 1.6 0.3 0.1 0.05 0.025 0.01 0.04 0.57
AISI 316 16.7 10.7 1.7 0.3 2.1 0.05 0.025 0.01 0.04 0.62
AISI 310S 25.2 19.2 1.6 0.7 0.1 0.045 0.025 0.01 0.04 0.80
INFLUENCE OF INTRINSIC SOLIDIFICATION BEHAVIOUR ON QUALITY OF CAST AND ROLLED PRODUCTS OF
STAINLESS STEEL , “A Century of Stainless steels”, Prof. S.K. Ray, M.O.S. Chair Professor, IIT Madras
Chemistry and Computed Ratio of Nickel Equivalent and Chromium Equivalent

6. Experiment
• The phase distribution during and subsequent to solidification of the standard stainless steels have been
experimentally measured by thermo-analysis in the present investigation.
• Cast slab samples with secondary dendrite arm spacing in the range of 60 – 90 µm and cooling rate of 1⁰C / s have
been used for the study.
• The liquidus and actual solidus , as well as temperature range for δ and γ for the four standard stainless steel
grades during and subsequent to solidification have been measured.

7. Effect of chemistry on actual solidus, and temperature range
of δ and γ in standard stainless steel grades
• The mechanical strength of the strand during
solidification is influenced by the distribution
of δ and γ in the solid shell, as well as the
extent of solid / liquid region, which depends
on extent of micro-segregation.
• Micro-segregation results in the increase of
concentration of an alloying element from the
original value Co to that in the last-solidifying
liquid, CL as defined by modified Scheil
equation.
CL = C o [ 1 – f s / ( α k + 1) ] k – 1
f s is solid fraction,
k is equilibrium distribution coefficient,
back-diffusion parameter α = D s t f ( λ / 2 ) – 2
D s is diffusion coefficient in solid,
local solidification time tf = ∆ TLS / ( δ T / δ t ),
∆ TLS is temperature interval between liquidus
and solidus, secondary dendrite arm spacing λ is
proportional to ( δ T / δ t ).
• Lower values of k , DS and ∆ TLS result in high
CL, and consequently large micro-segregation

8. Comparative solidification behaviour of
different stainless steel grades
301 304 316 310S
Liquidus 1465 C 1455 C 1450 C 1420 C
Actual Solidus 1400 C 1395 C 1370 C 1195 C
Range of δ 1465-1375 C 1455-1410 C 1450-1425 C --
Range of δ + γ 1375-1255 C 1410-1320 C 1425-1350 C --
Range of γ < 1255 C < 1320 C < 1350 C <=1420 C
Solidification Mode Entirely δ δ till fs ~ 0.75
δ + γ at fs > 0.75
δ till fs ~ 0.3
δ + γ at fs > 0.3
Entirely γ
Solid Shell Thick Thick Moderate Thin
Mushy Zone Thin Thin Moderate Thick

9. Strength and Ductility of Solidifying Shell
The solid shell is subjected to different kinds of thermomechanical and transformation strains in this temperature
region. Strains can typically originate from
• Solidification and thermal shrinkage
• Transformation of δ to γ and γ to α
• Bending and bulging due to ferrostatic pressure
• Reheating of surface in secondary cooling zone
• Misalignment of guide rolls in secondary cooling zone
• Bending of strand for vertical straight mould
• Unbending of strand
It is essential for the steel shell to withstand or accommodate the generated strain , otherwise cracks would form
causing quality deterioration

10. ZST, ZDT, LIT
• The temperature corresponding to the solid fraction of 0.7, Tfs=0.7 has been found to correspond to the zero
strength temperature (ZST).
• On the other hand , the actual solidus temperature , TSA i.e. the temperature at fs=1.0 has been
experimentally found to correspond to the zero ductility temperature (ZDT).
• When two adjacent growing dendrites touch against each other (which has been found to correspond to the
temperature at fs ~ 0.9 ) , the surrounding liquid cannot penetrate and reach the crack site. This temperature
has been designated as the liquid impenetrable temperature (LIT)

11. Evolution of strength and ductility in the
solidifying shell
• It has been observed that till about 50 ⁰C below ZDT the
ductility of the solid shell is poor, and the strand cannot
withstand the different stresses which develop during
casting.
• The temperature region above ( TSA – 50 ⁰C ) is therefore
susceptible to brittle crack formation in the solidifying
strand , because the ductility of the solid shell at this
region is either zero or nominal.
• Cracks(mainly, intergranular in nature) formed in the
solidifying dendrites are accessible to the surrounding
liquid, the cracks can get healed up.

12. Grain size in solid shell
• This difference in tendency for sticking or depression of solid shell surface has a profound influence on the cast
grain size.
• Slower heat transfer inside the mould results in pronounced grain coarsening in case of the depression grades like
AISI-304 stainless steel.
• In presence of two phases, either δ and γ, or γ and liquid , rate of growth of solid grains has been found to be
constrained due to pinning by 2nd phase particle.
• Hence, higher the temperature for complete transformation to γ, more is the possibility of grain coarsening.
• Depression grade like AISI-304 stainless steel develops coarser grains of austenite.
• Coarser grains of austenite under hot spots, depression lines or deep oscillation marks are found to be responsible
for inter-granular failure and consequent embrittlement. - formation of sub-surface cracks in the cast material.

13. • Contraction strain is maximum , and bulging strain is
minimum for the ratio of 0.55.
• In contrast , bulging strain is more than contraction strain
for the ratio of less than 0.3 , resulting in prominent
sticking tendency.
Schematic on the influence of ratio of nickel and chromium
equivalents on tendency for sticking vis-à-vis depression

14. Relative sticking vis-à-vis depression behaviour related to solidification characteristics
301 304 316 310S
Solidification Mode Entirely δ δ till fs ~ 0.75
δ + γ at fs > 0.75
δ till fs ~ 0.3
δ + γ at fs > 0.3
Entirely γ
Solid Shell Thick Thick and strong Moderate Strong but thin
Mushy Zone Thin Thin Moderate Thick
δ→γ in
Brittle zone
around solidus
Minor Large Moderate --
Mould Sticking -- -- -- High
Bulging -- -- -- High
Depression Minor Large Moderate Minor

15. Important conclusions for AISI 304 and 316
• The solidification behaviour of AISI 304 grade in the stainless steel family is similar to that of the 0.1 % carbon grade
-a large fraction of δ gets transformed to γ in the sensitive temperature range between fs = 0.9 and 50 C below TSA .
• Incidence of surface crack and depression in cast material of this grade - traced to coincidence of δ → γ
transformation with the brittle temperature range in this grade.
• The contraction of the mushy zone in this crack-sensitive temperature zone has been calculated to be as high as
1.35 % for AISI 304.
• The contraction strain has been found to be 0.9% for AISI 316.
• This value is much less for AISI 301 grades solidifying completely through δ mode.

16. Important conclusions for AISI 304 and 316
• Solidification is initially through δ mode, but the δ→γ transformation starts above actual solidification
temperature, TSA , and a large fraction of δ transforms to γ in the sensitive brittle temperature region
between LIT and ( TSA – 50 ).
• Cast material of γ is thick and has high strength.
• Cast material is prone to formation of shrinkage, depression, and deep oscillation mark, along with cracks on
surface and at sub-subsurface, and coarse grains
• Tendency for non-uniform shell growth and non-uniform distribution of phases in solid material is high.

17. Solutions for AISI 304 and 316
• High (0.9 – 1.0% / m) average taper of mould.
• Low heat transfer is achieved in mould using flux with high melting (break) point and high ( > 1 ) basicity.
• Relatively higher fraction of solid slag layer with thick crystalline component is thus ensured.

18. Important conclusions for AISI 309 and 310
• Solidification, completely, through γ mode- associated with high shrinkage.
• Tendency for micro-segregation-high –large mushy zone, thin solid shell.
• Tendency for macro-segregation-high- columnar and centreline crack.
-Solutions
• Average mould taper of ~ 0.9 % / m.
• Low melting point and low (< 1) basicity of mould flux ensure better lubrication and better control on mould friction.
• Smaller dia rolls in secondary cooling zone placed at low pitch can minimise bulging
• Residuals should be low, and Mn/S ratio has to be high to reduce incidence of microsegregation , and related crack
formation at intercolumnar and intergranular sites.

19. References
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20. THANK YOU