Are High Strength Bolts Weldable? Facts Before You Weld

Are high strength bolts weldable? Yes — most of them can be welded, but almost never in the way people assume, and the details matter enough that doing it wrong can crack the bolt or violate structural codes.

High strength fasteners like ASTM A325 and A490 structural bolts are made from medium-carbon steels with a carbon equivalent that makes them prone to hardening and hydrogen cracking when you strike an arc on them. That doesn't make welding impossible; it makes procedure everything.

This guide walks through which bolt grades tolerate welding, why welding a nut or bolt head is usually a bad idea, how to attach material to a bolt legally under AWS D1.1, and the preheat and electrode choices that keep welds from cracking days after they look fine.

Are High Strength Bolts Weldable? The Short Answer

Yes — technically, high strength bolts are weldable, because most of them are medium-carbon or low-alloy steels that accept arc welding. But there's an important catch: the American Institute of Steel Construction and AWS D1.1 treat bolted structural connections as friction-bearing or shear-bearing assemblies, and welding on a bolt basically voids its engineered properties.

The practical answer is more nuanced than a simple yes or no. If you weld directly to a bolt head, shank, or nut, the heat input changes the heat-treated microstructure that gives the bolt its strength. A Grade 8 or A490 bolt is quenched and tempered for a reason — arc welding can locally anneal it back to a softer condition, or worse, create a brittle heat-affected zone that cracks under load.

Here's what you're actually allowed and not allowed to do in most structural situations:

• Welding to a bolt (around it, tack-welding a nut, or attaching a plate): commonly accepted in structural practice, with procedures in AWS D1.1 Section 7.
• Welding through or fusing the bolt itself as a structural weld: not recognized, and the bolt's rated strength no longer applies.
• Using the bolt as a consumable electrode or fusing threads: strictly prohibited by code.

So the honest answer: the steel is weldable, but the application usually isn't — unless you follow a qualified procedure.

What Makes a Bolt Weldable or Not: The Metallurgy

Weldability in steels comes down to one number above almost everything else: carbon content, evaluated through the carbon equivalent (CE) formula. Low-carbon steel under about 0.30% carbon welds happily with little precaution. High strength bolts don't live in that neighborhood.

ASTM A325 bolts (now superseded by F3125 Grade A325) run roughly 0.30–0.45% carbon depending on diameter. A490 and Grade 8 bolts are alloy steels with added manganese, molybdenum, and sometimes chromium or boron, pushing the carbon equivalent well into the zone where rapid cooling from welding produces martensite — a hard, brittle phase that cracks under welding hydrogen stress.

Three mechanisms threaten a welded bolt:

1. Hydrogen-induced cracking (cold cracking): hydrogen from damp electrodes, moisture, or oil diffuses into the hot weld and cracks the heat-affected zone as it cools — sometimes 24 to 48 hours later.
2. Softening: welding heat can over-temper or re-anneal the quenched microstructure, dropping local strength below the bolt's rating.
3. Stress concentration: threads act like tiny notches; a weld toe ending on a threaded region multiplies stress and invites fatigue failure.

None of these make welding impossible. They make preheat, low-hydrogen electrodes, and post-weld handling required when the application truly demands it. (See Also: Are Galvanized or Stainless Steel Bolts More Durable)

Bolt Grades and Weldability Compared

Not every high strength fastener behaves the same under an arc. The table below ranks the common grades by how much care welding demands and where each is realistically usable.

The takeaway: the higher the grade, the less the steel wants to be welded, and the more the manufacturer's heat treat investment is at risk. For new designs, the correct move is usually to specify a weld stud or weldable threaded rod instead of forcing a structural bolt into a welded application.

When Welding Bolts Is Actually Done in Practice

There are three real-world situations where welding around high strength bolts is routine and code-sanctioned — knowing them saves you from reinventing a risky workaround.

Tack-welded nuts on structural bolts. AWS D1.1 Section 7 explicitly permits welding a nut to the base metal (and per procedure, engaging the bolt threads) to prevent loosening in structures subject to vibration or where a bolt might back off. The weld attaches the nut to the supporting steel, not the bolt to the joint load path. This is the classic "tack the nut" detail used on bridge and building work where impact loads exist.

Welding washer plates or keeper bars over a bolt head. A low-profile plate welded around — not through — the fastener locks the assembly without touching the bolt's load-bearing surfaces. Shipbuilding and heavy equipment repairs use this constantly.

Stud welding as an alternative. When the goal is a threaded attachment point welded to steel, drawn-arc or capacitor-discharge stud welding gives you a full-strength fusion weld in milliseconds or seconds, avoiding the prolonged heating that damages heat-treated hardware.

What is never accepted: fillet or groove welds designed to carry load through the bolt itself, welding on the threads to "lock" them, or fusing a bolt into a joint as a substitute for a designed weld. A bolt's clamp load comes from tensile preload — welding disrupts exactly that mechanism.

How to Weld to a High Strength Bolt Safely: Step by Step

If your situation legitimately calls for welding near or to a high strength bolt, follow a sequence like this one:

1. Identify the bolt grade. Check the head marking: three radial lines for A325/Grade 5 territory, six lines for A490/Grade 8. No marking means treat it as unknown and take maximum precautions.
2. Decide what the weld must do. Locking function? Then weld the nut or a keeper plate to the base metal only. Structural load transfer? Stop — that's a redesign, not a weld.
3. Clean to bare metal. Remove oil, galvanizing, and paint within 2 inches of the weld area. Zinc coating on hot-dip galvanized hardware produces porosity and toxic fumes — grind it back before arcing.
4. Preheat based on carbon equivalent. For A325-class steel, 250–400°F (roughly 120–200°C) is a working range; heavier sections and alloy bolts push toward the high end. Verify with a temperature-indicating crayon.
5. Use a low-hydrogen electrode. E7018 (freshly baked and kept in a heated rod oven) or an appropriate low-hydrogen wire with the right gas mix. Never weld hydrogen-controlled joints with damp or left-open electrodes.
6. Keep heat input low. Small stringer beads, short arcs, no weaving. Skip welding directly on the bolt shank or threads unless the procedure specifically calls for it.
7. Slow-cool the assembly. Cover with insulating blanket or allow furnace-style gradual cooling if practical. Rapid quenching in cold air is what forms brittle martensite.
8. Inspect after a delay. Check for cracks 24–48 hours post-weld — hydrogen cracking is a delayed act. Dye penetrant or a close visual with a bright light is a sensible minimum.

This sequence follows the same logic AWS D1.1 applies to weldable structural steel, adapted for hardware that wasn't designed to be welded at all.

Key Factors: Preheat, Electrodes, and Hydrogen Control

Three variables decide whether your weld on high strength bolt steel holds or cracks, and each has real numbers behind it. (See Also: Are High Powered Bolt Action Rifles Legal)

Preheat temperature. The thicker the section and higher the carbon equivalent, the more preheat you need. As a working guide: light sections of A307-class steel may need little or none; A325 hardware typically wants 250–400°F; A490 and Grade 8 alloys demand the top of that range or more, plus interpass temperature control so the joint never cools and reheats unevenly. Preheat slows cooling, giving diffusable hydrogen time to escape and tempering the HAZ as it forms.

Hydrogen control. Low-hydrogen electrodes (E7018, E7016, or suitable wire processes) must be dry. Electrodes exposed to humid air for more than a few hours pick up moisture that becomes weld hydrogen. Keep them in a rod oven at manufacturer-recommended temperature, typically around 250°F for low-hydrogen stick electrodes, and discard damaged ones.

Heat input and cooling rate. High amperage with slow travel dumps heat in and softens the bolt material; too little heat with fast cooling quenches it brittle. The target is the narrow middle: moderate amps, short beads, controlled interpass temperature, and slow cool-down.

Also consider post-weld heat treatment where feasible — a stress relief at around 1,100°F followed by slow cooling restores toughness to the HAZ. On a small bolt in the field that's rarely practical, which is exactly why stud welding exists as the engineered alternative.

Because exact requirements vary by project specification and jurisdiction, confirm the governing numbers with the applicable code (AWS D1.1 for structural steel) and the project engineer before welding on anything load-bearing.

Common Mistakes That Crack or Weaken Welded Bolts

Most failures in this area trace back to a handful of repeatable errors:

• Welding over galvanizing. Zinc produces porous welds and zinc fume fever. Grind the coating off first — no shortcuts.
• Welding through the threads "to lock them." This notches the highest-stress region of the fastener and destroys future adjustability. Use thread locker, a tack on the nut-to-steel interface, or a jam nut instead.
• No preheat on alloy hardware. Grade 8 and A490 bolt steel quenches itself in ambient air after a cold weld. Cracks often appear a day or two later, which is why crews wrongly blame "bad rods."
• Using rusted or damp E6013 electrodes. A rutile rod has no hydrogen control. On medium-carbon hardware it's a cracking kit, not a welding electrode.
• Expecting the weld to carry bolt loads. A fillet weld tacked onto a bolt head carries nothing reliably. The bolt's strength rating is void once it's fused; design the weld path separately or use the bolt unmodified.
• Ignoring the delayed-crack window. Walking away right after welding means missing hydrogen cracks that show up 24–48 hours out. Schedule the follow-up inspection.

One more subtle mistake: tack-welding the nut but bridging weld onto the bolt threads themselves. The intent is anti-loosening, but the arc on the thread flank hardens and notches it. Keep every weld on the nut-to-steel or plate-to-steel interface and the bolt stays a bolt.

Special Cases: Studs, Anchor Rods, and Welding to Nuts

A few scenarios deserve their own handling because the general rules bend.

Weld studs. If you're designing from scratch, skip the bolt entirely. Arc studs and CD studs are made from weldable low-carbon steel or stainless, come with the ferrule or flux tip engineered for the process, and produce a fusion weld stronger than the stud shank. This is the correct answer to "I need a welded threaded fastener."

Anchor rods and anchor bolts. Foundation anchor rods embedded in concrete are frequently welded to base plates — and that's legitimate when the rod material is weldable grade, such as ASTM F1554 Grade 36 or Grade 55 with supplemental weldability requirements ("S1"). F1554 Grade 105, the high strength option, is generally not considered readily weldable and needs special procedures. Check the grade on the mill cert, not by eye. (See Also: Are Honda 2 4l Cylinder Head Bolts Tight on Entry)

Welding to nuts. A weld on a hex nut's flats to base metal is acceptable as a locking method under structural procedures, but note that the nut's heat-treated hardness can still crack under a heavy bead. Use short tacks on opposite flats, small rod, and preheat.

Welding bolts in tensioned joints. Never weld on an assembly under tension. The local heat expands the shank, releases preload, and the joint relaxes — often permanently. Loosen, weld, then re-torque if the design allows, or better, don't weld a preloaded joint at all.

Common Questions About Welding High Strength Bolts

Can You Weld a Nut to a Bolt to Keep It From Loosening?

You can tack-weld the nut to the surrounding steel or base plate, and AWS D1.1 recognizes this as a locking method. Don't weld the nut to the bolt threads — that fuses the two and notches the fastener. Short tacks on the nut-to-steel interface, with preheat on A325-class hardware, do the job without damaging the bolt's properties.

What Happens If You Weld a Grade 8 Bolt?

The weld heat locally destroys the quench-and-tempered microstructure that gives Grade 8 its 150 ksi strength. The heat-affected zone can either soften below rating or harden into brittle martensite that cracks, often 24–48 hours later from hydrogen. For anything structural, replace the Grade 8 bolt with a weld stud or weldable threaded rod.

Can A325 Structural Bolts Be Welded?

The steel can be welded with proper procedure — preheat around 250–400°F, low-hydrogen electrodes, and slow cooling. But code-wise, A325 bolts are bolted connections; their rated strength is not valid after welding. Permitted structural practice is welding a nut or keeper plate around the bolt, not welding the bolt itself into the load path.

Is A490 Harder to Weld Than A325?

Yes, significantly. A490 is a higher-strength alloy steel with a greater carbon equivalent, making it more prone to hydrogen-induced cracking and HAZ hardening. Many fabricators simply refuse to weld on A490 hardware and redesign with weld studs. If welding is unavoidable, rigorous preheat, baked low-hydrogen electrodes, and delayed inspection are mandatory.

Can You Use a Bolt as an Electrode to Weld It in Place?

No. Using a bolt as a consumable electrode is expressly prohibited by structural welding codes. It produces an unpredictable weld with uncontrolled dilution of the bolt alloy, destroys the fastener's heat treatment, and creates a joint with no code basis. If you need a welded threaded element, use a purpose-made weld stud.

Do You Need to Preheat Before Welding on a Bolt?

For low-carbon hardware like A307, often not. For medium-carbon A325-class bolts, yes — roughly 250–400°F is the practical range depending on thickness. For A490 and Grade 8, expect the high end of that range and strict interpass control. Preheat slows cooling, prevents martensite formation, and lets hydrogen escape. Confirm the exact requirement from AWS D1.1 and your project specs.

What's the Best Alternative to Welding a High Strength Bolt?

Three options cover nearly every case: specify a weld stud (drawn arc or CD) for new work requiring a welded threaded attachment; use an anchor rod in a weldable grade such as F1554 Grade 36 or 55 (S1) for foundations; or use mechanical thread-locking — a jam nut, proper torque, or a tack weld on the nut-to-steel interface — when the existing bolted joint just needs to stay put.

Conclusion

Are high strength bolts weldable? The metal itself is, but the fastener's engineered strength is not — welding on an A325, A490, or Grade 8 bolt destroys the heat treatment behind its rating, and structural codes treat a welded bolt as no longer valid.

That doesn't leave you stuck. Tack-welding a nut or keeper plate to the base metal is code-sanctioned for locking, preheat and low-hydrogen electrodes keep field repairs from cracking, and weld studs or weldable F1554 anchor rods cover new designs properly.

Match the method to the grade, respect the preheat numbers, and inspect 48 hours later — then the weld and the bolt both do their jobs.