New process for predicting cold cracking in welding of high-strength steels could lower development times and costs
Cold cracking in the weld metal and the heat-affected zone (HAZ) when welding high-strength steel is a well-known problem that industry has spent decades trying to prevent through various mechanisms. Cold cracks usually form when the weld cools down below 250 °C (hence the term “cold crack”). Cold cracking is also known as hydrogen cracking or delayed cracking, and is associated with the formation of brittle microstructures as martensite in the presence of diffusible hydrogen as well as of tension stresses.
Whether such cold cracking occurs, and how quickly, depends on how high the concentration of hydrogen in the steel is, how the residual stress turns out, and how its microstructure is configured. Predicting the probability of cracking has been difficult up to now. This has presented major quality assurance challenges for sectors such as automotive, which is looking to the lighter weight, higher strength steels as a means to reduce vehicle weight and hence fuel consumption.
Now, scientists at the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg, in conjunction with the Chair of Joining and Welding Technology LFT at Brandenburg University of Technology Cottbus (BTU) developed a new process for making cold cracking more predictable.
The new process can determine, as early as the design stage, if critical conditions for such damage can be prevented. This lowers development times and costs.
Manufacturers used to conduct expensive testing, for example by applying an increasingly higher tensile stress to a sample component, and then analyse what stress level would cause cracking. Not only are these tests time-consuming and cost-intensive, the findings can not be applied to subsequent components on a one-to-one basis because the geometry of the component has a decisive influence on crack formation. Even currently available computer simulations failed to deliver the desired predictive accuracy for real components.
The new approach could markedly reduce such costly methods in the future—thus lowering production costs while shortening development phases.
The experts at LFT set up a special test to determine precisely the cracking criterion on samples of high-strength steel. Beside typical influencing factors such as hydrogen content, residual stresses and material structures that can be adjusted in at the same time, they also take into account the temperature gradients that emerge in the welding process.
The experts at IWM then feed a computer simulation with this criterion in order to analyze the threat of cold cracking in random components and geometries. The researchers can also get a preliminary look at the effects of any countermeasures, and make the necessary adjustments. To do so, they transfert the results back into the simulation, in order to fine-tune them there.
In the future, with the aid of this process, manufacturers of vehicles and machines could be able to define non-critical welding parameters and limiting conditions for their materials in advance and thus establish a substantially more efficient and safer production process.
This is especially relevant to materials that are difficult to weld, with very narrow processing windows regarding welding parameters or the pre- and post-heating temperatures. Fraunhofer IWM and LFT, in cooperation with Robert Bosch GmbH and ThyssenKrupp Steel Europe AG, are currently testing their new process on laser beam-welded demonstration models made of high-strength steels.
O. Dreibati et al. (2012) Hydrogen Charging of High Strength Steel Specimens and Physical Simulation of Cold Cracking under Laser Beam Welding Conditions. Materials Science Forum, 706-709, 1391 doi: 10.4028/www.scientific.net/MSF.706-709.1391