Views: 1 Author: Site Editor Publish Time: 2026-02-04 Origin: Site
In key industrial fields such as petrochemicals, power energy, pressure vessels and low-temperature equipment, and pressure pipelines, the performance, applicability, and service life of fasteners directly affect the safety and stability of the entire system. As a key basic fastener, it is very important to choose the appropriate material for stud bolts correctly. The American Society for Testing and Materials (ASTM) standard, with its strict grading and globally recognized authority, has become an important criterion for fastener selection in the industrial field. This article will delve into the two core standards of ASTM A193 and A320, interpreting the performance characteristics of key material grades such as B7, B8, B8M, B16, L7, and providing professional guidance for the selection of high-performance ASTM stud bolts.
The full name of ASTM A193 standard is "Alloy Steel and Stainless Steel Bolt Materials for High Temperature and High Pressure Equipment". This standard specifies the chemical composition, mechanical properties, and heat treatment requirements for threaded fasteners such as alloy steel and stainless steel bolts and bolts suitable for high temperature or high pressure services. This standard is the core basis for the connection system of equipment such as power plant boilers, hydrogenation reactors, and high-temperature valves.
ASTM A193 B7: This is the most widely used high-strength alloy steel stud material. It is usually made of chromium molybdenum alloy steel, which undergoes a heat treatment process of quenching and tempering to achieve a minimum tensile strength of 125 ksi (about 860 MPa) and a minimum yield strength of 105 ksi (about 725 MPa). B7 material can maintain good anti relaxation and anti creep performance at temperatures up to 450 ° C, making it an essential fastener for high-temperature and high-pressure carbon steel and alloy steel flange connections
Chemical Composition
| CHEMICAL | LIMITS | C | Mn | P | S | Si | Cr | Mo |
| ASTM A193 B7 | MIN | 0.38 | 0.75 | 0.15 | 0.80 | 0.15 | ||
| MAX | 0.48 | 1.00 | 0.035 | 0.040 | 0.35 | 1.10 | 0.25 |
Mechanical Properties
| MATERIAL | Diameter | T.S (MPA) | Y.S (MPA) | EL % | R/A % | HARDNESS |
| ASTM A193 B7 | M64 and under | 860 min | 720 min | 16 min | 50 min | 321 max |
| over M64 to M100 | 795 min | 655 min | 16 min | 50 min | 321 max | |
| over M100 tu M180 | 690 min | 515 min | 16 min | 50 min | 321 max |
ASTM A193 B7M: As an "improved version" of B7, B7M specifically limits the hardness range (235 max) and increases the impact toughness requirements while maintaining the same strength level. This characteristic gives it excellent resistance to sulfide stress corrosion cracking (SSC) and is designed specifically for acidic environments rich in wet hydrogen sulfide (H ₂ S), such as oil and gas extraction and refining facilities. It is a safe choice for dealing with harsh corrosion conditions.
Chemical Composition
| CHEMICAL | LIMITS | C | Mn | P | S | Si | Cr | Mo |
| ASTM A193 B7M | MIN | 0.38 | 0.75 | 0.15 | 0.80 | 0.15 | ||
| MAX | 0.48 | 1.00 | 0.035 | 0.040 | 0.35 | 1.10 | 0.25 |
Mechanical Properties
| MATERIAL | Diameter | T.S (MPA) | Y.S (MPA) | EL % | R/A % | HARDNESS |
| ASTM A193 B7M | M100 and under | 690 min | 550 min | 18 min | 50 min | 235 max |
| over M100 to M180 | 690 min | 515 min | 18 min | 50 min | 235 max |
ASTM A193 B8: Corresponding to austenitic stainless steel 304 (UNS S30400). Its supply status is divided into Class 1 (solution annealing) and Class 2 (cold processing to enhance strength). B8 material provides excellent universal corrosion resistance and low-temperature toughness, with a wide temperature range (-200 ° C to+700 ° C), suitable for food, chemical, and general corrosive environments.
Chemical Composition
| CHEMICAL | LIMITS | C | Mn | P | S | Si | Cr | Ni |
| ASTM A193 B8 | MIN | 18.00 | 8.00 | |||||
| MAX | 0.08 | 2.00 | 0.045 | 0.030 | 1.00 | 20.00 | 11.00 |
Mechanical Properties
| MATERIAL | Class | T.S (MPA) | Y.S (MPA) | EL % | R/A % | HARDNESS |
| ASTM A193 B8 | Class 1 | 515 min | 205 min | 30 min | 50 min | 223 max |
| Class 2 | 860 min | 690 min | 12 min | 35 min | 321 max |
ASTM A193 B8M: corresponds to austenitic stainless steel 316 (UNS S31600) containing molybdenum. The addition of molybdenum significantly enhances its ability to resist pitting and crevice corrosion, especially in environments containing chloride ions such as coastal areas, offshore platforms, and chemical plants, where it outperforms B8. B8M is a reliable solution to address moderate to high chloride corrosion issues.
Chemical Composition
| CHEMICAL | LIMITS | C | Mn | P | S | Si | Cr | Ni | Mo |
| ASTM A193 B8M | MIN | 16.00 | 10.00 | 2.0 | |||||
| MAX | 0.08 | 2.00 | 0.045 | 0.030 | 1.00 | 18.00 | 14.00 | 3.0 |
Mechanical Properties
| MATERIAL | Class | T.S (MPA) | Y.S (MPA) | EL % | R/A % | HARDNESS |
| ASTM A193 B8M | Class 1 | 515 min | 205 min | 30 min | 50 min | 223 max |
| Class 2 | 760 min | 655 min | 15 min | 45 min | 321 max |
ASTM A193 B16 is a chromium molybdenum vanadium alloy steel. It has a creep fracture strength far exceeding that of B7 material in the ultra-high temperature range above 540 ° C. Although its room temperature strength is similar to B7, its special alloy composition and heat treatment make it the preferred choice for double headed bolts in extreme temperature and pressure equipment such as ultra supercritical power plants and high-temperature cracking furnaces.
Chemical Composition
| CHEMICAL | LIMITS | C | Mn | P | S | Si | Cr | Mo | V | Al |
| ASTM A193 B16 | MIN | 0.36 | 0.45 | 0.15 | 0.80 | 0.50 | 0.25 | |||
| MAX | 0.47 | 0.70 | 0.035 | 0.040 | 0.35 | 1.15 | 0.65 | 0.35 | 0.015 |
Mechanical Properties
| MATERIAL | Diameter | T.S (MPA) | Y.S (MPA) | EL % | R/A % | HARDNESS |
| ASTM A193 B16 | M64 and under | 860 min | 720 min | 18 min | 50 min | 321 max |
| over M64 to M100 | 760 min | 655 min | 17 min | 45 min | 321 max | |
| over M100 tu M180 | 690 min | 585 min | 16 min | 45 min | 321 max |
When the operating temperature of the equipment drops below freezing point, the brittle transformation of the material becomes the main threat. The ASTM A320 standard "Alloy Steel and Stainless Steel Bolt Materials for Low Temperature Equipment" has emerged, which requires all materials to pass the specified Charpy V-notch impact test at low temperatures to ensure their toughness at low temperatures.
ASTM A320 L7: Its chemical composition is similar to A193 B7, but it must meet strict low-temperature impact toughness criteria (typically tested at -101 ° C). It is the standard fastener material for liquefied natural gas (LNG) storage tanks, low-temperature separation devices, low-temperature valves, and other equipment, ensuring that brittle fracture does not occur in cryogenic environments.
Chemical Composition
| CHEMICAL | LIMITS | C | Mn | P | S | Si | Cr | Mo |
| ASTM A320 L7 | MIN | 0.38 | 0.75 | 0.15 | 0.80 | 0.15 | ||
| MAX | 0.48 | 1.00 | 0.035 | 0.040 | 0.35 | 1.10 | 0.25 |
Mechanical Properties
| MATERIAL | T.S (MPA) | Y.S (MPA) | EL % | R/A % | HARDNESS |
| ASTM A320 L7 | 860 min | 725 min | 16 min | 50 min | 321 max |
ASTM A320 L7M: Its relationship with L7 is similar to that of B7M and B7. It controls hardness and chemical composition to resist sulfide stress corrosion cracking while ensuring low-temperature toughness (typically tested at -73 ° C). Suitable for working conditions with low temperature and simultaneous presence of H ₂ S acidic media, such as the low-temperature units in some oil and gas processing plants.
Chemical Composition
| CHEMICAL | LIMITS | C | Mn | P | S | Si | Cr | Mo |
| ASTM A320 L7M | MIN | 0.38 | 0.75 | 0.15 | 0.80 | 0.15 | ||
| MAX | 0.48 | 1.00 | 0.035 | 0.040 | 0.35 | 1.10 | 0.25 |
Mechanical Properties
| MATERIAL | T.S (MPA) | Y.S (MPA) | EL % | R/A % | HARDNESS |
| ASTM A320 L7M | 690 min | 550 min | 18 min | 50 min | 235 max |
Choosing the correct American standard double headed stud material requires a systematic working condition evaluation:
Temperature is the primary factor: clarify the upper and lower limits of the working temperature. Choose A193 (B7/B16) for high temperature and A320 (L7/B8) for low temperature.
The corrosiveness of the medium determines the type of material: for alkaline or acidic corrosive environments, it is necessary to choose stainless steel materials with strong corrosion resistance, such as B8, B8M, etc.
Pressure and load determine strength level: Calculate the required preload based on the design pressure and select the corresponding strength level (such as B7, B8 Class 2).
Follow project specifications: The final selection must strictly adhere to the material standards and grades specified in the project design documents (such as ASME Boiler and Pressure Vessel Code).
In addition to the raw materials themselves, high-quality double headed bolts also need to pay attention to:
Thread machining accuracy: It is usually required to achieve 3A level (American system) or 6g level (metric system) accuracy to ensure smooth meshing and uniform load.
Heat treatment and testing: Complete heat treatment records and hardness and metallographic reports are guarantees of performance.
Material Traceability: Each batch of products must be accompanied by a Material Certificate (MTC) issued by the supplier, which is a written proof that the material meets ASTM standards.
