Doctor of Philosophy
Mechanical and Materials Engineering
Anthony Gerald Straatman
As two important austenitic steels, the austenitic stainless steel (SS) and austenitic high-Mn steel have been seeking a decent yield strength to match their versatile exceptional advantages. However, the traditional, severe, and high-strain-rate plastic deformation techniques currently used to strengthen the austenitic steels, are limited by their ability to produce ultrastrong steel, bulk sized steel, and the fracturing risk they pose to the steel, respectively. In this research, we propose an industry-applicable 3-axis stress-released impact process, which addresses the challenges facing the current techniques and successfully prepares the body-thorough ultrastrong and ductile austenitic steels without fracturing risk.
The bulk 316L SS treated by this process achieves a to date highest yield strength (YS) of 1552 MPa and an ultimate tensile strength (UTS) of 1640 MPa, and a reasonable elongation to fracture (El) of 5.2%. More appealing tensile property, with the YS, UTS and El becoming 1274 MPa, 1398 MPa and 10.3%, respectively, is achieved by the 316L SS after we make an optimization on this novel process. The bulk Fe-20Mn-0.88C steel treated by this process achieves a YS of 735 MPa, a UTS of 1054 MPa, and an El of 40.2%, which is the ever best strength and ductility synergy achieved by this steel by a simple process. The high strengths of the austenitic steels are mainly attributed to the extraordinary grain-refining ability of this novel process, and the elements segregation induced by the annealing treatment in the SS contributes extra strengthening effects. The great ductilities of the austenitic steels are attributed to the high volume of twins produced by this novel process.
The high-Mn steel experiencing the uniaxial impact plus annealing treatment displays severe PLC effect. By examining the structures and the concentrations of C-Mn atom pair at different stages of an individual serration, we prove that the PLC effect is not caused by the direct interaction between the dislocations and the C atoms, as the previous models proposed. Instead, it is caused by two separate processes involving the returning of the C atoms from the tetrahedral sites to the octahedral sites, and the accumulation of the depleted dislocations.
Summary for Lay Audience
The austenitic stainless steel (SS) and the austenitic high-Mn steel gain vast applications with their multiple excellent advantages. However, these two types of steels are limited by a weak strength which has become the Achilles' heel for their broader application. The current processes used to strengthen these two steels have problems either in the limited strengthening ability, the limitation on the size of the steel, or the fracturing risk they pose to the steel. In this research, we address these problems by applying a novel 3-axis impact process with intermediate stress-releasing annealing treatment.
A bulk austenitic 316L SS after treatment by our process achieves a record high yield and ultimate tensile strength (YS and UTS), and a reasonable elongation to fracture (El) without facing fracturing risk. An even better strength and ductility combination including an almost doubled El and gently reduced YS and UTS, are achieved by the SS after we make an optimization on this process. A bulk Fe-20Mn-0.88C steel treated by our process also obtains an ever-best strength-ductility synergy that can be achieved by a simple method. The high strength of the two steels is attributed to the extraordinary ability of our process in producing small nano sized grains and twins in the materials. The elements segregation induced by the annealing treatment in the SS contributes extra strengthening effects. The great ductility of the two steels is attributed to the high volume of twins this process produces in the materials.
Except the novel processes, we reveal a new mechanism explaining the PLC effect based on the research on the large serrations appearing in tensile curves of the Fe-20Mn-0.88C steel. We prove that the PLC effect is not caused by the direct interaction between the dislocations and the solute atoms, as the previous models proposed. Instead, it is caused by two separate processes involving the returning of the C atoms from the tetrahedral sites to the octahedral sites, and the accumulation of the depleted dislocations.
Zhang, Busheng, "Mechanical property of body-thoroughly enhanced austenitic stainless steel and high-Mn steel by industry-applicable 3-axis stress-released impact" (2023). Electronic Thesis and Dissertation Repository. 9151.