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·Materials Characterization and testing
The Non-Destructive Testing of the Quality of Hard Alloy ProductsYu Fengnian
The Heilongjiang North Tools Factory
Hard alloy is a powder metallurgy material,which is made by pressing metal powder to shape and then sintering in the high temperature.
Because of raw material,the process (mixing,shaping, sintering,heat treatment etc.) and soon hard alloy products have defects,different grain size and different mechanical property.This kind of products stand very high pressure when using.If its mechanical property dose not reach the standard,its using time will be reduced,or even it break to uselessness when it is used only once.In order to pledge the quality of products,we must conduct a hundred per cent test to them,which is to finished by only non-destructive testing. We have made research of non-destructive testing method of the hard alloy products,
using mainly ultrasonic method and magnetic method:removing the flawed by detecting the flaws of products;evaluating grain size and properties of products by measuring sound speed,ultrasound attenuating and coercivity,and then evaluating the quality of products by the comprehensive results of ultrasound testing and measuring coercivity.This non-destructive testing,which conducts the complete test to products,pledges the quality of products.
CL is the longitudinal wave sound speed of the materials;E is the elastic modulus of the materials;r is the density of the materials,and s is Poisoon ratio. The sound speed of the materials is related to elastic modulus of the materials, so it also related to the strength of the materials. Therefore, measuring the sound speed can estimate the strength of the materials in a certain field.
Measure the time t, which is spent from the mth bottom echo of products to the nth, by the thickness of products s and t, it is that the longitudinal wave sound speed is
|a= ad + asc + aa||(3)|
In this formula, a d is spread attenuation coefficient; a sc is scattering attenuation coefficient; a a is absorption attenuation coefficient. In the course of measurement,the ultrasonic field produced by the same probe in products which the same length and shape may be regarded as the similar, therefore, the scattering attenuation may be regarded the same. (The absorption attenuation of solid materials is very small )In the formula of the scattering attenuation coefficient of materials
|asc = C2Fd3f4 (d«l)||(3)|
C2 is scattering coefficient, F is anisotropic coefficient, d is grain diameter, f is ultrasonic frequency, l is ultrasonic wave length. ( Among hard alloy products,5MHz ultrasonic longitudinal wave is approximately 1.35mm, grain diameter is largest 2 m m, which is further smaller than wave length) In the course of measuring attenuation, ultrasonic frequency is the same. The scattering coefficient C2 and anisotropic coefficient F in every products may be regarded similar, therefor, scattering attenuation is determined by grain diameter d. From above,measuring ultrasonic attenuation can determine the grain degree, and ultrasonic attenuation may be determined by measuring the bottom echo highness of products. The bigger attenuation is, the lower bottom echo is (gain is bigger);the smaller attenuation is, the higher bottom echo is(gain is smaller). For example,there are two products with lower bottom echo respectively 43dB and 42dB,their grain degrees are bigger both between 1.6-2.0m m; and the other two products with higher bottom echo,is 34dB, their grain degrees are smaller, between 0.8- 1.0m m. Using immersion testing to measure bottom echo can assure the same incidence condition of every products.
The Table 1 shows that the consequences of conducting ultrasonic defection, sound speed measurement, bottom echo measurement and coercivity measurement of products can evaluate the quality of products. The contrast in quality among non-destructive consequences and dissection testing and using situation indicates that this method of non-destructive testing can pledge the quality of products.
|number||defecting results||soundspeed CL (m/s)||bottom echo highnees |
|coercivity Hc (KA/m)||grain degree (mm)||strength (MPa)||using consequence|
|1||flawful||6684.0||43||8.6||1.6- 2.0||1021||using once was crushed|
|2||flawless||6679.7||42||8.46||1.6- 2.6||1019||using once was crushed|
|3||laminated flaw||6762.5||41||15.02||1.0- 2.0||2065||using once was crushed|
|4||flawless||6731.9||34||14.49||0.8- 1.0||2130||using 3040th no problem|
|5||flawless||6758.3||33||13.49||0.8- 1.0||2097||using 1859th no problem|
|6||flawful||6775.1||34||15.89||0.8- 1.0||2053.3||using thrice was crushed|
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