Sheet metal forming is currently a fundamental manufacturing process, used to produce components for several industrial sectors. Especially under automotive and aerospace driving force, sheet steels with improved mechanical performances are increasingly demanded to meet lightweight and cost reduction targets.
However, these innovative materials involve a series of failure risks (cracks, stretching, thinning). Therefore, quality controls are more and more needed to assure part quality.
FormPlanet aims to offer the sheet metal forming industry UNIQUE testing methodologies for a near-zero defect production
To reach the goal of defect-free parts, in-line process monitoring and failure preventive detection techniques are required. Within FormPlanet project, innovative non-destructive inspection techniques will be developed: 3MA systems (Micromagnetic Multiparameter Microstructure and stress Analysis in cooperation with Fraunhofer IZFP Saarbrücken), laser tracking, advanced thermography and also an industrial on-line diffusible hydrogen measurement test associated to the Small Punch Test.
Hydrogen absorption in hot stamping
One of FormPlanet case studies will be focused on USIBOR 2000, an Advanced High Strength Steel (AHSS) dedicated to hot stamping processes. It combines a remarkable mechanical performance with the possibility to achieve complex geometries. Nevertheless, high strength makes the material to be potentially susceptible to hydrogen delayed fracture.
Delayed fracture is a phenomenon occurring when materials, environment and stress interact with each other, producing an embrittlement effect and, thus, component failure. It can occur during manufacturing or directly in service life.
Hydrogen delayed fracture is an unpredictable phenomenon. It can happen during manufacturing or during the service life of the part
Water vapour in the hot pressing furnace atmosphere can dissociate on steel surface, allowing a certain amount of atomic hydrogen to enter to the metal lattice. Metal surface oxidation and consequent hydrogen absorption is favored in the case of aluminum based coating. Moreover, high temperature promotes its diffusion through the steel. When back at room temperature, hydrogen remains trapped into steel because the Al-Si coating acts as a strong barrier, leaving hydrogen the possibility to diffuse and causing the hydrogen embrittlement phenomenon. Furnace process parameters as dew point and dwell time are critical and have a great influence on hydrogen absorption occurrence.
Online part quality assessment
Within part quality assessment and in-process measurements, the optimisation of a new equipment and methodology is carried out to check diffusible hydrogen content directly online on semi-products with a completely non-destructive method, avoiding any sample or surface preparation for the measurement, not even coating removal.
Check diffusible hydrogen content online with a completely non-destructive method, avoiding any sample or surface preparation
Today none of press hardening or AHSS manufacturers have any inspection procedure in terms of hydrogen content in production plant. A reliable procedure, involving non destructive test method will be developed within FormPlanet project and made available for the Test Bed.
Online hydrogen measurement is part of Test Bed service catalogue
It consists in a combination of a simplified mechanical test, the Small Punch Test, and a diffusible hydrogen measurement by means of a dedicated portable equipment: HELIOS 4 HOT PROBE.
Once the critical diffusibe hydrogen content of a metal is defined in laboratory, and specific material properties (thickness, coating, etc.) are calibrated, the instrument will be able to check the hydrogen content directly on components during plant production. Parts exceeding the threshold value, which may be able to induce a delayed fracture under a static load, will be identified and corrective actions applied.
HELIOS 4 HOT PROBE redesign is aimed at guaranteeing major measurement reliability and reach Industry 4.0 objectives.
Figure 1: HELIOS 4 HOT PROBE equipment and HELIOS Manager features.
Linda Bacchi
Master degree in mechanical engineering at the University of Pisa. She started working on Hydrogen Embrittlement during her graduation thesis, investigating the effect of tempering temperature on the hydrogen embrittlement susceptibility of supermartensitic stainless steels. Part of Letomec R&D team, her main research interests are HELIOS platform for hydrogen-steel interaction study (Letomec srl patent), material characterisation and mechanical testing.
