PG Industrial Metallurgy chapter 2

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/mm 2 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.

5. Properties and Characteristics of structure during TMP

6. 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

7. -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 . I n 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.

8. 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.

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

10. 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.

11. 2.4.1 Important features of Aus forming of steels

12. 2.4.2 Ausforming process variables Austenitizing temperature Rate of cooling form austenitizing temperature to deformation temperature Temperature of deformation Amount of deformation

13. 2.4.3 Structural changes Refinement of the martensite plates, or packets Increase in dislocation density (~10 13 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

14. 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)

15. 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.

16. 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.

17. -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.

18. 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.

19. -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

20. 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.

21. -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.

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