Effect of Melt Quenching on Martensite Transformation in Fe-Ni Alloy

The main features of martensite transformation in melt-quenched Fe-31.4% Ni alloy on cooling below room temperature have been studied. It is found that the ribbon 50～60 μm thick, prepared by spinning technique, is a natural composite in which isothermal and surface martensite are not formed, while athermal martensite forms at lower temperature, all factors being the same, as compared to the alloy of the same composition and grain size, prepared by recrystallization.

MARTENSITE TRANSFORMATION MICROSTRUCTURE OF 40Cr STEEL COMPLEXLY INDUCED BY LASER SHOCK

40Cr steel is laser quenched by the NEL-2500A rapidly axial flow CO2 laser. Then the martensite induced by laser quenched is shocked by Nd：YAG laser again. Through comparing and analyzing the appearance and size of martensite, the dislocation density in microstmcture between the treated zones by laser quenched and by laser quenched plus laser shock, the following results are shown： The second martensite obtained by laser compound treatment is more fmer compared with those obtained by laser quenched; In the hardened zones obtained by compound treatment, a lot of slender second twin crystal martensites are induced; A lot of more high density dislocation tangles and cellular dislocations are generated. From the transmission electron microscope （TEM） micrograph after compound treatment, there are not only long lath and short nubbly martensites arranged in cross direction, but also massive nubbly and small short nubbly martensites arranged in longitudinal direction. Some martensites look like the broken blocks of quenched martensites. These new martensites are inserted transversely in the quenched martensites with large tangle. And they make quenched martensites break into pieces. Compared with the quenched martensites, the size of fresh martensites are smaller, about 0.3-0.5 μm.

Analysis of Microstructure and Mechanical Properties of Ultrafine Grained Low Carbon Steel

A novel design scheme of hot stamping, quenching and partitioning process was conducted in a quenchable boron steel to obtain the nanometric duplex microstructure comprising ultrafine retained austenite and martensite. It is shown that the materials possess excellent mechanical properties and the ductility can be further improved without compromising the strength. The newly treated steel shows excellent mechanical properties and the total elongation of the steel increases from 6.6% to 14.8% compared with that of hot stamped and quenched steel. Therefore, this kind of steel has become another group of advanced high-strength steels. The microstructure which is mainly responsible for such excellent mechanical properties was investigated.

Microstructure and mechanical properties of 20Si2CrNi3MoV steel treated by HDQP process

A combined process of hot-deformation plus two-step quenching and partitioning （HDQP） treatment was applied to a low carbon 20Si2CrNi3MoV steel, and transmission electron microscopy （TEM）, scanning electron microscopy （SEM）, X-ray diffraction （XRD）, Vickers hardness and tension test were used to characterize the microstructure and mechanical properties. More stable retained austen ite due to fine microstructures and typical curved micromorphology is obtained, and the newly-treated steel obtains more retained austenite because of the effect of hot deformation. The retained austenite fraction increases and then decreases with the increasing quenching temperature from 200 to 350 ℃. The maximum retained austenite fraction （18.3 % ） and elongation （15 % ） are obtained to enhance the ductility.

Prediction and Experimental Validation of Forming Limit Curve of a Quenched and Partitioned Steel

Forming limit curve（FLC）is an effective tool to evaluate the formability of sheet metals.An accurate FLC prediction for a sheet metal is beneficial to its engineering application.A quenched and partitioned steel,known as QP980,is one of the 3rd generation advanced high strength steels and is composed of martensite,ferrite and a considerable amount of retained austenite（RA）.Martensite transformation from RA induced by deformation,namely,transformation induced plasticity（TRIP）,promotes the capability of work hardening and consequently formability.Nakazima tests were carried out to obtain the experimental forming limit strains with the aid of digital image correlation techniques.Scanning electron microscopy（SEM）was employed to examine the fracture morphologies of Nakazima specimens of the QP980 steel.The observed dimple pattern indicated that tensile stress was the predominant factor which led to failure of QP980 specimens.Therefore,maximum tensile stress criterion（MTSC）was adopted as the forming limit criterion.To predict the FLC of QP980 steel,Von-Mises yield criterion and power hardening law were adopted according to the tested mechanical properties of QP980 steel.Results were compared with those derived from other three representative instability theories,e.g.Hill criterion,Storen-Rice vertex theory and Bressan-Williams model,which shows that the MTSC based FLC is in better agreement with the experimental results.