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 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).
STUD BOLT (THREADED ROD)
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.
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.
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.
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 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.
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 Element||ASTM A193 grade B7 (AISI 4140)||ASTM A193 grade B8 (AISI 304)||ASTM A193 grade B8M (AISI 316)|
|Carbon||0.38 – 0.48%||0.08% max||0.08% max|
|Manganese||0.75 – 1.00%||2.00% max||2.00% max|
|Silicon||0.15 – 0.35%||1.00% max||1.00% max|
|Chromium||0.80 – 1.10%||18.0 – 20.0%||16.0 – 18.0%|
|Nickel||absent||8.0 – 11.0%||10.0 – 14.0%|
|Molybdenum||0.15 – 0.25%||absent||2.00 – 3.00%|
ASTM A193 Stud Bolts: Mechanical Properties
|ASTM A193||Size||Min Tensile Strength, in ksi||Min Yield Strength, in ksi||Min Elongation, in %||RA % min||Max HBW||Max HRC|
|ASTM A193 grade B7||Up to 2-1/2||125||105||16||50||321||35|
|2-5/8 – 4||115||95||16||50|
|4-1/8 – 7||100||75||18||50|
|ASTM A193 grade B8 Class 1||All||75||30||30||50||223||35|
|ASTM A193 grade B8M Class 1||All||75||30||30||50||223||96|
|ASTM A193 grade B8 Class 2||Up to 3/4||125||100||12||35||321||35|
|7/8 – 1||115||80||15||35|
|1-1/8 – 1-1/4||105||65||20||35|
|1-3/8 – 1-1/2||100||50||28||45|
|ASTM A193 grade B8M Class 2||Up to 3/4||110||95||15||45||321||35|
|7/8 – 1||100||80||20||45|
|1-1/8 – 1-1/4||95||65||25||45|
|1-3/8 – 1-1/2||90||50||30||45|
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 MATERIALS||MIN.||MAX.|
|CARBON STEEL||-29 (-20)||300 (572)|
|A193 B7, L7||-73 (-100)||400 (752)|
|A193 B6||0 (32)||500 (932)|
|A193 B8||-200 (-325)||575 (1067)|
|A193 B16||0 (32)||520 (968)|
|A193 B17B||-29 (-20)||650 (1202)|
|A913 Inconel 718||0 (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
|ELEMENT||ASTM A453 Grade 660||ASTM A453 Grade 651||ASTM A453 Grade 662||ASTM A453 Grade 665||ASTM A453 Grade 668|
|Carbon||0.08 max||0.28-0.35||0.08 max||0.08 max||0.08 max|
|Manganese||2.00 max||0.75-1.5||0.40-1.00||1.25-2.00||2.00 max|
|Phosphorus, max||0.040 max||0.040 max||0.040 max||0.040 max||0.040 max|
|Sulfur, max||0.030 max||0.030 max||0.030 max||0.030 max||0.030 max|
|Silicon||1.00 max||0.30-0.80||0.40-1.00||0.1-0.80||1.00 max|
|Aluminum||0.35 max||0.35 max||0.25 max||0.50 max|
|Copper||0.50 max||0.50 max||0.25 max||…|
ASTM A453 Stud Bolts: Mechanical Properties
|ASTM A453 Grade||Class||Tensile, Mpa||Yield, Mpa||Elong. %, min||RA %, min|
|ASTM A453 Grade 660||A, B , & C||895 min||585 min||15||18|
|D||895 min||725 min||15||18|
|ASTM A453 Grade 651||A||690 min||485 min||18||35|
|B||655 min||415 min||18||35|
|ASTM A453 Grade 662||A||895 min||585 min||15||18|
|B||860 min||550 min||15||18|
|ASTM A453 Grade 665||A||1170 min||830 min||12||15|
|B||1070 min||830 min||12||15|
|ASTM A453 Grade 665||A & B||895 min||858 min||15||18|
ASTM A453 Stud Bolts: Heat Treatments Requirements
|ASTM A453||Class||Solutiona Treatment||Hardening Treatment|
|660||A||1650 +/- 25 °F [900 +/-14 °C], hold 2 h, min and liquid quench||1325 +/- 25 °F [720 +/- 14 °C] 16h, air cool|
|B||1800 +/- 25 °F [980 +/-14 °C], hold 1 h, min and liquid quench||1325 +/- 25 °F [720 +/- 14 °C] 16h, air cool|
|C||1800 +/- 25 °F [980 +/-14 °C], hold 1 h, min and oil quench||1425 +/- 25 °F [775 +/- 14 °C] hold 16h, air cool followed by 1200 +/- 25 °F [650 +/- 14 °C], hold 16h, air cool|
|D||1650 +/- 25 °F [900 +/-14 °C], hold 2 h, min and liquid quench OR||1325 +/- 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|
|651||A||hot-cold worked at 1200 °F min with 15% min reduction in cross-sectional area, stress-relief anneal at 1200 °F [650 °C] min or 4h, min|
|B||hot-cold worked at 1200 °F min with 15% min reduction in cross sectional area, stress-relief anneal at 1350 °F [730 °C] min or 4h, min|
|662||A||1800 +/- 25 °F [980 +/-14 °C], hold 1 h, min and liquid quench||1350 to 1400 °F [730 to 760 °C], hold 20h, furnace cool to 1200 +/- 25 °F [650 +/- 14 °C], hold 20h, air cool|
|B||1950 +/- 25 °F [1065 +/-14 °C], hold 2 h, min and liquid quench||1350 to 1400 °F [730 to 760 °C], hold 20h, furnace cool to 1200 +/- 25 °F [650 +/- 14 °C], hold 20h, air cool|
|665||A||1800 +/- 25 °F [980 +/-14 °C], hold 3 h, min and liquid quench||1350 to 1400 °F [730 to 760 °C], hold 20h, furnace cool to 1200 +/- 25 °F [650 +/- 14 °C], hold 20h, air cool|
|B||2000 +/- 25 °F [1095 +/-14 °C], hold 3 h, min and liquid quench|
|668||A||1650 +/- 25 °F [900 +/-14 °C], hold 2 h, min and liquid quench||1325 +/- 25 °F [720 +/- 14 °C] 16h, air cool|
|B||1800 +/- 25 °F [980 +/-14 °C], hold 1 h, min and liquid quench||1325 +/- 25 °F [720 +/- 14 °C] 16h, air cool|
A453 Stud Bolts: Time to Rupture and Elongation
|Grade||Class||Test Temperature||Stress, Min||Time to Rupture||Elongation|
|Deg F [ Deg C ]||Ksi||Mpa||Min, h*||Min %|
|660||A, B & C||1200 ||56||385||100||5|
|651||A & B||1200 ||40||275||100||5|
|662||A & B||1200 ||55||380||100||5|
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 Grade||Diameter||Tensile, KSI, min||Yield, KSI, min||Charpy Impact 20-ft-LBF @ temp||Elong, %, min||RA, %, min|
|ASTM A320 Grade L7||Up to 21⁄2||125||105||-150° F||16||50|
|ASTM A320 Grade L43||Up to 4||125||105||-150° F||16||50|
|ASTM A320 Grade B8 Class 1||All||75||30||N/A||30||50|
|ASTM A320 Grade B8M Class 1||All||75||30||N/A||30||50|
|ASTM A320 Grade B8 Class 2||Up to 3⁄4||125||100||N/A||12||35|
|7⁄8 – 1||115||80||N/A||15||35|
|11⁄8 – 11⁄4||105||65||N/A||20||35|
|13⁄8 – 11⁄2||100||50||N/A||28||45|
|ASTM A320 Grade B8M Class 2||Up to 3⁄4||110||95||N/A||15||45|
|7⁄8 – 1||100||80||N/A||20||45|
|11⁄8 – 11⁄4||95||65||N/A||25||45|
|13⁄8 – 11⁄2||90||50||N/A||30||45|
ASTM A182 STUD BOLTS (Duplex and Super Duplex)
Duplex Steel Bolts
0.2% ksi (min)
|Elongation %||Hardness (HB) MAX|
(in x 10-6)
Super Duplex Bolts
|Ultimate Tensile Strength, ksi||16 min.|
|0.2% Offset Yield Strength 0.2%, ksi||80 min.|
|0.1% Offset Yield Strength 0.2%, ksi||91 min.|
|Elongation in 2 inches, %||15 min.|
|Hardness Rockwell C||32 max.|
|Impact Energy, ft.-lbs.||74 min.|
|Modulus of Elasticity||psi x 106||29|
|The coefficient of Thermal Expansion||x10-6/°F||7.2|
|Thermal Conductivity||Btu/h ft °F||8.7|
|Electrical Resistivity||W-in x 10-6||31.5|
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
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 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:
|FLANGE MATERIAL||STUD BOLT||HEAVY HEX STEEL NUTS|
|-195° to 102°C||ASTM A 182 Gr. F304, F304L, F316, F316L, F321, F347||A320 Gr. B8 Class 2||A194 Gr. 8A|
|-101° to -47°C||ASTM A 350 Gr. LF3||A 320 Gr. L7||A 194 Gr. 7|
|-46° to -30°C||ASTM A 350 Gr. LF2||A 320 Gr. L7||A 194 Gr. 7|
|-29° to 427°C||ASTM A 105||A 193 Gr. B7||A 194 Gr. 2H|
|428° to 537°C||ASTM A 182 Gr. F11, F22||A 193 Gr. B16||A 194 Gr. 2H|
|538° to 648°C||ASTM A182 Gr. F11, F22||A 193 Gr. B8 Class 1||A 194 Gr. 8A|
|649° to 815°C||ASTM A182 Gr. F304 H, F316 H||A 193 Gr. B8M Class 1||A 194 Gr. 8A|