Study On Reliability Of Vermicular Graphite Cast Iron Vermicular Rate Detected By Ultrasound

The machinability and shock absorption of vermicular graphite cast iron are similar to those of common gray cast iron,and it also has good casting properties.These excellent performances make it widely used in working environment withstanding high mechanical thermal stress,such as cylinder block,cylinder head,brake disc of large diesel engine,etc.The core of producing high quality vermicular graphite iron castings is to control the fluctuation of vermicular rate.Therefore,the detection of vermicular rate is particularly important.The methods of measuring vermicular rate include metallographic method,audio method,ultrasonic method,etc.Among them,measuring vermicular graphite cast iron vermicular rate by ultrasonic method is a promising method for on-line rapid detection.In this paper,the factors affecting the vermicular rate test results of vermicular graphite cast iron by ultrasonic method are studied.For example,the surface roughness of the tested sample,the plane parallelism of the tested sample,the perpendicularity between the ultrasonic probe and the testing surface of the sample,the types of the ultrasonic coupling agents,the thickness of the coupling layer,the matrix structure of the casting,etc.And large number of experiments and simulations have been carried out on these factors.The results show that it will bring a difference of 4.58m/s if the parallelism of the two planes is changed by 0.1 degree on average.The relationship between the perpendicularity of the probe and the surface of the sample and the ultrasonic velocity is approximately linear.The ultrasonic velocity decreases by about 4.92 m/s with the increase of the dip angle by about 0.1 degree.The ultrasonic velocity increases slightly with the increase of pearlite content,but the change is very small and almost negligible.Roughness(Ra?5)of workpiece obtained by general machining will not affect sound velocity measurement.The commonly used coupling agents have little effect on the sound velocity measurement of vermicular graphite cast iron and can be chosen freely among them.For each increase of the thickness of coupling agents 0.05 mm and the decrease of sound velocity 32.9 m/s,the measurement of sound velocity will be greatly affected.On the basis of the above research results,453 specimens were quantitatively analyzed for vermicular rate according to the standard GB/T 266656-2011 with the help of quantitative metallographic analysis software through field sampling and laboratory pouring specimens,Meanwhile,the sound velocity is measured by Echometer 1076 ultrasonic velocimeter.The sound velocity in[5060,5200]m/s,[5200,5400]m/s,(5400,5700]m/s are obtained by using the method of piecewise regression analysis in MATLAB.Linear relation model,exponential relation model and linear relation model of vermicular rate-sound velocity are established respectively.The variance R~2 of each relation model is greater than 0.9,which indicates that the fitting degree is good.Under laboratory conditions,33 groups of experiments were carried out to verify the model.The results show that when the error value is less than±5%based on metallographic method,the accuracy of prediction vermicular rate of the model is over91.7%,92%in the area of[5200,5400]m/s and 90.9%in the area of[5400,5700]m/s.