Preheating and Cutting Speed for Thick Plate in Oxy-Fuel Cutting

In oxy-fuel cutting of thick carbon steel plate, preheat time and cutting speed cannot be considered in isolation. This article looks at preheating and cutting speed for thick plate in oxy-fuel cutting, how the right balance is set, and how a wrong choice affects part quality.

The Purpose of Preheating on Thick Plate

Oxy-fuel cutting works by raising the material to its ignition temperature and then cutting it through an oxidation reaction driven by the oxygen jet. On thin plate this temperature is reached quickly, but on thick plate enough heat must spread through the full cross-section before the cut begins, and preheat time becomes critical for achieving clean piercing at the start of the cut. Insufficient preheating causes a delayed pierce, an irregular start hole and quality loss in the first section of the cut line.

The Relationship Between Preheat Time and Plate Thickness

As plate thickness increases, the preheat time needed for the full cross-section at the cut edge to reach ignition temperature also increases. However, this relationship is not linear with thickness; it depends on cumulative heat buildup. Preheating longer than necessary creates a wide heat-affected zone at the start of the cut and can leave unwanted oxide buildup on the surface. For this reason, preheat time for each thickness range should start from a verified baseline confirmed by material certification and shop experience.

The Role of Cutting Speed on Thick Plate

Balancing Speed and Heat Input

When cutting speed drops, heat acts longer on the same cross-section, which on thick plate can overheat the cut edge and widen the heat-affected zone. When speed is kept too high, full penetration may not be achieved, particularly near the bottom surface of the plate, leaving uncut bridges along the cut line. On thick plate, the correct speed must sustain a steady, uninterrupted oxidation reaction along the cut while keeping heat buildup limited.

Speed Staging at the Start of the Cut

Starting the cut directly at full speed on thick plate can cause irregularity at the pierce point. For this reason, completing the pierce at a lower speed and then stepping up gradually to the target cutting speed improves both edge quality and the appearance of the start hole. This staging becomes especially important above roughly 30 mm thickness.

Cracking Risk and Material Sensitivity

In steels with a high carbon equivalent, insufficient preheating combined with rapid cooling after the cut can cause stress buildup at the cut edge and, in some cases, micro-cracking. This risk is more pronounced on thick plate and in a cold shop environment. On critical applications, the chemical composition on the material certificate should be reviewed, and preheat time may need to be raised above the standard value. This approach reduces thermal stress at the cut edge and makes subsequent welding or machining steps safer. For a closer look at the heat-affected zone and edge hardening, see our article on CNC oxy-fuel cutting: heat-affected zone (HAZ) and edge hardening control.

The Effect of Ambient Temperature and Plate Surface Condition

During periods when shop ambient temperature is low, the plate surface takes longer to heat, which means preheat time should be re-evaluated seasonally. On rusty, oily or painted surfaces, heat transfer becomes uneven and the same preheat time can give different results. Surface cleaning before cutting is an important precondition for consistent preheating results, especially on thick and critical parts.

Comparison With Alternative Cutting Methods

On very thick plate, oxy-fuel cutting is generally more economical than plasma or fiber laser cutting, but the preheating requirement extends the preparation time before cutting begins. To evaluate which cutting method suits a given thickness and amperage range, see our article on plasma cutting amperage, speed and plate thickness.

A Practical Approach to Quality Control

Before moving to series production, preheat time and cutting speed should be tested on a sample from the same plate batch, checking the appearance of the pierce point and the straightness of the cut edge. On critical projects, edge hardness measurement can verify whether preheating and speed settings are adequate. Values obtained from these test cuts can serve as a reference for later jobs with the same thickness and material.

DMK Makina's Approach to Oxy-Fuel Cutting of Thick Plate

Based in Lüleburgaz and serving customers in Thrace and Marmara as well as industrial projects across Turkey and Europe, DMK Makina sets preheat time and cutting speed for thick carbon steel plate according to material thickness and carbon equivalent. On critical parts, test cuts and edge inspection are carried out so that parts are delivered ready for subsequent welding or machining steps.

Conclusion

Preheating and cutting speed for thick plate in oxy-fuel cutting are two complementary parameters; when they are not properly balanced, cracking risk, irregular piercing and loss of edge quality follow. Setting preheat time according to thickness and material chemistry, and choosing cutting speed to keep heat buildup limited, are decisive for both quality and production efficiency.