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Weld Nuts
A comprehensive engineering reference for EPC contractors, fabrication engineers, procurement heads and TPI agencies — covering weld nut types, welding processes, dimensional standards, property classes, material grades, surface treatments and full project documentation requirements.
Weld Nut Types, Engineering Function
& Welding Processes
Hex · Square · Tab · Round · T-Weld · Projection · Pipeline Types
Definition and Engineering Function
A weld nut is an internally threaded fastener specifically designed to be permanently attached to a base metal substrate by welding, providing a fixed threaded insert that does not require operator access from behind during bolt engagement. The integral weld feature — which may take the form of a pilot boss, projection weld points, flange tabs, or a flat base — controls heat input, aligns the nut perpendicular to the work surface, and concentrates the weld current (in resistance welding) or filler material (in arc and MIG welding) to produce a metallurgically sound joint without distorting the nut thread.
Weld nuts are the engineering solution in four principal application scenarios: (1) assemblies where the rear face of the weld plate is inaccessible after fabrication; (2) high-volume automated production where a single-operator tightening operation is required; (3) thin-gauge sheet metal where a standard nut would pull through the parent material under fastener preload; and (4) structural frameworks requiring permanent, vibration-resistant threaded inserts that cannot loosen over service life. The weld permanently replaces the anti-rotation and pull-out resistance function of a wrench hold or a captive nut.
A loose hex nut on the back of a plate relies on friction at the thread and nut face to resist rotation and pull-out. A weld nut substitutes the mechanical friction mechanism with a metallurgical bond between the nut base and the parent material — which, when correctly welded to a qualified WPS, can develop the full shear and tensile strength of the parent material at the weld interface, substantially exceeding the anti-rotation and pull-out capability of any friction-retained nut in dynamic or vibration-loaded service.
Weld Nut Types — Complete Engineering Descriptions
Welding Processes — Engineering Guide
The internal thread of a weld nut must be protected from weld spatter, heat-induced metallurgical change and mechanical distortion during welding. Standard practice: a thread plug (plastic or metal) is inserted before welding and removed after. For resistance-welded pilot nuts, the pilot boss acts as a heat sink but the thread bore must be verified with a Go gauge after welding — heat from the weld zone can cause bore distortion sufficient to reject thread engagement in sizes M6 and below at high welding currents.
| Type | Standard | Weld Method | Size Range | Base Sheet (mm) | Primary Application |
|---|---|---|---|---|---|
| Hex Pilot Weld Nut | DIN 929 / ISO 21670 | Resistance / projection | M4–M16 | 0.8–3.5 | Automotive, enclosures, sheet metal panels |
| Square Projection Weld Nut | DIN 928 | Resistance / projection | M4–M16 | 1.0–4.0 | Machinery bases, structural plates, heavy panels |
| Tab / Flange Weld Nut | Custom / OEM spec | Arc / MIG / plug weld | M6–M24 | 4.0–25+ | Structural steel, offshore frames, pipeline brackets |
| Round Base Weld Nut | Custom / AWS D1.1 | MIG / TIG fillet weld | M6–M24 | 4.0–20+ | High-load anchor, equipment mounting, pipe clamp |
| T-Weld Nut (Pipeline) | Project spec / WPS | Full-pen butt / fillet | M8–M24 | Pipe wall | Instrument tapping, drain, vent, branch connection |
| 3-Point Projection Nut | ISO 21670 / EN 28673 | Resistance projection | M3–M12 | 0.5–3.0 | High-volume stampings, OEM automotive, appliances |
| Weld Stud Nut (CD) | AWS D1.1 / ISO 14555 | CD / Drawn arc stud | M5–M24 | 0.7–25+ | Vessels, structural, precast, enclosures |
| Self-Clinching / PEM | PEM standard | Press + optional weld | M3–M12 | 0.5–4.0 | Electronic enclosures, HVAC, light structural |
Dimensional Data, Standards
& Property Classes
ISO 21670 · Pilot Diameter · Flange · Thread Data
DIN 929 Hex Weld Nut — Dimensional Data
| Thread Size | Pitch (mm) | Width Across Flats (mm) | Nut Height (mm) | Pilot Diameter d3 (mm) | Pilot Height h1 (mm) | Hole in Sheet (mm) | Property Class |
|---|---|---|---|---|---|---|---|
| M4 | 0.70 | 7.0 | 3.2 | 4.7 | 1.3 | 4.8–5.0 | 4, 6 |
| M5 | 0.80 | 8.0 | 4.0 | 5.7 | 1.5 | 5.8–6.0 | 4, 6 |
| M6 | 1.00 | 10.0 | 5.0 | 6.7 | 1.8 | 6.8–7.0 | 4, 6, 8 |
| M8 | 1.25 | 13.0 | 6.5 | 9.0 | 2.0 | 9.1–9.3 | 4, 6, 8 |
| M10 | 1.50 | 16.0 | 8.0 | 11.0 | 2.5 | 11.1–11.3 | 6, 8 |
| M12 | 1.75 | 18.0 | 10.0 | 13.0 | 3.0 | 13.1–13.3 | 6, 8 |
| M16 | 2.00 | 24.0 | 13.0 | 17.0 | 3.5 | 17.1–17.4 | 6, 8 |
| M20 | 2.50 | 30.0 | 16.0 | 21.0 | 4.0 | 21.2–21.5 | 6, 8 |
| M24 | 3.00 | 36.0 | 19.0 | 25.0 | 4.5 | 25.2–25.5 | 6, 8 |
DIN 929:1987 / ISO 21670:2003. Hole in sheet = pilot diameter + 0.1/0.3 mm clearance for slip-fit location. Tolerance class 6H on thread. All dimensions in mm unless stated. Verify current standard revision for procurement.
| Thread Size | Pitch (mm) | Width Across Flats (mm) | Nut Height (mm) | Projection Height (mm) | No. Projections | Weld Base Area (mm²) | Property Class |
|---|---|---|---|---|---|---|---|
| M4 | 0.70 | 7.0 | 3.2 | 0.5 | 2 | 40 | 4, 6 |
| M5 | 0.80 | 8.0 | 4.0 | 0.6 | 2 | 54 | 4, 6 |
| M6 | 1.00 | 10.0 | 5.0 | 0.7 | 2 | 80 | 4, 6, 8 |
| M8 | 1.25 | 13.0 | 6.5 | 0.8 | 2 | 140 | 4, 6, 8 |
| M10 | 1.50 | 16.0 | 8.0 | 0.9 | 4 | 216 | 6, 8 |
| M12 | 1.75 | 20.0 | 10.0 | 1.0 | 4 | 320 | 6, 8 |
| M16 | 2.00 | 26.0 | 13.0 | 1.2 | 4 | 548 | 6, 8 |
DIN 928:1985. Projection dimensions critical for weld current concentration — non-conforming projection height causes incomplete fusion or excess spatter. Weld base area = e² (width across flats squared).
| Thread Size | Sheet Thickness (mm) | Weld Current (kA) | Electrode Force (kN) | Weld Time (cycles at 50Hz) | Minimum Torque After Weld (N·m) |
|---|---|---|---|---|---|
| M4 | 0.8–1.5 | 6–9 | 1.5–2.5 | 3–6 | 4 |
| M5 | 1.0–2.0 | 7–11 | 2.0–3.0 | 4–8 | 8 |
| M6 | 1.0–2.5 | 9–14 | 2.5–4.0 | 5–10 | 12 |
| M8 | 1.5–3.0 | 12–18 | 3.5–5.5 | 6–12 | 22 |
| M10 | 2.0–4.0 | 15–22 | 5.0–7.5 | 8–15 | 44 |
| M12 | 2.5–5.0 | 18–26 | 6.0–9.0 | 10–18 | 76 |
| M16 | 3.0–6.0 | 22–32 | 8.0–12.0 | 12–22 | 184 |
Reference welding parameters per ISO 14270 and electrode supplier data. Actual parameters must be established and qualified per WPS for each nut-sheet material combination. Weld quality qualification by push-off or torque test per ISO 14270 on sample coupon before production run.
Governing Standards
DIN 929 is the European standard for hex weld nuts with pilot boss, covering metric sizes M3 to M20. It specifies the pilot diameter, pilot height, nut height and width across flats. The pilot diameter must match the pre-punched hole in the base sheet within the tolerances defined in DIN 929 Table 2. DIN 928 covers square weld nuts with embossed projection weld points, M3 to M16, specifying projection height and form critical for weld current concentration in resistance projection welding.
ISO 21670 is the international standard for hex weld nuts with flange, superseding DIN 929 in international EPC and OEM project specifications. It defines flange diameter, flange height, nut height and mechanical property requirements. EN 28673 covers projection weld nuts with annular or ring projection. AWS D1.1 (Structural Welding Code — Steel) provides the weld qualification framework for arc-welded tab and round base weld nuts in structural applications. ISO 14555 covers stud welding qualification for capacitor discharge weld studs and female stud nuts. ISO 15614-12 covers resistance welding procedure qualification applicable to projection and spot welding of pilot and projection weld nuts.
| Property Class | Proof Load Stress (MPa) | Min. Pull-Out Force M8 (kN) | Min. Pull-Out Force M12 (kN) | Bolt Pairing | Material |
|---|---|---|---|---|---|
| ISO 4 | 400 | 8.0 | 18.5 | 4.6 / 4.8 | Carbon steel |
| ISO 6 | 600 | 12.0 | 27.5 | 5.8 / 6.8 | Carbon steel |
| ISO 8 | 800 | 16.0 | 36.5 | 8.8 | Carbon steel Q&T |
| A2-70 | 700 | 14.0 | 32.0 | A2-70 bolt | SS 304 |
| A4-70 | 700 | 14.0 | 32.0 | A4-70 bolt | SS 316 |
| A4-80 | 800 | 16.0 | 36.5 | A4-80 bolt | SS 316L cold-worked |
Pull-out force values are indicative for weld nuts welded to 2.0 mm mild steel sheet per ISO 14270. Actual pull-out strength is dependent on sheet thickness, parent material grade, and weld quality — must be established by coupon testing per WPS qualification. Property class refers to nut thread properties, not weld joint strength.
A_nugget = π × (d_nugget / 2)² // Single nugget area; d_nugget typically 4–6 mm for M8–M12
τ_shear_CS ≈ 0.577 × σ_yield // Von Mises yield criterion; σ_yield ~ 250 MPa for S235/ASTM A36
// WORKED EXAMPLE: M10 hex weld nut, d_nugget = 5 mm, mild steel (σ_y = 250 MPa)
A_nugget = π × 2.5² = 19.6 mm²
τ_shear = 0.577 × 250 = 144 MPa
F_pullout_est = 19.6 × 144 = 2,823 N (2.8 kN) per nugget // Verify by coupon test per WPS
Material Grades, Weldability
& Surface Treatments
Weldability · HAZ Control · HDG · Zinc · Dacromet · Post-Weld Coating
| Grade | Standard | UTS (MPa) | Proof Load (MPa) | Weldability | Corrosion | Key Application |
|---|---|---|---|---|---|---|
| S235 / A108 CS | EN 10025 / ASTM A108 | 360–510 | 235 | Excellent | Low | General fabrication, automotive, enclosures |
| S355 / A36 | EN 10025 / ASTM A36 | 490–630 | 355 | Good | Low | Structural steel, heavy fabrication, offshore frames |
| ISO Cl.6 CS | ISO 898-2 | — | 600 | Good | Low | Standard weld nut — DIN 929 / DIN 928 default |
| ISO Cl.8 CS | ISO 898-2 | — | 800 | Marginal | Low | High-load; post-weld hardness must be verified |
| SS 304 (A2) | ISO 3506-2 / ASTM A194 | ≥700 | 700 | Good | High | Corrosion-resistant enclosures, food & pharma |
| SS 316L (A4) | ISO 3506-2 / ASTM A194 | ≥700 | 700 | Good | Very High | Marine, offshore topside, chemical plant |
| Duplex 2205 | ASTM A182 F51 | ≥620 | 450 | Requires WPS | Very High | Sour service, subsea, offshore structures |
| Inconel 625 | ASTM B564 | ≥827 | 414 | Good (all pos.) | Extreme | High-temp, corrosive, turbine housings |
| Monel 400 | ASTM B164 | ≥480 | — | Good | High — seawater | Marine, HF acid environments |
Weldability and Heat-Affected Zone (HAZ) Considerations
The weldability of a weld nut is primarily governed by its carbon equivalent (CE or CET) value. Carbon steel weld nuts per DIN 929 in property class 6 typically have a CE value of 0.25–0.35, placing them in the “easily weldable” category per ISO/TR 15608 — no preheat is required for sheet thicknesses below 25 mm at ambient temperature. For class 8 carbon steel weld nuts (CE typically 0.40–0.55), the HAZ can develop a hardened martensitic zone that is both crack-susceptible and (in sour service) non-compliant with NACE MR0175 if hardness exceeds 22 HRC. Post-weld hydrogen bake-out at 200–250°C is mandatory for class 8 weld nuts on sour service components.
For austenitic stainless steel weld nuts (304, 316L), the primary concern is sensitisation — chromium carbide precipitation at grain boundaries in the HAZ following slow cooling through the 425–815°C range, which depletes the chromium-rich zone adjacent to the weld and creates a susceptibility to intergranular corrosion. Low-carbon grades (L suffix) or stabilised grades (321, 347) are specified where sensitisation risk is elevated. Duplex 2205 weld nuts require careful heat input control (0.5–2.5 kJ/mm for arc welding) to maintain the correct 50:50 ferrite/austenite balance in the HAZ — deviation in either direction degrades corrosion resistance and toughness. WPS qualification per ISO 15614-1 with Charpy and ferrite content tests is mandatory.
For carbon steel weld nuts in corrosive environments: the preferred sequence is weld nut supplied in plain (uncoated) condition → weld nut installed by resistance or arc welding → assembly cleaned (shot blast or acid etch) → assembly coated as a whole (hot-dip galvanise, liquid paint, powder coat, or Dacromet). This avoids coating damage during welding and ensures complete coverage of the weld interface. Pre-coated (zinc-plated or galvanised) weld nuts may generate zinc fume during welding — a health hazard — and produce zinc-contaminated weld zones that are susceptible to liquid metal embrittlement in high-strength steels.
| Finish | Standard | Thickness (µm) | Pre-Weld Use | Post-Weld Application | Salt Spray (hrs) | Notes |
|---|---|---|---|---|---|---|
| Plain / Bare | — | — | Preferred | N/A — coat assembly after welding | <24 | Standard supply condition for weld nuts |
| Phosphate + oil | MIL-DTL-16232 | 5–15 | Acceptable | Use as pre-treatment base | 48–96 | Minimal weld interference; burns off in weld zone |
| Black oxide + oil | MIL-DTL-13924 | 0.5–2 | Acceptable | Cosmetic only post-weld | 24–48 | Widely used pre-weld for DIN 929; negligible weld impact |
| Zinc electroplate | ASTM B633 SC1 | 5–8 | Caution | N/A — coat post-weld only | 96 | Zinc fume and LME risk at weld — pre-coat not recommended |
| Hot-dip galvanise | ASTM A153 | 45–86 | Not suitable | Assembly-level post-weld | 500+ | Galvanise assembled part — re-tap threads after HDG |
| Dacromet / Geomet | ISO 10683 | 8–12 | Caution | Post-weld component level | 720+ | Apply post-weld; provides excellent corrosion resistance |
| Electroless nickel | ASTM B733 | 15–50 | Caution | Post-weld selective application | 300+ | Apply to thread bore post-weld for corrosion protection |
| Powder coat | OEM spec | 60–100 | Not suitable | Assembly-level post-weld | 500–1000 | Standard for enclosure and structural assemblies |
Zinc electroplate or galvanised coatings on weld nuts risk zinc fume generation (health hazard — COSHH / OSHA controlled substance) and liquid metal embrittlement of the base steel at weld temperatures above 420°C. Plain or phosphate-coated weld nuts are the engineering standard for pre-weld supply. Coat the completed assembly post-welding.
For weld nuts installed in H≶S-containing service zones, both the nut body and the weld HAZ must comply with the maximum 22 HRC (237 HB) hardness limit per NACE MR0175 / ISO 15156 Part 2. This excludes ISO Class 8 carbon steel weld nuts without post-weld heat treatment, as HAZ hardness in class 8 material can reach 350–450 HB without preheat or PWHT. The WPS must specify preheat, interpass temperature and PWHT where required to ensure HAZ hardness compliance. Hardness survey of the completed weld coupon (including HAZ) is required as part of WPS qualification.
Inspection, QC Protocols, Applications
& Export Documentation
Automotive · Structural · Offshore · Pipeline · OEM Production
Inspection and Quality Control
Prior to welding, weld nuts are inspected per DIN 929 / DIN 928 or ISO 21670 for: pilot diameter and height (DIN 929), projection height and form (DIN 928), width across flats, nut height, thread form (Go/No-Go per ISO 1502 or ASME B1.2), and bearing face flatness. Pilot diameter tolerance is critical — an oversized pilot will not locate in the sheet hole and a severely undersize pilot will not align the nut perpendicular to the sheet surface, producing an angled thread bore after welding.
Weld quality of resistance-welded pilot and projection weld nuts is verified by push-off testing (axial load applied to the nut to failure, minimum load per ISO 14270 by nut size and sheet thickness) or torque testing (a bolt is tightened to proof torque in the welded nut — no rotation, cracking or thread damage permitted). Sampling frequency: minimum one assembly per 100 units for production quality monitoring; 100% inspection for safety-critical applications. For arc and MIG-welded tab and round-base weld nuts, visual inspection per AWS D1.1 Section 6 (full fusion, no undercut, no crater crack) plus UT or MT on the weld fillet for structural applications is required.
After welding, a Go gauge (ISO 1502 or ASME B1.2) must pass through the full thread depth freely without interference. A No-Go gauge must not engage more than two full turns. Thermal distortion during resistance welding can reduce thread bore diameter by 0.05–0.15 mm in sizes M6 and below — this is the most common post-weld rejection cause and must be verified before dispatch to the assembly line or project site.
Type 3.1: Validated by the manufacturer's own authorised inspection representative. Minimum standard for standard EPC, OEM and automotive project supply. Must state material standard, grade, heat/lot number, chemical analysis, mechanical test results and heat treatment. Type 3.2: Co-validated by purchaser-nominated TPI body (Bureau Veritas, DNV, Lloyds, SGS, TÜV). Mandatory for offshore, NACE, nuclear-adjacent and structural Class 1 applications. For weld nuts where the weld qualification is part of the supply scope, the WPS/PQR documentation package is a mandatory addition to the 3.1/3.2 MTC package.
Positive Material Identification by XRF or OES spectrometry is mandatory for all stainless steel, duplex and super duplex and exotic alloy weld nuts. For mixed-grade project orders, PMI at the point of packing prevents grade substitution. It is also critical to PMI-verify that stainless weld nuts being welded to stainless sheet are matching alloy families — welding dissimilar stainless grades (e.g., 304 nut to 316 sheet) may produce weld zones susceptible to galvanic corrosion or reduced strength.
Applications by Industry
Weld nuts are ubiquitous in automotive body-in-white (BiW) and chassis fabrication. DIN 929 hex pilot weld nuts in M6–M12 property class 6 are resistance-welded to mild steel panels at rates of hundreds of units per minute in automated welding cells. They provide the threaded attachment points for door hinges, brackets, seat rails, engine mounts and trim components where a blind fastening point is structurally required. The push-off and torque test frequency is defined in IATF 16949 quality management for automotive supply.
Tab and round-base weld nuts per AWS D1.1 are MIG or arc-welded to structural sections (I-beams, RHS, CHS) for bracket attachment, equipment mounting and mechanical connection. They replace drilled and tapped holes in locations where through-drilling is not possible or structurally weakening. Weld qualification per AWS D1.1 or ISO 9606-1 is mandatory. For pre-galvanised or painted structural sections, weld nuts are installed prior to surface treatment; for in-situ modification, a Class D repair weld procedure is required.
Weld nuts on offshore structures, subsea equipment and topside piping supports are required to comply with NACE MR0175 in sour service zones, and must be manufactured from duplex 2205 or super duplex for splash zone and submerged applications. Full WPS/PQR documentation, EN 10204 3.2 MTC with TPI countersignature, PMI, and post-weld hardness survey are project-standard requirements. For instrument tapping weld nuts on piping, ASME B31.3 inspection requirements apply including RT or PT after welding.
T-weld nuts and boss-type weld fittings on pipelines are subject to the full pressure design code (ASME B31.3 for process piping, ASME B31.8 for gas transmission). The weld joint is inspected by radiography (RT) for butt-weld configurations, or by magnetic particle (MT) or liquid penetrant (PT) for fillet weld configurations, per the applicable code inspection level. Pressure test at 1.5× MAWP after welding is mandatory before commissioning.
Hex pilot weld nuts in M4–M8 property class 6, carbon steel or stainless, are the standard threaded insert for electrical switchgear enclosures, junction boxes, cable management panels and HVAC ductwork. In ATEX Zone 1/2 classified areas, the enclosure material, weld quality and thread engagement length must comply with IEC 60079-1 (flameproof) or IEC 60079-7 (increased safety) requirements. For stainless enclosures in food-grade or pharmaceutical environments, 316L weld nuts with a full post-weld electropolish or passivation treatment per ASTM A380 are specified.
Export Packaging and Preservation
- Weld nuts packed in heat-sealed polypropylene bags, batch-labelled with PO number, type (DIN 929 / DIN 928), grade, thread size, heat/lot number and quantity
- VCI (Volatile Corrosion Inhibitor) poly liner in each bag for plain/phosphate-coated weld nuts intended for sea freight or long-term site storage
- Bags packed in double-wall corrugated cartons with foam or kraft paper void fill; nut pilots and projections must not contact each other to prevent deformation
- Cartons secured on ISPM-15 heat-treated timber pallets with stretch wrap and minimum two steel strapping bands per pallet layer
- MTC, dimensional inspection report, thread gauge certificate and all project documents in waterproof sealed envelope attached to exterior of each crate
- Outer crate labels: PO number, item tag, material grade, standard, type, heat number, quantity, gross weight, country of origin, handling instructions
- Thread bores individually capped with polyethylene plugs on large weld nuts (M12 and above) for long-term storage to prevent bore contamination and oxidation
| # | Document | Standard / Reference | Minimum Requirement |
|---|---|---|---|
| 01 | Material Test Certificate (MTC) | EN 10204 3.1 / 3.2 | 3.2 mandatory for offshore / NACE / structural code applications |
| 02 | Dimensional Inspection Report | DIN 929 / DIN 928 / ISO 21670 | AQL 1.0 per ISO 2859; 100% pilot diameter for T-slot critical nuts |
| 03 | Thread Gauge Certificate | ISO 1502 / ASME B1.2 | Go/No-Go per heat lot; repeat post-weld on sample coupon |
| 04 | Weld Quality Test Report | ISO 14270 / AWS D1.1 | Push-off or torque test per nut size — mandatory for all weld nuts |
| 05 | WPS / PQR Documentation | ISO 15614-1 / ISO 15614-12 | Required where weld procedure is in project supply scope |
| 06 | Proof Load Test Report | ISO 898-2 / ASTM F594 | Per heat/lot — mandatory for all structural and pressure-rated grades |
| 07 | Hardness Test Report (HAZ) | ISO 6507 / NACE MR0175 | Cross-section HAZ mapping mandatory for sour service weld nuts |
| 08 | PMI Report (XRF / OES) | Project specification | 100% of SS, duplex, alloy and exotic grade weld nuts |
| 09 | Chemical Analysis Report | EN 10025 / ISO 898 / ASTM | Included in MTC — CE value must be stated for weldability verification |
| 10 | Heat Treatment Certificate | ISO 898-2 / ASTM spec | Required for Q&T grades and post-weld heat treatment records |
| 11 | Surface Coating Certificate | MIL-DTL-16232 / ISO 10683 | Required for phosphate, Dacromet, electroless nickel finishes |
| 12 | ISO 9001 Manufacturer Certificate | ISO 9001:2015 | Current; scope must include fastener and weld nut manufacture |
| 13 | ISPM-15 Phytosanitary Certificate | IPPC / FAO | All wood packing for international export |
RR Hydraulics manufactures and exports weld nuts in all types and grades — hex pilot (DIN 929), square projection (DIN 928), tab/flange, round base, projection and T-weld — in carbon steel, stainless steel 304/316L, duplex & super duplex, Inconel and Monel. Full EN 10204 3.1/3.2 MTC documentation, ISO 9001:2015 QMS, WPS/PQR documentation where in scope, TPI witness by BV/DNV/Lloyds/SGS/TÜV accommodated. M4–M24 metric and inch series. NACE MR0175-compliant grades with HAZ hardness survey on request. 48-hour express dispatch on in-stock sizes.