Phase Transformations Of The 08F Steel During Nitrocarburization And Subsequent Heat Treatment

Ferritic nitrocarburizing(below 592?)is a surface-chemical heat treatment method by which the nitrogen and carbon atoms diffuse into the surface of ferrous materials,which can improve obviously surface hardness,wear resistance,corrosion resistance and bearing capacity properties of workpiece.So it is widely applied in the industry.The nitrocarburized layer of steels consists generally of a compound layer and a diffusion layer,in some cases,a transition layer can be formed between the compound layer and the diffusion layer.During the conventional nitrocarburizing process,the compound layer composed of ?-Fe2-3(C,N)phase and ?'-Fe4(C,N)phase is formed firstly on the surface of steels,which enhances surface hardness and wear resistance.Howerver,the compound layer is usually thin and brittle in essence,and sometimes contains many of pores,which reduces the fatigue properties of materials so that the compound is avoided in some cases.So far,the formation mechanism of compound layer is a controversial issue,and study on subsequent heat treatment of nitrocarburized layer is very limit.In the present work,a transition layer formed between the compound layer and diffusion layer was performed to investigate the phase transformations occurring in the nitrocarburized surface of 08F steel by a controlled nitrocarburizing process(in the range of 560-580?).In addition,the subsequent heat treatment on nitrocarburized layer of 08F steel was investigated.The microstructures in nitrocarburized layer of 08F steel and its subsequent strengthening layer have systematically been studied by means of optical microscope,X-ray diffraction,electron probe X-ray microanalysis,scanning electron microscope and transmission electron microscope,in association with the first-principles energy calculations.Phase transformations occurring in nitrocarburizing process and subsequent heat treatment process were also investigated.Meanwhile,the mechanical properties of strengthening layers formed under different stages were measured by Vicker hardness tester and nanoindentation.From the obtained results,the followings can be concluded:(1)A transition layer is formed between the compound layer and diffusion layer by a controlled nitrocarburized process.This transition layer not only improves remarkably surface hardness and ductility,but also provides the microstructures under different nitrocarburizing processes to trace back what had happened in the nitrocarburized layer occurring during nitrocarburizing processes,and provides believably experimental evidence for phase transformations.(2)The microstructure features of the transition layer provides crucial information about the phase transformation as follows:(i)the observed lamellar pearlite colonies in the transition layer indicate that all the surface sublayers prior to this pearlite colony layer had undergone a y-N/C phase state;(ii)the equil-axial bulk-like ?'-grains existing around the pearlite colonies and the ?'-sublayer having plenty of two-fold and three-fold microtwins suggest that the ?'-grains are the proeutectoid ?'-grains,and are directly formed from the y-N/C phase at the nitrocarburizing temperature;(iii)three-fold microtwins in ?'-grains indicates that the Y'phase does not derive from the decomposition of the ? phase due to orientation relationship between them,i.e.{111}?'//{0001}?;(iv)the precipitation of the ?-phase lamellas observed at the twin interfaces of ?'-microtwins demonstrates that at the nitrocarburizing temperature the ?-carbonitrides can nucleate and grow in the?'-grains.(3)The first-principles energy calculations show that the formation enthalpy of?-Fe3CxNi1-x carbonitride with the same C:N atomic ratio is always less than that of?'-Fe4CxN1-x carbonitride,which indicates that phase transformation occurring in nitrocarburizing process,i.e.,?'+N/C??,is a nature process.(4)The phase transformation mechanism or sequence of the nitrocarburizing surface of carbon steels can concisely be described as:?-Fe + N/C ??-N/C + N/C ??' + N/C ??.This phase transformation mechanism not only interprets the formation of the transition layer,but also for the compound layer,as well as for the grain boundary precipitates in the diffusion layer.Hence,the currently accepted phase transformation sequence or mechanism for the nitrocarburized surface of carbon steels,i.e.,?-Fe + N/C ??',has been developed in the present work.(5)The nitrocarburized 08F steel is performed to austenitize at 700? for different time,and Fe-C-N austenitic layers with low C content and high N content are prepared successfully.These austenitic layers are retained by cooling in water,indicating that Fe-C-N austenitic displays excellently thermal stability.(6)The decomposition mechanism of nitrocarburized compound layer occurring in austenitizing process can be depicted as follows:?+?'??'+?-N/C??-N/C,releasing a large number of active N and C atoms.These active N and C atoms diffuse into adjacent ?-Fe phase and make it to transform into ?-N/C phase,increasing the thickness of austenitic layer.Because decomposition of nitrocarburized compound layer belongs to diffusional phase transformation,so it needs long time to complete austenitic transformation.In present work,it will cost 5h for nitrocarburized layer to transform completely into austenitic layer.(7)The Fe-C-N austenite decomposes into ultra-fine bainitie during aging at 225? for 14h.The hardness of this bainitic layer is up to1200HV0.245N and it has excellent ductility.The microstrucutre of ultra-fine bainitic structure and its propertey are dependent on holding time of austenitizing process.During the unfinished austenitizing process,the thickness of bainitic layer and surface hardness of experimental materials increase with the prolonging of austenitizing time;when austenitizing time reaches 5h,the original compound layer transforms fully into austenitic layer containing even N and C contents,and thickness of bainitic layer as well as surface hardness of its aged specimen reach peak values;when the holding time exceeds 5h,surface hardness of aged specimen decreases with the increasing of austenitizing time,but thickness of bainitic layer almost remains constant.(8)During the subsequent heat treatment of nitrocarburized specimens,the grain boundary area in diffusion layer of austenitized specimens has also transformed into ultra-fine bainite,and the bainite at grain boundary connects with each other to form a large number of strengthen networks.(9)The bainitic phase transformation mechanism of Fe-C-N austenite aged at 225? is described as follows:First,Fe-C-N austenite decomposes into N/C-rich zones and N/C-depleted zones by a spinodal decomposition;secondly,when the contents of N and C atoms in these N/C-rich zones reach critical values,which will transforms to pre-?' phase by ordering of N and C atoms,and subsequently converts into ?' phase;Finally,these N/C-depleted zones can transform into ?-Fe phases with supersaturated N and C atoms by a shear transforamtion mechanism,and then the N and C atoms cluster at some planes of the ?-Fe phase with supersaturated N and C atoms,resuting in precipitating of high density ?" precipitates from the ?-Fe phase.In addition,the upper bainite with feather featured morphology and the low bainite without specific featured morphology are observed due to different diffusion speeds of N and C atoms in intergranular and defects such as grain boundary,dislocation and so on.Whether upper bainitic structure or low bainitic strucutre,the ?' phase is always a leading phase during the bainitic reaction,and the ?' phase keeps up cubic-cubic orientation relationship with original the ? phase.