ASTM A194 Grade 4 Heavy Hex Nuts are high-strength carbon-molybdenum alloy steel nuts designed for bolting assemblies in high-pressure and high-temperature service. Manufactured through hot forging, precision machining, and controlled quenching & tempering, these nuts deliver excellent mechanical strength, reliable thread engagement, and consistent performance under demanding conditions.ASTM A194 Grade 4 Heavy Hex Nuts serve as a robust higher-strength alternative in critical bolting applications and are commonly paired with ASTM A193 Grade B7 and B16 stud bolts. They are widely used in oil & gas, petrochemical, power generation, refining, and pressure vessel industries where durability and reliability are essential.
ASTM A194 Grade 4 Chemical Components
| Grade | Material | UNS Number |
C, % | Mn, % | Ph, % | S, % | Si, % | Cr, % | Ni, % | Mo, % |
|---|---|---|---|---|---|---|---|---|---|---|
| 4 | carbon, molybdenum | 0.40-0.50 | 0.70-0.90 | 0.035 | 0.040 | 0.15-0.35 | - | 0.20-0.30 |
ASTM A194 Grade 4 Hardness Requirement
| Grade & Type | Brinell Hardness | Rockwell Hardness | |
|---|---|---|---|
| C Scale | B Scale | ||
| 4, 7, 16 | 248-327 | 24 - 35 | - |
Mechanical Properties
|
Mechanical Property |
Up to 1-1/2" Diameter |
Over 1-1/2" Diameter |
|
Proof Load Stress (Heavy Hex) |
175 ksi (1205 MPa) |
150 ksi (1034 MPa) |
|
Proof Load Stress (Standard Hex) |
150 ksi (1034 MPa) |
120 ksi (827 MPa) |
|
Hardness (Rockwell C) |
24 to 35 HRC |
24 to 35 HRC |
|
Hardness (Brinell) |
248 to 327 HBW |
248 to 327 HBW |
Applications & Industries
ASTM A194 Grade 4 nuts are selected when operating temperatures exceed the safe limits of standard carbon steel grades such as 2H, yet do not justify the higher cost of austenitic stainless steels.
They are widely used in:
- Power Generation - Bolting for high-pressure steam lines, boiler feed pumps, and turbine housings
- Petrochemical Processing - Catalytic cracking units, high-temperature distillation columns, and flare system flanges
- Oil & Gas Exploration - High-temperature, high-pressure (HTHP) wellhead equipment
- Industrial Furnaces- Structural bolting for heat treatment facilities and incinerator exhaust manifolds
FAQ
1. What are the main differences between Grade 7 and Grade 4 nuts?
The main difference lies in the material composition:
· Grade 4: Carbon-Molybdenum steel, containing no significant chromium.
· Grade 7: Chromium-Molybdenum alloy steel (typically AISI 4140/4142 series), containing chromium and molybdenum, offering better overall performance.
Both have similar hardness ranges and guaranteed load stresses (approximately 24–35 HRC, 175 ksi), but Grade 7 generally excels in high-temperature performance and corrosion resistance. Grade 4 was marked as withdrawn in some standards in 2017, and Grade 7 is more common in practical use.
2. Which bolts or studs are typically used with Grade 4 nuts?
Grade 4 nuts are typically used with the following bolts/studs:
· ASTM A193 Grade B7
· ASTM A193 Grade B16
They can also be used with other high-strength alloy steel studs depending on engineering requirements.
3. What is the maximum operating temperature of a Grade 4 nut?
Due to the addition of molybdenum alloy, Grade 4 nuts can operate reliably in environments up to approximately 482°C to 510°C (900°F to 950°F), outperforming standard Grade 2H nuts.
4. How to identify an ASTM A194 Grade 4 nut?
Identification methods:
· The nut is usually marked with a "4" on the top or side (if machined from bar stock, it may be marked "4B").
· The manufacturer's markings will also be present.
· Further confirmation can be made by combining the Material Certificate of Materials (MTC) and hardness test results.
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