Laser Cutting Kerf and HAZ: Widths, Depths, and Compensation

Kerf is the width of the slot the cutting beam removes as it travels, and the heat-affected zone (HAZ) is the narrow band of metal beside that slot whose structure the heat changed without melting it. On a fiber laser, kerf runs 0.10 to 0.65mm depending on material, thickness, and assist gas, with 0.15 to 0.30mm typical on thin to medium sheet. The HAZ measures 0.13 to 0.25mm on mild steel, 0.10 to 0.20mm on stainless, and 0.20 to 0.40mm on aluminum. Both numbers are small, and both still decide outcomes: kerf silently changes every dimension on the drawing, because holes grow and tabs shrink by the width of the removed slot unless the toolpath is offset by half the kerf, and the HAZ is a band of altered metal sitting exactly where welds, fatigue cracks, and corrosion problems tend to start. This page carries the deep treatment: the values by process and material, the compensation workflow in CAD and nesting, a worked example, and the mitigation choices when heat damage cannot be tolerated.

Other pages in this family use these two numbers where a comparison needs them. Laser vs plasma and laser vs waterjet lean on kerf and HAZ as decision factors, fiber laser cutting and CO2 laser cutting carry the process context, and the materials and thickness guide covers tolerance by thickness. The data table, the compensation math, and the mitigation decisions live here.

What kerf is and what changes its width

A laser does not cut a line of zero width. It melts and partly vaporizes a narrow channel, and the assist gas blows the molten metal out. The finished slot ends up wider than the focused spot, because heat spreads into the walls and molten metal erodes them before it clears. Kerf is the width of that slot, measured across the cut.

How the kerf forms

The focused fiber spot sits at 0.1 to 0.4mm depending on lens and power, and production kerfs land between 0.10 and 0.65mm across the common sheet materials. The slot is also slightly tapered, about 0.02 to 0.05mm wider at the top than the bottom on thin sheet, because the beam and the melt stream widen slightly as they exit the bottom face. The number that matters for a job is the effective width at the surface, which is what the toolpath compensation uses.

What widens or narrows the kerf

Four factors move the number most. Thickness: kerf grows roughly 0.02 to 0.05mm for every millimeter of material. Assist gas: oxygen on mild steel cuts a kerf 10 to 20 percent wider than nitrogen would, because the exothermic iron-oxygen reaction adds heat to the cut. Focus: defocused cutting, common when pushing thick material, widens the kerf 20 to 40 percent. Nozzle diameter: a larger nozzle widens the gas stream and the kerf with it. Because the value depends on the setup, the honest kerf for a job is measured on the machine, not read from a table. Cut a coupon, measure, and set the compensation from the result.

Kerf widths by process and material

The table below collects kerf widths and HAZ depths for fiber laser on the three common sheet metals, with CO2, plasma, and waterjet rows for contrast. The fiber values come from the shared process atlas kerf tables, verified in-house, and the contrast rows carry the same figures used across the site.

Fiber laser kerf by material

Stainless 304 with nitrogen cuts the narrowest slot: 0.12 to 0.18mm at 1mm sheet, rising to 0.32 to 0.42mm at 10mm, and as fine as 0.10 to 0.14mm on 0.5mm foil-gauge sheet. Mild steel with oxygen runs slightly wider at equal thickness, 0.15 to 0.20mm at 1mm and 0.50 to 0.65mm at 20mm. Aluminum 5052 with nitrogen starts near the mild steel values at thin gauge, 0.15 to 0.22mm at 1mm, but climbs to 0.48 to 0.65mm at 15mm, because its high thermal conductivity pulls heat out of the cut in every direction and demands more input per unit of thickness. Across thin to medium sheet the working shorthand stays 0.15 to 0.30mm, the same figure used on the laser family pages.

CO2, plasma, and waterjet for contrast

A CO2 beam focuses wider than a fiber beam, so its kerf runs 0.20 to 0.50mm with a heat-affected zone roughly double the fiber value on mild steel. Plasma is a different regime: the jet melts a 2 to 5mm slot, an order of magnitude wider than the laser kerf, which is why fine features and tight nests push plasma toward structural plate work. Waterjet erodes rather than melts, leaving a 0.75 to 1.15mm kerf and no heat-affected zone at all. Its slot is wider than a laser slot, but the edge is metallurgically untouched. The full process trade-offs live on the comparison pages linked above.

Kerf compensation in CAD and nesting

The machine follows the programmed line with the beam centered on it, so material is removed half a kerf to each side of the line. Without compensation, every outside dimension comes out one full kerf undersize and every hole one full kerf oversize. At a 0.20mm kerf that is a 0.20mm error on each dimension, which is larger than the ±0.15mm band a fiber laser holds on 3 to 6mm mild steel.

The offset workflow

Compensation moves the toolpath, not the part geometry. Offset outside contours outward by half the validated kerf, and offset holes and slots inward by half the kerf, so the inner edge of the removed band lands on the drawn line. Nesting software applies this automatically once the kerf is entered for the job, and it adds the kerf to the spacing between nested parts at the same time. Machine builders such as ACCURL recommend planning around 0.1 to 0.2mm per side for production fiber work, then validating with a first-article measurement before releasing the batch, because material lot, thickness, focus, and gas all shift the real value.

Worked example: a 0.20mm kerf on a nested layout

Take a nest of 3mm mild steel brackets cut with oxygen assist. The atlas band for that job is 0.20 to 0.25mm, and a first-article coupon validates the machine at 0.20mm. Each bracket has a 100.00mm outline and a 10.00mm mounting hole.

With no compensation, the beam centerline rides the drawn lines, so the bracket outline cuts to 99.80mm and the hole cuts to 10.20mm. Both miss a ±0.15mm print. With compensation, the nesting software offsets the outline outward by 0.10mm and the hole inward by 0.10mm, and the parts cut to nominal inside the process tolerance.

The coupon math doubles as the validation. Cut a nominal 10.00mm hole and measure both the hole in the sheet and the drop-out plug: the hole reads 10.20mm and the plug reads 9.80mm, so the kerf is (10.20 minus 9.80) divided by 2, which confirms the 0.20mm entered in the software. Re-measure whenever thickness, material, or assist gas changes.

Nesting gains follow from the same number. The web between parts is one kerf wide plus whatever spacing the nest allows, so a 0.20mm kerf lets parts sit nearly edge to edge where a 2 to 5mm plasma kerf forces ten times the gap or more. Common-line cutting shares a single cut between neighboring parts instead of cutting two parallel lines, saving a kerf width of material on each shared edge, which matters most on dense nests of small parts in expensive material.

What the heat-affected zone is

Cutting heat does not stop at the melt line. Heat soaks into the metal beside the cut, and every temperature band it crosses leaves a different microstructure behind: grain-coarsened metal against the cut, then refined, partially transformed, and tempered bands as the temperature falls toward the parent metal. None of that metal melted, but all of it was heat-treated by accident, and its properties no longer match the base metal.

Plain-language metallurgy

The direction of the change depends on the alloy. TWI, the UK materials research body, describes carbon steel HAZ often hardening and becoming more crack-sensitive, aluminum HAZ softening and losing strength, and stainless HAZ at risk of sensitisation, where chromium carbides form at grain boundaries and corrosion resistance drops once local chromium falls below about 10.5 percent. TWI also notes that the HAZ is a common origin point for component failures, for exactly this reason: it is the band where properties change. On stainless, the heat tint on the cut edge, light yellow near 290°C through dark blue near 600°C, is the visible footprint of the same heating cycle.

What drives HAZ depth

The drivers are heat input, time at temperature, and cutting speed: more energy per millimeter of cut means a deeper band. Fiber’s advantage is concentration, a small spot moving fast, which is why its mild steel band is 0.13 to 0.25mm against CO2’s 0.25 to 0.50mm and plasma’s 1 to 5mm. Aluminum carries the deepest fiber band of the three common sheet metals at 0.20 to 0.40mm, the price of feeding enough heat into a metal that conducts it away in every direction.

Why the HAZ matters by application

A 0.10 to 0.20mm band of altered metal matters differently on a welded bracket, a rotating shaft collar, and a food-contact stainless panel: the application decides whether the band is a footnote or a rejection.

Stainless steel and corrosion

On a 3mm part in dry indoor service, the band rarely decides anything. It decides things on thin gauge, on chloride or chemical service, and on sealing or food-contact edges, where a sensitised band can become the corrosion path through the part. Where it matters, the fixes are mechanical or chemical: machine or grind the band off, or passivate the stainless edge, which restores surface corrosion resistance without changing dimensions.

Welding preparation

Assist gas leaves its signature on the edge. A nitrogen-cut edge is oxide-free and normally welds as cut, while an oxygen-cut mild steel edge carries an oxide layer that many welding procedures remove first, and an oxidized stainless edge is usually cleaned before welding to keep contamination out of the pool. The HAZ adds a second consideration, but a smaller one here: weld heat re-treats the band anyway, so a heavily welded edge cares less about the cutting HAZ than a finished, load-bearing edge does.

Fatigue and formed edges

On a part that flexes or rotates, the property gradient at the HAZ acts as a crack initiator, and the observation that failures often start in the HAZ applies directly. Two defenses work. Cut the part with waterjet, which leaves no band at all, or machine the edge back so the finished surface sits in clean parent metal: a machining allowance of twice the worst-case band, 0.50mm on mild steel and 0.80mm on aluminum, clears the affected metal with margin.

Reducing kerf and HAZ: parameters, gas, and process choice

Parameter and gas choices

The shop-side levers are concrete. Cut with nitrogen rather than oxygen when the kerf and edge chemistry allow it, since nitrogen kerfs run 10 to 20 percent narrower and leave no oxide. Keep focus set for the thickness, because defocus widens the kerf 20 to 40 percent. Run the parameters the cut development calls for rather than slowing down for safety, because slowing raises the heat delivered per millimeter and deepens the HAZ. Use the smallest nozzle diameter the job allows. The drawing-side lever is to specify rather than assume: when cut-edge geometry must be held, reference ISO 9013:2017, the standard that classifies thermal cuts and their quality tolerances. It covers oxyfuel flame cutting from 3 to 300mm, plasma from 0.5 to 150mm, and laser from 0.5 to 32mm thick.

When waterjet is the right answer

Waterjet cuts cold: no HAZ, no sensitisation, no heat tint, no thermal distortion of thin details. Its costs are equally concrete, a 0.75 to 1.15mm kerf that limits the smallest feature, best-case tolerance around ±0.05 to 0.10mm, and higher operating cost per meter from abrasive and water. Choose it for heat-treated parts that must keep their properties, thick plate headed for critical welding, stainless in chloride service under fatigue load, reflective metals like copper, and any part whose edge must be metallurgically identical to the parent metal. The laser vs waterjet page carries the full trade-off.

A kerf and HAZ checklist

Before releasing a nested laser job, confirm each item. Get the validated kerf for the exact material, thickness, and gas, measured on the machine rather than assumed from a table. Enter half of it as the toolpath offset, outward on profiles and inward on holes, and let the nesting software apply it to every part in the nest. Validate with the coupon method: kerf equals hole dimension minus plug dimension, divided by two, then adjust the offset from the first article. Keep features comfortably larger than the kerf, and hold to the fiber feature rules, holes and slots at least 1x the material thickness, which the fiber laser page covers. Decide whether the HAZ band matters for this part: heavily welded edges are re-treated by the weld anyway, but finished load-bearing, sealing, and corrosion-critical edges may not tolerate the band. If it matters, machine it off with a double-band allowance, passivate stainless, or move the part to waterjet. And when edge geometry must be held to print, reference ISO 9013:2017 on the drawing so cut quality is specified rather than inherited.