steel-cast-steel-node-design
Steel Cast Steel Node: G20Mn5 Material & Casting Process
Close-up of a cast steel node, a central casting with five tubular branch sockets extending in different directions, as-cast texture visible, silver-gray metallic finish, cool side lighting emphasizing the three-dimensional form and wall-thickness transitions, blurred industrial shop background, no text in frame.
At a complex steel joint — where six tubes meet at odd angles, or a brace frames into a column web with high stress concentration — welding plates and gussets by hand creates a stress pocket. A cast steel node solves it in one piece: the foundry pours the exact geometry you need, with smooth internal transitions that welded plates cannot match. But casting brings its own risks: porosity, cold shuts, and a material that needs different weld procedures than rolled steel.
A steel cast steel node design trades welding complexity for casting quality control — you get the geometry you want, but you must inspect the casting. This article covers G20Mn5 material, the sand-casting process, node forms, NDT and fatigue, and the comparison with welded built-up nodes.
Our general connection design article covers standard beam-column and baseplate connections; our welding process article covers how to weld rolled steel. Cast steel nodes are a different animal: the joint is a single casting, not an assembly of plates.
When to Choose Cast Steel Over Welded Nodes
Where Cast Steel Shines
Cast steel nodes earn their keep when the geometry defeats plate fabrication:
- Multi-brace intersections — three or more hollow sections meeting at angles that welded gussets cannot cleanly frame.
- Long-span space structures — stadium roofs, airport terminals, exhibition centers where every joint is unique.
- High-stress, fatigue-sensitive zones — smooth internal fillets beat cut-plate corners.
- Architecturally exposed steel — the node is part of the design language, and welds must disappear. These four cases are where a steel cast steel node design earns its cost.
Where Cast Steel Is Overkill
- Simple beam-to-column rigid joints — standard welded or bolted details work.
- Cost-sensitive industrial buildings — a steel workshop or warehouse does not need castings.
- One-off, low-complexity nodes — the pattern cost cannot be amortized.
Typical Project Types
Long-span space trusses, curved roof structures, bridge nodal zones, and exposed architecture. For the broader structural context, see long-span steel structure; for standard joint types, see steel structure connection design; for how rolled steel is welded in the shop, see steel building welding process.
G20Mn5 Material and Sand Casting Process
G20Mn5 Mechanical Properties
G20Mn5 (DIN 17182 / EN 10293) is the workhorse low-carbon manganese cast steel for structural nodes.
- Yield strength: ≥ 300 MPa (43.5 ksi)
- Tensile strength: 500–650 MPa (72.5–94 ksi)
- Elongation: ≥ 22%
- Charpy V-notch impact: ≥ 27 J at −20°C (−4°F)
- Carbon equivalent CEV: ≤ 0.45%, giving weldability comparable to mild structural steel
The closest North American equivalent is ASTM A148 Grade 80-50. For Chinese export projects, G20Mn5N+T (normalized and tempered) is the default specified grade. Specifying G20Mn5N+T is the baseline for any steel cast steel node design intended for export.
The Sand-Casting Process
A structural casting travels through these steps:
- Pattern making — wood or CNC-machined polystyrene pattern, 1:1 geometry.
- Mold making — sand mold (green sand or resin sand) packed around the pattern.
- Pouring — molten steel at ~1,600°C (2,900°F) poured into the mold.
- Shakeout — sand removed after cooling.
- Heat treatment — normalizing and tempering.
- Cleaning and fettling — risers and gates ground off.
- Machining — socket faces and weld prep edges are CNC-machined.
- NDT and inspection.
Typical casting weights range from 50 kg to 50 t (110–11,000 lb). Minimum wall thickness is 8–10 mm (0.3–0.4 in); complex structural nodes usually run 15–30 mm (0.6–1.2 in). Shrinkage porosity and gas porosity are the main intrinsic defects and must be found by NDT.
Heat Treatment
- Normalizing at 880–920°C (1,615–1,690°F), air-cooled, refines the grain.
- Tempering at 600–650°C (1,110–1,200°F), relieves residual stresses.
The as-cast + normalized + tempered condition (G20Mn5N+T) gives the best combination of strength, toughness and weldability.
Table 1: G20Mn5 Mechanical Properties
| Property | Metric | Imperial | Notes |
|---|---|---|---|
| Yield strength (min) | 300 MPa | 43.5 ksi | G20Mn5N+T |
| Tensile strength | 500–650 MPa | 72.5–94 ksi | By wall thickness |
| Elongation (min) | 22% | 22% | On gauge length |
| Charpy V-notch | ≥ 27 J at −20°C | ≥ 20 ft-lbf at −4°F | V-notch |
| Carbon equivalent CEV | ≤ 0.45% | ≤ 0.45% | Weldability control |
| Typical casting weight | 50–50,000 kg | 110–11,000 lb | Structural range |
| Minimum wall thickness | 8–10 mm | 0.3–0.4 in | Practical limit |
| Typical node wall | 15–30 mm | 0.6–1.2 in | Structural nodes |
For material grade choices and substitutions, see steel material substitution. Surface protection for cast nodes follows the same rules as rolled steel; see steel structure corrosion protection and steel structure painting.
Node Forms — Tubular, Spherical and Box
Tubular Cast Steel Node
A tubular node replaces the complex intersecting welds between several circular hollow sections. Each tube slides into a machined socket on the casting, and a full-penetration groove weld is made in the shop.
- Typical tube diameter: 114–500 mm (4.5–20 in)
- Node weight: 100–2,000 kg (220–4,400 lb)
- Best for: space trusses, tubular roof structures, bridge trusses.
Spherical Node
A spherical node is a hollow cast sphere with radial tube stubs. It replaces welded hollow spherical nodes (the "Raschel ball" system) with a casting that has uniform wall thickness and a smooth stress flow.
- Best for: grid space frames, large-span roofs, suspended ceilings.
- Advantage: equal wall thickness in all directions; no weld seam around the sphere.
Box / Irregular Node
A box node frames box-sections columns and beams into a single casting, replacing a stack of welded plates with varying stiffeners. Irregular shapes follow architectural lines.
- Best for: exposed architecture, facades, bespoke geometries.
- Advantage: one casting, one weld prep, no field gusset cutting. That single-piece logic is the structural reason a steel cast steel node design beats a built-up gusset at complex junctions.
Design Details That Matter
- Internal fillet radius ≥ 1.5 × wall thickness to keep stress concentration low.
- Wall thickness should transition gradually — sudden steps invite hot tears and stress concentrations.
- Weld consumables must match G20Mn5 — low-hydrogen electrodes (E7018 class) for joining to rolled sections.
For thin-walled member interactions with cast nodes, see steel thin walled member design. General joint design principles are in steel structure connection design, and long-span applications are covered in long-span steel structure.
Table 2: Cast Steel Node Forms Comparison
| Node Form | Typical Use | Weight Range | Wall Thickness | Notes |
|---|---|---|---|---|
| Tubular socket node | Space trusses, tubular roof | 100–2,000 kg (220–4,400 lb) | 15–30 mm (0.6–1.2 in) | Tube slides into machined socket |
| Spherical node | Grid space frames, large-span roof | 50–800 kg (110–1,760 lb) | 10–25 mm (0.4–1.0 in) | Hollow sphere, radial tubes |
| Box / irregular node | Exposed architecture, facades | 200–5,000 kg (440–11,000 lb) | 20–60 mm (0.8–2.4 in) | Bespoke geometry per project |
| Plate-to-cast transition | Column base, heavy brace joint | 500–10,000 kg (1,100–22,000 lb) | 30–80 mm (1.2–3.1 in) | Heavy loaded joints |
Designing a Complex Joint That Welded Plates Can't Handle?
We design cast steel nodes in G20Mn5: tubular sockets for multi-brace intersections, spherical nodes for space trusses, and box nodes for exposed architecture. Each one comes with NDT inspection and fatigue detail class. Tell us your geometry and load case.
NDT Inspection and Fatigue Design
Non-Destructive Testing
Castings carry intrinsic porosity that rolled steel does not. Three NDT methods are standard:
- Ultrasonic testing (UT) — detects internal shrinkage porosity, inclusions and cracks.
- Magnetic particle testing (MT) — detects surface and near-surface cracks.
- Radiographic testing (RT) — detects internal flaws in thick sections, used for critical heavy-wall nodes.
Acceptance rates:
- Critical, fatigue-sensitive nodes: 100% UT + 100% MT.
- Secondary nodes: 20% sample UT + 100% MT on welded areas.
- Acceptance criteria: EN 13920 casting clauses or AWS D1.1 supplement for castings.
Fatigue Design
Cast steel fatigue performance depends on surface quality and internal soundness, not on the nominal grade.
- Stress concentration factor Kt: cast nodes 1.2–1.8; welded built-up gussets 2.0–3.5.
- Grinding internal fillets can raise the fatigue detail class by 1–2 grades.
- Constant-amplitude fatigue detail classes: FAT 80–125 per IIW / Eurocode 3, depending on surface finish and NDT level.
Repair and Re-inspection
If NDT finds defects beyond the acceptance threshold, the casting can be weld-repaired: preheat to 150–200°C (300–390°F), weld with matching low-hydrogen consumables, and post-heat-treat. A single location may be repaired no more than twice. After repair, the affected zone must be re-inspected by the same NDT method. Writing the NDT percentage into the spec is what makes a steel cast steel node design auditable rather than a promise.
Fatigue principles are detailed in steel structure fatigue design and steel structure fatigue assessment. Independent inspection of castings is part of steel building third party inspection.
Table 3: Cast Steel Node NDT and Fatigue Summary
| Item | Method / Class | Acceptance | Frequency | Notes |
|---|---|---|---|---|
| Internal porosity / shrinkage | UT | EN 13920 / AWS D1.1 casting clause | 100% critical, 20% secondary | Primary casting defect |
| Surface cracks | MT | EN 13920 | 100% | All machined and as-cast surfaces |
| Internal flaws (thick wall) | RT | EN 13920 | By agreement, heavy sections | Used with UT for critical nodes |
| Fatigue detail class | FAT 80–125 | IIW / EC3 Annex | By design | Depends on surface finish |
| Stress concentration Kt | 1.2–1.8 cast vs 2.0–3.5 welded | — | Calculated | Smooth fillets lower Kt |
| Weld repair preheat | 150–200°C (300–390°F) | WPS per project | Max 2 repairs per location | Post-heat after repair |
Cast Steel vs Welded Built-Up Nodes
| Dimension | Cast Steel Node | Welded Built-Up Node |
|---|---|---|
| Geometry freedom | Arbitrary, smooth internal transitions | Limited by plate cutting and bevel access |
| Stress flow | Continuous fillets, Kt 1.2–1.8 | Weld toes, Kt 2.0–3.5 |
| Weight | Can vary wall thickness locally | Uniform plate thickness |
| Pattern / setup cost | High one-time pattern fee | Minimal, plate cut from stock |
| Unit cost per ton | $5,000–15,000/t FOB | $3,000–6,000/t |
| Lead time | 8–12 weeks incl. pattern | 2–4 weeks |
| Best scale | 20+ identical nodes amortizes pattern | One-off or simple joints |
Decision Rule
- Three or more complex intersections, fatigue-critical, or architecturally exposed → cast steel.
- Cost-sensitive, simple beam-column, or single geometry → welded built-up.
For the broader choice between bolted and welded joints, see bolted vs welded steel connection. Welding shrinkage and distortion control for built-up gussets is covered in steel welding distortion control. For fatigue-sensitive multi-brace joints, a steel cast steel node design also lowers the stress concentration factor.
When the multi-brace intersection is simple enough to avoid a casting but the plate still has to survive block shear and Whitmore yielding, the welded built-up option becomes our steel truss gusset plate design guide: a 10–25 mm A36 or A572 plate with drilled (not punched) holes, edge distance at 1.5 db minimum, and the 30-degree Whitmore strip checked for both gross yield and net fracture per AISC 360.
Specification, Cost and Supply Chain
Design Output
Cast node drawings must call out:
- Material grade and condition — G20Mn5N+T.
- Heat treatment — normalizing + tempering, with certificates.
- NDT scope and standard — UT/MT percentages, acceptance class.
- Casting tolerance — ISO 8062 CT10–CT12.
- WPS for welding the casting to rolled sections in the field or shop.
Cost Snapshot
- G20Mn5 cast nodes: $5,000–$15,000/t ($2.3–$6.8/lb) FOB, depending on complexity.
- One-time pattern cost: $8,000–$25,000 per unique geometry.
- Welded built-up node comparison: $3,000–$6,000/t.
- Break-even: roughly 20+ identical nodes, when the pattern cost spreads and field welding savings appear.
Supply Chain
Major Chinese structural casting capacity sits in Jiangsu, Zhejiang and Shandong. Export castings should be released only after independent NDT by SGS, BV or equivalent. For supplier selection, see how to select steel structure supplier; for shop evaluation, see steel factory audit checklist; for cost breakdowns, see steel building quote breakdown.
Conclusion
A cast steel node is a single piece that delivers complex geometry, smooth stress flow and a controlled casting quality. G20Mn5 is the default material; sand casting plus normalizing and tempering is the standard route; and NDT plus fatigue detail classification is the acceptance gate. Cast steel is not the fallback "when welding fails" — it is the better choice when multiple members meet at odd angles, the joint is fatigue-sensitive, or the node is architecturally exposed. The NDT percentage and repair rules must be written into the contract. Treat a steel cast steel node design as a controlled foundry product, not a welded plate assembly.
Send us your node geometry and load case, and our engineers will issue the G20Mn5 technical specification, NDT plan and fatigue detail class for your review.
Complex Geometry, Smooth Stress Flow, Inspected Quality — One Casting Instead of Ten Plates.
We design G20Mn5 cast steel nodes for multi-brace intersections, exposed architecture and fatigue-critical joints — with full NDT inspection and fatigue detail classification. Tell us your node geometry and load case.
🏭 Explore: Steel Factory · Steel Workshop
Case Example
A 14,000 m² (≈151,000 sq ft) exhibition hall in northern Europe, with a 70 m (≈230 ft) tubular roof truss meeting at eight heavily loaded multi-planar nodes.
Key challenges: several tubular branches converge at high force, where a welded built-up node would need 30+ stiffener plates and carry a known fatigue risk.
Solution: G20Mn5 cast steel nodes (normalized) were selected, sand-cast with UT and MT inspection, and the weld transitions between the cast cone and the rolled tube ends were carefully detailed.
Results: each cast node replaced an estimated 45 kg (≈100 lb) of welded plate work, fatigue design met the 2-million-cycle category, fabrication lead time was 12 weeks, and field welding time at each node dropped about 40%. See truss gusset plate design and connection design fundamentals.
Reference Links
- AISC 360 Specification for Structural Steel Buildings
- ASCE 7 Minimum Design Loads and Associated Criteria for Buildings and Other Structures
- ISO 12944 Corrosion protection of steel structures by protective paint systems
- AWS D1.1/D1.1M Structural Welding Code—Steel
- Eurocode 3 (EN 1993) Design of Steel Structures
About the Author
Senior Structural Engineer
With over 20 years of hands-on experience in steel structure design and prefabricated building engineering, our in-house senior structural engineer has personally contributed to more than 500 steel building projects—including warehouses, industrial factories, aircraft hangars, agricultural buildings, and commercial structures. The focus is on translating design codes such as AISC 360, ASCE 7, and Eurocode 3 into buildable, cost-effective steel solutions that balance structural performance, fabrication efficiency, and total project cost.
Learn more about our engineering team
Frequently Asked Questions
Q1: What is G20Mn5 cast steel?
G20Mn5 is a low-carbon manganese cast steel (DIN 17182 / EN 10293) with a minimum yield strength of 300 MPa (43.5 ksi) and tensile strength of 500–650 MPa (72.5–94 ksi). It has good weldability (carbon equivalent ≤ 0.45%) and notch toughness down to −20°C. It is the most common material for structural cast steel nodes in Europe and is routinely specified for Chinese export projects.
Q2: When should I use a cast steel node instead of a welded built-up node?
Choose cast steel when: (1) three or more members meet at complex angles that welded gussets cannot handle, (2) the joint is fatigue-critical and smooth stress flow matters, (3) the node is architecturally exposed and needs clean geometry, or (4) you have 20+ identical nodes so the pattern cost is amortized. For simple beam-column joints or one-off nodes, welded plates are cheaper and faster.
Q3: How are cast steel nodes inspected?
Three NDT methods are standard: (1) Ultrasonic testing (UT) for internal porosity and shrinkage, (2) Magnetic particle testing (MT) for surface and near-surface cracks, and (3) Radiographic testing (RT) for thick-section internal flaws. Critical nodes get 100% UT + 100% MT; secondary nodes may be sampled at 20%. Acceptance criteria follow EN 13920 or AWS D1.1 casting clauses.
Q4: How do you weld a cast steel node to rolled steel tubes?
G20Mn5 welds well to rolled carbon steel (Q355 / A992) using low-hydrogen electrodes (E7018 or matching). Preheat to 150–200°C (300–390°F) for thick sections. The node typically comes with machined sockets or weld prep edges so the tube slides in and the groove weld is made in the shop. Field welding is minimized — most nodes are pre-assembled and shipped.
Q5: How much do cast steel nodes cost?
G20Mn5 cast steel nodes run $5,000–15,000/t ($2.3–$6.8/lb) FOB, plus a one-time pattern/mold cost of $8,000–25,000 per unique geometry. A welded built-up node costs $3,000–6,000/t. Cast steel becomes competitive when you have 20+ identical nodes — the pattern cost spreads out, and you save on field welding hours and fatigue risk.
Reference links: EN 10293 Steel Castings for General Engineering · IIW Fatigue Design Recommendations
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