evaluation of stainless steel fasteners …docs.trb.org/prp/17-03530.pdfwilliams, xia, darby, and...

Williams, Xia, Darby, and Sharp 1

EVALUATION OF STAINLESS STEEL FASTENERS FOR BOLTED FIELD SPLICE 1

CONNECTIONS OF ASTM A1010 CORROSION-RESISTANT STEEL PLATE 2

GIRDERS 3

4

5

6

Thomas R. Williams 7 University of Virginia 8

Thornton Hall B228, 351 McCormick Road, Charlottesville, VA 22903 9

Tel: 434-293-1913; Fax: 434-293-1990; Email: [email protected] 10

11

Xuemeng Xia 12 University of Virginia 13

Wilsdorf Hall B20, McCormick Road, Charlottesville, VA 22903 14


16

Thomas E. Darby III, P.E. 17 Virginia Department of Transportation 18

6200 Elko Tract Road, Sandston, VA 23150 19


21

Stephen R. Sharp, Ph.D., P.E., Corresponding Author 22 Virginia Transportation Research Council 23

530 Edgemont Road, Charlottesville, VA 22903 24


26

27

Word count: 5205 words text + 9 tables/figures x 250 words (each) = 7,455 words 28 29

30

31

32

33

34 Submission Date: November 15, 2016 35

mailto:[email protected]





ABSTRACT 1 The Virginia Department of Transportation initiated this study to compare the mechanical 2

properties, availability, and costs of stainless steel fastener materials for use with ASTM A1010 3

stainless steel plate. The investigation focused on fastener materials included in ASTM A193 4

and compared them to ASTM A325 bolts. The ASTM A193 bolts tested were the B6, B8, and 5

B8M. 6

Test results indicated that the ASTM A193 B8 fasteners provided the most economic 7

combination of mechanical strength, corrosion resistance, and cost. Uniaxial tension tests and 8

Skidmore-Wilhelm rotational capacity tests revealed that the A193 B6 fasteners had high 9

strength but lower ductility whereas the ASTM A193 B8M fasteners had lower strength but 10

higher ductility. The ASTM A193 B8 fastener had an ideal combination of strength and 11

ductility. The mechanical performance of the ASTM A193 fasteners was further improved by 12

the use of hardened washers. Because the bolts are hot forged, sensitization, which can reduce 13

corrosion resistance, was of concern. Standard testing indicated that the B8 and B8M as-14

received bolts were unsensitized. 15

The cost of ASTM A193 B8 fasteners compared favorably with that of other stainless 16

steel fasteners, and the B8 fasteners are available from manufacturers complying with Buy 17

America requirements. Although the cost of stainless steel fasteners are higher than of ASTM 18

A325 steel fasteners, larger orders as usage increases may help reduce prices. The initial 19

research indicates that stainless fasteners can be a structurally sound option for building reduced 20

maintenance bridges. 21

22

Keywords: ASTM A1010, ASTM A193, ASTM A194, Buy America, plate girder, bolt tension, 23

corrosion, rotational capacity, slip-critical, splice plate connection, stainless steel 24

25


INTRODUCTION 1 After World War II, the standard practice in the United States with regard to steel bridge 2

members was to specify the use of ASTM A36 steel with multiple coats of lead-based paint for 3

protection against corrosion. Weathering steel was developed in other industries and adapted to 4

bridge use in accordance with ASTM A709. Weathering steel did not require coating and was 5

believed to add life-cycle cost savings by also avoiding periodic recoating in-service. After 6

limitations with weathering steel were discovered, the FHWA issued a list of contributing factors 7

identifying conditions that should be avoided (1). The use of ASTM A1010 (A1010) steel 8

girders in bridges may provide the combination of corrosion resistance and life-cycle cost 9

savings to address those factors. 10

Several years ago, Fletcher performed a study to compare the corrosion resistance of 11

several steel plate products (2). This study showed that based on life-cycle analysis, the A1010 12

steel plate material had a 90% chance of being more economical than conventional coated steel 13

after 20 years of service and a 100% chance after 40 years of service (2). The savings resulted 14

from the corrosion resistance of the steel, which makes painting and other corrosion-related 15

maintenance unnecessary. 16

In 2004, as part of the Innovative Bridge Research and Construction Program, a multi-17

cell box girder bridge design in Williams, California, used A1010 steel because of the close 18

proximity to water beneath the bridge. An A1010 steel producer used the material to fabricate 19

the first plate girders to support a bridge at their Coatesville Steel Mill in Pennsylvania in 2007. 20

Although these efforts demonstrate the viability of this technology, the Oregon Department of 21

Transportation is particularly notable in their efforts to use A1010 steel as a more corrosion-22

resistant alternative to weathering steel (3). As a result of this work, several other highway 23

agencies are pursuing the use of A1010 steel plate girders. 24

The Virginia Department of Transportation (VDOT) has applications where corrosion-25

resistant steel plate girders would be a competitive solution compared with other materials. 26

However, unlike previously built bridges, the first VDOT A1010 steel plate girder bridge would 27

require the use of bolted splice plate connections. Therefore, with the improved corrosion-28

resistant properties of the A1010 steel plate girders, the type of steel fastener to be used required 29

careful consideration. 30

VDOT specifications require the use of high-strength (ASTM A325 (A325)) bolts. The 31

current specifications, however, do not recognize ASTM A193 fasteners. In addition, the design 32

specifications require bolted splices to meet strict slip-critical behavior limits in design and 33

construction, and field testing is one component. Faying surfaces must meet prescribed friction 34

requirements, and field-tightening requirements must provide the specified clamping force 35

tension. An additional challenge is that the fasteners must meet the Buy America (BA) 36

requirements (4). Therefore, the question arose as to which stainless steel fastener would 37

appropriately balance the requirements of mechanical strength, economy, and corrosion 38

resistance in the assembly of A1010 steel plate girders. 39

40

PURPOSE AND SCOPE 41 To support the maintenance-free properties of A1010 steel plate girders, it is desirable to use 42

corrosion-resistant fasteners at the girder connections. This study used laboratory testing to 43

investigate the feasibility of using stainless steel fasteners with A1010 steel plate girders and to 44

determine if a reliable turn-of-the-nut tightening procedure could be developed. The study 45


assessed mechanical properties, corrosion resistance, shipping time, cost, and material 1

availability in accordance with BA requirements. 2

Bolt installation acceptance criteria were then developed around the splice plate design 3

for the replacement of the Route 340 Bridge over the South River in Waynesboro, Virginia. This 4

information provided appropriate information needed for selecting a fastener and determining the 5

turn-of-nut tightening procedure for splicing A1010 steel plate girders. 6

7

METHODS 8 9

Literature Review 10 The researchers reviewed current specifications regarding stainless steel bolting and bridge 11

construction to determine the material options available for testing. The literature included 12

ASTM, VDOT, and FHWA standards. 13

14

Selection and Ordering of Materials 15 Existing VDOT specifications allow the use of high-strength A325 bolts and matching nuts and 16

washer material (5). A325 bolts were tested alongside the stainless fastener assemblies for the 17

purpose of comparison (Table 1). ASTM A193 (A193) bolts manufactured from stainless steel 18

alloys were known to approach the VDOT specified strength for high-strength bolts. 19

20 TABLE 1 Matching ASTM and SAE Stainless Steel Material Types 21 Assembly

ID

Quantity

ASTM Bolt Designation

(SAE Bolt Designation)

ASTM Nut Designation

(SAE Bolt Designation)

Washer

B6 20 A193 B6 (Type 410) A194 Grade 6 (Type 410) SAE 410

B8 20 A193 B8 Class 2 (Type 304) A194 Grade 8 (Type 304) SAE 304

B8M 20 A193 B8M Class 2 (Type 316) A194 Grade 8M (Type 316) SAE 316

A325

Uncoated

20 A325 Type I Plain A563 Grade DH Plain, ASTM F436

A325

Galvanized

20 A325 Type I Galvanized A563 Galvanized, Grade

DH

Galvanized, ASTM

F436

22 The primary experimental concern with the A193 bolts was their ability to achieve 23

adequate mechanical properties. The fasteners selected would provide an alternative to standard 24

steel fasteners and needed to achieve the necessary clamping force with standard installation 25

methods. 26

Although mechanical properties are critical, A193 fasteners are produced to meet high 27

temperature service applications needed in the mechanical engineering field, but BA 28

requirements are not an industry issue in many of these applications (6). However, for federally 29

funded highway programs, BA requirements guide the procurement process for locating potential 30

suppliers and placing orders for steel and iron goods (4). Therefore, this study needed to confirm 31

procurement of fasteners complied with the BA requirements. To assess costs, the initial order 32

provided the values for the cost comparison. In addition, the researchers recorded the delivery 33

times of the initial and subsequent fastener orders. 34

Finally, since minimizing maintenance is important, the long-term durability of the 35

fasteners is critical. The inherent corrosion properties of the A193 bolts in comparison to A1010 36

was not of concern since the A193 bolts contain more chromium than A1010, making them more 37

corrosion resistant. However, for the manufacturing of B8 and B8M Class 2 bolts, it is common 38

in the United States to hot forge. The bolt head is placed on a threaded rod by heating the rod 39


red hot and forming the hex head. This can potentially lead to the release of tension that 1

occurred during strain hardening and a phenomenon called sensitization to occur (7). 2

Sensitization refers to the migration of chromium from the center of the grain to the grain 3

boundaries to bind with carbon to form chromium carbides because of improper heating and 4

cooling. This migration of chromium causes the material to become susceptible to intergranular 5

corrosion, which becomes a problem following installation and cause premature bolt failure. 6

7

Design of Route 340 Bridge 8 The bridge replacement project in Waynesboro was designed to allow for the use of 7/8-inch 9

stainless steel fasteners with a minimum tensile strength of 100 ksi at the splices. The design 10

followed AASHTO design guidelines and incorporated factors such as the number of slip planes, 11

surface condition factor, and hole size factor. Based on this analysis, a required minimum bolt 12

tension of 30 kips per bolt was used, rather than the conventional minimum bolt tension of 39 13

kips for a 7/8-inch A325 bolt, to determine if the fasteners were acceptable and would provide 14

the clamping load needed for a splice plate connection. 15

16

Testing 17 Uniaxial Tension Testing 18

Tension testing of the full-size fasteners in accordance with ASTM A370 determined ultimate 19

strength values and the elongation at failure (8). 3 ½ inch bolts were used for testing with the 20

distance between the grip length varying, with the variability in length being approximately 1 – 3 21

threads exposed above the lower grip. The data from the tension testing provided force-22

displacement graphs that illustrated the strength and ductility properties of each fastener type. A 23

laser extensometer measured the bolt elongation as the bolt was pulled in a tension frame. 24

25

Incremental Rotation Testing 26

VDOT’s Road and Bridge Specifications, Section 226.02(h), specify high-strength fastener A325 27

shall meet Rotational Capacity testing, and provides requirements for minimum tension, rotation 28

and maximum torque (5). This test was used on our control group of A325 fasteners and was the 29

basis for developing the incremental rotation test used to study the stainless alloys of A193 30

fasteners. 31

After receiving the fasteners, researchers grouped together fastener assemblies for testing, 32

clearly labeling each assembly to facilitate date recording. To study the rotation-tension 33

relationship in the sample fasteners, a fastener assembly was placed in the Skidmore-Wilhelm 34

device adding enough washers to leave one or two threads sticking out beyond the face of the 35

nut. A pea sized amount of anti-seizing lubricant was applied to the threads before placing the 36

nut at the end followed by tightening the nut to a tension of 4 kips (an industry standard for 7/8 37

inch diameter high-strength fasteners). A reference mark at zero and additional marks at twenty 38

degree increments were made from this position of the nut before turning the nut in the test. 39

The nut was turned twenty degrees and the tension and torque (using a dial type torque 40

wrench) were recorded. This process continued until the nut was turned sufficiently beyond 41

what would be allowed during field installation. 42

To analyze the data, the tension and the torque was plotted against the rotation for each 43

sample lot and these graphs analyzed to determine where the relationships exhibited non-linear 44

plastic behavior. It is difficult to determine an analytical basis because the bolt is under 45

combined tension and shear (torsion) stress, but the torque is resisted by both the bolt and the nut 46


at the interface with the washer. For this reason, the researchers decided to rely on the empirical 1

results to affirm the design values were safe. 2

After the first batch of tests, the researchers formalized the testing procedure by writing 3

an acceptance test using specific rotational capacity requirements for the A193 fastener 4

assemblies. The requirements were based on the test results of the first batch of A193 fastener 5

assemblies. The tension and torque requirements were adjusted to account for the different 6

properties of the A193 fasteners as compared with the A325 fasteners. The revised procedure 7

was used for the testing of subsequent bolt batches. 8

9 Hardness Testing 10

11 Rockwell Hardness Testing Rockwell B hardness testing was performed in accordance with 12

ASTM E18 (10). Rockwell hardness testing of the tested nuts was performed at 12 points, at 13

each corner, and at the midpoint between corners. Hardness testing was performed on untested 14

washers at 12 locations, 3 in each cardinal direction, when permissible. 15

A tested B8 bolt was cut longitudinally into three pieces. The samples were polished to a 16

final polish of 1200 grit in accordance with ASTM E3 (9). Hardness tests were then performed 17

down the length of the bolt. 18

19

Vickers Hardness Testing Bolts were cut in the same way as with the tested B8 bolt. The 20

samples were polished to 1200 grit in accordance with ASTM E3 (9). Microhardness tests were 21

performed using the Buehler Micromet 5101 hardness tester with a load of 1 kilogram and a 22

distance of 0.05 inch between data points across the diameter of the bolt. The testing was done 23

in accordance with ASTM E384 (11). The data were reported as the average of the three middle 24

points. 25

26

Optical Microscopy 27

The optical images of the bolt threads were taken on an as-received bolt with the Hirox KH-7700 28

Macro Lens with standard settings and lighting. The microscopy images of the sectioned bolt 29

threads were polished to a 3-micron finish in accordance with ASTM E3 (9). The images were 30

then taken at 100x magnification. 31

32

Scanning Electron Microscopy 33

The bolt fracture samples were first cut to fit inside the scanning electron microscope chamber 34

and then sonicated in ethanol for 5 minutes to remove any remaining stainless steel anti-seize 35

compound that remained on the samples from previous testing. All images were taken in the 36

secondary electron mode. 37

38

Sensitization Testing of B8 and B8M Bolts for Corrosion Resistance 39

The bolts were cut longitudinally into three pieces. These samples were then prepared and tested 40

in accordance with ASTM A262 (12). A second set of samples was prepared with a final polish 41

using a 3-micron diamond polish. All images were taken on the Hirox KH-7700 using the 10C 42

lens with a magnification of 490x unless otherwise specified. 43

The B8 and B8M bolt head samples were polished to 3 microns both to remove the 44

previous etching and to provide a clean surface to sensitize. The two bolt heads were then cut in 45

half. One-half of each bolt head was heated to 675°C (1247°F) in a furnace for 1 hour as guided 46


by ASTM A262 (12). Each sample was allowed to cool to room temperature in the closed 1

furnace environment. The samples were etched in accordance with ASTM A262 (12). 2

3

RESULTS AND DISCUSSION 4

5

Selection and Ordering of Materials 6 To assess costs, an initial order included several bolt options. The B6 bolt assembly order 7

included 20 A193 bolts; 20 ASTM A194 (A194) Grade 6 nuts; and 40 SAE Type 410 washers. 8

The B8 bolt assembly order included 30 A193 bolts; 30 A194 Grade 6 nuts; and 60 SAE Type 9

304 washers. The B8M bolt assembly order included 20 A193 bolts; 20 A194 Grade 6 nuts; and 10

40 SAE Type 316 washers. Galvanized and uncoated A325 bolt assemblies meeting the VDOT 11

specification were also ordered to serve as a baseline for comparison during testing. All items 12

discussed in this report met BA requirements; a fourth type of bolt was not included in this study 13

because a supplier that met the BA requirements for A193 B8MN bolts could not be located. 14

Therefore, it is important to know that not all A193 bolts will meet BA requirements. 15

The cost breakdown of each stainless fastener assembly is provided in Table 2. Although 16

it is possible that the price per item was affected by the relatively small quantities of material 17

purchased, it was surprising how substantial the difference in cost was between the B6 and the 18

other fasteners. Earlier work has shown the sensitivity of stainless pricing to alloying elements 19

such as nickel and molybdenum, so it might be expected that the B6 bolt would have a lower 20

cost compared to the B8 and B8M fasteners (13). It was also interesting that the cost of the 21

A193 bolts was around the same order of magnitude, but the cost of the nut and washer for the 22

B6 fasteners was much higher. 23

24 TABLE 2 Costs for Each Stainless Steel Fastener Assemblies Item Listed 25 Item (Dimensions) A325 Uncoated B6 B8 B8M

Bolt (⅞-in x3-½ in ) $1.88 $28.25 $14.65 $14.50

Nut (for ⅞-in bolt) $1.90 $131.00 $16.50 $17.80

Washer (for ⅞-in bolt) $0.31 $130.00 $2.60 $1.90

Total Stainless Steel Fastener Assembly:

1 Bolt, 1 Nut, and 1 Washer $4.09 $289.25 $33.75 $34.20

26

The delivery times for various vendors were also tracked and compared for the initial and 27

subsequent orders. Vendor 1 provided 7/8-inch A325 assemblies, both plain and galvanized, in 6 28

days, which was used as a baseline to evaluate the delivery of the other stainless steel nut, bolt, 29

washer assemblies. Vendor 2 provided 7/8-inch and 1-inch A193 bolts and A194 nuts in 7 days. 30

Vendor 3 provided 7/8-inch 303 stainless steel washers in 5 days. Vendor 3 provided 7/8-inch 31

304 stainless steel washers in 16 days. The first three vendors had reasonably close delivery 32

times as compared to those for an A325 assembly. Vendor 4 required nearly 3 times as long to 33

provide the stainless product when compared to the delivery time for A325 fasteners. Moreover, 34

a second order placed with Vendor 2 for stainless fasteners required 45 days from the time the 35

order was place until it was delivered. It is clear that additional time can be required to receive 36

A193 and A194 stainless fasteners. 37

38

Test Results 39

40 Uniaxial Tension Behavior of Fastener Assemblies 41


The A193 standard required the 7/8-inch-diameter stainless fasteners to reach an ultimate 1

strength of up to 115 ksi (6). Figure 1(a) shows the average test results for the uniaxial tension 2

tests of three bolts (7/8-inch diameter, 3.5-inch length) from each of the three stainless fastener 3

types. Each fastener type reached its required strength. The B8M fastener showed the most 4

ductility, and the B6 fasteners the least. The B8 fastener had a combination of high strength and 5

ductility. 6

There were difficulties with the uniaxial tension tests concerning equipment. A laser 7

extensometer recorded the elongation of the bolt in the tension frame. At low elongation and 8

stress values, there were inconsistencies in the test data, which was attributed to the sensitivity of 9

the laser. However, as the displacement value became significant, the inconsistencies stopped. 10


1 FIGURE 1 Stress vs. displacement curves showing (a) average response for A193 B8, B8M and B6 stainless 2 fastener assemblies, and (b) graph of variability in uniaxial tension testing for different A193 B8 fasteners, 3 with each line representing results from A193 B fastener testing. 4 5

(a)

(b)


A193 B8 Fastener Uniaxial Test Variability The A193 B8 fasteners appeared to show limited 1

variability in the uniaxial tension tests and rotational capacity tests. Figure 1(b) compares the 2

uniaxial test results of three individual A193 B8 fasteners with the overall average. The figure 3

compares only stresses above 90 ksi because of measuring inconsistencies at low displacement 4

values. The solid line represents the average, and the dashed lines represent individual fasteners. 5

Near yielding, the fastener stresses range from approximately 110 ksi to more than 120 ksi. The 6

ultimate strength range generally was between 135 ksi and 140 ksi. 7 8 Incremental Rotation Testing 9

Researchers conducted initial rotational capacity tests on a single batch of stainless steel 10

fasteners and compared the results with those of control tests of A325 fasteners. The stainless 11

steel fasteners tested were as follows: eight B8 assemblies, seven B8M assemblies, seven B6 12

assemblies, eight plain A325 assemblies, and five galvanized A325 assemblies. 13

During initial testing of A193 bolts, after the A325 bolts were tested to develop a baseline 14

for comparison, researchers used a stainless alloy washer that was not specified as a hardened 15

washer. The recorded torques were higher than anticipated, and the cause was the developed 16

friction as the nut ground into the washer. The subsequent distress in the washer was clearly 17

visible without magnification. Soon after these first few bolts were tested, hardened stainless 18

steel washers were obtained for the remaining testing which eliminated this problem and 19

demonstrated that hardened washers must be used. 20

Results were plotted each day after testing using the rotation of the nut as the independent 21

variable and the tension and torques as the dependent variables. The trend indicated on the 22

graphs was studied for consistency to support the validity of the testing method. Researchers 23

expected to note a portion of the graph indicating plasticity since earlier direct tension testing 24

showed a yield point in that mechanism. The levels of stress and rotation necessary to achieve 25

that stress were noted. Next, the amount of rotation beyond the “break point” in the linearity of 26

the graph was noted to ensure the bolts would have reserve strength beyond the installation point. 27

The trend toward the end of the graph was observed to determine if there was a distinct loss of 28

load-carrying capacity prior to failure. Finally, the rotation vs. torque curves were studied to 29

ensure that something unforeseen or unanticipated had not occurred that would be considered 30

detrimental to the long-term load-carrying capacity of the bolt, such as unexplained spikes in the 31

torque during tightening. 32

Figure 2(a) shows the average results of the initial rotational capacity tests. The figure 33

shows the development of tension in the bolted assemblies as the angle of rotation of the nut was 34

increased. The angle of rotation was measured relative to the snug tight condition at 0 degrees. 35

In general, the stainless steel fastener assemblies had lower tension values than the plain steel 36

assemblies at high angles of rotation. The line shows the average data from the A325 uncoated 37

assemblies, serving as a basis for comparison. The relationship between the two material types 38

was not as clear at lower angles of rotation. The B6 and B8 assemblies appeared to develop the 39

highest tension values up to about 150 degrees of rotation. The tension in the A325 galvanized 40

and uncoated assemblies significantly increased between about 125 and 250 degrees. Among the 41

stainless fastener assemblies, the B6 assembly appeared to reach the highest overall tension, 42

followed closely by the B8 assembly. Skidmore-Wilhelm testing showed that the B8 and B8M 43

assemblies were capable of reaching high angles of rotation. 44

The amount of torque required for the B6 and B8 assemblies to reach a given angle of 45

rotation was greater than for the A325 uncoated assembly, as shown by Figure 2(b). Despite 46

this, the stainless assemblies all remained below the current torque limit set by VDOT. 47


Researchers gathered the data shown in the figure using a wall-mounted Skidmore-Wilhelm 1

testing device with the aid of a torque multiplier. Over the course of testing, the researchers also 2

used a free-standing Skidmore-Wilhelm test setup. The torque values recorded during testing 3

with the free-standing setup appeared to be consistently higher for all fastener materials than 4

when the wall-mounted device was used. This was attributed to using a torque multiplier for the 5

wall-mounted device which reduced the leverage required while gathering data from the torque 6

wrench. 7

8


1 FIGURE 2 Rotational capacity test results showing (a) the tension vs. angle of rotation for fastener 2 assemblies, and (b) the torque vs. angle of rotation for fastener assemblies. Galv. = galvanized. 3 4

Bolt tension did not develop as rapidly in the A193 bolts as it did in the standard A325 5

bolts. However, the A193 bolts reliably attained the 30 kips per bolt assumed in the design. The 6

rotation required to attain the 30 kips was not a concern when compared to the amount of 7

rotation the bolts were able to undergo without fracturing. Because of the expense of obtaining 8

another Skidmore-Wilhelm testing device, the nuts were not turned enough to cause the bolt to 9

(a)

(b)

0

10

20

30

40

50

60

0 100 200 300 400 500 600

Ten

sio

n,

kip

Angle of Rotation, Degrees

B8 B6 B8M 325 Plain 325 Galv. 325 Trendline

0

100

200

300

400

500

600

700

0 100 200 300 400 500 600

To

rque,

ft*

lb


B6 B8 B8M B8 Hybrid 325 Plain 325 Galv.


fracture, which could have damaged the testing equipment. Instead, the nuts were turned well 1

beyond twice the angle of rotation anticipated to be used during installation. 2

The researchers often noted cracks developed in the root of the thread, as anticipated. In 3

addition, the researchers often noted a second zone just under the head of the bolt where necking 4

was beginning to occur. Discussions with workers at the manufacturing facility indicated that 5

these bolts are often manufactured by heating the round stock to orange hot before forming the 6

head and allowing the bolt to cool. This is thought to relieve some stresses but also reduces the 7

yield point of the steel. Standard practice for A325 bolts uses a greater degree of cold working. 8

This second zone of necking is thought to contribute to the reduction in the linearity of the 9

rotation vs. tension relationship. 10

At the end of the testing program, the researchers were comfortable that the bolts would 11

reliably achieve the 30 kips necessary to support the assumptions used during design and that the 12

bolts would have plenty of reserve capacity to justify the rotations specified to obtain the 13

specified clamping force. However, the tests were conducted on a fairly limited number of bolts 14

from each specified alloy, and only two production lots of bolts were tested. To establish a 15

recommended value for lower clamping force AASHTO splice designs, a statistically significant 16

number of bolt lots will be required to establish a design table. 17

A193 B8 Fastener Tension Variability Figure 3(Left) shows the variability of tension during 18

the tightening of the initial eight A193 B8 fasteners. Figure 3(Right) shows the tension 19

variability of eight A325 plain fasteners. The A325 fasteners showed significantly more 20

variability in tension between about 100 to 250 degrees of rotation from the “snug tight 21

condition.” 22

23

24 25 FIGURE 3 Comparison of variability in tension development in (Left) A193 B8 fastener, and (Right) A325 26 plain fastener. 27 28 All fasteners showed significant torque variability during testing. In general, the A325 29

fasteners required less torque than the A193 fasteners to reach a given angle of rotation, which 30

might in the stainless steel be attributed to thread galling (14). For any fastener type, torque at a 31

given angle of rotation could vary by as much as 100 ft-lb between fasteners of the same 32

production lot. 33

34

Hardness 35

0

10

20

30

40

50

60

70

0 100 200 300 400

Ten

sio

n i

n k

ip


0

10

20

30

40

50

60

70

0 100 200 300 400

Ten

sio

n i

n k

ip



In the standard carbon steel nut, washer, and bolt set, the nut was significantly less hard than the 1

washer. However, in the case of the stainless steel fasteners, the opposite was true. The washers 2

were significantly less hard than the nuts. This resulted in gouges being formed in the washers 3

during initial testing, which means that not all of the torque applied to the nut is used to tighten 4

the nut on the washer. Thus, to minimize the galling phenomenon, the hardness of the washer 5

should be maximized, while still meeting the necessary standards. 6

The B6 bolt showed the most consistent hardness through the length of the bolt. The 7

hardness values of the B8 and B8M bolts, however, varied greatly from the bolt head to the bolt 8

threads. The bolt head (Point 1) was at least 100 HV10 less than the threads (the last two points) 9

were ultimately. The greatest variability was in the bolt shank and the beginning of the threads, 10

most likely because of the manufacturing process. 11

A difference can be seen in the hardness values in the first and second sets of B8 bolts. 12

In the first, the B8 bolts tended to break just below the shank region; in the second, the B8 bolts 13

broke further down in the thread region. The locations of the fractures correlated with the drops 14

in hardness shown in Figure 4. The hardness values of the first set of B8 bolts dropped 15

significantly at Point 4, which is just below the heat mark on the bolt. The hardness values of the 16

second set dropped slightly at Point 5, which is about midway into the threads. These two drops 17

were also seen in the hardness testing of the tested B8 bolt. 18 19

20 FIGURE 4 Vickers microhardness values along bolt length. 21 22 Optical Microscopy 23

Figure 5(a) depicts the typical rough threads that can occur in the bolt manufacturing process. It 24

was determined that the metal smearing effect was due to a machining error as opposed to 25

microcracking that occurred on the surface. Figures 5(b) and 5(c) support the idea that the 26

material in the bolt threads was merely smeared as opposed to cracked. The rough threads did 27

not have an effect on the ability of the bolts to perform as needed. 28


1 FIGURE 5 Thread condition showing (a) rough threads on B6 bolt, (b) rough threads looking 2 perpendicularly at longitudinally cut B6 bolt, and (c) smooth threads looking perpendicularly at 3 longitudinally cut B8M bolt. 4 5

Scanning Electron Microscopy 6

Figure 6(b) shows the two phases of microvoid coalescence cracking. As a ductile material is 7

being tensioned, microvoids begin forming. As more tension is placed on the sample, the 8

microvoids coalesce and form the structure in the region highlighted by the smaller oval. 9

Finally, the microvoids form into a crack, as shown by the region highlighted by the larger oval. 10

Microvoid coalescence cracking is most often seen in ductile fractures, such as that of the B8 11

bolt. The smooth “rock candy” structures that dominate in Figure 6(c) are indicative of an 12

intergranular cleavage fracture. However, the rough surfaces, highlighted by the ovals, show 13

regions of dimpling, which are indicative of some, though minimal, ductility in the material. 14

Figure 6(d) depicts dimples on the B8M fracture surface, which are indicative of a ductile 15

fracture. However, the B8M bolt (Figure 6d) did not show any signs of microvoid cracking on 16

the fracture surface. 17

A second set of B8 bolts was ordered and tested. These bolts also broke in a ductile 18

fashion. However, the fracture occurred lower in the threads and had more microvoid cracking 19

(a)

(b) (c)


than in the first set. The increase in microvoid cracking may be explained by the location of the 1

fracture. 2

The three different bolts fractured as expected. The austenitic stainless steel bolts (B8 3

and B8M) fractured in a typical ductile fashion, necking before ultimately failing and covered in 4

dimples. The B6 bolt fractured in a typical cleavage fracture. However, the fact that it fractured 5

in an intergranular manner as opposed to a transgranular manner may be cause for concern. 6 7

8 FIGURE 6 Fracture surfaces of different bolts after uniaxial tensile testing showing (a) A325 showing 9 ductile (left side) transitioning to brittle (right side) (b) microvoid coalescence in B8 bolt, (c) intergranular 10 brittle fracture in B6 bolt, and (d) dimple on ductile fracture B8, B8M bolt. 11


Sensitization of B8 and B8M Bolts 1

The B8 and B8M microstructures were evaluated in accordance with ASTM A262. However, 2

because scratches obscured the surface, the decision was made to polish the sample to 3 microns. 3

Figure 7(a) shows the B8 bolt head microstructure, where sensitization due to the manufacturing 4

process is most likely to occur. Although some ditch structures can be seen in the grains 5

highlighted by the ovals, grains are not completely surrounded by ditches. The streaks that are 6

seen across the image are forge lines that appeared only after electroetching. Neither the sample 7

prepared in accordance with ASTM A262 nor that was polished to 3 microns showed 8

sensitization. The as-received B8M bolt did not show any signs of sensitization. 9

A section of the B8 bolt was then intentionally sensitized and compared to an as-received 10

section. Figure 7(b) shows the unsensitized material on the left and the sensitized material on the 11

right in this photograph. The difference between the grain boundaries of the two can be easily 12

seen with grains on the sensitized bolt showing grains that are completely surrounded by ditches. 13

The as-received B8M bolt that received a similar additional heat treatment as the B8 did not 14

show any signs of sensitization. 15

16


1 FIGURE 7 Sensitization of (a) B8 bolt head region polished to 3 microns, and (b) comparison of B8 bolt head 2 after sensitization treatment on bolt section (unsensitized on left side of vertical black line and sensitized on 3 right side) at 210x magnification. 4

5

CONCLUSIONS 6

● The A193 stainless fasteners will develop lower tension values than traditional A325 7

steel fasteners at equivalent installation rotations. 8

● The A193 B8 fasteners tested met the minimum bolt splice tension requirement of 30 9

kips per bolt. 10

● The use of hardened washers helps achieve better mechanical performance. 11

(a)

(b)


● Bolt installation should be guided by rotation values rather than torque values. 1

● Not all A193 bolts or A194 nuts will meet BA requirements. 2

● Compared to A325 fasteners, additional delivery time should be anticipated when 3

specifying A193 fasteners. 4

● Sensitization was not detected in the as-received B8 or B8M bolts. 5

● A193 B8 bolts can meet the Route 340 splice plate connection design requirements. 6

ACKNOWLEDGMENTS 7 Justin Ocel of FHWA , Hormoz Seradj of Oregon DOT, Andrew Zickler, Keith Harrop, William 8

Via Jr., Larry Lundy, Keith Williams, Trey Simpson, and Brian Simmons of VDOT, and Bill 9

Ordel and James Copeland of VTRC are recognized for their contributions. 10

11

REFERENCES 12 1. Willett, T. O. Uncoated Weathering Steel in Structures. Publication Technical Advisory 13

5140.22. FHWA, U.S. Department of Transportation, 1989. 14

https://www.fhwa.dot.gov/bridge/t514022.cfm. Accessed July 11, 2016. 15

2. Fletcher, F. B. Improved Corrosion-Resistant Steel for Highway Bridge Construction. 16

Publication FHWA-HRT-11-062. FHWA, U.S. Department of Transportation, 2011. 17

3. Hormoz, S. Oregon’s Experience with ASTM A1010. In TRB 94th Annual Meeting 18

Compendium of Papers, Transportation Research Board of the National Academies, 19

Washington, D.C., 2015. 20

4. FHWA. Construction Program Guide: Buy America, March 16, 2016. 21

https://www.fhwa.dot.gov/construction/cqit/buyam.cfm. Accessed Feb. 5, 2016. 22

5. Virginia Department of Transportation. 2007 Road and Bridge Specifications. Richmond, 23

2007. 24

6. ASTM International. ASTM A193/A193M-16 Standard Specification for Alloy-Steel and 25

Stainless Steel Bolting for High Temperature or High Pressure Service and Other Special 26

Purpose Applications. West Conshohocken, Pa., 2016. 27

7. Portland Bolt & Manufacturing Company. Ordering A193 Class 2 Bolts, January 25, 2011. 28

http://www.portlandbolt.com/technical/faqs/ordering-astm-a193-class-2-bolts/. Accessed 29

Feb. 23, 2016. 30

8. ASTM International. ASTM A370-16 Standard Test Methods and Definitions for Mechanical 31

Testing of Steel Products. West Conshohocken, Pa., 2016. 32

9. ASTM International. ASTM E3-11 Standard Test Methods for Rockwell Hardness of 33

Metallic Materials. West Conshohocken, Pa., 2011. 34

10. ASTM International. ASTM E18-16 Standard Guide for Preparation of Metallographic 35

Specimens. West Conshohocken, Pa., 2016. 36

11. ASTM International. ASTM E384-16 Standard Test Method for Knoop and Vickers 37

Hardness of Materials. West Conshohocken, Pa., 2016. 38

12. ASTM International. ASTM A262-15 Standard Practice for Detecting Susceptibility to 39

Intergranular Attack in Austenitic Stainless Steels. West Conshohocken, Pa., 2015. 40

13. Presuel-Moreno, F., J. R. Scully, and, S. R. Sharp. Identification of Commercially Available 41

Alloys for Corrosion-Resistant Metallic Reinforcement and Test Methods for Evaluating 42

Corrosion-Resistant Reinforcement. Publication VTRC 08-R21. Virginia Transportation 43

Research Council, Charlottesville, 2008. 44

https://www.fhwa.dot.gov/bridge/t514022.cfm

https://trid.trb.org/view/1336382

https://trid.trb.org/view/1336382


14. Fastenal Company. Technical Reference Guide, September 13, 2005. 1

http://www.fastenal.com/content/documents/FastenalTechnicalReferenceGuide.pdf. 2

Accessed November 15, 2016. 3

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