ASTM

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).

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).

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.

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.

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.

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.

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.

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

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
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
Phosphorus, max 0.035% 0.045% 0.045%
Sulfur, max 0.040% 0.030% 0.030%
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 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
Product Name 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).

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

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 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 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 25 100 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