In oxy-fuel cutting, material is heated to its ignition temperature before being cut, which creates a zone adjacent to the cut line whose metallurgical structure changes. This zone is called the heat-affected zone (HAZ), and its width and hardness profile directly determine whether a part is suitable for downstream processing. In this article, we look at how the heat-affected zone forms in CNC oxy-fuel cutting and how edge hardening can be controlled.
What Is the Heat-Affected Zone (HAZ)?
The heat-affected zone is the narrow band of material adjacent to the cut line whose grain structure and mechanical properties change due to the high heat generated by the oxidation reaction during cutting. The material in this zone does not melt, but because it is briefly exposed to high temperature and then cools rapidly, its microstructure transforms. In carbon steels, this transformation can produce a noticeable increase in hardness at the cut edge. Depending on the cutting method, material thickness, and parameters used, HAZ width typically ranges from a few tenths of a millimeter to several millimeters.
Parameters That Affect HAZ Width
Cutting Speed and Heat Input Relationship
As cutting speed decreases, heat acts on the same section of the cut line for longer, widening the heat-affected zone. Cutting too fast, on the other hand, can prevent full penetration, so speed selection must balance both cut quality and HAZ width. When parameter and speed settings by material thickness are set correctly, HAZ width also stays within predictable limits.
Preheat Time and Temperature Distribution
Keeping preheat time longer than necessary causes a wider area of the material to heat up before cutting begins, widening the HAZ as a result. Limiting preheating to the minimum time required for the thickness keeps the heat effect concentrated closer to the cut line. Excessive preheating can also negatively affect the material's overall hardness and toughness balance.
Material Thickness and Carbon Content
Thicker materials require greater total heat input, which widens the HAZ. In addition, steels with higher carbon content show a more pronounced hardening tendency (martensite formation) during rapid cooling; for this reason, edge hardness after cutting should be specifically checked on materials with a high carbon equivalent (CE). A similar sensitivity is observed in low-alloy high-strength (HSLA) steels.
The Mechanism of Edge Hardening
During cutting, the edge region reaches a high temperature and is then rapidly cooled by the surrounding cold material. This rapid cooling can produce a hard, brittle microstructure (martensite or bainite), particularly in medium- to high-carbon steels. On 30-40 mm thick carbon steel plates, the hardness measured at the cut edge can be noticeably higher than that of the base material. A hardened edge can increase tool wear in subsequent drilling or machining operations and, in some applications, creates a risk of cracking.
Impact of HAZ and Edge Hardening on Downstream Processing
On parts prepared for welding, a hardened edge zone can negatively affect weld quality and penetration; for this reason, critical welded joints may require the cut edge to be ground or annealed. Similarly, in machining steps such as drilling or tapping after cutting, the hardened zone can shorten tool life and extend machining time. This effect should be factored into cost planning, especially in series production.
Methods to Reduce HAZ Width
Cutting Sequence and Heat Management
The arrangement of parts on the sheet affects heat buildup between adjacent cut lines. When cuts are closely spaced, the material may be exposed to heat again before fully cooling, causing HAZ width to increase cumulatively; the cutting path should therefore be planned to limit heat buildup. On larger parts, varying the cutting sequence can distribute total heat input more evenly, reducing both HAZ width and part distortion.
Comparison with Alternative Methods
For tight-tolerance projects where HAZ sensitivity is high, alternative methods such as plasma or fiber laser cutting can offer a smaller heat-affected zone. The oxy-fuel, plasma and fiber laser cutting comparison covers in detail which method is best suited to which thickness and tolerance range.
Measuring HAZ in Quality Control
In critical applications, hardness testing (such as Vickers or Rockwell) on sample parts after cutting can verify HAZ width and hardness profile. A metallographic cross-section can also be taken for microstructure examination, which visually confirms the HAZ boundaries and microstructural change. In series production, these measurements are repeated periodically to detect parameter drift in a timely manner.
DMK Makina's Approach to HAZ Management in Oxy-Fuel Cutting
Based in Lüleburgaz and serving customers across the Thrace and Marmara regions, DMK Makina controls heat input and cutting sequence in oxy-fuel cutting of thick carbon steel parts to deliver predictable heat-affected zone (HAZ) width and edge hardness. On critical projects, post-cut hardness verification and edge grinding, when required, are also provided to ensure parts are suitable for subsequent welding or machining steps.
Conclusion
The heat-affected zone and edge hardening directly affect whether parts produced by oxy-fuel cutting are suitable for downstream processing and welding. Correctly managing cutting speed, preheat time, and cutting sequence keeps HAZ width limited and predictable, improving both production quality and cost efficiency.
