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Stud Bolt for Flanges (ASME B16.5)

What is a stud bolt? Stud bolts for flanges consist of a fully threaded steel rod and two heavy hexagonal steel nuts. Stud bolts are inserted in the flange holes and tightened to seal a flanged joint. The number, the length, the diameter of the stud bolts required for a flanged connection depend on the flanges type, diameter, and rating (as per the ASME flange bolt chart).

STUD BOLT AND NUTS

A stud bolt consists of one threaded steel rod and two (matching) hexagonal heavy steel nuts. Stud bolts and nuts are essential components of flanged joints, as they are key to seal flanged joints properly.

Stud bolts materials

Stud bolts are available in multiple diameters and lengths combinations, metric and imperial, and in a variety of materials from carbon steel to alloy, stainless and nickel alloys (common grades are: ASTM A193 B7, L7, B8, B8M, B16, Alloy 20, Monel, Hastelloy, Inconel, 17-4PH, titanium, etc.). These grades are discussed below in this article.

The material of the hex steel nuts shall match the material of the threaded rod (generally, stud materials ASTM A193 match with nuts materials ASTM A194).

The ASME B16.5 specification covers stud bolts and nuts for ASME flanges (number, the diameter and the length of required stud bolts, i.e “flange bolt chart”) by flange diameter, pressure class and face finish (RF, FF, RTJ).

STUD BOLT (THREADED ROD)

The Stud (threaded rod)

A stud is a threaded rod that is inserted into the holes of the mating flanges and then tightened, at both ends, by applying a specific torque to the steel nuts.

Stud bolt length (OAL/FTF)

The length of the stud can be measured either as overall length (i.e., “OAL”) or as “first useable thread to first useable thread” (i.e., “FTF”).

The FTF length can be calculated by subtracting a quarter of an inch to the OAL length, and it the standard stud length measurement for piping applications.

Flanges of different diameter and rating require stud of different lengths and diameters.
OAL and FTF stud length

Stud bolt thread pitch and thread series

The stud is threaded according to the specifications set by ASME B1.1.
The term “thread pitch” refers to the steepness of the angle of thread, which is measured in threads per inch.

The most used thread is the symmetrical thread pitch type with a “V-profile” (a 60-degree angle), as it is easier for the manufacturer to inspect compared to the non-symmetrical types.

Thread series relate to the diameter and pitch combinations, measured by the number of threads per inch (“TPI”) applied to a single diameter:

  • Coarse thread series (UNC/UNRC): UNC is the most common type for screws, bolts, and nuts. Coarse threads are used for threads in low strength materials such as iron, mild steel, copper and softer alloy, aluminum, etc. The coarse thread is also more tolerant of adverse conditions and it facilitates quick assembly
  • Fine thread series (UNF/UNRF): UNF is widely used for precision duties and when higher tensile strength is required (vs. the coarse thread series)
  • 8-Thread series (8UN) is the elective threading method for several ASTM standards including A193 B7, A193 B8/B8M, and A320. This series is widely used for diameters from one inch upwards

Stud bolts covered by the ASTM A193 or A320 Specifications use 8UN for all diameters 1” and above, which means that there are 8 threads per inch for these sizes. For any other material grade, the buyer shall specify the thread pitch unless it is given by the applicable ASTM norm.

bolt thread pitch .       v pitch 60 degrees

 

NUTS FOR STUD BOLTS

To fasten a stud, two heavy hex steel nuts have to be bolted at the opposite ends of the rod.

The stud and the steel nuts join perfectly due to the friction existing between their respective threads, by a slight stretching of the bolt, and the compression of the two joined parts.

heavy hex nut

In the past, steel nuts had a squared head. Nowadays, hexagonal head nuts (featuring 6 sides instead of 4) have fully replaced the older shape as they ensure quicker and more effective screwing on the stud.

Nuts for petrochemical applications are tightened to a specific torque using special torque wrenches. The mechanical strength of the steel nut material shall be compatible with the strength of the mated bolt.

The dimensions and weights of heavy steel nuts for stud bolts are covered by the ASME B18.2.2 specification.

 

STUD VS BOLT

A common question is what is the difference between stud and bolts?

The answer is that a stud is a metal rod or shaft that features threads on both sides (as shown in the image above) and requires two heavy hexagonal nuts to be serrated; a bolt is a fastener with a “built-in” head nut at one side and that requires a single nut to be serrated.

Stud bolts Bolt
Stud vs. bolt (stud on the left image, bolt on the right image)

 

STUD BOLT MATERIALS

The most common materials for stud bolts (for flanges) are ASTM A193 (grade B7, B8, B8M, B8T), ASTM A453 (grade 660), ASTM A320 (grade L7, L7M), and ASTM A182 (duplex and super duplex bolting). For aggressive fluids and environments, stud bolts can be coated with Xylan, Xylar and other materials.

ASTM A193 STUD BOLTS (HIGH-TEMP.)

The ASTM A193 specification covers alloy-steel and stainless steel stud bolts materials for high temperature or high-pressure service.

ASTM A193 stud bolts are available in national coarse (UNC) thread pitches, generally used in traditional applications, which means that there are 8 threads per inch (“thread per inch”) for rod diameters above 1 inch. B7 is the most common specification grade for stud bolts.

stud bolt materials
threaded stud bolt

The most common stud bolts materials covered by ASTM A193 are:

  • ASTM A193 B5
  • ASTM A193 B6
  • ASTM A193 B7: Alloy steel, AISI 4140/4142 quenched and tempered
  • ASTM A193 B7M
  • ASTM A193 B16
  • ASTM A193 B8: Class 1 Stainless steel, AISI 304, carbide solution treated.
  • ASTM A193 B8A
  • ASTM A193 B8M: Class 1 Stainless steel, AISI 316, carbide solution treated.
  • ASTM A193 B8MA
  • ASTM A193 B8T (SS 321)
  • ASTM A193 B8cl2: Class 2 Stainless steel, AISI 304, carbide solution treated, strain hardened
  • ASTM A193 B8Tcl2
  • ASTM A193 B8Mcl2: Class 2 Stainless steel, AISI 316, carbide solution treated, strain hardened

ASTM A193 Stud Bolts: Chemical Composition

Physical ElementASTM A193 grade B7 (AISI 4140)ASTM A193 grade B8 (AISI 304)ASTM A193 grade B8M (AISI 316)
Carbon0.38 – 0.48%0.08% max0.08% max
Manganese0.75 – 1.00%2.00% max2.00% max
Phosphorus, max0.035%0.045%0.045%
Sulfur, max0.040%0.030%0.030%
Silicon0.15 – 0.35%1.00% max1.00% max
Chromium0.80 – 1.10%18.0 – 20.0%16.0 – 18.0%
Nickelabsent8.0 – 11.0%10.0 – 14.0%
Molybdenum0.15 – 0.25%absent2.00 – 3.00%

ASTM A193 Stud Bolts: Mechanical Properties

ASTM A193SizeMin Tensile Strength, in ksiMin Yield Strength, in ksiMin Elongation, in %RA % minMax HBWMax HRC
ASTM A193 grade B7Up to 2-1/2125105165032135
2-5/8 – 4115951650
4-1/8 – 7100751850
ASTM A193 grade B8 Class 1All7530305022335
ASTM A193 grade B8M Class 1All7530305022396
ASTM A193 grade B8 Class 2Up to 3/4125100123532135
7/8 – 1115801535
1-1/8 – 1-1/4105652035
1-3/8 – 1-1/2100502845
ASTM A193 grade B8M Class 2Up to 3/411095154532135
7/8 – 1100802045
1-1/8 – 1-1/495652545
1-3/8 – 1-1/290503045

The material chosen for the threaded stud and the hex nuts should be compatible. Materials for bolting sets shall be selected based on the process’s working temperatures, as shown in the table below:

TEMPERATURE °C ( °F )
STUD BOLTS MATERIALSMIN.MAX.
CARBON STEEL-29 (-20)300 (572)
A193 B7, L7-73 (-100)400 (752)
A193 B60 (32)500 (932)
A193 B8-200 (-325)575 (1067)
A193 B160 (32)520 (968)
A193 B17B-29 (-20)650 (1202)
A913 Inconel 7180 (32)750 (1382)
A453 Gr. 660-29 (20)538 (1000)

Stud bolts can be produced also with “dual certification,” i.e. they conform to multiple sets of ASTM standards (A193 B7 stud bolts can meet the requirements of A320 L7; B8 and B8m stud bolts also typically conform to both A193 and A320).

ASTM A453 STUD BOLTS (HIGH-TEMP.)

The ASTM A453 specification covers standards for Grade 660 (Class A, B, C, and D), Grade 651 (Class A and B), Grade 662 (Class A and B), and Grade 665 (Class A and B) of bolting materials, with ten classes of yield strength ranging from 50 to 120 KSI [345 to 827 MPa], for use in high-temperature service such as fasteners, pressure vessels and flanges.

Bolting materials in ASTM A453 are covered rolled, forged, or hot-extruded bars, and also bolts, nuts, screws, washers, studs, and stud bolts.
Materials shall adhere to specified contents of carbon, manganese, phosphorus, sulfur, silicon, nickel, chromium, molybdenum, tungsten, titanium, columbium, aluminum, vanadium, boron, and copper.

According to ASTM A453 materials shall be subjected to tension, stress-rupture, and hardness tests. Materials shall conform to yield strength, tensile strength, elongation, reduction of area, Brinell hardness, and Rockwell hardness requirements.

Hardening and solution treatment requirements for each material class are also given. The most common grades under ASTM A453 are:

  • ASTM A453 660A
  • ASTM A453 660B
  • ASTM A453  660C
  • ASTM A453 660D

ASTM A453 Chemical Composition

ELEMENTASTM A453 Grade 660ASTM A453 Grade 651ASTM A453 Grade 662ASTM A453 Grade 665ASTM A453 Grade 668
%%%%%
Carbon0.08 max0.28-0.350.08 max0.08 max0.08 max
Manganese2.00 max0.75-1.50.40-1.001.25-2.002.00 max
Phosphorus, max0.040 max0.040 max0.040 max0.040 max0.040 max
Sulfur, max0.030 max0.030 max0.030 max0.030 max0.030 max
Silicon1.00 max0.30-0.800.40-1.000.1-0.801.00 max
Nickel24.0-27.08.0-11.024.0-28.024.0-28.017.5-21.5
Chromium13.5-16.018.0-21.012.0-15.012.0-15.013.5-16.0
Molybdenum1.00-1.501.00-1.752.0-3.501.25-2.251.50 max
Tungsten1.00-1.75
Titanium1.9-2.350.1-0.351.80-2.102.70-3.302.20-2.80
Columbium *0.25-0.6
Aluminum0.35 max0.35 max0.25 max0.50 max
Vanadium0.10-0.500.50 max
Boron0.001-0.0100.001-0.0100.01-0.070.001-0.010
Copper0.50 max0.50 max0.25 max

ASTM A453 Stud Bolts: Mechanical Properties

ASTM A453 GradeClassTensile, MpaYield, MpaElong. %, minRA %, min
ASTM A453 Grade 660A, B , & C895 min585 min1518
 D895 min725 min1518
ASTM A453 Grade 651A690 min485 min1835
415 min
 B655 min415 min1835
345 min
ASTM A453 Grade 662A895 min585 min1518
 B860 min550 min1518
ASTM A453 Grade 665A1170 min830 min1215
 B1070 min830 min1215
ASTM A453 Grade 665A & B895 min858 min1518

ASTM A453 Stud Bolts: Heat Treatments Requirements

ASTM A453 ClassSolutiona TreatmentHardening Treatment
660A1650 +/- 25 °F [900 +/-14 °C], hold 2 h, min and liquid quench1325 +/- 25 °F [720 +/- 14 °C] 16h, air cool
B1800 +/- 25 °F [980 +/-14 °C], hold 1 h, min and liquid quench1325 +/- 25 °F [720 +/- 14 °C] 16h, air cool
C1800 +/- 25 °F [980 +/-14 °C], hold 1 h, min and oil quench1425 +/- 25 °F [775 +/- 14 °C] hold 16h, air cool followed by 1200 +/- 25 °F [650 +/- 14 °C], hold 16h, air cool
D1650 +/- 25 °F [900 +/-14 °C], hold 2 h, min and liquid quench OR1325 +/- 25 °F [720 +/- 14 °C] hold 16h, air cool followed by 1200 +/- 25 °F [650 +/- 14 °C], hold 16h, air cool
if necessary to achieve properties, second age : 1200 +/- 25 °F [650 +/- 14 °C] hold 16h, air cool
1800 +/- 25 °F [980 +/-14 °C], hold 1 h, min and liquid quench
651Ahot-cold worked at 1200 °F[650] min with 15% min reduction in cross-sectional area, stress-relief anneal at 1200 °F [650 °C] min or 4h, min
Bhot-cold worked at 1200 °F[650] min with 15% min reduction in cross sectional area, stress-relief anneal at 1350 °F [730 °C] min or 4h, min
662A1800 +/- 25 °F [980 +/-14 °C], hold 1 h, min and liquid quench1350 to 1400 °F [730 to 760 °C], hold 20h, furnace cool to 1200 +/- 25 °F [650 +/- 14 °C], hold 20h, air cool
B1950 +/- 25 °F [1065 +/-14 °C], hold 2 h, min and liquid quench1350 to 1400 °F [730 to 760 °C], hold 20h, furnace cool to 1200 +/- 25 °F [650 +/- 14 °C], hold 20h, air cool
665A1800 +/- 25 °F [980 +/-14 °C], hold 3 h, min and liquid quench1350 to 1400 °F [730 to 760 °C], hold 20h, furnace cool to 1200 +/- 25 °F [650 +/- 14 °C], hold 20h, air cool
B2000 +/- 25 °F [1095 +/-14 °C], hold 3 h, min and liquid quench1350 to 1400 °F [730 to 760 °C], hold 20h, furnace cool to 1200 +/- 25 °F [650 +/- 14 °C], hold 20h, air cool
668A1650 +/- 25 °F [900 +/-14 °C], hold 2 h, min and liquid quench1325 +/- 25 °F [720 +/- 14 °C] 16h, air cool
B1800 +/- 25 °F [980 +/-14 °C], hold 1 h, min and liquid quench1325 +/- 25 °F [720 +/- 14 °C] 16h, air cool

A453 Stud Bolts: Time to Rupture and Elongation

GradeClassTest TemperatureStress, MinTime to RuptureElongation
Deg F [ Deg C ]KsiMpaMin, h*Min %
660A, B & C1200 [650]563851005
651A & B1200 [650]402751005
662A & B1200 [650]553801005
665A1200 [650]755151003
B1200 [650]704851005

ASTM A320 STUD BOLTS (LOW-TEMP.)

The ASTM A320 specification covers alloy steel and Stainless Steel stud bolts materials for low-temperature service.

Each alloy under ASTM A320 shall conform to prescribed chemical requirements. The material, as represented by the tension specimens, shall conform to specific requirements in terms of tensile strength, yield strength, elongation, and hardness. The stud bolt material shall meet the prescribed impact energy absorption requirements and the target test temperature.

According to the ASTM A320 specification, manufacturers shall execute at least the following mechanical tests on the material: impact test, tension test, and hardness test.

The most common stud bolts materials under ASTM A320 are listed below (low-temperature service):

  • ASTM A320 L7: Alloy steel, AISI 4140/4142 Quenched and tempered 
  • ASTM  A320 L7M
  • ASTM A320 L43: Alloy steel, AISI 4340 Quenched and tempered 
  • ASTM A320 B8 Class 1: Stainless steel, AISI 304, carbide solution treated 
  • ASTM A320 B8A
  • ASTM A320 B8T
  • ASTM A320 B8TA
  • ASTM A320 B8C
  • ASTM A320 B8M: Class 1 Stainless steel, AISI 316, carbide solution treated 
  • ASTM A320 B8MA
  • ASTM A320 B8cl2: Stainless steel, AISI 304, carbide solution treated, strain hardened 
  • ASTM A320 B8Mcl2: Stainless steel, AISI 316, carbide solution treated, strain hardened

ASTM A320 Stud Bolts: Mechanical Properties

ASTM A320 GradeDiameterTensile, KSI, minYield, KSI, minCharpy Impact 20-ft-LBF @ tempElong, %, minRA, %, min
ASTM A320 Grade L7Up to 212125105-150° F1650
ASTM A320 Grade L43Up to 4125105-150° F1650
ASTM A320 Grade B8 Class 1All7530N/A3050
ASTM A320 Grade B8M Class 1All7530N/A3050
ASTM A320 Grade B8 Class 2Up to 34125100N/A1235
78 – 111580N/A1535
118 – 11410565N/A2035
138 – 11210050N/A2845
ASTM A320 Grade B8M Class 2Up to 3411095N/A1545
78 – 110080N/A2045
118 – 1149565N/A2545
138 – 1129050N/A3045

ASTM A182 STUD BOLTS (Duplex and Super Duplex)

Duplex Steel Bolts

Chemical Composition:

CMnSiPSCrMoNiN
Duplex 2205
(S31803)
0.03
max
2.0
max
1.0
max
0.03
max
0.02
max
min: 21.0
max: 23.0
min: 2.5
max: 3.5
min: 4.5
max: 6.5
min: 0.08
max: 0.20
Duplex 2205
(S32205)
0.03
max
2.0
max
1.0
max
0.03
max
0.02
max
min: 22.0
max: 23.0
min: 3.0
max: 3.5
min: 4.5
max: 6.5
min: 0.14
max: 0.20

Mechanical Properties:

GradeTensile Strength
ksi (min)
Yield Strength
0.2% ksi (min)
Elongation %Hardness (HB) MAX
2205906525217

Physical Properties:

Density
lbm/in3
Electrical
Resistivity
mW•in
Thermal
Conductivity
(BTU/hr•ft•°F)
Heat
Capacity
BTU/lbm•°F
Electrical
Resistivity
(in x 10-6)
at 68°F0.27827.68.70.112
at 212°F26.19.20.11935.4
at 392°F25.49.80.12737.4
at 572°F24.910.40.13439.4

Super Duplex Bolts

Chemical Composition:

CCrNiMoNOthers
0.0225740.27S=0.001

Mechanical Properties:

Ultimate Tensile Strength, ksi16 min.
0.2% Offset Yield Strength 0.2%, ksi80 min.
0.1% Offset Yield Strength 0.2%, ksi91 min.
Elongation in 2 inches, %15 min.
Hardness Rockwell C32 max.
Impact Energy, ft.-lbs.74 min.

Physical Properties:

Densitylb/in30.28
Modulus of Elasticitypsi x 10629
The coefficient of Thermal Expansionx10-6/°F7.2
68-212°F/°F
Thermal ConductivityBtu/h ft °F8.7
Heat CapacityBtu/lb/°F0.12
Electrical ResistivityW-in x 10-631.5

COATED STUD BOLTS

Stud bolts can be coated to increase the resistance to corrosion. The most common types of coatings for stud bolts are listed below:

  • Electro zinc plating
  • Electro cadmium plating
  • Hot dip galvanizing
  • PTFE Coating
  • Phosphate coating
  • Electroless nickel plating
  • Zinc-nickel coating
  • Aluminum coating
  • Silver coating
  • Zinc/Nickel by electrodeposition
  • Dacromet
  • Geomet

In addition, also XYLAN and Xylar coatings are available:

  • XYLAN 1070
  • XYLAN 1024
  • Xylar 1

Xylan and Xylar coatings for stud bolts have multiple advantages:

  • Lower frictions vs. uncoated bolts (CoF is as low as 0.02)
  • Increased wear resistance for the bolt even under extreme pressures.
  • Strong corrosion and chemical resistance in most demanding environments
  • Increased resistance to adverse weather conditions (such as extreme sunlight, salt-water exposure to chemicals)
  • Wider operating range in terms of temperature (from -420° to +550°F, i.e. -250° to 285°C).
  • Color coding for easier traceability
  • Pliability: Xylan coatings bend easily and repeatedly and do not break
  • Machinability: multiple Xylan coatings can be applied to the stud bolts
  • Strong adhesion to bolting materials

Stud bolts coating materials

STUD BOLTS AND NUTS SELECTION (SERVICE vs. GRADE)

The material to use for stud bolts depends on multiple factors, the main ones are the material of the flanges and the pipeline design temperature:

DESIGN
TEMPERATURE
FLANGE MATERIALSTUD BOLTHEAVY HEX STEEL NUTS
-195° to 102°CASTM A 182 Gr. F304, F304L, F316, F316L, F321, F347A320 Gr. B8 Class 2A194 Gr. 8A
-101° to -47°CASTM A 350 Gr. LF3A 320 Gr. L7A 194 Gr. 7
-46° to -30°CASTM A 350 Gr. LF2A 320 Gr. L7A 194 Gr. 7
-29° to 427°CASTM A 105A 193 Gr. B7A 194 Gr. 2H
428° to 537°CASTM A 182 Gr. F11, F22A 193 Gr. B16A 194 Gr. 2H
538° to 648°CASTM A182 Gr. F11, F22A 193 Gr. B8 Class 1A 194 Gr. 8A
649° to 815°CASTM A182 Gr. F304 H, F316 HA 193 Gr. B8M Class 1A 194 Gr. 8A

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