Cut edge cracking
Cut edge cracking is a phenomenon that is closely related to hydrogen cracking in welds and occurs when thermal cutting methods are used. If cut edge cracks should occur, they will become visible between 48 hours and up to several weeks after the cutting. So, cut edge cracking can be regarded as delayed cracking. The risk of cut edge cracking increases with the steel hardness and plate thickness. How to reduce the risk of cut edge cracking is described below.
Cut edge cracking is closely related to the hydrogen content and residual stresses in the steel plate. It is therefore of interest to reduce the hydrogen content as well as the residual stresses, which can be done in different ways:
- Preheating the plate
- Post heating
- Reduced cutting speed
- Combination of preheating, post heating and reduced cutting speed together with a prolonged cooling process of HAZ
One method to avoid hydrogen cracking when cutting is to preheat the material and cut it while the material is warm. Preheating can preferably be used prior to oxy-fuel cutting and plasma cutting with oxygen as plasma gas.
Regarding all type of laser cutting and plasma cutting with nitrogen, preheating is not recommended due to its negative effect of the cut edge quality.
Depending on the situation, either part of the plate or the entire plate can be heated. The way to do this can be:
- Heating furnace
- >Preheating torches
- Electrical mats
Heating in furnaces is the best way to preheat due to that it results in an even temperature of the entire plate. Preheating torches can also be applied for preheating of Hardox and Strenx plates see figure 3. It is of importance that the torches are in motion so that the temperature of the plate does not exceed maximum preheating temperature. Further, the preheating temperature is measured on the opposit side of where the preheating is applied.
Electrical mats is a slow preheating method, so to preheat to 150-200 °C a good practice is to preheat overnight and begin the cutting operation the next morning.
Post heating is a reliable method in order to avoid cut edge cracking. This can either be done in a furnace or with torches. The easiest method is to use torches since they are widely spread in industry, furnaces are not so common. It is important that the post heating process takes place as soon as possible after the part has been cut out. The maximum time is 30 minutes between start of cutting and start of post heating procedure. It is of importance not to heat the material too much.
Using furnaces the temperature should not exceed the maximum allowable temperature listed in table 2 and the plate has to stay in the furnace until it reaches this temperature. Depending on the thickness of the plate the time will vary, but as a general rule of thumb the time of post heating should be at least 5 minutes for every mm of plate thickness (i.e. 50 minutes for a 10 mm thick plate).
Using torches, figure 4, it is of importance to not overheat. The temperature of the cut edge shall not exceed 700 °C. Normally post heat treatment using torches is done manually and in this case it is of importance to know how to control the temperature. This is done by looking at the color of the cut edge just behind the torch, it should just start to glow (very dark red). If the color is bright sherry or dark orange the temperature is too high and the post heating will not be successful and has to be redone. If the post heating is done in strong light (outside in the sun) it is harder to determine the temperature, so if possible perform the post heating indoors.
Reduced cutting speed
When cutting speed is reduced, the material heats up around the cut front and the heat affected zone will be wider. This affects the residual stresses in such a way that the risk of cut edge cracking is reduced. One should though bear in mind that reduced cutting speed is not as reliable as preheating or post heating and should only be used as a substitute if, for instance, the workshop does not have appropriate pre/post heating equipment.
If reduced cutting speed is used it is important that the cutting speed doesn’t exceeds the one listed in in this document, otherwise the risk for cutting cracks won’t be reduced at all.
Regardless of whether or not preheating of the cut parts are employed, a slow cooling rate will reduce 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 are covered with an insulating blanket. Allow the parts to cool slowly down to room temperature.
Sharp corners will act as stress raisers and since hydrogen cracking is closely related to residual stresses, sharp corners will increase the risk for cut edge cracking. This is true for all cutting methods both thermal and cold cutting methods like AWJ cutting. If the following actions are considered the risk for cracks will decrease:
- >If possible avoid sharp “inward facing” corners
- If possible use smooth geometries
- When sharp corners can’t be avoided, make a circular loops around “outward facing” corners.
- >If the cutting operation is to be stopped (I.e. overnight) make a clean cut to remove stress raisers