Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Evaluating risk of hydrogen embrittlement: new simulation of cold cracks in high-strength steels

03.05.2018

High-strength steels play a vital role in the construction of modern vehicles and machines. If these steels are welded during the production of components, mobile hydrogen atoms can cause problems within the material: the atoms accumulate slowly at highly stressed areas of a component, resulting in the steel becoming brittle at these locations. This can result in so-called cold break formations which can lead to component failure. Dr. Frank Schweizer of the Fraunhofer Institute for Mechanics of Materials IWM has developed a simulation method with which component manufacturers can assess cold break tendencies and adjust their production accordingly.

Manufacturers of vehicle and machine components often use high-strength steels to save material in lightweight construction and for crash-relevant structural components that require exceptionally high durability. When welding these components, various factors may lead to the unwanted formation of fine cracks, which may spread and even lead to component failure.


Light microscopy image of a welded connection’s weld structure.

© Fraunhofer IWM


Laser welded connection’s weld structure (left); calculated locally diffusible hydrogen concentration depending on temperature-time-development and welding residual stress (right).

© Fraunhofer IWM

Unfortunately it is very hard or impossible to assess these factors with experiments, for example hydrogen concentration would have to be measurable in highly localized points inside the component during welding. Another difficulty: the period of time during which a break may forms is relatively long – it can form within seconds during the welding process or after a few days. This is why the reject rate of laser welded high-strength steel components to this day remains uncomfortably high.

Illustrating the influence of hydrogen traps

To enable component manufacturers to reduce the reject rate of high-strength steels, Dr. Frank Schweizer, a member of the »Microstructure, Residual Stress« group at the Fraunhofer IWM, refined industrially used methods of numerical welding simulation in his dissertation. Now he can recreate any events at highly localized component points on his computer. This even functions for very quick changes between room and melting temperatures, as they occur during welding.

»Now we can calculate the temporal development of influencing factors and their intersections accurately and monitor them virtually«, explains Schweizer. These factors include hardening structures, residual stress and localized hydrogen concentration, which can lead to crack formation in the welded component. Alongside the hydrogen which is introduced during the welding process, previously existing hydrogen in the steel is loosened up by the welding heat and rendered movable and diffusible.

»The special feature of this new method is that it also takes into account the effect of so-called hydrogen traps«, says Schweizer. He discovered that hydrogen traps greatly influence the »movable« hydrogen occurring in low hydrogen concentration for different laser welding connections. With higher hydrogen content the thermomechanical behavior of the material grows more relevant for the formation of breaks.

»The hydrogen atoms slowly collect in the narrow area of the heat-affected zone where tensile residual stress is especially high«, says Schweizer. Even after the steel has cooled down, hydrogen can collect at these points and the steel grows brittle. »Thus even after hours or days cracks may form which must lead to the component’s rejection«, explains Schweizer.

Simulation as basis for optimization of laser welding process

Simulation results are the basis for optimizing laser welding processes and sustainably preventing component failure: »Now the laser process parameters can be adjusted to keep interactions of cold crack risk factors as low as possible«, says Schweizer. More precise pre-heating and post-heating temperatures, as well as custom-fit annealing durations can be determined from simulations.

»During the planning of components a simulation is useful as well: Following the data, better component shapes can be deduced to improve local stress points and avoid cracks«, explains Schweizer. In the next step of his research, Schweizer wants to examine the influence of various materials and component surfaces on the so-called effusion of hydrogen more closely in order to more precisely interpret pre-heating and post-heating procedures in the future. Furthermore he will use this new methodology on more steels and different welding procedures.

As a data basis for Schweizer’s expanded numerical welding simulations, characteristic material values of three different high-strength steels were used: bearing steel, martensitephase steel and fine-grained steel. Characteristic material values were calculated experimentally as well as using both new and his own numerical evaluation methods. Schweizer tested his simulation models successfully in three industrially used components which were joined using different welding procedures: heat conduction welding as well as deep welding via fiber lasers and CO2 lasers.

Weitere Informationen:

https://www.iwm.fraunhofer.de/en/press/press-releases/24_04_18_evaluatingriskhyd... - Link to Press Release

Katharina Hien | Fraunhofer-Institut für Werkstoffmechanik IWM

More articles from Machine Engineering:

nachricht MOONRISE: Bringing 3D printing to the moon – Moondust melted under lunar conditions
01.07.2020 | Laser Zentrum Hannover e.V.

nachricht Strong and ductile Damascus steels by additive manufacturing
25.06.2020 | Max-Planck-Institut für Eisenforschung GmbH

All articles from Machine Engineering >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Electrons in the fast lane

Solar cells based on perovskite compounds could soon make electricity generation from sunlight even more efficient and cheaper. The laboratory efficiency of these perovskite solar cells already exceeds that of the well-known silicon solar cells. An international team led by Stefan Weber from the Max Planck Institute for Polymer Research (MPI-P) in Mainz has found microscopic structures in perovskite crystals that can guide the charge transport in the solar cell. Clever alignment of these "electron highways" could make perovskite solar cells even more powerful.

Solar cells convert sunlight into electricity. During this process, the electrons of the material inside the cell absorb the energy of the light....

Im Focus: The lightest electromagnetic shielding material in the world

Empa researchers have succeeded in applying aerogels to microelectronics: Aerogels based on cellulose nanofibers can effectively shield electromagnetic radiation over a wide frequency range – and they are unrivalled in terms of weight.

Electric motors and electronic devices generate electromagnetic fields that sometimes have to be shielded in order not to affect neighboring electronic...

Im Focus: Gentle wall contact – the right scenario for a fusion power plant

Quasi-continuous power exhaust developed as a wall-friendly method on ASDEX Upgrade

A promising operating mode for the plasma of a future power plant has been developed at the ASDEX Upgrade fusion device at Max Planck Institute for Plasma...

Im Focus: ILA Goes Digital – Automation & Production Technology for Adaptable Aircraft Production

Live event – July 1, 2020 - 11:00 to 11:45 (CET)
"Automation in Aerospace Industry @ Fraunhofer IFAM"

The Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM l Stade is presenting its forward-looking R&D portfolio for the first time at...

Im Focus: AI monitoring of laser welding processes - X-ray vision and eavesdropping ensure quality

With an X-ray experiment at the European Synchrotron ESRF in Grenoble (France), Empa researchers were able to demonstrate how well their real-time acoustic monitoring of laser weld seams works. With almost 90 percent reliability, they detected the formation of unwanted pores that impair the quality of weld seams. Thanks to a special evaluation method based on artificial intelligence (AI), the detection process is completed in just 70 milliseconds.

Laser welding is a process suitable for joining metals and thermoplastics. It has become particularly well established in highly automated production, for...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

International conference QuApps shows status quo of quantum technology

02.07.2020 | Event News

Dresden Nexus Conference 2020: Same Time, Virtual Format, Registration Opened

19.05.2020 | Event News

Aachen Machine Tool Colloquium AWK'21 will take place on June 10 and 11, 2021

07.04.2020 | Event News

 
Latest News

Rising water temperatures could endanger the mating of many fish species

03.07.2020 | Life Sciences

Risk of infection with COVID-19 from singing: First results of aerosol study with the Bavarian Radio Chorus

03.07.2020 | Studies and Analyses

Efficient, Economical and Aesthetic: Researchers Build Electrodes from Leaves

03.07.2020 | Power and Electrical Engineering

VideoLinks
Science & Research
Overview of more VideoLinks >>>