The Formation Mechanism Of Reverse Austenite Of 15-5PH Stainless Steel And Its Influence On Propertie

15-5PH stainless steel has been widely used in aerospace,oil and gas mining and other fields owing to its high strength,good toughness and excellent corrosion resistance and other comprehensive properties.The reversed austenite（A_r）is found inside most early failure 15-5PH valve boxes.As a soft and tough phase,although it can improve the material toughness,but it reduces the strength of the material,causing an adverse effect on the actual production.Therefore,it is of practical significance to study the formation of reversed austenite in 15-5PH stainless steel and its influence on the properties.The reversed austenite of 15-5PH stainless steel was studied in this work.The effects of plasma rapid heating and long aging（12-72 h）on the transformation of reversed austenite in 15-5PH stainless steel was studied by means of X-ray diffraction（XRD）,scanning electron microscopy（SEM）and transmission electron microscopy（TEM）.This work focus on the formation mechanism of reversed austenite,and investigated the relationship between reversed austenite content,microstructural characteristics,the mechanical properties and hydrogen damage resistance of 15-5PH stainless steel by tensile,impact,hydrogen penetration and hydrogen blistering tests.It was found that the formation temperature（A_s）of reversed austenite for the15-5PH stainless steel samples by plasma rapid heating is between 700°C and 725°C.The amount of reversed austenite increases quickly with elevating of the heating temperature,and reaches its maximum at 775°C.The microstructure of this sample contains 91%of martensite（M）and 9%of A_r.Subsequently,the increase of heating temperature has no effect on the content of A_r.Due to the suppression of the decomposition of martensite by plasma rapid heating,the microstructure of all samples after rapid heating consists of martensite and reversed austenite.With elevating of the heating temperature,however,the thin-film reversed austenite and lath martensite transformed to the short rod-like reversed austenite and polygonal martensite,respectively.The investigations of TEM and SAED patterns further confirmed that the orientation relationship between the old phase（M）and the new phase（A_r）follows the K-S relationship,that is,（111）γ∥（011）α’and[110]γ∥[111]α’.Similarly to the plasma rapid heated samples,the content of A_rincreases rapidly and then remains stable with increasing of aging time.The results showed that the 0～48h of aging time is the rapid growth stage of the content of A_r.The content of A_rin the48 h aged sample greatly increased to 10%.After that,the content of reversed austenite hold steady with further increasing of aging time.Unlike the rapid heated samples,the microstructure of the long aged samples is composed of tempered martensite,reversed austenite and nano-sizedε-Cu phase.The investigations of XRD and EDX indicated that Cu element almost entirely precipitated in the sample after 12 h of aging.Moreover,it was found that the size ofε-Cu precipitates increased with increasing of the aging time.For example,the average particle size increased from 20.8 nm for the 12 h aged sample to 33.6 nm for the 72 h aged sample.The observations of TEM revealed that the morphology of reversed austenite transformed from thin sheet to thick sheet with increasing of aging time.Based on Johnson-Mehl-Avrami equations,the kinetics model for the formation of reversed austenite in the 15-5PH stainless steel samples after long aging was proposed as following:f（28）0.971[-exp(-1.3×10^-6t^0.94)].The reversed austenite played an important role in the properties of 15-5PH stainless steel.On the one hand,the tensile and yield strengths of the samples after plasma rapid heating and long aging reduced with increasing of reversed austenite;on the other hand,the A_rcontribute to the improvement of toughness.In terms of yield ratio,the value of long aged sample is higher,which can be beneficial to raising the service life of the 15-5PH fracturing pump boxes.The results of hydrogen penetration and hydrogen blistering tests showed that（i）the hydrogen diffusion coefficient of the15-5PH stainless steel samples decreases with increasing of heating temperature and aging time,（ii）the critical hydrogen blistering current density increases with increasing of heating temperature and aging time.This clearly indicates that the increase of reversed austenite and the formation ofε-Cu precipitates play a significant role in improving the hydrogen damage resistance of the 15-5PH stainless steel.