Field Welding Structural Steel: Sequencing, Preheat & Distortion Control

Welcome

Hello, and welcome. This is Super Structures General Contractors — a national general contractor headquartered in Powhatan, Virginia — here to help you and your clients build something that lasts. We're glad you're with us, and we look forward to connecting with you.

Alright, Field Welding Structural Steel: Sequencing, Preheat & Distortion Control. Don't let the plain title fool you. Here's the part that actually matters on the job: Field welds go in only after the frame is plumbed and braced, in a planned sequence that balances shrinkage, with preheat and interpass temperature held to the WPS, because heat moves steel and the order you weld decides where it ends up. Learn it well and your columns stay plumb, your welds stay crack-free, and the erector never has to pull a frame back after you locked it in the wrong place.

Shop welding happens on a fixture in a building. Field welding happens 40 feet up, in wind, on a frame that is still moving, with the erector and the inspector both waiting on you. The rules that keep that work right are about order, heat, and restraint.

Welding comes after plumb-up, not before

The erector sets the steel, bolts the connections snug, plumbs and aligns the tier within the AISC 303 erection tolerances, then final-tightens the bolts. Only then do column splices and moment connections get welded. Welding earlier locks the frame in whatever position it was in, and a frame welded out of plumb cannot be pulled back. Temporary bracing stays until the permanent connections are complete and the erection plan says it can come out. On moment connections, the web bolts are tightened before the flange welds so the beam cannot shift.

Preheat and interpass control

Preheat slows the cooling rate so hydrogen can escape and the heat-affected zone does not harden and crack. The D1.1 prequalified table sets the minimum by steel group and thickness. For common structural steels with low-hydrogen processes, the table runs roughly from no preheat on thinner material, to 50 degrees F above 3/4 in., 150 degrees F above 1-1/2 in., and 225 degrees F above 2-1/2 in.; the WPS carries the exact number, and high-strength or quenched-and-tempered steels need more. Field rules that go with the table:

• Base metal below 32 degrees F must be preheated to at least 70 degrees F, and no welding is done when the ambient temperature is below 0 degrees F.
• Preheat is measured at least 3 in. from the joint in every direction, so the heat has soaked through rather than warmed the surface.
• Preheat is held throughout welding, and the interpass temperature stays between the WPS minimum and maximum. Too cold invites cracking; too hot ruins toughness. Heat with a rosebud torch, resistance blankets, or induction; measure with temperature crayons or a contact pyrometer. Low-hydrogen consumables are part of the same system: E7018 out of the holding oven has a limited exposure time, 4 hours for the common E70 class, before it goes back to be re-dried.

Sequence inside a joint and across the frame

Inside a joint: root, fill, cap, with every pass cleaned of slag before the next. Double-bevel joints are welded alternately from each side to balance pull. Long seams use backstep or skip sequencing so shrinkage does not accumulate in one direction. On a moment connection, each flange weld is completed as a unit, in the order the project's weld sequence sheet gives. Across the frame: weld from the stiff, plumbed center of a bay outward, finish splices on a column line in a balanced pattern instead of running all one side first, and split the welders so opposite sides of the building shrink together.

How heat moves steel

Every weld shrinks as it cools, and the steel moves with it:

• Transverse shrinkage pulls the plates together across the weld; a single-V butt in 1/2 in. plate can close on the order of 1/16 in.
• Longitudinal shrinkage along the weld bows a member toward the welded side.
• Angular distortion rotates a plate about the weld, worse with more passes and single-sided grooves. Controls, in order of value: do not overweld, because weld volume is the distortion; use double-sided joints to balance pull; use fewer, larger passes within the WPS limits; preset the joint against the expected movement; clamp or strongback where the detail allows. When a member still moves, heat straightening with a torch is an engineering-approved repair, kept below about 1,200 degrees F for ordinary carbon steel and lower for quenched-and-tempered material, with no water quenching.

Working the field weld safely and well

Gas-shielded processes need a windscreen above 5 miles per hour; self-shielded flux-core and stick tolerate wind, which is why they own the field. No welding on wet steel or in rain or snow without shelter. The work clamp goes on the member being welded, close to the joint, never through a bolted connection, a crane cable, or rigging; stray current pits bearings and burns cables. At height, the welder is tied off, the leads are managed so they cannot pull a lift, and screens protect the trades below. Peening is allowed on intermediate layers only, never on the root or the cap.

Going Deeper (Intermediate)

Restraint is the quiet variable. A thick column flange splice high in a frame is restrained in every direction, so the shrinkage cannot move the steel and builds stress instead. That is where hydrogen cracks appear hours or days later. Pros raise preheat above the table on heavily restrained thick joints and complete the joint in one shift rather than leaving a half-filled groove overnight. A weld sequence sheet for the frame, drawn by the erector with the engineer's input, is standard on multi-story and seismic work.

Advanced / Pro-Level

On seismic projects, AWS D1.8 limits interpass temperature on demand-critical welds and adds welder supplemental testing for bottom-flange welds through access holes. Heavy welded sections may require a post-heat soak to drive out hydrogen; the engineer specifies it. AISC's guidance on heavy shapes includes avoiding welds in the k-area of rolled columns, where mill straightening has reduced toughness, so continuity plate and doubler welds are stopped short of it. Pros plan cold-weather welding months ahead: heating equipment, enclosures, and schedule float.

Practice Challenge

A 2 in. thick A992 column flange splice is scheduled for a December morning at 25 degrees F with a low-hydrogen flux-core WPS. What preheat does the crew need, and how do they prove it? (Answer: the steel is below 32 degrees F, so at minimum it must be brought to 70 degrees F, but the prequalified table for low-hydrogen processes on material over 1-1/2 through 2-1/2 in. requires 150 degrees F. Heat both members to at least 150 degrees F measured at least 3 in. from the joint on all sides, confirm with a temperature crayon, hold it through welding, check interpass between passes, and shelter the joint from wind and moisture.)

In Practice

A crew that welds to a sequence sheet, checks preheat with a crayon before every joint, and finishes each thick splice in one shift leaves a plumb frame and sound welds behind. A crew that welds whatever is handy and trusts a torch pass for preheat finds the cracks on the inspector's UT report.

Common Mistakes to Avoid

• Welding column splices or moment connections before the tier is plumbed and bolted
• Warming the surface with a torch pass and calling it preheat
• Welding all one side of a column line and bowing the frame
• Overwelding, which adds distortion and cost without adding strength

Takeaway: Field welds go in only after the frame is plumbed and braced, in a planned sequence that balances shrinkage, with preheat and interpass temperature held to the WPS, because heat moves steel and the order you weld decides where it ends up.

Educational content only — not engineering, design, inspection, or safety advice, and not a substitute for hands-on training, a qualified welding procedure, or a licensed professional. Weld to the WPS and the drawings, follow AWS D1.1, AISC, and OSHA requirements as adopted and applied on your project, and follow the manufacturer's instructions for your equipment and consumables.