Selon la spécification RCSC 2025 Specification for Structural Joints Using High-Strength Bolts, différentes nuances d'écrous sont autorisées pour les assemblages non revêtus et revêtus. Par exemple, un boulon A325 Type 1 non revêtu offre davantage d'options de nuances d'écrous ASTM A563 que l'assemblage revêtu correspondant. Un boulon A490 Type 3 présente encore une exigence différente.
Pour les achats et le contrôle qualité, cela crée deux questions distinctes : Le grade d'écrou est-il autorisé pour la configuration de boulon spécifiée ? Et l'écrou livré satisfait-il aux exigences applicables de produit et d'assemblage ? Les deux points doivent être clarifiés avant réception.
Identifier le boulon avant de choisir l'écrou
A325 et A490 restent des désignations familières, mais les grades de boulons structurels à tête hexagonale haute sont désormais couverts par l'ASTM F3125/F3125M. L'ASTM A563/A563M traite des écrous, tandis que la spécification RCSC traite de leur utilisation dans les assemblages boulonnés structurels. Ces normes remplissent des fonctions différentes et ne doivent pas être considérées comme des documents de réception interchangeables.
La RCSC 2025 identifie les boulons à tête hexagonale haute ASTM F3125 Grade A325 comme Groupe 120 et les boulons à tête hexagonale haute Grade A490 comme Groupe 150. Ces groupes distinguent les catégories de résistance du boulonnage structurel ; ce ne sont pas des désignations de grade d'écrou ASTM A563.
Avant de consulter une référence de compatibilité des écrous, établir la configuration réelle du boulon :
- Grade et type de boulon : F3125 Grade A325 ou A490, et si l'article est un boulon conventionnel à tête hexagonale haute ou un autre produit d'assemblage apparié.
- Type de boulon : Type 1 ou Type 3. Le Type 3 est associé à l'acier résistant aux intempéries et présente des composants d'accouplement autorisés distincts.
- Diamètre nominal et désignation du filetage : Ceux-ci déterminent les dimensions d'accouplement requises et la compatibilité du filetage.
- État de surface : Nu, galvanisé à chaud, galvanisé mécaniquement ou un autre revêtement spécifiquement autorisé pour l'assemblage.
- Spécification et édition applicables : Le projet ou le bon de commande peut faire référence à une édition particulière de l'ASTM ou du RCSC plutôt qu'à la publication la plus récente.
Un boulon décrit uniquement comme “ A325, 3/4 inch ” ne fournit pas assez d'informations pour finaliser le choix de l'écrou. Le type et l'état de revêtement peuvent modifier la nuance autorisée, même si le diamètre nominal reste identique.
Le style d'écrou compte également. Le RCSC 2025 spécifie généralement des écrous hexagonaux lourds conformes à l'ASTM A563 pour les assemblages de boulons structuraux hexagonaux lourds concernés, sous réserve de ses dispositions de substitution. Un écrou hexagonal standard portant une désignation de nuance familière ne doit pas être accepté simplement parce qu'il se visse sur le boulon. La géométrie d'écrou requise et la norme dimensionnelle doivent également correspondre à l'assemblage spécifié.
Pour des informations générales sur les nuances d'écrous hexagonaux et la compatibilité des boulons, voir la présentation technique SUNHY séparée.
La distinction importe au-delà de l'écrou lui-même. Le RCSC 2025 exige également que le type de rondelle, le cas échéant, corresponde au type de boulon. L'approvisionnement doit donc identifier le boulon, l'écrou et la rondelle comme un système compatible plutôt que comme trois articles de catalogue sans lien.
Quelles nuances d'écrous ASTM A563 sont autorisées ?
Le point de départ pour un assemblage structural selon le RCSC 2025 est le Tableau 2.2, qui établit les nuances d'écrous ASTM A563 autorisées selon le groupe, le type et l'état de revêtement du boulon. L'explication suivante résume les conséquences du choix sans remplacer le tableau applicable.
A325 Type 1 : les assemblages nus et revêtus diffèrent
Pour un boulon nu ASTM F3125 Grade A325 Type 1, le RCSC 2025 autorise les nuances ASTM A563 C, C3, D, DH et DH3.
Cette plage d'options est parfois condensée en une combinaison préférée du fabricant. La nuance DH, par exemple, est un choix courant, mais ce n'est pas la seule nuance autorisée pour la configuration nue selon cette édition.
La situation change lorsque le boulon A325 Type 1 est revêtu conformément à la section 2.8 de la RCSC. Ici, le tableau 2.2 identifie ASTM A563 Grade DH.
Un acheteur ne doit pas reconduire une spécification d'écrou de grade C précédemment approuvée simplement parce que le grade et le diamètre du boulon restent inchangés. La modification de la spécification de revêtement peut changer le grade d'écrou autorisé et la vérification d'assemblage requise.
A490 Type 1 : une sélection plus restreinte
Pour les boulons nus ASTM F3125 grade A490 Type 1, la RCSC 2025 autorise les grades DH et DH3. Pour les assemblages Type 1 revêtus conformément à la section 2.8, le grade ASTM A563 autorisé est DH.
Cette distinction évite deux erreurs courantes.
Premièrement, un écrou A563 grade C ne doit pas être approuvé pour un assemblage A490 en arguant qu'il convient à certaines applications A325. Les combinaisons autorisées sont différentes.
Les boulons auto-patinables Type 3 exigent leur propre décision d'appariement
Pour les configurations Type 3 nues, la RCSC 2025 autorise des écrous C3 ou DH3 avec les boulons du groupe 120 (A325), et des écrous DH3 avec les boulons du groupe 150 (A490).
La désignation “ 3 ” est importante car le choix d'un matériau auto-patinable n'est pas uniquement une question de résistance mécanique. Un assemblage conçu autour d'un acier auto-patinable exige une prise en compte appropriée des composants d'appariement et de leurs conditions d'exposition.
Le tableau RCSC ne fournit pas, dans ces lignes, de combinaison générale Type 3 revêtue. Par conséquent, un acheteur ne doit pas étendre les autorisations Type 3 nues à un assemblage auto-patinable revêtu sans base de spécification applicable et sans l'approbation d'ingénierie requise.
L'écrou ASTM A194 grade 2H peut-il remplacer un écrou A563 ?
RCSC 2025 Section 2.3.2 permits ASTM A194 Grade 2H nuts as substitutes for ASTM A563 Grades C, D, and DH.
Il s'agit d'une autorisation définie, et non d'une affirmation générale selon laquelle tout écrou A194 peut remplacer n'importe quel écrou A563. En particulier, elle ne doit pas être étendue automatiquement aux exigences de nuance résistante aux intempéries C3 ou DH3.
Where Grade 2H is proposed, the buyer still needs to confirm the heavy-hex configuration, correct thread fit, applicable coating requirements, documentation, and complete assembly conditions. A permissible substitute grade does not eliminate the need to verify the delivered component.
ASTM A563/A563M also contains provisions concerning stronger-grade substitutions with notice to the purchaser. Those provisions must be read alongside any more specific structural-joint and purchase-order restrictions. A supplier’s proposed upgrade is not an automatic authorization to change a specified structural assembly.
The practical selection rule is straightforward: identify a permitted grade first, then verify that the proposed nut has the required style, finish, and supporting evidence.
In an illustrative finish revision, an A325 Type 1 order changes from plain to a permitted coated assembly while the original nut line remains unchanged. A nut permitted for the plain configuration may not satisfy the coated row. Recheck the governing pairing, nut treatment and complete assembly evidence. The nut-grade guide supplies background; the cited structural pairing table controls this decision.
Pourquoi les revêtements modifient la compatibilité des écrous
Coating selection can alter the mechanical behavior of a bolting assembly even when the bolt and nut have unchanged nominal sizes.
For galvanized or certain zinc-flake-coated assemblies, the coating adds material to the mating thread surfaces. To accommodate that thickness, structural nuts are commonly manufactured with intentionally oversized internal threads, known as overtapping.
Overtapping creates clearance for the coated bolt threads, but it also reduces the amount of thread engagement. This matters because the nut must resist thread stripping while the bolt is subjected to the tensile loading associated with installation and service.
A nut that is adequate in a plain-thread configuration cannot automatically be treated as adequate after its internal threads have been enlarged. The relevant nut strength requirements, coating conditions, and assembly tests must be satisfied.
Hot-dip galvanizing and mechanical galvanizing are not interchangeable processes
RCSC 2025 Section 2.8 distinguishes hot-dip galvanizing from mechanical galvanizing. The two processes have separate specification requirements, and the threaded components of a galvanized assembly must use the same applicable process.
For Group 120/A325 heavy-hex bolting, permitted galvanized assemblies require careful control of the corresponding nut treatment and lubrication. Hot-dip galvanized nuts are overtapped according to ASTM A563 requirements, and mechanically galvanized nuts have their own applicable overtapping requirements.
The assembly should not be created simply by purchasing galvanized bolts from one source and nuts from another without establishing responsibility for compatibility testing.
RCSC commentary explains why galvanized structural bolting is treated as a matched assembly. Changes to coating thickness, thread engagement, and lubrication can affect the ability to achieve the required tightening behavior without thread damage or stripping.
For a galvanized assembly governed by these requirements, the supplier must provide the applicable matched and tested components. If separately sourced components are proposed, the purchaser must establish how the necessary assembly verification and certification responsibilities will be fulfilled.
A490 coating restrictions need precise wording
A490/Group 150 heavy-hex bolts are not permitted to be hot-dip galvanized or mechanically galvanized under RCSC 2025.
That restriction should not be simplified into “A490 bolts cannot be coated.”
Section 2.8 separately permits qualifying zinc-flake coating systems complying with the specified ASTM F3393 classification and other applicable conditions. This is a defined coating route, not a general permission to use any commercial zinc-flake, zinc-plated, or galvanized finish.
For a proposed coated A490 assembly, the buyer should verify the coating standard and classification, the supplier’s assembly configuration, the required treatment of the nuts, and the relevant product and assembly evidence.
Older technical literature may refer to different zinc-flake standards used in previous editions. A historic designation or a supplier’s informal coating description is not sufficient evidence that a delivered assembly conforms to the requirements of RCSC 2025.
Hypothetical example: Changing the finish changes the nut decision
Assume an EPC purchaser is preparing a request for 3/4-inch ASTM F3125 Grade A325 Type 1 heavy-hex bolts.
The original specification calls for plain bolts, and the supplier offers plain ASTM A563 Grade C heavy-hex nuts. Grade C is one of the permitted A563 grades for this plain combination under RCSC 2025, provided the other product and project requirements are satisfied.
The design is then revised to require hot-dip galvanized bolting.
The buyer cannot simply keep the original Grade C nut specification and add galvanizing. Under the coated Group 120 Type 1 conditions in RCSC Table 2.2, the specified A563 grade is DH.
The revised procurement request must address the permitted Grade DH nut, correct galvanizing process, overtapped thread configuration, lubrication, matching components, and applicable rotational-capacity evidence.
The bolt diameter has not changed. The required selection and verification work has.
This assumed case illustrates a specification decision; it is not a recorded project, a tested product result, or proof of any supplier’s manufacturing capability.
Vérifier le marquage des écrous, les filetages et les dossiers d'inspection
A permitted grade on a purchase order establishes what should be supplied. Receiving inspection establishes whether the actual shipment matches the specified requirements.
These are related but different decisions, and neither can be completed by checking the nut grade marking alone.
Confirm the nut’s identity
Inspect the nut for the applicable grade identification and manufacturer’s identification, using the marking requirements of the governing standard and the permitted marking arrangements.
The markings help identify whether the supplied item is represented as Grade C, C3, D, DH, DH3, or another permitted specification grade. A194 Grade 2H has its own designation and should not be silently relabelled as A563 DH.
The inspector must also confirm the required heavy-hex style and nominal size. The grade marking is useful evidence of identification, but it does not independently establish that the nut passed every chemical, hardness, proof-load, or dimensional acceptance requirement.
If the marking is missing, inconsistent with the purchase order, or not traceable to the supplier’s documentation, the discrepancy should be resolved before the component is accepted for the specified structural assembly.
Verify thread fit against the actual finish
Thread compatibility is more than matching the nominal diameter and pitch.
A plain nut and an overtapped coated nut can have different finished internal-thread conditions. That difference may be intentional and necessary for the specified assembly.
Receiving inspection should confirm the relevant thread designation, applicable dimensional requirements, coating condition, and whether overtapping is required. Where gauging or dimensional records are supplied, their acceptance basis should correspond to the specified finished nut condition rather than an assumed uncoated configuration.
An assembler should not attempt to correct a questionable fit by uncontrolled thread chasing, removal of the coating, or applying an unapproved lubricant. Such actions could change the supplied assembly condition and invalidate the basis of its verification.
A bolt and nut turning freely by hand does not demonstrate that the assembled fastener system meets the specified load, coating, or installation requirements.
Distinguish product testing from assembly verification
Three forms of evidence are especially important, but they establish different things.
| Preuve | What it helps establish | What it does not establish alone |
|---|---|---|
| Nut product conformity and test report | Compliance with applicable ASTM A563 or A194 material, mechanical, and dimensional requirements, within the reported scope | That the nut was tested as part of the actual supplied bolt–nut–washer assembly |
| Rotational-capacity evidence, where required | Compatibility of the applicable coated assembly, including its lubricant and capacity to undergo the specified rotation without unacceptable failure | That the completed structural joint was installed and inspected correctly |
| Pre-installation verification records, where required by the installation method | That the specified assembly and installation procedure demonstrate the required pretensioning behavior before installation | That every nut lot independently satisfies all product specification requirements |
ASTM F606/F606M provides mechanical testing methods for threaded fastener products, including applicable nut proof-load and hardness testing. The product standard and governing purchase requirements determine the actual acceptance criteria.
A proof-load test evaluates the nut’s response to a defined axial test load under specified conditions. It is not the same as the pretension achieved in a completed structural joint. Similarly, a hardness result cannot automatically be substituted for the required proof-load evidence.
The exact numerical test limits, sampling arrangements, and acceptance conditions must be checked against the contractual ASTM A563/A563M edition and the particular product configuration. Published data from an older edition should not be presented as current ASTM A563/A563M-26 acceptance requirements without verification.
RCSC 2025 Section 2.9 addresses the availability of applicable product and assembly test reports to the Engineer of Record and Inspector. Pre-installation verification is separately associated with the relevant installation procedure. These responsibilities should remain distinct in the project’s inspection plan.
Use receiving inspection to close actual discrepancies
Consider a shipment labelled “A563 DH heavy hex nuts” for coated A325 Type 1 bolts.
The receiving decision should not stop at the DH identification. The inspector should determine whether the specified coating was applied, whether the threads were processed for that coating, whether the required lubrication and matched-assembly conditions were maintained, and whether the required records are available.
The documentation should identify the actual supplied product and the applicable lot or other traceability identifiers required by the purchase order. Traceability statements must be consistent with the records; a generic catalogue certificate is not proof of testing for a particular delivered lot.
If a record refers to a different grade, finish, or assembly configuration, the buyer should request clarification or hold the affected material rather than infer compliance from visual similarity.
A hypothetical delivery contains DH-marked nuts with no clear finished-thread or lubricant basis for the ordered coated assembly. The grade mark does not resolve those conditions. Reconcile the product records, coating, overtapping and matched components, retaining the lot-specific evidence relationship. Do not correct an unexplained tight fit by uncontrolled thread chasing.
Spécifier les écrous et les assemblages corrects dans une demande de prix
A useful RFQ does more than state “ASTM A563 nuts for A325 bolts.” It defines enough information for a supplier to propose the correct items and for the purchaser to verify the delivered assembly without making assumptions.
The following checklist separates selection information from verification evidence.
| RFQ or receiving item | What to specify or confirm |
|---|---|
| Governing documents | Applicable ASTM F3125/F3125M, ASTM A563/A563M, RCSC, and project editions |
| Bolt configuration | Grade A325 or A490, Type 1 or Type 3, style, nominal diameter, length, and thread designation |
| Nut requirement | Permitted A563 grade or approved substitute, heavy-hex style, size, and marking requirements |
| Washer configuration | Required washer specification, Type, finish, and special configuration where applicable |
| Revêtement | Exact coating method, governing coating specification, classification, and finish condition |
| Traitement du filetage | Whether overtapping is required and how finished-thread conformity will be verified |
| Assembly condition | Matched component requirements, specified lubrication, packaging, and handling controls |
| Acceptance evidence | Product conformity reports, applicable lot identification, and assembly test records required for the configuration |
| Changes and substitutions | Written identification and approval of any departure from the specified nut grade, finish, or assembly |
Not every project requires an identical document package. The evidence requirement should follow the applicable standards, connection and installation conditions, and contract documents. A buyer can request additional traceability or inspection records, but those requests should not be represented as universal ASTM requirements.
For coated assemblies, it is useful to ask the supplier to identify who is responsible for the complete assembly verification, rather than simply providing separate bolt, nut, and washer certificates.
For a plain A490 Type 3 assembly, the focus changes to confirming the permitted weathering-grade nut and mating component configuration. Asking for an unrelated galvanizing record would not solve the relevant acceptance question.
The final procurement decision should follow a clear sequence: establish the bolt identity, confirm the permitted nut configuration, verify the coating and thread conditions, and review the actual product and assembly evidence. If a proposed substitution changes that basis, obtain the necessary specification and project approval before accepting it.
SUNHY’s structural bolt product information may serve as a starting point for a technical inquiry. Buyers should provide the bolt grade and Type, nut grade, dimensions, finish, quantity, and required documentation for a configuration-specific review. Availability, manufacturing capability, and conformity must be confirmed for the particular order rather than inferred from this guide.
Technical reference basis
- Research Council on Structural Connections (RCSC). Spécification pour les assemblages structuraux utilisant des boulons à haute résistance, June 5, 2025. Particularly the bolt Group definitions, permitted nut-grade requirements, coated assembly provisions, and test-report requirements.
- ASTM International. ASTM A563/A563M-26 — Standard Specification for Carbon and Alloy Steel Nuts (Inch and Metric).
- ASTM International. ASTM F3125/F3125M-26 — Standard Specification for High Strength Structural Bolts and Assemblies.
- ASTM International. ASTM F606/F606M-26a — Standard Test Methods for Determining the Mechanical Properties of Externally and Internally Threaded Fasteners, Washers, Direct Tension Indicators, and Rivets.
The 2025 RCSC document is the edition used for the pairing explanations in this guide. Actual acceptance requirements must follow the standards and editions governing the project; this article does not reproduce the complete ASTM product specifications.
Questions fréquemment posées
Quel écrou A563 faut-il utiliser avec un boulon A325 ?
Identifier d'abord le type de boulon et la finition, puis utiliser la nuance autorisée dans le tableau applicable. Les combinaisons nues et revêtues ne disposent pas des mêmes options.
Un écrou A194 2H peut-il remplacer tous les écrous A563 ?
Non. L'autorisation de substitution RCSC citée est limitée aux nuances A563 spécifiées. Ne l'étendez pas automatiquement aux exigences de nuance résistante aux intempéries.
Un écrou de résistance supérieure suffit-il à valider une substitution ?
Non. Vérifiez l'autorisation de nuance applicable, le style, l'état du filetage, le revêtement, la documentation et toute approbation requise pour l'assemblage réel.
