Every shop foreman has a story about the crack that looked harmless on Friday and turned into a costly shutdown by Monday. Heavy equipment lives hard. Loader towers flex on rutted haul roads, excavator booms twist under asymmetric loads, forklifts slam pallets, and trailers rack with every pothole. Steel remembers. Microfractures grow, paint blisters, a hairline opens, and suddenly a hinge plate tears off or a bucket ear elongates. That is when a mobile welder with structural chops pays for itself, not just with a clean bead, but with judgment: where to cut, where to stitch, how to stop a crack from coming back.
I have repaired fractured booms on freezing job sites, gouged out and rebuilt forklift mast rails in cramped warehouses, and nursed bent loading dock plates back into service between shift changes. The principles do not change, but the trade-offs do. Repairing heavy equipment cracks is not just about depositing metal. It is choosing process and filler, controlling heat input, respecting metallurgy, and designing the fix so the next failure happens somewhere else, preferably much later.
Cracks in structural members are feedback, sometimes subtle, sometimes blunt. A crack at the termination of a weld toe on a truck frame crossmember usually points to stress concentration and poor toe blending. A crack radiating from a bolt hole in a trailer hanger bracket screams out about hole edge condition, over-torque, or misalignment. Longitudinal cracks on the underside of a backhoe dipperstick often trace back to heat-affected zones from prior repairs and an abrasive work cycle.
Cracks propagate for a reason. Cyclic loading, impact, corrosion, undercut, lack of fusion, and design stress risers each play a role. Before striking an arc, I look for the load path. Where does the member carry tension and compression, and where does it flex? If the repair stiffens a local area without addressing load distribution, the crack may simply move to the next weakest link. Good welding saves the day only when paired with good diagnosis.
Mobile structural welding is part craft, part logistics. The call comes in: dozer blade corner cracked through the gusset, machine stuck on a right-of-way with rain moving in the next day. I load the truck welding rig with carbon arc gouging rods, 7018, 8018-C3, low-hydrogen stainless buttering rods for dissimilar zones, aluminum spool gun for odds and ends, preheat torches, induction blankets for larger mass, clamps, air supply, die grinder, N95s, and a simple dye penetrant kit. The goal is not to bring the whole shop, but to bring a kit that allows options. Out on a job site you cannot plan for every variable, yet you can plan to make the right decision once you see the steel.
Access matters. Sometimes the only way to reach a torn loader tower plate is to remove a guard or support the boom, pin it, and crib it safely. I refuse to weld under suspended loads without proper cribbing and lockout. Safety aside, a stable setup produces better welds, better tie-ins, and fewer residual stresses. It is also faster than fighting a bouncing part.
Paint hides cracks and contaminates welds. Grease pulls gas into the puddle. I start by cleaning down to bright metal six to ten inches beyond the visible crack, using a needle scaler for the scaling and a flap wheel to finish. On structural components with potential hidden branching, I run dye penetrant after the first cleaning. It takes minutes and can reveal hairlines that would have been left to grow under fresh paint.
I drill stop holes, but only after inspection. A 3/16 to 1/4 inch stop hole at each extremity helps reduce crack tip stress intensity, provided it is centered on the crack path and deburred. Then I chase the crack with carbon arc gouging or a grinder to a U or V profile, depending on thickness, targeting root access without removing unnecessary base metal. For mild carbon steel components, a cut back of one and a half to two times material thickness on each side is typical for full penetration.
Surface prep changes when dealing with stainless steel or aluminum. Stainless steel welding benefits from segregated brushes and tools to avoid ferrous contamination. Aluminum demands a fresh, scrubbed oxide-free joint and meticulous cleanliness. Cross-contamination from a grinder that touched carbon steel is a common reason for pitty, porous aluminum welds in the field.
It is tempting to default to what the machine is set for. The right process, however, follows material, thickness, environment, and position.
Shielded metal arc welding, commonly called stick, is the workhorse for outdoor structural repair. Low-hydrogen electrodes like E7018 or E7018-1 provide toughness and predictable properties. For higher-strength steels commonly found in heavy equipment frames and modern trailers, E8018-C3 or E9018-M can match or outrun base metal tensile strength. I carry fresh, oven-dried rods in sealed tubes in the truck and monitor exposure time. Moisture in flux means hydrogen, and hydrogen means delayed cracking.
Flux-cored arc welding, especially gas-shielded wires like E71T-1 or E71T-8 for self-shielded, speeds deposition on thicker sections and performs well in flat and horizontal positions. On windy sites, self-shielded flux-core can outperform MIG, but it brings more spatter and requires careful parameter control to avoid lack of fusion.
Gas metal arc welding, or MIG, makes sense for shop-side sections, guard repairs, and lighter brackets when I can shield the joint. With spray transfer on thicker mild steel, MIG lays in quickly with good fusion. On aluminum welding, a spool gun or push-pull setup with 4043 or 5356 wire can save a day, but I do not attempt structural aluminum without proper fit-up, power, and cleanliness.
Gas tungsten arc welding, TIG, sees use in intricate stainless steel welding on railings, food-grade stainless components around loading dock equipment, and thin aluminum panels. TIG is slow and unforgiving in the wind, but it offers unrivaled control when distortion and cosmetic quality matter.
I choose filler based on the steel class and service. Many heavy equipment parts are A36 or similar mild steels, which pair well with 7018. Others are higher-strength low-alloy steels. Excavator booms and modern truck frames might be built from quenched and tempered steels. In those cases, matching or slightly undermatching the filler can be wise, coupled with preheat and controlled cooling. For dissimilar joints, such as repairing a worn pin boss with a sleeve, stainless buttering with 309 can serve as a buffer before capping with a compatible steel filler. AWS classifications guide the match, but field judgment fills the gap when the data plate is missing and the part has seen years of service.
Hydrogen cracking ruins good welds after the welder leaves. It loves thick sections, restrained joints, higher-strength steels, and low temperatures. I rarely skip preheat on structural repair. Even 150 to 250 F makes a difference on mild steel in cool weather. For thicker members, or when working with 80 to 100 ksi steels, 300 to 400 F is common, guided by carbon equivalent if known. I measure preheat with temperature crayons or a contact thermometer and keep interpass within a range to avoid tempering issues.
Control the sources. Keep rods dry. Avoid welding over galvanized without removing the zinc and ventilating thoroughly. Purge oil from cracks with heat and time. When in doubt, extend preheat and slow cool the part under an insulating blanket. The cost is minutes, the benefit is a repair that does not pop on the next cold start.
I plan the weld sequence to balance heat and stress. Short beads, backstepping, and alternating sides on double-sided joints help limit distortion. On a thick boom plate, I might run stringers rather than wide weaves to control heat input and improve toughness. For a through-crack in a plate, I aim for full penetration and then grind and cap the opposite side where accessible. If access is limited, I use back gouging to ensure no unfused root remains.
Toe blending matters. A beautiful bead with a sharp toe can be the next crack starter. After the weld cools, I often dress the toe with a flapper wheel to a smooth transition, especially on dynamic members. On high-cycle parts like trailer spring hangers or loading dock hinges, a radius at the weld toe reduces stress concentration dramatically.

Gusseting and fishplates have to be chosen carefully. Adding a gusset at a corner can shift the fatigue hot spot inches away and solve the problem, but it can also lock in distortion or create a new rigid node that forces the parent plate to flex elsewhere. I prefer reinforcement that follows the load path. Fishplates with generous tapering and rounded ends, not square cut plates, spread stress instead of trapping it.
A quarry loader came in with a cracked tower plate near the boom pivot. The original weld tied a stiffener directly into a corner with a hard, square stop. The crack shot out from the toe like a drawn map. After cleaning and dye-checking, we found branching under paint. We drilled stop holes, gouged, and performed a double-sided 7018 repair with 300 F preheat. Then we redesigned the stiffener end into a scalloped profile, added a tapered doubler with rounded terminations, and blended toes. That machine ran another two seasons before it came back for unrelated pin wear.
A tandem trailer with a fractured spring hanger had elongated holes and cracking around the hanger welds. Cutting off and reusing the old hanger would have been faster, but the steel showed microcracking. We removed it, repaired the frame rail with back gouging and 8018-C3, then welded a new hanger with stitch sequencing to avoid pull. We reamed the bolt holes to the next nominal size and installed new hardware. The customer wanted to add a stiff backing plate. We did, but with tapered ends and ground transitions, not a square-edged trap.
A warehouse forklift mast rail had a longitudinal crack halfway up, hidden behind rollers. The mast sees shock loads and the crack sat in the high-stress zone. Painting over it would have been cosmetic. Instead, we pulled the mast, stripped the segment, put it on stands, and executed a controlled preheat, root pass with 7018-1, multiple low-heat stringers, and slow cool, then machined the rail face to restore roller alignment. Turnaround took a day, but the mast tracked true and the repair held.
Aluminum on flatbeds, toolboxes, and certain boom covers often cracks at corners where vibration lives. Aluminum welding demands cleanliness and correct filler. 4043 flows smoothly and reduces the chance of hot cracking. 5356 offers higher strength and better color match for anodized parts, but it is less forgiving. For structural aluminum repairs, joint design is king, and heat input control is critical to avoid a softened heat-affected zone. If a cracked aluminum ramp shows widespread fatigue, replacing the panel is smarter than chasing cracks.
Stainless appears in industrial environments, especially around loading dock equipment, food processing guards, and railings. Stainless steel welding removes corrosion concerns but brings its own pitfalls. Distortion is pronounced and sensitization at the heat-affected zone can occur in certain grades. For 304, 308L filler usually fits. If joining stainless to carbon steel on a guard mount, a 309L buffer weld reduces dilution problems. Clean tools and post-weld passivation keep the piece looking and performing right.
A repair needs verification. In the field, visual inspection and simple dye penetrant are practical. I look for uniform bead appearance, proper tie-in, lack of undercut, and the absence of porosity. Where full penetration is required and only one side is accessible, I will back gouge and re-weld, then apply penetrant to the cap after cool-down. On critical gear, especially lifting points and aerial devices, I recommend magnetic particle testing or ultrasonic inspection as budget allows, and I document settings, filler classification, and preheat for the customer record. Certified welds, backed by AWS procedures and welder qualifications, protect both the operator and the owner.
Judgment is part of the service. Some cracks can be chased for years, others mark the end of life. If a component shows multiple cracks, widespread thinning, and elongation at pin bores, it becomes a cost comparison. Machining oversize, installing a bushing, and adding a doubler might buy time, but not safety. On a rusted wrought iron fencing panel or an old gate hinge with deep pitting, a fresh section of steel, properly primed and painted, can outlast a patched relic. Structural repair is a craft, not a magic trick.
The quality of on site welding services depends on the equipment just as much as the hands guiding it. A well-outfitted truck or trailer with a diesel welder-generator, air for carbon arc gouging, and enough auxiliary power to run grinders and lights turns a muddy lot into a workable shop. I carry dedicated leads for stick, a wire feeder for flux-core and MIG, a TIG torch for stainless touch-ups, and a spool gun for aluminum. With that, I can pivot between heavy equipment frame repair, fence welding at a commercial yard, or pipe repair on a small industrial run, without rescheduling. Emergency welder calls at night rarely wait for perfect weather or perfect power, so redundancy counts.
The work is not finished at the last cap pass. Grinding transitions smooth and removing sharp corners reduce stress risers. For outdoor equipment, I prime and paint the area. On galvanized components, I burn back the zinc for the weld, then cold galvanize spray the repair and surrounding area. For railings and gates, fit and finish matter. Good stainless work gets cleaned and passivated. Aluminum benefits from a seal coat if it will live near salt or fertilizers.
Lubrication and alignment are part of the repair. A fresh weld on a boom ear means little if the pin fights the bore or the grease zerk is clogged. I often chamfer pin bores, chase threads, and ensure the assembled geometry does not preload the weld. Little steps like aligning a loading dock hinge or ensuring railings land square on their bases keep cracks from coming back.
Owners ask two questions: how soon can it run, and how long will it last. The honest answer balances speed and durability. A forklift rail crack can be stabilized within hours to get a shift through, then brought down for a full repair the next day. A trailer crossmember patch can be welded from underneath to get to a job, but the permanent fix involves cut-out and a fitted replacement with proper plug welds and flanges. For industrial customers, the plan often involves staged repairs during scheduled lulls. Having a mobile welder who communicates options, costs, and risks clearly is as valuable as the weld itself.
On commercial properties, the same truck that fixes a loader can be called to restore wrought iron fencing, gates, and railings after a delivery truck hooks a panel. Those repairs share the same fundamentals of fit, prep, and welding process selection. For ornamental iron, the finish hides sins or exposes them. MIG with short-circuit transfer works well for thin fence pickets and cap rails, while TIG is ideal for stainless handrails where distortion control and clean beads matter. When replacing posts, set them below frost depth, align the rails, and stagger splices to avoid a visible seam. If the job includes a loading dock bumper bracket or a bent bollard, I treat them like structural tasks rather than decoration. Trucks hit hard, and those fixtures are there to take abuse.
Certified welders bring more than a stamp. AWS certifications demonstrate a welder has passed bend tests and understands procedure adherence. For structural steel, maintaining procedure qualification records and welder performance qualifications builds a trail of accountability. In my practice, I note the process, electrode, amperage range, preheat temperature, and position for significant structural repairs. This habit aligns with industrial customers’ expectations and simplifies future audits, insurance reviews, or resale inspections.
Skipping preheat to save minutes is the single most frequent cause of a repair that fails cold. Chasing only the visible portion of a crack leaves a live defect under the cap. Over-welding, with heavy, wide beads and high heat input, looks heroic but often softens surrounding metal and invites distortion. On aluminum, using the wrong filler can produce a pretty weld that cracks in service. For pipe welding or pipe repair near equipment, ignoring internal buildup or misalignment can introduce turbulence or stress that shortens life. The fix is simple, if not always easy: respect the metallurgy, prep thoroughly, and weld to a plan.
Shops do excellent work, but getting a disabled excavator or a racked trailer to a bay costs time and money. A portable welder with the right tools and judgment can turn a breakdown into a scheduled repair window. That flexibility matters to construction crews racing weather, warehouse managers coordinating shifts, and industrial plants that cannot halt a line for long. It is the difference between replacing a boom section and rebuilding a cracked plate on site, between scrapping a dock leveler and restoring its hinge with proper fit, between shutting a yard gate for days and hanging a new post before opening.
The craft is not glamorous. It is heavy, hot, and precise. It calls for knowledge that lives in the hands as much as in manuals: the sound of a puddle with the right amount of heat, the feel of a rod biting, the way a plate relaxes when you sequence beads correctly. That is what turns a mobile welder into a structural problem solver.
If your equipment or infrastructure shows the telltale signs of fatigue, a quiet line at the edge of a weld, a rattle where there used to be none, a hinge working loose, do not wait for it to become drama. A careful assessment, a grounded repair plan, and disciplined welding can keep your heavy equipment, trailers, gates, and loading docks working, without surprises.
On Call Mobile Welding
917 J Pl Suite 2, Plano, TX 75074
(469) 750-3803