Chapter 2: HTMT and LTMT processes

1. Chapter –II: High temperature & Low temperature
thermo-mechanical Processes
2.1 Introduction
2.2 Controlled rolling
2.3 Hot-cold working
2.4 Ausforming
2.4.1 Important features of Aus forming of steels
2.4.2 Aus forming process variables
2.4.3 Structural changes
2.4.4 Strengthening factors
2.4.5 Important applications
2.5 Iso –forming
2.6 Cryo-forming or zerolling
2.7 Mar-straining

2. 2.1 Introduction
High Temperature Thermo-mechanical Treatment
 HTMT, in contrast to LTMT , can be performed on any steel of moderate
hardenability. The deformation forces are significantly lower at the temperatures
for HTMT, and the 20% to 40% deformation required for optimum properties is
usually less than half that required for LTMT .
 The need for an auxiliary furnace to cool down to the deformation temperature is
less critical since deformation is usually performed either at the austenitizing
temperature or slightly below.

3. -cont’d-
 Recent research on HTMT includes detailed characterization of the mechanical
behavior, as well as a refinement of processing variables.
 For example , a calibration curve was developed based on several low alloy steels
whereby the hardenability for HTMT can be determined from available SHT data.
 HTMT was found to increase the elastic limit and decrease the shear modulus.
HTMT increases toughness, elongation, reduction of area, impact energy, and
possibly strength (yield and tensile) while decreasing the transition temperature.
 Temper embrittlement (450°C to 600°C) is reduced by a change from intergranular
(along prior austenite grain boundaries) to a ductile, fibrous fracture. The tensile
strength rarely exceeds 300 kg/mm2 due to the onset of recrystallization.

4. -cont’d-
 For carbon , low alloy, and medium alloy steels , where the recrystallization kinetics are
sufficiently rapid , there is a definite limit to deformation level, deformation temperature
and/or deformation rate.
 For high alloy steels dynamic recrystallization is not a serious problem although heavy
reductions can produce excessive work hardening, similar to LTMT, so that ductility
eventually drops with increasing strength.
 The quenching requirements on completion of HTMT also differ for these steels. Rapid
quenching after deformation is required of carbon and low alloy steels to retain the
dynamic polygonized structure. Medium and high alloy steels are in a work-hardened
condition, and therefore some holding time is required before quenching for static
polygonization.

7. Properties and Characteristics of structure during TMP

8. 2.2 Controlled rolling
 Very high strength levels are obtained by controlled rolling. This process consists
of heating steel above the upper critical temperature, i.e., stable austenitic
temperature range. Austenite thus obtained is deformed, and conditions are so
maintained that fine grains of recrystallized austenite are obtained.
 The grain growth tendency is checked by the hot working process variables and by
the presence of second phase particles.
 Second phase particles are generally carbides of micro-alloying elements such as
niobium, vanadium and titanium. Fine austenitic grains will result in fine ferritic
grains in the final structure. Ferritic grains nucleate at austenitic grain boundaries

9. -cont’d-
 Carbides of micro-alloying elements not only control the growth of austenitic
grains but also retard the rate of recrystallization. However, the carbide of micro-
alloying elements are effective only up to about 1050°C, and so rolling should be
performed below this temperature.
 Heavy deformation during rolling elongates the austenitic grains, thereby
increasing the grain boundary area. This results in the availability of larger number
of nucleation sites for ferrite.
 In order to have maximum strengthening, heavy deformation and low finishing
temperature should be chosen. The process is widely employed for high strength
low alloy steels.

10. 2.3 Hot-cold working
 Hot-Cold working process consists of heating steel above the upper critical
temperature. Stable austenite present at this temperature is deformed heavily in such
a way that no recrystallization takes place.This non-recrystallized austenite is
transformed into martensite by rapid quenching.
 In this process, work is carried out at minimum possible temperature above the
austenitizing temperature. In order to control recrystallization, alloying elements
such as vanadium, titanium or niobium are added to steel. The steel so obtained
strong directional properties.
 Mechanical properties, such as strength, ductility, impact and fatigue strength are
considerably improved by this process.

11. -cont’d-
Schematic representation of HTMT and LTMT processes Hot –Cold working TMT cycle

12. 2.4 Ausforming
 Ausforming is consists of heating steel above the upper critical temperature so as
to get austenite. This austenite is supercooled to a temperature below the
recrystallization temperature of the steel. The austenite so supercooled is deformed
heavily. It is then quenched to obtain completely martensitic structure and then
tempered.
 Not all steels can be given this treatment. Only steels which possess sufficient gap
between pearlitic and bainitic C-curves are suitable for this purpose. In addition
the pearlitic and bainitic C-curves should have sufficiently long incubation period.
This ensures availability of sufficient time for deformation.

13. 2.4.1 Important features of Aus forming of steels

14. 2.4.2 Ausforming process variables
 Austenitizing temperature
 Rate of cooling form austenitizing temperature to deformation temperature
 Temperature of deformation
 Amount of deformation

15. 2.4.3 Structural changes
 Refinement of the martensite plates, or packets
 Increase in dislocation density (~1013 cm-2) in martensite. Martensite plates may
have inherited fine dislocation substructures from austenite.
 Change in the size, amount and distribution of carbides.
 Development of texture in the martensite

16. 2.4.4 Strengthening factors
 Major contribution is due to fine dispersion of alloy carbides associated with
dislocations.
 The presence of alloying elements which raise the stacking fault energy (SFE) of
austenite, for example Ni, raises SFE, reduces the strengthening effect.
 In contrast, the strengthening effect associated with ausforming is increased
considerably in the presence of elements which reduces the SFE of the austenite.
(Mn lower SFE, raises rate of work hardening)

17. 2.4.5 Important applications
 Unusual high fatigue and torsional strengths of ausformed steels suggest that they
can be used in vehicle suspension systems, such as, torsion bars, coil springs,
etc.
 Their high hardness, toughness and elevated temperature strength recommend
them for use in tools, such as punches, dies, cutting tools and shears.
 Other possible applications are in high strength bolts. Aircraft parts, such as
landing gears, structural panels, high strength forgings, and also in agricultural
and earth moving equipments.

18. 2.5 Iso –forming
 The isoforming process consists of deforming steel below the lower critical
temperature during transformation. The resultant product of transformation may
be either fine pearlite or bainite, depending on the prevailing conditions. The
process is called isoforming because transformation proceeds isothermally.
 The steel is first heated above upper critical temperature and then quenched
immediately to a temperature of about 650°C, i.e., in the vicinity of nose of the
TTT curve . Mechanical working is carried out at this temperature.

19. -cont’d-
 Sufficient time should be available at this temperature for
carrying out the deformation process and for the
metastable austenite to transform isothermally to pearlite.
Just after the completion of the transformation, steel is
quenched. The larger the deformation or lower the
deformation temperature, the greater is the level of
strength developed in the steel.
 Bainitic structure can be achieved in the final product in
the same way as discussed above with minor
modifications. In this case stable austenite is supercooled
to a temperature range where it transforms to bainite,
steel is deformed during the transformation of metastable
γ to bainite.

20. 2.6 Cryo-forming or zerolling
 It consists of heating steel above the upper critical temperature. From this
temperature, steel is rapidly quenched to sub-zero temperature. Then it is
plastically deformed at sub-zero temperature, which is accompanied by high rate
of work hardening.
 The transformation of a part of austenite to martensite takes place during
deformation, and martensite thus produced has better yield strength, tensile
strength and hardness.
 When austenite gets transformed into martensite at sub-zero temperature, a noise
similar to crying is produced. This crying like sound is produced because both
deformation and transformation proceed simultaneously.

21. -cont’d-
 The process is well suited to steels which
cannot be strengthened by cold working
because of the high rate of work hardening ,
resulting in loss of ductility in rapid rate.
 The only drawback associated with the process
is that a part of austenite is stabilized. This in
turn transforms to hard and brittle martensite
during service at room temperature.
 Martensite so formed may cause brittleness

22. 2.7 Mar-straining
 In the marstraining process, steel is heated above austenitizing temperature, followed by
rapid quenching so as to get a martensitic structure. Since as as-quenched is very hard and
brittle, it is partially tempered to restore ductility.
 The ductility martensite thus obtained is cold worked. Only small deformations can be
employed in this case because of the rapid rate of work hardening of martensite. This cold
worked structure is re-tempered.
 The second tempering temperature should be lower than the first one. The process produces
strain ageing and results in significant improvement in yield strength and tensile strength
levels. It is believed that epsilon carbide formed at low tempering temperature dissolves
during deformation.

23. -cont’d-
 The dislocation-carbon interaction thus obtained hinders the movement of
dislocations on re-tempering, and mechanical strength of the steel is improved.
Since bainite is relatively soft as compared to martensite, it can be cold worked
easily.
 The strain tempering response of bainite is found to be better than that of
martensite in the sense that, for a given strength value, better ductility can be
obtained.
 The first stage of the process, i.e., pre-tempering, which is carried out to impart
some ductility to the steel for cold working, can be dispensed with in the case of
strain tempering of bainite.

24. End of Chapter 2: High temperature & Low temperature
thermo-mechanical Processes