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In operating conditions combining extreme low temperatures with strong corrosion, selecting high-performance fasteners is critical. A320 L7M bolts and A194 7M nuts, based on ASTM A320/A194 standards, are high-strength carbon steel fasteners specifically designed for low-temperature service. They can be produced via hot forging, cold forging, or machining from bar stock followed by heat treatment. Their excellent performance ensures stability and safety for mechanical equipment operating under extreme conditions.
ASTM A320 Grade L7M is a standard material specification for alloy steel bolting materials for pressure vessels, valves, flanges, and fittings for low temperature services.
ASTM A194 Grade 7M is a standard material specification for high tensile nuts and other internally threaded fasteners, made from chromium molybdenum alloy steel. Grade 7M nuts can be produced by means of hot forging, cold forging or machining from bar stock (Grade Type AISI 4140/4142/4145/4140H/4142H/4145H) followed by heat treatment. ASTM A194 specification covers requirements for chemical composition, heat treatment, hardness, proof load, etc., for Grade 7M nuts.
I. Mechanical Properties of A320 L7M Bolts and A194 7M Nuts
A320 Grade L7M bolts and A194 Grade 7M nuts utilize a low-temperature alloy steel base material (C 0.37~0.49%, Cr 0.75~1.20%, Mo 0.15~0.25%), offering outstanding low-temperature performance. Through an optimized quenching and tempering heat treatment process, we maintain high strength and toughness in the bolts and nuts. This ensures they retain excellent tensile strength (≥517 MPa), yield strength (≥207 MPa), and impact toughness even in ultra-low temperature environments. This prevents the risk of low-temperature brittle fracture down to -73°C (-100°F), making them an ideal choice for liquefied natural gas (LNG) storage tanks and cryogenic pressure vessels.
ASTM A320 Chemical Composition
| Element | Composition, % A |
|---|---|
| Chromium Molybdenum B | |
| 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 |
ASTM A194 Chemical Composition
Element | Alloy Steel |
|---|---|
| Chromium Molybdenum Steel | |
| CompositionA, % | |
| Carbon | 0.37 – 0.49 |
| Manganese | 0.65 – 1.10 |
| Phosphorus | 0.035 max |
| Sulfur | 0.04 max |
| Silicon | 0.15 – 0.35 |
| Chromium | 0.75 – 1.20 |
| Molybdenum | 0.15 – 0.25 |
ASTM A320 Mechanical Properties
| Diameter, in. [mm] | Tensile Strength, min, ksi [MPa] | Yield Strength min, ksi [MPa] (0.2% offset) | Elongation in 2 inch or 50mm, min, % | Reduction of Area, min, % | Hardness, max | |
|---|---|---|---|---|---|---|
| HBW | HRC | |||||
| 2 1/2 [65] and under A | 100 [690] | 80 [550] | 18 | 50 | 235 | 99 |
Heat Treatment : Quenched and Tempered at 1150°F [620°C] minimum.
ASTM A320 Impact Test Requirements (Notch Toughness)
| Size of Specimen, mm | Test Temperature | Minimum impact value required for average of each set of three specimens, ft-lbf [J] | Minimum impact value permitted for one specimen only of a set, ft-lbf [J] | |
|---|---|---|---|---|
| °F | °C | |||
| 10 X 10 | -100 | -73 | 20 [27] | 15 [20] |
| 10 X 7.5 | 16 [22] | 12 [16] | ||
ASTM A194 Grade 7M Hardness
| Grade | Completed Nuts | Sample Nut After Heat Treatment | |||
|---|---|---|---|---|---|
| Brinell Hardness, BHN | Rockwell Hardness | Brinell Hardness, BHN, min | Rockwell Hardness, HRB, min | ||
| HRC | HRB | ||||
| 7M | 159 – 235 | … | 84 – 99 | 159 | 84 |
Heat Treatment: Grade 7M shall be quenched and tempered at a temperature of at least 1150 °F [620 °C].
II. PTFE Xylan Coated Stud Bolts, ASTM A320 L7M Xylan PTFE Coated Stud bolt
Surface: yellow, black, galvanized, HDG, blue. Fluoro Carbon Coated
Whitford Quality Approved Coater Xylan 1070 Coated Stud Bolt, Xylan Coated Bolts, PTFE Fluorocarbon Coated A193 B7 Stud Bolt &A194 2H NUTS
To withstand harsh corrosive environments, the surface treatment of A320-L7M bolts and A194-7M nuts employs a composite coating technology: a base layer of zinc-nickel electroplating and a top layer of yellow PTFE (Polytetrafluoroethylene), establishing a dual anti-corrosion barrier.
PTFE (polytetrafluoroethylene) is a chemical that is applied to common bolting materials, such as B7 stud bolts, to provide corrosion and chemical resistance.
When we talk about PTFE-coated stud bolts and other fasteners, we are usually referring to Teflon® and Xylan®.
PTFE-coated studs have been used for many years in the bolting industry, particularly in applications requiring corrosion resistance or in offshore environments.
PTFE-coated stud bolts (including Xylan-coated stud bolts) are corrosion-resistant.
PTFE, also known as Teflon, is a fluorocarbon solid, consisting wholly of carbon and fluorine. PTFE is hydrophobic: neither water nor water-containing substances wet PTFE, as fluorocarbons demonstrate mitigated London dispersion forces due to the high electronegativity of fluorine. PTFE has one of the lowest coefficients of friction of any solid.
PTFE is used as a non-stick coating for hardware and is non-reactive, partly due to the strength of carbon–fluorine bonds. Consequently, it is often used in containers and pipework for reactive and corrosive chemicals.
Why to Coat Bolts with PTFE Coatings?
- Cleaning and painting bare steel bolts on site is likely to be difficult and expensive, and sometimes not feasible.
- Once inserted into the holes, the plain bolts are expected to be exposed to the weather for an extended period of time, during which they will dry out and rust, making correct tightening difficult or impossible.
- It may be necessary to release or retighten the bolt within the foreseeable future.
- Due to its unique benefits, fluoropolymer coating has been applied to various types and grades of fastener. The water industry takes advantage of the superior corrosion-resistant properties of coating hex-head bolts for underground use. Stainless steel fasteners, which are used in many different industries, are coated to improve lubricity and prevent galling.
- Fluoropolymer coatings are extremely durable and difficult to remove. However, the coating can sometimes be damaged during assembly of fasteners in the field. With most other fluoropolymer coatings, this results in exposed bare metal which quickly corrodes, causing the coating to fail. Our metallic base coat, however, ensures superior corrosion resistance and continues to provide protection even under the harshest conditions.
If your project operates in a low-temperature corrosive environment, A320 Grade L7M bolts and A194 Grade 7M nuts are your ideal solution. C&N Group offers a variety of A320 L7M bolts and A194 7M nuts to choose from and supports customization. Each batch of products is accompanied by complete material certificates and shipped with professional protective packaging to ensure high-quality delivery. Please contact us immediately if required.
