PhD opportunities

Investigation of embrittlement mechanisms of a martensitic stainless steel under extreme cooling conditions

Thesis proposal

Area of expertiseMaterials science and engineering
Doctoral SchoolSMI - Sciences des Mtiers de l'Ingnieur
SupervisorMme Anne-Franoise GOURGUES
Research unitCentre of materials
KeywordsStainless steel , Microstructure, Resistance spot welding , Fracture
AbstractThis research project aims at improving the current understanding of fracture mechanisms of the molten zone in high-strength, martensitic stainless steel resistance spot welds. A quantitative link is to be established between cooling conditions, microstructural development and fracture toughness, by characterizing phase transformations and applying the local approach to fracture.

Context and challenges: Lightweighting and improving safety of automobile vehicles requires the development of high strength steels with high forming ability. To this aim, two kinds of steel have been developed during the last decade by steelmakers, namely, cold-forming steels with high work hardenability and high strength, and hot stamping steels deformed at about 950C, that is hardened during the martensitic press-quench.
Thanks to their high chromium content (>10.5 wt%), martensitic stainless steels possess a high hardenability. They are therefore good candidates for press hardening as well as for indirect quenching (i.e., stamping + austenitization + quenching). For this reason, APERAM has defined a stainless steel family that suits this kind of applications. These steels are given their final microstructure and properties by a high-temperature (950 - 1050C) heat treatment for a few minutes, followed by press-quenching during stamping. Besides their hardenability, these steels exhibit yield strength and tensile strength that are robust with respect to the cooling rate. On the other hand, their fracture toughness seems more sensitive to this parameter, which affects the mechanical strength of resistance spot welds under Mode I loading.
The development of automotive stainless steel grades strongly requires their ability to be welded together, as well as with low alloy steels. This PhD project aims at improving current understanding of the link between the cooling rate, the microstructure and the toughness, especially of spot welds that experience very severe cooling rates. It will make use of experienced gained by the Centre des Matriaux and by Aperam concerning characterization of fracture of spot welds, in relation with their microstructure [Krajcarz et al. 2013 and 2016] and concerning the local approach to fracture (see e.g. [Tankoua et al. 2014, Mithieux et al. 2018]).

* Scientific objectives: The objective of this PhD project is to refine understanding of the origins of brittleness of weld metal, by linking cooling conditions, microstructural evolution and local toughness. This link is poorly known for these grades up to now. The results will allow proposing metallurgical or operating solutions in order to improve mechanical properties of resistance spot welds.

Methodology: Internal work at APERAM has already shown that the crack path was very reproducible and evidenced local heterogeneity in mechanical properties of the weld metal. The brittleness could originate from a change in precipitation or in solidification-induced localized segregations. The work will start with a survey of knowledge that will be of interest for the project. In particular, microstructural analysis methods that seem most efficient to characterize these microstructural features, metallurgical tools that can be used to vary the corresponding microstructural parameters, as well as most relevant mechanical tests will be critically assessed. This survey will make use of open literature data, of internal work in the company and of know-how that is available from the two laboratories, namely, APERAM and the Centre des Matriaux. The results of this survey will be capitalized in a report to be issued a few months after the beginning of this project.
In parallel, preliminary characterization is to be made of molten zones available from APERAM, in order to become familiar with their microstructure and fracture mechanisms.

Based on this information, mechanical tests will be carried out and analyzed in order to identify the crack path under well-controlled stress states. The solidification mode of the weld metal will be determined by scanning electron microscopy and electron backscatter diffraction (EBSD). Partitioning of alloying elements during solidification will be characterized, in particular using electron microprobe analysis, in order to evidence (or to reject) features that could be responsible for major segregation. Modelling of the solidification process could be carried out to better understand underlying mechanisms. The nature and the role of precipitation after cooling, in the weld metal and in the heat-affected zone, will also be quantified in detail using available tools (mainly, scanning electron microscopy observations of fracture surfaces and extractive replicas, transmission electron microscopy). Any other characterization method that will be proposed by the PhD student and that could be useful will be considered. This part of the project will determine first-order microstructural parameters that govern the fracture toughness of these welded zones.

The local approach to fracture will be applied in order to quantify the links between the microstructure, the fracture mechanisms and the cracking resistance of the weld metal, for a variety of microstructures to be produced during the project. A testing and analysis methodology will be set up in order to efficiently determine optimized microstructures, leading to spot welds with improved properties.

At the Centre des Matriaux, the work will be carried out within a team with several ongoing thesis projects on the fracture of martensitic steels, which will favor dynamic interactions between the students.

Expected results:
From an industrial point of view, this work will propose metallurgical and operating solutions to brittleness issues concerning welds made of this steel.
From the scientific point of view, the work will improve the link between the martensitic microstructure, chemistry, and toughness for extremely severe cooling rates.
As for the training of the PhD student, excellent skills in metallurgy and in microstructural investigations will be developed during the project; skills in mechanics and especially in fracture mechanics will also constitute a major training. Working in a high-level industrial R&D environment will be of particular interest for a candidate wanting to work in industry after the PhD completion. Combined metallurgy + mechanics skills are sought for by a number of industries. Initiation to teaching is possible. Scientific communication skills will be developed thanks to oral presentations and writing scientific journal articles.
ProfileEngineer and / or Master of Science - Good level of general and scientific culture.
Good level of knowledge of French (B2 level in French is required) and English. (B2 level in English is required)
Good analytical, synthesis, innovation and communication skills.
Qualities of adaptability and creativity. Teaching skills. Motivation for research activity. Coherent professional project.

Prerequisite (specific skills for this thesis): Metallurgy with a strong basis in mechanics of materials. Commitment in experiments and expertise in a context facing with strong industrial challenges.

Applicants should supply the following :
- a detailed CV
- a covering letter explaining the applicants motivation for the position
- detailed exam results
- two references : the name and contact details of at least two people who could be contacted to provide an appreciation of the candidate

to be sent to
FundingConvention CIFRE