Ultrasound Detection Of Solidification Structure And Defects Of 42CrMo Molded Ingot

As the equipment manufacturing industry develops in the direction of scale,integration and specialization,the quality requirements for large-scale forgings are getting higher and higher,and the quality requirements for upstream product ingots are also increasing.But the steel ingot will inevitably have internal defects during the casting process,which has varying degrees of impact on the quality of downstream products,and even lead to the scrapping of forgings in severe cases.Therefore,it is of great significance to improve the utilization rate of steel ingots,improve the quality of forgings and process stability to carry out ultrasonic testing on steel ingots and formulate targeted forging processes according to the testing results.Taking the 42 CrMo octagonal ingot as the research object,the ultrasonic inspection of the local curved surface of the ingot body was carried out by the A-type pulse reflection ultrasonic inspection system.According to the ultrasonic testing time domain waveform,it was preliminarily determined that there was a continuity defect in the range of303～603mm from the riser inside the ingot.Slicing at predicted locations with defect on the ingot,through low magnification inspection,it was determined that the internal defect of the steel ingot was the central loose defect.Combined with the low magnification inspection results,the ultrasonic signal was analyzed by spectrum,and a method was proposed to characterize the position and size of the loose defect inside the steel ingot by using the spectral characteristic parameters of the ultrasonic signal.Finally,it was determined that there are loose defects in the range of 237～803mm from the riser,which greatly improved the identification accuracy of defects compared with the ultrasonic time-domain signal judging defect boundary method.The solidified structure inside the ingot was obtained by metallographic examination,and it was determined that it was composed of fine grain area on the surface,columnar crystal area,coarse equiaxed crystal area and slightly finer equiaxed crystal area in the center.The relationship between the morphology,size and distribution characteristics of the solidified structure inside the steel ingot and the spectral characteristic parameters was studied.A method to characterize the inner columnar grain boundary of the steel ingot was proposed by using the characteristic parameters such as the amplitude peak value,the center frequency and the peak frequency of the amplitude spectrum.By this method,the columnar grain boundary was determined to be 80～100mm away from the outer edge.According to national standards,statistical analysis of the grain size inside the ingot was carried out.The results showed that the grain size in the transition region between columnar crystals and coarse equiaxed crystals was the lowest,about 2～3 grains per square centimeter.Except for the chilled crystal in the outer layer,the finer equiaxed crystal area near the center of the ingot was the highest grain size,with more than 6 grains per square centimeter.Take any position where the internal structure and defects of the steel ingot unknown,and carried out ultrasonic testing.The range of columnar grain boundaries and low-magnification defects obtained according to the ultrasonic detection echo spectral characteristics is consistent with the experimental results,which verified the feasibility of the ultrasonic signal spectral parameters to characterize the columnar grain boundaries and loose defects.