Study On Embrittlement Mechanisms Of Interstitial-free And 2.25Cr1Mo Steels

The interstitial-free(IF)and 2.25Cr1 Mo low alloy steels both have low content of carbon equivalent and characterize high toughness and plasticity.Based on respective compositions and properties,the low alloy steels are adopted by different fields.For instance,IF steel with great deep drawability is mainly employed in the manufacture of some automobile body coverings and structural parts.2.25Cr1 Mo steel has become the most important material of pressure vessel and pipe because of its excellent toughness and high resistances of oxidation and temperature.However,the phosphorus added for solid solution strengthen in the IF steel could segregate at grain boundaries in hot rolling,which further leads to non-hardening embrittlement of the steel.Meanwhile,the hardening embrittlement of the IF steel occurs during deep drawing and cold rolling.In another way,the grain boundary segregation of some impurities in the 2.25Cr1 Mo steel is non-negligible after its long time service at a medium temperature,resulting in non-hardening embrittlement of the steel.In practice,the hardening embrittlement of 2.25Cr1 Mo steel may be caused by some processes such as welding and aging precipitation.Last but not least,the grain size has ability to affect both hardening and non-hardening embrittlements of these two low alloy steels.Therefore,the increased brittleness of the steel is adopted because of the combined hardening and non-hardening embrittlement but there is no available reference about this.Hence,this study mainly focuses on the combined effect of the phosphorus grain boundary segregation,strengthening(hardening)and grain size on the brittleness of the IF and 2.25Cr1 Mo steels.It aims to clarify the mechanism of hardening and non-hardening embrittlement for the low alloy steel.This study mainly explores the embrittlements of IF and 2.25Cr1 Mo steels.By means of high temperature heat treatment,aging heat treatment,room temperature tensile and welding thermal simulation,various combinations of the grain size,phosphorus boundary segregation concentration and yield strength of the samples are obtained.Then the fracture appearance transition temperatures(FATTs)of samples are determined by the impact tests along with scanning electron microscope(SEM)observations of the fractures.Several characterization methods,such as hardness measurement,metallographic,Auger electron spectroscopy(AES)analysis,field emission gun scanning transmission electron microscopy(FEGSTEM)and electron back scattering diffraction(EBSD)are employed to analyze the microstructures,chemical compositions of the grain boundary,grain boundary character distributions,and Vickers hardness of the experimental steels.Based on these analysis results,the relationships between FATT with grain size,strength(hardness)and P segregation concentration are established and the mechanism of combined hardening and non-hardening embrittlement of the low alloy steel are clarified.It has been discovered that the segregation of phosphorus at a constant temperature 600 oC presents a non-equilibrium character in the IF steel quenched from a higher point.This non-equilibrium segregation vanishes after 20 hours during aging heat treatment.Furthermore,the equilibrium segregation concentrations of phosphorus in both IF and 2.25Cr1 Mo steels increase with the rising grain size while decreases with the aging temperature going up.Further study indicates that the dependence of P segregation concentration on the grain size mainly results from the increasing content of large angle and high ? CSL grain boundaries in the coarser grain steels.Finally,based on the theoretical analysis of segregation thermodynamics,the segregation free energy increases with the rising grain size;meanwhile there exists a linear relationship between segregation entropy and enthalpy.The FATTs of the IF steel with different combinations of grain size,P boundary segregation concentration and yield strength are determined.The result shows that the FATT has positive linear relationships with P segregation concentration(Cp,at.%)and yield strength(?s,10MPa)but negative linear relationship with grain size(d-1/2,mm-1/2).What's more,the linear relationship between FATT and P segregation concentration or between FATT and yield strength is affected by grain size,which however has no correlation.Based on the Taylor expansion model,the combined hardening and non-hardening embrittlement equation for the IF steel is established: FATT(oC)= 2.1Cp + 3.48?s-22.36d-1?2 + 0.64(Cp-14)(d-1?2-3.06)+0.896(?s-14)(d-1?2-3.06)-13.7,where Cp is the phosphorus grain boundary concentration in at.%;?s is the yield strength in 10 MPa and d is the grain size in mm.It is indicated that there is some differences between the embrittlement mechanism of 2.25Cr1 Mo steel and that of IF steel.The major one is that the FATT of 2.25Cr1 Mo steel is independent on its average grain size.This can be attributed to their different microstructures which result in different sensitivities of the brittleness to their grain boundary structures.On the other hand,similar to the IF steel,the FATT of 2.25Cr1 Mo steel also has linear relations with the P segregation concentration and yield strength.The embrittlement equation is set up to describe the relationship between FATT and P segregation concentration and yield strength for the 2.25Cr1 Mo steel: FATT(oC)=0.25?s+2.64CP-270,where ?s is the yield strength in MPa and CP is the grain boundary concentration of P in at.%.The brittleness of welding heat affected zone in the 2.25Cr1 Mo steel is suggested to be enhanced by the peak temperature,while it comes down significantly after tempering.By observation of the fracture morphology,it shows that the welding heat affected zone has the intergranular fracture.However,the tempered one presents the intergranular fractures.The embrittlement of the as-welded heat affected zone can be ascribed to the existence of the M/A islands and the welding residual stress so that it belongs to the hardening embrittlement.Furthermore,the M/A island becomes coarser when the peak temperature is improved,increasing the brittleness of the heat affected zone at higher peak temperature.The M/A island in heat affected zone disappears after tempering for 800 h at 520 oC;in the meantime a lot of small carbides emerge and mainly distributes near some interfaces of the microstructure.On the other hand,the increased P segregation concentration with the increasing peak temperature results from the grain size coarsening during the improvement of the peak temperature.Consequently,contrary to the as-welded heat affected zone,the embrittlement of tempered heat affected zone belongs to non-hardening embrittlement because of the major impact of the grain boundary segregation of phosphorus.