What is post-heating of steel?

Postheating can be applied directly after thermal cutting in particular for high-strength and hard steels. The purpose is to reduce residual stresses and to make hydrogen diffuse out of the cut pieces, thereby reducing the risk for hydrogen-induced cracking. A furnace or a gas flame can be used for post-heating. Using a flame calls for careful temperature control to prevent undesired changes in steel properties.

Is it necessary to preheat before cutting steel?

The purpose of preheating is normally to reduce the risk for hydrogen-induced cracking when applying thermal cutting methods. This risk grows with increasing plate thickness and hardness of the steel plates to be cut. This makes preheating an important factor to consider when cutting Hardox ® and most Strenx ® grades. Detailed recommendations can be found in the SSAB brochures found at SSAB Download center.

How can I avoid hydrogen-induced cracks from cutting steel?

You can minimize the risk for hydrogen-induced cracks in cut edges of Hardox ® and Strenx ® grades by following SSAB’s cutting recommendations. The recommendations consist of additional processes such as preheating, post-heating and slow cooling. Each additional process has recommended parameters that can be found in the SSAB cutting recommendation .

How does the surface quality effect the cut edge quality?

A dirty, painted or corroded plate surface is detrimental for the cut edge quality, although this depends on the method. For oxy-fuel and plasma cutting, the surface condition has a limited influence, during oxy-fuel cutting the speed might need to be reduced slightly if the surface is painted or corroded. For laser cutting, surface quality is very important and a very clean (naked) plate surface near the intended cutting line is needed.

What are hydrogen-induced cracks in steel?

A hydrogen-induced crack is a delayed crack phenomena that might occur during cutting of steel using thermal methods. The cutting process will contribute with some hydrogen, with temperatures stimulating diffusion of hydrogen and residual tensile stresses. The concentration of hydrogen in the steel, close to the cut, will increase. In hard steel in particular, the conditions necessary for hydrogen-induced cracking are now present. Since diffusion of hydrogen in the steel needs some time, cracking can be delayed for some days.

Finishing

Postheating is a reliable method for avoiding cut edge cracking. It is important that the postheating process occurs as soon as possible after the piece has been cut out. The time between the start of the cutting procedure and the start of the postheating procedure should be minimized and never exceed 60 minutes. Postheating can be accomplished either in a furnace or with torches.

How to postheat after thermal cutting?

Postheating in furnaces

When using furnaces, the temperature should not exceed the maximum allowable temperature listed in Table 6, and the plate must remain in the furnace until it reaches this temperature.
As a rule of thumb, the duration of postheating should be at least 5 minutes for every millimeter of plate thickness (i.e., 50 minutes for a 10 mm thick plate).
Postheating in a furnace allows more hydrogen migration from the HAZ, as well as a small as possible reduction of tensile stresses in the HAZ.
The minimum and maximum temperatures are the same as presented in Preheating temperature recommendations.
Heating in furnaces is the best way to preheat the plate, since it provides an even temperature distribution across the entire plate.
Alternatively, a simple and cost-effective solution for postheating is to use electrical heating elements and insulation blankets (such as Rockwool) or to build a heat treatment box for use with electrical heating.

An inexpensive and practical solution, a heat treatment box used with electrical heating

Postheating with a torch

Normally postheat treatment using torches is done manually and it is important to know how to control the temperature. When using torches, it's essential not to overheat. The temperature of the cut edge should not exceed 700 °C (1,292°F), preferably maintained between 300 to 500 °C (572-932 °F). This can be achieved by observing the color of the cut edge at the torch; it should just start to glow a very dark red. If the color is bright cherry red or dark orange, the temperature is too high, and the postheating may not be successful.

Manual postheat treatment using a torch

Glow very dark red at the torch

Infrared thermometer

Alternatively, the temperature can be controlled with an infrared thermometer, pointed directly to the cut edge at the flame. The heat from the torch will temper the re-quenched zone of the HAZ, which will reduce the tensile stresses at the cut edge. The torch used should have a rather big flame with low intensity. This will allow the heat to reach further into the material without heating it too much.

Slow cooling

Regardless of whether the cut parts are preheated, slow cooling reduces the risk of cut edge cracking. Slow cooling can be achieved if the parts are stacked together while still warm from the cutting process and then covered with an insulating blanket, allowing the parts to gradually cool down to room temperature.

Slow cooling by stacking the hot parts and covering them with insulating blanket.

Postheating soaking time recommendations

To achieve effective postheating, the entire cross-section of the plate must reach the target temperature, generally for all steel grades. A certain amount of time is required for the heat to spread throughout the plate. It's important to note that the surface should be maintained at the target temperature consistently during the soaking period. The graphs provided can be used to determine the required soaking time.

For example, when preheating a 100mm plate to 175 °C from both sides, when a surface temperature of 175 °C is reached, this temperature should be maintained for 11 minutes to ensure the core of the plate has the same temperature.

Required soaking time when heat is applied from both sides, for example heating in a furnace

Required soaking time when the heat is supplied from one side only, for example, heating with propane torches or heating with electrical mats

FAQs about cutting

More questions and answers about cutting

Advance to the next stage of steel processing

After achieving a clean, precise finish, welding is the key to building strength and structural integrity. Learn how to prepare, join, and optimize your steel components for reliable performance.

Start welding guidance

Didn't find what you were looking for?

Our team is just a message away — contact us and we’ll point you in the right direction.

Contact Technical support

The information in this report is only applicable to SSAB’s products and should not be applied to any other products than original SSAB products.

This report provides general results and recommendations for SSAB steel products. This report is subject to SSAB’s Terms of Use. It shall be the user's responsibility to verify that the information contained herein is correct and is suitable to be used for the particular purpose and application of the user. The report is intended to be used by professional users only who possess adequate expertise, qualification and knowledge for the safe and correct use of the results and recommendations in this report. This report is provided “as is”. The use of the report is at user’s own discretion and risk and that users will be solely responsible for any use of this report. SSAB disclaims any liability for the content or potential errors of this report, including but not limited to warranties and condition of merchantability or fitness for a particular purpose or suitability for individual applications. SSAB shall not be liable for any kind of direct or indirect damages and/or costs related to or arising therefrom, whether special, incidental, consequential or directly or indirectly related to the use of, or the inability to use, the report or the content, information or results included therein.