Preparation And Properties Of Gradient Dual Phase Steel Via Torsion Process

Strengthening and toughening of metals and alloys are the key topics in the field of materials research.Unfortunately,most methods that effectively increase the strength of the materials can lead to drop of elongation.This phenomenon is called“strength–ductility trade-off” relationship.In recently years gradient structure materials have attracted much attention due to solving this problem.Gradient structured materials show a superior high strength,high plasticity,good surface wear resistance and fatigue resistance.However,there are still many limitations in the preparation methods of gradient structured materials,such as low universality,expensive equipment,small sample size and poor interface bonding.In addition,the new technique for introduction of gradient structure and the influence of gradient structure on mechanical properties are few reported at home and abroad.When studying the phase transformation strengthening of AISI 304 austenitic stainless steel(304 SS),we proposed a novel method which simultaneously strengthens and toughens austenitic steels by introducing gradient martensite phase.Commercial cold rolled 304 SS rods with a diameter of 10 mm were selected as the starting materials.The starting materials were treated by quenching after solution treatment at 1050 °C for 0.5 h.The as-quenched samples have a fully uniform austenitic structure.In this work,we found that gradient deformation induced martensite(?'-M)particles,with a volume fraction increasing from core to surface,can be obtained in the cylindrical rods by applying free-end-torsion(FET).We compared the microstructures and tensile properties of gradient-structured 304 SS prepared by unidirectional-torsion(UT)and cyclic forward/reverse torsion(CFRT).The results shows that piled-up dislocations formed near the core region play a key role in the subsequent tensile deformation,and control the strain-hardening ability of the FET treated samples.The gradient ??-M enhances the strength of the surface layer and improves the tensile properties of the FET treated samples.CFRT torsion process have been developed on the basis of UT.It was found that under CFRT the 304 SS can be twisted 1000 cycles without fracture,and the cumulative torsion angle increase by 1-2 orders of magnitude compared to UT.Compared to UT,CFRT is more effective in inducing martensitic transformation,and enhances the gradient distribution of the ??-M.These findingsprovide a pathway for developing high strength and good ductility steels and other alloyed metals via gradient distributed second phase particles.Additionally,the 304 SS rods were also processed by CFRT treatments under low-speed and high-speed torsion at room temperature.Microstructures in the core and surface layers of the CFRT-treated samples were systematically characterized,and the effect of twisting speed on torsion behavior and the strain-hardening mechanisms in tensile of the gradient structured steels was also discussed.Results show that the CFRT treatment can introduce martensite phase on the surface of the rods via strain-induced martensitic transformation.Compared to low-speed torsion,high-speed torsion sample surfaces have higher martensite content,and lower stacking faults(SFs),NTs,and dislocation entanglements(DTs)in the sample surface.That finding suggests that high-speed torsion is more effective in inducing martensite in the surface layer during CFRT treatment.During the stretching process,the overall strain-hardening behavior of the gradient material is related to the content of its gradient defects.Higher gradient martensite content results in a higher surface hardness of the material,but less overall tensile properties.All the results show that the introduction of gradient microstructures in 304 SS,such as gradient martensite distribution,gradient NTs and gradient dislocations,can effectively enhance and toughen austenitic stainless steels.Compared with the traditional plastic deformation technology,the gradient structure introduced by the torsion process can make the 304 SS have higher yield strength and good plasticity.It is expected to achieve lightweight design in the field of high-strength steel for automobiles.Moreover,the torsion process is simple,low-cost,and allows for large-scale sample processing.Different heat treatment and surface treatment processes can be used before and after torsion to achieve different application purposes.Therefore,the torsion process has a broad research value and a wide range of application prospects in preparing high-performance gradient materials.