Welding AR400 Steel: Preheat, Filler Metals, Procedures

A maintenance welder gets called to reline a rock crusher hopper, cuts the new AR400 wear strips, tacks them hot and fast with the settings he uses for mild steel — and by morning a hairline crack runs across the heat-affected zone. AR400 rewards patience and punishes shortcuts, and understanding why is the difference between a repair that lasts years and one that fails in a week.

Why Welding AR400 Steel Is Not Like Welding Mild Steel

A maintenance welder gets a call to reline a rock crusher hopper. The new AR400 wear strips are cut, fit-up looks clean, and he runs his usual mild-steel routine: tack fast, no preheat, weave it in, move on. By morning, a hairline crack has crawled across the heat-affected zone beside the last tack. The plate is scrap, the crusher is down another shift, and nobody can quite explain what went wrong.

Nothing about that repair was careless in the ordinary sense. It was simply mild-steel muscle memory applied to a steel that does not forgive it. AR400 rewards patience and punishes shortcuts, and understanding why is the difference between a repair that lasts years and one that fails in a week.

So, can you weld AR400? Yes. AR400 is entirely weldable, and it gets welded every day on crushers, buckets, truck beds, and chutes all over the world. But it is weldable only with a controlled procedure — managed preheat, low-hydrogen consumables, disciplined heat input, and a joint plan. Weld it like A36 and you are essentially gambling against metallurgy.

What AR400 Actually Is

Strip away the trade names and AR400 is defined by hardness, not by a single chemistry. “AR” stands for abrasion resistant, and the “400” refers to a nominal Brinell hardness of 400. In practice the band runs wider than the name suggests: typical ranges sit around 360–440 BHN, with Steel Warehouse listing roughly 380–440 BHN for its AR400 plate. To put that in a scale most welders carry in their heads, 400 BHN is approximately 42 HRC, a figure Modern Casting cites in its guidance on welding abrasion-resistant plate. That is roughly three times the hardness of the mild steel plate sitting next to it on the rack.

That hardness is not rolled into the steel. It is heat-treated in. AR400 arrives from the mill in a quenched-and-tempered condition, a point emphasized in JC Steele’s technical sheet on welding AR plate. The plate was austenitized, quenched hard, then tempered back to buy the toughness that keeps it from shattering on impact. The result is a carefully engineered balance: hard enough to resist gouging abrasion, tough enough to survive rock impact.

Here is the part that catches people out. Welding does not just add metal to that plate. It locally re-heat-treats it. Every arc pass drives the steel immediately behind the fusion line past its critical temperature, re-austenitizing it, and the surrounding mass of cold plate then quenches that zone like a die. The steel that was carefully tempered in the mill comes back as fresh, untempered martensite — hard, brittle, and full of residual stress.

Two Failures Every Welder Fears

That localized re-heat-treatment creates two distinct failure modes, and they pull in opposite directions.

• Hydrogen-induced cold cracking in the HAZ. Hard, brittle martensite plus hydrogen from the electrode flux, damp backing, rust, or oil, plus shrinkage stress from a restrained joint — that combination is the classic recipe for an underbead or toe crack that may not appear for hours after the weld cools.
• Loss of wear resistance from over-softening. Push too much heat into the plate and you over-temper it. The HAZ and the region beyond it drop in hardness, and a wear plate that was supposed to last three seasons wears through in one.

The uncomfortable truth is that the usual shop habits produce both problems at once. Fast tacks with no preheat chill the arc into a fast-cooling, hydrogen-hungry hard zone. Wide weaves and cranked-up amperage dump heat that softens plate for inches in every direction. Dirty 70-series rods, damp low-hydrogen electrodes that were never stored in a rod oven, and oily or painted surfaces all load hydrogen into a joint that is primed to crack.

What This Article Covers

The rest of this guide walks through the full AR400 welding picture in the order you actually need it. We start with the steel itself — composition, hardness bands, carbon equivalent, and typical service — then move into the heat-affected zone metallurgy that explains the cracking. From there we get concrete about preheat and interpass numbers, filler metal selection, heat input and bead placement, joint design and cutting, the mistakes that show up in real shops, and finally inspection, hardness testing, and procedure qualification. It closes with a step-by-step shop workflow you can take straight to the plate.

None of it requires exotic equipment. It requires knowing which numbers matter and actually checking them. That hopper repair could have been saved by a $12 temperature crayon and twenty minutes of patience.

Understanding AR400 Steel: Composition, Hardness, and Typical Uses

Before you strike an arc on AR400, you need to know exactly what is in the clamp. “AR400” gets thrown around like a part number, but it is not one. It is a hardness classification, and that distinction changes everything about how you weld it.

What “AR400” Actually Means

AR stands for abrasion resistant, and the 400 refers to the steel’s typical hardness on the Brinell scale. Steel Warehouse puts the typical AR400 hardness band at roughly 380–440 BHN, while Associated Steel lists typical production at 400–440 BHN with a tensile strength around 180,000 psi. The number is a target hardness, not a chemistry.

That leads to the single most important fact about this material: AR400 has no single universal ASTM chemistry. There is no “AR400 spec” the way there is an A36 spec. AR400 is a mill-defined wear grade, and different producers hit the 400 BHN target with different alloy recipes. Some tonnage is supplied under A514-family or other quenched-and-tempered specifications; much of it is sold under proprietary mill names. As JADCO’s technical staff points out, you should request mill certification and chemistry from your supplier for every plate you plan to weld. Two plates that both test 420 BHN can behave differently under the torch.

Typical Chemistry and Carbon Equivalent

Recipes vary, but most AR400 steels are low-alloy, quenched-and-tempered grades built around a modest carbon content with manganese for hardenability, plus small additions of chromium, nickel, and sometimes molybdenum. The weldability-relevant number is the carbon equivalent (CE). Associated Steel lists a typical AR400 carbon equivalency of 0.47.

What CE 0.47 means in practice: AR400 sits squarely in the “weldable, but not casually” category. Steels below about 0.40 CE are generally forgiving. Above that, hardenability rises, the heat-affected zone can form fresh martensite on fast cooling, and the risk of hydrogen-induced cracking climbs with it. The Associated Steel AR400 data sheet is a good reference for these baseline numbers.

Available Thicknesses and What You’ll See in the Shop

AR400 is most commonly supplied as plate from about 1/8 inch up to 2 inches (roughly 3 to 50 mm), with 1/4″ to 1″ the workhorse range for wear liners. Thicker quenched plate carries more through-thickness restraint and steeper thermal gradients, which is why heavier sections push preheat and heat-input requirements upward — a theme we return to in the temperature section.

Where AR400 Is Used

You’ll find AR400 wherever material is beaten, scraped, or slid rather than bent:

• Crusher and shredder liners — impact-plus-abrasion duty in rock crushing and recycling
• Hopper and chute liners — Coremark Metals specifically cites hoppers and general contact surfaces as core AR400 applications
• Bucket wear edges, cutting edges, and adapter plates on excavators and loaders
• Screener decks and grizzly bars in aggregate processing
• Cement mixer drums and fins — another Coremark-listed use
• Dump truck beds and trailer liners
• Mining and agricultural equipment — dozer blade wear plates, grain handling, and soil-engaging tools

The pattern is clear: AR400 shines in moderate-impact abrasion. For extreme impact, welders usually step down to tougher, softer grades; for pure sliding abrasion with no impact, harder overlays win.

The Hardness–Toughness Trade-Off

Hardness and toughness sit on opposite ends of a seesaw. A quenched-and-tempered 400 BHN plate is hard enough to resist cutting and gouging by abrasive particles, but that same martensitic structure that gives it wear life leaves it less tolerant of two things: localized over-tempering, which softens it and costs wear life, and hydrogen plus restraint, which cracks it. Hobart Brothers’ technical guidance on welding abrasion-resistant plate frames this as the core challenge: the wear life comes from hardness, but that hardness is exactly what makes the HAZ vulnerable when welding heat re-austenitizes it.

This is why an identical repair can fail two ways. Weld too hot or too slow, and you widen a soft band around the weld that wears out first. Weld too cold or too fast on a restrained joint with a dirty rod, and you get an underbead crack. Preheat, filler choice, bead placement, and heat input are all about threading that needle.

The same tempering and re-hardening logic appears across alloy steel work, so if you’re building out your fundamentals, our guide to alloy steel welding electrodes covers filler selection principles that carry over directly.

What to Take Into the Next Steps

1. AR400 is defined by hardness (~380–440 BHN), not chemistry — get the mill cert.
2. Typical tensile strength is around 180,000 psi, with a typical CE near 0.47: weldable, but crack-sensitive under the wrong conditions.
3. It’s a quenched-and-tempered plate; welding locally re-heat-treats the steel.
4. It’s built for moderate-impact abrasion: hoppers, crusher liners, bucket edges, mixer drums, truck beds.

With the material defined, the next question is what actually happens inside the steel when heat hits it — and that’s where AR400 welds are won or lost.

What Happens in the Heat-Affected Zone: The Metallurgy Behind the Cracks

Cracks in AR400 almost never start in the weld bead itself. They start in the narrow band of base metal right next to it, the heat-affected zone, where the arc has rewritten the steel’s microstructure without ever melting it. That zone can be just a few millimeters wide, and it is where welding AR400 either succeeds or fails.

AR400 leaves the mill quenched and tempered: the plate was heated, rapidly cooled, then reheated to a controlled temperature to trade hardness for toughness. The arc undoes that careful heat treatment locally. Every pass reheats the adjacent steel to a peak temperature that depends on how far it sits from the fusion line, then cools it at a rate set by heat input, plate thickness and ambient temperature. The result is a patchwork of microstructures with different hardnesses sitting millimeters apart, and hardness differences are where cracks find a foothold.

The Three Peak-Temperature Zones

Move outward from the fusion line and you pass through three recognizable regions:

• Grain-coarsened HAZ. Steel heated well above roughly 1,100°C (about 2,000°F). Austenite grains grow large, and large grains transform into coarse, hard products on cooling. This is the most crack-prone band.
• Grain-refined HAZ. Peak temperatures just above the upper critical temperature. The grains re-form fine and the steel here is often tougher than the parent metal.
• Intercritical HAZ. Peak temperatures between the lower and upper critical points, where only part of the structure transforms. This band can end up mixed and locally softened, which shows up as a dip in hardness on a survey.

The grain-coarsened band is the danger zone. Above the critical temperature the quenched-and-tempered structure re-austenitizes, and on cooling it has nowhere to go but into fresh, untempered martensite. Preheating AR plate slows that cooling rate, which reduces the chance of forming martensite, the hard, brittle microstructure that promotes cracking, according to Welding Answers. That single sentence is the entire argument for preheat in miniature.

Carbon Equivalent: Why This Steel Hardens So Readily

Hardenability is not a mystery, it is chemistry. Carbon, manganese, chromium, nickel and molybdenum all push the transformation curve to the right, making it easier for the steel to form martensite at cooling rates a mild steel would shrug off. The usual shorthand for combining those elements is carbon equivalent, and AR400 sits at roughly 0.47 CE per Associated Steel. That value signals moderate-to-high hardenability, which is exactly why controlled preheat is required rather than optional.

CE is also the input that lets you scale requirements upward. Thicker plate, colder shop air, or a more restrained joint all shift the cooling curve in the wrong direction, and raising preheat is how you shift it back.

The Cracking Triangle: Hardness, Hydrogen, Restraint

Hydrogen-induced cracking in AR plate shows up in the weld metal or the HAZ, driven by rapid cooling and highly restrained joints, as Hobart Brothers describes. It needs three conditions at once, and removing any one of them breaks the chain:

• Hard, brittle microstructure — controlled by preheat, interpass temperature and heat input.
• Available hydrogen — introduced by electrode coatings and flux, damp backing bars, backing welds, condensation, rust, cutting fluids, paint and oils. Low-hydrogen consumables and proper conditioning are the countermeasure; for a deeper look at how coatings and classifications affect hydrogen pickup, see our guide to alloy steel welding electrodes.
• High tensile stress and restraint — built up by heavy fixturing, tight fit-up, long fillet runs and shrinkage the joint cannot relieve.

The mitigation logic follows directly: heat the plate to slow cooling and soften the transformation, keep hydrogen out with dry low-hydrogen consumables and clean surfaces, and design the joint and weld sequence so shrinkage is distributed rather than locked in. Cracking often appears hours after the weld has cooled, once diffused hydrogen reaches a stressed, hard region, so a weld that looks fine at quitting time is not yet cleared.

Get those three variables under control and the HAZ stops being the weak link. They are also the reason AR400 work is written as a procedure rather than improvised at the bench, which is what the next section puts into numbers.

Preheat and Interpass Temperature: The Numbers That Matter

If you only control one variable when welding AR400, make it temperature. Preheat and interpass temperature decide how fast the weld and heat-affected zone cool, and cooling speed is what determines whether hydrogen escapes harmlessly or gets trapped in a hard, brittle microstructure. Everything else in this article depends on getting these numbers right.

Where the Published Numbers Come From

There is no single universal AR400 preheat table, because AR400 is a mill-defined grade rather than a chemistry-specific ASTM specification. What you get instead is a cluster of manufacturer recommendations that all point the same direction. JC Steele’s technical service bulletin recommends preheating AR400 to 150–300°F (66–149°C) specifically to slow the cooling rate. Associated Steel states that a preheat and interpass temperature of 250–350°F is generally sufficient for AR400, rising to 350–450°F for 500 BHN plate.

On the upper end, ArcelorMittal guidance cited by Iowa DOT recommends maximum interpass temperatures around 210–225°F for certain quenched-and-tempered plate. And in controlled shop practice documented in AWS technical forums, AR400 has been welded at 200°C (392°F) preheat with E7018-H4 when a specific weld-metal hardness had to be held. Notice the pattern: preheat lands somewhere between about 150°F and 400°F depending on thickness, restraint, and how much hydrogen control you need, while the interpass ceiling stays modest so the plate is never over-tempered.

Deriving Your Number From Thickness, Weather, and CE

Those ranges are a starting point, not a substitute for thinking. Three things push your required preheat toward the top of the band:

• Section thickness. Heavy plate acts as a heat sink and also raises joint restraint. Thin 1/4-inch wear strips often sit comfortably near 150°F; 1-inch plate in a boxed-in hopper repair may need 300°F or more.
• Ambient conditions. Cold shop floors, drafts, and outdoor wind chill steal heat between passes. Welding AR400 in a 40°F breeze is not the same job as welding it in a 70°F shop, even with identical settings.
• Carbon equivalent and restraint. Higher CE means higher hardenability, which means a faster-cooling HAZ reaches martensite more readily. Heavily restrained joints, where shrinkage stress cannot relieve itself, also push you toward the higher end.

Hobart Brothers’ guidance on abrasion-resistant plate makes the same point from the cooling-rate side: preheat exists to slow the HAZ cool-down and give hydrogen time to diffuse out before the steel goes below roughly 300°F, where diffusion effectively stalls.

How Far Out, and How to Measure

Preheat is an area, not a spot. Common practice is to bring at least 3 inches (75 mm) beyond the weld toe up to temperature, and on heavier or highly restrained joints, 3× the joint thickness is a safer spread. Heat the joint uniformly, not just the bevel face, or the far side of the plate will pull heat away before you strike an arc.

Guessing is the most common failure here. A hand held over the plate tells you it is “warm,” not that it is 250°F. Use a calibrated contact thermometer, a digital IR gun aimed at clean bare metal, or thermal crayons/tempilstiks that melt at a stated temperature. Measure at multiple points along the joint and recheck as you go, because a long root pass will pull the leading edge down while the trailing end climbs.

Don’t Forget the Consumables

Preheat is only half the hydrogen story. A bone-dry electrode at perfect preheat still underperforms if the coating is damp. Store low-hydrogen consumables in a holding oven and follow the manufacturer’s baking schedule for E7018 and other basic-coated rods, and prefer the H4 and H8 suffix designations, which certify lower diffusible hydrogen levels. Alloy Steel Welding Electrodes: Selection, Use & Safety covers the classification codes and handling rules in detail.

As Modern Machine Shop’s seven tips for welding abrasion-resistant plate put it plainly: measure, record, and repeat. Write the preheat and interpass values on the job traveler, and check them the same way you would check fit-up, because on AR400 they are structural dimensions.

Choosing the Right Filler Metal for AR400

Filler selection on AR400 is not a strength-matching exercise. It is a service-duty decision. Hobart Brothers identifies filler-metal selection, joint restraint, and heat input as the three common challenges when welding abrasion-resistant plate, and filler is what you commit to before the arc strikes.

Strength Matching vs. Service-Duty Logic

The default habit — match the filler to the base metal — is usually wrong on AR plate. AR400 sits near 400 Brinell hardness, and no production filler welds a structural joint at that hardness without becoming brittle enough to crack. The real question is what the joint must survive:

• Impact-loaded joints — crusher frames, hopper supports, bucket shanks, anything taking shock. Undermatching, tougher weld metal is preferred: a soft, ductile bead absorbs energy, a hard, brittle one starts cracks.
• Wear surfaces — liner edges, chute bottoms, patch plates. These get matching or overmatching hardness fillers as overlay beads after the joint is structurally sound.
• Structural attachments — brackets, wear strips, tie-ons. A moderate-strength, low-hydrogen filler covers most of these.

Low-Hydrogen Stick Electrodes: E7018 and E7018-H4

E7018 is the workhorse for AR400 field repairs. It deposits relatively soft, tough ferritic weld metal, runs in all positions, and carries inherently low hydrogen — provided the rods are handled correctly. That handling matters: a compromised coating can pick up enough moisture to undo the advantage.

Use the H4 variant when restraint is high or the plate chemistry pushes hardenability. AWS classifies E7018-H4 at a maximum of 4 ml of diffusible hydrogen per 100 g of deposited weld metal, roughly half the H8 limit. On steel whose heat-affected zone already sits near the cracking threshold, that margin matters. Store rods in a holding oven near 250–300°F, re-bake per the data sheet, and never pull them from a damp caddy. For classification codes and storage practice, see our guide to alloy steel welding electrodes.

Flux-Cored Wire: E71T-1 in 100% CO2

For shop procedures and production work, E71T-1 flux-cored wire with 100% CO2 shielding is the most commonly cited qualifying combination for AR400. Shop procedures under AWS D1.1 call for at least 150°F (66°C) preheat extending six inches from the joint in every direction.

The advantages are real: higher deposition rates than stick, consistent deposited chemistry, and weld metal around 70 ksi tensile with usable toughness. But FCAW runs hotter than SMAW, so heat-input discipline becomes part of the filler decision itself. Self-shielded flux core (E71T-8, E71T-11) saves the gas bottle but carries more diffusible hydrogen than gas-shielded wire — on AR400 that pushes you toward stricter preheat and storage discipline, not looser.

Hardfacing Overlay Fillers: AWS A5.5

To build a wear surface rather than join two parts, you use AWS A5.5 classified abrasion-resistant overlay electrodes. High-chromium, high-carbon deposits reach 400+ BHN as welded, but they are brittle with almost no ductility, so they never serve as structural fillers. Typical practice: complete the joint with a soft low-hydrogen filler first, then overlay the wear face. Many procedures butter the cut edge with a soft bead before the hard overlay, giving the brittle deposit a tougher transition zone — standard practice when welding AR plate in industrial equipment service.

Filler Comparison at a Glance

Filler	Typical use	Pros	Limitations
E7018	Field repairs, structural attachments	Tough, all-position, low hydrogen	Lower deposition; rod handling discipline
E7018-H4	Impact joints, restrained fit-ups	Diffusible H₂ ≤ 4 ml/100 g	Higher cost; same nominal strength
E71T-1 (100% CO2)	Shop procedures, production	High deposition, consistent chemistry	Hotter; heat-input control critical
Self-shielded FCAW (E71T-8/-11)	Field work, no gas bottle	Portable, no cylinder to drag	Higher hydrogen; preheat discipline
AWS A5.5 overlay rods	Wear faces, liner surfaces	400+ BHN, excellent wear life	Brittle; never structural

Hydrogen Control, Shielding, and Final Weld Hardness

Whatever filler you pick, diffusible hydrogen decides whether a sound-looking weld cracks overnight. AWS designations (H4, H8, H16) state the maximum millilitres of hydrogen per 100 g of deposited metal, and on AR400 lower is always better. Hydrogen also enters through damp backing, mill scale, rust, oil, paint, and moisture in the gas line — so shielding gas must be dry and the joint genuinely clean, not just wiped.

Filler choice also sets final weld hardness. A soft E7018 or E7018-H4 bead typically lands around 180–220 BHN, far softer than the plate but tough enough to tolerate impact. A hardfacing overlay pushes the weld face past 400 BHN while sacrificing ductility. No filler gives you AR400 hardness and AR400 toughness in the same bead, which is why the service-duty logic above beats any number on a spec sheet.

Heat Input, Bead Placement, and Temper Bead Techniques

If preheat is the foundation of welding AR400, heat input is the discipline that keeps the job honest. You can dial in perfect preheat and use the right low-hydrogen filler, then undo all of it in ten minutes with a wide weave on high amps. AR400 is quenched and tempered, so every pass you lay down is a localized heat treatment. The work is controlling what that treatment does.

The Heat Input Formula, in Plain Terms

Heat input is the energy delivered into the joint per inch of weld. The standard formula is:

Heat Input (kJ/in) = (Amps × Volts × 60) ÷ (Travel Speed in inches per minute × 1000)

Run your machine at 260 amps, 28 volts, and travel at 12 ipm, and you are putting roughly 36 kJ/in into the plate. Drop travel speed to 6 ipm and that number doubles. This is why travel speed, not just dial settings, is the variable most welders underestimate. As Modern Casting’s guidance on welding abrasion-resistant plate points out, excessive heat input widens the heat-affected zone and increases the volume of softened base metal, which is exactly the band that wears out first in service.

The risk cuts both ways. Too much heat over-tempers the plate, turning a 400 Brinell surface into a soft, chewed-up strip weeks into service. Too little heat, or a cold fast-cooling bead, produces a brittle untempered martensite structure in the HAZ that invites underbead cracking. Welding Answers summarizes the standard mitigation cleanly: preheat plus a controlled interpass temperature is the accepted method of slowing the cooling rate so hard martensite doesn’t form in the first place.

Stringer Beads, Not Weaves

Here is the uncomfortable truth: the big, beautiful weaves that win you compliments on mild steel are the enemy on AR plate. A wide weave dumps a long, slow heat cycle into one spot, widens the coarse-grained HAZ, and creates a tall, convex bead profile that concentrates stress at the toes. On abrasion-resistant plate, run stringer beads, or at most a slight weave no wider than 2 to 2.5 times the electrode diameter. Multiple narrow, overlapping beads distribute heat more evenly and leave a flatter profile with better toe blending.

Multi-pass sequencing matters as much as bead profile. Stagger your bead placement rather than stacking passes in one spot, and alternate sides of a joint where geometry allows so shrinkage stresses balance instead of accumulating. If you need a guide on maintaining consistent bead control across positions, our All Position Welding Rod Guide covers rod behavior that translates directly to this kind of controlled work. For stick applications specifically, the consumable handling fundamentals in our Alloy Steel Welding Electrodes resource are worth reviewing before you strike an arc on AR400.

Buttering the Joint Edge

Before running root passes into hardened AR400 edges, consider buttering: laying a single controlled stringer of filler along the bevel face first, then welding the root into that softer, already-tempered layer. Buttering gives the root pass a metallurgical buffer instead of depositing directly onto a hard, quench-sensitive edge. It also lets you verify heat input and preheat response on a low-stakes pass before committing to the joint root.

Temper Bead Technique

Temper-bead (or temper-pass) welding is the deliberate version of the same idea. Each successive bead’s heat input is controlled so that it tempers the untempered martensite in the layer beneath it, progressively building a joint whose HAZ has effectively heat-treated itself without a furnace. The discipline here is strict: each layer’s heat input must be held within roughly 10% of the value used during procedure qualification. That tight tolerance is what reproduces the hardness results the qualification coupon demonstrated. Wander outside it and you are welding an untested metallurgy while pretending to follow a qualified procedure.

Interpass Temperature: The Hidden Variable

Interpass temperature is the number most often ignored and the one that quietly ruins AR400 welds. Every pass starts from wherever the last one left the plate. Let the interpass climb past the recommended ceiling and you are preheating yourself into over-tempering territory. Let it drop below the minimum and the next bead cools too fast and hardens. Monitor it with a contact thermometer or thermal crayon, not your hand or your memory, and stay inside the band your procedure specifies. Section 4 covers the specific temperature ranges for AR400 plate in detail.

Control heat input, keep beads narrow, sequence passes deliberately, and treat interpass temperature as a measured variable rather than a guess. That combination is what separates a repair that holds its hardness from one that quietly softens and fails.

Joint Design, Cutting, and Fit-Up for AR400 Plate

Most AR400 welds are decided before the arc is struck. The crack that ruins a crusher liner usually traces back to a cut edge that was never ground, a bevel that left a partial-penetration root, or a part held so rigidly in the fixture that shrinkage had nowhere to go. Fit-up on abrasion-resistant plate is not just mechanical housekeeping, it is the first step of the welding procedure.

Cutting AR400: Manage Heat and Hardness at the Edge

AR400 is through-hardened plate, which cuts both ways. The good news is that hardness runs through the full thickness rather than sitting in a thin surface skin, so a wear part keeps protecting itself after years of abrasion. The bad news is that any cutting method adding heat can locally re-austenitize the edge, and when that band cools fast it re-hardens into a brittle layer that behaves like a crack starter. Mechanical methods have the opposite problem: a dull blade or an aggressive bite notches the edge, and a notch in 400 BHN steel is a stress raiser with no tolerance for error. Associated Steel’s AR400 processing data is explicit about this, advising that cut edges be ground back before welding consumables are even opened.

In practice, the cutting choices break down like this:

• Plasma cutting is the everyday standard for AR400 plate. It is fast, produces a narrow thermal zone, and leaves a manageable edge that still needs a grinding pass.
• Abrasive saw or waterjet gives the cleanest result because no heat is introduced, so no re-hardened layer forms. It is slower, but for critical wear parts it removes an entire variable.
• Carbide milling or machining produces a smooth, controlled edge, though tooling wears quickly against 400 BHN material. This aligns with Coremark Metals’ guidance that AR400 plate is suitable for welding but is not a good candidate for forming and machining operations.
• Oxy-fuel cutting works, but only with caution. It dumps far more heat into the plate than plasma, and the resulting edge must be treated as suspect until it is ground to bright metal. Follow standard torch safety practices as outlined by OSSHA’s welding and cutting guidance.

The rule that covers all four: cut slightly oversize, then grind or machine back 1/16 to 1/8 inch (roughly 1.5 to 3 mm) to remove the heat-affected or work-hardened skin. If a defect has to be removed by air arc gouging or carbon arc cutting, treat that gouged surface exactly like a fresh cut and grind it before welding. Never run a bead directly onto an unground, unpreheated hardened edge and expect it to hold.

Bevel Geometry and Why Full Penetration Matters

A typical single-V preparation is a 45-degree bevel with a small 1/16-inch land (root face), which gives enough room for a root pass without forcing an excessively wide gap. Double-V joints are useful on thicker plate because they balance the deposited metal on both sides and cut total shrinkage.

Full penetration matters more on AR400 than on mild steel because a partial-penetration root acts as a built-in notch. That notch sits at the bottom of a hard, high-restraint joint, which is precisely where hydrogen-induced cracking initiates. Where full penetration is genuinely impossible, the joint should be engineered as a fillet or partial-penetration detail with a defined root, not left to chance.

Tacks, Fixturing, and Managing Restraint

Hobart Brothers identifies restrained joints as one of the three common challenges when welding abrasion-resistant plate, and that restraint is usually built during fit-up. A part clamped flat across every inch of its length has nowhere to contract, so the shrinking weld metal pulls against cold, rigid plate and cracks the underbead.

Practical restraint control looks like this: tack with the same low-hydrogen filler and the same preheat you will use for the final weld; keep tacks short and well spaced; place them where the root pass will consume them rather than at the ends of a bead, where they become crater cracks; and sequence the welds so shrinkage is distributed instead of concentrated. Back-step and skip sequences, plus alternating beads from side to side on a double-sided joint, keep distortion even. JADCO makes the broader point that the higher alloy and carbon content of AR grades demands precise preheat and procedure before fabrication begins — fit-up is where that discipline either starts or gets skipped.

Forming Limits and Layout Discipline

Because AR400 arrives quenched-and-tempered, cold bending after hardening is where parts crack. Form the shape first, then weld it into assembly, and keep bend radii generous — sharp cold bends in 400 BHN plate concentrate strain and can initiate surface cracks that only appear later under load. For tight or complex geometry, the sound answer is to form and machine in a softened condition and re-heat-treat, or to select a formable grade for that component instead. Layout should be marked with soapstone, soft pencil, or a washable marker; hard stamping, scratch awl lines, and center punches all leave notches in the plate surface, and on abrasion-resistant steel a small notch is a future crack.

Get cutting, beveling, tacking and restraint right and the welding itself becomes far more forgiving. Get them wrong and even a perfect bead on a perfect schedule will not save the joint.

Common AR400 Welding Mistakes and How to Troubleshoot Them

Most AR400 failures in the shop aren’t metallurgical mysteries. They’re habits carried over from mild steel that nobody questioned until a crack showed up. The good news: nearly every common mistake has a clear symptom and a straightforward fix.

The Ten Shop Mistakes, Matched to Symptoms and Fixes

Use this as a diagnostic table. Find the symptom you’re seeing, trace it back to the habit that caused it, and correct the process rather than the patch.

Mistake	Typical Symptom	The Fix
Welding cold with mild-steel habits	Underbead or toe cracks overnight	Preheat to 150°F (66°C) minimum and verify with a thermometer
Skipping preheat entirely	Cracks in highly restrained joints	Apply 150–300°F (66–149°C) per procedure, extending well beyond the joint
Dirty or non-low-hydrogen consumables	Random underbead cracking	Switch to E7018-H4 or equivalent; store rods in a holding oven
Weaving too wide	Soft band, wide HAZ, distortion	Run stringer beads with controlled travel speed
Excessive heat input	Reduced hardness, accelerated wear	Lower amperage/voltage or speed up; monitor interpass
Over-preheating	Over-tempered, softened plate	Cap preheat and interpass; use a contact pyrometer, not guesswork
Tacking without preheat	Cracks starting at tack toes	Preheat before the first tack; treat tacks as real welds
Welding over paint, hardfacing, or rust	Porosity, inclusions, cracks	Clean to bright metal; grind off old hardfacing layers
Ignoring interpass temperature	Cumulative softening or hard zones	Check temperature between passes; let the plate recover
Wrong filler (hard, brittle mismatch)	Cracking under impact loading	Use a tougher, undermatching filler for impact service

Two of these deserve extra emphasis. According to Hobart Brothers, cracking in AR plate weld metal or HAZ is commonly driven by rapid cooling or highly restrained joints, and both are within the welder’s control. And underbead cracking is strongly tied to hydrogen picked up from moisture, rust, and non-low-hydrogen consumables, which is an entirely avoidable cause. For context on rod selection, our Alloy Steel Welding Electrodes guide covers low-hydrogen handling practices in depth.

The overheating mistake is just as costly as cracking. As Modern Casting notes, overheating AR400 can reduce hardness in the heat-affected zone, creating a soft band that actually wears faster than the surrounding plate. You’ve repaired the crack and quietly created a new wear point.

Troubleshooting a Crack Found After Welding

Discovering a crack the next morning is discouraging, but the repair sequence is well established:

1. Stop and verify. Use dye penetrant to map the full crack length, including any branching beyond what’s visible.
2. Grind it out completely. Dig to sound metal. Stop-crill holes at crack ends help, but only if you remove the entire indication.
3. Clean and dry the area. Remove oil, paint, and mill scale; moisture trapped in a repair is hydrogen waiting to happen.
4. Re-preheat correctly. Bring the area to at least 150°F (66°C) and confirm with a contact thermometer or thermal crayon.
5. Rerun with qualified parameters. Same filler classification, documented heat input, stringer beads, monitored interpass temperature.
6. Cool slowly. No drafts, no water, no compressed-air quenching. Cover the repair if the shop is cold.

For the reasoning behind interpass limits on this grade, Associated Steel’s AR400 welding data gives specific bands worth keeping at the bench, and J.C. Steele’s technical bulletin on AR plate walks through joint preparation and repair sequence.

One last habit worth breaking: welding over painted, primed, or previously hardfaced surfaces. Burnt coatings introduce hydrogen and inclusions, and hardfacing over a soft root pass traps stress where it does the most damage. Clean first, always. If you’re cutting and prepping plate alongside this work, our surface prep and cleaning guide reinforces the same principle across materials.

Catch the mistake early, fix the process not the symptom, and the crack never gets the chance to become tomorrow’s emergency call-out.

Inspection, Hardness Testing, and Procedure Qualification

You can follow every preheat and interpass rule here and still not know whether the weld is good until someone proves it. AR400 hides its problems well: a hydrogen crack can sit under a bead for hours before it surfaces, and a softened band in the heat-affected zone looks identical to sound metal to the naked eye. Verification turns a weld from “looks fine” into a documented, defensible result.

Visual Inspection Comes First

Start simple. Clean the bead, remove slag completely, and examine the weld toe and surrounding HAZ for hairline indications, underbead shadowing, spatter-bonded cracks, and undercut. Cracks in AR plate almost always track the HAZ or start at the toe, so trace those areas carefully. If anything looks suspicious, do not grind and re-run over it yet. Move to penetrant testing.

Alloy Steel Welding Electrodes: Selection, Use & Safety is worth reading alongside this section if your shop juggles multiple low-hydrogen consumables across grades.

Dye Penetrant Testing for Surface-Breaking Cracks

Liquid penetrant testing (PT) is the shop-standard second step for AR400 because hydrogen-induced cracking is frequently surface-breaking and far too fine for unaided vision. The process is straightforward: clean and degrease the area, apply a red penetrant, let it dwell per the manufacturer’s instructions, wipe it off, apply white developer, and read the indications. Bright red lines against white developer are cracks. PT is inexpensive, needs no power, and works on any cleaned surface, which is why fabricators welding wear plate reach for it far more often than radiography.

Portable Hardness Testing Across the Weld Zone

Visual and PT confirm that nothing cracked; hardness testing confirms that nothing silently lost its wear resistance. Portable Leeb (rebound) and portable Brinell testers give readings directly on the plate in minutes. Take a transect: several points in weld metal, several in the HAZ at varying distances from the fusion line, and a baseline reading in unaffected base metal.

The benchmark that matters for hydrogen cracking risk is keeping peak HAZ hardness below roughly 350 HV. Research on quenched-and-tempered plate uses that threshold because cracking susceptibility climbs sharply as untempered martensite hardens in the underbead zone. Readings above it mean your preheat, hydrogen control, or heat input fell short.

For wear surfaces, also compare post-weld readings against the plate’s delivered 380–430 BHN band. A broad drop into the 250s BHN means the heat input over-tempered the plate and that zone will now wear faster than the surrounding liner.

Macro-Sections in Qualification Coupons

Before production welding, good shops qualify the procedure on a coupon, cut it, etch it, and examine the macro-section. It reveals actual penetration profile, lack of fusion, HAZ width, and where the hard zones sit, information no surface test provides. Cheap insurance against a $40,000 liner coming out of a crusher with a rejectable weld.

Procedure Qualification Under AWS D1.1

AWS D1.1 includes supplementary provisions for quenched-and-tempered steels, and they are strict for a reason: these steels are welding-sensitive by nature. Qualification requires a written Welding Procedure Specification (WPS) backed by a Procedure Qualification Record (PQR), with the essentials locked down:

• Filler metal classification and hydrogen designation (H4/H8)
• Preheat and maximum interpass temperature
• Heat input limits, controlled through amperage, voltage, and travel speed
• Joint geometry, pass sequence, and cleaning between passes
• Post-weld cooling rate and any restraint conditions

Every variable outside those essentials needs requalification. Log actual readings during welding, not planned values, because the PQR is only as honest as the log behind it.

What a Complete AR400 Inspection Record Contains

A finished wear-plate record should include: the WPS/PQR reference, consumable lot numbers, actual preheat and interpass readings, calculated heat input per pass, visual sign-off, PT results with timing, and hardness survey data with locations mapped to the joint. Add coupon macro-section photographs if one was run.

That packet is what lets you hand a mining or aggregate client a repair they can trust, and it separates a documented AR400 procedure from a welder’s best guess. The next section pulls every requirement in this article into one shop-floor workflow.

Step-by-Step AR400 Welding Workflow and Final Takeaways

Everything in this article only pays off if it happens in the right order. Preheat means little if the cut edge was never ground, and a perfect filler choice won’t save a joint welded cold with a wide weave. Below is the full sequence condensed into one workflow you can tape inside the welding hood.

The AR400 Shop Workflow

1. Verify the material. Confirm the plate is actually AR400 and pull the mill certificate before striking an arc. Check the stated hardness range, thickness, and carbon equivalent. If the cert is missing or the plate is unmarked, treat it as unknown steel and test a scrap piece first.
2. Engineer the joint. Decide the bevel geometry, required penetration, and weld size based on the load the part will see. Full penetration matters more on wear plate than most welders assume, because an unfused root becomes the crack initiator under impact loading.
3. Cut and grind. Cut with plasma, an abrasive saw, or carbide tooling, then grind the cut face to remove the re-hardened or notched layer. Clean back at least an inch on both sides of rust, paint, oil, and old hardfacing — all of them are hydrogen or contamination sources.
4. Plan the sequence. Sketch bead order, start and stop points, and how shrinkage will distribute across the joint. Stagger tacks and balance the welds on both sides of the joint so the plate isn’t pulled into a restraint you’ll fight all the way through.
5. Set and verify preheat. Heat to 150–300°F (66–149°C) and hold it through welding so the steel cools slowly enough to avoid fresh untempered martensite. Measure with a contact thermometer or temperature-indicating crayon across the full preheat zone — never judge by color or by hand.
6. Condition consumables. Use low-hydrogen consumables only, and keep stick electrodes in their held or re-baked condition until the moment they’re used. Confirm wire is dry and clean, and that shielding gas lines are free of moisture and leaks.
7. Select filler and set parameters. Lock in the filler from your procedure, dial in amperage, voltage, and travel speed, and do a quick trial on scrap to confirm your heat input window before touching the actual part.
8. Weld in stringers with monitored interpass. Run short, staggered stringers rather than wide weaves, checking interpass temperature between passes. Associated Steel notes that maintaining preheat during welding with 250–350°F interpass is generally sufficient for 400 BHN plate, so keep a crayon in your glove and stop when you hit the ceiling.
9. Control heat input pass by pass. Stay inside the window you established: too much current softens the base metal and widens the heat-affected zone, while too little produces the fast-cooling hard zone that invites underbead cracking. If you drift, correct on the next pass rather than trying to fix it with a hotter bead.
10. Cool slowly. Cover the finished weld and surrounding area to let it drift down in still air. Avoid fans, compressed-air blow-offs, drafts, and water quenching — thermal shock can crack a restrained AR400 joint even when the welding itself was flawless.
11. Inspect. Start with visual examination for toe cracks and undercut, then run dye-penetrant testing on critical seams. Any crack found gets ground out fully to sound metal, the area dried and re-preheated, and the pass rerun with verified parameters — never welded over.
12. Hardness-test and document. Take portable hardness readings across weld metal, HAZ, and base metal on wear-critical repairs. Then record everything: material cert, preheat and interpass values, heat input, filler classification, operator, and date.

When to Call a Welding Engineer

Bring in qualified engineering support when the joint is highly restrained, when the plate chemistry is unknown or the carbon equivalent runs high, when service involves low-temperature impact loading, or when the repair is structural rather than a wear replacement. Procedure qualification under quenched-and-tempered requirements is also a job for someone who can produce a PQR and a qualified WPS. If your shop needs a starting point on consumable selection, our guide to alloy steel welding electrodes covers classification, hydrogen designations, and storage practice in detail.

The Payoff

AR400 is entirely weldable — the plate is in service in crushers, buckets, and truck beds worldwide, and weld repairs are routine when the procedure drives the work instead of habit. The payoff for following this workflow is concrete: wear surfaces that reach their intended life, heat-affected zones free of cold cracks, fewer emergency shutdowns, and a repair record that proves the job was done right. Patience at the preheat stage is cheap insurance compared with a cracked liner pulled out mid-season. Weld it like the hardened steel it is, and AR400 rewards you with years of service instead of a week of excuses.

Frequently Asked Questions

Is AR400 steel weldable?

Yes. AR400 is regularly welded for wear liners, buckets, and repair work, but it requires a controlled procedure — preheat, low-hydrogen consumables, and managed heat input — because it arrives quenched and tempered and is prone to hard, crack-sensitive heat-affected zones.

What preheat temperature should I use for AR400?

Common guidance is 150–300°F (66–149°C), with preheat and interpass temperatures around 250–350°F typical for 400 BHN plate. Thicker sections, colder weather, or higher carbon equivalents push the requirement up, so measure with a thermometer rather than guessing.

Which filler metal works best for welding AR400?

Low-hydrogen options such as E7018 or E7018-H4 and gas-shielded flux-cored wires like E71T-1 in 100% CO2 are common choices. The right selection depends on service: tougher, slightly softer weld metal suits impact-loaded joints, while harder fillers are used where the weld itself must resist abrasion.

Why does the heat-affected zone crack when welding AR400?

Cracking usually comes from three factors together: a hard martensitic HAZ, hydrogen from moisture or consumables, and high restraint in the joint. Preheat slows cooling, low-hydrogen consumables limit hydrogen, and sensible joint sequencing reduces restraint.

Does welding ruin the hardness of AR400 plate?

Excessive heat input widens the HAZ and can over-temper the plate, creating a softer band that wears faster. Too little heat does the opposite and leaves a hard, brittle zone. Controlling heat input, bead size, and interpass temperature keeps both problems in check.

Can I hardface AR400 with a hardfacing overlay?

Yes, AR400 is a common base plate for hardfacing overlays, but the weld procedure and consumable must still be chosen for the service. Use a buttering or transition layer where dissimilar hardness could otherwise promote cracking at the interface.

Do I need to post-weld heat treat AR400?

Post-weld heat treatment is generally not used on quenched-and-tempered plate because it can over-temper and soften the base metal. Instead, controlled preheat, temper-bead techniques, and slow cooling are used to manage hardness and hydrogen.

How do I know if my AR400 weld is acceptable?

Verify it with visual inspection, dye-penetrant testing for surface cracks, and portable hardness readings across weld, HAZ, and base metal. Critical work should be backed by a qualified welding procedure with recorded preheat, interpass, heat input, and filler classification.