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Kowalkowski and Varma

EFFECTS OF HEAT STRAIGHTENING ON THE MICROSTRUCTURE AND

STRUCTURAL PROPERTIES OF BRIDGE STEEL

Keith J. Kowalkowski

Structural Engineer

Ruby and Associates, P.C.

E-Mail: [email protected]

Amit H. Varma

Assistant Professor

School of Civil Engineering

Purdue University

E-Mail: [email protected]

Submission Date: August 1, 2006

Word count: Text + Tables + Figures = 4500 + 1000 + 2000 = 7500

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mailto:[email protected]

mailto:[email protected]


ABSTRACT

Metallographic investigations were conducted to determine the influence of damage-heat straightening repair cycles on the microstructure of ASTM A36, A588, and A7 steel. The results of these investigations were used to derive explanations explain changes that occur in the structural properties and fracture toughness of these steels. Ninety-one specimens made from A36, A588, and A7 steel were subjected to multiple damage-repair cycles with either a recommended maximum heating temperature of 650C or an overheated temperature (A36 only) of 760C or 870C. Microscopic photographs were taken of the undamaged steel, the steel after damage, and after the final repair of each specimen. These photographs indicate a large density of dislocations (slip planes) when damaged. After repair, these slip planes are no longer visible in the microstructure indicating that recovery and recrystallization is occurring in heat-straightened steel. Results indicate that specimens subjected to larger damage strains and overheated temperatures have better structural property results. Literature indicates that larger damage strains increase the rates of nucleation and growth when heating. Larger damage strains also result in finer grain structures. Overheated temperatures result in finer and more uniform grain distributions as apposed to heating with a maximum temperature of 650C.

Keywords: heat treatment, steel bridges, notch toughness, stress, strain, hardness, microstructure

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INTRODUCTION

Composite steel bridges are occasionally damaged by collisions with over-height trucks or other loads. The damage primarily consists of out-of-plane bending and twisting of the steel fascia beam, and in several cases, it includes fracture of the diaphragm-to-beam connections, and denting, gouging, or cracking of the beam flange close to the impact location. The out-of-plane bending and twisting of the steel beam typically dominates the damage imposed by the over-height collision, and is characterized as a category T damage by FHWA (1) and (2).

Heat straightening is a cost-effective repair technique that can be used in-situ to repair the damage (out-of-plane bending and twisting) of steel beams. It is conducted by subjecting the damaged regions of the beam to restraining forces and heating in specific patterns like Vee, strip, or spot heats. The restraining forces are applied close to the point of impact in the direction opposite to the damaging force. Heating is applied to the damaged (plastically deformed) regions of the beam using oxygen-fuel torches. Heat straightening causes thermo-plastic deformations that are opposite to (reverse) the damage deformations, and thus ‘repair’ the beam.

Several heating cycles are required to repair the damaged beams within tolerances. The most commonly used heating patterns are: (a) Vee heats to repair the out-of-plane deformation of the beam flanges, (b) Strip heats to repair beam webs, and (c) Spot heats to repair local buckling. The maximum heating temperature is limited to 650o C for mild steels and 600o C for quenched and tempered steels. These limits are recommended by FHWA (1) and NCHRP (3) based on limited research conducted in the past.

Both the damage and the heat straightening repair will have an influence on the structural properties, fracture toughness, and microstructure of the bridge steel. This raises some concerns regarding the state and acceptability of the steel after damage and repair. Additionally, heat straightening subjects the steel to elevated temperatures, which raises concerns regarding the heat treatment imparted unintentionally to the bridge steel. This paper addresses some of these fundamental questions regarding the effects of damage and heat straightening repair on the microstructure and consequently the structural properties and fracture toughness of bridge steels.

BACKGROUND: PRIOR RESEARCH

Significant research has been conducted on the heat straightening repair of damaged steel plates and beams. Prior research has focused on: (i) developing efficient heat straightening repair techniques and implementing them in the field, (ii) evaluating the effects of heat straightening on the structural properties of bridge steels, (iii) the residual stresses produced by the damage and heat straightening repair process, and (iv) the development of heat straightening repair guidelines. A detailed review of all prior research has been presented elsewhere (4, 5), and will not be repeated here for brevity.

This section will summarize relevant prior research focusing on the structural properties, fracture toughness, and microstructure of damaged-repaired steel. However, most of this earlier research, for example (6, 7, and 8) focused on the effects of heat straightening on the structural properties of undamaged steel, i.e., the steel was subjected to limited (about four to five) cycles of heating without any initial damage. The findings from these studies have limited or no applicability to steels subjected to significant damage followed by heat straightening repair.

Avent et al. (9, 10) conducted the first comprehensive studies of the effects of initial damage followed by heat straightening repair on the structural properties of steel. The major findings from these studies indicate that heat straightening does not have a significant influence on the elastic modulus of steel, which is as expected. Heat straightening reduces the yield stress of quenched and tempered steels, and it increases the yield and tensile stress of mild steels. The increase in tensile stress is not as significant as the increase in yield stress for mild steels. Significant variations were found in the yield and tensile stresses of steel coupons taken from apex, mid, and open end of the Vee heat. The increases in the yield

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and tensile stresses were much higher for coupons taken from the apex of the Vee heat. Heat straightening reduces the ductility of damaged-repaired steels. This reduction is even more significant for steels subjected to multiple damage-heat straightening repair cycles.

The effects of heat straightening on the fracture toughness of undamaged steel plates have also been studied by several researchers (11, 12, 13). These studies indicate that heating mild steels with maximum temperature (Tmax) limited to 650oC produces insignificant changes in the fracture toughness. Quenched and tempered steels were found to have a positive shift in the upper shelf energy absorption, which indicates an increase in the fracture sensitivity. However, these findings have limited applicability for steels subjected to damage followed by heat straightening repair. Till (13) investigated the effects of different heating temperatures (650, 760, and 870oC) and cooling methods on the fracture toughness of undamaged A36 steel. The experimental results indicated that heating typically increases the Charpy V-notch (CVN) fracture toughness of A36 steel. Metallographic investigations revealed that the processes of recovery, recrystallization, and grain growth had occurred in the heated specimens, which is a major finding that will be discussed later.

RECENT RESEARCH

It is evident from the discussion above that currently there is a lack of research of the effects of damage followed by heat straightening repair on the structural properties, fracture toughness, and microstructure of bridge steels. Prior research indicates that the structural properties etc. are altered due to the changes that occur in the microstructure of plastically deformed and heat-treated steel. However, very limited research has been conducted to correlate the damage and repair parameters with the changes in microstructure and consequently the structural properties of bridge steels.

A research project focusing on the effects of multiple damage-heat straightening repair cycles on the structural properties and fracture toughness of various bridge steels was recently conducted by the authors. The research was conducted in two phases. Phase I conducted extensive laboratory-scale tests to determine the effects of various damage and repair parameters on the structural properties and fracture toughness of various bridge steels. Phase II conducted six large-scale beam tests to validate the findings from phase I for realistic bridge beams.

The bridge steels included in the research were A36, A588, and A7 steel. The damage and repair parameters considered in the research were the damage strain (d), the restraining stress (r), the number of damage-repair cycles (Nr), and for A36 steel only the heating temperature (Tmax). The experimental results and major findings from Phase I were presented in detail in 2005 (14, 4, 5), and the experimental results and major findings from Phase II were presented in detail in 2006 (15, 5).

OBJECTIVE AND SCOPE

This paper summarizes the relevant experimental results from Phase I of the research project, and presents a plausible explanation of the effects of damage and heat straightening repair on the microstructure and consequently the structural properties and fracture toughness of steel. The paper will also present some microstructure-based rationale for the apparent effects of various damage and repair parameters on the structural properties etc. of bridge steels. The paper will focus on steels subjected to only one damage-heat straightening repair cycle. The specimens subjected to multiple damage-repair cycles have not been included because they are beyond the objective and focus of this paper.

EXPERIMENTAL INVESTIGATIONS

As presented earlier in (14, 4, 5), the experimental investigations were conducted on laboratory-scale (200 x 1175) dog-bone shaped specimens with a reduced section identified as the test-area. The test-area of each specimen was about 82 x 127mm in size and had the same thickness (25 mm typical) as the rest of the specimen. The specimen test-areas were subjected to multiple damage-heat straightening repair cycles as follows.

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The specimen was damaged by subjecting it to monotonically increasing uniaxial tension producing the target inelastic damage stain (d) in the test-area. It was repaired by subjecting the specimen to uniaxial compression producing the target restraining stress (r) in the test area followed by strip heating in a serpentine path from the bottom of the test area to the top. The maximum heating temperature was typically limited to 650oC. In some cases, it was increased to 760 or 870oC as discussed later. The restraining stress and heating were repeated until the residual (post-cooling) deformation of the test-area was within tolerance limits. Several heating cycles were required to repair the specimens subjected to the larger damage strains. This constituted one damage-repair cycle for the specimen. It was repeated the specified number (Nr) of times to subject the specimen to multiple damage-repair cycles, if needed.

After subjecting the specimens to the damage-repair cycles, material coupons were fabricated from the test-area and tested according to applicable ASTM standards. The material test results were used to determine the structural properties, fracture toughness, surface hardness, and microstructure grain size of the damaged-repaired steels. Two uniaxial tension coupons (with 50 mm gage length) were fabricated from the test-area of each specimen, and tested according to ASTM E8 to determine the elastic modulus (E), yield stress (y), ultimate stress (t), and % elongation (e) of the damaged-repaired steels. Additionally, six Charpy V-notch (CVN) specimens were also fabricated from the test-area of each specimen, and tested according to ASTM E23 to determine the fracture toughness of the damaged-repaired steels. These CVN specimens typically included three quarter and three mid-thickness specimens that were used to compute the average quarter and mid-thickness (FT-Q and FT-M) fracture toughness of the damaged-repaired steels.

Rockwell hardness tests were conducted on the top (outside facing) surface of a quarter-thickness CVN specimen to determine the surface hardness (Hd) of the damaged-repaired steel. Additionally, the top (outside facing) surface of the quarter-thickness CVN specimen was polished and etched (using a 0.2% nitol solution) to conduct metallographic inspections according to ASTM E3 (10). Photographs of the steel microstructure were taken at 100, 240, and 480X magnifications. The 100X picture was used to compute the grain size according to the line intercept procedure in ASTM E112 (11).

All the material tests mentioned above, namely, the uniaxial tension, CVN fracture toughness, Rockwell surface hardness, and microstructure grain size test were also conducted on material coupons taken from the test-area of an untested, i.e., undamaged or virgin steel specimen. The results from these material tests were used to define the undamaged steel material properties (Eo, yo, uo, %eo, FTo-Q, FTo-M, and GSo) for the bridge steels. These undamaged steel properties were used to compare and evaluate the effects of damage-repair cycles on steel material properties.

The experimental investigations focused on three steels, namely, ASTM A36, A588, and A7 steel. The undamaged steel material properties for the A36, A588, and A7 steels are presented in Table 1. As shown in the Table, two A36 plates were used in this research, which are designated as A36-1 and A36-2, respectively. The A36-1, A588, and A7 steel specimens were subjected to damage-repair with the maximum heating temperature (Tmax) limited to 650oC. The A36-2 steel specimens were subjected to heat straightening repair with Tmax allowed to reach 760 and 870oC, respectively.

The test-matrix for the A36-1, A588, A7, and A36-2 steel specimens is shown in Tables 2-5. As shown in Table 2, six A36-1 steel specimens were tested with three damage strains (d = 30, 60, or 90 times the yield strain y) and two restraining stresses (r=25 or 50% of the yield stress y). As shown in Table 3, six A588 specimens were tested with three damage strains (d = 20, 40, or 60 y) and two restraining stresses (r=25 or 50% of y). As shown in Table 4, six A7 steel specimens were tested with three damage strains (d = 30, 60, or 90 times y) and two restraining stresses (r=25 or 50% of y). As shown in Table 5, eight A36-2 steel specimens were tested with two damage strains (d = 60 or 90 times y), two restraining stresses (r=25 or 50% of y), and two maximum heating temperatures (Tmax=760 or 870oC). All these (twenty-six) specimens were subjected to one damage-repair cycle with the damage and repair parameters (d, r, and Tmax) indicated in the test matrices.

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STRUCTURAL PROPERTIES AND FRACTURE TOUGHNESS

Material coupons were fabricated from the test-areas of the damaged-repaired specimens and tested according to ASTM standards as explained in the previous section. The results from the standard material tests were used to determine the structural properties, fracture toughness, and microstructure of the damaged-repaired steel specimens. These material properties were normalized with respect to the undamaged steel properties to evaluate the effects of damage and heating repair. The normalized properties for the damaged-repaired steel specimens are presented in Tables 2-6. In these Tables E/Eo, y/yo, u/uo, e/eo, Hd/Hdo, FT-Q/FT-Qo, FT-M/FT-Mo, and GS/GSo are the normalized elastic modulus, yield stress, ultimate stress, % elongation (ductility), surface hardness, average quarter-thickness fracture toughness, average mid-thickness fracture toughness, and microstructure grain size, respectively.

The normalized experimental results in Tables 2-6 indicate that damage followed by heat straightening repair typically increases the yield stress of the steels. This increase is about 4-13% for A36-1 steel, about -1-8% for A588 steel, -4-10% for A7 steel, and about 21-27% for A36-2 steel that was heated beyond the 650oC limit, which is significant. Heat straightening has a small influence on the elastic modulus and ultimate stress of all steels. The elastic modulus is typically within ±10% of the undamaged values, and the ultimate stress typically increases by about 5-7%. The surface hardness of A36-1, A588, and A7 steel increase slightly (about 2-12% of the undamaged value), while the surface hardness of A36-2 steel decreased slightly (about 0-3% of the undamaged value). In summary, heat straightening has a small influence on the elastic modulus, ultimate stress, and surface hardness of steels. It increases the yield stress of the steel slightly. This increase is much greater for the steels heated beyond 650oC.

The normalized results in Tables 2-6 indicate that heat straightening has a reasonable influence on the ductility (% elongation). Heat straightening reduces the ductility of steels. For A36-1 steel, the ductility reduces to about 75-89% of the undamaged ductility after one damage-repair cycles. The ductility of A588 steel reduces to about 74-88% of the undamaged ductility. The ductility of A7 steel reduces to about 77-89% of the undamaged ductility, and the ductility of A36-2 steel reduces to about 76-92% of the undamaged ductility. The reduction in ductility is highest for the A7 steel and lowest for the overheated A36-2 steel. However, in all cases the raw ductility (% elongation) values satisfied AASHTO requirements, which can be ascertained by computing e by multiplying the ratio of e/eo from Tables 2-6 with the values of eo in Table 1, and comparing it with the AASHTO requirements for the steel.

The results in Tables 2-6 indicate that heat straightening has a significant influence on the fracture toughness of bridge steels. It decreases the CVN fracture toughness in some cases, and in other cases increases the fracture toughness above the undamaged values. The CVN fracture toughness rarely reduces below 50% of the undamaged toughness (exception: Specimen A36-90-50-1). The fracture toughness (of all specimens) never reduces below the AASHTO recommended limit (20 J at 40oF) for mild steels, probably because the initial undamaged toughness is very high (see Table 1). As seen in Tables 2-6, the normalized fracture toughness at the mid-thickness (FT-M/FTo-M) is lower than the normalized fracture toughness at quarter thickness (FT-Q/FToM), which implies that heat straightening does not produce a uniform change through the thickness of the material.

The results in Table 2 indicate that for A36-1 steel, smaller damage strains (d=30 y) reduce the fracture toughness to about 45-70% of the undamaged value. Increasing the restraining stress (while keeping the damage strain constant) typically reduces the fracture toughness. Specimen A36-90-50-1 had low normalized fracture toughness probably due to the large damage strain and restraining stress. The results in Table 3 indicate that for A588 steel, smaller damage strains (d=20 y) reduce the fracture toughness to about 42-70% of the undamaged toughness. Larger damage strains result in normalized fracture toughness close to or greater than the undamaged toughness, which is remarkable. Increasing the restraining stress typically reduces the fracture toughness. The results in Table 4 indicate that for A7 steel, larger damage strains (d=90 y) result in fracture toughness greater than or close to the undamaged

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fracture toughness. Smaller damage strains (d=30 or 60 y) result in fracture toughness values in the range of 63-89% of the undamaged values.

The results in Table 5 for A36-2 steel indicate that heat straightening with the Tmax equal to 760oC results in fracture toughness values that are much greater than the undamaged values. The fracture toughness values are approximately 172-443% of the undamaged values, which is remarkable. Increasing the damage strain or the restraining stress does not seem to have a direct trend with the change in fracture toughness values. The results also indicate that heat straightening with Tmax equal to 870oC also results in fracture toughness values that are much greater (approximately 188-417%) than the undamaged fracture toughness values. Once again, increasing the damage strain or the restraining stress does not seem to have a direct trend with the fracture toughness values.

MICROSTRUCTURE

The results in Tables 2-6 indicate that heat straightening has an influence on the grain size (GS) of steels. As shown in Table 2, the normalized grain size (GS/GSo) of A36-1 steel is about 92-112%, i.e., the grain size after damage-repair is approximately 92-112% of the undamaged steel grain size. As shown in Table 3, heat straightening slightly increases the grain size of A588 steel. The normalized grain size of A588 steel is approximately 110-133%, i.e., the grain size increases after damage-repair to approximately 110-133% of the undamaged grain size. As shown in Table 4, heat straightening generally reduces the grain size of A7 steel. The normalized grain size of A7 steel is about 75-106%. Additionally, as shown in Table 5, heat straightening with Tmax greater than 650oC always reduces the normalized grain size of A36-2 steel. The grain size after damage-repair is about 66-85% of the undamaged steel grain size.

Figures 2-5 show the microstructures of A36-1, A588, A7, and A36-2 steel. Each figure includes a picture of the steel microstructure (a) before damage, (b) after damage, and (c) after heat straightening repair. In these microstructure pictures, the lighter grains are pro-eutectoid ferrite and the darker grains are pearlite.

Figure 2(a) shows the microstructure of A36-1 steel before damage. The average grain size is 0.0303 mm. Figure 2(b) shows the microstructure after damage (d=90y). This figure illustrates the formation of slip bands within the ferrite grains. These slip bands represent planes across which slip occurred as the material undergoes plastic strains. Plastic deformations (and strains) are produced by the movements of individual crystal defects called dislocations (12). These individual movements aggregate to give rise to a variety of large-scale deformations including slipping and twinning. The slip bands in Figure 2(b) are visible traces of slip planes that formed in the plastically deformed steel (12). Figure 2(c) shows the microstructure of specimen A36-90-25-1, which was damaged to 90y and repaired with a restraining stress of 0.25y and a maximum heating temperature of 650C. As shown in Figure 2(c), the slip planes, which were clearly identified after damage (Figure 2(b)), are no longer visible in the microstructure. Figure 3(c) shows the microstructure of an A588 specimen damaged to 60y and repaired with a restraining stress of 0.25y. Again, the slip planes identified in Figure 3(b) are no longer visible in the microstructure. The grain size of A588 steel increases after repair. This result is different in comparison to other steel types.

Figure 4(c) shows the microstructure of an A7 specimen damaged to 90y and repaired with a restraining stress of 0.40y. Again, the slip planes identified in Figure 4(b) are no longer visible in the microstructure. Figure 5(c) shows the microstructure of an A36 specimen damaged to 90y and repaired with a restraining stress of 0.25y and a maximum heating temperature of 870C. The damaged-repaired microstructure possesses finer grains and a more uniform distribution as compared to the undamaged microstructure shown in Figure 5(a). Slip planes indicated in Figure 5(b) are again no longer visible in the microstructure after repair.

Figures 6-8 provide further microstructure comparisons of interest for the results of Kowalkowski and Varma (1). Figure 6(a) shows the microstructure of A36 steel withd=30y and Figure 6(b) shows the microstructure after repair with r = 0.40y and Tmax=650C. Fewer slip planes are found in the microstructure of Figure 6(a) as apposed to Figure 2(b). A larger grain size results after a lower d in

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Figure 6(b) as apposed to Figure 2(c). Figure 7(a) shows the microstructure of an A7 steel specimen with d = 30y and Figure 7(b) shows the microstructure after repair with r = 0.25y. Similar comparisons are found in the results of A7 steel subjected to a smaller d as for A36. Figure 8 provides a comparison of two microstructures of A36 steel with d = 60y and repaired with r = 0.50y and Tmax of 760C (a) or 870C (b). Both specimens were subjected to a lower d and a higher r as apposed to the microstructure seen in Figure 7(c). However, all three microstructures have similar grain sizes and grain distributions. Therefore, the effects of the various damage-repair parameters do not have a significant influence on the results of the overheated temperature specimens. The results in Figures 6-8 will be discussed in more detail later.

HEAT STRAIGHTENING VS. HEAT TREATMENT OF STEEL

The applications of common steel heat treatments (purpose, effects, and parameters) were reviewed and compared to the applications of heat straightening. This review indicated that two common annealing processes termed as process annealing and normalizing annealing relate well to heat straightening using either recommended temperature limits or overheating temperatures:

Process annealing is usually applied to hypoeutectoid steels with up to 0.3% C (valid for steels in research). The steel is heated to temperatures of typically 550-650C, 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 frequently referred to as a “stress-relief” or “recovery” treatment since it partially softens cold-worked steels by relieving internal stresses (residual stresses) from cold working (8). This process does not cause phase changes (i.e. changes to austenite) but recrystallization may occur (9). Process annealing is fairly related to heat straightening as both focus on relieving or repairing cold-worked material. Similar temperature ranges are used to achieve the desired outcome. However, process annealing generally requires longer holding times to achieve the desired affect. Totten and Howes (9) indicate holding times from one to forty-eight hours.

Normalizing annealing is a heat treatment process consisting of austenitizing (heating to temperatures that cause an austenite microstructure) followed by slow cooling (9). This annealing process is used to refine grains and produce a more uniform and desirable grain size distribution (7). From this description, and guided by the microstructure comparisons presented later, heat straightening A36 steel with Tmax=870C has a very similar effect as normalizing. During heat straightening, the temperatures exceed the Ac3 for only a matter of seconds as apposed to hours, which is generally required for normalizing.

Recovery is the relief of some of the stored internal strain energy of a previously cold-worked metal by heat treatment. During recovery, the physical and mechanical properties of the cold-worked steel begin to revert to the properties prior to cold-working (12). Early in the recovery process, some internal stresses are relieved and the number of dislocations reduces. As recovery proceeds, dislocation interaction results in an increase of dislocation density as dislocation arrays are formed. The temperature of grain recovery (TGR) correlates with the recrystallization temperature (TR) and the melting temperature (TM) of the same material according to the equation: TGR = TR – 300 = 0.4TM – 300 [C]. Assuming TM=1400C for steel, TGR and TR are computed as 260C and 560C, respectively. Therefore, TGR is much lower than temperatures applied during heat straightening. It is often difficult to draw a clear separation between recovery and recrystallization as the two processes often overlap. Usually, the restoration of mechanical properties to annealed values is 1/5 achieved during recovery (14).

Recrystallization refers to the formation of a new set of strain-free grains in the microstructure of previously cold worked steel (7). An annealing heat treatment such as process annealing is necessary for recrystallization to occur. At the end of this process, the restoration of mechanical and physical properties is completed. The most important parameters that influence the rate of recrystallization in metals and alloys are the; (1) amount of prior deformation, (2) heating temperature, (3) time, (4)

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initial grain size, (5) composition, and (6) rate of heating (9). Beginning from a critical deformation (cr), an increase in plastic strain causes the nucleation and growth rates to increase and therefore the rate of recrystallization to grow. The grain size at the end of primary recrystallization is smaller after greater deformations. Values of the critical strain (cr) for steel are lacking from the literature.

The kinetics of primary recrystallization is both time and temperature dependent. The recrystallization behavior of a particular alloy is sometimes specified in terms of TM defined as the temperature at which recrystallization just reaches completion in 1 hr. (9). Totten and Howes (9) indicate that TM is approximately 540C for carbon steel. However, TM decreases for greater prior deformation and an increase in annealing time. Steel subjected to heat straightening is held above the recrystallization temperature for only a short period. Recrystallization occurs in heat-straightened steel as evident by the microscopic photographs presented in this paper. Experimental data reveals that the surface temperature of A36 specimens heated to 650C, 760C, and 870C, is held above 540C (TR) for approximately 140 s, 320 s, and 410 s, respectively. However, each specimen is subjected to numerous heating cycles. A specimen subjected to a higher d and a lower r requires more heating cycles for repair. Therefore, the recovery of internal strain energy and recrystallization of new grains are more likely to occur.

Grain growth is referred to as the increase in average grain size of a polycrystalline material (7). This process typically follows recovery and recrystallization. However, it may occur simultaneously before recrystallization is complete. Grain growth occurs by the migration of grain boundaries as larger grains grow at the expense of smaller grains. The results included in this paper indicate that grain growth evolves in the microstructure of A588 steel, which also indicates that recovery and recrystallization occur rapidly. These processes may be related to the good fracture toughness and ductility results of A588 steel regardless of the various damage-repair parameters (1).

DISCUSSION MICROSTRUCTURE, HEAT STRAIGHTENING, HEAT TREATMENT AND EFFECTS OF DAMAGE AND REPAIR PARAMETERS

The most influential damage-heat straightening repair parameters identified by Kowalkowski and Varma (1) were the initial damage strain for A36 and A7 steels and the maximum heating temperature for A36 steel. In addition, applying a higher restraining stress appears to be more detrimental to the fracture toughness of steel. However, clear differences were not identified in the microstructure investigations when applying a higher restraining stress as apposed to a lower restraining stress (see Tables 2-5). Kowalkowski and Varma (1) unpredictably concluded that repairing an A36 or A7 specimen subjected to a lower initial damage strain results in a lower fracture toughness after repair as apposed to a higher initial damage strain. They also concluded that using a higher maximum temperature of 760C or 870C results in much higher fracture toughness as apposed to repairing with the recommended limit of 650C.

Smith (13) indicates that a coarse-grained structure is not as desirable as a fine-grain structure for most steels as it leads to lower strengths and decreased ductility. Finer-grained steels have more grain boundaries that act as barriers to dislocations. Therefore, a higher density of grain boundaries will produce higher yield and tensile stresses. Decreasing the grain size significantly decreases the transition temperature, which is the temperature that governs the ductile to brittle fracture (9). Hence, refining the grain size increases both the strength and the fracture toughness of steels. The grain size of overheated A36 steel decreases significantly as compared to the undamaged A36 steel. These steels also indicate a large increase in strength and exceptionally high fracture toughness (1). Smaller grain sizes and more uniform grain distributions result after the repair of A7 and A36 steel subjected to higher damage strains. These steels generally exhibit better fracture toughness and ductility results as apposed to lower damage strains.

Kowalkowski and Varma (1) indicate that smaller damage strains are more detrimental to the fracture toughness of A36 and A7 steels. The discussion in this section indicates two possible reasons for these results. The first reason is that higher damage strains increase the rates of nucleation and grain

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growth. The second reason is that less heating cycles are required to repair the damaged steel as apposed to repairing higher damage strains. Therefore, for both of these reasons, it appears that recrsytallization of the grains has less probability to transpire for lower damage strains. Applying a higher restraining stress results in a similar effect as less heating cycles are required. The microstructure does not have as much opportunity for recovery and recrystallization to progress, which may be related to lower fracture toughness results.

For overheated A36 specimens, there were negligible trends in the various parameters and the experimental results presented by Kowalkowski and Varma (1). There are also no significant differences identified from the microstructure investigations (Figures 5(c) and 8) with the exception that the grain sizes of specimens heated to 870C were lower that the grain sizes of specimens heated to 760C (Table 5). Overheated temperatures have a significant impact on the microstructure of A36 steel. The new microstructures possess a much finer grain structure. Overheated temperatures increase the strength and toughness substantially and only produces moderate decreases in ductility. Therefore, the steel is in much better condition than repairing with the recommended limit of 650C. In addition, fewer heating cycles are required to straighten the member as apposed to 650C.

SUMMARY AND CONCLUSIONS

Metallographic investigations were conducted to determine the influence of damage-heat straightening repair cycles on the microstructure of ASTM A36, A588, and A7 steels. Micrographic photographs were taken of each undamaged steel plate, after each damage strain level, and after the respective damage-heat straightening repair cycles of each specimen. Changes in the grain size and distribution were related to the structural property results presented by Kowalkowski and Varma (1). The photographs of the damaged steel clearly indicate dislocations in the form of slip planes that increase with an increase in damage strain. Slip planes are not present after the heat straightening applications. These comparisons indicate that recovery and recrystallization occur in the grain structure, even though the holding times are much shorter than other annealing procedures such as process annealing or normalizing annealing.

The results indicate that damage-heat straightening repair cycles decrease the grain size of A36 and A7 steels. Damage-repair cycles of larger damage strains result in lower grain sizes indicating that more recovery and recrystallization occur at the grain boundaries and at the slip planes. These results are likely related to lower damage strains resulting in less ductility and fracture toughness after heat straightening as apposed to higher damage strains. The microstructure has less time to recover the internal strain energy and fewer locations for the formation of new grains during recrystallization.

The results also indicate that damage-repair cycles increase the grain size of A588 steel indicating that recovery, recrystallization, and grain growth are occurring during the repair of this steel type simultaneously. These predictions are likely related to the good ductility and fracture toughness results of damaged-repaired A588 steel.

A more fine-grained and uniform microstructure evolves when using overheated temperatures of 760C or 870C during heat straightening as apposed to the recommended limit of 650C. The grain sizes of all overheated specimens were lower than the undamaged grain size. Therefore, the strength and fracture toughness of the steel was substantially higher. It appears that using overheated temperatures allows for full recrystallization of the grains to occur. Overall, from the results of Kowalkowski and Varma (1) and the results in this paper, it appears that heat straightening A36 steel with overheated temperatures is more beneficial than the recommended limit of 650C.

ACKNOWLEDGEMENTS

This research was sponsored by the Michigan Department of Transportation (Roger Till–cognizant official).

10


11


REFERENCES

1. Kowalkowski, K.J. and Varma, A.H. (2005), “Structural Properties and Fracture Toughness of Steels Subjected to Multiple Damage-Heat Straightening Repair Cycles,” Journal of Structural Engineering, Accepted for publication.

2. Avent, R.R., Mukai, D.J., and Robinson, P.F. (2000), “Effect of Heat Straightening on Material Properties of Steel,” Journal of Materials in Civil Engineering, 12(3), 188-195.

3. Till, R. D. (1996), Effect of Elevated Temperature on Fracture Critical Steel Members, MDOT Research Report No. R-1344, Materials and Technology Division, Michigan Department of Transportation, pp. 1 – 17.

4. AASHTO (2002), Standard Specifications for Highway Bridges, 17th Edition, American Association of State and Highway Transportation Officials, Washington, DC.

5. Varma, A.H. and K.J. Kowalkowski (2004). “Effects of Multiple Damage-Heat Straightening Repair on the Fundamental Properties of Bridge Steels.” MDOT Report No. RC-1456, Michigan Department of Transportation, Construction and Technology Division, Lansing, MI, 418 pp.

6. Avent, R.R., and Mukai, D.J. (1998), Heat-Straightening Repairs of Damaged Steel Bridges - A Technical Guide and Manual of Practice, Report No. FHWA-IF-99-004, Federal Highway Administration, U.S. Department of Transportation.

7. Callister, W.D. (1997), Material Science and Engineering, an Introduction, Fourth Edition, John Wiley & Sons Inc., New York, New York.

8. Smith, W.F. (2004), Foundations of Material Science and Engineering, Third Edition, McGraw-Hill Inc., New York, New York.

9. Totten, G.E. and Howes, M.A. (1997), Steel Heat Treatment Handbook, Marcel Dekker, Inc., New York, New York.

10. ASTM (1999a), "Standard Practice for Preparation of Metallographic Specimens," E3, American Society for Testing and Materials, West Conshohocken, PA.

11. ASTM (1999b), "Standard Test Methods for Determining Average Grain Size," E112, American Society for Testing and Materials, West Conshohocken, PA.

12. ASM (1973), Metals Handbook, American Society of Metals, Volume 8, Metals Park, Ohio, pp 211-218.

13. Smith, W.F. (1993), Structure and Properties of Engineering Alloys, Second Edition, McGraw-Hill Inc., New York, New York.

14. Byrne, J.G. (1965), Recovery, Recrystallization, and Grain Growth, The Macmillan Company, New York, New York.

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LIST OF TABLES

TABLE 1 Undamaged Steel Structural Properties

TABLE 2 A36 Steel Test Matrix and Normalized Experimental Results



TABLE 5 Overheated A36 Steel Test Matrix and Normalized Experimental Results

LIST OF FIGURES

FIGURE 1 Temperatures located on the iron-iron carbide phase diagram for 0.15% C steel

FIGURE 2 Microstructure of undamaged, damaged, and repaired A36 steel (Plate 1) (240X)

FIGURE 3 Microstructure of undamaged, damaged, and repaired A588 steel (480X)


FIGURE 5 Microstructure of undamaged, damaged, and repaired overheated A36 (Plate 2) (480X)

FIGURE 6 Comparisons of A36 steel with d = 30y and then repaired with r = 0.40y

FIGURE 7 Comparisons of A7 steel with d = 30y and then repaired with r = 0.25y

FIGURE 8 Comparison of A36 steel repaired with elevated temperatures

TABLE 1 Undamaged Steel Structural Properties13


Steelyo

(MPa)

(GPa)uo

(MPa)

eo

%

Hd

Rock.

FTo-Q

(J)

FTo-M

(J)

GSo

(mm)

A36-1 271 211 445 39.2 64.5 186 182 0.0303

A588 396 195 573 33.6 87.2 141 107 0.0132

A7 267 215 428 44.1 69.7 -*- 53 0.0340

A36-2 270 209 460 38.9 78.4 82 73 0.0393

* A7 steel had mean fracture toughness values for mid thickness only

14



Specimen

Damage-Repair

Parameters

Normalized damaged-repaired

structural properties

Name

Nr Tm

FT-Q

FTo-Q

FT-M

FTo-M

A36-30-40-1 30 40 1 650 1.13 1.03 1.06 0.75 1.09 0.45 0.51 0.92

A36-30-70-1 30 70 1 650 1.12 1.04 1.05 0.86 1.12 0.66 0.70 0.97

A36-60-25-1 60 25 1 650 1.06 1.05 0.99 0.88 1.04 1.47 0.58 1.12

A36-60-50-1 60 50 1 650 1.07 1.05 1.01 0.89 1.02 1.09 0.29 0.92

A36-90-25-1 90 25 1 650 1.04 0.98 1.00 0.81 1.05 1.44 0.83 0.81

A36-90-50-1 90 50 1 650 1.06 0.97 0.98 0.83 1.02 0.20 0.18 1.01

15



Specimen

Damage-Repair

Parameters



Name

Nr Tm

FT-Q

FTo-Q

FT-M

FTo-M

A588-20-25-1 20 25 1 650 1.04 1.05 1.01 0.88 1.01 0.70 0.55 1.10

A588-20-50-1 20 50 1 650 1.07 1.08 1.04 0.83 1.03 0.70 0.42 1.14

A588-40-25-1 40 25 1 650 0.99 1.08 0.98 0.82 1.05 1.44 1.24 1.11

A588-40-50-1 40 50 1 650 1.08 1.07 1.01 0.74 1.05 1.23 0.82 1.24

A588-60-25-1 40 25 1 650 1.00 1.08 0.99 0.85 1.04 1.27 0.60 1.25

A588-60-50-1 60 50 1 650 1.05 1.08 1.02 0.84 1.06 1.15 1.00 1.33

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TABLE 4. A7 Steel Test Matrix and Normalized Experimental Results

Specimen

Damage-Repair

Parameters



Name

Nr Tm

FT-Q

FTo-Q

FT-M

FTo-M

A7-30-25-1 30 25 1 650 0.97 0.93 0.99 0.89 1.02 --- 0.89 0.98

A7-30-40-1 30 40 1 650 1.00 0.93 1.00 0.79 1.11 --- 0.67 0.89

A7-60-25-1 60 25 1 650 0.99 0.86 0.97 0.77 1.07 --- 0.63 1.06

A7-60-40-1 60 40 1 650 1.03 0.88 0.99 0.77 1.08 --- 0.63 0.86

A7-90-25-1 90 25 1 650 1.10 -* 0.98 0.86 1.11 --- 1.03 0.78

A7-90-40-1 90 40 1 650 0.86 -* 0.96 0.77 1.04 --- 1.11 0.75

* E could not be measured accurately due to the initial curvature in the tension coupons

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TABLE 5 A36-2 Steel Test Matrix and Normalized Experimental Results

SpecimenDamage-Repair

ParametersNormalized damaged-repaired


Name

Nr Tm

FT-Q

FTo-Q

FT-M

FTo-M

A36-60-25-1-760 60 25 1 760 1.23 0.98 1.07 0.85 0.99 3.21 4.43 0.85

A36-60-50-1-760 60 50 1 760 1.21 1.04 1.05 0.84 0.97 2.98 3.05 0.66

A36-90-25-1-760 90 25 1 760 1.25 1.01 1.08 0.86 0.98 3.02 1.72 0.69

A36-90-50-1-760 90 50 1 760 1.27 1.03 1.08 0.93 0.99 3.56 3.69 0.66

A36-60-25-1-870 60 25 1 870 1.24 1.01 1.07 0.92 0.94 2.07 4.17 0.73

A36-60-50-1-870 60 50 1 870 1.23 0.97 1.08 0.92 0.97 3.18 2.03 0.79

A36-90-25-1-870 90 25 1 870 1.22 1.01 1.07 0.76 1.01 3.75 1.88 0.78

A36-90-50-1-870 90 50 1 870 1.21 1.00 1.06 0.92 0.97 3.83 2.25 0.85

18


1 2 3400

600

800

1000

2000

0.760.022

+ Fe3C

, Ferrite

Austentite

727C (1340 F)

912C

Composition (wt% C)

1500

0(Fe)

0.15% C (A36)

760 C (1400 F)

1000

Tem

pera

ture

(C

)

Tem

pera

ture

(F)

870C (1600 F)

650 C (1200 F)

1 2 3400

600

800

1000

2000

0.760.022

+ Fe3C

, Ferrite

Austentite

727C (1340 F)

912C

Composition (wt% C)

1500

0(Fe)

0.15% C (A36)

760 C (1400 F)

1000

Tem

pera

ture

(C

)

Tem

pera

ture

(F)

870C (1600 F)

650 C (1200 F)

FIGURE 1 Temperatures located on the iron-iron carbide phase diagram for 0.15% C steel

19


a) Undamaged b) After Damage of 90y c) After Repair, Tmax=650C

FIGURE 2 Microstructure of undamaged, damaged, and repaired A36 steel (Plate 1) (240X)






20


FIGURE 5 Microstructure of undamaged, damaged, and repaired overheated A36 (Plate 2) (480X)

21


a) After Damage of 30y b) After Repair, Tmax=650C

FIGURE 6 Comparisons of A36 steel withd = 30y and then repaired with r = 0.40y

a) After Damage of 30y b) After Repair, Tmax=650C

FIGURE 7 Comparisons of A7 steel withd = 30y and then repaired with r = 0.25y

a) Tmax = 760C b) Tmax = 870C

FIGURE 8 Comparison of A36 steel repaired with elevated temperatures

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