Investigation On The Properties Of Steel Based On Laser-Induced Breakdown Spectroscopy

Steel is one of the most widely used materials,and it is regarded as the primary raw material in national economy and social development.In practice,kinds of trace elements are added in the process of steel-making to obtain special properties.In some instances,hardenability of steel can be improved by adding manganese in steel-making;the heat resistance of steel becomes better when titanium is mixed;effect of hydrogen corrosion resistance of iron is increased by mixing molybdenum.The technologies of detection and analysis for mixing components is extremely important for improving steel quality because these concentrations of mixing elements determine the properties and types of steel.There are a few analysis methods for element composition of steel,such as X-ray fluorescence micro-quantitative analysis,atomic absorption spectrometry and inductively coupled plasma atomic emission spectrometry.However,some of them can not measure all elements,suffer tedious pretreatment,or are time-consuming and expensive etc.In this paper,laser-induced breakdown spectroscopy(LIBS)is employed to detect and analyze several mixing element of steel rapidly and simultaneously.The main contents of this dissertation are as follows: 1.The LIBS system for detecting steel samples is built and it mainly consists two parts: light path section and spectral signal analysis section.The light path includes pulse laser,optical beam collimation and focusing,and plasma light collection.And spectral analysis includes pulse delay generator,spectrometer and CCD.The difficulty of building the system is how to align the beam and collect the plasma beam.It is required the beam is reflected by a mirror at 45o and focused on the surface of sample through a lens.It is found that stronger signal is obtained when fiber collector is placed at 4cm above sample and 45o relative to the laser beam.2.The parameters of LIBS system are very important for the qualitative and quantitative analysis.In this dissertation,the factors such as the selecting characteristic wavelength,energy of pulse laser,delay of CCD acquisition,distance between the focused beam point and the surface of sample,etc.have been studied to obtain the optimum parameters in LIBS measurement.Spectral intensities of atomic lines are related to transition probabilities of excited energy states.Ti I 334.19 nm,Fe I 438.35 nm,Fe I 427.12 nm,and Mo I 438.16 nm have been selected as analytical lines.The variation of spectral signals is observed at the laser energy of 30 mJ,35mJ,40 mJ,45mJ,and 50 mJ.It is found that spectral intensity is proportional to the laser energy.At the same time,the spectra were obtained at the delay time set to 0.1 ?s,1 ?s,10 ?s,and 100 ?s,respectively.The recorded spectrum shows that the optimum spectral signal is obtained at 1 ?s.The intensity of observed spectra is maximum when the convergence of beam is focused on the surface of sample.3.The optimum parameters(laser energy 50 mJ,time delay 1 ?s,focal point position on the sample surface)are employed to quantitatively analyze the Ti element in the homemade iron alloy.In comparison with the traditional method and the internal standard method,we have first proposed the double-line average internal standard method to establish a calibration model for steel samples.The fitting correlation coefficient(R)obtained by the internal standard method is 0.9978 and 0.9939,respectively,which was superior to the R(0.9563)obtained by the conventional method.The R-value was 0.9984 derived by the double-line average internal standard method fitted.In addition,the relative error of the traditional calibration method in the range of 0.063%-1.9% is 23.7%,the relative error of the internal standard method is 6.0%,and the relative error of the average internal standard method is 3.9%.It can be seen that the double-line average internal standard method can effectively remove the influence of the matrix effect on the measurement result,reduce the analysis error,and improve the accuracy of measurement.4.D2 steel samples prepared by different heat treatment processes have different microstructures.The plasma characteristics and spectral characteristics of different D2 steel samples were obtained through measuring the spectra of Mo I 438.16 nm in different microstructures of D2 steel.The data shows that the metallographic structure has a certain correlation with the spectral intensity,and this property can be used to identify the metallographic state of the metal.