Machine frame materials, and what each one changes
Which grade to name, how thick plate behaves, when tube beats plate and when a casting or an extrusion beats both. Values are minimums from the standards or typical figures, labelled as such. Grade cross-references live in the steel grade equivalents tooland the process side in fabrication technology.
Structural steel: which grade to name
Five grades cover most welded machine frames. Minimum yield is for plate up to 16 mm. The choice is mostly about impact toughness and what the fabricator stocks, because every grade here has the same stiffness. Background on the material family is on Wikipedia.
| Grade | Standard | Min yield, MPa | Impact test | Name it when |
|---|---|---|---|---|
| S235JR | EN 10025-2 | 235 | 27 J at +20 °C | Brackets, covers, lightly loaded frames where thickness is set by stiffness, not strength |
| S355J2 | EN 10025-2 | 355 | 27 J at -20 °C | Loaded frames, gantries and anything with fatigue or cold service. The safe default for Europe. |
| A36 | ASTM A36 | 250 | Only if ordered | US drawings, general frames. Name supplementary impact testing if the frame is dynamically loaded. |
| Q235B | GB/T 700 | 235 | 27 J at +20 °C | Plate commonly stocked by Asian mills and fabricators. Equivalent class to S235JR. |
| Q355B | GB/T 1591 | 355 | 34 J at +20 °C | Asian stock grade in the S355 class. Older certificates still say Q345B. |
Older equivalents (St37-2, St52-3, SS400, SM490A and more) are in the equivalents tool and the standards reference. See also choosing a steel grade for a machine frame. See also s355j2 or s355jr: the impact grade question on machine frames.
Plate thickness: yield steps down, preheat goes up
EN 10025-2 lowers the minimum yield as plate gets thicker, and the other standards do the same in their own bands. On a stiffness-driven base this rarely matters. On a loaded frame checked at 355 MPa, an 80 mm base plate is checked against the wrong number.
Thick plate also needs preheat before welding, set from thickness, hydrogen level and carbon equivalent per EN 1011-2 or AWS D1.1. Plate thickness tolerances follow EN 10029 (or ASTM A6), which is why a machined pad needs stock above the nominal thickness.
| Nominal thickness | S355, MPa | S235, MPa |
|---|---|---|
| Up to 16 mm | 355 | 235 |
| Over 16 to 40 mm | 345 | 225 |
| Over 40 to 63 mm | 335 | 215 |
| Over 63 to 80 mm | 325 | 215 |
| Over 80 to 100 mm | 315 | 215 |
Typical plate by member on a welded machine frame. The network works plate from 6 to 200 mm; these are common ranges, not design values.
| Member | Typical thickness | Why |
|---|---|---|
| Base plates and machined pads | 30 to 80 mm | Thick enough to keep machining stock and flatness after stress relief |
| Main beams and side walls | 15 to 40 mm | Sized by stiffness first; box sections where torsion matters |
| Ribs and diaphragms | 10 to 25 mm | Spaced to stiffen under pads and stop wall buckling |
| Gussets, brackets, covers | 6 to 16 mm | Laser cut where fit-up matters, little or no preheat |
Plate weight and nesting: plate weight calculator. Read: plate thickness tolerances and why your frame drawing needs them. Read: plate availability is what sets your lead time.
Tube and RHS frames
Square and rectangular hollow sections are cold formed to EN 10219 or hot finished to EN 10210, and to ASTM A500 in the US, commonly in S355 or A500 grade C. They give high torsional stiffness for little weld length, which suits automation stations, conveyor frames, guarding and light machine frames. Their limits on a precision frame: wall thickness rarely goes above 12 to 16 mm, so machined pads are welded on as plate, and closed sections must be vented for galvanising and sealed or drained for washdown.
Read: square tube or i-beam for a light machine frame.
Welded steel, cast iron or aluminium extrusion
The family decision comes before the grade. The family overview with mineral casting and stainless is on the technology page; this table adds the numbers. Values are typical for each material class.
| Property | Welded steel | Cast iron | Aluminium extrusion |
|---|---|---|---|
| Elastic modulus | About 210 GPa | About 100 to 140 GPa (grey), 170 GPa (ductile) | About 70 GPa |
| Density | 7.85 g/cm³ | About 7.1 to 7.3 g/cm³ | 2.7 g/cm³ |
| Thermal expansion | About 12 µm/m·K | About 10 to 11 µm/m·K | About 23 µm/m·K |
| Damping | Low, raised by ribbing, mass or a filled core | High in grey iron | Low; joints add some |
| Tooling | None | Pattern, so repeat volume | Die for custom profiles; catalogue profiles need none |
| Change after first unit | Easy | Costly | Very easy |
| Stress relief | Needed before precision machining | Ageing or annealing of the casting | Usually none on a bolted frame |
Aluminium extrusion frames, usually 6063-T6 or 6005A-T6 T-slot profiles, are bolted rather than welded and suit light automation cells that change often. With a third of the stiffness of steel and twice its thermal expansion, they are a poor base for a cutting machine. Cast iron wins on damping and repeat volume; welded steel wins on lead time, size and design changes.
Cast iron or welded steel for a machine base · Aluminium extrusion frames versus welded steel frames · Mineral casting versus welded steel for a machine base
What the material means for stress relief
The material decides whether, how and at what temperature a frame is relieved before machining. Methods and equipment are on the technology page; the cycle can be worked out with the stress relief calculator.
| Material | Typical route | Watch for |
|---|---|---|
| Carbon steel weldments (S235, S355, A36, Q235, Q355) | Thermal stress relief, typically 550 to 650 °C depending on the code, about 1 hour per 25 mm, or vibratory where the drawing allows it | Furnace size limits the frame length. Say which method you accept on the drawing. |
| Grey and ductile iron castings | Thermal ageing of the rough casting, typically 500 to 600 °C with slow cooling | Done by the foundry before machining. Ask for the cycle record. |
| Quenched and tempered or normalised plate | Stress relief must stay below the tempering or normalising temperature | A wrong cycle can lower strength. Name the plate condition on the drawing. |
| Aluminium extrusion frames | None on a bolted frame; welded aluminium loses temper strength near the weld | Precision faces are machined on fixed plates bolted to the frame. |
| Stainless weldments | Rarely thermally relieved; distortion is controlled by sequence and fixturing | Pickling and passivation after welding matter more for washdown frames. |
Read: stress relieving welded machine bases: thermal, vibratory, or neither.
Read: normalised plate and why it machines more predictably.
Questions about frame materials
S355 or S235 for a machine frame?
If the frame thickness is set by stiffness, which is common on machine bases, S235 or Q235 is enough and slightly easier to weld. If the frame carries real load, fatigue or cold service, name S355J2 or its equivalent. Strength does not add stiffness: both grades have the same modulus.
Can a Chinese or Asian stock grade replace the European one on my drawing?
Often, within the same strength class, but impact temperature and chemistry limits differ. Name one grade with its standard, say whether an equivalent is acceptable, and ask for an EN 10204 3.1 certificate. Cross-references are in the steel grade equivalents tool.
When is tube or RHS better than plate?
For light and medium frames, automation stations and covers, hollow sections give good torsional stiffness for little welding. For heavy machine bases with machined pads, plate weldments are usually stiffer per kilogram where it matters and easier to machine.
Does an aluminium extrusion frame need stress relief?
Not when it is bolted. The extrusions are supplied in a stable temper. Welding aluminium lowers strength near the weld, so precision features are usually machined on separate plates bolted to the frame.
Cite or link this page
Free to quote, reference or link with attribution. The calculation method is stated above so it can be checked rather than taken on trust.
Material settled on the drawing?
Send it with the grade, the plate list and the stress relief you accept. It gets matched with shops in the network that already work that material at that size. Reply within 24 business hours.
Become a sourcing partner
The heavy-fabrication network is open to new shops. If you build large weldments, machine frames or precision-machined bases and you have export experience, send a capability profile. We reply within 24 business hours.