Microstructure And Mechanical Properties Of Cold-rolled And Cryogenic Fe-32Ni Alloys After Annealing

In this paper,Fe-32 Ni alloy was used as the initial material,the microstructure and mechanical properties of the material after cold rolling and cryogenic annealing were studied.The microstructure of Fe-32 Ni alloy is completely austenitic at room temperature,and the martensitic transformation point is below room temperature,cryogenic cooling in liquid nitrogen can cause martensitic transformation of the tissue,it is an ideal material for martensite and martensitic transformation research.The microstructure of the test steel was characterized and analyzed by metallographic microscope,scanning electron microscope,transmission electron microscope and X-ray diffraction,the hardness and strength of the test steel were tested by microhardness tester and tensile tester.(1)First,the test steel was subjected to cold rolling deformation of 0 %,42 %,55 %,70 %,85 %,90 % and cryogenic(-196 °C)treatment.Cold rolling deformation produces fine grain strengthening and work hardening effects,meanwhile the martensitic transformation in cryogenic process is inhibited.The larger the cold rolling deformation,the higher the strength,the less martensite production.The yield strength of undeformed cryogenic sample is 420MPa and martensite content is 68 %,the yield strength of 90 %cold-rolled and cryogenic samples is 618MPa,and the martensite content is 32 %.The larger the cold rolling deformation,the smaller the martensite size.(2)Then,90 % cold-rolled and cryogenic samples were annealed at450-850 °C(50 °C is a gradient).When the annealing temperature is between 450 °C and600 °C,tissue response occurs,when the annealing temperature is 650 °C,the recrystallization is completed,as the temperature continues to rise,the average grain size increased gradually.When the annealing temperature is 620 °C,The composite structure of recovered layer and recrystallized grain was obtained,the yield strength and elongation were 360MPa and 27.4 %,respectively.(3)After the 90 % cold-rolled and cryogenic samples were annealed at 620 °C for 1 h,the second liquid nitrogen cryogenic rolling was carried out with a deformation of 40 %.The second cryogenic process caused martensitic transformation in both the recovered layer and the recrystallized grains,the yield strength is increased to 366MPa and the martensite content is 17.8 %.40 % cold rolling not only increases the strength of the sample,but also has martensitic transformation.the yield strength is 770MPa and the martensite content is 29.7 %.(4)Finally,reverse phase transition annealing was performed on the samples with double cryogenic and 40 % cold rolled deformation.The starting and ending temperatures of martensite reversal of 40 % cold-rolled samples are 410 °C and 430 °C,respectively.When the sample is annealed at 420 °C,the structure recovers at the same time of martensite reverse transformation.With the increase of annealing time,the strength increases first and then decreases.When the annealing time is 1 h,the yield strength reaches the maximum value of 810MPa.