Views: 3 Author: Site Editor Publish Time: 2025-11-19 Origin: Site
Stud bolts and nuts are used in conjunction with flanges and gaskets to connect pipelines, and are one of our main products for sale.
Last month, we received an order from an old customer for hot-dip galvanized bolts and nuts. Material requirements include ASTM A320 L7 full thread hot-dip galvanized bolts and ASTM A194 GR.7 hot-dip galvanized nuts; The bolt size is 3 inches, the length is 570mm, and the quantity is 400 sets. Nuts need to be used together with bolts.
Although the order product is single, we fully understand their needs. During the communication process, the client clearly stated that the delivery time for this batch of hot-dip galvanized bolts and nuts is urgent and cannot affect the project schedule. Therefore, they need a fast delivery date.
We have identified 'delivery time' as a key factor in winning this order to meet the customer's critical requirements. Therefore, we sought support from upstream suppliers through various means and obtained competitive prices and delivery times. Shortly after sending the quotation to the customer, we received their order.
ASTM A320/A320M is a bolt material standard developed by the American Society for Testing and Materials, and its full name is "Standard Specification for Alloy Steel and Stainless Steel Bolt Materials for Low Temperature Use". This standard specifies the chemical composition, mechanical properties, heat treatment, and testing requirements for fasteners such as bolts, screws, and studs used in low-temperature environments.
The A320 standard includes various alloy steel and stainless steel grades to meet the requirements of different low-temperature levels. Common levels include:
Alloy Steel:
L7/L7A: commonly used in extremely low temperature environments (such as -150 ° F/-101 ° C).
L43: It also has good low-temperature toughness.
B8: Usually refers to chromium alloy steel.
These grades typically require quenching and tempering heat treatment to achieve the required strength and high toughness.
Stainless Steel
B8: The most commonly used austenitic stainless steel (such as 304 type), but classified according to the processing method:
B8 Class 1: Strain hardening (work hardening), suitable for medium low temperatures.
B8 Class 2: Solid solution annealing treatment, with better corrosion resistance but lower strength.
B8M: Molybdenum reinforced austenitic stainless steel (such as 316 type) has better corrosion resistance.
B8T: Stabilized stainless steel (such as 321 type).
In order to ensure the toughness of materials at low temperatures, the A320 standard has strict regulations on heat treatment:
Alloy steel materials must undergo quenching and tempering treatment, and their tempering temperature must not be lower than 1000 ° F (537 ° C). This is a key requirement as higher tempering temperatures help improve the impact toughness of the material.
This is one of the most significant differences between A320 and ordinary bolt standards such as ASTM A193.
All A320 materials must undergo Charpy V-notch impact testing.
The standard specifies the impact energy values that materials of different grades and sizes must achieve at specific low temperatures.
This is the core test to verify the material's ability to resist brittle fracture at low temperatures.
To better understand A320, it can be compared with similar standards:
Standard | Designation | Applications | Difference |
ASTM A320 | Low temperature alloy/stainless steel bolts | Low-temperature environment | Low temperature impact test |
ASTM A193 | Alloy steel and stainless steel bolts for high temperature/high pressure | High temperature and high pressure environment | Usually no impact testing is required, or different requirements are required |
ASTM A194 | Nut compatible with high temperature and high pressure bolts | Used with A193 bolts | The matching requirements for nuts have been specified |
SAE J429 / ASTM A307 | Standard carbon steel bolt | Normal temperature universal environment | Low requirements for chemical composition and low-temperature performance |
ASTM A320 L7 is a specific alloy steel grade under the ASTM A320 standard. It is specifically used to manufacture bolts, studs, screws, and other pressure vessel fasteners that require operation in extremely low temperature environments.
Its core value lies in its ability to maintain excellent strength and extremely high impact toughness at low temperatures, thereby preventing brittle fracture.
| Element | Composition,%A |
| Chromium MolybdenumD | |
| Carbon,max | 0.38-0.48 |
| Manganese | 0.75-1.00 |
| Phosphorus,max | 0.035 |
| Sulfur,max | 0.040 |
| Silicon | 0.15-0.35 |
| Chromium | 0.80-1.10 |
| Molybdenum | 0.15-0.25 |
L7 is a medium carbon, low-alloy steel that typically contains alloying elements such as chromium and molybdenum, which enable it to achieve deep hardenability and high strength through heat treatment.
Main alloying elements: chromium (Cr) and molybdenum (Mo).
Common similar grades: Their chemical composition is very similar to AISI 4140 or 4142 steel.
Temperature Grade
| Material | Temperature °c ( °f ) | |
| Minumum | Maximum | |
| Carbon Steel | -29 (-20) | 300 (572) |
| B7, L7 | -73 (-100) | 400 (752) |
| B6 | 0 (32) | 500 (932) |
| B8 | -200 (-325) | 575 (1067) |
| B16 | 0 (32) | 520 (968) |
| B17B | -29 (-20) | 650 (1202) |
| INC 718 | 0 (32) | 750 (1382) |
| 660 | -29 (20) | 538 (1000) |
Performance Parameters
| bolts Tensile Class | Test Temperature °F [°C] | Stress, min | Time to Rupture, min, h [Note] | Elongation, min, % | |
| ksi | Mpa | ||||
| A, B and C | 1200 [650] | 56 | 385 | 100 | 5 |
| D | 1200 [650] | 40 | 275 | 100 | 5 |
High Strength Grade: L7 typically provides the same mechanical performance requirements as ASTM A193 B7 that comply with ASTM A320 standards. This means that its tensile strength, yield strength, and other indicators are equivalent to those of B7 grade.
The key difference between L7 and B7 lies in their impact toughness.
This is where the soul of L7 level resides. According to ASTM A320 standard, L7 material must undergo the following treatments:
Quenching and tempering: to achieve high strength.
Tempering temperature: must not be lower than 1250 ° F (677 ° C).
This is the fundamental difference between L7 and ordinary high-temperature bolt materials (such as A193 B7, with a tempering temperature of 1100 ° F/593 ° C).
Higher tempering temperatures can significantly improve the toughness and ductility of materials, especially their impact resistance at low temperatures. This is a prerequisite for it to be capable of low-temperature service.
All ASTM A320 L7 materials must pass the Charpy V-notch impact test.
Test temperature: -150 ° F (-101 ° C). This is the standard test temperature for L7.
Standard requirement: The standard specifies the average value of three samples and the absorbed energy value of a single sample to ensure that the material does not become brittle under extreme cold conditions.
L7 is widely used in the following fields due to its excellent low-temperature performance:
Liquefied natural gas (LNG) facilities: The storage temperature of LNG is approximately -162 ° C, and L7 is a key fastener material for flange connections in such applications.
Petrochemical industry: equipment, valves, pipelines, and storage tanks used for processing low-temperature media such as liquid ammonia, propylene, propane, and dry ice (carbon dioxide).
Low temperature refrigeration system: Industrial grade large-scale refrigeration equipment.
Ensure that the diameter of the bolt is within the specified tolerance range to ensure smooth nut engagement.
Verify whether the total length of the bolts meets the order specifications. Especially for double headed bolts, dimensional accuracy is crucial for achieving appropriate preload in flange connections.
Ensure that the threaded portion is long enough to fully engage the nut and achieve the required preload force.
The ASTM A320 L7 bolt specification covers alloy steel bolt materials and bolt components for pressure vessels, valves, flanges, and fittings in low-temperature environments. The definition of bolted connection is specified in specifications A962/A962M.
