The edge of a part separated by oxy-fuel cutting is different material than it appears. The high temperature generated during cutting permanently alters the internal structure of the steel in a narrow band right next to the cut line. Known as the heat-affected zone (HAZ), this band is invisible to the naked eye but directly affects subsequent welding, bending, and machining operations. This article covers why the HAZ forms, what risks it creates, and how it is managed.
What Kind of Thermal Process Is Oxy-Fuel Cutting?
Oxy-fuel cutting works on a different principle from other thermal methods: here, the material is separated not by melting but by burning. First, the flame brings the steel's surface to ignition temperature; then high-purity cutting oxygen is directed at it, and the steel burns through oxidation. The resulting molten oxide is blown out of the kerf by the pressure of the oxygen jet.
This process delivers a significant amount of heat to the area around the cut line. As cutting progresses, that region first heats up rapidly, then cools rapidly as the surrounding cool material draws the heat away. This heating-cooling cycle reshapes the steel's internal structure, much like a heat treatment.
What Is the Heat-Affected Zone?
The HAZ is the band extending from the cut edge into the material whose internal structure has changed due to heat. This zone hasn't melted, but because it heated up sufficiently and then cooled rapidly, it has transformed into a different metallurgical structure.
The underlying mechanism is this: when steel reaches high temperature, its internal structure transforms; rapid cooling then prevents this structure from returning to its soft form, resulting in a hard, brittle structure. The hardness at the cut edge can rise noticeably above the hardness of the base material.
The main factors that determine HAZ width:
- Material thickness: thicker material draws heat away faster, making cooling harsher
- Cutting speed: slower cutting delivers more heat and widens the HAZ
- The power and duration of the preheat flame
- The steel's chemical composition, particularly carbon content
- Ambient temperature and the plate's starting temperature
Carbon Equivalent: The Number That Determines Risk
The most practical metric for predicting how much a steel will harden after cutting is carbon equivalent. This value combines the contribution of carbon and other alloying elements to hardenability into a single number.
In mild structural steels with low carbon equivalent, HAZ hardening generally isn't a problem. As this value rises, edge hardening and crack risk increase; above a certain threshold, preheating becomes mandatory. For this reason, checking the material certificate before cutting thick, high-carbon steel prevents problems that would otherwise surface later.
What Problems Does the HAZ Create?
A hardened cut edge shows up at several different stages:
- In welding: a hard, brittle edge raises the risk of cold cracking
- In bending: cracking can occur when bending from a work-hardened, embrittled edge
- In machining: a hard edge quickly dulls cutting tools
- In fatigue: the brittle region can become a crack initiation point
- In drilling: a drill bit may fail to advance through a hard edge, or may break
This is especially critical before welding. If a weld groove prepared by thermal cutting is welded directly on its hardened surface, crack risk rises noticeably.
Controlling the HAZ with Preheating
The most effective way to control HAZ hardness is preheating. When material is heated before cutting, the post-cut cooling rate drops, and formation of a hard structure is largely prevented.
Preheating is especially required for high carbon-equivalent steels, thick sections, cutting in low ambient temperatures, and parts that will be welded after cutting. Heating needs to be uniform along the cut line and through the material thickness; heating just the surface is not enough. Temperature should be verified with a contact thermometer or temperature-indicating crayon.
Slow cooling after cutting serves the same purpose. Rather than placing the part on a cold floor or exposing it to wind, cooling it slowly under insulation lowers hardness and reduces residual stress.
Machining Allowance: A Practical Solution
In critical applications, the most reliable approach is to fully remove the hardened edge. This is done by cutting the part slightly larger than its finished dimension and then removing the region covering the HAZ through machining or grinding.
The allowance left should be enough to cover the HAZ depth, which varies with thickness and material. This approach adds processing cost but prevents far more expensive problems like weld cracking or bend fracture. On surfaces where a weld groove will be prepared, lightly grinding the cut edge is often sufficient and low-cost.
Oxy-Fuel Cutting Also Has a Thermal Advantage
Although the HAZ looks like a disadvantage, it doesn't change oxy-fuel cutting's standing on thick sections. On very thick carbon steel, oxy-fuel cutting remains both the economically and technically most suitable method. The point here isn't to avoid the method — it's to treat the HAZ as a known, planned parameter. When what happens after cutting is known in advance, the HAZ is a manageable issue.
CNC Oxy-Fuel Cutting with DMK Makina
DMK Makina evaluates material grade and post-cut operations together on thick plate oxy-fuel cutting projects at its Lüleburgaz facility. Preheating is applied when needed, appropriate machining allowance is planned for edges that will be welded, and parts are cooled in a controlled manner. We are here for your thick-plate cutting needs in the Thrace and Marmara region.