Heat Treatment and its Applications for Rehabilitating Steel Bridges in Indiana

1. Heat Treatment and its Applications for Rehabilitating Steel Bridges in Indiana Amit Varma and Matt Lackowski School of Civil Engineering Purdue University 11/12/2006

2. OUTLINE • Heat treatment and its relation to heat straightening • Heat straightening survey findings • Feasibility study report

3. Heat Treatment • Introduction • Steel Metallurgy • Steel Micro-Constituents • Effects of Plastic Deformations • Effects of Heat Treatment • Implications

4. Introduction • Heat treatment is used to change the metallurgy and the related material, structural, and surface properties of steel • Different heat treatment processes have different effects on the resulting microstructure and structural properties including strength, fracture toughness, and ductility of steel. • Heat treatment has commonly been used during the steel manufacturing process to improve the quality or properties of steel, but it can also be used to repair damage or rehabilitate older or used steel.

5. Steel Metallurgy: LC vs. HSLA steel • Low-carbon steels contain up to 0.25% carbon along with various other elements • manganese (up to 1.65%), sulfur (up to 0.05%), phosphorous (up to 0.04%), silicon (up to 0.60%), and copper (up to 0.60%) • High-strength low-alloy (HSLA) steels have lower carbon contents (0.05-0.25% C) • Manganese contents up to 2.0%, and small quantities of chromium, nickel, molybdenum, copper, nitrogen, vanadium, niobium, titanium and zirconium added in various combinations • High-Strength Low-Alloy steels provide better mechanical properties and/or greater resistance to atmospheric corrosion, better formability, and weldability. • The microstructure of low-carbon and HSLA steel is similar in terms of its micro-constituents • The difference is that HSLA steels typically have smaller grain sizes.

6. Steel Metallurgy: Phase Transformation • Low-carbon and HSLA steels contain less than 0.8% carbon, which classifies them as hypoeutectoid steels • Hypoeutectoid steels consist of pearlite and proeutectoid ferrite

7. Steel Metallurgy: Phase Transformation • At high temperatures exceeding the Ac3 temperature, the hypoeutectoid structure is in a stable -austenite phase • As the steel cools below the Ac3 temperature: • Proeutectoid ferrite will nucleate and grow at the austenite grain boundaries • The amount of proeutectoid ferrite formed will continue to grow and the austenite will increase in percent carbon content. • At the Ac1 temperature (approximately 1340F): • The remaining austenite will transform into pearlite. It consists of both ferrite and cementite (Fe3C) layers

8. Steel Metallurgy: Alloy Effects • In HSLA steels, the addition of different alloying elements changes: • the physical and structural properties • the eutectoid temperature (Ac1) • the temperature that defines stable austenite (Ac3) • Empirical equations relate the effects of alloying elements to the critical transformation points • Manganese (Mn) and nickel (Ni) are distinguished as austenite-stabilizing elements that lower the eutectoid temperature expanding the temperature range over which austenite is stable • Silicone (Si), chromium (Cr), molybdenum (Mo), arsenic (As), niobium (Nb), and tungsten (W) are distinguished as ferrite stabilizing elements which raise the eutectoid temperature of the steel and reduce the range over which austenite is stable

9. Steel Metallurgy: Grain Size • The grain size focuses on the size of the a-ferrite grains. HSLA steels usually have smaller (fine) graints • Fine grains are preferred over coarse (large) grains because: • Higher strength • Slightly increased ductility • Better fracture toughness • Finer-grained steels have more grain boundaries that act as barriers to dislocations • A higher density of grain boundaries will produce higher yield and tensile strengths • Plane strain fracture toughness normally increases with a reduction in grain size when composition and other microstructure variables are maintained constant • Decreasing the grain size significantly decreases the transition temperature governing the ductile to brittle fracture

10. Steel Micro-Constituents • When steel is cooled from temperatures higher than the phase transformation temperatures (Ac1 or Ac3), a number of micro-constituents form depending on the cooling rate • Two types of time-temperature-transformation (TTT) diagrams are used to predict the formations of different micro-constituents in steel after cooling below the Ac1 temperature • An isothermal TTT diagram, which plots temperature vs. logarithm of time for steel alloys of definite composition. It is used to determine when transformation begins and ends for an isothermal (constant temperature) heat treatment. • A continuous cooling transformation (CCT) diagram is a plot of temperature vs. logarithm of time for a steel alloy of definite composition. It is used to determine when transformations begin and end when a previously austenized alloy is cooled continuously at a specified rate.

11. Superimposed Continuous Cooling Diagram of a AISI (0.12%C) Steel

12. Steel Micro-Constituents: Martensite • Martensite is formed from austenized iron-carbon alloys that are rapidly cooled or quenched to a relatively low temperature • For alloys containing less than 0.6 wt% Carbon (LC and HSLA), the martensite grains form as long and thin plates that are aligned side by side and parallel to each other. • Martensite is the hardest, strongest, and the most brittle form of micro-constituent for an iron-carbon alloy • It has extremely low ductility and fracture toughness • The formation of martensite during heat treatment of a steel bridge needs to be strictly avoided. • Bainite is extremely difficult to form. It is usually only seen in labs rather than as the results of manufacturing or heat treatment process

13. Steel Micro-Constitutents: Pearlite • Pearlite • Consists of layers of -ferrite and cementite (Fe3C) layers • Cementite is much harder and more brittle than ferrite • Pearlite increases the hardness, yield strength, and ultimate strength of steel but decreases the ductility and toughness • Dependent on the cooling period, the resulting pearlite may be fine or coarse grained • Coarse-grained pearlite will form upon longer cooling periods. Fine pearlite is harder and stronger, but more brittle.

14. Effects of Plastic Deformations on Steel Microstructure • Plastic deformation can be produced by cold-working or hot-working • The change in microstructure produced by both are different. One involves elevated temperature effects. • Plastic deformations are produced by the movement of individual crystal defects called dislocations (ASM 1973) • A very large number of dislocations exist in deformed metal • The crystallographic plane along which the dislocation line transverses is the slip plane • A slip line represents a transfer of the material on opposite sides of the slip plane • Due to random crystallographic orientations, the direction of slip varies from one grain to another • Plastic deformations should result in slip planes and grain elongation along the direction of extension.

15. Plastic Deformation and Microstructure

16. Plastic Deformation and Microstructure

17. Plastic Deformation and Microstructure

18. Heat Treatment and Microstructure • Heat treatment of damaged steel is different from heat treatment of undamaged steel • The damaging process changes the microstructure by forming several slip planes • These planes act as grain boundaries, and allow greater effects of heat treatment • Additionally, some new processes like recovery, recrystallization, and grain growth occur • As such there are several HT processes. The two that are relevant to this study are: • Process Annealing • Normalizing Annealing

19. Heat Treatment and Microstructure • The most commonly used heat treatments include: • Soft or spheroidizing annealing, tempering, and isothermal annealing which are used to improve the ductility of new steel (formed by quenching) • Isoforming, low and high-temperature thermomechanical treatments which are used to modify steels • Process annealing or normalizing annealing which are used to relieve the effects of cold working, residual stresses, etc. • These heat treatment processes were reviewed to identify their maximum heating temperature, heating duration, time-temperature path, mechanical loads, etc.

20. Heat Treatment and Microstructure

21. Heat Treatment and Microstructure • Process annealing is applied to hypo-eutectoid steels (with up to 0.3% Carbon) by heating it to temperatures in the range of 500-650C, which are below the Ac1 phase transformation temperature. • The steel is held at these temperatures for the necessary time, and cooled at a desired rate. • Process annealing is referred frequently as stress-relief or recovery treatment since it partially softens cold-worked (inelastic deformed) steels by relieving internal stresses (residual stresses) • This process does not cause any phase changes (i.e. changes to austenite), but it induces recovery and recrystallization of the damaged steel microstructure, which will slowly eliminate the effects of damage (slip bands etc.) • Closely related to heat straightening with Tmax = 1200F • The holding time is the major difference.

22. Heat Treatment and Microstructure • Process annealing is closely related to heat straightening because they both focus on relieving or repairing cold- worked material, and similar temperature ranges are used to achieve the desired outcome. • Process annealing requires temperature soaking (holding) times from several minutes to hours. • Heat straightening involves subjecting the damaged material to several heating cycles. In each heating cycle, the damaged material spends some time between the temperature range of 500-650C. • This time duration cumulates over several heats to subject the damaged material to a reasonable duration of heating in the process annealing (500-650C) temperature range.

23. Heat Treatment and Microstructure

24. Heat Treatment and Microstructure • Normalizing annealing subjects the steel to heating above the phase transformation temperature (Tmax > Ac1) followed by slow cooling • Heating above the Ac1 leads to the partial austenitization, i.e., transformation of pearlite to austenite, and heating above the Ac3 temperature leads to complete austenitization, i.e., transformation of both ferrite and pearlite to austenite. • The ferrite and pearlite microstructures reform as the austenite microstructure steel is cooled slowly. • Normalizing annealing leads to considerable recovery and recrystallization of the damaged microstructure, and results in a refined grain sizes with uniform distribution. • This results in better structural properties and fracture toughness values.

25. Heat Treatment and Microstructure • Heat straightening with overheating Tmax= 760 or 870oC leads to normalizing annealing of the damaged steel. • Normalizing annealing requires temperature soaking (holding) times from several minutes to hours (22). • Heat straightening repair involves subjecting the damaged material to several heating cycles. • In each cycle, the steel is heated to the target temperature Tmax and cooled continuously. The time spent above the Ac1 temperature cumulates over several heating cycles to subject the material to reasonable duration in the normalizing range. • Heat straightening with overheating can result in refined microstructure producing better structural properties and fracture toughess.

26. Heat Straightening with Microstructure

27. Recovery • Recovery is the relief of some of the stored internal strain energy of a previously cold-worked metal by heat treatment. • The physical and mechanical properties of the cold worked steel begin to revert to the properties that existed prior to cold-working • The rate of recovery is a thermally activated process that decreases with increasing time and decreasing temperature • Early in the recovery process, some internal stresses are relieved and the number of dislocations slightly reduces • As recovery proceeds, dislocation interaction results in an increase of dislocation density as dislocation arrays are formed. • These arrays constitute the walls of new cells (subgrains) of recovered steel and have lower energy configurations than the dislocation tangles, which made up the cold-worked metal • Recovery constructs the dislocations in a more stable arrangement forming small angle grain boundaries.

28. Recovery • The temperature of grain recovery correlates with the recrystallization temperature, which relates to the melting temperature of the same material according to Equation • TGR = TR – 300 = 0.4TM – 300 [C] • Where, TGR = temperature of grain recovery, • TR = temperature of recrystallization • TM = melting point • Assuming a melting temperature of 1400C (2550F), the temperature of grain recovery and the recrystallization temperature is computed as 260C (500F) and 560C (1040F), respectively • This means that both recovery and recrystallization occur when heat straightening is conducted within limits.

29. Recovery and Recrystallization • It is difficult to draw a clear division between recovery and recrystallization as the two processes often overlap • Recrystallization refers to the formation of a new set of strain-free grains within a previously cold worked material. • Formation and growth occur in a deformed matrix of new grains, which are distortion-free and appreciably more perfect than the matrix after polygonization (Gorelik 1981). • An annealing heat treatment such as process annealing is necessary for recrystallization to occur. • During recrystallization, the restoration of mechanical and physical properties is completed

30. Recrystallization • The most important factors that affect recrystallization are • Amount of prior deformation (damage) • Temperature and time • Initial grain size • Composition of the metal or alloy • The recrystallized volume in the material increases during annealing due to the growth of the nuclei, where the rate is described by two parameters known as the rate of nucleation, N, and the rate of growth, G. • These processes above depend on the amount of prior cold deformation

31. Recrystallization • Amount of deformation : • Beginning from a critical deformation, an increase in plastic strain causes N and G to increase and therefore the rate of recrystallization to grow • The size of the grains at the end of primary recrystallization is smaller after greater deformations • Elevated temperatures and slower heating shifts cr towards greater values • Heating temperature: • The recrystallization behavior is sometimes specified in terms of a temperature (TM) at which recrystallization just reaches completion in 1 hour • The TM of low carbon steel is approximately 1000F • A greater prior deformation decreases the temperature for recrystallization • The grains that form at the end of primary recrystallization at typical heating rates become noticeably coarser with a further increase in temperature

32. Recrystallization • Time of annealing • The average rate of growth G, in contrast to N is independent of annealing time until the growing recrystallization nuclei begin to collide • It has also been established by direct observations that the rate of growth does vary with time. • When deformations slightly exceed the critical value, the two processes of growth of initial grains and growth of recrystallization nuclei can occur in parallel. • Initial grain size • A finer grain size increases the area of grain boundaries, which increases the probability of nucleation sites to form • A finer initial grain size accelerates the process of primary recrystallization.

33. Recrystallization • Rate of Heating • Rapid heating to the recrystallization temperature prevents full recovery prior to recrystallization and causes a large driving force for recrystallization to remain • With rapid heating, higher temperatures are reached before recrystallization has time to begin, which facilitates nucleation of new grains • Higher the heating rate, the higher the recrystallization temperature (Totten and Howes 1997) • The rate of heating when heat straightening is not beneficial for recrystallization.

34. Grain Growth • After recrystallization is complete the strain free grains will continue to grow if the steel specimen is left at elevated temperatures • Grain growth is referred to as the increase in average grain size of a polycrystalline material. It generally follows recovery and recrystallization • Extended annealing at a high temperature can cause a few grains to grow to a very large size, which is known as secondary recrystallization or abnormal grain growth • It appears that long holding periods would be required for the full process of recrystallization.

35. Survey Results

36. Feasibility Analysis • Identifying heating and control equipment required for conducting heat treatment in the field • The capital expenses for purchasing such equipment, the extent of control available, and the power requirements at the site • Quality control and techniques for inspecting the heat treatment performed • Examining the need for bridge lane closures or traffic redirection while heat treatment is being performed • Issues related to fracture and non-fracture critical bridges • Evaluating the reduced strength of the bridge during the heat treatment for rehabilitation • Anticipating the mechanical properties of the steel after rehabilitation

37. Identifying heating and control equipment • In order to repair a steel bridge girder to its proper shape and maintain the steel’s material properties, it is recommended that the steel be heat straightened and then heat treated through two separate and distinct processes • The processes are separated in this manner to minimize working time in the field, to reduce overall costs, and to maximize the steel shape integrity and steel properties • It is recommended that heat straightening practitioners begin the heat straightening process as usual • In order to enhance the desirable steel properties, a heat treatment process is then recommended

38. Identifying Heating and Control Equipment • In order to provide the heat treatment process, (one) flat panel ceramic fiber heater will be needed, a control device to regulate the temperature of the flat panel ceramic fiber heater, and an electric generator to supply the on-site power for this heater. • There are some regulations for the use of the flat panel ceramic fiber heater that must be set in place to ensure the safety of its operation: • Only one flat panel ceramic fiber heater be used per span • The heater should be moved to an adjacent heat straightened area progressively until all of the heat straightened steel bridge girder has been heat treated • This is done to prevent weakening of significant portions of the bridge girder during treatment

39. Capital Expenses and Power Requirements • The power requirements at the site are derived from the practical availability of electrical power by heat straightening practitioners and by the flat panel ceramic fiber heater device. • The recommended flat panel ceramic fiber heater size is: • 40” X 16” X 2” • Under 20 lbs. • 11.5 kW through a parallel series of a 240 volt connection. • The power requirement for the flat panel ceramic fiber heaters is within the acceptable range of the heat straightening practitioners. • Survey results indicate that the heat straightening practitioners can easily provide an on-site generator • These generators can provide a significant level of electric power to adequately run the heaters

40. Quality Control and Inspection • The heat energy from the flat panel ceramic fiber heater will not be localized on the steel bridge girder as the flame from the heat straightening torch • A flat panel ceramic fiber heater will have some heat transfer through the section and length of the member duerto conduction • This heat transfer will be more extensive as time goes by, and must be evaluated • The flat panel ceramic fiber heater must maintain the appropriate temperature as well as maintain the appropriate distance (2’’-4”) between the surface of the steel and the surface of the heater for the required time • The appropriate temperature, distance, and time have to be designed prior to placement

41. Quality Control and Inspection • The effectiveness of the heat treatment must be evaluated at a quantitative level. • The resulting micro-constituents and microstructure of the steel must be examined and a variety of tests to determine its material properties must be researched thoroughly

42. Transportation Management: Lane Closures • During the heat treatment, there may be a need to close the traffic on the bridge close to the girder being treated. • The effects of heat treatment on the serviceability of the bridge should be evaluated. The effects of heating on the strength and stiffness should be considered in this evaluation. • The redundant nature of bridge design should allow for lane opening • The magnitude of dead load and traffic loading must be considered • It will not be necessary to close traffic under the bridge, but the lanes used or head clearance available will have to be reduced to allow adequate space for the heat straightening practitioners to perform the job • The main concern with allowing traffic under the bridge during the heat treatment operation is the risk of falling objects (such as the flat panel ceramic fiber heater) • Both of these risks present a dangerous situation for traveling automobiles, and must be evaluated before major decisions

43. Fracture Critical Bridges • Whenever fracture of the bridge is a concern extra precautions need to be made to reduce the bridges susceptibility to this failure mode • A member that is non-redundant is deemed as fracture critical because the fracture of this member during the heat straightening or heat treatment process will force a failure of the whole structure • In general, if the member satisfies the fracture critical criteria for heat straightening than it should also be acceptable for the same member to be heat treated.

44. Expected Properties after Heat Treatment • Upon completion of the recovery and recrystallization, the steel material will regain it strength, ductility, and fracture toughness. • The improvement in fracture toughness will be better than just for heat straightening. • The yield strength and ultimate strength of the steel are increased only slightly • Additional research is needed to verify these findings

45. Conclusion • Heat treatment in conjunction with heat straightening will substantially benefit the microstructure, material properties, and fracture toughness of steels.