Fasteners, Stud Bolts, Studs, Bolting, Screws, Nuts, Washers
API 20E standard specifies requirements for the qualification, production and documentation of Alloy and Carbon Steel Bolting for Use in the Petroleum and Natural Gas Industries. API 20E standard states the requirements for three bolting specification levels (BSL) and nine bolting types. The manufacturer may qualify to one or more of the bolting types listed in 1.4 and to one or more BSLs. Each individual bolting type shall be qualified. Qualification to a higher BSL shall qualify to a lower BSL. The following paragraphs describe the conditions which, when met, allow the bolting to meet the appropriate bolting type and BSL classification level.
Manufacturing processes shall be performed so as to avoid the introduction of stress risers that can occur from sharp angles and tool marks. Threads may be cut or rolled. Unified National Threads shall be "R" (UNR controlled radius root) series. Furnace calibration shall be in accordance with API 6A, Annex M, SAE AMS 2750 or SAE AMS-H-6875. For induction or direct resistance heat treatment, calibration shall be in accordance with the manufacture’s written procedure.
ASTM A320 Grade L43 and ASTM A540, Grade B23 shall be double tempered. When threads are rolled, parts shall subsequently be stress relieved at a temperature within 50oF (28oC) of, but not exceeding the final tempering temperature.
Austenitizing temperatures shall not exceed 1700F (925C). Tempering temperatures for ASTM A193, Grade B7, ASTM A320, Grade L7, and Grade L43, and ASTM A540 Grade B23 shall not exceed 1300oF (700oC).
The hardness test shall conform to the requirements of ASTM A193, A194, A320 or A540 as applicable except that maximum hardness for Grades B7, L7, 2H, 4, 7, L43, B22 and B23 shall be 34 HRC (321 HBW).
| Material Designation | Marking |
|---|---|
| ASTM A193 GRADE B7 | TB B7 |
| ASTM A193 GRADE B7M | TB B7M |
| ASTM A320 GRADE L7 | TB L7 |
| ASTM A320 GRADE L7M | TB L7M |
| ASTM A320 GRADE L43 | TB L43 |
| ASTM A194 GRADE 2H | TB 2H |
| ASTM A194 GRADE 4 | TB 4L |
| ASTM A194 GRADE 7 | TB 7L |
| ASTM A194 GRADE 2HM | TB 2HM |
| ASTM A194 GRADE 7M | TB 7M |
| ASTM A540 GRADE B22 | TB B22 |
| ASTM A540 GRADE B23 | TB B23 |
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Each piece 1 inch (25.4 mm) nominal diameter and larger shall be marked. For bolting less than 1 inch (25.4 mm) nominal diameter, the bolting shall be securely containerized to maintain heat lot identification and traceability. Multiple heat lots shall not be mixed in a single container. Containers used in the processing, storing and shipping of bolting not individually marked shall be clearly labeled with all marking information required by the ASTM specification, as applicable, and this standard.
Plating and coating shall be applied only when specifically stated in the purchasing documents. Instructions for plating or coating shall consider the effect of build-up on threads.
For plating and coating, low temperature bake shall be specified for prevention of embrittlement in accordance with ASTM B633 and ASTM B850, as applicable.
Metallic coatings are applied to fasteners for corrosion resistance, decorative purposes, and extended shelf-life. Metallic coatings are primarily sacrificial in nature. The most common electrodeposited metallic coatings for use on fasteners are zinc, cadmium, and zinc-nickel. Corrosion resistance is limited due to the minimal coating thickness (typically .2 - .3 mils, although thicker deposits can be achieved). Hot dipped galvanizing is also a fastener coating but is used less because of interference issues that arise due to inconsistent coating thickness and tight tolerances.
Cermet coatings, typically aluminum filled, are used extensively as base coats, but require top coats (typically fluoropolymer) for lubricity required for uniform torque.
The electroplating process varies on types and desired results. Most plating processes generate hydrogen. Post-baking after plating is used to diffuse internal hydrogen. If the post-bake temperature is not high enough, the hydrogen does not diffuse. If the post-bake temperature is too high, the plating can be compromised. If plating temperature exceeds the optimized operating limits of the bath or if improper temperature control during post-bake occurs, it can lead to premature bolt failures due to hydrogen embrittlement and loss of corrosion protection.
Non-metallic coatings for fasteners typically are fluoropolymers offering a combination of corrosion resistance, wear resistance and lubricity. They can be applied over phosphate etched substrate where lubricity only is required, or as a topcoat to zinc plate or cermet base coats for more corrosion resistant coating systems. Some examples are:
Corrosion Protection: Fluoropolymer coatings offer corrosion resistance where the film is not damaged or breached. However, they can be subject to extensive damage during the installation (Mechanical or UV exposure), thus exposing the substrate to corrosion.
Lubricity: Fluoropolymer fastener coatings are used primarily as a dry film lubricant, offering consistent torque values. Typically, no additional lubricants, greases, or otherwise, are required with fluoropolymer coatings due to the PTFE content.
Marine epoxy topcoats can be applied to fasteners after assembly, for additional protection (specifically for offshore environments) but are not considered fastener class coatings due to thicknesses which may impair fit and function.