How to weld RHS steel for site frames with a MIG welder

🔥 MIG Welding RHS Steel for Site Frames — Practical Trade Guide for Australian Fabricators

What is involved in MIG welding RHS steel for site frames?

Welding RHS mild steel for site frames with a MIG/GMAW welder is a standard fabrication task across structural supports, plant frames, brackets and general construction steelwork. Success comes down to correct joint prep, fit-up, tacking sequence, heat control and using proven MIG settings matched to the RHS wall thickness and welding position.

For any load-bearing or structural frame, welding procedure, weld category, inspection and acceptance criteria should align with AS/NZS 1554 requirements, and site installation should also consider project specifications, engineer details and any relevant WPS.

What skill level is required for MIG welding RHS site frames?

Intermediate — Estimated time: 4–8 hours for a small site frame, depending on frame size, fit-up complexity, weld category and finishing requirements.

What essential tools are needed for MIG welding RHS site frames?

Materials Required

Safety Requirements

Step-by-Step Guide

Step 1: Review drawings, weld details and frame tolerances

Start by confirming the fabrication drawings, member schedule, connection details, overall dimensions, diagonals and any nominated weld sizes. Check whether the frame is classed as structural, whether a WPS applies, and whether welds are required as continuous fillet welds, intermittent welds, or full-penetration joints.

For site frames, also check:

If tolerances are not stated on the job pack, confirm acceptable shop tolerances before cutting. Small errors in early fit-up quickly stack up across multi-member RHS frames.

Step 2: Select the right MIG setup for RHS thickness

Choose consumables and machine setup based on wall thickness and weld size.

Typical workshop setup for mild steel RHS:

Typical starting settings for flat/horizontal fillet welds on mild steel:

These are starting points only. Fine-tune for:

For most RHS frame fabrication in the shop, use short-circuit transfer on lighter sections and spray/pulsed transfer where machine capability and thickness justify it.

Step 3: Cut RHS accurately and identify each member

Cut all members to length using a saw that gives a square, repeatable cut. On site frame work, poor cut accuracy creates gaps, out-of-square corners and time-wasting rework during assembly.

Mark every member clearly:

If mitres are specified, verify the angle before cutting production quantities. Check first-off parts against the drawing and confirm the measured diagonal dimensions where rectangular frames are involved.

Step 4: Prepare the weld zones properly

Grind or clean the weld area back to bright metal where contamination, heavy mill scale, surface rust, paint, oil or moisture is present. Although MIG will tolerate more than TIG, dirty RHS still causes:

For 3 mm and under, square butt joints may be acceptable depending on detail and required penetration. For thicker RHS, introduce a bevel if required by design or WPS.

As a guide:

If the joint is load-bearing, confirm the edge prep and root gap meet the nominated welding procedure.

Step 5: Set up the frame on a flat bed and clamp for squareness

Lay out the frame on a flat table, bench or fabrication bed. Use stops, clamps, packers and magnetic aids only as temporary setup tools—do not rely on magnets alone to control distortion or maintain dimension.

Check:

For larger frames, use strongbacks or temporary braces to hold the assembly stable during tack-up. If site tolerance is tight, record diagonal measurements before welding starts.

Step 6: Tack in sequence to lock the frame without pulling it out

Use sound, substantial tacks rather than small cosmetic tacks. Place tacks where they will either be consumed by final welds or removed and remade if unsuitable.

A good sequence is:

For heavier or long-span frames, use balanced tacks on opposing faces to reduce movement. If the frame begins to pull, cut and retack early rather than trying to weld out a bad setup.

💡 Tack size should suit the weld size and member stiffness. Weak tacks crack during handling and introduce misalignment before final welding.

Step 7: Dial in the MIG arc on offcuts before welding the frame

Before welding the actual frame, test on matching offcuts in the same position and thickness. Aim for a stable arc with minimal spatter, a flat-to-slightly-convex fillet profile and clear fusion into both legs.

Check:

Adjust as needed:

For solid wire MIG on mild steel, use DCEP polarity.

Step 8: Weld the frame using a balanced sequence

Use a welding sequence that spreads heat evenly and limits distortion. Avoid fully welding one side of the frame before touching the opposite side.

A typical approach:

For most RHS site frames, fillet welds are common at:

Technique notes:

Where accessible, weld symmetrically on opposing faces to counter shrinkage. If joining thicker base plates or heavy cleats to lighter RHS, favour heat into the heavier section but avoid overheating the tube wall.

Step 9: Control heat input and distortion

RHS can pull quickly, especially in rectangular frames with long continuous welds. Heat control is what separates clean fabrication from constant straightening work.

To minimise distortion:

For lighter wall RHS such as 2–3 mm, do not dwell in corners or pause excessively at starts and stops. For thicker sections, monitor interpass temperature if called up by WPS or project specification.

Step 10: Weld plates, cleats and attachments last with fit-up checks

Once the main frame is dimensionally stable, fit remaining:

Check each attachment for:

Plate-to-RHS welds often sink heat into the plate while overheating the tube edge if technique is poor. Direct the puddle into the thicker plate first, then wash into the RHS to maintain fusion without undercut or burn-through.

Step 11: Inspect welds and verify frame dimensions

After welding, carry out visual inspection before grinding or coating. Structural weld acceptance should follow the project requirement and applicable AS/NZS 1554 criteria.

Inspect for:

Then recheck:

If welds need dressing for fit-up or appearance, avoid reducing throat thickness below requirement.

Step 12: Clean, repair if needed, and prepare for dispatch or site install

Remove spatter, clean the frame and repair any defects properly. Defect removal should be by grinding back to sound metal followed by rewelding to approved practice.

If the frame is to be painted or galvanised:

Tag the frame clearly for installation and protect finished surfaces during transport. For site work, pack with lifting points, bolt sets and marked orientation to reduce install delays.

Common Mistakes to Avoid

Using oversized fillet welds on 2–3 mm wall section creates distortion, burn-through and unnecessary heat input.

Trying to fill large gaps with MIG instead of recutting or refitting leads to shrinkage, weak weld profiles and ugly fabrication.

Running all welds on one side before balancing the opposite side almost guarantees pull, twist and out-of-square frames.

Mill scale, oil, moisture, paint and gal residue cause porosity, unstable arc performance and failed visual inspection.

Waiting until the frame is fully welded before checking diagonals and squareness usually means difficult rework or scrap.

Pro Tips

This gives a realistic check of penetration, bead shape and machine settings before touching the actual frame.

On repetitive frame work, this keeps all boilermakers using the same sequence and improves dimensional consistency.

Cracked or contaminated tacks should be ground out and remade, especially on structural work.

Catching movement early saves heat-straightening later.

Even with gas flow 14–18 L/min, drafts can strip shielding gas and cause porosity. Screens often help more than simply increasing gas flow.

Recommended Tools

⚠️ Safety

Use an auto-darkening welding helmet compliant with AS/NZS 1338.1, leather gloves, safety glasses and FR jacket/apron at all times. Provide fume extraction, manage hot work with extinguisher and permit controls, and isolate bystanders from UV/arc flash with screens.

For load-bearing site frames, confirm welding procedures, weld categories and inspection requirements to AS/NZS 1554 and the project engineer’s documentation. Only suitably competent personnel should complete structural fabrication and any required weld certification should match the job specification.

If purchasing a welder package or fabrication equipment over $1,000, consider equipment finance through Local Knowledge Finance to manage workshop cash flow.