Thick plate work separates a capable welding company from a real steel fabricator. Once sections climb past 1 inch, sometimes past 4 or 6 inches, heat stops behaving politely. Distortion becomes structural, not cosmetic. Hydrogen turns from a concept in a textbook into a crack you can hear. Production windows, joint prep, and consumable choices pull on each other. Shops that do this well blend written procedures with tribal knowledge, and they document to a standard that auditors, inspectors, and customers trust.
What follows is the playbook I wish someone had handed me when I first moved from railings and frames to pressure shells, mill housings, crane hooks, and heavy machine bases. It’s written for the working side of a metal fabrication shop or machine shop, but it respects code language and the reality of cost and schedule. If you’re in custom industrial equipment manufacturing, contract manufacturing, or industrial machinery manufacturing, these are the patterns, numbers, and traps you will see every week.
The thick plate problem set
A 2 inch plate weld looks similar to a 3/8 inch weld from a distance. Up close, everything changes. Heat affected zones grow. Restraint loads hike residual stress. Moisture tolerance narrows. Low hydrogen rigor becomes non-negotiable. Preheat that felt optional becomes part of the weld pool chemistry. And all of it is more expensive per inch than most buyers expect.
Three realities drive procedure design:
- Heat input and diffusion govern microstructure and distortion. You balance energy to avoid lack of fusion without pushing tensile strength and impact properties off target. Hydrogen control is insurance you actually use. Dry electrodes, controlled interpass temperatures, and bake cycles stop cold cracking. Access and sequence matter as much as amperage. You can specify the perfect pass count and still create shrinkage you cannot machine out.
Keep those in mind as we get into practical details.
Codes and paperwork that actually help
On production work, the most useful document in the building is a qualified WPS. If your shop does contract manufacturing, you’ve seen AWS D1.1 for structural steel, ASME Section IX for pressure work, and sometimes EN ISO 15614 if you support a European Manufacturer or an Industrial design company. The standards differ in scope but agree on one thing: thick plate welding needs procedure qualification, not just prequalified tables.
A good WPS for thick plate does more than list filler metal and amperage. It gives a welder enough guardrails to make good decisions when the fit-up isn’t textbook.
- Base metal group and chemistry: For example, A572 Grade 50 behaves differently from A514 quenched and tempered steel at the same thickness. The WPS should call out group numbers and any restrictions like heat input limits for TMCP or Q&T steels. Filler metal choices with diffusion hydrogen rating: E7018-H4R versus standard E7018 matters if you run at 40 Fahrenheit in a damp shop. For high strength steels, specify matching or overmatching alloys only if the code and design justify it. Preheat and interpass: Min and max. Include how to measure and where. On plate over 2 inches, preheat uniformity is a real project. Heat input window: Many codes let you specify kJ/in. Use it. Tie it to travel speed ranges that a welder can use in the booth without a calculator. Position and technique: Stringer versus weave, and why. Some steels and thicknesses demand stringers to control heat input and slag inclusions, especially in deep grooves. Backing and access: Clarify if backing bars, copper shoes, ceramic tiles, or run-off tabs are part of the procedure.
A PQR backs the WPS, and it’s worth doing a real one, not a shortcut. If you anticipate impact testing in service, put it in the PQR. Film your bend tests if you are training new people, because nothing teaches a better lesson than watching a coupon fail.
Material behavior, from mild to Q&T
On thick plate, “steel is steel” gets expensive. Three families show up regularly in a cnc metal fabrication environment.
Mild and low alloy structural steels, like A36 and A572, are forgiving. Preheat becomes mandatory past 1.5 to 2 inches, with common values ranging from 150 to 300 Fahrenheit depending on restraint and hydrogen risk. Most shops run E7018 or equivalent wire for GMAW and FCAW. The main risk is lack of fusion in deep grooves caused by sluggish slag and low travel speeds. The cure is heat input discipline and proper joint geometry.
High strength low alloy plate, like A514 and A709 HPS, punishes sloppy heat. These are quenched and tempered. Weld them too hot, for too long, and you temper the base metal in the heat affected zone. That destroys toughness, especially in cold climates. Here, heat input limits show up in the WPS, often below 45 kJ/in. Preheat happens, but interpass must be limited, commonly in the 400 to 450 Fahrenheit range. Filler selection must match the base metal’s strength without pushing diffusible hydrogen.
Abrasion resistant plate, like AR400 and AR500, welds fine in cosmetic applications but bites back under load if you ignore the HAZ. Frequent approach: keep heat input low, use stringer beads, and limit interpass temperature. Consumables that match base metal strength aren’t necessary unless the weld is loaded. In many custom metal fabrication jobs you can use E7018 or a 70 series wire with a compatible chemistry, then protect the Industrial manufacturer joint by design rather than brute hardness.
Joint design that stacks the odds
Heavy section grooves do not scale linearly from thin plate joints. Geometry drives fusion, slag removal, and shrinkage management.
Open root single bevels are common up to 1.5 inches. Beyond that, double bevels or double V grooves reduce volume and halve the path for hydrogen to get trapped. I favor a 60 to 70 degree included angle on very thick joints when using stick or flux cored, even though it looks wasteful, because it bites the sidewalls and makes slag evacuation less frustrating. With GMAW or SAW, smaller included angles sometimes work because the process is more forgiving of sidewall wetting.
Root faces should be enough to avoid burn through but not so big they create a cold shelf. On hand welding, 1/8 to 3/16 inch roots with land are typical. For SAW on thick plate, land can run larger with a backing strip or ceramic backing.
Access for back gouging matters. If you cannot flip the part because the weldment weighs 12 tons, design a joint that you can gouge from one side. Carbon arc gouging (CAC-A) is a necessary skill at thick sizes. Get comfortable with its travel speeds, air pressure, and only use grinders to finish, not to remove inches of steel.
Here’s a small real-world detail: if your cnc metal cutting operation leaves hard scale on bevels, your welders will fight it. Build time into the routing to clean bevels to bright metal for at least 1/2 inch from the edge. Mill scale lifts hydrogen, contaminates the weld pool, and hides tight fit-up defects that become lack of fusion.
Process selection by need, not habit
Shops drift into favorite processes, but thick plate rewards matching the process to the job. Each process carries specific strengths.
Shielded metal arc welding (SMAW). The all-arounder in a steel fabrication environment, especially when the joint is awkward and interpass control matters. Low hydrogen electrodes like E7018 or E8018-C3 for certain low alloy steels are standard. Rod ovens are not optional. For multi-inch joints, stick excels at root and hot passes, and at tie-ins where you need to burn out slag pockets. Slow, yes, but very controllable.
Flux cored arc welding (FCAW). If I have the option in a welding company with modern wire feeders, FCAW with T-5 or T-8 low hydrogen wires is often the production choice for fill passes. It carries high deposition, tolerates some joint irregularities, and handles vertical up well. Watch hydrogen values and bake flux cored wire if the manufacturer allows it. Many codes accept an H8 classification in thick joints with proper preheat.
Gas metal arc welding (GMAW) and pulsed spray. On thick plate with good fit-up and position favorable to flat or horizontal, GMAW can be fast and clean. But with thick sections, avoid short circuit mode in structural root passes unless prequalified and controlled. Pulsed spray gives you fusion at lower heat input than conventional spray. On double-sided work, do the root with stick or FCAW, then switch to pulsed GMAW for fills if the WPS allows.
Submerged arc welding (SAW). If your Machine shop or machining manufacturer also fabricates heavy rolled shells or long seams in position, SAW is a production monster. Deposition rates climb past 20 lb/hour per arc with paired wires. It takes fixturing, planning, and consistent prep. SAW pays off when you run long flat passes, such as column splices or machine base plates with long double V joints. Many industrial machinery manufacturers keep a SAW station just for these seams.
Gouging and buttering as supporting acts. Carbon arc gouging is not a weld process, but it is part of thick plate welding. Use it to remove backer strips, open roots, and find defects early. Buttering with compatible filler metal can rescue a hard or high alloy edge, building a layer that welds like a mild steel joint and isolates the base metal.
Preheat and interpass: where rigor beats luck
Hydrogen cracks do not care about your deadline. The preventive steps are not mysteries, but they require discipline every day.
How to choose preheat. Variables include carbon equivalent (CE), thickness, restraint, process hydrogen level, and service temperature. Most shops use CE formulas like CEIIW or CET. As CE climbs, preheat goes up. On a 2 inch A572 plate with CE around 0.43, preheat of 200 to 250 Fahrenheit is typical, with higher in winter or on highly restrained welds. For a 3 inch A514 plate, you may preheat 300 to 350 Fahrenheit but set an interpass ceiling near 400 to protect the HAZ.
Where and how to measure. Surface thermometers lie on cold days. Infrared guns lie on shiny surfaces. Use temperature crayons or contact thermocouples on cleaned, dull metal near the joint, both sides, and at least 3 inches from the edge to gauge soak, not just skin temperature. On plates over 2 inches, allow time for uniform through-thickness heat. I have waited 20 to 40 minutes between heavy passes on massive joints to avoid a cold core.
Interpass control. Set a minimum to keep hydrogen solubility favorable and a maximum to protect toughness. Record them. Thick work benefits from real-time data. Some shops use logging devices clipped to the part during critical welds. At a minimum, assign an accountable person to own those numbers, not just the welder in the hood trying to make arc time.
Hydrogen discipline. Low hydrogen electrodes remain in heated quivers. Open flux cored wire spools are stored in dry cabinets if required by the manufacturer. Compressed air for cleaning is dry and oil free. No water quenching of spatter on the joint. These small habits separate trouble-free nights from 2 a.m. repair shifts.
Pass sequencing that controls shrinkage
The shape of the weld bead and the path of shrinkage set your final geometry. Thick plate welding rarely allows unrestricted movement. You are welding inside fixtures, next to machined surfaces, under clamps that simulate how the part will live.
I like to plan the weld in three phases. First, root and hot passes set the foundation. They establish the fusion line and block hydrogen. On a double-sided joint, back gouge and inspect the root after flipping or from one side if designed for it. Second, fill passes distribute heat and begin to balance shrinkage. Alternate sides if possible. Use block sequences that mirror each other. Short beads shrink less than long beads, so I prefer short, staggered passes to continuous runs on highly restrained welds. Third, cap passes dress the surface and blend edges without excess reinforcement. The cap shape should match the intended load path. A tall, narrow cap can concentrate stress. Aim for a slight crown with good toe transitions.
One small shop habit that saves time: map your pass sequence on the drawing or the part. Even chalk lines help. In a well-run cnc metal fabrication cell, the welder knows which passes to run, in what order, and which pauses are enforceable. This prevents the common failure of blasting heat into one quadrant while the opposite side stays cold.
Distortion and restraint: move the part, or it moves you
Every shrinkage battle on thick plate belongs in the planning meeting, not on the floor after the first arc. Machining manufacturer schedules and tolerances depend on weld strategy.
Fixture smart, not hard. A rigid fixture can lock in residual stress that shows up when you unclamp, or worse, it can tear tack welds and roots mid-pass. Design restraint that supports the joint but allows limited movement. Add pre-camber to long members where predictable shrinkage will pull them straight. The more linear inches of weld, the more pre-camber matters.
Preserve machining stock. If the Machine shop needs to hit 0.002 inch flatness over 30 inches after welding, you cannot give them a banana. Leave measurable stock to skim out distortion. On a machine base weldment I worked on, leaving 0.050 inch per side created enough room to machine out the unavoidable lift after 1,200 inches of weld.
Sequence between operations. Sometimes the best distortion control is to interrupt welding with rough machining. For example, weld half the joint, rough mill critical faces, then complete the weld. This lowers restraint during the second half and allows stress relief by steel removal. It adds setup cost but can save the assembly.
Predict with experience. Software helps, but nothing beats a record of how similar parts behaved. Good Manufacturers keep a playbook of real shrinkage numbers. Document camber targets, clamp setups, and final measurements. In contract manufacturing, that book is a profit center.
Inspection and NDE that finds problems early
Thick plate hides defects. Don’t wait for final UT to learn about lack of fusion in pass five.
Visual inspection is the first filter. Cleanliness, bead shape, undercut, toe blending, and slag removal are real clues. Train your leads to judge arc strikes and craters as defects, not style points. Arc strikes on Q&T plate can initiate cracks that propagate later in service.
Magnetic particle testing catches surface and near-surface cracks. Use it after back gouging to verify you’ve removed root defects. On heavy welds, I like MT right after root validation, then again before capping. This timing finds hydrogen cracks that show up hours after welding.
Ultrasonic testing is the standard for volumetric inspection on thick sections. Be realistic about UT capability. Small planar lack of fusion can hide at bevel angles that are unfriendly to your probes. Use weld profiles that allow probe angles to access suspect regions. Partner with your NDE vendor early to tune joint geometry if the part’s function depends on it.
Radiography appears less often on thick plate but remains valuable on certain pressure boundary welds or critical attachments. In custom industrial equipment manufacturing, film quality often becomes a scheduling anchor. Plan it into your takt time.
Process controls on the floor, not just on paper
A WPS does nothing if a welder cannot follow it in the booth. Translation from document to daily practice is the difference between a paper program and a real one.
Consumables. Electrode ovens are powered and calibrated. Rods move from oven to portable quiver to arc within stated exposure times. FCAW wire is stored per manufacturer instructions. Shielding gas composition is correct and documented. For example, if a pulsed GMAW procedure requires 90/10 Ar/CO2, do not substitute 75/25 because the rack ran low.
Equipment calibration. Machines in a cnc metal fabrication cell that require calibrated meters get their stickers and records. Portable feeders remain set with voltage and wire speed ranges marked on the dial or in the booth. On SAW, flux is dry, sieved, and recycled per the WPS.
Fit-up control. Tack welds match the WPS filler and heat input when possible. If fitters use GMAW short circuit to tack a joint meant for low hydrogen process, re-tack or grind before production passes. Root openings stay within specified tolerance. If they don’t, stop and ask for engineering disposition. Nothing ruins schedule like forcing a root into a joint that has closed up due to heat.
Record keeping. Weld maps tie welders to passes and parameters. Interpass logs exist. Hold points for inspection are known. This sounds bureaucratic until you have to prove a repair was not systemic. Good records shorten arguments and keep owners happy.
Anecdote from the floor: why preheat enforcement paid for itself
A client sent a batch of 3 inch A572 Grade 50 plates for a crane turntable base, a large ring with heavy brackets. The initial quote was tight. During kickoff we interrupted the habit of “preheat when you feel it.” We set a strict 225 Fahrenheit minimum and a 400 Fahrenheit maximum interpass, enforced with crayons and a handheld data logger. We posted a pass sequence for each bracket and alternated sides even when it meant changing fixtures mid-pass.
Two welders grumbled about the wait times. They were not wrong, it slowed the pace. But the UT results came back clean on first pass, zero repairs. On similar parts six months earlier, before we tightened preheat, we had 6 to 8 percent repair rate by length, mostly lack of fusion near the root. Even if each defect took only an hour to gouge and refill, the scrap and rework costs were far larger than the pauses for temperature control. The Machine shop also thanked us, because the final machining took 40 percent less time than forecast due to lower distortion. That job paid for a new induction preheating coil set.
Induction preheat and when it pencils out
Oxyfuel torches and propane burners work, but they are blunt tools. For plate above 2 inches or in cold shops, induction preheat often makes sense. It heats through thickness more uniformly, lowers the risk of surface overheating, and pairs well with automated temperature control. The cost hurdle is real. A small system can run tens of thousands of dollars, and large coils cost more. Look at the jobs where you see repeated hydrogen cracking, long preheat times, or complex geometries. If you do regular heavy repairs on industrial machinery or process equipment, induction pays for itself through schedule compression and fewer defects.
A caution: induction can hide itself. The surface looks cool, the core is hot, or vice versa depending on setup. Use thermocouples, not just infrared, and place them at multiple depths if possible on procedure trials. Build your WPS with induction in mind and include coil placement to minimize temperature gradients.
Automation and mechanization without losing control
Not every thick plate weld needs a robot. But mechanized travel, simple seam trackers, and positioners change the game. A rotator that turns a 10,000 pound weldment into flat position saves heat input and improves bead consistency. A simple weld carriage for long plate seams brings travel speed into a tight window that manual hands struggle to hold for hours.
For cnc metal cutting shops adding welding, this is where investment pays. The same mindset that produces accurate bevels can control travel and heat if you bring in the right tools. Mechanization also helps your WPS compliance, because you set the machine to a speed and it stays there. Train welders to listen to the arc and to adjust voltage and wire speed as they see joint variability. Mechanization is not a substitute for skill, it multiplies it.
Repair philosophy: fix fast, fix right, and do not hide it
Repairs are not shameful. Hidden repairs are. On thick plate, a poor repair can embed stresses you don’t see until field failure. Write a simple repair procedure that mirrors your WPS structure. Include how to excavate, how far to chase defects, when to stop and re-evaluate, and which NDE to use before re-welding. Control preheat on repairs more strictly than on original welds. The risk of hydrogen cracking climbs in heat-affected zones on thick parts, especially a day or two after a rushed repair.
I once watched a crew grind a crater crack on a 2.5 inch plate until the groove reached past the original bevel. The part looked like a sculpture. They were minutes from scrapping a $20,000 assembly. A carbon arc gouge and a proper dye penetrant check would have removed the crack in ten minutes with far less damage. Tools and training matter, especially on night shift.
Collaboration with machining and design
When a steel fabricator and a designer talk early, everything goes better. If the Industrial design company gives you a joint that cannot be accessed for back gouging, ask for a change. If the Machinery parts manufacturer metal fabrication canada needs a boring mill setup that conflicts with your clamp positions, adjust the weld sequence to free that surface earlier. Cross-train planners from both sides. A custom industrial equipment manufacturing team that sits welders and machinists at the same table will avoid at least half of the nasty surprises that creep into shop schedules.
Share numbers. If you learned that a long double V on 3 inch plate shrinks 0.030 inch per foot in width, hand that number to design. They can build pre-camber and machining stock into the model. If they specify radii that your weld toes cannot achieve without grinding, tell them and adjust the spec. Tolerance that ignores the weld is a fantasy.
When to stress relieve and when not to
Postweld heat treatment is not free. It requires ovens large enough for your part and time that most schedules do not like. But for heavily restrained thick weldments subject to fatigue or precise machining, a stress relieve can pay off. Typical temperatures for carbon steels sit between 1,100 and 1,250 Fahrenheit, held for an hour per inch of thickness with a minimum hold time, followed by controlled cooling. For quenched and tempered steels, follow the base metal manufacturer’s guidance to avoid temper embrittlement or property loss. On A514, for example, postweld heat treatment is limited and often prohibited. If the drawing calls for stress relief on a Q&T steel, stop and resolve the conflict before you light a burner.
If you skip PWHT, you can still manage residual stress by smart sequencing, interpass control, and rough machining between weld phases. Many machine base weldments in the 2 to 4 inch range achieve excellent stability through those methods alone.
Cost control without quality shortcuts
Thick plate welding is cost sensitive. Material, time, consumables, and inspection stack quickly. The temptation is to cut corners on prep or preheat. Better to save money where risk is low and efficiency gains are reliable.
- Minimize groove volume with double bevels and optimized included angles that still allow fusion and access. Less volume means fewer pounds of wire and fewer passes. Use run-off tabs and backer materials that reduce start-stop defects. The time to fit a tab is less than the time to repair crater cracks. Standardize bevels from your cnc metal cutting equipment to reduce grinding. A good plasma or oxyfuel bevel cut that is dressed, not remade, saves hours. Consolidate process changes. For example, do roots with SMAW, do fills with FCAW, and do caps with GMAW pulsed. Each changeover has a cost; plan it once per joint, not three times. Schedule NDE tightly with production. Waiting a day for UT on a critical joint while the part blocks a bay is expensive. Book the inspector with realistic windows.
These savings preserve quality because they support consistency. Cutting preheat or rushing interpass does the opposite. Good buyers and Manufacturers understand that and will accept the logic if you explain it early.
Training the crew and keeping knowledge alive
A thick plate program dies when the one senior welder retires. Build depth. Run mock-ups, not just lectures. A 12 inch long double V coupon teaches more than a seminar. Record parameters that actually produced good welds on your equipment, not just catalog values. Teach fitters how to prep bevels that welders want to see, and teach welders how to reject bad prep politely but firmly.
Pair new people with veterans on real work. Let them own the back gouge and the MT checks. Let them run the hot passes under supervision. Measure distortion together and guess the next move. The goal is not to create a hero, it is to create a crew that can split a job and get the same answer.
Where cnc and data help without turning the shop into a lab
Cnc metal cutting already supplies bevels and consistent joint fit. Extend that mindset into welding. Simple data capture on heat input and interpass temperature makes your WPS more than a binder. Use QR codes on the part to pull up the right WPS on a tablet. Log amperage, voltage, and travel speed on long seams with a clip-on device. None of this needs to be fancy, but it should be reliable.
When a customer returns with a repeat order, you will have hard numbers to duplicate the previous success. For an Industrial design company that wants traceability, this builds confidence. For a Machinery parts manufacturer that lives on repeatability, it shortens the learning curve on every re-order.
Final thoughts from the bay
Thick plate rewards patience and punishes bravado. The right procedures are not there to slow you down, they exist so you can go fast without tripping. Every weld is a negotiation between heat and restraint, and the best steel fabricator learns how to make both work in their favor. The work is physical and exacting, but it is also a craft that blends cnc precision with hand skill. When you watch a welder tie a perfect cap on a 3 inch joint after a day of planned preheat, measured passes, and smart sequencing, you see why this business keeps pulling people in.
Whether you build machine frames, shipyard components, pressure boxes, or custom carriers for a mining line, the principles hold. Get the joint right. Control the heat. Respect the material. Inspect before it is too late. Keep your records honest. And remember that your best processes start as stories on the floor, sharpened by the few who have burned enough rod to know what really works.
Waycon Manufacturing Ltd
275 Waterloo Ave, Penticton, BC V2A 7N1
(250) 492-7718
FCM3+36 Penticton, British Columbia
Manufacturer, Industrial design company, Machine shop, Machinery parts manufacturer, Machining manufacturer, Steel fabricator
Since 1987, Waycon Manufacturing has been a trusted Canadian partner in OEM manufacturing and custom metal fabrication. Proudly Canadian-owned and operated, we specialize in delivering high-performance, Canadian-made solutions for industrial clients. Our turnkey approach includes engineering support, CNC machining, fabrication, finishing, and assembly—all handled in-house. This full-service model allows us to deliver seamless, start-to-finish manufacturing experiences for every project.