ASTM A194 Grade 7 Hex Nuts — API 20E Qualified Bolting for Oil & Gas

NUBOTECK manufactures ASTM A194 Grade 7 hex nuts for high-pressure applications in the petroleum, natural gas, refinery and petrochemical industries. These nuts manufactured by NUBOTECK can withstand low temperature impact requirements upto -101⁰C.

ASTM A194 Grade 7 is one of the bolting grades covered by API Specification 20E for use in the petroleum and natural gas industries. API 20E does not replace ASTM A194 Grade 7; rather, it establishes additional qualification and production requirements of the applicable API 20E Bolting Specification Level (BSL), to these ASTM grade nuts.

What is ASTM A194 Grade 7?

ASTM A194 Grade 7 is the most widely specified alloy steel bolting material extensively used for pressure vessels, valves, flanges, and fittings especially for low-temperature service. Grade 7 hex nuts provide excellent strength, toughness and reliability, making them the preferred choice throughout the oil & gas industry.

ASTM A194 standard governs the requirements specifically for Grade 7. However, the requirements given in ASTM A194 are said to be fully achieved, only when the generic/ common bolting requirements of ASTM A962 have been fully met.

Grade 7 Nuts in Oil & Gas Applications

When these Nuts are used in critical service of Oil and Gas the additional requirements of API 20E need to be applied over and above the the regular requirements given for this grade by the ASTM standards (ASTM A194, ASTM A962). Specification API 20E introduces enhanced requirements for manufacturing process control, qualification testing, traceability, metallurgical examination and documentation.

Manufacturing Process

ASTM A194 Grade 7 hex nuts are manufactured from chromium-molybdenum alloy steel which is quenched and tempered to achieve the required mechanical properties. Proper heat treatment ensures optimum strength, toughness and reliability for high-pressure service conditions.

Typical manufacturing sequence:

Raw Material Verification
Heat Treatment - Austenitizing, Quenching, Tempering
Mechanical Property - Verification
Hot Forming
Machining
Threading - Rolling/ Cutting
Inspection & Testing
Documentation and Certification

API-20E requirements for Hardness Testing of Bars:

In case of heat treatment using induction process, all readings for hardness must be within 3 HRC. Conversion between hardness scales shall be in conformance with ASTM E140. Test frequency shall conform to ASTM F1470-19, Table 3, sample size A

Raw Material (Steel) Manufacturing

Manufacturing Parameter	ASTM/ API-20E BSL-1	API-20E BSL-2	API-20E BSL-3
Melting Practice	Basic-oxygen, Electric-furnace, Vacuum Induction Melting (VIM)	Electric-furnace, Vacuum Induction Melting (VIM)
	Molten steel may be vacuum treated prior to or during pouring of the ingot or strand casting. The primary melting may incorporate separate degassing or refining and may be followed by secondary melting such as electroslag remelting or vacuum remelting
	The steel shall be fully killed. When steel of different grades is continuously cast identification of the resultant transition material is required. The steel producer shall remove the transition material by an established procedure that positively separates the grades
		Secondary refining or ladle refining, shall be performed. When multiple heats of the same chemical composition range are sequentially strand cast, the heats shall be separated by an established procedure such that intermix material shall not be supplied (ASTM A962/A962M)
Casting	Continuous, Ingot	Continuous, Ingot	Ingot
		Top pouring during Ingot casting not permitted unless the VIM is used
Reduction Ratio	4.0:1 Min	4.0:1 Min	10.0:1 Min

Cleanliness (Metallurgy)

Testing	ASTM/ API-20E BSL-1	API-20E BSL-2, BSL-3	Test Technique
Inclusion content		Severity Level Number 2 for thin, 1.5 for thick, all categories	ASTM E45 Method A
Macrostructure	S2, R2, C3	S1, R1, C2	ASTM E381 and ASTM A962
Grain size		Size 5 or finer	ASTM E112
Microstructure	Predominately tempered martensite.		After heat treatment longitudinal direction at ¼T
Banding		Figure 1 is not permitted Band hardness > 470 HK is not permitted	ASTM E384, after heat treatment longitudinal direction at ¼T

Chemical Composition

Element	ASTM/ API-20E BSL-1	API-20E BSL-2	API-20E BSL-3
Carbon	0.38 – 0.48
Sulfur	0.040 Max	0.025 Max	0.015 Max
Phosphorus	0.035 Max	0.025 Max	0.015 Max
Manganese	0.75 – 1.00
Silicon	0.15 – 0.35
Chromium	0.80 – 1.10
Molybdenum	0.15 – 0.25
Vanadium			0.05 Max
Niobium			0.05 Max
Boron (residual)		0.0005 Max	0.0005 Max

Mechanical Properties

Property	ASTM Requirement	API-20E Requirement
Hardness	24 - 35 HRC	24 - 34 HRC
Proof Load	Sample tested based on the number of pieces manufactured from the same heat treatment lot for each nominal diameter of nut and the steel mill heat number. Nuts must be able to sustain minimum load defined per nominal diamater
for Low Temperature (Charpy Impact)	Min 20J Avg 27J @ -101⁰C

Available Sizes

Diameter Range

1/2" to 2-1/2"

M12 to M64

Product Forms

Heavy Hex Nuts
Hex Nuts
Custom Thickness Nuts

Stud Bolts Complementing ASTM A194 Grade 7 Nuts

There is no specific restrictions applied by any of the specifications. So popularly, ASTM A193 B7, ASTM A320 L7 gets paired with this grade of Nut. Choice is based on the service design.

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